## Posts Tagged ‘Higgs boson’

### Knowledge and the Higgs Boson

Friday, May 17th, 2013

This essay makes a point that is only implicit in most of my other essays–namely that scientists are arro—oops that is for another post. The point here is that science is defined not by how it goes about acquiring knowledge but rather by how it defines knowledge. The underlying claim is that the definitions of knowledge as used, for example, in philosophy are not useful and that science has the one definition that has so far proven fruitful. No, not arrogant at all.

The classical concept of knowledge was described by Plato (428/427 BCE – 348/347 BCE) as having to meet three criteria: it must be justified, true, and believed. That description does seem reasonable. After all, can something be considered knowledge if it is false? Similarly, would we consider a correct guess knowledge? Guess right three times in a row and you are considered an expert –but do you have knowledge? Believed, I have more trouble with that: believed by whom? Certainly, something that no one believes is not knowledge even if true and justified.

The above criteria for knowledge seem like common sense and the ancient Greek philosophers had a real knack for encapsulating the common sense view of the world in their philosophy. But common sense is frequently wrong, so let us look at those criteria with a more jaundiced eye. Let us start with the first criteria: it must be justified. How do we justify a belief? From the sophists of ancient Greece, to the post-modernists and the-anything-goes hippies of the 1960s, and all their ilk in between it has been demonstrated that what can be known for certain is vanishingly small.

Renee Descartes (1596 – 1960) argues in the beginning of his Discourse on the Method that all knowledge is subject to doubt: a process called methodological skepticism. To a large extend, he is correct. Then to get to something that is certain he came up with his famous statement: I think, therefore I am.  For a long time this seemed to me like a sure argument. Hence, “I exist” seemed an incontrovertible fact. I then made the mistake of reading Nietzsche[1] (1844—1900). He criticizes the argument as presupposing the existence of “I” and “thinking” among other things. It has also been criticized by a number of other philosophers including Bertrand Russell (1872 – 1970). To quote the latter: Some care is needed in using Descartes’ argument. “I think, therefore I am” says rather more than is strictly certain. It might seem as though we are quite sure of being the same person to-day as we were yesterday, and this is no doubt true in some sense. But the real Self is as hard to arrive at as the real table, and does not seem to have that absolute, convincing certainty that belongs to particular experiences. Oh, well back to the drawing board.

The criteria for knowledge, as postulated by Plato, lead to knowledge either not existing or being of the most trivial kind. No belief can be absolutely justified and there is no way to tell for certain if any proposed truth is an incontrovertible fact.  So where are we? If there are no incontrovertible facts we must deal with uncertainty. In science we make a virtue of this necessity. We start with observations, but unlike the logical positivists we do not assume they are reality or correspond to any ultimate reality. Thus following Immanuel Kant (1724 – 1804) we distinguish the thing-in-itself from its appearances. All we have access to are the appearances. The thing-in-itself is forever hidden.

But all is not lost. We make models to describe past observations. This is relatively easy to do. We then test our models by making testable predictions for future observations. Models are judged by their track record in making correct predictions–the more striking the prediction the better. The standard model of particle physics prediction of the Higgs[2] boson is a prime example of science at its best. The standard model did not become a fact when the Higgs was discovered, rather its standing as a useful model was enhanced.  It is the reliance on the track record of successful predictions that is the demarcation criteria for science and I would suggest the hallmark for defining knowledge. The scientific models and the observations they are based on are our only true knowledge. However, to mistake them for descriptions of the ultimate reality or the thing-in-itself would be folly, not knowledge.

[2] To be buzzword compliant, I mention the Higgs boson.

### Petite chronique d’un prof au CERN (II)

Thursday, May 2nd, 2013

A l’occasion de l’ouverture de l’appel à candidature 2013 de “Sciences à l’Ecole” pour l’accueil d’enseignants français au CERN durant une semaine, nous publions ces jours-ci le journal quotidien plein d’humour de Jocelyn Etienne qui a suivi ce programme l’année dernière, au mois de novembre dernier.

Immersion au pays des particules
Lundi 05 novembre 2012

C’est moi ou la pièce de 5 francs est énorme ?

Grosse journée, petit déjeuner au restaurant du CERN, bon café, tartine beurrée confiturée pour 1 franc suisse ! A tester. Tiens d’ailleurs, c’est moi ou la pièce de 5 francs est énorme ?

L’hôtel-foyer du CERN

Il ne pleut pas ce matin (ça ne va pas durer, la promenade post-déjeuner s’est faite sous la pluie) alors j’en profite pour prendre une photo de l’hôtel-foyer qui m’héberge. C’est une des fenêtres du 1er étage derrière laquelle se trouve ma chambre, mais inutile de zoomer pour chercher à m’apercevoir. Qui prend la photo à votre avis ?

Daniel Denegri aurait peut-être vu le boson de Higgs…

Après une première présentation de l’in2p3 par Arnaud Marsollier, suivi de Mick Storr pour le CERN, c’est Daniel Denegri qui nous présente l’expérience CMS, incroyable projet de détection de particule qui s’étend sur 20 ans. Denegri lui-même est un brillant chercheur croate qui parle parfaitement le français, l’anglais entre autres, il aurait peut-être vu le boson de Higgs qui semble plus facile à détecter que son bras droit, tellement le bonhomme est énergique. L’après-midi, c’est au tour de Simone Gilardoni, théoricien des « accélérateurs collisionneurs » de nous montrer que les prouesses nécessaires pour maintenir un faisceau de protons dans un tube de 27 km de long, ne sont pas à la portée des bricoleurs du dimanche. Ou devrais-je dire 2 faisceaux dans 2 tubes qui se croisent de temps en temps ?…

Simone Gilardoni, théoricien des “accélérateurs collisionneurs”

Le petit point visible derrière Simone est visible ici en direct, si le LHC n’est pas à l’arrêt. Il y en a même deux, comme je l’ai dit précédemment ; nos deux faisceaux de protons dont on contrôle l’état notamment par des miroirs qui renvoient le rayonnement qu’il diffuse… enfin, c’est ce que j’ai compris…

D’ailleurs le LHC va bientôt être arrêté pour quelques mois (il est actuellement arrêté, ndlr, voir ici pourquoi en vidéo). J’espère que ce n’est pas lié à ce bouton sur lequel j’ai appuyé en pensant que c’était l’éclairage de ma salle de bains. Il reprendra ensuite de plus belle pour tenter d’atteindre les 13-14 TeV contre 7 TeV actuellement. Je sais, ça fait beaucoup…

L’après –midi se poursuit par une présentation des masterclasses par Nicolas Arnaud, chercheur à Orsay et organisateur de notre French Teacher Programme au CERN. Puis il nous initie à la détection de particules à l’aide d’un logiciel et de vraies mesures.

Atelier “masterclasses”: J’ai trouvé les W qui se désintègrent, donc j’ai le droit de prendre une photo de mes collègues en plein effort.

Pour finir, je me rends à une conférence tardive sur les sondes Voyager 1 et 2 donnée par Edward Stone, responsable scientifique de ces sondes depuis 1972.

Sur le chemin, j’immortalise la version suisse du principe de superposition d’état, ou comment un vélo peut être en deux endroits différents au même moment…

A suivre…

Jocelyn Etienne est enseignant au lycée Feuillade de la ville de Lunel.

Pour soumettre sa candidature pour la prochaine session du stage au CERN, c’est par ici.

Principe de superposition d’état…

…ou comment un vélo peut être en deux endroits au même moment !

### Higgs update, HCP 2012

Thursday, November 22nd, 2012

Last week, Seth and I met up to discuss the latest results from the Hadron Collider Physics (HCP) Symposium and what they mean for the Higgs searches. We have moved past discovery and now we are starting to perform precision measurements. Is this the Standard Model Higgs boson, or some other Higgs boson? Should we look forward to a whole new set of discoveries around the corner, or is the Higgs boson the final word for new physics that the LHC has to offer? We’ll find out more in the coming months!

### How to tell a Higgs from another boson?

Thursday, September 20th, 2012

On July 4, when CERN announced “the observation of a new particle” and not the discovery of the Higgs boson, many wondered why be so cautious. It was simply too early to tell what kind of boson we had found. The Higgs boson is the last missing piece of the Standard Model of particle physics, a model that has enabled theorists to make extremely precise predictions. But to fully trust this model, it should have all its pieces. Who would want to complete a 5000-piece puzzle with the wrong piece?

Both the CMS and ATLAS experiments have been conducting several checks since July:

1) Are all possible decay modes predicted by the Standard Model observed?

2) Is each observed decay happening at the right rate?

3) What are the fundamental properties of the new boson?

The first checks (based on half the data now available) indicate that the new boson is compatible with being the Higgs boson. But the precision is still too low to tell as shown on the plots below (the signal strength and σ/σSM H are the same quantity).

The Higgs boson can decay in many ways and the plot shows which decays have been observed and at what rates. A signal strength (of 1 means the signal corresponds exactly to what is expected for a Higgs boson.  Zero would mean there is no signal seen for this particular decay channel. The black points represent the measured values and the horizontal bar, the error margin.

At this point, we cannot tell unambiguously if the first two measurements are more compatible with 0 (the decay does not exist) or 1 (yes, it decays at the predicted rate).  Both CMS and ATLAS need to analyze more data to say if the new boson decays into two b quarks (H → bb) and two tau leptons (H → ττ).

The other three decay modes, namely WW, two photons (H → γγ) and ZZ occur at about the rate or slightly more often than expected by the Standard Model.

The decisive test will come by measuring its spin and parity, two “quantum numbers” or properties of fundamental particles. The spin is similar to the angular momentum of a spinning object. But for fundamental particles, only discrete values can be used. For bosons (the particles carrying the various forces), these values can be 0, ±1, ±2 and so on. For fermions, the building blocks of matter like quarks and leptons (electron, muon, tau and neutrinos), it can only be +½ or -½.

Aidan Randle-Conde has compiled all possibilities on his blog. A particle with spin 1 cannot decay into two photons. Since we have seen the new boson decaying into photons, spin 1 is already ruled out in the table below. Moreover, a spin 2 boson could not decay into two taus, which is why it is so important to look for this decay in the latest data.

(from Aidan Randle-Conde’s blog)

The Standard Model predicts that the spin and parity of the Higgs boson will be 0+. To distinguish between 0+ and 0-, as well as 2+ and 2-, the only way is to carefully measure the angles at which all the decay products fly apart. So if we observe the new boson decaying into photons, we must measure the angle between the photons and the beam axis. If it decays into two Z, each one going into two electrons or two muons, we must carefully measure the angles of these four particles and their combined mass. Here is what Sara Bolognesi and her colleagues predict for Higgs bosons decaying into ZZ, WW or two photons. We must measure specific quantities, namely the mass and angles of the decay products, to distinguish them. If they match the red curve, we will know it is the Higgs boson, but it they look like one of the other curves, it will mean the new boson corresponds to a different theoretical model.

Each experiment now has about 14 fb-1 of data on tape and expects about 25 fb-1 in total by the end of the year. With the 5 fb-1 collected last year, it should be sufficient to unmask the new comer. “All” we need to do is measure these extremely complex quantities.

Pauline Gagnon

For more info, see these two CERN news videos  on CERN YouTube (part 1 and part 2) on the Higgs boson spin.

### Understanding the Higgs search

Wednesday, August 15th, 2012

It’s been over a month since CERN hosted a seminar on the updated searches for the Higgs boson. Since then ATLAS and CMS and submitted papers showing what they found, and recently I got news that the ATLAS paper was accepted by Physics Letters B, a prestigious journal of good repute. For those keeping score, that means it took over five weeks to go from the announcement to publication, and believe it not, that’s actually quite fast.

Crowds watch the seminar from Melbourne, Australia (CERN)

However, all this was last month’s news. Within a week of finding this new particle physicists started on the precision spin measurement, to see if it really is the Higgs boson or not. Let’s take a more detailed look at the papers. You can see both papers as they were submitted on the arXiv here: ATLAS / CMS.

### The Higgs backstory

In order to fully appreciate the impact of these papers we need to know a little history, and a little bit about the Higgs boson itself. We also need to know some of the fundamentals of scientific thinking and methodology. The “Higgs” mechanism was postulated almost 50 years ago by several different theorists: Brout, Englert, Guralnik, Hagen, Higgs, and Kibble. For some reason Peter Higgs seems to have his name attached to this boson, maybe because his name sounds “friendliest” when you put it next to the word “boson”. The “Brout boson” sounds harsh, and saying “Guralnik boson” a dozen times in a presentation is just awkward. Personally I prefer the “Kibble boson”, because as anyone who owns a dog will know, kibble gets everywhere when you spill it. You can tidy it up all you like and you’ll still be finding bits of kibble months later. You may not find bits often, but they’re everywhere, much like the Higgs field itself. Anyway, this is all an aside, let’s get back to physics.

It helps to know some of history behind quantum mechanics. The field of quantum mechanics started around the beginning of the 20th century, but it wasn’t until 1927 that the various ideas started to get resolved into a consistent picture of the universe. Some of the greatest physicists from around the world met at the 1927 Solvay Conference to discuss the different ideas and it turned out that the two main approaches to quantum mechanics, although they looked different, were actually the same. It was just a matter of making everything fit into a consistent mathematical framework. At that time the understanding of nature was that fields had to be invariant with respect to gauge transformation and Lorentz transformations.

The Solvay Conference 1927, where some of the greatest physicists of the 20th century met and formulated the foundations of modern quantum mechanics. (Wikipedia)

A gauge transformation is the result of the kind of mathematics we need to represent particle fields, and these fields must not introduce new physics when they get transformed. To take an analogy, imagine you have the blueprints for a building and you want to make some measurements of various distances and angles. If someone makes a copy of the blueprints, but changes the direction of North (so that the building faces another direction) then this must not change any of the distances or angles. In that sense the distances and angles in blueprint are rotation-invariant. They are rotation-invariant because we need to use Euclidean space to represent the building, and a consequence of using Euclidean space is that any distances and angles described in the space must be invariant with respect to rotation. In quantum mechanics we use complex numbers to represent the field, and a gauge transformation is just a rotation of a complex number.

The Lorentz transformation is a bit simpler to understand, because it’s special relativity, which says that if you have a series of events, observers moving at different speeds and in different directions will agree on the causality of those events. The rest of special relativity is just a matter of details, and those details are a lot of fun to look at.

By the time all of quantum mechanics was coming together there were excellent theories that took these symmetries into account. Things seemed to be falling into place, and running the arguments backwards lead to some very powerful predictions. Instead of observing a force and then requiring it to be gauge and Lorentz invariant, physicists found they could start with a gauge and Lorentz invariant model and use that to predict what forces can exist. Using plain old Euclidean space and making it Lorentz invariant gives us Minkowski space, which is the perfect for making sure that our theories work well with special relativity. (To get general relativity we start with a space which is not Euclidean.) Then we can write the most general description of a field we can think of in this space as long as it is gauge invariant and that’s a valid physical field. The only problem was that there were some interactions that seemed to involve a massive photon-like boson. Looking at the interactions gave us a good idea of the mass of this particle, the $$W$$ boson. In the next few decades new particles were discovered and the Standard Model was proposed to describe all these phenomena. There are three forces in the Standard Model, the electromagnetic force, the weak force, and the strong force, and each one has its own field.

### Inserting the Higgs field

The Higgs field is important because it unifies two of the three fundamental fields in particle physics, electromagnetism and the weak fields. It does this by mixing all the fields up (and in doing so, it mixes the bosons up.) Flip Tanedo has tried to explain the process from a theorist’s point of view to me privately on more than one occasion, but I must admit I just ended up a little confused by some of the finer points. The system starts with three fields which are pretty much all the same as each other, the $$W_1$$, $$W_2$$, and the $$W_3$$. These fields don’t produce any particles themselves because they don’t obey the relevant physical laws (it’s a bit more subtle in reality, but that’s a blog post in itself.) If they did produce their own fields then they would generate massless particles known as Goldstone bosons, and we haven’t seen these, so we know there is something else going on. Instead of making massless bosons they mix amongst themselves to create new fields, giving us massive bosons, and the Goldstone bosons get converted into extra degrees of freedom. Along comes the Higgs field and suddenly these fields separate and mix, giving us four new fields.

The Higgs field, about to break the symmetry and give mass (Flip Tanedo)

The $$W_1$$ and $$W_2$$ mix to give us the $$W^+$$ and $$W^-$$ bosons, and then the $$W_3$$ field meets the $$B$$ field to give us the $$Z$$ boson and the photon. What makes this interesting is that the photon behaves well on its own. It has no mass and this means that its field is automatically gauge invariant. Nature could have decided to create just the electromagnetic field and everything would work out fine. Instead we have the photon and three massive bosons, and the fields of these massive bosons cannot be gauge invariant by themselves, they need something else to make it all balance out. By now you’ve probably guessed what this mystery object is, it’s the Higgs field and with it, the Higgs boson! This field fixes it all up so that the fields mix, we get massive bosons and all the relevant laws (gauge invariance and Lorentz invariance) are obeyed.

Before we go any further it’s worth pointing a few things out. The mass of the $$W$$ boson is so large in comparison to other particles that it slows down the interactions of a lot of particles, and this is one of the reasons that the sun burns so “slowly”. If the $$W$$ boson was massless then it could be produced in huge numbers and the rate of fusion in the sun would be much faster. The reason we have had a sun for billions of years, allowing the evolution of life on Earth (and maybe elsewhere) is because the Higgs field gives such a large mass to the $$W$$ boson. Just let that thought sink in for a few seconds and you’ll see the cosmic significance of the Higgs field. Before we get ahead ourselves we should note that the Higgs field leads to unification of the electromagnetic and weak forces, but it says nothing about the strong force. Somehow the Higgs field has missed out one of the three fundamental forces of the Standard Model. We may one day unite the three fields, but don’t expect it to happen any time soon.

### “Observation” vs “discovery”, “Higgs” vs “Higgs-like”

There’s one more thing that needs to be discussed before looking at the papers and that’s a rigorous discussion of what we mean by “discovery” and if we can claim discover of the Standard Model Higgs boson yet. “Discovery” has come to mean a five sigma observation of a new resonance, or in other words that probability that the Standard Model background in the absence of a new particle would bunch up like this is less than one part in several million. If we see five sigma we can claim a discovery, but we still need to be a little careful. Suppose we had a million mass points, what is the probability that there is one five sigma fluctuation in there? It’s about $$20\%$$, so looking at just the local probability is not enough, we need to look at the probability that takes all the data points into account. Otherwise we can increase the chance of seeing a fluctuation just by changing the way we look at the data. Both ATLAS and CMS have been conscious of this effect, known as the “Look Elsewhere Effect”, so every time they provide results they also provide the global significance, and that is what we should be looking at when we talk about the discovery.

Regular readers might remember Flip’s comic about me getting worked up over the use of the word “discovery” a few weeks back. I got worked up because the word “discovery” had been misused. Whether an observation is $$4.9$$ or $$5.1$$ sigma doesn’t matter that much really, and I think everyone agrees about that. What bothered me was that some people decided to change what was meant by a discovery after seeing the data, and once you do that you stop being a scientist. We can set whatever standards we like, but we must stick to them. Burton, on the other hand, was annoyed by a choice of font. Luckily our results are font-invariant, and someone said “If you see five sigma you can present in whatever durn font you like.”

Getting angry over the change of goalposts. Someone has to say these things.

In addition to knowing what we mean by “discovery” we also need to take hypothesis testing into account. Anyone who claims that we have discovered the Higgs boson is as best misinformed, and at worst willingly untruthful. We have discovered a new particle, there’s no doubt about that, but now we need to eliminate things are not the Higgs until we’re confident that the only thing left is the Higgs boson. We have seen this new particle decay to two photons, and this tells us that it can only only have spin 0 or spin 2. That’s eliminated spin 1, spin 3, spin 4… etc for us, all with a single measurement. What we are doing now trying to exclude both the spin 0 and spin 2 possibilities. Only one of these will be excluded, and then will know for sure what the spin is. And then we know it’s the Standard Model Higgs boson, right? Not quite! Even if we know it’s a spin 0 particle we would still need to measure its branching fractions to confirm that it is what we have been looking for all along. Bear this in mind when thinking about the paper- all we have seen so far is a new particle. Just because we’re searching for the Higgs and we’ve found something new it does not mean that it’s a the Higgs boson.

### The papers

Finally we get to the papers. From the titles we can see that both ATLAS and CMS have been suitably agnostic about the particle’s nature. Neither claim it’s the Higgs boson and neither even claim anything more than an “observation”. The abstracts tell us a few useful bits of information (note that the masses quoted agree to within one sigma, which is reassuring) but we have to tease out the most interesting parts by looking at the details. Before the main text begins each experiment dedicates their paper to the memories of those who have passed away before the papers were published. This is no short list of people, which is not surprising given that people have been working on these experiments for more than 20 years. Not only is this a moving start to the papers, it also underlines the impact of the work.

Both papers were dedicated to the memories of colleagues who did not see the observation. (CMS)

Both papers waste no time getting into the heart of the matter, which is nature of the Standard Model and how it’s been tested for several decades. The only undiscovered particle predicted by the Standard Model is the Higgs boson, we’ve seen everything else we expected to see. Apart from a handful of gauge couplings, just about every prediction of the Standard Model has been vindicated. In spite of that, the search for the Higgs boson has taken an unusually long time. Searches took place at LEP and Tevatron long before the LHC collided beams, and the good news is that the LEP limit excluded the region that is very difficult for the LHC to rule out (less than $$114GeV$$). CDF and D0 both saw an excess in the favored region, but the significance was quite low, and personally I’m skeptical since we’ve already seen that CDF’s dijet mass scale might have some problems associated with it. Even so we shouldn’t spend too long trying to interpret (or misinterpret) results, we should take them at face value, at least at first. Next the experiments tell us which final states they look for, and this is where things will get interesting later on. Before describing the detectors, each experiment pauses to remind us that the conditions of 2012 are more difficult than those of 2011. The average number of interactions per beam crossing increased by a factor of two, making all analyses more difficult to work with (but ultimately all our searches a little more sensitive.)

At this point both papers summarize their detectors, but CMS goes out of their way to show off how the design of their detector was optimized for general Higgs searches. Having a detector which can reconstruct high momentum leptons, low momentum photons and taus, and also tag b-jets is not as easy task. Both experiments do well to be able to search for the Higgs bosons in the channels they look at. Even if we limit ourselves to where ATLAS looked the detectors would still have trigger on leptons and photons, and be able to reconstruct not only those particles, but also the missing transverse energy. That’s no easy task at a hadron collider with many interactions per beam crossing.

The two experiments have different overall strategies to the Higgs searches. ATLAS focused their attention on just two final states in 2012: $$\gamma\gamma$$, and $$ZZ^*$$, whereas CMS consider five final sates: $$\gamma\gamma$$, $$ZZ^*$$, $$WW^*$$, $$\tau\tau$$, and $$b\bar{b}$$. ATLAS focus mostly on the most sensitive modes, the so-called “golden channel”, $$ZZ^*$$, and the fine mass resolution channel, $$\gamma\gamma$$. With a concerted effort, a paper that shows only these modes can be competitive with a paper that shows many more, and labor is limited on both experiments. CMS spread their effort across several channels, covering all the final states with expected sensitivities comparable to the Standard Model.

### $$H\to ZZ^*$$

The golden channel analysis has been presented many times before because it is sensitive across a very wide mass range. In fact it spans the range $$110-600GeV$$, which is the entire width of the Higgs search program at ATLAS and CMS. (Constraints from other areas of physics tell us to look as high as $$1000GeV$$, but at high masses the Higgs boson would have a very large width, making it extremely hard to observe. Indirect results favor the low mass region, which is less than around $$150GeV$$.) Given the experience physicists have had with this channel it’s no surprise that the backgrounds are very well understood at this point. The dominant “irreducible” background comes from Standard Model production of $$Z/\gamma*$$ bosons, where there is one real $$Z$$ boson, and one “off-shell”, or virtual boson. This is called irreducible because the source of background is the same final state as the signal, so we can’t remove further background without also removing some signal. This off-shell boson can be an off-shell $$Z$$ boson or an off-shell photon, it doesn’t really matter which since these are the same for the background. In the lower mass range there are also backgrounds from $$t\bar{t}$$, but fortunately these are well understood with good control regions in the data. Using all this knowledge, the selection criteria for $$8TeV$$ were revisited to increase sensitivity as much as possible.

The invariant mass spectrum for ATLAS's H→ZZ* search (ATLAS)

Since this mode has a real $$Z$$ boson, we can look for two high momentum leptons in the final state, which mames things especially easy. The backgrounds are small, and the events are easy to identify, so the trigger is especially simple. Events are stored to disk if there is at least one very high momentum lepton, or two medium momentum leptons which means that we don’t have to throw any events away. Some triggers fire so rapidly that we can only store some of the events from them, and we call this prescaling. When we keep $$1$$ in $$n$$ events then we have a prescale of $$n$$. For a Higgs search we want to have a high efficiency as possible so we usually require a prescale of $$1$$. Things are not quite so nice for the $$\gamma\gamma$$ mode, as we’ll see later.

The invariant mass spectrum for CMS's H→ZZ* search (CMS)

After applying a plethora of selections on the leptons and reconstructing the $$Z$$ and Higgs boson candidates the efficiency for the final states vary from $$15\%-37\%$$, which is actually quite high. No detector can cover the whole of the solid angle, and efficiencies vary with the detector geometry. The efficiency needs to be very high because the fraction of Higgs bosons that would decay to these final states is so small. At a mass of $$125GeV$$ the branching fraction to the $$ZZ^*$$ state is about $$2\%$$, and then branching fraction of $$Z$$ to two leptons is about $$6\%$$. Putting that all together means that only $$1$$ in $$10,000$$ Higgs bosons would decay to this final state. At a mass of $$125GeV$$ the LHC would produce about $$15,000$$ Higgs bosons per $$fb^{-1}$$. So for $$10fb^{-1}$$ we could expect to have about $$11$$ Higgs bosons decaying to this final state, and we could expect to see about $$3$$ of those events reconstructed. This is a clean mode, but it’s an extremely challenging one.

The selection criteria are applied, the background is estimated, and the results are shown. As you can see there is a small but clear excess over background in the region around $$125GeV$$ and this is evidence supporting the Higgs boson hypothesis!

CMS see slightly fewer events than expected, but still see a clear excess (CMS)

### $$H\to\gamma\gamma$$

Out of the $$H\to ZZ^*$$ and $$H\to\gamma\gamma$$ modes the $$\gamma\gamma$$ final state is the more difficult one to reconstruct. The triggers are inherently “noisy” because they must fire on something that looks like a high energy photon, and there are many sources of background for this. As well as the Standard Model real photons (where the rate of photon production is not small) there are jets faking photons, and electrons faking photons. This makes the mode dominated by backgrounds. In principle the mode should be easy: just reconstruct Higgs candidates from pairs of photons and wait. The peak will reveal itself in time. However ATLAS and CMS are in the middle of a neck and neck race to find the Higgs boson, so both collaborations exploit any advantage they can, and suddenly these analyses become some of the most difficult to understand.

A typical H→γγ candidate event with a striking signature (CMS)

To get a handle on the background ATLAS and CMS each choose to split the mode into several categories, depending on the properties of the photons or the final state, and each one with its own sensitivity. This allows the backgrounds to be controlled with different strategies in each category, leading to increased overall sensitivity. Each category has its own mass resolution and signal-to-background ratio, each is mutually independent of the others, and each has its own dedicated studies. For ATLAS the categories are defined by the presence of two jets, whether or not the photon converts (produces an $$e^-e^+$$ pair) in the detector, the pseudorapidity of the photons, and a kinematic quantity called $$p_{T_T}$$, with similar categories for CMS.

When modelling the background both experiments wisely chose to use the data. The background for the $$gamma\gamma$$ final state is notoriously hard to predict accurately, because there are so many contributions from different backgrounds, from real and fake photon candidates, and many kinematic or detector effects to take into account. The choice of background model even varies on a category by category basis, and choices of model vary from simple polynomial fits to the data, to exponential and skewed Gaussian backgrounds. What makes these background models particularly troublesome is that the background has to be estimated using the signal region, so small deviations that are caused by signal events could be interpreted by the fitting algorithm as a weird background shape. The fitting mechanism must be robust enough to fit the background shapes without being fooled into thinking that a real excess of events is just a slightly different shape.

ATLAS's H→γγ search, where events are shown weighted (top) and unweighted (bottom) (ATLAS)

To try to squeeze even more sensitivity out of the data CMS use a boosted decision tree to aid signal separation. A boosted decision tree is a sophisticated statistical analysis method that uses signal and background samples to decide what looks like signal, and then uses several variables to return just one output variable. A selection can be made on the output variable that removes much of the background while keeping a lot of the signal. Using boosted decision trees (or any multivariate analysis technique) requires many cross checks to make sure the method is not biased or “overtrained”.

CMS's H→γγ search, where events are shown weighted (main plot) and unweighted (inset) (CMS)

After analyzing all the data the spectra show a small bump. The results can seem a little disappointing at first, after all the peak is barely discernable, and so much work has gone into the analyses. Both experiments show the spectra after weighting the events to take the uncertainties into account and this makes the plots a little more convincing. Even so, what matters is the statistical significance of these results, and this cannot be judged by eye. The final results show a clear preference for a boson with a mass of $$125GeV$$, consistent with the Higgs boson. CMS see a hint at around $$135GeV$$, but this is probably just a fluctuation, given that ATLAS do not see something similar.

ATLAS local significance for H→γγ (ATLAS)

(If you’ve been reading the blog for a while you may remember a leaked document from ATLAS that hinted at a peak around $$115GeV$$ in this invariant mass spectrum. That document used biased and non peer-reviewed techniques, but the fact remains that even without these biases there appear to be a small excess in the ATLAS data around $$115GeV$$. The significance of this bump has decreased as we have gathered more data, so it was probably just a fluctuation. However, you can still see a slight bump at $$115GeV$$ in the significance plot. Looking further up the spectrum, both ATLAS and CMS see very faint hints of something at $$140GeV$$ which appears in both the $$ZZ^*$$ and $$\gamma\gamma$$ final states. This region has already been excluded for a Standard Model Higgs, but there may be something else lurking out there. The evidence is feeble at the moment, but that’s what we’d expect for a particle with a low production cross section.)

### $$H\to WW^*$$

One of the most interesting modes for a wide range of the mass spectrum is the $$WW(*)$$ final state. In fact, this is the first mode to be sensitive to the Standard Model Higgs boson searches, and exclusions were seen at ATLAS, CMS, and the Tevatron experiments at around $$160GeV$$ (the mass of two on-shell $$W$$ bosons) before any other mass region. The problem with this mode is that it has two neutrinos in the final state. It would be nice to have an inclusive sample of $$W$$ bosons, including the hadronic final states, but the problems here are the lack of a good choice of trigger, and the irreducible and very large background. That mean that we must select events with two leptons and two neutrinos in them. As the favored region excludes more and more of the high mass region this mode gets more challenging, because at first we lose the mass constraint on the second $$W$$ boson (as it must decay off-shell), and secondly because we must be sensitive in the low missing transverse energy region, which starts to approach our resolution for this variable.

While we approach our resolution from above, the limit on the resolution increases from below, because the number of interactions per beam crossing increases, increasing the overall noise in the detector. To make progress in this mode takes a lot of hard work for fairly little gain. Both papers mention explicitly how difficult the search is in a high pileup scenario, with CMS stating

“The analysis of the $$7TeV$$ data is described in [referenced paper] and remains unchanged, while the $$8TeV$$ analysis was modified to cope with more difficult conditions induced by the higher pileup of the 2012 data taking.”

and ATLAS saying

“The analysis of the $$8TeV$$ data presented here is focused on the mass range $$110<m_H<200GeV$$ It follows the procedure used for the $$7TeV$$ data described in [referenced paper], except that more stringent criteria are applied to reduce the $$W$$+jets background and some selections have been modified to mitigate the impact of the high instantaneous luminosity at the LHC in 2012.”

It’s not all bad news though, because the final branching fraction to this state is much higher than that of the $$ZZ^*$$ final state. The branching fraction for the Standard Model Higgs boson to $$WW^*$$ is about $$10$$ times higher than that for $$ZZ^*$$, and the branching fraction of the $$W$$ boson to leptons is also about $$3$$ times higher than the $$Z$$ boson to leptons, which gives another order of magnitude advantage. Unfortunately all these events must be smeared out across a large spectrum. There is one more trick we have up our sleeves though, and it comes from the spin of the parent. Since the Standard Model Higgs boson has zero spin the $$W$$ bosons tend to align their spins in opposite directions to make it all balance out. This then favors one decay direction over another for the leptons. The $$W^+$$ boson decays with a neutrino in the final state, and because of special relativity the neutrino must align its spin against its direction of motion. The $$W-$$ boson decays with an anti-neutrino, which takes its spin with its direction of motion. This forces the two leptons to travel in the same direction with respect to the decay axis of the Higgs boson. The high momenta of the leptons smears things out a bit, but generally we should expect to see one high momentum lepton, and a second lower momentum lepton n roughly the same region of the detector.

The transverse mass for ATLAS's H→WW* search (ATLAS)

ATLAS did not actually present results for the $$WW^*$$ final state on July 4th, but they did show it in the subsequent paper. CMS showed the $$WW^*$$ final state on July 4th, although it did somewhat reduce their overall significance. Both ATLAS and CMS spend some of the papers discussing the background estimates for the $$WW^*$$ mode, but ATLAS seem to go to more significant lengths to describe the cross checks they used in data. In fact this may help to explain why ATLAS did not quite have the result ready for July 4th, whereas CMS did. There’s a trade off between getting the results out quickly and spending some extra time to understand the background. This might have paid off for ATLAS, since they seem to be more sensitive in this mode than CMS.

The invariant mass for CMS's H→WW* search (CMS)

After looking at the data we can see that both ATLAS and CMS are right at the limits of their sensitivity in this mode. They are not limited by statistics, they are limited by uncertainties, and the mass point $$125GeV$$ sits uncomfortably close some very large uncertainties. The fact that this mode is sensitive at all is a tribute to the hard work of dozens of physicists who went the extra mile to make it work.

CMS's observed and expected limits for H→WW*, showing the dramatic degradation in sensitivity as the mass decreases (CMS)

### $$H\to b\bar{b}$$

At a mass of $$125GeV$$ by far the largest branching fraction of the Standard Model Higgs boson is to $$b\bar{b}$$. CDF and D0 have both seen a broad excess in this channel (although personally I have some doubts about the energy scale of jets at CDF, given the dijet anomaly they see that D0 does not see) hinting at a Higgs boson of $$120-135GeV$$. The problem with this mode is that the background is many orders of magnitude larger than the signal, so some special tricks must be used to remove the background. What is done at all four experiments is to search for a Higgs boson that is produced in associated with a $$W$$ or $$Z$$ boson, and this greatly reduces the background. ATLAS did not present an updated search in the $$b\bar{b}$$ channel, and taking a look at the CMS limits we can probably see why, the contribution is not as significant as in other modes. The way CMS proceed with the analysis is to use several boosted decision trees (one for each mass point) and to select candidates based on the output of the boosted decision tree. The result is less than $$1$$ sigma of significance, about half of what is expected, but if this new boson is the Higgs boson then this significance will increase as we gather more data.

A powerful H→bb search requires a boosted decision tree, making the output somewhat harder to interpret (CMS)

It’s interesting to note that the $$b\bar{b}$$ final state is sensitive to both a spin 0 and a spin 2 boson (as I explained in a previous post) and it may have different signal strength parameters for different spin states. The signal strength parameter tells us how many events we see compared to how many events we do see, and it is denoted with the symbol $$\mu$$. A there is no signal then $$\mu=0$$, if the signal is exactly as large as we expect then $$\mu=1$$, and any other value indicates new physics. It’s possible to have a negative value for $$\mu$$ and this would indicate quantum mechanical interference of two or more states that cancel out. Such an interference term is visible in the invariant mass of two leptons, as the virtual photon and virtual $$Z$$ boson wavefunctions interfere with each other.

### $$H\to\tau\tau$$

Finally, the $$\tau\tau$$ mode is perhaps the most enlightening and the most exciting right now. CMS showed updated results, but ATLAS didn’t. CMS’s results were expected to approach the Standard Model sensitivity, but for some reason their results didn’t reach that far, and that is crucially important. CMS split their final states by the decay mode of the $$\tau$$, where the final states include $$e\mu 4\nu$$, $$\mu\mu 4\nu$$, $$\tau_h\mu 3\mu$$, and $$\tau_h e3\nu$$, where $$\tau_h$$ is a hadronically decaying $$\tau$$ candidate. This mode has at least three neutrinos in the final state, so like the $$WW^*$$ mode the events get smeared across a mass spectrum. There are irreducible backgrounds from $$Z$$ bosons decaying to $$\tau\tau$$ and from Drell-Yan $$\tau\tau$$ production, so the analysis must search for an excess of events over these backgrounds. In addition to the irreducible backgrounds there are penalties in efficiency associated with the reconstruction of $$\tau$$ leptons, which make this a challenging mode to work this. There are dedicated algorithms for reconstructing hadronically decaying $$\tau$$ jets, and these have to balance out the signal efficiency for real $$tau$$ leptons and background rejection.

CMS's H→τtau; search, showing no signal (CMS)

After looking at the data CMS expect to see an excess of $$1.4$$ sigma, but they actually see $$0$$ sigma, indicating that there may be no Standard Model Higgs boson after all. Before we jump to conclusions it’s important to note a few things. First of all statistical fluctuations happen, and they can go down just as easily as they can go up, so this could just be a fluke. It’s a $$1.5$$ sigma difference, so the probability of this being due a fluctuation if the Standard Model Higgs boson is about $$8\%$$. On its own that could be quite low, but we have $$8$$ channels to study, so the chance of this happening in any one of the channels is roughly $$50\%$$, so it’s looking more likely that this is just a fluctuation. ATLAS also have a $$\tau\tau$$ analysis, so we should expect to see some results from them in the coming weeks or months. If they also don’t see a signal then it’s time to start worrying.

CMS's limit of H→ττ actually shows a deficit at 125GeV. A warning sign for possible trouble for the Higgs search! (CMS)

### Combining results

Both experiments combine their results and this is perhaps the most complicated part of the whole process. There are searches with correlated and uncorrelated uncertainties, there are two datasets at different energies to consider, and there are different signal-to-background ratios to work with. ATLAS and CMS combine their 2011 and 2012 searches, so they both show all five main modes (although only CMS show the $$b\bar{b}$$ and $$\tau\tau$$ modes in 2012.)

When combining the results we can check to see if the signal strength is “on target” or not, and there is some minor disagreement between the modes. For the $$ZZ^*$$ and $$WW^*$$ modes, the signal strengths are about right, but for the $$\gamma\gamma$$ mode it’s a little high for both experiments, so there is tension between these modes. Since these are the most sensitive modes, and we have more data on the way then this tension should either resolve itself, or get worse before the end of data taking. The $$b\bar{b}$$ and $$\tau\tau$$ modes are lower than expected for both experiments (although for ATLAS the error bars are so large it doesn’t really matter), suggesting that this new particle may a non-Standard Model Higgs boson, or it could be something else altogether.

Evidence of tension between the γγ and fermionic final states (CMS)

While the signal strengths seem to disagree a little, the masses all seem to agree, both within experiments and between them. The mass of $$125GeV$$ is consistent with other predictions (eg the Electroweak Fit) and it sheds light on what to look for beyond the Standard Model. Many theories favor a lower mass Higgs as part of a multiplet of other Higgs bosons, so we may see some other bosons. In particular, the search for the charged Higgs boson at ATLAS has started to exclude regions on the $$\tan\beta$$ vs $$m_{H^+}$$ plane, and the search might cover the whole plane in the low mass region by the end of 2012 data taking. Although a mass of $$125GeV$$ is consistent with the Electroweak Fit, it is a bit higher than the most favored region (around $$90GeV$$) so there’s certainly space for new physics, given the observed exclusions.

The masses seem to agree, although the poor resolution of the WW* mode is evident when compared to the ZZ* and γγ modes (ATLAS)

To summarize the results, ATLAS sees a $$5.9$$ sigma local excess, which is $$5.1$$ sigma global excess, and technically this is a discovery. CMS sees a $$5.0$$ sigma local excess, which is $$4.6$$ sigma global excess, falling a little short of a discovery. The differences in results are probably due to good luck on the part of ATLAS and bad luck on the part of CMS, but we’ll need to wait for more data to see if this is the case. The results should “even out” if the differences are just due to fluctuations up for ATLAS and down for CMS.

ATLAS proudly show their disovery (ATLAS)

If you’ve read this far then you’ve probably picked up on the main message, we haven’t discovered the Standard Model Higgs boson yet! We still have a long road ahead of us and already we have moved on to the next stage. We need to measure the spin of this new boson and if we exclude the spin 0 case then we know it is not a Higgs boson. If exclude the spin 2 case then we still need to go a little further to show it’s the Standard Model Higgs boson. The spin analysis is rather complicated, because we need to measure the angles between the decay products and look for correlations. We need to take the detector effects into account, then subtract the background spectra. What is left after that are the signal spectra, and we’re going to be statistically limited in what we see. It’s a tough analysis, there’s no doubt about it.

We need to see the five main modes to confirm that this is what we have been looking for for so long. If we get the boson modes ($$ZZ^*$$, $$WW^*$$, $$\gamma\gamma$$) spot on relative to each other, then we may have a fermiophobic Higgs boson, which is an interesting scenario. (A “normal” fermiophobic Higgs boson has already been excluded, so any fermiophobic Higgs boson we may see must be very unusual.)

There are also many beyond the Standard Model scenarios that must be studied. As more regions of parameter space are excluded, theorists tweak their models, and give us updated hints on where to search. ATLAS and CMS have groups dedicated to searching for beyond the Standard Model physics, including additional Higgs bosons, supersymmetry and general exotica. It will be interesting to see how their analyses change in light of the favored mass region in the Higgs search.

A favored Higgs mass has implications for physics beyond the Standard Model. Combined with the limits on new particles (shown in plot) many scenarios can be excluded (ATLAS)

2012 has been a wonderful year for physics, and it looks like it’s only going to get better. There are still a few unanswered questions and tensions to resolve, and that’s what we must expect from the scientific process. We need to wait a little longer to get to the end of the story, but the anticipation is all part of the adventure. We’ll know is really happening by the end of Moriond 2013, in March. Only then can we say with certainty “We have proven/disproven the existence of the Standard Model Higgs boson”!

I like to say “We do not do these things because they are easy. We do them because they are difficult”, but I think Winston Churchill said it better:

This is not the end. It is not even the beginning of the end, but it is perhaps the end of the beginning.” W. Churchill

### References etc

Plots and photos taken from:
“Webcast of seminar with ATLAS and CMS latest results from ICHEP”, ATLAS Experiment, CERN, ATLAS-PHO-COLLAB-2012-014
Wikipedia
“Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC”, ATLAS Collaboration, arXiv:1207.7214v1 [hep-ex]
“Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC”, CMS Collaboration, arXiv:1207.7235v1 [hep-ex]
Flip Tanedo

It’s been a while since I last posted. Apologies. I hope this post makes up for it!

### Higgs Seminar postgame discussion

Friday, July 6th, 2012

Following the Higgs seminar on Wednesday July 4th (Higgsdependence Day), fellow bloggers Steve Sekula and Seth Zenz joined me to discuss the results. We discussed all sorts of topics from the measurements themselves, to the nature of the work, to the future of the study of the Higgs boson. Enjoy!

### New boson could pave the way towards new discoveries

Thursday, July 5th, 2012

After the spectacular results reported yesterday at CERN on the discovery of a new boson, the largest particle physics conference of the year started today in Melbourne. Such announcement put the bar high for all the speakers.

As many people have pointed out already, it is still early to call the new boson a “Higgs boson” although the odds are really high. First we must check that it behaves exactly like the Higgs boson. Is it produced as often as the Standard Model predicts, and does it decay in the same proportions as expected? Verifying these properties with the highest accuracy will be the main task in the coming months and years. It’s not that physicists are compulsive about precision, but this is exactly where we might find the opening to the “secret passage”.

Theorists like Peter Higgs, François Englert and Robert Brout in 1964 showed us the way when they postulated the existence of the Higgs boson and Higgs mechanism. Today still, theorists are trying to guide the experimentalists in the right direction.

All theorists today agree that our current theoretical model has its limits. The Standard Model appears to be to the world of particle physics what the four basic operations are to mathematics. Most daily tasks are achieved using only additions, subtractions, multiplications and divisions. But we all know that there is more to mathematics: geometry and trigonometry for example are needed to solve more complex problems.

All this to say that there are clear signs that the Standard Model is only the first layer of a more complex theory. Many believe the next layer is a theory called supersymmetry or SUSY.

One major difficulty with this theory is that is has more than 100 free parameters, making it impossible to obtain predictions without assigning fixed values to some of these parameters. This lead to more manageable models, like the Constrained Minimal Supersymmetric Model or CMSSM.

Today, at the International Conference on High Energy Physics, several theorists discussed the impact of the recently revealed mass of the Higgs boson on the CMSSM model. For example, Dmitri Kanikov showed that one can use the intrinsic interconnections within the theory to see how the current limits obtained from the most recent experiments substantially constrain the parameters of the CMSSM.

Nazila Mahmoudi took this approach one step further by imposing constraints not to the CMSSM model but rather to the whole set of free SUSY parameters. This lead her and her colleagues to realize that with the actual searches and mostly, the stringent constraint coming from the Higgs mass at about 126 GeV, many of the constrained models are nearly ruled out.

The vertical axis shows the Higgs boson mass. If one assumes a Higgs mass between 123-129 GeV, scenarios such a minimal Gauge Mediated SUSY Breaking Model and no-scale (shown in gray and magenta) are excluded.

She was very optimistic even though the current searches at the LHC have not yet revealed any new SUSY particles. She showed that in fact there are plenty of values still allowed for the many parameters of SUSY. As she stated, if we have not found SUSY particles yet, it does not mean they are not there but simply that they must be much heavier or belong to more complex configurations, making them harder to find. By eliminating models like that, it helps zoom on the right one.

Nice optimistic way to close this first day of the conference.

Pauline Gagnon

### Higgs Seminar liveblog from CERN

Tuesday, July 3rd, 2012

CERN are holding a seminar for the latest results for the ATLAS and CMS Higgs searches. This is the first such update since December 2011, and there is a reasonable chance that at least one of the experiments could show a 5 sigma excess. This is my liveblog, follow along for live updates!

## “Observation of a new particle consistent with a Higgs Boson (but which one…?)”

Thank you to all who joined me on this liveblog and on twitter!

The seminar is webcast live so that you can watch from anywhere in the world. The link is http://cern.ch/webcast. The seminar will begin at 09:00 CERN time (00:00 US West Coast, 03:00 US East Coast, 08:00 UK, 17:00 Melbourne.)

This is my liveblog and I will be providing updates as the seminar proceeds. Most recent updates at the top of the page. Also follow me on twitter (@aidanatcern) and Seth Zenz (@sethzenz). Ken Bloom is also liveblogging from ICHEP, and my boss, @drsekula is liveblogging for SMU.

## The liveblog

10:59: Rolf: We can all be proud of this day. Enjoy it! (Applause)

10:55: Any comments from the theorists? (Applause) Many congratulations!

10:50: Many thanks offered from the front row.

10:48: Any questions from Melbourne? Any applause from Melbourne?! (Applause from Melbourne.) Any remarks? A: Grateful to take part in this historic event and wish you the best.

## Overview (Rolf Heuer)

10:44: “Speaking as a layman: I think we have it.” We have a discovery consistent with a Higgs boson (but which one?) This is the beginning. “Global implications for future. Standing applause!

## ATLAS talk (Fabiola Gianotti)

10:42: Local excess of 5.0 sigma, dominated by gamma gamma and ZZ* final states.

10:41: Only recorded on third of 2012 data. More data to come. The LHC is working beyond expectation. Theorists: please be patient!

10:40: Next steps: publish paper, then gather more data.

10:38: Evolution of excess with time. December saw 3.5 sigma peak. Seeing a nice 5 sigma peak today!

10:37: Excess compatible with Standard Model Higgs boson.

10:34: Excluded all points in the Higgs mass spectrum now, except around 125GeV and at very high mass.

10:33: Observe 3.4 local (2.5 global) sigma excess at 125GeV.

10:30: Slight excess above background + Standard Model signal at 125Gev. (Expect 10.4 +- 1.1 total, observe 13)

10:29: Z->4 leptons seen in the spectrum.

10:28: 1.3 times more ZZ events in data at higher masses.

10:26: Total reconstruction efficiency for electrons 98% flat in eta, pt and pileup. Required for low transverse momentum objects. 60% gain in acceptance times efficiency electrons. 45% gain for muons.

10:24: H->ZZ*->4 leptons final state. Backgrounds suppressed using isolation requirements. High efficiency needed, down to low transverse momentum objects. Gain in sensitivity of 20-30% since 2011.

10:21: 4.5 local (3.6 global) sigma excess in gamma gamma. Signal strength is 1.9 +/- 0.5. Cross section seems a little high, but consistent with Standard Model within 2 sigma.

10:19: Background model taken from data, using sidebands. Both 2011 and 2012 exclusions show compatible shapes.

10:18: Isolation of photon used to reject jets. Subtraction algorithm used to remove some pileup dependent effects.

10:17: Rejection of jets is 1 part in 10^4, at 90% signal efficiency.

10:15: Need to know the position of the vertex to get the angle of the photons and the mass. Do not use tracking information, in order to be insensitive to pileup. Use longitudinal and lateral segmentation of the electromagnetic calorimeter to point the photons.

10:14: Important to have powerful gamma identification to reject jet backgrounds. Energy scale known to 0.3% at the mass of the Z. Linearity known to better than 1% up to a few 100 GeV. Mass resolution not seriously affected by pileup.

10:11: Gamma gamma final state. Large backgrounds, split signal into 10 categories, depending on the kinematics and conversion variables. Expect gain in sensitivity by 15%. Signal to background ratio is very small. (170 signal events for 6340 background events.)

10:09: Use experience with the detector from 2011 to inform analyses in 2012. Improved reconstruction and identification of physics objects.

10:07: Previous results show exclusions except near 116GeV and 125GeV.

10:06: As center of mass energy changes from 7TeV to 8TeV, cross section increases by a factor of 1.3. Irreducible background cross sections increase by a factor of 1.2-1.25, whereas reducible backgrounds increase by a factor of 1.4-1.5. This gives an increase of sensitivity of 10%.

10:05: Many electroweak results , with cross sections of rare and rarer processes. Small amounts of tension in measurements.

10:04: Analysis not possible without dedicated computing resources. Usually 100,000 jobs in parallel at a time.

10:02: Trigger thresholds rise and luminosity rises. This keeps the good physics events for lower mass objects. Efficiency of electron trigger is flat and 94%. Stable performance required with respect to changes in pileup. Pileup changes as the run progresses.

10:00: Pileup showing big challenges for the continued analysis of data. Missing transverse energy resolution rises linearly with pileup, but is fine and flat after pileup suppression using information from the detector.

09:58: Pileup is increasing quickly. Average of 30 collisions per bunch crossing (with 50ns bunch spacing, rather than 25ns which is design performance.)

09:56: Integrated luminosity of 6.3fb^-1. 94% efficiency. 90% of delivered luminosityy is recorder to disk, in spite of very fresh data and harsher conditions.

09:55: Results are preliminary, data taking stopped two weeks ago. Pileup increased, harsher conditions. Present the highest sensitivity and best resolution modes (gamma gamma and ZZ*.) Other channels contains missing energy, poorer mass resolution and sensitive to pileup.

## CMS talk (Joe Incandela)

09:51: Following lots of applause, acknowledgements. Lots of people to thank.

09:49: Event yields are self consistent across the topologies. Ratio of WW* and ZZ* states consistent. Couplings consistent with Standard Model at 95% confidence, we need more data. “We have observed a new boson with a mass of 125.3 +/- 0.6 GeV at 4.9sigma significance.”

09:48: Combined mass is 125.3 +/- 0.6 GeV. Now we need to see if it is compatible with Standard Model Higgs boson. Signal strength is 0.8+/-0.2.

09:46: Observed limit 1.06 x Standard Model cross section. Low statistics may cause some slight bias. Needs investigation. “Very interesting channel.” (Nice to hear open and candid discussion about results. Responsible science.)

09:44: tau tau channel. Challenging, lots of sub modes. 2 times improvement in sensitivity since 2011. “Use a very fancy fit that I won’t explain in detail…”

09:42: Current limits are compatible with signal or background.

09:42: Now bb, large branching fraction but huge background. Look for associated production mode. (W+H, Z+H; H->bb)

09:41: Still working on combination.

09:39: WW* analysis. Very difficult channel at low mass. DeltaPhi between leptons and invariant mass of two leptons used as discriminators.

09:37: Combined result for gamma gamma and ZZ* is 5.0 sigma. That’s a discovery!

09:35: Broader distribution for mass of Z bosons. Needs to be watched in the future…

09:34: Z->4l peak seen in the final mass spectrum! Also a bump at 126GeV.

09:32: Moving to ZZ* search. 20% improvement since 2011. Using all four (light) lepton final states. Backgrounds estimated from data. Angular analysis of leptons performed. 8 degrees of freedom in this angular analysis.

09:30: 4.2 sigma local significance, 3.2 sigma global. 1.56 +/- 0.43 x Standard Model cross section.

09:28: Peak clearly visible at 125GeV at the 2.3 sigma leve.

09:28: Classes combined weighted by signal to background ratio. Impressive bump appears!

09:27: Background model comes from data. Bias must be less than 20% of statistical error in the data.

09:25: Multivariate analysis used with kinematic variables, identification and per event mass resolution and vertex probability. Classes arranged in decreasing order of purity.

09:24: Photons selected using kinematic variables (transverse energy and mass of diphoton system.) Mass reconstruction depends on the vertex position. Aim to be within 1cm of the correct vertex. Correct to 83%(80%) in 2011 (2012).

09:23: Different algorithms for electron reconstruction, including brem recovery. Slightly better performance in Monte Carlo compared to data, so smear the data.

09:22: Analysis performed blind in 2012. Most studies are data driven.

09:21: Multivariate analysis used, using boosted decision trees. Classify different kinds of events, end up with four event classes. Crosschecked using an alternate background model, using sideband subtraction. Also a cut based crosscheck.

09:20: Standard Model cross sections well measured, including ttbar.

09:19: Jets a challenging but performing well. Shape differences are evident for pileup jets. Jet resolution good to within 15% up to the TeV scale.

09:18: Muon efficiency appears flat a function of pileup, as does isolation. 2012 has lower fake rates for electrons than 2011 for the same efficiency. Tau identification is ~70% with very low fake rates.

09:16: Particle flow used to great effect at CMS. Sophisticated electron reconstructed. Electron and photon calibrations show excellent performance. Gaining in sensitivity with identification algorithms.

09:15: Data recording and Monte Carlo production shown impressive performance and improvements.

09:14: Laser monitored correction for light loss in ECAL crystals. Resolution good to 1% using Z lineshape for calibration.

09:13: CMS detector, silicon tracker with 200m2 and 10M channels. Huge 3.8T solenoid (which is what CMS is named after.) Very fine granularity. Electromagnetic calorimeter a first for hadron experiment, using PbW04 75,000 crystals. Close to 100% up time for subsystems.

09:11: Luminosity increasing appreciably in 2012. 5.2fb^-1 collected so far in 2012.

09:10: Discovery potential: expect 5 to 6 sigma sensitivity for a Standard Model Higgs around 125GeV.

09:07: Constraints come from masses of top quark and W boson. Great exclusions coming from Tevatron.

09:08: In 2012 LHC moved from 7TeV to 8TeV. Dominant production mechanism is gluon gluon fusion. (Others include vector boson fusions, top radiation and associated produciton.

09:09: Main decay modes: WW, ZZ, bb, tautau, gammgamma.

09:05: “A lot of effort to combine all the work of thousands of people… it’s very tricky.”

09:06: Big challenge from pileup, about 50 interactions per event. Very rare particle, lots of sleepless nights.

## Before the talks

09:02: Rolf Heuer: “Good morning everybody at CERN. Good afternoon everybody at Melbourne.” The seminar is about to begin. “Today is a special day.”

08:59: It is time. May the announcements begin.

08:56: Peter Higgs just arrived! Applause.

08:48: Why the Higgs boson is the “God particle”: It gives us mass. Mass is the fundamental unit of Catholicism.

08:46: Less than 15 minutes to go. I hope my typing is good enough and fast enough! Apologies for any typos.

08:40: We can see our colleagues in Melbourne and they can see us. Jon Ellis just arrived. There are many cameras here. I’m waiting for Peter Higgs to show up…

08:29: ATLAS Spokesperson, Fabiola Gianotti has arrived. As far as I know CMS will present first, and ATLAS will present second. (Last time ATLAS presented first.)

8:13: Famous faces arriving. Rolf Heuer, Director General of CERN. Guido Tonelli, the former CMS Spokesperson. Eilam Gross, the ATLAS Higgs Convener and Bill Murray (not the actor, the former ATLAS Higgs Convener).

08:02: I waited in the lobby since 11pm last night, with food and blankets and books. There was a very communal atmosphere and people tweeted their experience (search for the #occupyCERN tag!) Now we reap the benefits of the wait.

08:01: A short while ago me and my mother were interviewed by an Israeli TV station!

07:45: I waited 8 hours to get a seat, and I have a wonderful view! I should be able to hear the speakers well, all questions being asked, and the answers. I’m sitting here with my mother to my right (she flew all the way from the UK to attend!) and my boss to my left.

### What you need to know about the Higgs seminar

Tuesday, July 3rd, 2012

The upcoming Higgs seminar could be the biggest announcement in particle physics for nearly 30 years. There have been several excellent blog posts and videos explaining what the Higgs is and what it does, so I’ll link to those at the bottom of the page. What I want to do here is give you the overview of what you really need to know to get the best from the talk.

Of course you should follow along with the liveblog as well!

### What’s happening with the webcast?

CERN have put in a lot of resources for the webcast. General users can get to the webcast at http://cern.ch/webcast. If you have a CERN login you can use a second webcast at http://cern.ch/webcast/cern_users.

The webcast will start around 09:00 CST (that’s 00:00 US West Coast, 03:00 US East Coast, 08:00 UK, and 17:00 Melbourne.

### What is the Higgs boson? What does it do?

The Higgs boson is part of the Standard Model of particle physics. The Standard Model includes the quarks and leptons (which make up all the matter see around us) and the photon, gluons, and $$W$$ and $$Z$$ boson (which carry all the forces in nature, except for gravity.) Three of these particles, the $$W^+$$, $$W^-$$ and $$Z$$ bosons, have mass, but according to our framework of physics, they should not have mass, unless the Higgs boson exists. The Standard Model of physics predicts that the $$W$$, $$Z$$, photon and Higgs all come as a package and they are all related to each other. If we don’t see a Higgs boson, we don’t understand the world around us.

People say that the Higgs boson gives particles mass, but this isn’t quite what happens. The Higgs boson allows some particles to have mass. The Higgs boson does not explain the mass that comes from binding energies (for example, most of the mass of the proton) and it does not explain the mass associated with dark matter. If the Higgs boson is discovered it will complete the Standard Model of physics, but it will not complete our picture of the universe. There will still be many unanswered questions.

### What would a discovery look like?

In order to claim a discovery an experiment would need to see a 5 sigma excess over the expected background. A sigma is a measure of uncertainty, and the chance of seeing a 5 sigma excess due to statistical fluctuations is about 1 in 3 million. If both experiments see an excess of 5 sigma in the same region the chances that this is due to a fluctuation is 1 in 9 million million!

The experiments produce “Brazil plots”, which show what they expect to see if there is no Higgs, and compare it to what they actually see. The green band shows 1 sigma deviations, the yellow bands show 2 sigma deviations, and then you have to use your imagination to see the remaining bands, and colors. When the green and yellow bands pass below the SM=1 line, and the central black line does too, then the Higgs is excluded in that region to 95% confidence. If the black line stays above the SM=1 line then we haven’t excluded the Higgs boson in that region yet. So when the green and yellow bands fall far below the SM=1 line, but the black line stays above or at the SM=1 line then we accumulate evidence for a Higgs boson.

### How do we search for the Higgs boson?

The search for the Higgs boson depends on its mass. At high mass it can decay to heavy particles with clean signatures, so the high mass region was the first region to see an exclusion. At very high mass the width of the Higgs boson is large, so the events get spread out over a large range, so the searches take a little longer. At low mass the decays get very messy, so we have to pick our decay modes carefully. The cleanest modes are the two photon mode (often called gamma gamma), the ZZ* mode and the WW* mode. Of these three, the gamma gamma and ZZ* modes are the most sensitive, so we can expect to see these presented tomorrow.

The data are collected that the detectors and stored to disk, and the physicists spend their time analyzing the data. This is a slow process, full of potential pitfalls, so the internal review process is long and stringent. This is one of the reasons why we need two experiments, so that they can check each other’s findings. The experiments at Tevatron have already presented their results and they see an excess in the same region. This is vital because they are sensitive to different final states, so between the Tevatron and the LHC we have all the analyses covered.

For each analysis there are two kinds of background, the “reducible” backgrounds where particles fake the particles we are looking for (for example, a high energy electron can look just like a high energy photon) and the “irreducible” backgrounds where particles are the same kind as the ones we are looking for. So when you see plots showing the gamma gamma searches, you can expect to see four categories: gamma gamma (irreducible Standard Model background), jet gamma, jet jet, and “other”. As we make more and more stringent requirements to eliminate these backgrounds we also lose signal events, so we have trade off background rejection against signal acceptance.

On top of all these problems we also have to take reconstruction and acceptance into account. We cannot record every event, so we pick and choose events based on how interesting they look. Does an event have two high energy photon candidates? If so, record it. Does an event have four leptons in the signal state? If so, record it. These trigger decisions are affected by definitions of “high energy”, by the algorithms we use, and by the coverage of the detectors. We have to take all of these biases into account with systematic uncertainties, and these can dominate for some of the searches.

When we put all this together we end up asking some simple questions: “How many background events do we expect?” “How many events do we see in data?” “What is the total uncertainty on the background and signal?” “How many signal events do we think we see?” “How much larger is this than the uncertainty?” This then gives us the “n sigma” for that mode across the mass range. We combine these sigmas within a single experiment, taking correlated uncertainties into account, and that’s how we get our Brazil plots.

### How likely is a discovery?

In 2011 we had about $$5fb^{-1}$$ of luminosity and we saw about 3 sigma for each experiment. In 2012 we had about $$6.5fb^{-1}$$ of luminosity at slightly higher energy (giving a factor of 1.25). So we can work out what to expect for 2012 sensitivity- just take the 3 sigma and add it in quadrature to $$(\sqrt{1.25\times 6.5/5})\times 3$$ sigma and that comes out at 4.9 sigma. If we’re lucky one or more experiments might see more than 5 sigma, meaning we could have a discovery!

### What next for the Higgs?

If we make a discovery, either now or in the coming weeks, then we need to measure the properties of the new particle. We can’t claim to have discovered the Standard Model Higgs boson until we’ve measured its branching fractions and spin. Fortunately, if the Higgs boson is at 125GeV then we have a rich variety of decay modes, and this could give us insights into all kinds of interesting measurements, such as the quark masses.

Now go and enjoy the seminar!

What comes next? (Richard Ruiz)

How difficult is it find the Higgs? (Richard Ruiz)

(Video) What is a Higgs boson? (Dom Lincoln)

(Video) Higgs boson – Latest update (Dom Lincoln)

### Higgsdependence Day

Monday, July 2nd, 2012

On July 4th CERN will hold a seminar where ATLAS and CMS will present their latest findings on the search for the Higgs boson. There’s a reasonable chance that either or both experiments will see a 5 sigma excess, and this would be enough to claim a “discovery”. One of my US friends at CERN called this day Higgsdependence Day, and all over the USA people will be celebrating with fireworks and barbecues. (Okay, perhaps they will be celebrating something else. My boss tells me he might tar and feather me as the token British member of the group…)

CERN is not the only lab to be holding a seminar. Today at 09:00 CDT Fermilab will be announcing the latest results from CDF and D0. Rumors suggest a 3 sigma excess (technically an “observation”) in the interesting region. So if you can spare the time I’d recommend you listen in on the announcement. You can see the webcast information here.

In anticipation of the CERN seminar, when I came to my office this morning I found a bottle of champagne with a label hastily pasted to the back. It seems these might be placed alongside fire extinguishers in every office at CERN! (You can get your own label here.)

No Higgs seminar is complete without a bottle of Champagne, just in case!

For those of us who can’t get enough of the Higgs boson and want to brush up on the basics I would recommend the following show, put out by the BBC. This contains the latest results from the 2011 searches and it goes into quite a bit of depth about why we think the Higgs boson exists and what to expect from the 2012 searches.

Finally for those keeping score I still have $20 riding on a non-discovery. If a 5 sigma excess is seen on Wednesday there is a bit more work that needs to be done to show that it is the Standard Model Higgs, and that would probably take until the end of 2012 running. So my$20 is safe… for now.