## Posts Tagged ‘Higgs boson’

### The International Linear Collider: More than Just a Higgs Factory

Friday, August 23rd, 2013

The ILC site has been chosen. What does this mean for Japan?

The two ILC candidate sites: Sefuri in the South and Kitakami in the North. Credit: linearcollider.org

Hi Folks,

It is official [Japanese1,Japanese2]: the Linear Collider Collaboration and the Japanese physics community have selected the Kitakami mountain range in northern Japan as the site for the proposed International Linear Collider. Kitakami is a located in the Iwate Prefecture and is just north of the Miyagi prefecture, the epicenter of the 2011 Tohoku Earthquake. Having visited the site in June, I cannot aptly express how gorgeous the area is, but more importantly, how well-prepared Iwate City is for this responsibility.

Science is cumulative: new discoveries are used to make more discoveries about how nature works, and physics is no different. The discovery of the Higgs boson at the Large Hadron Collider was a momentous event. With its discovery, physicists proved how some particles have mass and why others have no mass at all. The Higgs boson plays a special role in this process, and after finally finding it, we are determined to learn more about the Higgs. The International Linear Collider (ILC) is a proposed Higgs boson factory that would allow us to intimately understand the Higgs. Spanning 19 miles (31 km) [310 football pitches/soccer fields], if constructed, the ILC will smash together electrons and their antimatter partners, positrons, to produce a Higgs boson (along with a Z boson). In such a clean environment (compared to proton colliders), ultra-precise measurements of the Higgs boson’s properties can be made, and thereby elucidate the nature of this shiny new particle.

The general overview schematic of the International Linear Collider. Credit: linearcollider.org

However, the ILC is more than just a experiment. Designing, constructing, and operating the machine for 20 years will be a huge undertaking with lasting effects. For staters, the collider’s Technical Design Report (TDR), which contains every imaginable detail minus the actual blueprints, estimates the cost of the new accelerator to be 7.8 billion USD (2012 dollars). This is not a bad thing. Supposing 50% of the support came from Asia, 25% from the Americas, and 25% from Europe, that would be nearly 2 billion USD invested in new radio frequency technology in England, Germany, and Italy. In the US, it would be nearly 2 billion USD invested in coastal and Midwestern laboratories developing new cryogenic and superconducting technology. In Asia, this would be nearly 4 billion USD invested in these technologies as well as pure labor and construction. Just as the LHC was a boon on the European economy, a Japanese-based ILC will be a boon for an economy temporarily devastated  by an historic earthquake and tsunami. These are just hypothetical numbers; the real economic impact will be  larger.

I had the opportunity to visit Kitakami this past June as a part of a Higgs workshop hosted by Tohoku University. Many things are worth noting. The first is just how gorgeous the site is. Despite its lush appearance, the site offers several geological advantages, including stability against earthquakes of any size. Despite its proximity to the 2011 earthquake and the subsequent tsunami, this area was naturally protected by the mountains. Below is a photo of the Kitakami mountains that I took while visiting the site. Interestingly, I took the photo from the UNESCO World Heritage site Hiraizumi. The ILC is designed to sit between the two mountains in the picture.

The Kitamaki Mountain Range as seen from the UNESCO World Heritage Site in Hiraizumi, Japan. Credit: Mine

What I want to point out in the picture below is the futuristic-looking set of tracks running across the photo. That is the rail line for the JR East bullet train, aka the Tohoku Shinkansen. In other words, the ILC site neighbours a very major transportation line connecting the Japanese capital Tokyo to the northern coast. It takes the train just over 2 hours to traverse the 250 miles (406.3 km) from Tokyo station to the Ichinoseki station in Iwate. The nearest major city is Sendai, capital of Miyagi, home to the renown Tohoku University, and is only a 10 minute shinkansen ride from Ichinoseki station.

The Kitamaki Mountain Range as seen from the UNESCO World Heritage Site in Hiraizumi, Japan. Credit: Mine

What surprised me is how excited the local community is about the collider. After exiting the Ichinoseki station I discovered this subtle sign of support:

There is much community support for the ILC: The Ichinoseki Shinkansen Station in Iwate Prefecture, Japan. Credit: Mine

The residents of Iwate and Miyagi, independent of any official lobbying organization, have formed their own “ILC Support Committee.” They even have their own facebook page. Over the past year, the residents have invited local university physicists to give public lectures on what the ILC is; they have requested that more English, Chinese, Korean, and Tagalog language classes be offered at local community centers; that more Japanese language classes for foreigners are offered in these same facilities; and have even discussed with city officials how to prepare Iwate for the prospect of a rapid increase in population over the next 20 years.

Despite all this, the real surprises were the pamphlets. Iwate has seriously thought this through.

Pamphlets showcasing the Kitakami Mountain Range in Iwate, Japan. Credit: Mine

The level of detail in the pamphlets is impressive. My favourite pamphlet has the phrase, “Ray of Hope: Tohoku Is Ready to Welcome the ILC” on the front cover. Inside is a list of ways to reach the ILC site and the time it takes. For example: it takes 12 hours 50 minutes to reach Tokyo from Rome and 9 hours 40 minutes from Sydney. The brochure elaborates that the Kitakami mountains maintain roughly the same temperature as Switzerland (except in August-September) but collects much more precipitation through the year. Considering that CERN is located in Geneva, Switzerland, and that many LHC experimentalists will likely become ILC experimentalists, the comparison is very helpful. The at-a-glance annual festival schedule is just icing on the cake.

“Ray of Hope” pamphlet describing how to each different ILC campuses by train.  Credit: Mine

Now that the ILC site has been selected, surveys of the land can be conducted so that blue prints and a finalized cost estimate can be established. From my discussions with people involved in the site selection process, the decision was very difficult. I have not visited the Fukuoka site, though I am told it is a comparably impressive location. It will be a while still before any decision to break ground is made. And until that happens, there is plenty of work to do.

Happy Colliding

– Richard (@bravelittlemuon)

### Questioning the existence of God and the God Particle

Friday, June 7th, 2013

Does God exist?  This is one of the oldest questions in philosophy and is still much debated. The debate on the God particle is much more recent but searching for it has cost a large fortune and inspired people’s careers. But before we can answer the questions implied in the title, we have to decide what we mean when we say something exists. The approach here follows that of my previous essay that defines knowledge in terms of models that make successful predictions.

Let us start with a simple question: What does it mean when we say a tree exists? The evidence for the existence of trees falls into two categories: direct and indirect. Every autumn, I rake the leaves in my backyard. From this I deduce that the neighbour has a tree. This is indirect evidence. I develop a model that the leaves in my backyard come from a tree in the neighbour’s yard. This model is tested by checking the prediction that the leaves are coming from the direction of the neighbour’s yard. Observations have confirmed this prediction.  Can I then conclude that a tree exists? Probably, but it would be useful to have direct evidence. To obtain this, I look into my neighbour’s yard. Yup, there is a tree. But not so fast–what my eye perceives is a series of impressions of light. The brain then uses that input to construct a model of reality and that model includes the tree. The tree we see is so obvious that we frequently forget that it is the result of model construction, subconscious model construction, but model construction none-the-less. The model is tested when I walk into the tree and hurt myself.

Now consider a slightly more sophisticated example: atoms. The idea of atoms, in some form or other, dates back to ancient India and Greece but the modern idea of atoms dates to John Dalton (1766 – 1844). He used the concept of atoms to explain why elements always interact in the ratios of small whole numbers. This is indirect evidence for the existence of atoms and was enough to convince the chemists but not the physicists of that time. Some like Ernst Mach (1838 – 1916) refused to believe in what they could not see up until the beginning of the last century[1]. But then Albert Einstein’s (1879 – 1955) famous 1905 paper[2] on Brownian motion (the motion of small particles suspended in a liquid) convinced even the most recalcitrant physicists that atoms exist.  Einstein showed that Brownian motion could be easily understood as the result of the motion of discrete atoms. This was still indirect evidence but convincing to almost everyone. Atoms were only directly seen after the invention of the scanning electron microscope and even then there was model dependence in interpreting the scanning electron microscope results. As with the tree, we claim that atoms exist because, as a shown by Dalton, Einstein and others, they form an essential part of models that have strong track record of successful predictions.

Now on to the God particle. What a name! The God particle has little in common with God but the name does sound good in the title of this essay. Then again, calling it the Higgs boson is not without problems as people other than Peter Higgs[3] (1920 – ) have claimed to have been the first to predict its existence. Back to the main point, why do we say the God particle exists? First there is the indirect evidence. The standard model of particle physics has an enviable record of successful predictions. Indeed, many (most?) particle physicists would be happier if it had had some incorrect predictions. We could replicate most of the successful predictions of the standard model without the God particle but only at the expense of making the model much more complicated. Like the recalcitrant physicists of old who rejected the atom, the indirect evidence for the God particle was not good enough for most modern-day particle physicists. Although few actually doubted its existence, like doubting Thomas, they had to see it for themselves. Thus, the Large Hadron Collider (LHC) and its detectors were built and direct evidence was found. Or was it? Would lines on a computer screen have convinced the logical positivists like Ernst Mach? Probably not, but the standard model predicted bumps in the cross-sections and the bumps were found. Given the accumulated evidence and its starring role in the standard model of particle physics, we confidently proclaim that the God particle, like the tree and the atom, exists. But remember, that even for the tree our arguments were model dependent.

Having discussed the God particle what about God? I would apply the same criteria to His/Her/Its existence as for the tree, the atom, or the God particle. As in those cases, the evidence can be direct or indirect.  Indirect evidence for God’s existence would be, for example, the argument from design attributed to William Paley (1743 – 1805). This argument makes an analogy between the design in nature and the design of a watch. The question is then is this a good analogy? If we adopt the approach of science this reduces to the question: Can the analogy be used to make correct predictions for observations? If it can, the analogy is useful, otherwise it should be discarded. There is also the possibility of direct evidence: Has God or His messengers ever been seen or heard? But as the previous examples show, nothing is ever really seen directly but depends on model construction. As optical illusions illustrate, what is seen is not always what is there. Even doubting Thomas may have been too ready to accept what he had seen. As with the tree, the atom or the God particle, the question comes back to: Does God form an essential part of a model with a track record of successful predictions?

So does God exist? I have outlined the method for answering this question and given examples of the method for trees, atoms and the God particle. Following the accepted pedagogical practice in nuclear physics, I leave the task of answering the question of God’s existence as an exercise for you, the reader.

[1] Yes, 1905 was the last century. I am getting old.

[2] He had more than one famous 1905 paper.

[3] Why do we claim Peter Higgs exists?  But, I digress.

### The Definite Article Problem

Tuesday, June 4th, 2013

A Little Bit of the Higgs Boson for Everyone

Hi All,

This post is long overdue but nonetheless I am thrilled to finally write it. We have discovered the a some  ??? Higgs boson, and it is precisely my trouble writing this very sentence that inspires a new post. CERN‘s press office has keenly presented a new question in particle physics known as the Definite Article Problem:

Have we discovered “a” Higgs boson or “the” Higgs boson?

We can express the Article problem in another way:

Are there more Higgs bosons?

Before I touch upon that problem, I want to explain about why the Higgs boson is important. In particular, I want to talk about the Sun! Yes, the Sun.

## The Higgs Boson and Electroweak Symmetry Breaking is Important because the Sun Shines.

Okay, there is no way to avoid this: I really like the sun.

Slide Credit: Mine. Image Credit: GOES Collaboration

It shines. It keeps the planet warm. There is liquid water on Earth, and some very tasty plants too.

Slide Credit: Mine. Image Credit: NobelPrize.org

At the heart of the Sun is a ranging nuclear furnace and involves two types of processes: (1) those that involve the Strong nuclear force and (2) those that involve the Weak nuclear force (look for the neutrinos!). The two types of processes work together in a solar relay race to complete a circuit, only to do it over and over again for billions of years. And just like a real relay race, the speed at which the circuit is finished is set by the slowest member. In this case, the Weak force is the limiting factor and considerably slows down the rate at which the sun could theoretically operate. If we make the Weak force stronger, then the Sun would shine more brightly. Conversely, if we make the Weak force even weaker, the Sun would be dimmer.

Slide Credit: Mine. Image Credit: NobelPrize.org

From studying the decays of radioactive substances, we have learned that the rate of Weak nuclear processes is set by a physical constant called Fermi’s Constant. Fermi’s Constant is represented by symbol GF. From study the Higgs boson and the Higgs Mechanism, we have learned that Fermi’s Constant is literally just another constant, v, in disguise. This second physical constant (v) is called the Higgs “vacuum expectation value” , or “vev” for short, and is the amount of energy the Higgs field has at all times relative to the vacuum.

The point I want to make is this: If we increase the Higgs vev, Fermi’s Constant gets smaller, which reduces the rate of Weak nuclear interactions. In other words, a larger Higgs vev would make the sun shine less brightly. Going the other way, a smaller Higgs vev would make the sun shine more brightly. (This is really cool!)

Slide Credit: Mine. Image Credit: Jacky-Boi

The Higgs vev is responsible for some other things, too. It is a source of energy from which all elementary particles can draw. Through the Higgs Mechanism, the Higgs field provides mass to all elementary particles and massive bosons. One would think that for such an important particle we would have a firm theoretical understanding it, but we do not.

Credit: Mine

We have a very poor theoretical understanding of the Higgs boson. Among other things, according to our current understanding of the Higgs boson, the particle should be much heavier than what we have measured.

Credit: Mine

## The Definite Article Problem

There are lots of possible solutions to the problems and theoretical inconsistencies we have discovered relating to the Standard Model Higgs boson. Many of these ideas hypothesize the existence of other Higgs bosons or particles that would interact like the Higgs boson. There are also scenarios where Higgses have identity crises: the Higgs boson we have observed could be a quantum mechanical combination (superposition) of several Higgs bosons.

I do not know if there are additional Higgses. Truthfully, there are many attractive proposals that require upping the number Higgs bosons. What I do know is that our Higgs boson is interesting and merits much further studying.

Credit: Mine

Happy Colliding

– richard (@bravelittlemuon)

PS In case anyone is wondering, yes, I did take screen shots from previous talks and turn them into a DQ post.

### 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