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Posts Tagged ‘EPS’

Impressionnant, excitant et plein de nouvelles perspectives. Cela résume mon impression alors que se termine aujourd’hui la conférence de physique des particules de la Société européenne de physique (EPS) à Vienne.

Nous avons été exposés à une quantité impressionnante de nouvelles données. Non seulement les expériences du Grand collisionneur de hadrons (LHC) du CERN ont finalisé la plupart de leurs analyses sur l’ensemble des données recueillies avant l’arrêt début 2013, mais elles ont aussi déjà commencé à analyser les nouvelles données. Ceci confirme que tout, des détecteurs aux logiciels de reconstruction, fonctionne parfaitement après le vaste programme d’améliorations et de réparations.

conference-dinner

Souper de clôture de la conférence au magnifique palais Schönbrunn à Vienne (Photo: Gertrud Konrad)

Tous les outils nécessaires aux analyses de physique – simulations, systèmes d’acquisition de données, trigger, calibrations et algorithmes d’analyse – produisent déjà des résultats de haute qualité avec les données des collisions à une énergie de 13 TeV. Les expériences sont clairement en mesure de reprendre les analyses là où elles les avaient laissées avec les données collectées à 8 TeV. Bien sûr, il n’y a encore aucuns signes de nouveaux phénomènes mais les expériences LHCb, CMS et ATLAS ont toutes de petites anomalies qui devraient être élucidées avec les nouvelles données du LHC.

Durant cette conférence, on a pu apprécié aussi la variété des expériences en place et les nouveaux résultats qui commencent déjà à arriver sur la matière sombre et l’énergie sombre. De nouvelles avenues sont aussi explorées pour élargir les recherches dans l’espoir de découvrir les 95 % du contenu de l’Univers qui manquent toujours à l’appel. Les expériences ont fait des pas de géants et on s’attend à des percées majeures d’ici à peine quelques années. On peut aussi espérer des développements dans le secteur des neutrinos, un domaine de recherche prolifique mais aussi un des plus déconcertants et embrouillants depuis de nombreuses années.

Comme l’a souligné Pierre Binetruy, un théoricien travaillant en cosmologie : « Les découvertes simultanées du boson de Higgs et la confirmation de quelques unes des caractéristiques de l’inflation (la période marquée par une expansion fulgurante juste après le Big Bang) a ouvert une nouvelle ère dans la compréhension commune de la cosmologie et de la physique des particules ». Nous sommes clairement à la veille de percées majeures et de nouvelles découvertes dans plusieurs domaines. La prochaine conférence sera sans aucun doute un événement à ne pas manquer.

Pauline Gagnon

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution ou consultez mon site web.

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Impressive, exciting and eye-opening. This is how I would summarize the European Physics Society (EPS) particle physics conference that is ending today in Vienna.

The participants were treated to an impressive amount of new data. Not only had the Large Hadron Collider (LHC) experiments at CERN finalised most of their analyses on the entire set of data collected prior to the long shutdown of the last two years, but they had also already started analysing the new data. This confirms that everything, from hardware to software, is up and running after extensive upgrades, repairs and improvements.

All the tools for physics analysis – simulations, data acquisition systems, trigger menus, calibration and analysis algorithms – are already performing beautifully at the new collision energy of 13 TeV. The experiments are clearly in a position to take up the analyses where they had left them with the 8 TeV data. True, there are no signs for new physics anywhere yet but LHCb, CMS and ATLAS all have little hints that will soon be elucidated with the new data.

conference-dinner

Conference dinner in the beautiful Schönbrunn castle in Vienna (Credit: Gertrud Konrad)

A wealth of new experiments and results were also presented at the conference on dark matter and dark energy. New avenues are also explored to broaden the searches in the hope of accounting for the 95% of the content of the Universe that is still completely unknown. Giant steps have already been taken and major breakthroughs are expected in the very near future. Developments are also expected in the neutrino sector, a prolific research domain that has been most puzzling and confusing for many years.

As stated by Pierre Binetruy, a theorist working on cosmology: “The simultaneous discovery of the Higgs and confirmation of some of the basic features of inflation (the rapid expansion that followed the Big Bang) has opened a new era in the common understanding of cosmology and particle physics“. It is clear that we are on the eve of major advances and discoveries. The next conference is sure to be an event not to be missed.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline  or sign-up on this mailing list to receive an e-mail notification. You can also visit my website

 

 

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Hauptgebäude1

“Nous posons des questions depuis 1365”. Cette déclaration inspirante accrochée près de son imposant portail d’entrée, marque le 650ième anniversaire de l’Université de Vienne. Cette bannière accueillait ce matin les 730 physiciens et physiciennes venu assister à la principale conférence de physique des particules de l’année organisée par la Société Européenne de Physique. Pendant une semaine, les participants et participantes devront choisir parmi des centaines de présentations où on fera le tour des connaissances actuelles en physique des particules et des nouvelles avenues avancées. Première énorme surprise : le thermomètre affichait 39˚C hier, la plus haute température jamais enregistrée à Vienne.

Déjà, la première journée comportait son lot de résultats récents et excitants. Tel qu’annoncé la semaine dernière, la collaboration LHCb du CERN a découvert les tous premiers pentaquarks, des particules composées de cinq quarks. Les quarks sont quelques uns des grains de matière fondamentaux. Les physiciens et physiciennes observent déjà depuis des décennies des dizaines de particules faites de deux ou trois quarks. Par exemple, plusieurs particules sont faites d’une paire de quark et d’antiquark. D’autres particules, comme les protons et les neutrons, contiennent trois quarks. Tout récemment, quelques groupes expérimentaux avaient aussi rapporté la découverte de tétraquarks, des objets composés de quatre quarks. Et finalement, la semaine dernière, grâce à l’énorme quantité de données rendues disponibles par le Grand Collisionneur de Hadrons, ou LHC, les scientifiques de l’expérience LHCb ont fièrement annoncé la découverte de pentaquarks. Ils et elles ont ainsi pu réaliser ce que beaucoup d’autres groupes avaient en vain essayé de faire pendant des décennies. On s’attendait à leur existence, mais ils n’avaient jamais été observés auparavant. Ce qui prouve bien qu’il nous reste encore beaucoup à découvrir et à comprendre.

Autre belle nouvelle: l’expérience de neutrinos T2K, qui se déroule au Japon, a peut-être détecté les premiers signes d’oscillations d’antineutrinos. On connaît à ce jour trois types de neutrinos, chacun accompagnant sa propre particule, soit l’électron, le muon ou le tau. Le processus d’oscillation décrit comment des neutrinos d’un type particulier peuvent se changer en un autre type de neutrinos.  Ce phénomène a déjà été observé pour les neutrinos, mais ce serait une première avec les antineutrinos. Mais tout est loin d’être clair, au contraire. D’abord, l’équipe de T2K n’a que trois petits évènements à se mettre sous la dent et il n’y a encore aucune certitude qu’on ait bel et bien affaire à des antineutrinos et non pas des neutrinos. Il faudra encore attendre une année ou deux avant que suffisamment d’évènements soient accumulés pour qu’on puisse en avoir le cœur net. Mais si c’était le cas, cela nous en apprendrait davantage sur les similitudes ou différences entre matière et antimatière.

Plusieurs expériences essaient aussi d’établir s’il n’existerait pas un autre type de neutrinos, appelés neutrinos stériles, dont le spin serait l’inverse des autres neutrinos, c’est-à-dire qu’ils tourneraient sur eux-mêmes dans le sens inverse des neutrinos habituels. Bien sûr, toute découverte de nouvelles particule est à suivre. Mais la confirmation de l’existence de neutrinos stériles tout particulièrement. Cela enverrait une onde de choc en physique des particules car ce serait une observation directe d’une physique nouvelle bien plus vaste que le Modèle Standard actuel. Il faudrait alors tout revoir. Et qui sait? Les physiciens et physiciennes pourraient bien avoir de quoi continuer à se poser des questions pendant les 650 années à venir…

Pauline Gagnon

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution ou consultez mon site web

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Hauptgebäude1« We have been asking questions since 1365 ». This inspiring statement marking the 650th anniversary of the University of Vienna, is hanging near the imposing entrance of its main building. This sign welcomed this morning the 730 physicists who came to Vienna to participate to the main particle physics conference this year organised by the European Physics Society. For a week, the participants will have to choose among hundreds of presentations where the current status of knowledge in particle physics will be presented along with the newest avenues. and we are already making history: Vienna recorded yesterday its highest ever temperature with 39 ˚C.

And this first day brought recent and exciting results. As announced last week, the LHCb collaboration at CERN has discovered the first pentaquarks, composite objects made of five quarks. Quarks are some of the building blocks of matter. Physicists have observed for decades dozens of different particles made of two or three quarks. For example, many particles are made of a pairs of quark and antiquark, while others, like protons and neutrons, contain three quarks. However, in recent years, a few experimental groups also reported the discovery of tetraquarks, objects composed of four quarks. Finally, last week, thanks to the huge dataset made available by the Large Hadron Collider, scientists from the LHCb experiment achieved what many other groups had tried to do for decades without success, and proudly announced the discovery of pentaquarks. Such composite objects were expected but never observed before. It goes to show how much we still have to discover and understand.

Another nice piece of news: the T2K neutrino experiment, which takes place in Japan, may have detected the first signs for oscillations of antineutrinos. To this day, there are three known types of neutrinos, each one accompanying its own particle, namely the electron, the muon and the tau. The oscillation process describes how one type of neutrinos can change into another type. This phenomenon has already been observed for neutrinos, but it would be the first observation for antineutrinos. However, all is far from being set in concrete yet, quite the contrary. With only three events at hands, the T2K team still needs to verify if these events really involve antineutrinos and not just neutrinos. They therefore need to collect more date for another year or two before this issue can be settled. If it turns out to be indeed antineutrinos, we would learn more on the similarities or the differences between matter and antimatter.

Several experiments are also trying to establish if there could also be another type of neutrinos, called sterile neutrinos. Their spin would be the opposite of known neutrinos, meaning they would be spinning on themselves in the opposite direction. Clearly, any new type of particle is something worth watching for. The confirmation of the existence of sterile neutrinos would send a shock wave in particle physics since it would constitute an indisputable proof for the existence of a theory more encompassing than the current theoretical model, called the Standard Model. Everything would then have to be rethought. And who knows? Physicists could very well have enough to keep asking difficult questions for another 650 years…

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline, sign-up on this mailing list to receive an e-mail notification or consult my website.

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What If It’s Not The Higgs?

Sunday, August 21st, 2011

Updated: Monday, 2011 August 29, to clarify shape of angular distribution plots.

It’s the $10 billion question: If experimentalists do discover a bump at the Large Hadron Collider, does it have to be the infamous higgs boson? Not. One. Bit. Plainly and simply, if the ATLAS & CMS collaborations find something at the end of this year it will take a little more data to know we are definitely dealing with a higgs boson. Okay, I suppose I should back up a little an add some context. 🙂

The Standard Model of Particle Physics (or SM for short) is the name for the very well established theory that explains how almost everything in the Universe works, from a physics perspective at least. The fundamental particles that make up the SM, and hence our Universe, are shown in figure 1 and you can learn all about them by clicking on the hyperlink a sentence back. Additionally, this short Guardian article does a great job explaining fermions & bosons.

Fig 1. The Standard Model is composed of elementary particles, which are the fundamental building blocks of the Universe, and rules dictating how the particles interact. The fundamental building blocks are known as fermions and the particles which mediate interactions between fermions are called bosons. (Image: AAAS)

As great as the Standard Model is, it is not perfect. In fact, the best way to describe the theory is to say that it is incomplete. Three phenomena that are not fully explained, among many, are: (1) how do fermions (blue & green boxes in figure 1) obtain their mass; (2) why is there so little antimatter (or so much matter) in the Universe; and (3) how does gravity work at the nanoscopic scale? These are pretty big questions and over the years theorists have come up with some pretty good ideas.

The leading explanation for how fermions (blue & green boxes in figure 1) have mass is called the Higgs Mechanism and it predicts that there should be a new particle called the higgs boson (red box at bottom of figure 1). Physicist believe that the Higgs Mechanism may explain the fermion masses is because this same mechanism very accurately predicts the masses for the other bosons (red boxes in figure 1). It is worth nothing that when using the Higgs Mechanism to explain the masses of the bosons, no new particle is predicted.

Unfortunately, the leading explanations for the huge disparity between matter & antimatter, as well as a theory of gravity at the quantum level, have not been as successful. Interestingly, all three types of  theories (the Higgs Mechanism, matter/antimatter, and quantum gravity) generally predict the existence of a new boson, namely, the higgs boson, the Z’ boson (pronounced: zee prime), and the graviton. A key property that distinguishes each type of boson from the others is the intrinsic angular momentum they each carry. The higgs boson does not carry any, so we call it a “spin 0” boson; the Z’ boson carries a specific amount, so it is called a “spin 1” boson; and the graviton carries precisely twice as much angular momenta as the Z’ boson, so the graviton is called a “spin 2” boson. This will be really important in a few paragraphs but quickly let’s jump back to the higgs story.

Fig 2. Feynman Diagrams representing a higgs boson (left), Z’ boson (center), and graviton (right)
decaying into a b quark (b) & anti-b quark (b).

In July, at the European Physics Society conference, the CDF & DZero Experiments, associated with the Tevatron Collider in Illinois, USA, and the CMS & ATLAS Experiments, associated with the Large Hadron Collider, in Geneva, Switzerland, reported their latest results in the search for the higgs boson. The surprising news was that it might have been found but we will not know for sure until the end of 2011/beginning of 2012.

This brings us all the way back to our $10/€7 billion question: If the experiments have found something, how do we know that it is the higgs boson and not a Z’ boson or a graviton? Now I want to be clear: It is insanely unlikely that the new discovery is a Z’ or a graviton, if there is a new discovery at all. If something has been been discovered, chances are it is the higgs boson but how do we know?

Now, here is where awesome things happen.

The Solution.

In all three cases, the predicted boson can decay into a b quark (b) & anti-b quark (b) pair, which you can see in the Feynman diagrams in figure 2. Thanks to the Law of Conservation of Momentum, we can calculate the angle between each quark and the boson. Thanks to the well-constructed detectors at the Large Hadron Collider and the Tevatron, we can measure the angle between each quark and the boson. The point is that the angular distribution (the number of quarks observed per angle)  is different for spin 0 (higgs), spin 1 (Z’), and spin 2 (graviton) bosons!

To show this, I decided to use a computer program to simulate how we expect angular distributions for a higgs → bb, a Z’→ bb, and a graviton → bb to look. Below are three pairs of plots: the ones to the left show the percentage of b (or b) quarks we expect at a particular angle, with respect to the decaying boson; the ones on the right show the percentage of quarks we expect at the cosine (yes, the trigonometric cosine) of the particular angle.

 

Figure 3. The angular distribution (left) and cosine of the angular distribution (right) for the higgs (spin-0) boson, mH = 140 GeV/c2. 50K* events generated using PYTHIA MSUB(3).

Figure 4. The angular distribution (left) and cosine of the angular distribution (right) for a Z’ (spin-1) boson, mZ’ = 140 GeV/c2. 50K* events generated using PYTHIA MSUB(141).

Figure 5. The angular distribution (left) and cosine of the angular distribution (right) for a graviton (spin-2) boson, mG = 140 GeV/c2. 40K* events generated using PYTHIA MSUB(391), i.e., RS Graviton.

Thanks to the Law of Conservation of Angular Momentum, the intrinsic angular momenta held by the spin 0 (higgs), spin 1 (Z’), and spin 2 (graviton) force the quarks to decay preferentially at some angles and almost forbid other angles. Consequentially, the angular distribution for the higgs boson (spin 0) will give one giant hump around 90°; for the Z’ boson will have two humps at 60° and 120°; and the graviton (spin 2) will have three humps at 30°, 90°, and 150°. Similarly in the cosine distribution: the spin-0 higgs boson has no defining peak; the spin-1 Z’ boson has two peaks; and the spin-2 graviton has three peaks!

In other words, if it smells like a higgs, looks like a higgs, spins like a higgs, then my money is on the higgs.

A Few Words About The Plots

I have been asked by a reader if I could comment a bit on the shape and apparent symmetry in the angular distribution plots, both of which are extremely well understood. When writing the post, I admittedly glossed over these really important features because I was pressed to finish the post before traveling down to Chicago for a short summer school/conference, so I am really excited that I was asked about this.

At the Large Hadron Collider, we collide protons head-on. Since the protons are nicely aligned (thanks to the amazing people who actually operate the collider), we can consistently and uniformly label the direction through which the protons travel. In our case, let’s have a proton that come from the left be proton A and a proton that comes from the right be proton B. With this convention, proton A is traveling along what I call the “z-axis”; if proton A were to shoot vertically up toward the top of this page it would be traveling along the “x-axis”; and if it were to travel out of the computer screen toward you, the reader, the proton would be traveling in the “y direction” (or along the “y-axis”). The angle between the z-axis and the x-axis (or z-axis and the y-axis) is called θ (pronounced: theta). You can take a look at figure 6 for a nice little cartoon of the coordinate system I just described to you.

Figure 6: A coordinate system in which proton A (pA) is traveling along the z-axis and proton B (pB) in the negative z direction. The angle θ is measure as the angle between the z-axis and the x-axis, or equally, between the z-axis and the y-axis.

When the quarks (spin 1/2) inside a proton collide to become a higgs (spin 0), Z’ (spin 1), or graviton (spin 2), angular momentum must always be conserved. The simplest way for a quark in proton A and a quark in proton B to make a higgs boson is for the quarks to spin opposite directions, while still traveling along the z-axis, so that their spins cancel out, i.e., spin 1/2 – spin 1/2 = spin 0. This means that the higgs boson (spin 0) does not have any angular momentum constraints when decaying into two b-quarks and thus the cosine of the angle between the two b-quarks should be roughly flat and uniform. This is a little hard to see in figure 3 (right) because, as my colleague pointed out, the resolution in my plots are too small. (Thanks, Zhen!)

Turning to the Z’ boson (spin 1) case, protons A & B can generate a spin 1 particle most easily when their quarks, again while traveling along the z-axis, are spinning in the same direction, i.e., spin 1/2 + spin 1/2 = spin 1. Consequentially, the spin 1 Z’ boson and its decay products, unlike the higgs boson (spin 0), are required to conserve 1 unit of angular momentum. This happens most prominently when the two b-quarks (1) push against each other in opposite directions or (2) travel in the same direction. Therefore, the cosine of the angle made by the b-quarks is dominantly -1 or +1. If we allow for quantum mechanical fluctuations, caused by Heisenberg’s Uncertainty Principle, then we should also expect b-quarks to sometimes decay with a cosine greater than -1 and less than +1. See figure 4 (right).

The spin 2 graviton can similarly be explained but with a key difference. The spin 2 graviton is special because like the Z’ boson (spin 1) it can have 1 unit of angular momentum, but unlike Z’ boson (spin 1) it can also have 2 units of angular momenta. To produce a graviton with 2 units of angular momenta, rarer processes that involve the W & Z bosons (red W & Z in figure 1) must occur. This allows the final-state b-quarks to decay with a cosine of 0, which explains the slight enhancement in figure 5 (right).

It is worth noting that the reason why I have been discussing the cosine of the angle between the the quarks and not the angle itself is because the cosine is what we physicists calculate and measure. The cosine of an angle, or equally sine of an angle, amplify subtle differences between particle interactions and can at times be easier to calculate & measure.

The final thing I want to say about the angular distributions is probably the coolest thing ever, better than figuring out the spin of a particle. Back in the 1920s, when Quantum Mechanics was first proposed, people were unsure about a keystone of the theory, namely the simultaneous particle and wave nature of matter. We know bosons definitely behave like particles because they can collide and decay. That wavy/oscillatory behavior you see in the plots are exactly that: wavy/oscillatory behavior. No classical object will decay into particles with a continuous distribution; no classical has ever been found to do so nor do we expect to find one, at least according to our laws of classical physics. This wave/particle/warticle behavior is a purely quantum physics effect and would be an indicator that Quantum Mechanics is correct at the energy scale being probed by the Large Hadron Collider. 🙂

 

Happy Colliding.

– richard (@bravelittlemuon)

PS I apologize if some things are a little unclear or confusing. I traveling this weekend and have not had time to fully edit this post. If you have a question or want me to clarify something, please, feel free to write a comment.

PPS If you are going to be at the PreSUSY Summer School in Chicago next week, feel free to say hi!

*A note on the plots: I simulated several tens of thousands of events for clarity. According to my calculations, it would take four centuries to generate 40,000 gravitons, assuming the parameters I chose. In reality, the physicists can make the same determination as we did with fewer than four years worth of data.

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The following guest post is from Kostas Nikolopoulos, a postdoctoral researcher at Brookhaven National Laboratory. Nikolopoulos, who is analyzing data from the Large Hadron Collider at CERN, received his Ph.D. in experimental high-energy physics from the University of Athens in 2010.

Last Wednesday, I travelled three hours by train from Geneva, Switzerland to Grenoble, France to spend a week at the International Europhysics Conference on High Energy Physics. Here, I’m presenting some of the latest findings in the search for the Higgs boson at the Large Hadron Collider’s ATLAS detector, and joining the overarching conversation about the elusive particle. (more…)
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Life at the limit

Friday, July 22nd, 2011

At the moment anyone who has even a passing interest in particle physics is thinking about the results being presented that European Physical Society High Energy Physics 2011 conference (referred to as simply “EPS”.) It is at EPS that we hear news about the search the Higgs boson, and the news is tantalizing! The talks are all publicly available, and to understand them fully you need to know a little bit about how limits work.

What is a limit?

A limit is an upper or lower bound for a physical quantity, and we place limits when we don’t have enough information to estimate the value accurately or precisely. When we say something like “The lower limit for the mass of the Higgs boson is 114GeV” what we mean is that given the data we have had access to we can be confident that the mass of the Higgs boson is at least 114GeV.

One of the most interesting parts of the search for the Higgs is that we have several limits at the moment, so one experiment might say “The mass is at least 114GeV” and another experiment will say “It’s not between 155 and 190GeV”, and experts in electroweak physics will say “Don’t bother to look above 300GeV”. Each mass region requires a slightly different search, and that’s why some limits appear before others.

Confidence problems

Like anyone else, physicists have issues with “confidence”. To a physicist, “confidence” means the extent to which they trust a measurement, so it’s an important concept to get right! Our data are statistically limited, so we can never be 100% certain in any of our measurements. What we usually do is say something like “We’re 95% certain that the Higgs mass is not in the region 157-174GeV”. To understand what that really means you need to think backwards. We’ve got some data and the probability that we would get this data, given that the Higgs mass in the region 157-174GeV is 5%, or 1 in 20.

You can probably see why this gives us confidence problems… if we have 20 data points that show us measurements with 95% confidence then we expect 1 of them to be incorrect. As we look across one of our plots we can see lots of data points (generally, every time there’s a kink in the plot there’s another data point.) How do we know when we’ve got it right? The answer is that we don’t know, and the fluctuations can take the distributions up and down. At first this seems like a minor irritation, but it has serious implications.

Your typical limit plot

A lot of the talks at EPS contain plots like this:

ATLAS limit (K. Cranmer, on behalf of ATLAS. EPS HEP 2011)

ATLAS limit on Higgs mass (K. Cranmer, on behalf of ATLAS. EPS HEP 2011)

They look pretty, but they don’t look simple. The green and yellow bands show us the expected confidence bounds for some number, and that’s what we should look at first to get a feeling for what the plot is telling us. The line at the center of these bands shows us the expected limit. The “Observed” line shows us what we actually see in the experiment. If the “Observed” line stays within the bands then our expectations are about right.

The y-axis shows the production cross section of the Higgs boson, multiplied by the branching fraction to the final state, and some other factors. These numbers all vary as the mass of the Higgs boson, which is one of the reasons why the graphs look so wiggly. The exciting part is the horizontal line at 1. This is the line where we would expect to see the Higgs boson being produced. If the “Observed” line crosses the line at 1 then we can conclude that the Higgs boson probably does not exist at that mass, because our limit is already at 1 times the Standard Model. As the upper yellow band passes under the line at 1 we can be almost certain that the Higgs doesn’t exist there. (Remember the definition of the confidence: “At this mass point, we’re 95% sure that the Higgs production cross section is less than what the Standard Model predicts.”)

What the plots tell us

Exciting things start to happen when the limits change! As we gather more data the limits improve and we exclude more mass points. On the plot, we would see the green and yellow bands move down. If the Higgs doesn’t exist in a particular mass region then the “Observed” line would move down as well. But, if we see the bands move down and the “Observed” line get left behind then that’s a hint that the Higgs boson mass is in that region!

This is cause for major excitement for some physicists and skepticism for others. Remember the confidence problem of fluctuations and you can see that this kind of fluctuation would happen very often. When does a “fluctuation” turn into “evidence”? It’s a topic that’s not very well defined, but we’ve chosen to say three standard deviations (imagine a third colored band on the plot) is a good indication of evidence, and five standard deviations (a veritable rainbow of confidence!) is proof of new physics. When we see a fluctuation the answer is to add more data and see if it remains. If it stays there while the bands move down around it then there’s probably a particle there.

LEP? What is LEP?

In these talks you’ll often see “LEP” on the plots. This refers to a collection of four high-precision experiments that operated at CERN before the era of the LHC. LEP was an electron-positron collider and the LHC now occupies the tunnels where LEP were. The four experiments (ALEPH, Delphi, L3 and Opal) searched for the Higgs boson directly via a process known as Higgsstrahlung. They excluded the Higgs boson mass up to 114GeV, and their contribution to the hunt, although it looks small on these plots, is certainly substantial. (In fact, you can see how the limits at the LHC experiments get poorer and poorer at lower masses. The LEP experiments were better suited to these regions by design, and by design the LHC experiments are better at higher mass regions.)

What about electroweak?

The Higgs boson is just one of many particles, and since all the other particles interact with each other we expect the Higgs boson to interact as well. If it does it will affect all kinds of processes, most notably the electroweak processes, which are finely constrained. This allows us to identify the most likely place where the Higgs boson would be found, so if you see anything about electroweak fits and exclusions it’s usually the result of fitting a huge number of electroweak parameters in an attempt to limit the mass of the Higgs boson.

Find your own limit!

Now that you have all you need to read these plots you can go check out the limits yourself! Here are the talks from the main experiments:

ATLAS
CDF
CMS
D0

Electroweak fit

On an unrelated note, Happy Pi Approximation Day!

Errata

This post initially incorrectly credited the plot to A. Baroncelli. This plot was presented at EPS by Kyle Cranmer. Apologies to Kyle!

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