## Posts Tagged ‘ATLAS’

### Moriond 2014: New Results, New Explorations… but no New Physics

Friday, April 4th, 2014

Even before my departure to La Thuile in Italy, results from the Rencontres de Moriond conference were already flooding the news feeds. This year’s Electroweak session from 15 to 22 March, started with the first “world measurement” of the top quark mass, from a combination of the measurements published by the Tevatron and LHC experiments so far. The week went on to include a spectacular CMS result on the Higgs width.

Although nearing its 50th anniversary, Moriond has kept its edge. Despite the growing numbers of must-attend HEP conferences, Moriond retains a prime spot in the community. This is in part due to historic reasons: it’s been around since 1966, making a name for itself as the place where theorists and experimentalists come to see and be seen. Let’s take a look at what the LHC experiments had in store for us this year…

New Results­­­

Stealing the show at this year’s Moriond was, of course, the announcement of the best constraint yet of the Higgs width at < 17 MeV with 95% confidence reported in both Moriond sessions by the CMS experiment. Using a new analysis method based on Higgs decays into two Z particles, the new measurement is some 200 times better than previous results. Discussions surrounding the constraint focussed heavily on the new methodology used in the analysis. What assumptions were needed? Could the same technique be applied to Higgs to WW bosons? How would this new width influence theoretical models for New Physics? We’ll be sure to find out at next year’s Moriond…

The announcement of the first global combination of the top quark mass also generated a lot of buzz. Bringing together Tevatron and LHC data, the result is the world’s best value yet at 173.34 ± 0.76 GeV/c2.  Before the dust had settled, at the Moriond QCD session, CMS announced a new preliminary result based on the full data set collected at 7 and 8 TeV. The precision of this result alone rivals the world average, clearly demonstrating that we have yet to see the ultimate attainable precision on the top mass.

This graphic shows the four individual top quark mass measurements published by the ATLAS, CDF, CMS and DZero collaborations, together with the most precise measurement obtained in a joint analysis.

Other news of the top quark included new LHC precision measurements of its spin and polarisation, as well as new ATLAS results of the single top-quark cross section in the t-channel presented by Kate Shaw on Tuesday 25 March. Run II of the LHC is set to further improve our understanding of this

A fundamental and challenging measurement that probes the nature of electroweak symmetry breaking mediated by the Brout–Englert–Higgs mechanism is the scattering of two massive vector bosons against each other. Although rare, in the absence of the Higgs boson, the rate of this process would strongly rise with the collision energy, eventually breaking physical law. Evidence for electroweak vector boson scattering was detected for the first time by ATLAS in events with two leptons of the same charge and two jets exhibiting large difference in rapidity.

With the rise of statistics and increasing understanding of their data, the LHC experiments are attacking rare and difficult multi-body final states involving the Higgs boson. ATLAS presented a prime example of this, with a new result in the search for Higgs production in association with two top quarks, and decaying into a pair of b-quarks. With an expected limit of 2.6 times the Standard Model expectation in this channel alone, and an observed relative signal strength of 1.7 ± 1.4, the expectations are high for the forthcoming high-energy run of the LHC, where the rate of this process is enhanced.

Meanwhile, over in the heavy flavour world, the LHCb experiment presented further analyses of the unique exotic state X(3872). The experiment provided unambiguous confirmation of its quantum numbers JPC to be 1++, as well as evidence for its decay into ψ(2S)γ.

Explorations of the Quark-Gluon Plasma continue in the ALICE experiment, with results from the LHC’s lead-proton (p-Pb) run dominating discussions. In particular, the newly observed “double-ridge” in p-Pb is being studied in depth, with explorations of its jet peak, mass distribution and charge dependence presented.

New explorations

Taking advantage of our new understanding of the Higgs boson, the era of precision Higgs physics is now in full swing at the LHC. As well as improving our knowledge of Higgs properties – for example, measuring its spin and width – precise measurements of the Higgs’ interactions and decays are well underway. Results for searches for Beyond Standard Model (BSM) physics were also presented, as the LHC experiments continue to strongly invest in searches for Supersymmetry.

In the Higgs sector, many researchers hope to detect the supersymmetric cousins of the Higgs and electroweak bosons, so-called neutralinos and charginos, via electroweak processes. ATLAS presented two new papers summarising extensive searches for these particles. The absence of a significant signal was used to set limits excluding charginos and neutralinos up to a mass of 700 GeV – if they decay through intermediate supersymmetric partners of leptons – and up to a mass of 420 GeV – when decaying through Standard Model bosons only.

Furthermore, for the first time, a sensitive search for the most challenging electroweak mode producing pairs of charginos that decay through W bosons was conducted by ATLAS. Such a mode resembles that of Standard Model pair production of Ws, for which the currently measured rates appear a bit higher than expected.

In this context, CMS has presented new results on the search for the electroweak pair production of higgsinos through their decay into a Higgs (at 125 GeV) and a nearly massless gravitino. The final state sports a distinctive signature of 4 b-quark jets compatible with a double Higgs decay kinematics. A slight excess of candidate events means the experiment cannot exclude a higgsino signal. Upper limits on the signal strength at the level of twice the theoretical prediction are set for higgsino masses between 350 and 450 GeV.

In several Supersymmetry scenarios, charginos can be metastable and could potentially be detected as a long-lived particle. CMS has presented an innovative search for generic long-lived charged particles by mapping their detection efficiency in function of the particle kinematics and energy loss in the tracking system. This study not only allows to set stringent limits for a variety of Supersymmetric models predicting chargino proper lifetime (c*tau) greater than 50cm, but also gives a powerful tool to the theory community to independently test new models foreseeing long lived charged particles.

In the quest to be as general as possible in the search for Supersymmetry, CMS has also presented new results where a large subset of the Supersymmetry parameters, such as the gluino and squark masses, are tested for their statistical compatibility with different experimental measurements. The outcome is a probability map in a 19-dimension space. Notable observations in this map are that models predicting gluino masses below 1.2 TeV and sbottom and stop masses below 700 GeV are strongly disfavoured.

… but no New Physics

Despite careful searches, the most heard phrase at Moriond was unquestionably: “No excess observed – consistent with the Standard Model”. Hope now lies with the next run of the LHC at 13 TeV. If you want to find out more about the possibilities of the LHC’s second run, check out the CERN Bulletin article: “Life is good at 13 TeV“.

In addition to the diverse LHC experiment results presented, Tevatron experiments, BICEP, RHIC and other experiments also reported their breaking news at Moriond. Visit the Moriond EW and Moriond QCD conference websites to find out more.

Katarina Anthony-Kittelsen

### LHC and Tevatron share first joint result

Thursday, March 20th, 2014

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

Scientists working on the world’s leading particle collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron and CERN’s Large Hadron Collider. These machines are the past and current holders of the record for most powerful particle collider on Earth.

Scientists from the four experiments involved—ATLAS, CDF, CMS and DZero—announced their joint findings on the mass of the top quark today at the Rencontres de Moriond international physics conference in Italy.

Together the four experiments pooled their data analysis power to arrive at a new world’s best value for the mass of the top quark of 173.34 ± 0.76 GeV/c2.

Experiments at the LHC at the CERN laboratory in Geneva, Switzerland and the Tevatron collider at Fermilab in Illinois, USA are the only ones that have ever seen top quarks—the heaviest elementary particles ever observed. The top quark’s huge mass (more than 100 times that of the proton) makes it one of the most important tools in the physicists’ quest to understand the nature of the universe.

The new precise value of the top-quark mass will allow scientists to test further the mathematical framework that describes the quantum connections between the top quark, the Higgs particle and the carrier of the electroweak force, the W boson. Theorists will explore how the new, more precise value will change predictions regarding the stability of the Higgs field and its effects on the evolution of the universe. It will also allow scientists to look for inconsistencies in the Standard Model of particle physics—searching for hints of new physics that will lead to a better understanding of the nature of the universe.

“The combining together of data from CERN and Fermilab to make a precision top quark mass result is a strong indication of its importance to understanding nature,” says Fermilab director Nigel Lockyer. “It’s a great example of the international collaboration in our field.”

Courtesy of: Fermilab and CERN

A total of more than six thousand scientists from more than 50 countries participate in the four experimental collaborations. The CDF and DZero experiments discovered the top quark in 1995, and the Tevatron produced about 300,000 top quark events during its 25-year lifetime, completed in 2011. Since it started collider physics operations in 2009, the LHC has produced close to 18 million events with top quarks, making it the world’s leading top quark factory.

“Collaborative competition is the name of the game,” says CERN’s Director General Rolf Heuer. “Competition between experimental collaborations and labs spurs us on, but collaboration such as this underpins the global particle physics endeavor and is essential in advancing our knowledge of the universe we live in.”

Each of the four collaborations previously released their individual top-quark mass measurements. Combining them together required close collaboration between the four experiments, understanding in detail each other’s techniques and uncertainties. Each experiment measured the top-quark mass using several different methods by analyzing different top quark decay channels, using sophisticated analysis techniques developed and improved over more than 20 years of top quark research beginning at the Tevatron and continuing at the LHC. The joint measurement has been submitted to the arXiv.

A version of this article was originally issued by Fermilab and CERN as a press release.

### Oh brave new world, which has such physicists in it!

Monday, February 10th, 2014

In August I moved away from CERN, and I’ve been back and forth between CERN and Brussels quite a lot since then. In fact right now I’m sitting in the building 40 where people go to drink coffee and have meetings, and I can see the ATLAS Higgs Convener sitting on the next table. All this leaves me feeling a little detached from what is really happening at CERN, as if it’s not “my” lab anymore, and that actually sums up how many people think about particle physics at the moment. With LHC Run I we found the Higgs boson. It was what most people expected to see, and by a large margin it was the most probable thing we would have discovered. Things will be different for Run II. Nobody has a good idea about what to expect in terms of new particles (and if they say they do have a good idea, they’re lying.) In that sense it’s not “our” dataset, it’s whatever nature decides it should be. All we can do is say what is possible, not what is probable. (Although we can probably say one scenario is more probable than another.)

The problem we now face is that there is no longer an obvious piece that’s missing, but there are still many unanswered questions, which means we have to move from an era of a well constrained search to an era of phenomenology, or looking for new effects in the data. That’s not a transition I’m entirely comfortable with for several reasons. It’s often said that nature is not spiteful, but it is subtle and indifferent to our expectations. There’s no reason to think that there “should” be new physics for us to discover as we increase the energy of the LHC, and we could be unlucky enough to not find anything new in the Run II dataset. A phenomenological search also means that we’d be overly sensitive to statistical bumps and dips in the data. Every time there’s a new peak that we don’t expect we have to exercise caution and skepticism, almost to the point where it stops being fun. Suppose we find an excess in a dijet spectrum. We may conclude that this is due a new particle, but if we’re going to be phenomenologists about it we must remain open minded, so we can’t necessarily expect to see the same particle in a dimuon final state. It would then be prudent to ask if such a peak comes from a poorly understood effect, such as jet energy scales, and those kinds of effects can be hard to untangle if we don’t have a good control sample in data. At least with the discovery of the Higgs boson, the top quark, and the W and Z bosons we knew what final states to expect and what ratios they should exhibit. There’s also something a little unsettling about not having a roadmap of what to expect. When asked to pick between several alternative scenarios that are neither favoured by evidence nor disfavoured by lack of evidence it’s hard to decide what to prioritise.

Take your pick of new physics! Each scenario will have new phase space to explore in LHC Run II [CMS]

On the other hand there is reason to be excited. Since we don’t know what to expect in LHC Run II, anything we do discover will change our views considerably, and will lead to a paradigm shift. If we do discover a new particle, or even better, a new sector of particles, it could help frame the Standard Model as a subset of something more elegant and unified. If that’s the case then we can look forward to decades of intense and exciting research, that would make the Higgs discovery look like small potatoes. So the next few years at the LHC could be either the most boring or the most exciting time in the history of particle physics, and we won’t know until we look at the data. Will nature tantalise us with hints of something novel, will it give us irrefutable evidence of a new resonance, or will it leave us with nothing new at all? For my part I’m taking on the dilepton final states. These are quick, clean, simple, and versatile signatures of something new that are not tied down to a specific model. That’s the best search I can perform in an environment of such uncertainty and with a lack of coherent direction. Let’s hope it pays off, and paves the way for even more discoveries.

What’s happening at 325GeV at CDF? Only more data can tell us. Based on what the LHC has seen, this is probably a statistical fluctuation. (CDF)

### Getting to the Bottom of the Higgs

Thursday, January 30th, 2014

Updated Friday, January 31, 2014: Candidate event of Higgs boson decaying to bottom quarks has been added at the bottom.

CMS has announced direct evidence of the Higgs coupling to bottom quarks. This is special.

Last week, the Compact Muon Solenoid Experiment, one of the two general purpose experiments at the CERN Large Hadron Collider (LHC), submitted two papers to the arXiv. The first claims the first evidence for the Higgs boson decaying directly to tau lepton pairs and the second summarizes the evidence for the Higgs boson decaying directly to bottom quarks and tau leptons. (As an aside: The summary paper is targeted for Nature Physics, so it is shorter and more broadly accessible than other ATLAS and CMS publications.) These results are special, and why they are important is the topic of today’s post. For more information about the evidence was obtained, CERN posted a nice QD post last month.

Fig 1. Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the ATLAS detector.

There is a litany of results from ATLAS and CMS regarding the measured properties of the Higgs boson. However, these previous observations rely on the Higgs decaying to photons, Z bosons, or W bosons, as well as the Higgs being produced from annihilating gluons or being radiated off a W or Z. Though the top quark does contribute to the Higgs-photon and Higgs-gluon interactions, none of these previous measurements directly probe how fermions (i.e., quarks and leptons) interact with the Higgs boson. Until now, suggestions that the Higgs boson couples to fermions (i) proportionally to their masses and (ii) that the couplings possess no other scaling factor were untested hypotheses. In fact, this second hypothesis remains untested.

Fig. 2: Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

As it stands, CMS claims “strong evidence for the direct coupling of the 125 GeV Higgs boson” to bottom quarks and tau leptons. ATLAS has comparable evidence but only for tau leptons. The CMS experiment’s statistical significance of the signal versus the “no Higgs-to-fermion couplings” hypothesis is 3.8 standard deviations, so no rigorous discovery yet (5 standard deviations is required). For ATLAS, it is 4.1 standard deviations. The collaborations still need to collect more data to satisfactorily validate such an incredible claim. However, this should not detract from that fact that we are witnessing phenomena never before seen in nature. This is new physics as far as I am concerned, and both ATLAS and CMS should be congratulated on discovering it.

Fig. 3: Event display of a candidate Higgs boson decaying into a bottom quark and anti-bottom quark in the ATLAS detector. HT to Jon Butterworth for the link.

## The Next Step

Once enough data has been collected to firmly and undoubtedly demonstrate that quarks and leptons directly interact with the Higgs, the real tests of the Standard Model of particle physics start up. In the Standard Model, the strength at which a fermion interacts with the Higgs is proportional to the fermion mass and inversely proportional to the ground state energy of the Higgs field. There is no other factor involved. This is definitively not the case for a plethora of new physics models, including scenarios with multiple Higgs bosons, like supersymmetry, as well as scenarios with new, heavy fermions (heavy bottom quark and tau lepton partners). This is definitely a case of using newly discovered physics to find more new physics.

Happy Colliding.

- Richard (@bravelittlemuon)

PS I was unable to find an event display of a Higgs boson candidate decaying into a pair of bottom quarks. If anyone knows where I can find one, I would be very grateful.

PSS Much gratitude toward Jon Butterworth for providing a link to Higgs-bbar candidate events.

### One giant leap for the Higgs boson

Friday, December 6th, 2013

Both the ATLAS and CMS collaborations at CERN have now shown solid evidence that the new particle discovered in July 2012 behaves even more like the Higgs boson, by establishing that it also decays into particles known as tau leptons, a very heavy version of electrons.

Why is this so important? CMS and ATLAS had already established that the new boson was indeed one type of a Higgs boson. In that case, theory predicted it should decay into several types of particles. So far, decays into W and Z bosons as well as photons were well established. Now, for the first time, both experiments have evidence that it also decays into tau leptons.

The decay of a particle is very much like making change for a coin. If the Higgs boson were a one euro coin, there would be several ways to break it up into smaller coins, but, so far, the change machine seemed to only make change in some particular ways. Now, additional evidence for one more way has been shown.

There are two classes of fundamental particles, called fermions and bosons depending on their spin, their value of angular momentum. Particles of matter (like taus, electrons and quarks) belong to the fermion family. On the other hand, the particles associated with the various forces acting upon these fermions are bosons (like the photons and the W and Z bosons.).

The CMS experiment had already shown evidence for Higgs boson decays into fermions last summer with a signal of 3.4 sigma when combining the tau and b quark channels. A sigma corresponds to one standard deviation, the size of potential statistical fluctuations.  Three sigma is needed to claim evidence while five sigma is usually required for a discovery.

For the first time, there is now solid evidence from a single channel – and two experiments have independently produced it. ATLAS collaboration showed evidence for the tau channel alone with a signal of 4.1 sigma, while CMS obtained 3.4 sigma, both bringing strong evidence that this particular type of decays occurs.

Combining their most recent results for taus and b quarks, CMS now has evidence for decays into fermions at the 4.0 sigma level.

The data collected by the ATLAS experiment (black dots) are consistent with coming from the sum of all backgrounds (colour histograms) plus contributions from a Higgs boson going into a pair of tau leptons (red curve). Below, the background is subtracted from the data to reveal the most likely mass of the Higgs boson, namely 125 GeV (red curve).

CMS is also starting to see decays into pairs of b quarks at the 2.0 sigma-level. While this is still not very significant, it is the first indication for this decay so far at the LHC. The Tevatron experiments have reported seeing it at the 2.8 sigma-level. Although the Higgs boson decays into b quarks about 60% of the time, it comes with so much background that it makes it extremely difficult to measure this particular decay at the LHC.

Not only did the experiments report evidence that the Higgs boson decays into tau leptons, but they also measured how often this occurs. The Standard Model, the theory that describes just about everything observed so far in particle physics, states that a Higgs boson should decay into a pair of tau leptons about 8% of the time. CMS measured a value corresponding to 0.87 ± 0.29 times this rate, i.e. a value compatible with 1.0 as expected for the Standard Model Higgs boson. ATLAS obtained 1.4 +0.5 -0.4, which is also consistent within errors with the predicted value of 1.0.

One of the events collected by the CMS collaboration having the characteristics expected from the decay of the Standard Model Higgs boson to a pair of tau leptons. One of the taus decays to a muon (red line) and neutrinos (not visible in the detector), while the other tau decays into a charged hadron (blue towers) and a neutrino. There are also two forward-going particle jets (green towers).

With these new results, the experiments established one more property that was expected for the Standard Model Higgs boson. What remains to be clarified is the exact type of Higgs boson we are dealing with. Is this indeed the simplest one associated with the Standard Model? Or have we uncovered another type of Higgs boson, the lightest one of the five types of Higgs bosons predicted by another theory called supersymmetry.

It is still too early to dismiss the second hypothesis. While the Higgs boson is behaving so far exactly like what is expected for the Standard Model Higgs boson, the measurements lack the precision needed to rule out that it cannot be a supersymmetric type of Higgs boson. Getting a definite answer on this will require more data. This will happen once the Large Hadron Collider (LHC) resumes operation at nearly twice the current energy in 2015 after the current shutdown needed for maintenance and upgrade.

Meanwhile, these new results will be refined and finalised. But already they represent one small step for the experiments, a giant leap for the Higgs boson.

For all the details, see:

Presentation given by the ATLAS Collaboration on 28 November 2013

Presentation given by the CMS Collaboration on 3 December 2013

Pauline Gagnon

### Un pas de géant pour le boson de Higgs

Friday, December 6th, 2013

Les collaborations ATLAS et CMS du CERN ont maintenant l’évidence que la nouvelle particule découverte en juillet 2012 se comporte de plus en plus comme le boson de Higgs. Les deux expériences viennent en fait de démontrer que le boson de Higgs se désintègre aussi en particules tau, des particules semblables aux électrons mais beaucoup plus lourdes.

Pourquoi est-ce si important? CMS et l’ATLAS avaient déjà établi que ce nouveau boson était bien un type de boson de Higgs. Si tel est le cas, la théorie prévoit qu’il doit se désintégrer en plusieurs types de particules. Jusqu’ici, seules les désintégrations en bosons W et Z de même qu’en photons étaient confirmées. Pour la première fois, les deux expériences ont maintenant la preuve qu’il se désintègre aussi en particules tau.

La désintégration d’une particule s’apparente beaucoup à faire de la monnaie pour une pièce. Si le boson de Higgs était une pièce d’un euro, il pourrait se briser en différentes pièces de monnaie plus petites. Jusqu’à présent, le distributeur de monnaie semblait seulement donner la monnaie en quelques façons particulières. On a maintenant démontré qu‘il existe une façon supplémentaire.

Il y a deux classes de particules fondamentales, appelées fermions et bosons selon la valeur de quantité de mouvement angulaire. Les particules de matière comme les taus, les électrons et les quarks appartiennent tous à la famille des fermions. Par contre, les particules associées aux diverses forces qui agissent sur ces fermions sont des bosons, comme les photons et les bosons W et Z.

L”été dernier, l’expérience CMS avait déjà apporté la preuve avec un signal de 3.4 sigma que le boson de Higgs se désintégrait en fermions en combinant leurs résultats pour deux types de fermions, les taus et les quarks b. Un sigma correspond à un écart-type, la taille des fluctuations statistiques potentielles. Trois sigma sont nécessaires pour revendiquer une évidence tandis que cinq sigma sont nécessaires pour clamer une découverte.

Pour la première fois, il y a maintenant évidence pour un nouveau canal de désintégration (les taus) – et deux expériences l’ont produit indépendamment. La collaboration ATLAS a montré la preuve pour le canal des taus avec un signal de 4.1 sigma, tandis que CMS a obtenu 3.4 sigma, deux résultats forts prouvant que ce type de désintégrations se produit effectivement.

En combinant leurs résultats les plus récents pour les taus et les quarks b, CMS a maintenant une évidence pour des désintégrations en fermions avec 4.0 sigma.

Les données rassemblées par l’expérience ATLAS (les points noirs) sont en accord avec la somme de tous les évènements venant du bruit de fond (histogrammes en couleur) en plus des contributions venant d’un boson de Higgs se désintégrant en une paire de taus (la ligne rouge). En dessous, le bruit de fond est soustrait des données pour révéler la masse la plus probable du boson de Higgs, à savoir 125 GeV (la courbe rouge).

CMS commence aussi à voir des désintégrations en paires de quarks b avec un signal de 2.0 sigma. Bien que ceci ne soit toujours pas très significatif, c’est la première indication pour cette désintégration jusqu’ici au Grand collisionneur de hadrons (LHC). Les expériences du Tevatron avaient rapporté l’observation de telles désintégrations à 2.8 sigma. Bien que le boson de Higgs se désintègre en quarks b environ 60 % du temps, il y a tant de bruit de fond qu’il est extrêmement difficile de mesurer ces désintégrations au LHC.

Non seulement les expériences ont la preuve que le boson de Higgs se désintègre en paires de taus, mais elles mesurent aussi combien de fois ceci arrive. Le Modèle Standard, la théorie qui décrit à peu près tout ce qui a été observé jusqu’à maintenant en physique des particules, stipule qu’un boson de Higgs devrait se désintégrer en une paire de taus environ 8 % du temps. CMS a mesuré une valeur correspondant à 0.87 ± 0.29 fois ce taux, c’est-à-dire une valeur compatible avec 1.0 comme prévu pour le boson de Higgs du Modèle Standard. ATLAS obtient 1.4 +0.5-0.4, ce qui est aussi consistent avec la valeur de 1.0 à l‘intérieur des marges d’erreur.

Un des événements captés par la collaboration CMS ayant les caractéristiques attendues pour les désintégrations du boson de Higgs du Modèle Standard en une paire de taus. Un des taus se désintègre en un muon (ligne rouge) et en neutrinos (non visibles dans le détecteur), tandis que l’autre tau se désintègre en  hadrons (particules composées de quarks) (tours bleues) et un neutrino. Il y a aussi deux jets de particules vers l’avant (tours vertes).

Avec ces nouveaux résultats, les expériences ont établi une propriété de plus prédite pour le boson de Higgs du Modèle Standard. Reste encore à clarifier le type exact de boson de Higgs que nous avons. Est-ce bien le plus simple des bosons, celui associé au Modèle Standard? Ou avons nous découvert un autre type de boson de Higgs, le plus léger des cinq bosons de Higgs prévus par une autre théorie appelée la supersymétrie.

Il est encore trop tôt pour écarter cette deuxième hypothèse. Tandis que le boson de Higgs se comporte jusqu’ici exactement comme ce à quoi on s’attend pour le boson de Higgs du Modèle Standard, les mesures manquent encore de précision pour exclure qu’il soit de type supersymétrique. Une réponse définitive exige plus de données. Ceci arrivera une fois que le LHC reprendra du service à presque deux fois l’énergie actuelle en 2015 après l’arrêt actuel pour maintenance et consolidation.

En attendant, ces nouveaux résultats seront affinés et finalisés. Déjà ils représentent un petit pas pour les expériences et un bond de géant pour le boson de Higgs.

Pour tous les détails (en anglais seulement)

Présentation donnée par la collaboration ATLAS le 28 novembre 2013

Présentation donnée par la collaboration CMS le 3 décembre 2013

Pauline Gagnon

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### Higgs Convert

Friday, November 29th, 2013

Since 4th July 2012, the physicists at CERN have had a new boson to play with. This new boson was first seen in the searches that were optimised to find the world famous Higgs boson, and the experiments went as far as to call it a “Higgs-like” boson. Since then there has been an intense program to study its spin, width, decay modes and couplings and so far it’s passed every test of Higgs-ness. Whether or not the new boson is the Standard Model Higgs boson is one of the most pressing questions facing us today, as there is still room for anomalous couplings. Whatever the answer is, a lot of physicists will be pleased. If we find that the properties match those of a Standard Model Higgs boson exactly then we will hail it as a triumph of science and a fitting end to the quest for the Standard Model which has taken the work of thousands of physicists over many decades. If we find some anomaly in the couplings this would be a hint to new physics hiding “just around the corner” and tease is with what we may see at higher energies when the LHC turns on again in 2015.

A candidate for a Higgs boson decaying to two tau leptons (ATLAS)

For those who have read my blog for a long time, you may remember that I wrote a post saying how I was skeptical that we would find the Standard Model Higgs boson. In fact I even bet a friend $20 that we wouldn’t find the Standard Model Higgs boson by 2020, and until today I’ve been holding on to my money. This week I found that ATLAS announced the results of their search for the Higgs boson decaying to two tau leptons, and the results agree with predictions. When we take this result alongside the decays to bosons, and the spin measurements it’s seems obvious that this is the Higgs boson that we were looking for. It’s not fermiophobic, and now we have direct evidence of this. We have see the ratio of the direct ferimonic couplings to direct bosonic couplings, and they agree very well. We’d had indirect evidence of fermionic couplings from the gluon fusion production, but it’s always reassuring to see the direct decays as well. (As a side note I’d like to point out that the study of the Higgs boson decaying to two tau leptons has been the result of a huge amount of very hard work. This is one of the most difficult channels to study, requiring a huge amount of knowledge and a wide variety of final states.) Now the reason for my skepticism was not because I thought the Standard Model was wrong. In fact the Standard Model is annoyingly accurate in its predictions, making unexpected discoveries very difficult. What I objected to was the hyperbole that people were throwing around despite the sheer lack of evidence. If we’re going to be scientists we need to rely on the data to tell us what is real about the universe and not what some particular model says. If we consider an argument of naturalness (by which I mean how few new free terms we need to add to the existing edifice of data) then the Higgs boson is the best candidate for a new discovery. However that’s only an argument about plausibility and does not count as evidence in favour of the Higgs boson. Some people would say things like “We need a Higgs boson because we need a Brout-Englert-Higgs mechanism to break the electroweak symmetry.” It’s true that this symmetry needs to be broken, but if there’s no Higgs boson then this is not a problem with nature, it’s a problem with our models! The fact that we’ve seen the Higgs boson actually makes me sad to a certain extent. The most natural and likely prediction has been fulfilled, and this has been a wonderful accomplishment, but it is possible that this will be the LHC’s only new discovery. As we move into LHC Run II will we see something new? Nobody knows, of course, but I would not be surprised if we just see more of the Standard Model. At least this time we’ll probably be more cautious about what we say in the absence of evidence. If someone says “Of course we’ll see strong evidence of supersymmetry in the LHC Run II dataset.” then I’ll bet them$20 we won’t, and this time I’ll probably collect some winnings!

### Prix Nobel de physique 2013 : “le succès de toute une communauté”

Friday, October 11th, 2013

Jacques Martino, Directeur de l’Institut national de physique nucléaire et des particules du CNRS, adresse ses félicitations à François Englert et Peter Higgs pour le Prix Nobel de physique 2013, et rappelle la contribution en France du CNRS à la découverte du fameux boson.

Enthousiasme général des physiciens et ingénieurs des expériences Atlas et CMS lors de l’annonce du Prix Nobel de Physique 2013. © CERN

« Au nom du CNRS, je veux féliciter François Englert et Peter Higgs pour l’intuition extraordinaire dont ils ont fait preuve il y a presque 50 ans, en “inventant” le “boson de Higgs”. Le boson de Higgs a été théorisé dans les années 1960, notamment pour expliquer pourquoi certaines particules ont une masse alors que d’autres n’en ont pas. Il est alors devenu un véritable Graal pour nos physiciens. Il est en effet la clé de voûte du Modèle standard de la physique des particules, un ensemble théorique cohérent permettant de décrire le monde des particules subatomiques. Sans nul doute, la découverte d’un boson de Higgs vient donc de manière éclatante conforter ce modèle standard !

Il est indéniable que cette prédiction a animé des milliers de chercheurs durant toutes ces années, et je veux saluer aussi le travail titanesque accompli par les chercheurs,  ingénieurs et techniciens qui ont construit le LHC au CERN ainsi que les détecteurs Atlas et CMS. Ce prix Nobel célébré aujourd’hui, il nous appartient un peu aussi, car nos chercheurs français ont participé de manière très importante à cette grande quête collective qu’a été la traque du boson de Higgs.

Il aura fallu relever des défis technologiques colossaux qu’il s’agisse de l’accélérateur, des détecteurs ou bien encore des infrastructures de calcul permettant de traiter l’énorme quantité de données produites. Car rechercher le boson de Higgs revient véritablement à chercher une aiguille dans une botte de foin !

Plusieurs centaines de personnes du CNRS ont apporté leur pierre à la construction des  expériences du LHC et joué un rôle décisif dans l’exploitation scientifique des données. L’action déterminante du CNRS dans ce domaine serait sans aucun doute impossible sans l’expertise reconnue de l’IN2P3 qui fédère l’ensemble de ces activités et qui participe ainsi avec force au rayonnement national et international du CNRS. Ces recherches rappellent aussi de manière remarquable combien la collaboration internationale peut être porteuse de réussite.

Cette découverte majeure est le premier succès du LHC et vient ainsi couronner le succès de toute une communauté. Pour toute cette communauté, aujourd’hui est un jour de fête. Et pour le CNRS, cette découverte récompense 20 années d’investissements technologiques et humains dans lesquels une douzaine de laboratoires de CNRS, ont joué un rôle majeur aux côtés du CERN, ainsi que 200 chercheurs français.

La vie du LHC ne fait que commencer et cette réussite est certainement porteuse d’un avenir riche de nouvelles découvertes qui mobiliseront nos équipes dans les années qui viennent. Le Higgs a encore bien des secrets à nous livrer, nous l’avons pour l’instant seulement “aperçu”, et il convient de préciser sa nature et ses caractéristiques. Il s’agit là d’un énorme chantier à venir. Mais le programme de recherche du LHC dépasse largement ce cadre !  Le Modèle standard de la physique des particules s’il se voit conforté, laisse de nombreuses questions en suspens. Matière noire, supersymétrie… La recherche d’une nouvelle physique au-delà du Modèle standard va ainsi se poursuivre dans les années pour repousser toujours les frontières de notre compréhension de la matière et de l’Univers. »

À voir également :

- Jacques Martino réagit à l’annonce du Prix Nobel de Physique 2013

- Comment chasse-t-on le boson ?

- et pour tout savoir sur le LHC et le boson de Higgs (actus, BDs, vidéos): http://lhc-france.fr/higgs

### And the 2013 Nobel Prize in Physics goes to…

Tuesday, October 8th, 2013

Today the 2013 Nobel Prize in Physics was awarded to François Englert (Université Libre de Bruxelles, Belgium) and Peter W. Higgs (University of Edinburgh, UK). The official citation is “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.” What did they do almost 50 years ago that warranted their Nobel Prize today? Let’s see (for the simple analogy see my previous post from yesterday).

The overriding principle of building a theory of elementary particle interactions is symmetry. A theory must be invariant under a set of space-time symmetries (such as rotations, boosts), as well as under a set of “internal” symmetries, the ones that are specified by the model builder. This set of symmetries restrict how particles interact and also puts constraints on the properties of those particles. In particular, the symmetries of the Standard Model of particle physics require that W and Z bosons (particles that mediate weak interactions) must be massless. Since we know they must be massive, a new mechanism that generates those masses (i.e. breaks the symmetry) must be put in place. Note that a theory with massive W’s and Z that are “put in theory by hand” is not consistent (renormalizable).

The appropriate mechanism was known in the beginning of the 1960′s. It goes under the name of spontaneous symmetry breaking. In one variant it involves a spin-zero field whose self-interactions are governed by a “Mexican hat”-shaped potential

It is postulated that the theory ends up in vacuum state that “breaks” the original symmetries of the model (like the valley in the picture above). One problem with this idea was that a theorem by G. Goldstone required a presence of a massless spin-zero particle, which was not experimentally observed. It was Robert Brout, François Englert, Peter Higgs, and somewhat later (but independently), by Gerry Guralnik, C. R. Hagen, Tom Kibble who showed a loophole in a version of Goldstone theorem when it is applied to relativistic gauge theories. In the proposed mechanism massless spin-zero particle does not show up, but gets “eaten” by the massless vector bosons giving them a mass. Precisely as needed for the electroweak bosons W and Z to get their masses!  A massive particle, the Higgs boson, is a consequence of this (BEH or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism and represents excitation of the Higgs field about its new vacuum state.

It took about 50 years to experimentally confirm the idea by finding the Higgs boson! Tracking the historic timeline, the first paper by Englert and Brout, was sent to Physical Review Letter on 26 June 1964 and published in the issue dated 31 August 1964. Higgs’ paper, received by Physical Review Letters on 31 August 1964 (on the same day Englert and Brout’s paper was published)  and published in the issue dated 19 October 1964. What is interesting is that the original version of the paper by Higgs, submitted to the journal Physics Letters, was rejected (on the grounds that it did not warrant rapid publication). Higgs revised the paper and resubmitted it to Physical Review Letters, where it was published after another revision in which he actually pointed out the possibility of the spin-zero particle — the one that now carries his name. CERN’s announcement of Higgs boson discovery came 4 July 2012.

Is this the last Nobel Prize for particle physics? I think not. There are still many unanswered questions — and the answers would warrant Nobel Prizes. Theory of strong interactions (which ARE responsible for masses of all luminous matter in the Universe) is not yet solved analytically, the nature of dark matter is not known, the picture of how the Universe came to have baryon asymmetry is not cleared. Is there new physics beyond what we already know? And if yes, what is it? These are very interesting questions that need answers.

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

Wednesday, May 29th, 2013

À 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.

Dans les cavernes des géants
Mercredi 07 novembre 2012

La matinée est animée par un physicien autrichien guide alpin hyperactif dont je n’ai pas saisi le nom mais que je devrais pouvoir retrouver avant la fin du séjour dans un lieu où même le boson de Higgs est détectable (edit : Michael Hoch en fait). Il nous amène voir les sites où se trouvent deux gigantesques détecteurs de particules, CMS et ATLAS, placés à l’endroit où les faisceaux de protons du LHC se rencontrent.

Avant cela, rapide visite dans un site où un bout du LHC est exposé. On y voit les deux conduits dans lesquels les faisceaux de protons circulent quasiment à la vitesse de la lumière, et dans des sens opposés.

Quatre fois sur les 27 km, ces 2 tuyaux se croisent pour causer les collisions qui sont analysées par CMS et ATLAS (mais aussi LHCb et ALICE). Le module sur lequel je m’appuie sur la photo comporte aussi des électroaimants supraconducteurs refroidis à -271°C par de l’hélium liquide. Les aimants servent plus ou moins à diriger et comprimer le faisceau, son accélération se faisant en d’autres points à l’aide de champ électrique haute fréquence. Mais tout ça ne peut-être vu en fonctionnement car cela se situe à 100 m sous terre et de plus, les radiations émises pourraient nuire à mon cuir chevelu.

À CMS, c’est le physicien Jean Fay qui nous fait visiter les locaux avec grandes compétence et gentillesse. Bien que l’on ne puisse pas approcher le détecteur (mais l’affiche de la photo donne une idée de sa taille), une salle de contrôle de la bestiole nous est accessible.

Le système d’exploitation est linux car les pannes windows sont à proscrire… C’est le monsieur qui me l’a dit. Je résume sa pensée : « Vindoze, c’est bon pour les présentations poveurpoïnt, et encore… »

Attends, je dois vérifier un truc… non, c’est bon en fait !

Vite, il nous faut retourner vers ATLAS. Il se situe en fait vers le CERN, alors que CMS est diamétralement opposé, et en France si j’ai bien tout compris.

C’est un physicien retraité à l’esprit vif comme un neutrino qui nous guide : Klaus Bätzner. Le site ATLAS est plus orienté vers le public car il est proche du CERN et sans doute plus accessible. Une salle de projection 3D est mise à notre disposition. Équipés de lunettes et d’un casque, la vidéo qu’on nous présente est impressionnante.

La salle de contrôle est pleine de grands écrans, de petits écrans, de claviers, et de gens qui regardent des écrans tout en pianotant sur les claviers. Ils sont comme dans un aquarium et on peut les observer sans trop interférer avec leur comportement.

Après le déjeuner avalé en vitesse, direction la salle du conseil pour écouter l’excellent Fabrice Piquemal du CNRS nous parler des neutrinos. Ça tombe bien, les détecteurs précédents ne font qu’extrapoler la présence de neutrinos lors d’une collision, par calcul de l’énergie manquante. Les neutrinos ont la fâcheuse tendance à traverser la matière comme qui rigole, et ne vont pas plus vite que la lumière contrairement à une idée faussement répandue.

Le soir, nous nous retrouvons à Genève après avoir sagement suivi la ligne 14. Le dîner se déroule dans un restaurant où des musiciens jouent avec tout ce qui leur passe sous la main : scie, cuillère, cloche, parfois même des instruments de musique à condition qu’ils fassent plus de 3 mètres. Exténué, retour vers 23 h au CERN.

À 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.