• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • USLHC
  • USLHC
  • USA

  • James
  • Doherty
  • Open University
  • United Kingdom

Latest Posts

  • Andrea
  • Signori
  • Nikhef
  • Netherlands

Latest Posts

  • CERN
  • Geneva
  • Switzerland

Latest Posts

  • Aidan
  • Randle-Conde
  • Université Libre de Bruxelles
  • Belgium

Latest Posts

  • TRIUMF
  • Vancouver, BC
  • Canada

Latest Posts

  • Laura
  • Gladstone
  • MIT
  • USA

Latest Posts

  • Steven
  • Goldfarb
  • University of Michigan

Latest Posts

  • Fermilab
  • Batavia, IL
  • USA

Latest Posts

  • Seth
  • Zenz
  • Imperial College London
  • UK

Latest Posts

  • Nhan
  • Tran
  • Fermilab
  • USA

Latest Posts

  • Alex
  • Millar
  • University of Melbourne
  • Australia

Latest Posts

  • Ken
  • Bloom
  • USLHC
  • USA

Latest Posts

Posts Tagged ‘Higgs’

Wrapping it up on the Higgs boson

Friday, December 14th, 2012

As the Large Hadron Collider (LHC) is preparing to shut down for the end of the year holidays, the LHC experiments presented on Thursday morning a summary of the last three years of operation. For CMS and ATLAS, the highlight was of course the discovery of what looks more and more like the Higgs boson.

The certainty for the presence of a new boson has been reinforced. As Sara Bolognesi, speaking on behalf of the CMS collaboration, put it: “The signal is so strong, the probability of having it wrong is as low as the chance of flipping a coin 40 times and getting 40 heads in a row”. Marumi Kado, representing ATLAS, showed that even when using a single decay channel, the signal is strong enough to claim a discovery. Hence, the focus is now on finding the exact properties of this new boson to reveal its identity.

ATLAS showed their first results on the spin and parity of the new boson. The parity seems positive, as expected for the Standard Model Higgs boson, reaching the same observation as CMS. But the jury is still out on the value of its spin although the results are more compatible with 0, the value expected by the Standard Model, but a value of 2 is still possible. A clearer answer might come once the 23 inverse femtobarns of data delivered this year by the LHC will have been processed and combined for the two experiments.

What’s new on the more-and-more-Higgs-like new boson? CMS showed the first results on a Higgs boson decaying into a Z boson and a photon. This decay channel should be very small unless there are contributions from processes predicted by theories going beyond the Standard Model, and these could be huge. Nothing is seen so far but this is a promising avenue.

A few facts are nevertheless puzzling. For example, ATLAS measures two different masses when the Higgs decays to two photons as opposed to four leptons, the two decay channels giving the best precision on the mass measurement.

Each one of these decay channels represents one way the Higgs boson can break apart. It is very much like making change for one dollar. No matter if you give the change with coins of ten, twenty or fifty cents, the total sum should always add up to one dollar. As it stands, it is as if ATLAS obtains $1.05 and $0.95 when adding up all the coins, despite having checked each channel with extreme scrutiny for a possible mistake.

This is most likely due to a statistical fluctuation since the data gives only one mass value in the global combination but it might take more data than is at hand to resolve this apparent discrepancy. CMS obtains similar masses in both channels but the results need to be updated with more data for the two-photon channel.

Another slightly intriguing fact: both experiments measure more Higgs boson decays into two photons than what is predicted by the Standard Model. I summarized the situation in the table below.

The error margins are still fairly large which means more data will be needed to sort it all out. The LHC will undergo a major upgrade starting in March 2013, to restart at higher luminosity and higher energy beginning of 2015.  It takes a lot of patience to do high energy physics!

Pauline Gagnon

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

 

 

Share

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!

Share

Depuis la découverte en juillet dernier de ce qui pourrait être le boson de Higgs, les physiciennes et physiciens des expériences CMS et ATLAS essaient de trouver sa véritable identité. Est-ce vraiment le boson de Higgs prédit par le  modèle standard de la physique des particules ou une autre type de boson de Higgs relié à une théorie différente ?

Pour en avoir le cœur net, nous devons vérifier toutes ses propriétés, par exemple comment et dans quelles proportions il se désintègre. On doit aussi établir son spin et sa parité, deux propriétés des particules fondamentales.

Le nouveau boson a une courte durée de vie, il se désintègre tout de suite après avoir été créé. Il peut alors se briser de cinq façons différentes observables au Grand Collisionneur de Hadrons (LHC): en produisant deux photons, deux bosons Z ou W, deux quarks b ou encore deux leptons taus (une particule semblable à l’électron mais 3500 fois plus lourd). Il nous faut établir si chaque mode de désintégration existe et s’il se produit au taux prévu.

L’été dernier, lors de l’annonce de sa découverte, les deux expériences n’avaient des résultats clairs que pour les trois premiers modes. L’échantillon de données était alors trop petit pour voir des désintégrations en une paire de quarks b ou de taus.

Avec maintenant plus de données, les deux expériences ont pu montrer des résultats dans tous les canaux à une conférence aujourd’hui à Kyoto comme on peut le voir sur les graphes ci-dessous. La figure de gauche montre les résultats de CMS et celle de droite, ceux d’ATLAS.

 

 

 

 

 

 

 

 

 

Les valeurs de “σ ⁄ σSM” et “μ” sont équivalentes et représentent le rapport entre ce qui est observé et ce que le modèle standard prévoit. Une valeur de 1 signifie que tout concorde avec la théorie, et zéro implique que ce canal de désintégration n’est pas observé. Toute autre valeur implique que l’on voit ce canal mais qu’il se produit à un taux différent de celui auquel on s’attendait. Il faut bien sûr tenir compte des marges d’erreur avant de tirer une quelconque conclusion.

Les deux expériences ont maintenant des résultats pour les canaux de désintégrations en paires de quarks b ou en taus et les marges d’erreurs sont réduites pour plusieurs canaux. Pour l’instant, CMS obtient une valeur combinée de 0.88 ± 0.21 tandis qu’ATLAS mesure 1.3 ± 0.3. Les deux mesures sont donc compatibles avec 1.

La présence de ces cinq canaux serait compatible avec un boson de spin 0. Si en plus les taux de désintégration correspondent, ce nouveau boson aurait de plus en plus l’air du boson de Higgs mais ce ne serait toujours pas suffisant. Il faudra aussi qu’il soit de parité positive, comme le prédit le modèle standard.

Le spin d’une particule fondamentale réfère à sa rotation sur elle-même. La parité est reliée à ce qui arrive quand on inverse une direction dans l’espace. Voit-on la même chose lorsqu’on l’observe directement ou à travers un miroir quand la droite et la gauche sont inversées? Les particules possédant une parité positive agissent de la même façon qu’on les regarde directement ou dans un miroir.

On peut déterminer la parité d’une particule en observant la direction prise par ses débris quand elle se désintègre. Dépendamment de sa parité, ils s’éloigneront de préférence dans une direction plutôt qu’une autre. Par exemple, CMS a mesuré les angles entre les quatre électrons ou muons produits quand un boson se désintègre d’abord en deux bosons Z, eux-mêmes donnant une paire d’électrons ou de muons. Puis ils-elles ont comparé les distributions avec deux standards : l’un établi pour une parité négative, l’autre positive comme on le voit sur la figure ci-dessous.

La courbe de gauche en bleu montre la probabilité que l’on mesurerait pour un point en particulier pour une particule de parité positive alors que celle de droite en rose donne cette probabilité pour une parité positive. La valeur mesurée par CMS, indiquée par la flèche verte, indique clairement que le nouveau boson a plus certainement une parité positive telle que prescrite par le modèle standard.

CMS a aussi commencé à chercher d’autres bosons au-delà de la limite de 600 GeV exclue jusqu’à maintenant. Si de nouveaux bosons apparaissent, cela pourrait signifier que celui qui a été trouvé n’est qu’un des cinq bosons prévus par la supersymmétrie, une autre théorie, et non pas l’unique boson de Higgs du modèle standard.

Alors, où en est-on? Avec plus du double de données utilisées en juillet, les scientifiques sont passé-e-s de la quête d’un boson élusif aux premières mesures de ses propriétés. Lorsqu’on aura établi sans équivoque tous les canaux de désintégration, leur taux, le spin et la parité de cette particule, on en saura plus sur son identité.

Pour l’instant, bien qu’il soit encore trop tôt pour se prononcer, ce boson semble avoir de plus en plus l’air et la chanson du boson de Higgs. On en saura encore un peu plus en mars prochain quand toutes les données auront été analysées et améliorées. Mais cela pourra prendre du temps avant qu’il ait dit son dernier mot.

Pauline Gagnon

Pour être averti-e 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

 

 

Share

The mystery remains on the Higgs boson

Thursday, November 15th, 2012

Ever since the discovery of what might be the Higgs boson last July, physicists from the CMS and ATLAS experiments have been trying to pinpoint its true identity. Is this the Higgs boson expected by the Standard Model of particle physics or some “Higgs-like boson” befitting a different theoretical model?

To tell the difference, we must check all its properties, like how often this boson decays into different types of particles, and determine its spin and parity, two properties of fundamental particles.

Since the new boson has a short lifetime, it breaks apart immediately after being created. There are five ways a Standard Model Higgs boson should decay that we can study at the Large Hadron Collider (LHC): breaking into two photons, two W or two Z bosons, two b quarks or two tau leptons in well defined proportions.  We must check both the presence of and the rate at which each decay mode occurs.

Last summer, just after the discovery of the new boson, both experiments reported unambiguous observations in only three channels. Unfortunately, the data sample was still too small to really be able to check if the new boson could decay into a pair of b quarks or tau leptons.

With more data available, the two experiments have just shown results for all channels today at a conference held in Kyoto as shown on the two figures below.

 

 

 

 

 

 

 

 

The left figure is for CMS and the right one for ATLAS. The values “σ/σSM” and “μ” are equivalent and represent the ratio of what is seen to what is expected from the Standard Model. So if μ is exactly one for a given channel, it means that channel decays at the rate expected from the theory. A value of zero would imply this particular decay channel is not seen at all, contrary to expectation. If μ has any other value, it implies the new boson does not behave quite as predicted. But one must take into account the error margin (the horizontal bar) before drawing any conclusion.

Both experiments now measured decays into two b quarks and two tau leptons and the errors have gone down for several channels. For now, CMS obtains a combined value of 0.88 ± 0.21whereas ATLAS mesures 1.3 ± 0.3. Both are compatible with 1.

The confirmed presence of all five modes would be compatible with a spin-zero particle. Having in addition all the correct decay rates would make the new boson look much more like a Higgs boson but it would still not be quite sufficient. The new boson must also have positive parity as the Standard Model predicts.

The spin of a fundamental particle refers to its rotation on itself, as the name suggests. Parity has to do with flipping direction in space, exactly like what happens when we watch an event directly or through a mirror where the left and right directions are inverted. Particles with a positive parity look the same when you observe them directly or through a mirror.

The parity can be determined by looking at the direction taken by all fragments after the boson decays. Depending on its parity, its debris will fly in a preferred direction. For example, CMS measured all angles between the four electrons or muons when the “Higgs-like” bosons decay into two Z bosons, each one ending in a pair of electrons or muons. Then they compared the distributions with two standards: one for positive, one for negative parity as shown on the figure below.

The left curve in blue shows the probability one would measure for a particular point on the horizontal axis if the new boson had a negative parity. The right curve in pink shows the same for a particle with positive parity. The value measured by CMS (green arrow) indicates the new boson most likely has a positive parity as expected by the Standard Model.

CMS also started looking for other bosons with masses beyond 600 GeV, the current excluded limit. If new bosons turn up, it could mean we have found one of the five Higgs bosons predicted by supersymmetry, a new theoretical model, and not the single Higgs boson predicted by the Standard Model.

So where do we stand? With more than twice as much data as shown in July, scientists have moved from searching for this elusive particle to starting to measure its properties. Once the decay channels, decay rates, spin and parity are clearly established, we will be able to determine its identity.

It is still too early to tell but the new boson looks like, sings like and dances more and more like a Higgs boson. More certainty will come out next March at a winter conference with still more data and improved analyses. But it will take a long time to figure out beyond any doubt if the discovered boson was really the Standard Model Higgs boson.

Pauline Gagnon

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

 

 

Share

Are the Higgs Rumors True?

Monday, October 22nd, 2012

What Higgs rumors, you may ask? Well, there aren’t any that I know of, yet. But there might be soon…

There might be rumors soon because we are about to do another round of updates, for the 2012 Hadron Collider Physics Symposium (HCP). There aren’t any yet because our results (at least on CMS) are still “blinded,” which means that we haven’t actually looked at the “places” in the data where we see signs of our new boson. What we’re doing instead is looking at simulated data to see how much our results might improve when we add in the collisions we’ve recorded since ICHEP. We’re also putting in a few new analysis techniques, and checking them in the same way. And of course we are looking at data in other “places,” and we’re comparing it to simulations to make sure they’re doing a good job.

There will be several weeks between the moment we “unblind” — that is, look for the first time at what our signal looks like with the new data — and when results are shown at HCP. This is just as things were at ICHEP, and during those few weeks there were a lot of rumors around. It’s not possible to confirm or deny rumors when you know the status of ongoing work but haven’t yet agreed with your colleagues that it’s finished and ready to talk about publicly. So this time, I’m going to get in some general comments about rumors before I know anything at all about actual results. These comments will apply just as well to future updates.

What are we doing during the gap between unblinding and the conference? We’re checking our results, and putting them in a final presentable form. This is already compressed into a very short, hectic time, as I’ve written about before.

Are the rumors true? They are definitely not our official results, but they might turn out to be close. Or they might not. Specifically, the possibilities are:

  • A rumor is pretty much right. It’s no secret that particle physicists are bad at keeping secrets, and we really don’t want to be good at it. If one in 3,000 physicists decides to tell the Internet what our first-pass internal results look like, we can’t really stop them. Of course they’re breaking the rules, and we wish they wouldn’t, because it’s a collaborative effort and we’d prefer to agree together that we’re finished before announcing our results — because we want to make sure we did everything as well as we can. But still, our first-pass results are usually pretty close to final.
  • A rumor isn’t quite right. This could happen if we do find small mistakes or make refinements in the last few weeks of analyzing the data. This changes the answer by a bit, so the rumor is out of date. You could also make up a “not-quite-right” rumor just by making an educated guess based on our last results and how much new data we’ve taken!
  • A rumor is just plain wrong. Nobody says rumors have to be based on anything. Or they could be based on a misunderstanding of far-from-complete internal results.

We physicists working on this stuff don’t find it easy to wait for the answers either, and as Jon Butterworth has pointed out, rumors of other experiments’ results are actually dangerous for our work! For everybody else who’s tempted to indulge in rumors, just remember: you might be getting part of the picture early, or you might not. The only way to be sure is to wait for the next real update.

Share

I like talking about science. I like talking about religion. I even like talking about the relationship and boundaries between the two. These are all fascinating subjects, with many questions that are very much up for debate, so I am very pleased to see that CERN is participating in an event in which scientists, philosophers, and theologians talk together about the Big Bang and other questions.

But this quote, at least as reported by the BBC, simply doesn’t make any sense:

Co-organiser Canon Dr Gary Wilton, the Archbishop of Canterbury’s representative in Brussels, said that the Higgs particle “raised lots of questions [about the origins of the Universe] that scientists alone can’t answer”.

“They need to explore them with theologians and philosophers,” he added.

The Higgs particle does no such thing; it is one aspect of a model that describes the matter we see around us. If there is a God, CERN’s recent observations tell us that God created a universe in which the symmetry between the photon and the weak bosons is probably broken via the Higgs Mechanism. If there is not, they tell us that a universe exists anyway in which the symmetry between the photon and the weak bosons is probably broken via the Higgs Mechanism. It doesn’t raise any special questions about the origins of the universe, any more than the existence of the electron does.

There are many interesting philosophical questions to ask about the relationships between models of scientific observations on the one hand, and notions of absolute Truth on the other. You can also talk about what happened before the times we can make scientific observations about, whether there are “other universes” with different particles and symmetries, and so on. Theologians and philosophers have much to say about these issues.

But in regard to searches for the Higgs boson in particular, the people we need to explore questions with are mostly theoretical physicists and statisticians.

Share

“Snowmass” (Not Snowmass)

Saturday, October 13th, 2012

Every so often, perhaps once or twice a decade, particle physics in the United States comes to some kind of a crossroads that requires us to think about the long-term direction of the field. Perhaps there is new experimental data that is pointing in new directions, or technology developments that make some new facility possible, or we’re seeing the end of the previous long-term plan and it’s time to develop the next one. And when this happens, the cry goes up in the community — “We need a Snowmass!”

Snowmass refers to Snowmass Village in Colorado, just down the road from Aspen, the home of the Aspen Center for Physics, a noted haunt for theorists. During the winter, Snowmass a ski resort. During the summer, it’s a mostly empty ski resort, where it’s not all that expensive to rent some condos and meeting rooms for a few weeks. Over the past few decades there have been occasional “summer studies” held at Snowmass, typically organized by the Division of Particles and Fields of the American Physical Society (and sponsored by a host of organizations and agencies). It’s a time for the particle-physics community to come together for a few weeks and spend some quality time focusing on long-range planning.

The last big Snowmass workshop was in 2001. At the time, the Fermilab Tevatron was just getting started on a new data run after a five-year shutdown for upgrades, and the LHC was under construction. The top quark had been discovered, but was not yet well characterized. We were just beginning to understand neutrino masses and mixing. The modern era of observational cosmology was just beginning. A thousand physicists came to Snowmass over the course of three weeks to plot the future of the field. (And I was a lot younger.) Flash forward eleven years: the Tevatron has been shut down (leaving the US without a major high-energy particle collider), the LHC is running like gangbusters, we’re trying to figure out what dark energy is, and just in the past year two big shoes have dropped — we have measured the last neutrino mixing angle, and, quite famously, observed what could well be the Higgs boson. So indeed, it is time for another Snowmass workshop.

This week I came to Fermilab for a Community Planning Meeting for next year’s Snowmass workshop. Snowmass 2013 is going to be a bit different than previous workshops in that it will not actually be at Snowmass! Budgetary concerns and new federal government travel regulations have made the old style of workshop infeasible. Instead, there will be a shorter meeting this summer hosted by our colleagues at the University of Minnesota (hats off to thee for having us), so this time we won’t have as much time during the workshop to chew over the issues, and more work will have to be done ahead of time. (But I suspect that we’re still going to call this workshop “Snowmass”, just as the ICHEP conference was “the Rochester conference” for such a long time, even if it’s now the “Community Summer Study”.)

This Snowmass is being organized along the three “frontiers” that we’re using to classify the current research efforts in the field — energy, intensity and cosmic. As someone who works at the LHC, I’m most familiar with what’s going on at the energy frontier, and certainly there are important questions that have only come into focus this year. Did we observe the Higgs boson at the LHC? What more do we have to know about it to believe that it’s the Higgs? What are the implications of not having observed any other new particles yet for particle physics and for future experiments? The Snowmass study will help us understand how we answer these questions, and specifically what experiments and facilities are needed to do so. There are lots of interesting ideas that are out there right now. Can the LHC tell us what we need to know, possibly with an energy or luminosity upgrade? Is this the time to build a “Higgs factory” that would allow us to study measure Higgs properties precisely? If so, what’s the right machine for that? Or do we perhaps need an accelerator with even greater energy reach, something that will help us create new particles that would be out of reach of the LHC? What kind of instrumentation and computing technologies are needed to make sense of the particle interactions at these new facilities? The intensity and cosmic frontiers have equally big and interesting questions. I would posit that the scientific questions of particle physics have not been so compelling for a long time, and that it is a pivotal time to think about what new experiments are needed.

However, we also have the bracing reality that we are looking at these questions in a budget environment that is perhaps as constrained as it has ever been. Presentations from our champions and advocates at the Department of Energy and the National Science Foundation, the agencies that fund this research (and that sponsor the US LHC blog) were encouraging about the scientific opportunities but also noted the boundary conditions that arise from the federal budget as a whole, national research priorities, and our pre-existing facilities plan. It will continue to be a challenge to make the case for our work (compelling as it may be to us, and to someone who might be interested in looking at the Quantum Diaries site) and to envision a set of facilities that can be built and used given the funding available.

The first (non-native) settlers of Snowmass, Colorado, were miners, who were searching for buried treasure under adverse conditions. They were constrained by the technology of the time, and the facilities that were available for their work. I shouldn’t suggest that what we are doing is exactly like mining (it’s much safer, for one thing), but hopefully when we go to Snowmass (or really “Snowmass”) we will be figuring out how to develop the technology and facilities that are needed to extract an even greater treasure.

Share

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

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

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

Share

Le 4 juillet, le CERN annonçait avoir «observé une nouvelle particule » et non « découvert le boson de Higgs. » Pourquoi faire preuve de tant de retenue? Simplement parce ce qu’il était trop tôt pour se prononcer. Le boson de Higgs est la dernière pièce manquante au Modèle Standard de la physique des particules, un modèle qui a permis aux théoriciennes et théoriciens de faire des prédictions d’une extrême précision. Mais qui voudrait compléter un casse-tête de 5000 morceaux en y insérant la mauvaise pièce?

Les expériences CMS et ATLAS ont déjà attaqué les questions suivantes:

1) Voit-on tous les modes de désintégration prédits par le Modèle Standard?

2) Est-ce que chacun se produit aussi souvent que prévu?

3) Quelles sont les propriétés fondamentales de ce nouveau boson?

Bien que les premières vérifications effectuées (basées sur la moitié des données disponibles aujourd’hui) indiquent que le nouveau boson aie tout l’air du Higgs, la précision actuelle est encore trop faible pour trancher comme le montre les graphes suivants. (signal strength et σ/σSM H représentent la même quantité).

Le boson de Higgs peut se désintégrer de plusieurs façons et le graphe montre les différents canaux observés ainsi que leur fréquence. Une « force de signal » (signal strength) de 1 implique que le signal correspond exactement à ce que prédit le modèle pour un boson de Higgs. Et zéro veut dire que ce canal de désintégration n’est pas observé. Les points en noir représentent les mesures faites et la barre horizontale, la marge d’erreur associée.

Comme on le voit bien, il est encore impossible de dire si les deux premiers canaux sont compatible avec 0 (non, ce canal n’est pas observé) ou 1 (oui, on le voit au taux prévu). ATLAS et CMS doivent analyser plus de données pour déterminer si ce boson se désintègre en deux quarks b (H → bb) et deux leptons tau (H → ττ). Les trois autres canaux sont bel et bien observés mais à des taux légèrement supérieurs à ceux prévus par le Modèle Standard.

Le test décisif viendra des mesures de son spin et de sa parité, deux « nombres quantiques » (ou particularités mesurables) attachés aux particules fondamentales. Le « spin » est semblable à la quantité de mouvement angulaire qu’on associe à un corps en rotation. Sauf que pour les particules fondamentales, cette quantité ne peut prendre que certaines valeurs bien précises. Pour les bosons, les particules associées aux champs de forces, la valeur doit être 0, ±1, ±2 etc. Pour les fermions, les grains de matière tels que les quarks et les leptons (électron, muon, tau and neutrinos), le spin est soit +½, soit -½.

Aidan Randle-Conde résume bien toutes les possibilités dans son blog (en anglais). Seule une particule de spin 0 ou 2 peut se désintégrer en deux photons. Puisqu’on a vu que le nouveau boson se désintègre en deux photons, il ne peut avoir qu’un spin 0 ou 2. De plus, un boson de spin 2 ne peut se désintégrer en deux taus. Il est donc crucial de mesurer si c’est le cas ou pas en utilisant toutes les données accumulées récemment.

(tiré du blog d’Aidan Randle-Conde)

Le Modèle Standard impose que le spin et la parité du boson de Higgs soit 0+. Reste donc à déterminer si le nouveau boson est de type 0+ ou encore 0, 2+ ou 2. Le seul moyen est de mesurer les angles auxquels les produits de désintégration s’échappent. Si on observe une désintégration en deux photons, on doit mesurer l’angle entre les photons et la direction des faisceaux du LHC. Lorsque le boson se brise en deux Z, chacun donnant  à son tour deux électrons ou deux muons, il faut mesurer les angles et la masse combinée des quatre particules finales.

Voici ce que Sara Bolognesi et ses collègues prédisent pour un boson de Higgs se désintégrant soit en ZZ, WW ou deux photons. En mesurant la masse et les angles des produits de désintégration, on pourra déterminer le spin et la parité du nouveau boson. Si leur distribution correspond aux courbes en rouge dans les diagrammes suivants, c’est qu’on a bel et bien trouvé le boson de Higgs. Si cela ressemble plutôt aux autres courbes, celles associées à d’autres modèles, c’est qu’il s’agit d’un autre type de boson.

Chaque expérience a maintenant en main 14 femtobarn inverse (fb-1) de données et on espère atteindre 25 fb-1 au total d’ici la fin de l’année. Avec les 5 fb-1 accumulés l’an dernier, ce devrait être suffisant pour arriver à démasquer le nouveau venu. Il ne reste « plus » qu’à mesurer toutes ces quantités assez complexes.

Pauline Gagnon

Pour être averti-e 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

Pour plus d’info sur le spin du boson de Higgs, regardez ces deux récents vidéos sur CERN YouTube (première et seconde partie) (en anglais seulement)

Share

The art of data mining is about searching for the extraordinary within a vast ocean of regularity. This can be a painful process in any field, but especially in particle physics, where the amount of data can be enormous, and ‘extraordinary’ means a new understanding about the fundamental underpinnings of our universe. Now, a tool first conceived in 2005 to manage data from the world’s largest particle accelerator may soon push the boundaries of other disciplines. When repurposed, it could bring the immense power of data mining to a variety of fields, effectively cracking open the possibility for more discoveries to be pulled up from ever-increasing mountains of scientific data.

Advanced data management tools offer scientists a way to cut through the noise by analyzing information across a vast network. The result is a searchable pool that software can sift through and use for a specific purpose. One such hunt was for the Higgs boson, the last remaining elementary particle of the Standard Model that, in theory, endows other particles with mass.

With the help of a system called PanDA, or Production and Distributed Analysis, researchers at CERN’s Large Hadron Collider (LHC) in Geneva, Switzerland discovered such a particle by slamming protons together at relativistic speeds hundreds of millions of times per second. The data produced from those trillions of collisions—roughly 13 million gigabytes worth of raw information—was processed by the PanDA system across a worldwide network and made available to thousands of scientists around the globe. From there, they were able to pinpoint an unknown boson containing a mass between 125–127 GeV, a characteristic consistent with the long-sought Higgs.

An ATLAS event with two muons and two electrons - a candidate for a Higgs-like decay. The two muons are picked out as long blue tracks, the two electrons as short blue tracks matching green clusters of energy in the calorimeters. ATLAS Experiment © 2012 CERN.

The sheer amount of data arises from the fact that each particle collision carries unique signatures that compete for attention with the millions of other collisions happening nanoseconds later. These must be recorded, processed, and analyzed as distinct events in a steady stream of information. (more…)

Share