• John
  • Felde
  • University of Maryland
  • USA

Latest Posts

  • 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

  • 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
  • USA

Latest Posts

Warning: file_put_contents(/srv/bindings/215f6720ac674a2d94a96e55caf4a892/code/wp-content/uploads/cache.dat): failed to open stream: No such file or directory in /home/customer/www/quantumdiaries.org/releases/3/web/wp-content/plugins/quantum_diaries_user_pics_header/quantum_diaries_user_pics_header.php on line 170

Archive for December, 2013

CERN Christmas Play

Tuesday, December 24th, 2013


Note on the door separating the recently renewed HR department and the theory department.

Note on the door separating the recently renewed HR department and the theory department.

It’s that time of the year again, the time for CERN’s theory group’s yearly christmas play.
If’ you’d wander around in CERN’s corridors and happen to pass the theory department, you can notice the remarkably high concentration of jokes flying around in the corridor. See for example the note on the door separating the (recently renewed) HR department and the (still looking like a lab in the 60s) theory department.

Of course I am not the first one to blog about the CERN christmas play, see for example here or here. In fact, this silly play has a tradition that goes 30 years back. I could find some records of it on the cern document server going back to 1984. I didn’t have the time to check them all out, but let me know if you can spot any famous physicists in their young days, I am sure there are a lot!


Invitation for the 'Theory christmas party', hinting this year's theme as a quest for the holy grail and a reference to CERN's official 'thank you  drinks' called 'Bosons & more'.

Invitation for the ‘Theory christmas party’, hinting this year’s theme as the quest for the holy grail and a reference to CERN’s official ‘thank you drinks’ called ‘Bosons & more’. The king’s head has been replaced by the head of our director general, and the knights by the spokespersons of the biggest experiments.



Anyway, back to this year’s play. Each year, a group of theorists gathers together during their lunch breaks for about a week to practice their script, written and directed by John Ellis, to perform after the christmas dinner, on an improvised stage, set up in the main cafeteria.

The theme of the play varies every year (see last year’s Bond themed one), and this year there was chosen a Monty Python parody (I love Monty Python) on a quest for the (un)holy grail.

I will post the video here as soon as it is available (which may take until after the christmas break). In the meanwhile, here are some of the highlights (picture credits go to Mike Struik):

The scene shifters practiced a different silly walk for each scene shift. It was amazing to see how good they got at this.

The scene shifters practiced a different silly walk for each scene shift. It was amazing to see how good they got at this. Note the -not so representative- version of the LHC in the back.



Referencing the oppressed peasant sketch . The theorists/peasants have no idea that something like a DG (director general) exists. CERN’s DG, Rolf Heuer has become quite a public figure in the last years.

Epic appearance of Professor Ellis as God, explaining 'Rolfur', king (DG) of the CERNois, to look for the unholy grail of particle physics, namely 'dark matter'

Epic appearance of Professor Ellis as God.



Myself in the middle as an ex-perimentalist, referring to the dead parrot sketch.

Myself in the middle as an ex-perimentalist, referring to the dead parrot sketch. We truly could not find a real experimentalist willing to play a role 😉

John Ellis made an epic appearance playing ‘God’, explaining ‘Rolfur’, king (DG) of the CERNois, to look for the unholy grail of particle physics, namely dark matter and dark energy.
Now the Higgs boson is found, it seems  that our DG tries to promote the search for dark matter as one of CERN’s most important tasks.
Unfortunately for CERN, we are not the only one looking for dark matter and there are many ways to search for it without the need of a particle accelerator. In fact, we would only be able to detect it here, if dark matter is made up of WIMPs (weakly interacting massive particles). A recent post that I particularly liked on this can be found here.

Sir LHC-alot being wooed by the damsels.

Sir LHC-alot being wooed by the damsels.


A French guy defending the bridge to stockholm, trying to prevent sir LHCalot to claim a Nobel Prize.

A French guy defending the bridge to stockholm, trying to prevent sir LHCalot from claim a Nobel Prize.

Video will appear soon!


Which is the Real CERN?

Thursday, December 19th, 2013

Is this CERN...?

Is this CERN…?

Or is this CERN...?

Or is this CERN…?

A few weeks ago, at the very real peril of spending our weekend on something that was a little like work for both of us, I went with my wife to the Collider exhibit at the Science Museum in London.

Collider a detailed, immersive exhibit about the Large Hadron Collider and the people who work on it. It’s amazing to hear video interviews from real physicists and see real places at CERN reproduced. A lot of the information is on realistic-looking whiteboards, and there’s real stuff lying everywhere just like in real offices. (The real stuff is glued and stapled down; my wife, a museum curator interested in the implementation of the exhibit, checked that detail personally.) One thing that bothered me that might not bother you: the videotaped physicists are clearly actors, with stories told just a bit too dramatically. One thing that might bother you but didn’t bother me, because I can skip reading signage and just explain to my wife what I think it should say: not all of the amazing things you could see are explained very well.

But the fun part really is the feeling of actually being in the midst of where the science is done. For example, at right, you can see a picture of me in one of the CERN hallways recreated for the exhibit, and you can see a picture of me in front of the real version of the same office. But which is which?


Mu2e attracts magnet experts

Wednesday, December 18th, 2013

This article appeared in symmetry on Dec. 16, 2013.

By tapping into specialized knowledge around the world, the Mu2e collaboration will undertake a first-of-its-kind experiment. Image courtesy of Lawrence Berkeley National Laboratory

By tapping into specialized knowledge around the world, the Mu2e collaboration will undertake a first-of-its-kind experiment. Image courtesy of Lawrence Berkeley National Laboratory

Fermilab’s Mu2e experiment is unlike anything ever attempted. So when the collaboration needed a first-of-its-kind magnet prototype built, they turned to an institution known for its magnet expertise: the Genoa section of the Italian Institute for Nuclear Physics, or INFN, located in the University of Genoa in Italy.

Earlier this year, INFN-Genoa became the sixth Italian institution to join the Mu2e collaboration, which now sports more than 150 members from 28 labs and universities in the United States, Italy and Russia. The team of magnet experts there has decades of experience working on high-energy physics experiments—they helped design and build magnets for BaBar at SLAC and, more recently, the CMS detector at CERN.

Now they’re putting that knowledge toward building prototypes of the years-in-development magnets that will be used for for Mu2e, an experiment intended to study whether charged particles called leptons can change from one type to another. According to Doug Glenzinzki, the deputy project manager for Mu2e, the experiment’s goal is to narrow down the possibilities for completing physicists’ picture of the universe, by amassing evidence for one theory over others.

“We know the Standard Model is incomplete,” Glenzinski says. “The number one goal of particle physics is to elucidate what a more complete model looks like. There are a lot of theories, and we are looking for data that tells us which is right.”

The Mu2e apparatus includes a detector solenoid, a transport solenoid and a production solenoid. Image courtesy of: Mu2e Collaboration

The Mu2e apparatus includes a detector solenoid, a transport solenoid and a production solenoid. Image courtesy of: Mu2e Collaboration

It turns out, Glenzinski says, “charged lepton flavor violation”—the phenomenon Mu2e is being built to study—is a powerful way of discriminating between possible models. Seeing this violation would also open up new questions about a theory of nature that has stood for 80 years. In short, this experiment could point the way toward the future of particle physics.

Mu2e will use a 92-foot-long detector with a unique design. It will be built in three sections, each its own superconducting solenoid, which is a set of electromagnetic coils that generates the particular magnetic fields required for the experiment. The detector consists of a production solenoid, a detector solenoid and a snake-like transport solenoid connecting them. Fermilab’s accelerators will fire a beam of protons into the production solenoid, where they will strike a target to produce pions. It’s the job of the transport solenoid to winnow down that beam of pions as it moves through, herding negatively charged muons to the detector solenoid and sending other unwanted particles out of the way.

The transport solenoid—a 42-foot-long curved pipe—will use 50 different magnets to accomplish this. The Genoa team will build prototypes of these magnets, working from years of design and engineering by Fermilab’s Technical Division, an effort led by Giorgio Ambrosio, Mike Lamm and Tom Page.

Pasquale Fabbricatore is one of the leaders of the Genoa team—he worked on both the BaBar and CMS magnets. He says that though the Mu2e magnets will use similar technology to large detector magnets, their unusually small size—about 6.5 feet in diameter—makes applying that technology tricky.

This sample holder is used to test the prototype conductor for the Mu2e experiment's transport solenoid. Photo: INFN-Genoa Mu2e Collaboration

This sample holder is used to test the prototype conductor for the Mu2e experiment’s transport solenoid. Photo: INFN-Genoa Mu2e Collaboration

“Superconducting magnets are so particular that each one is a prototype,” Fabbricatore says. “Each unique magnet has unique problems.”

For example, Fabbricatore says, the prototype magnet will consist of a module containing two electromagnetic coils, installed close together through a shrink-fitting operation. While placing the first one should be easy, he says, warming the second coil up to the right temperature to install it without damaging the first could prove to be difficult.

“This is a problem we have never encountered before,” he says.

INFN-Genoa is just the latest Italian institution to join the Mu2e team. Glenzinski says the experiment has received strong support from Italy since the project’s inception. Italy is now contributing to Mu2e with four INFN groups from Frascati, Pisa, Udine and Lecce. It also leads the building of the calorimeter system, which helps measure the momentum of electrons and identify background signals. Glenzinski says the Genoa group makes a fine addition to a growing collaboration.

“Pasquale and his team are world-class magnet experts,” Lamm says. “They’re a great addition to the Mu2e collaboration and we’re excited to have them join us.”

The work on the new magnet began in September, and Fabbricatore says the prototype will be delivered to the collaboration in July 2014. Glenzinski says that fits the experiment’s timeline nicely. The collaboration will test the prototype, then send it out to a vendor to create the 50 magnets needed for the project. Assembly of the Mu2e detector should begin in 2016, with the experiment ready to take data by the end of 2019.

Andre Salles


Excitement from IceCube

Friday, December 13th, 2013

After a rather long hiatus (I was writing my PhD dissertation), I am getting back into the habit of posting about interesting things happening in particle physics. Since finishing my degree at UC Davis, I made an arduous cross country drive to start a new adventure as a postdoc at the University of Maryland working on the IceCube neutrino experiment at the South Pole. I have joined this collaboration at a particularly exciting time since the full detector was completed in May of 2011.

COVER Hit distribution (red, early; green, late) of a neutrino interaction with the Antarctic IceCube neutrino detector on 14 July 2011. Light from this transfer of 250 teraelectron volts of energy fills a sphere 600 meters across. This event, among the highest-energy neutrino interactions ever observed, forms part of the first evidence for a high-energy neutrino flux of astrophysical origin.

Back in June of this year, two neutrino events were reported with energies slightly above 1 PeV (peta-electronvolt). To put this number in context, the protons circulating in the Large Hadron Collider (LHC) at CERN have energies of about 4 TeV (tera-electronvolt) each. A PeV is 1,000 times greater than a TeV. Although we would love to be able to produce these higher energies at colliders like the LHC, it simply isn’t feasible at this time. As a result, we must rely on nature to produce these high energy particles for us, and hope that she flings a few our way so we can detect them. This is the job of the IceCube detector, a huge, 1 cubic kilometer, neutrino detector instrumented deep within the Antarctic ice. The enormous size is necessary since few of these particles are produced at such high energies, and even then the neutrino interaction probability is miniscule. Unfortunately, the physicist has no control over nature, nor physics, and so our only recourse is to build big! For those interested in more details about the detector, see the website at the University of Wisconsin – Madison here.

Today the collaboration reports findings from a new neutrino search published in Science. The new search includes neutrino events at lower energies as well, down to about 30 TeV. The results of this search indicate that it is highly unlikely that these neutrinos were produced by any mechanism at Earth. Many high energy neutrinos are produced in Earth’s atmosphere, but not this many, and not at these energies.

Of particular interest to the community is a very fundamental question: “Where do these particles come from anyway?” Since the neutrino interactions preserve some information about the neutrino’s direction, the hope is that these neutrino events will all be coming from a particular place in the universe. Looking for this, the results are tantalizing. Since not all of the events provide exact position information, our best guess of the particle’s direction can be a little fuzzy. So far, however, the most significant clustering of events can be seen below in the full skymap (bottom left side). This location roughly corresponds to the center of our galaxy, but the fuzziness of the event locations does not permit us to say where exactly these neutrinos are coming from.

Luckily, the detector continues to collect additional neutrino events, even possibly as you read this. Our fingers are crossed that more events will be detected in this regime, filling in our understanding of extraterrestrial neutrinos and the cosmos in general.

In celebration of these results, the online magazine, Physics World, has named the IceCube  findings the 2013 breakthrough of the year! A discussion will be held via Google hangout, and shown on the Physics World youtube channel to explain the results, and take questions form the audience today at 4pm UTC (11:00 EST).

Skymap of the IceCube neutrino events. The purple regions indicate more likely locations of neutrino sources (darker is more likely). The plane of the galaxy is shown as a grey line, and the center of the galaxy is denoted by a filled grey square (near the event marked #14).

Skymap of the IceCube neutrino events. The purple regions indicate more likely locations of neutrino sources (darker is more likely). The plane of the galaxy is shown as a grey line, and the center of the galaxy is denoted by a filled grey square (near the event marked #14). The best guess for each event’s location are indicated with either a + (shower like events) or a X (muon tracks).


Yes, once!

Paradigm and paradigm shift are so over used and misused that the world would benefit if they were simply banned.  Originally Thomas Kuhn (1922–1996) in his 1962 book, The Structure of Scientific Revolutions, used the word paradigm to refer to the set of practices that define a scientific discipline at any particular period of time. A paradigm shift is when the entire structure of a field changes, not when someone simply uses a different mathematical formulation. Perhaps it is just grandiosity, everyone thinking their latest idea is earth shaking (or paradigm shifting), but the idea has been so debased that almost any change is called a paradigm shift, down to level of changing the color of ones socks.

The archetypal example, and I would suggest the only real example in the natural and physical sciences, is the paradigm shift from Aristotelian to Newtonian physics. This was not just a change in physics from the perfect motion is circular to an object either is at rest or moves at a constant velocity, unless acted upon by an external force but a change in how knowledge is defined and acquired. There is more here than a different description of motion; the very concept of what is important has changed. In Newtonian physics there is no place for perfect motion but only rules to describe how objects actually behave. Newtonian physics was driven by observation. Newton, himself, went further and claimed his results were derived from observation. While Aristotelian physics is broadly consistent with observation it is driven more by abstract concepts like perfection.  Aristotle (384 BCE – 322 BCE) would most likely have considered Galileo Galilei’s (1564 – 1642) careful experiments beneath him.  Socrates (c. 469 BC – 399 BC) certainly would have. Their epistemology was not based on careful observation.

While there have been major changes in the physical sciences since Newton, they do not reach the threshold needed to call them a paradigm shifts since they are all within the paradigm defined by the scientific method. I would suggest Kuhn was misled by the Aristotle-Newton example where, indeed, the two approaches are incommensurate: What constitutes a reasonable explanation is simply different for the two men. But would the same be true with Michael Faraday (1791 – 1867) and Niels Bohr (1885–1962) who were chronologically on opposite sides of the quantum mechanics cataclysm?  One could easily imagine Faraday, transported in time, having a fruitful discussion with Bohr. While the quantum revolution was indeed cataclysmic, changing mankind’s basic understanding of how the universe worked, it was based on the same concept of knowledge as Newtonian physics. You make models based on observations and validate them through testable predictions.  The pre-cataclysmic scientists understood the need for change due to failed predictions, even if, like Albert Einstein (1879 – 1955) or Erwin Schrödinger (1887 – 1961), they found quantum mechanics repugnant. The phenomenology was too powerful to ignore.

Sir Karl Popper (1902 – 1994) provided another ingredients missed by Kuhn, the idea that science advances by the bold new hypothesis, not by deducing models from observation. The Bohr model of the atom was a bold hypothesis not a paradigm shift, a bold hypothesis refined by other scientists and tested in the crucible of careful observation. I would also suggest that Kuhn did not understand the role of simplicity in making scientific models unique. It is true that one can always make an old model agree with past observations by making it more complex[1]. This process frequently has the side effect of reducing the old models ability to make predictions. It is to remedy these problems that a bold new hypothesis is needed. But to be successful, the bold new hypothesis should be simpler than the modified version of the original model and more crucially must make testable predictions that are confirmed by observation. But even then, it is not a paradigm shift; just a verified bold new hypothesis.

Despite the nay-saying, Kuhn’s ideas did advance the understanding of the scientific method. In particular, it was a good antidote to the logical positivists who wanted to eliminate the role of the model or what Kuhn called the paradigm altogether. Kuhn made the point that is the framework that gives meaning to observations. Combined with Popper’s insights, Kuhn’s ideas paved the way for a fairly comprehensive understanding of the scientific method.

But back to the overused word paradigm, it would be nice if we could turn back the clock and restrict the term paradigm shift to those changes where the before and after are truly incommensurate; where there is no common ground to decide which is better. Or if you like, the demarcation criteria for a paradigm shift is that the before and after are incommensurate[2]. That would rule out the change of sock color from being a paradigm shift. However, we cannot turn back the clock so I will go back to my first suggestion that the word be banned.

To receive a notice of future posts follow me on Twitter: @musquod.


[1] This is known as the Duhem-Quine thesis.

[2] There are probably paradigm shifts, even in the restricted meaning of the word, if we go outside science. The French revolution could be considered a paradigm shift in the relation between the populace and the state.


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

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



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

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.