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Archive for January, 2014

This article appeared in Fermilab Today on Jan. 29, 2014.

The Department of Energy recently gave the Muon g-2 experiment approval to proceed to the next phase of design. Photo: Cindy Arnold

The Department of Energy recently gave the Muon g-2 experiment approval to proceed to the next phase of design. Photo: Cindy Arnold

2013 was a big year for the Muon g-2 experiment.

Over the summer, the 52-foot-wide electromagnet that forms the core of the experiment was transported from New York to Illinois in a flurry of publicity. Construction began on the building that will house that device and should be completed in the next couple of months.

And in December, the Department of Energy granted Critical Decision 1 approval to the experiment, marking a major milestone and charting the path forward.

Chris Polly, project manager for Muon g-2, said this approval process was the first time that DOE officials have reviewed the entire scope of the experiment, from the design to the cost to the timeline. In order to get to this stage, the collaboration developed a 500-page report, designing and costing every element of the project and then laying those elements out in a schedule consisting of 1,500 activities spanning four years.

“It was an incredible amount of work that required everyone on the collaboration to really focus, thoroughly think through the whole experiment and document it all for the reviewers,” Polly said.

The reviewers were pleased with the work and only had a few recommendations. Most notably, the committee suggested that the experiment team work with the DOE to develop an accelerated schedule.

The review took place in September, and the intervening months were spent working out the timeline and funding profile. The work that had already been done to transport the electromagnet and begin construction of the MC-1 Building helped convince the reviewers that the team could keep to such a schedule.

“CD-1 approval is a very important milestone for the experiment, and we appreciate all the strong support that we received from DOE and the laboratory management in getting us to this point,” said Lee Roberts, co-spokesperson for the experiment.

The Muon g-2 collaboration received more good news this month as well: The omnibus budget bill signed into law on Jan. 17 includes funding to continue the design and begin construction of the experiment. (That funding is not explicitly spelled out in the bill but is covered.)

2014 will be another big year with the reassembly of the storage ring in its new home, the development of detectors for the experiment and the start of construction for the muon source. And this summer the Muon g-2 team will undergo the next step in the approval process, an extensive CD-2 review.

Andre Salles


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.

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

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.

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

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.


Anti-beam me up, Scotty!

Tuesday, January 28th, 2014

While the CERN accelerator complex was being revamped in 2013, the ASACUSA experiment took time to carefully review the data taken in 2012 at the Antiproton Decelerator (AD) facility. This painstaking work paid off and they just announced in Nature having produced the first ever beam of antihydrogen atoms.

In laboratory experiments like the ones conducted at CERN, matter and antimatter are always created in equal amounts. The Big Bang theory predicts that the same quantities of matter and antimatter were also created at the origin of the Universe. However, nowadays, one sees absolutely no trace of this “primordial” antimatter. So what happened to all the antimatter that once was in the Universe?

To answer this question, CERN has a full antimatter program underway at the AD to check if antimatter has the same properties as matter. One of the best ways to do so is to compare antihydrogen atoms with hydrogen atoms. This is the simplest of all atoms, having only one electron orbiting around one single proton.

Antihydrogen atoms are replica of hydrogen atoms but with an anti-electron – called positron – and an antiproton replacing the electron and proton of regular atoms.

All matter emits light when excited just as a piece of metal shines when heated up. The light emitted gives a unique signature for each atom. For example, hydrogen emits and absorbs light of specific frequency when an electron jumps from one energy level to another. It also has a “hyperfine structure” corresponding to magnetic interactions between the nucleus and the electron.

The ASACUSA experiment aims to check the hyperfine structure of antihydrogen. This can be done by observing which frequencies antihydrogen atoms absorb.

asacusa-realThe ASACUSA experiment at CERN (Image: Yasunori Yamakazi )

So here is what ASACUSA did: they produced antihydrogen atoms by first decelerating and cooling antiprotons down to very low temperature. Then they mixed antiprotons with positrons and combined them in a strong non-uniform magnetic field. These strong magnetic fields are necessary to keep antiprotons and positrons from touching any matter. That would cause their immediate annihilation and prevent the formation of antihydrogen atoms.

The next problem was to move the antihydrogen atoms away from this field to be able to study their hyperfine structure. Otherwise, the strong non-uniform magnetic field would mask the tiny effects generated by the magnetic interaction between the antiproton and the positron responsible for the hyperfine structure.

But atoms are neutral and cannot be controlled by electric fields. However, antihydrogen atoms are like tiny magnets. So by using non-uniform magnetic fields, the scientists were able to manipulate these tiny magnets and create a beam of antihydrogen atoms. It was directed towards a small detector located after a microwave cavity and a sextupole magnet.

The sextupole magnet focuses or defocuses antihydrogen atoms on the detector depending on the direction of the antihydrogen tiny magnets.


The ASACUSA setup. From left to right: the magnets (grey) used to produce antihydrogen atoms, the microwave cavity (green) to induce hyperfine transitions, the sextupole magnet (red and grey) and the antihydrogen detector (gold). Credit: Stefan Meyer Institute.

The detector reveals the number of antihydrogen atoms passing through the device after they go through a microwave cavity. It was turned off in 2012 but will be on in the future.  The antihydrogen atoms will only absorb microwave photons having exactly the energy corresponding to its hyperfine transitions. This process will alter the trajectory of antihydrogen atom in the sextupole magnet, and eventually the number of antihydrogen atoms reaching the detector will be reduced.

By counting how many antihydrogen atoms reach the target when the microwave cavity is tuned to specific frequencies, the scientists will determine the frequencies of the hyperfine structure.

ASACUSA now has the proof that 80 antihydrogen atoms made it to their detector. The next step is to see if fewer are observed when the microwave cavity is turned on at the right frequency.

And then we will know if antihydrogen is the exact mirror image of hydrogen. This may reveal if antimatter differs from matter and explain why it has all vanished.

Pauline Gagnon

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Scotty, anti-faisceau!

Tuesday, January 28th, 2014

Tandis que le complexe des accélérateurs du  CERN était à l’arrêt en 2013, l’expérience ASACUSA en a profité pour soigneusement analyser les données prises en 2012 au Décélérateur d’antiprotons (AD). Ce travail minutieux leur a permis d’annoncer dans la revue Nature la toute première production d’un faisceau d’antihydrogène.

Dans les expériences de laboratoire comme celles du CERN, la matière et l’antimatière sont toujours créées en quantités égales. La théorie du Big Bang prévoit que des quantités égales de matière et d’antimatière existaient à l’origine de l’Univers. Cependant, de nos jours, il ne reste aucune trace de cette antimatière “primordiale”. Qu’est-il arrivé à toute l’antimatière qui se trouvait dans l’Univers ?

Pour répondre à cette question, le CERN a un vaste programme d’étude de l’antimatière en cours au AD. On veut vérifier si l’antimatière a les mêmes propriétés que la matière. Une des façons de procéder est de comparer des atomes d’antihydrogène avec des atomes d’hydrogène. On choisit l’hydrogène puisque c’est le plus simple de tous les atomes, avec seulement un électron orbitant autour d’un unique proton.

Les atomes d’antihydrogène sont la réplique des atomes d’hydrogène, mais avec un antiélectron – appelé positron – et un antiproton remplaçant l’électron et le proton des atomes normaux.

Toute matière émet de la lumière quand elle est excitée. C’est ce qui se passe quand on chauffe une pièce métallique. La lumière émise est comme la signature de chaque atome. Par exemple, l’hydrogène émet ou absorbe de la lumière d’une fréquence spécifique quand un électron saute d’un niveau d’énergie à un autre. Il existe aussi “une structure hyperfine ” correspondant aux interactions magnétiques entre le noyau et l’électron.

L’expérience ASACUSA a pour but de vérifier la structure hyperfine de l’antihydrogène. Ceci se fait en observant quelles  fréquences les atomes d’antihydrogène peuvent absorber.

asacusa-realL’expérience ASACUSA au CERN (Photo: Yasunori Yamakazi )

Voici ce qu’ASACUSA a réussi: l’équipe a d’abord produit des atomes d’antihydrogène en ralentissant des antiprotons à très basse température. Puis on a mélangé ces antiprotons avec des positrons pour les combiner à l’aide d’un champ magnétique fort et non-uniforme. Ce champ est nécessaire pour éviter que les antiprotons et les positrons n’entre en contact avec de la matière. Cela causerait leur annihilation immédiate et empêcherait la formation d’atomes d’antihydrogène.

Puis il a fallu déplacer les atomes d’antihydrogène en dehors de ce champ pour pouvoir étudier leur structure hyperfine. Autrement, le fort champ magnétique masquerait les effets minuscules produits par l’interaction magnétique entre l’antiproton et le positron à l’origine de la structure hyperfine.

Mais comme les atomes sont neutres, ils ne peuvent pas être contrôlés par des champs électriques. Cependant, un atome d’antihydrogène ressemble à un minuscule aimant. Les scientifiques ont donc manipulé ces aimants microscopiques en utilisant un champ magnétique non-uniforme, et créé un faisceau d’atomes d’antihydrogène.  Ce faisceau a ensuite été dirigé vers un détecteur situé après une cavité à micro-ondes et un aimant sextupole.

L’aimant sextupole focalise ou défocalise les atomes d’antihydrogène sur le détecteur, dépendamment de l’orientation de leurs minuscules aimants.


L’expérience ASACUSA. De gauche à droite : les aimants (en gris) utilisés pour  produire les atomes d’antihydrogène, la cavité à micro-ondes (en vert) qui induit des transitions hyperfines, l’aimant sextupole de focalisation (en rouge et gris) et le détecteur à antihydrogène (en jaune). Crédit : Stefan Meyer Institute.

Le détecteur compte le nombre d’atomes d’antihydrogène qui l’atteignent après avoir traversé une cavité à micro-ondes. Cette cavité était éteinte en 2012, mais sera allumée à l’avenir. Les atomes d’antihydrogène absorberont alors seulement les photons micro-ondes ayant exactement l’énergie correspondant aux transitions de la structure hyperfine. L’absorption d’un photon changera la trajectoire de l’atome d’antihydrogène dans l’aimant sextupole et donc réduira le nombre d’atomes d’antihydrogène atteignant le détecteur.

En comptant combien d’atomes d’antihydrogène atteignent le détecteur lorsque la cavité à micro-ondes émettra des photons de fréquences spécifiques, les scientifiques détermineront quelles sont les fréquences de la structure hyperfine.

ASACUSA a maintenant la preuve que 80 atomes d’antihydrogène ont atteint  le détecteur. La prochaine étape consistera à déterminer si moins d’antiatomes seront observés lorsque la cavité à micro-ondes sera allumée à la bonne fréquence.

Nous saurons alors si l’antihydrogène est exactement l’image inversée de l’hydrogène. Ceci révélera si l’antimatière diffère de la matière et pourrait expliquer pourquoi elle est complètement disparue de l’Univers.

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



This originally appeared in Fermilab Today on Jan. 23, 2014.

Fermilab docent Toni Mueller shows students a model of a beamline. The Midwest Conference for Undergraduate Women in Physics coordinators offered participants a tour of Fermilab or Argonne. Photo: Amanda Solliday

Fermilab docent Toni Mueller shows students a model of a beamline. The Midwest Conference for Undergraduate Women in Physics coordinators offered participants a tour of Fermilab or Argonne. Photo: Amanda Solliday

Seventy female college students in hard hats descended into the MINOS cavern, walked through the Tevatron tunnel and explored the Linac beamline as part of the Midwest Conference for Undergraduate Women in Physics Friday, Jan. 17.

“The first time I toured Fermilab, it wasn’t what I was expecting at all, even after two years of college-level physics,” said Savannah Thais, a senior physics major at The University of Chicago. “I had no idea what it was like to do science all day, every day.”

Thais attended the 2013 conference and this year volunteered for the local organizing committee. She hopes participants will see the scientists and engineers at national laboratories as potential role models. The conference organizers also aim to provide female physics students a chance to connect with each other.

“Many times, especially at smaller colleges and universities, there are not many women in physics departments. You might be the only girl in your classes,” Thais said. “So we hope the participants can meet other female undergrads who share some of the same goals as they do.”

Sahar Jalal, a senior math and physics double major at Grinnell College, says she enjoys learning about the large-scale research projects.

“I didn’t know there were so many international collaborations at Fermilab,” Jalal said during lunch at Wilson Hall.

In between tour stops, 28 Fermilab scientists, engineers, science writers and docents met with students over the noon meal.

The Conferences for Undergraduate Women in Physics rotate each year to different sites nationwide. The University of Chicago hosted this year’s Midwest conference, partnering with other area universities and institutions.

The location allowed organizers to offer a Fermilab tour for the first time. The 250 Midwest participants could also choose to visit Argonne, while students at the other regional conferences visited Berkeley, Brookhaven and Livermore national laboratories.

Particle physicists play a particularly active role in the conferences, said Kevin Pitts, a physics professor at the University of Illinois. He notes the two national co-chairs and three Midwest organizing committee members work in particle physics.

Sam Zeller, a Fermilab staff scientist on the local committee, welcomed the chance to offer young scientists a glimpse into the life of a researcher.

“Seeing a national laboratory was a big thing for me as an undergraduate,” Zeller said. “It made me think about physics as a career, so it’s nice to give that opportunity back to the next generation of students.”

Amanda Solliday


A beam of your own

Thursday, January 16th, 2014

As part of its 60th anniversary celebration and to help keep us young at heart, CERN has launched a special competition for students called: Beam line for schools.

CERN is inviting students aged 16 and upward from anywhere in the world to submit a proposal to do an experiment with a beam of particles from the Proton Synchrotron beam line. Each team can be composed of up to 30 students with at least one adult supervisor. This summer, up to nine students of the selected team will be invited to CERN to run the team’s experiment. Travelling and living expenses for the selected group will be covered by CERN.

PSA view of the Proton Synchrotron beam line.

The proposals will be pre-selected by a group of CERN scientists, and will then be reviewed by the same committee that validates all proposals for experiments at the laboratory’s Super Proton Synchrotron and Proton Synchrotron accelerators.

So what could you be doing? Essentially, you can investigate how beams of particles interact with matter. For example, you could study what happens when beams containing different types of particles hit targets made of various materials. The proposals will be judged on creativity, motivation, feasibility and adherence to the scientific method.

To help you understand what can be done, we have put together a short presentation that explains the basics about particles and beams. These short talks are available in English, French, Italian, Spanish and German and are part of a YouTube playlist that includes recordings of Google hangouts in English, French, Italian, Spanish and German, in which CERN scientists answer questions.

Here is your chance to come to run your own experiment at CERN. This will last about a week and take place in July, August or September. CERN physicists will be helping you to refine your idea before and during your stay at CERN.

Interested? Then you can stay up-to-date via the CERN website, #bl4s on Twitter, Facebook, Google+ or YouTube.

Don’t hesitate and fill out the registration form before 31 January 2014. All you need to do at this point is send us the name of the school and of the participants as well as a tweet-of-intent stating why you think you should win this competition. You will still have until 31 March to prepare your full project, including a 1-minute video giving the highlights. Here is your chance!

Pauline Gagnon

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Un faisceau juste pour vous

Thursday, January 16th, 2014

Dans le cadre de son 60ème anniversaire et pour nous aider à rester jeunes d’esprit, le CERN a lancé une compétition spéciale pour les étudiant-e-s appelée : Un faisceau  pour les écoles.

Le CERN invite donc les étudiant-e-s de 16 ans et plus de n’importe quel pays à soumettre une proposition pour venir effectuer une expérience avec un faisceau de particules du synchrotron à protons (PS). Chaque équipe peut compter jusqu’à 30 étudiant-e-s avec au moins un-e adulte responsable. Cet été, tout au plus neuf étudiant-e-s de l’équipe choisie seront invité-e-s au CERN pour réaliser l’expérience pour l’équipe. Les frais de déplacement et d’hébergement du groupe seront pris en charge par le CERN.

PSVue de l’accélérateur du synchrotron à protons ou PS.

Les propositions seront pré-sélectionnées par un groupe de scientifiques du CERN puis passées en revue par le même comité qui valide toutes les demandes d’expériences des laboratoires opérant au synchrotron à protons et au supersynchrotron à protons.

Alors que pourriez-vous faire? Essentiellement, examiner comment les faisceaux de particules interagissent avec la matière. Vous pourriez par exemple étudier comment des faisceaux contenant différentes particules interagissent avec des cibles de matériaux divers. On jugera les propositions sur leur créativité, leur motivation, leur faisabilité et l’adhésion à la méthode scientifique.

Pour vous aider à comprendre ce qui peut être fait, nous avons préparé de courtes présentations expliquant l’essentiel sur les particules et les faisceaux. Ces présentations sont disponibles en anglais, français, italien, espagnol et allemand. Vous les trouverez sur une liste de sélections sur YouTube qui comprend aussi les enregistrements de discussions sur Google en cinq langues où des scientifiques du CERN répondent à différentes questions sur le projet.

Courrez donc la chance de venir réaliser votre propre expérience au CERN. Le séjour durera environ une semaine et aura lieu en juillet, août ou septembre. Des physicien-ne-s vous aideront à raffiner vos idées avant et pendant votre stage au CERN.

Intéressé-e-s? Vous pouvez rester à jour via le site Web du CERN ou en suivant #bl4s sur Twitter, Facebook, Google+ ou YouTube.

N’hésitez pas et inscrivez-vous avant le 31 janvier 2014. Tout ce que vous avez faire à pour l’instant est de nous envoyer le nom de l’école et des participant-e-s, ainsi qu’un tweet expliquant pourquoi vous pensez que vous devriez gagner cette compétition. Vous aurez encore jusqu’au 31 mars pour compléter votre application, y compris une vidéo d’une minute soulignant l’essentiel du projet. Une occasion à ne pas manquer!

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


A whole Universe to be discovered

Wednesday, January 15th, 2014

The past two years have been rather exceptional for CERN: first in 2012, the CMS and ATLAS experiments discovered the Higgs boson, confirming the mechanism elaborated 48 years earlier by Robert Brout, François Englert and Peter Higgs. Then in 2013, Englert and Higgs received the Nobel Prize for Physics for their theory.

2014 is also going to be special year since CERN is going to turn 60. But beyond this anniversary, CERN is preparing the Large Hadron Collider (LHC) to explore new territories.

With the Higgs boson discovery, we have completed the Standard Model, the current theory that explains what makes all visible matter around us. But that is just a mere 5% of the total content of the Universe. The existence of dark matter tells us our current model is incomplete. So far, the various analyses of the data taken at 8 TeV has not yet revealed traces of dark matter or any new particles. To push all our searches further and faster, we need to increase the reach of the LHC by going to higher energies.

This is why since February last year all accelerators and experiments at CERN began a long shutdown for maintenance and consolidation. This will continue in 2014 for the LHC but many accelerators of CERN complex will be coming back to life starting this summer.


The starting point of the chain of accelerators is a simple hydrogen bottle. The electrons are stripped from the hydrogen atoms using an electric field to leave single protons. These are then accelerated in a small linear accelerator (LINAC 2 at the bottom centre of the diagram below). The Low Energy Ion Ring (LEIR) plays a similar role but with heavy ions.


The protons get an extra kick in the Booster before being injected into what is CERN’s oldest circular accelerator still in operation, the Proton Synchrotron (PS). Then the protons head for the Super Proton Synchrotron (SPS), where they reach 450 GeV in energy (that is 450 billion electronvolts). This is the final stage before injection into the LHC where the energy will get nearly thirty times larger, namely 13 TeV.

The beams from the accelerator chain are also delivered to various other experimental areas, such as ISOLDE and n-TOF where a huge number of experiments involving nuclei are conducted. Other protons hit a target to produce antiprotons for the Antiproton Decelerator (AD), a facility dedicated to antimatter studies. These experiments will all resume their activities in 2014.


All consolidation work for the LHC and its experiments will take place in parallel. ATLAS and CMS plan to complete all repairs and upgrades to their detector by November, ALICE at the beginning of December and LHCb in early January 2015.

Meanwhile, all physicists not involved with hardware are either completing the many ongoing analyses of all data taken up to 2013, preparing new simulations at higher energies, improving the data reconstruction algorithms or designing the new trigger selection criteria. Everybody is preparing to meet the challenge of dealing with more data at higher energy. All in the hope that we might be rewarded once more with new discoveries since there is still a whole new world to explore out there.

Pauline Gagnon

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Tout un Univers à découvrir

Wednesday, January 15th, 2014

Les deux dernières années ont été plutôt exceptionnelles pour le CERN. En 2012, les expériences CMS et ATLAS  ont découvert le boson de Higgs, confirmant le mécanisme élaboré 48 ans auparavant par Robert Brout, François Englert et Peter Higgs. Et en 2013, Englert et Higgs se sont vus décerner le Prix Nobel de physique pour leurs travaux.

2014 sera également une année spéciale, puisque le CERN célébrera ses 60 ans. Mais au-delà de son anniversaire, cette année le CERN prépare le Grand collisionneur de hadrons (LHC) à explorer de nouveaux territoires.

Avec la découverte du boson de Higgs, nous avons complété le Modèle Standard, la théorie actuelle qui explique de quoi toute la matière visible est faite. Mais ce type de matière ne compte que pour 5 % du contenu total de l’Univers. L’existence de matière sombre nous prouve que le modèle actuel est incomplet. Jusqu’ici, l’analyse des données prises à 8 TeV ne révèle pas pour l’instant de traces de cette matière sombre. Pour pousser nos recherches plus loin et plus vite, nous devons augmenter la portée du LHC en allant à plus haute énergie.

C’est pourquoi depuis février 2013 tous les accélérateurs et expériences du CERN sont à l’arrêt afin d’effectuer des travaux de maintenance et de consolidation. Ceci se poursuivra en 2014 pour le LHC, mais plusieurs accélérateurs du complexe du CERN reprendront du service dès cet été.


Le point de départ de la chaîne d’accélérateurs est une simple bouteille d’hydrogène. Les électrons sont arrachés aux atomes d’hydrogène par un champ électrique pour ne laisser que les protons. Ceux-ci sont ensuite accélérés dans un petit accélérateur linéaire (LINAC 2 en bas, au centre du diagramme ci-dessous). L’anneau d’ions de basse énergie (LEIR) joue le même rôle, mais avec des ions lourds.


Les protons obtiennent une poussée supplémentaire dans le Booster avant d’être injectés dans le plus vieil accélérateur du CERN encore en service, le synchrotron à protons (PS). Puis les protons sont dirigés vers le supersynchrotron à protons (SPS) où ils atteignent une énergie de 450 GeV (soit 450 milliards d’électronvolts). C’est l’étape finale avant l’injection dans le LHC où des énergies près de trente fois plus grandes seront atteintes en 2015, soit 13 TeV.

Les faisceaux issus de la chaîne d’accélérateur alimentent aussi d’autres zones expérimentales comme ISOLDE et n-TOF où un très grand nombre d’expériences nucléaires prennent place. D’autres protons sont dirigés vers une cible pour produire des antiprotons pour le Décélérateur d’Antiprotons (AD), un laboratoire consacré à l’étude de l’antimatière. Ces expériences reprendront toutes leurs activités en 2014.
Tous les travaux de consolidation du LHC et de ses expériences s’effectuent en parallèle. ATLAS et CMS prévoient d’achever leurs travaux sur les détecteurs avant novembre. ALICE sera prêt début décembre et LHCb début janvier 2015.

Dans le même temps, tous les physicien-ne-s qui ne sont pas impliqué-e-s dans ces travaux finalisent les analyses des données prises jusqu’en 2013, préparent de nouvelles simulations à plus haute énergie, améliorent les algorithmes de reconstruction des données ou rendent les critères de sélection du système de prise de données plus performant. Tout le monde doit relever le défi d’être prêt à traiter plus de données récoltées à plus haute énergie. Tout ça dans l’espoir que nous serons peut-être récompensé-e-s encore une fois par de nouvelles découvertes puisqu’il reste encore tout un monde à découvrir.

Pauline Gagnon

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Accelerating Down Under

Tuesday, January 14th, 2014

Believe it not, there are particle accelerators to be found beyond the outer-Geneva area – even in such far flung locations as Melbourne, Australia. You may recall from a previous blog post that I befriended some Aussie particle physicists at CERN during the summer who kindly invited me to a two-week accelerator school at the Australian Synchrotron in Melbourne. And here I am!

The Australian Synchrotron opened in 2007 and is the largest stand-alone piece of scientific infrastructure in the southern hemisphere. It is a source of highly intense light which is used for a wide range of research purposes.

The Australian Synchrotron.

The Australian Synchrotron.

Synchrotrons are circular machines which accelerate electrons to extremely high energies, producing electron beams which travel at almost the speed of light. As the beam of electrons takes a circular path around the machine, the electrons emit intense radiation known as synchrotron light. This light is really useful for imaging, analysis and in a wide range of scientific experiments.

In some accelerators, operators attempt to minimize the emission of synchrotron radiation so that particles retain maximum energy for high-energy collisions. For example, in the Large Hadron Collider (LHC), protons are accelerated as they have a much larger mass than electrons and so suffer less from loss of synchrotron radiation. Also, the larger the circumference of the circular path which the particles take, the weaker the synchrotron radiation emission – that’s why the LHC was built with a huge 27 km circumference. Linear accelerators, such as the Stanford Linear Accelerator Center (SLAC) in the US, avoid the emission of synchrotron radiation altogether as particles travel in a straight line.

So a synchrotron’s key output – synchrotron light – is the very same thing which operators of other accelerators voraciously try to minimize.

The Australian Synchrotron’s accelerator school is an intensive two-week course on particle physics and accelerators. It attracts student physicists from across Australia (and occasionally the UK!), as well as lecturers and tutors from leading institutions from across the world.

Stay tuned over the next few weeks to hear about my adventures at the Australian Synchrotron.