## Posts Tagged ‘LHC’

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

Monday, February 10th, 2014

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

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

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

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

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

### Getting to the Bottom of the Higgs

Thursday, January 30th, 2014

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

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

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

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

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

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

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

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

## The Next Step

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

Happy Colliding.

- Richard (@bravelittlemuon)

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

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

### 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

### 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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### No cream, no sugar

Monday, January 6th, 2014

My first visit to CERN was in 1997, when I was wrapping up my thesis work. I had applied for, and then was offered, a CERN fellowship, and I was weighing whether to accept it. So I took a trip to Geneva to get a look at the place and make a decision. I stayed on the outskirts of Sergy with my friend David Saltzberg (yes, that David Saltzberg) who was himself a CERN fellow, and he and other colleagues helped set up appointments for me with various CERN physicists.

Several times each day, I would use my map to find the building with the right number on it, and arrive for my next appointment. Invariably, I would show up and be greeted with, “Oh good, you’re here. Let’s go get a coffee!”

I don’t drink coffee. At this point, I can’t remember why I never got started; I guess I just wasn’t so interested, and may also have had concerns about addictive stimulants. So I spent that week watching other people drink coffee. I learned that CERN depends on large volumes of coffee for its operation. It plays the same role as liquid helium does for the LHC, allowing the physicists to operate at high energies and accelerate the science. (I don’t drink liquid helium either, but that’s a story for another time.)

Coffee is everywhere. In Restaurant 1, there are three fancy coffee machines that can make a variety of brews. (Which ones? You’re asking the wrong person.) At breakfast time, the line for the machines stretches across the width of the cafeteria, blocking the cooler that has the orange juice, much to my consternation. Outside the serving area, there are three more machines where one can buy a coffee with a jeton (token) that can be purchased at a small vending machine. (I don’t know how much they cost.) After lunch, the lines for these machines clogs the walkway to the place where you deposit your used trays.

Coffee goes beyond the restuarants. Many buildings (including out-of-the-way Building 8, where my office is) have small coffee areas that are staffed by baristas (I suppose) at peak times when people who aren’t me want coffee. Building 40, the large headquarters for the CMS and ATLAS experiments, has a big coffee kiosk, where one can also get sandwiches and small pizzas, good when you want to avoid crazy Restaurant 1 lunchtimes and coffee runs. People line up for coffee here during meeting breaks, which usually puts us even further behind schedule.

Being a non-drinker of coffee can lead to some social discomfort. When two CERN people want to discuss something, they often do it over coffee. When someone invites me for a chat over coffee, I gamely say yes. But when we meet up I have to explain that I don’t actually drink coffee, and then sit patiently while they go to get a cup. I do worry that the other person feels uncomfortable about me watching them drink coffee. I could get a bottle of water for myself — even carbonated water, when I feel like living on the edge — but I rarely do. My wife (who does drink coffee, but tolerates me) gave me a few jetons to carry around with me, so I can at least make the friendly gesture of buying the other person’s coffee, but usually my offer is declined, perhaps because the person knows that he or she can’t really repay the favor.

So, if you see a person in conversation in the Restaurant 1 coffee area, not drinking anything but nervously twiddling his thumbs instead, come over and say hello. I can give you a jeton if you need one.

### Higgs Convert

Friday, November 29th, 2013

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

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

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

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

Friday, October 11th, 2013

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

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

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

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

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

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

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

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

À voir également :

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

- Comment chasse-t-on le boson ?

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

### Can the LHC solve the dark matter mystery?

Friday, July 12th, 2013

Last part in a series of four on Dark Matter

After reviewing how dark matter reveals its presence through gravitational effects, the lack of direct evidence of interaction with regular matter and the cosmological evidence supporting its existence, here is what the Large Hadron Collider (LHC) at CERN can do.

We can find dark matter with the LHC but only if dark matter interacts with regular matter. Since we do not know how this may happen, we design traps suited for as many beasts as there are theories. Here are a few.

Supersymmetry

The current theory describing particle physics is the Standard Model. It has been extremely successful, explaining just about everything observed so far. Unfortunately, at higher energy, its equations start to break down.

This is why theorists developed Supersymmetry (or SUSY), building on the Standard Model and extending it further. What is truly remarkable is that this new theory invented to fix the flaws of the Standard Model predicts the existence of particles with the properties expected from dark matter, hence its great popularity.

All would be perfect except that no one has detected any of the many expected supersymmetric particles. This might simply mean that these particles are heavier than the current LHC reach. We will have more chances of discovering them once the LHC resumes in 2015 at much higher energy.

The lightest supersymmetric particle

In the LHC, protons collide, producing large amounts of energy. Since energy, E, and mass, m, are two forms of the same essence as stated by the famous E = mc2, energy can materialise into new particles.  Heavy particles are unstable and quickly decay into lighter ones.

Some variants of SUSY predict that all supersymmetric particles must decay into other supersymmetric particles. Under this assumption, the lightest SUSY particle cannot decay into anything else and remains stable, not interacting with anything else just like dark matter is expected to be.

A typical decay chain is shown above. A supersymmetric quark decays into another SUSY particle, χ2, and a normal quark, q. At the two subsequent stages, an electron or muon (denoted l+ and l-) and lighter SUSY particles are produced. The lightest one, in this case a particle called neutralino χ1, cannot decay into anything else and escapes the detector leaving no signal behind.

Seeing the invisible

An event is a snapshot capturing all lighter particles emitted when an unstable particle decays. And within each event, the energy needs to be balanced. So even when a particle flies across the detector leaving no signal, it can still be detected through the energy imbalance in the event. Invisible particles such as the lightest supersymmetric particles can be detected this way.

Both the CMS and ATLAS collaborations have been looking for events containing large amounts of unbalanced energy accompanying a single photon or a single jet (a jet is a bundle of particles made of quarks).

This figure displays an event from the ATLAS experiment containing a single photon (the energy deposit is shown in yellow around 4 o’clock on the left picture) and the missing energy represented by the pink dashed line around 10 o’clock.

This is exactly what an event containing the lightest supersymmetric particle and a photon would look like. But an event containing a Z boson and a photon would look just the same if the Z boson decayed into two neutrinos (other particles that do not interact with the detector).

Unfortunately, nothing has been observed in any of the channels studied so far that is in excess of what is expected from the background, i.e. other known types of events giving similar signatures.

Unlike the direct dark matter searches, the LHC analyses are sensitive to light dark matter particles. Remember the messy plot I showed about direct searches for dark matter? CMS and ATLAS can help clarify the situation, although their results depend on theoretical assumptions when the direct searches don’t.

Below are the CMS results for a search of events containing a single jet and missing energy.  The horizontal axis gives the mass of the dark matter candidate and the vertical axis, the allowed interaction rate with ordinary matter. Everything above the various lines is excluded. CMS (solid red line) exclude light dark matter particles for large interaction rates, a region inaccessible to XENON100, (solid blue curve) the most powerful experiment for direct dark matter searches.

The Higgs boson and dark matter

Another approach to find dark matter relies on some theories that predict that the Higgs boson could decay into dark matter particles. Higgs bosons can be produced with another boson, such as with a Z boson. If the Higgs boson decays to any type of dark matter, we would only see the decay products of the Z and missing energy for the Higgs boson. Searches for such decays have so far not revealed anything above the expected background level.

A dark parallel world

A group of theorists developed an amazing Theory of Dark Matter incorporating ideas of a Hidden Valley where two worlds would evolve in parallel: our world with Standard Model and the yet undiscovered supersymmetric particles, and a dark world populated with dark particles as depicted below, where each horizontal line represents a particle of a given mass.

The idea is that the LHC could produce heavy supersymmetric particles. These particles would decay in a cascade into lighter ones down to the lightest SUSY one. That particle would be a “messenger” capable of crossing over the Hidden Valley, escaping into the dark sector and becoming invisible to us.

In the dark sector, this particle could decay in a cascade into lighter dark particles until it reaches the lighest supersymmetric dark particle, another messenger capable of tunnelling back to our world where it would reappear into many pairs of electrons or muons.

This may sound like pure science fiction but it is all rooted in sound, but still unproven, physics as a quick check with the original papers cited above will demonstrate.

I was until recently one of the experimentalists looking for signs of this Hidden Valley, selecting events containing regrouped pairs of electrons and muons but so far, nothing has been found.

Experimentalists are still looking, there and in many other places, constantly refining their searches and trying new strategies. If dark matter interacts with matter, we ought to find it.

First part in a Dark Matter series:        How do we know Dark Matter exists?

Second part in a Dark Matter series:   Getting our hands on dark matter

Third part in a Dark Matter series:      Cosmology and dark matter

Pauline Gagnon

### Le LHC résoudra-t-il le mystère de la matière sombre?

Friday, July 12th, 2013

Dernier volet d’une série de quatre sur la matière sombre

Après avoir examiné comment la matière sombre révèle sa présence à travers des effets gravitationnels, l’absence de preuves directes d’interaction avec la matière ordinaire et comment la cosmologie soutient aussi son existence, voici ce que le Grand collisionneur de hadrons (LHC) du CERN peut accomplir.

Nous pourrons peut-être trouver la matière sombre avec le LHC mais seulement si la matière sombre interagit avec la matière ordinaire. Comme nous ne connaissons pas le processus exact, nous élaborons des pièges adaptés à autant de bestioles qu’il y a de théories. En voici quelques-unes.

La supersymétrie
Le Modèle standard, la théorie actuelle décrivant la physique des particules, réussi à expliquer presque tout ce qui a été observé jusqu’à présent. Malheureusement, à plus haute énergie, ses équations ne tiennent plus la route.

C’est pourquoi des théoricien-ne-s ont développé la  supersymétrie  (ou SUSY pour les intimes) qui englobe le modèle standard mais va plus loin. Ce qui est vraiment remarquable, c’est que cette nouvelle théorie élaborée pour corriger les défauts du modèle standard prédit l’existence de particules ayant les caractéristiques de la matière sombre, d’où sa grande popularité.

Tout serait parfait, sauf qu’aucune des nombreuses particules supersymétriques postulées n’a encore été détectée. Est-ce simplement parce que ces particules sont hors de la portée actuelle du LHC ? Nous aurons plus de chances de les découvrir après son redémarrage en 2015 à bien plus haute énergie.

La plus légère des particules supersymétriques
Dans le LHC, les protons entrent en collision, produisant de grandes quantités d’énergie. Puisque l’énergie, E, et la masse, m, sont deux formes d’une même essence comme le stipule la célèbre E = mc2, l’énergie peut se matérialiser en nouvelles particules. Les particules lourdes sont instables et se désintègrent rapidement en plus légères.

Certaines variantes de SUSY prédisent que toutes les particules supersymétriques doivent se désintégrer en d’autres particules supersymétriques. Suivant cette assomption, la particule supersymétrique la plus légère ne peut pas se désintégrer et reste stable, incapable d’interagir avec quoi que ce soit d’autre, exactement comme on s’y attend pour la matière sombre.

Voici une chaîne de désintégration typique. Un quark supersymétrique se désintègre en une autre particule supersymétrique, χ2, et en un quark ordinaire, q. Lors des deux étapes suivantes, un électron ou muon (notés l+ and l-) et des particules supersymétriques plus légères sont produites. La plus légère, dans ce cas particulier une particule appelée neutralino, χ1 ne peut se désintégrer en quoi que ce soit d’autre et s’échappe du détecteur sans laisser de trace.

Voir l’invisible
Un événement est un cliché révélant toutes les particules plus légères émises lors des désintégrations de particules instables. Pour chaque évènement, l’énergie doit être balancée. Ainsi, même lorsqu’une particule traverse le détecteur en ne laissant aucun signal, elle peut être détectée grâce au déséquilibre de l’énergie de cet événement. On détecte donc les particules supersymétriques les plus légères et invisibles de cette façon.

Les collaborations CMS et ATLAS cherchent donc des événements ayant un fort déséquilibre en énergie accompagné soit d’un unique photon soit d’un jet (une gerbe de particules constituées de quarks).

Ci-dessus, on voit un événement capté par l’expérience ATLAS contenant un seul photon (le dépôt d’énergie indiqué en jaune vers 4 heures à gauche et aussi à droite) et l’énergie manquante représentée par la ligne pointillée rose vers 10 heures.

C’est exactement ce à quoi un événement contenant la particule supersymétrique la plus légère et un photon ressemblerait. Mais un événement contenant un boson Z et un photon a la même allure quand le boson Z se désintègre en deux neutrinos (autres particules qui n’interagissent pas avec le détecteur).

Malheureusement, jusqu’à présent, pour les multiples scénarios étudiés, rien n’a été trouvé sauf le bruit de fond attendu, c’est à dire tous les autres types d’événements connus ayant la même signature.

Contrairement aux recherches directes de matière sombre, les analyses du LHC sont sensibles aux particules de matière sombre même légères. Rappelez-vous le diagramme très fouillis que j’ai montré sur les recherches directes de matière sombre? CMS et ATLAS peuvent aider à clarifier la situation, même si leurs résultats dépendent d’hypothèses théoriques contrairement aux recherches directes.

Voici les résultats de l’expérience CMS pour les recherches d’évènements contenant un seul jet et de l’énergie manquante. L’axe horizontal donne la masse du candidat de matière sombre et l’axe vertical, le taux d’interaction avec la matière ordinaire. Toutes les valeurs au-dessus des différentes courbes sont exclues. CMS (ligne rouge) exclue les particules de matière sombre légère ayant un taux d’interaction élevé, une région inaccessible à XENON100 (courbe bleue), l’expérience la plus puissante pour la recherche directe de la matière sombre.

Boson de Higgs et matière sombre
Une autre approche visant à trouver la matière sombre repose sur certaines théories prédisant que le boson de Higgs pourrait se désintégrer en particules de matière sombre.

Les bosons de Higgs sont parfois produits avec un boson Z. Si le boson de Higgs se désintègre en matière sombre, nous verrions seulement les débris du boson Z et de l’énergie manquante pour le boson de Higgs. Les recherches en ce sens ont jusqu’ici rien révélé de plus que le bruit de fond attendu.

Des mondes parallèles
Des théoricien-ne-s ont développé une étonnante théorie de la matière sombre incorporant les idées d’une vallée cachée où deux mondes évolueraient en parallèle: notre monde avec les  particules du modèle standard et celles de la supersymétrie (bien qu’encore inconnues), et un monde complètement séparé peuplé de particules sombres comme illustré ci-dessous. Ici, chaque ligne horizontale représente une particule d’une masse donnée.

L’idée est que le LHC pourrait produire des particules supersymétriques lourdes. Ces particules se désintégreraient en cascade. La plus légère des particules de SUSY serait un “messager” capable de traverser la vallée cachée et de s’échapper dans le secteur sombre, devenant invisible pour nous.

Dans le secteur sombre, cette particule se désintégrerait en une cascade de particules sombres jusqu’à ce qu’elle atteigne la plus légère des particules supersymétriques sombres, un autre messager capable de réapparaître dans notre monde en émettant de nombreuses paires d’électrons ou de muons.

Même si cela ressemble à de la science-fiction, il s’agit bien de physique non vérifiée mais très sérieuse comme en attestent les articles cités ci-dessus.

J’étais jusqu’à tout récemment l’une des expérimentatrices et expérimentateurs à la recherche de signes de cette vallée cachée. Nous sélectionnions des événements contenant des paires regroupées d’électrons et de muons, mais n’avons rien trouvé de plus que le bruit de fond.

Les recherches continuent, là et dans de nombreux autres endroits, tout en raffinant constamment les méthodes et en essayant de nouvelles stratégies. Si la matière sombre interagit avec la matière, nous devrions la trouver.

Premier volet:     Comment sait-on que la matière sombre existe?

Deuxième volet: Comment mettre la main sur la matière sombre

Troisième volet: Cosmologie et matière sombre

Pauline Gagnon

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### How to find a needle in 78 haystacks?

Friday, June 21st, 2013

The Large Hadron Collider (LHC) started a vast consolidation program in March 2013 that will last well into 2015. Everybody at CERN on the accelerators or the experiments is now working hard to complete all needed tasks in time.

The experimental collaborations are currently deploying huge efforts on many fronts. One major task is preparing to deal with the increased data volume the revamped LHC will bring in 2015.

The LHC will resume at higher energy and luminosity, i.e. more intense beams. For the LHCb experiment, since it operates at constant luminosity, higher energy will translate into more tracks per event and almost twice the signal rate. Same situation for the other experiments, ALICE, CMS and ATLAS, but they will also have higher luminosity, meaning having to cope with more collisions occurring simultaneously every time bunches of protons collide in the LHC, making it increasingly difficult to disentangle each recorded event.

To give you an idea, here are three snapshots captured by the ATLAS detector in successive years. The event on the left was taken at low luminosity at the start of the LHC. Very few collisions happened at the same time yielding very few tracks per event as seen on the picture.

Then in 2011, the average number of simultaneous collisions increased to around 12 (centre) and reached up to 40 by the end of 2012 (right).  In 2015, there will be between 60 and 80 superimposed collisions in each event depending on the operating scheme that will be retained. The challenge will be to extract a collision of interest from the huge quantity of tracks in each event.

Hence, much effort is spent improving the simulation, calibration and reconstruction of such events. Physicists are building on the existing techniques to be able to cope with the expected data volume.

The picture above shows a zoomed view of an event in the centre of the CMS detector where 78 proton-proton collisions took place simultaneously (the bright dots on the horizontal axis). The scale here is a few centimetres.

Here, each track corresponds to a charged particle. And each and every one of these tracks must be associated with only one vertex, namely, the point in space where it was created in a proton collision. This way, only the tracks associated to the main collision point will be retained to reconstruct the event.

In the picture above, most tracks come from collisions where the protons barely grazed each other and can be ignored. Only the energetic collisions have a chance to produce the heavy and rare particles we are interested in.

In parallel, all groups are using the opportunity of the shutdown to replace or repair electronic modules, power supplies and other components that failed or showed signs of deterioration during the past three years. New sub-detectors are even being added to increase the detectors performance. For example, the CMS collaboration is extending its muon detector coverage and the ATLAS experiment is adding a fourth layer on its pixel detector. LHCb is replacing its beam pipe and ALICE is doing major upgrades to its calorimeters.

But the main effort for all LHC experiments is still to finalize all analyses using the full data collected so far. Everyone seems to be following my mother’s advice: We must tirelessly revisit our work until it is perfect. (Cent fois sur le métier, remettez votre ouvrage). This is precisely what is happening right now. Each aspect of the data analysis is revisited to reach the full potential of the current data set: calibration, particle identification, background evaluation and signal extraction.

Every collaboration already has dozens of new results ready for the upcoming major summer conferences such as the European Physics Society meeting in mid-July.

Pauline Gagnon