## Posts Tagged ‘LHC’

### 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

### The Definite Article Problem

Tuesday, June 4th, 2013

A Little Bit of the Higgs Boson for Everyone

Hi All,

This post is long overdue but nonetheless I am thrilled to finally write it. We have discovered the a some  ??? Higgs boson, and it is precisely my trouble writing this very sentence that inspires a new post. CERN‘s press office has keenly presented a new question in particle physics known as the Definite Article Problem:

Have we discovered “a” Higgs boson or “the” Higgs boson?

We can express the Article problem in another way:

Are there more Higgs bosons?

Before I touch upon that problem, I want to explain about why the Higgs boson is important. In particular, I want to talk about the Sun! Yes, the Sun.

## The Higgs Boson and Electroweak Symmetry Breaking is Important because the Sun Shines.

Okay, there is no way to avoid this: I really like the sun.

Slide Credit: Mine. Image Credit: GOES Collaboration

It shines. It keeps the planet warm. There is liquid water on Earth, and some very tasty plants too.

Slide Credit: Mine. Image Credit: NobelPrize.org

At the heart of the Sun is a ranging nuclear furnace and involves two types of processes: (1) those that involve the Strong nuclear force and (2) those that involve the Weak nuclear force (look for the neutrinos!). The two types of processes work together in a solar relay race to complete a circuit, only to do it over and over again for billions of years. And just like a real relay race, the speed at which the circuit is finished is set by the slowest member. In this case, the Weak force is the limiting factor and considerably slows down the rate at which the sun could theoretically operate. If we make the Weak force stronger, then the Sun would shine more brightly. Conversely, if we make the Weak force even weaker, the Sun would be dimmer.

Slide Credit: Mine. Image Credit: NobelPrize.org

From studying the decays of radioactive substances, we have learned that the rate of Weak nuclear processes is set by a physical constant called Fermi’s Constant. Fermi’s Constant is represented by symbol GF. From study the Higgs boson and the Higgs Mechanism, we have learned that Fermi’s Constant is literally just another constant, v, in disguise. This second physical constant (v) is called the Higgs “vacuum expectation value” , or “vev” for short, and is the amount of energy the Higgs field has at all times relative to the vacuum.

The point I want to make is this: If we increase the Higgs vev, Fermi’s Constant gets smaller, which reduces the rate of Weak nuclear interactions. In other words, a larger Higgs vev would make the sun shine less brightly. Going the other way, a smaller Higgs vev would make the sun shine more brightly. (This is really cool!)

Slide Credit: Mine. Image Credit: Jacky-Boi

The Higgs vev is responsible for some other things, too. It is a source of energy from which all elementary particles can draw. Through the Higgs Mechanism, the Higgs field provides mass to all elementary particles and massive bosons. One would think that for such an important particle we would have a firm theoretical understanding it, but we do not.

Credit: Mine

We have a very poor theoretical understanding of the Higgs boson. Among other things, according to our current understanding of the Higgs boson, the particle should be much heavier than what we have measured.

Credit: Mine

## The Definite Article Problem

There are lots of possible solutions to the problems and theoretical inconsistencies we have discovered relating to the Standard Model Higgs boson. Many of these ideas hypothesize the existence of other Higgs bosons or particles that would interact like the Higgs boson. There are also scenarios where Higgses have identity crises: the Higgs boson we have observed could be a quantum mechanical combination (superposition) of several Higgs bosons.

I do not know if there are additional Higgses. Truthfully, there are many attractive proposals that require upping the number Higgs bosons. What I do know is that our Higgs boson is interesting and merits much further studying.

Credit: Mine

Happy Colliding

- richard (@bravelittlemuon)

PS In case anyone is wondering, yes, I did take screen shots from previous talks and turn them into a DQ post.

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

Wednesday, May 29th, 2013

À l’occasion de l’ouverture de l’appel à candidature 2013 de “Sciences à l’Ecole” pour l’accueil d’enseignants français au CERN durant une semaine, nous publions ces jours-ci le journal quotidien plein d’humour de Jocelyn Etienne qui a suivi ce programme l’année dernière, au mois de novembre dernier.

Dans les cavernes des géants
Mercredi 07 novembre 2012

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

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

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

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

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

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

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

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

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

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

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

À suivre…

Jocelyn Etienne est enseignant au lycée Feuillade de la ville de Lunel.

Pour soumettre sa candidature pour la prochaine session du stage au CERN, c’est par ici.

### Tweeting the Higgs

Wednesday, January 23rd, 2013

Back in July two seminars took place that discussed searches for the Higgs boson at the Tevatron and the LHC. After nearly 50 years of waiting an announcement of a $$5\sigma$$ signal, enough to claim discovery, was made and all of a sudden the twitter world went crazy. New Scientist presented an analysis of the tweets by Domenico et al. relating to the Higgs in their Short Sharp Scient article Twitter reveals how Higgs gossip reached fever pitch. I don’t want to repeat what is written in the article, so please take a few minutes to read it and watch the video featured in the article.

The distribution of tweets around the July 2nd and July 4th announcements (note the log scale)

Instead of focusing on the impressive number of tweets and how many people were interested in the news I think it’s more useful for me as a blogger to focus on how this gossip was shared with the world. The Higgs discovery was certainly not the only exciting physics news to come out of 2012, and the main reason for this is the jargon that was used. People were already familiar with acronyms such as CERN and LHC. The name “Higgs” was easy to remember (for some reason many struggled with “boson”, calling it “bosun”, or worse) and, much to physicists’ chagrin, “God particle” made quite a few appearances too. It seems that the public awareness was primed and ready to receive the message. There were many fellow bloggers who chose to write live blogs and live tweet the event (I like to think that I started bit of a trend there, with the OPERA faster than light neutrinos result, but that’s probably just wishful thinking!) Following the experiences of December 2011, when the webcast failed to broadcast properly for many users had twitter on standby, with tweets already composed, hungry for numbers. The hashtags were decided in advance and after a little jostling for the top spot it was clear which ones were going to be the most popular. Despite all the preparation we still saw huge numbers of #comicsans tweets. Ah well, we can’t win them all!

The point is that while the world learned about the Higgs results I think it’s just as important that we (the physicists) learn about the world and how to communicate effectively. This time we got it right, and I’m glad to see that it got out of our control as well. Our tweets went out, some questions were asked and points clarified and the news spread. I’m not particularly fond of the phrase “God particle” , but I’m very happy that it made a huge impact, carrying the message further and reaching more people than the less sensational phrase “Higgs boson”. Everyone knows who God is, but who is Higgs? I think that this was a triumph in public communication, something we should be building on. Social media technologies are changing more quickly each year, so we need to keep up.

A map of retweets on July 4th, showing the global spread.

But moving back to the main point, the Higgs tweets went global and viral because they were well prepared and the names were simple. Other news included things like the search for the $$B_s$$ meson decaying to two muons and the limits that places on SUSY, but how does one make a hashtag for that? I would not want to put the hashtag #bs on my life’s work. It’s always more exciting to announce a discovery than an exclusion too. The measurement of $$\theta_{13}$$ was just as exciting in my opinion, but that also suffered the same problem. How is the general public supposed to interpret a Greek character and two numbers? I should probably point out that this is all to do with finding the right jargon for the public, and not about the public’s capacity to understand abstract concepts (a capacity which is frequently underestimated.) Understanding how $$\theta_{13}$$ fits in the PMNS mixing matrix is no more difficult than understanding the Higgs mechanism (in fact it’s easier!) It’s just that there’s no nice nomenclature to help spread the news, and that’s something that we need to fix as soon as possible.

As a side note, $$\theta_{13}$$ is important because it tells us about how the neutrinos mix. Neutrino mixing is beyond the Standard Model physics, so we should be getting more excited about it! If $$\theta_{13}$$ is non-zero then that means that we can put another term into the matrix and this fourth term is what gives us matter-antimatter asymmetry in the lepton sector, helping to explain why we still have matter hanging around in the universe, why we have solid things instead of just heat and light. Put like that is sounds more interesting and newsworthy, but that can’t be squeezed into a tweet, let alone a hashtag. It’s a shame that result didn’t get more attention.

It’s great fun and a fine challenge to be part of this whole process. We are co-creators, exploring the new media together. Nobody knows what will work in the near future, but we can look back what has already worked, and see how people passed on the news. Making news no longer stops once I hit “Publish”, it echoes around the world, through your tweets, and reblogs, and we can see its journey. If we’re lucky it gets passed on enough to go viral, and then it’s out of our control. It’s this kind of interactivity that it so rewarding and engaging.

You can read the New Scientist article or the original paper on the arXiV.

### Gluon Walls: A New Form of Matter?

Tuesday, January 8th, 2013

Theoretical physicist Raju Venugopalan

We sat down with Brookhaven theoretical physicist Raju Venugopalan for a conversation about “color glass condensate” and the structure of visible matter in the universe.

Q. We’ve heard a lot recently about a “new form of matter” possibly seen at the Large Hadron Collider (LHC) in Europe — a state of saturated gluons called “color glass condensate.” Brookhaven Lab, and you in particular, have a long history with this idea. Can you tell me a bit about that history?

A. The idea for the color glass condensate arose to help us understand heavy ion collisions at our own collider here at Brookhaven, the Relativistic Heavy Ion Collider (RHIC)—even before RHIC turned on in 2000, and long before the LHC was built. These machines are designed to look at the most fundamental constituents of matter and the forces through which they interact—the same kinds of studies that a century ago led to huge advances in our understanding of electrons and magnetism. Only now instead of studying the behavior of the electrons that surround atomic nuclei, we are probing the subatomic particles that make up the nuclei themselves, and studying how they interact via nature’s strongest force to “give shape” to the universe today.

We do that by colliding nuclei at very high energies to recreate the conditions of the early universe so we can study these particles and their interactions under the most extreme conditions. But when you collide two nuclei and produce matter at RHIC, and also at the LHC, you have to think about the matter that makes up the nuclei you are colliding. What is the structure of nuclei before they collide?

We all know the nuclei are made of protons and neutrons, and those are each made of quarks and gluons. There were hints in data from the HERA collider in Germany and other experiments that the number of gluons increases dramatically as you accelerate particles to high energy. Nuclear physics theorists predicted that the ions accelerated to near the speed of light at RHIC (and later at LHC) would reach an upper limit of gluon concentration—a state of gluon saturation we call color glass condensate.* The collision of these super-dense gluon force fields is what produces the matter at RHIC, so learning more about this state would help us understand how the matter is created in the collisions. The theory we developed to describe the color glass condensate also allowed us to make calculations and predictions we could test with experiments. (more…)

### Higgs update, HCP 2012

Thursday, November 22nd, 2012

Last week, Seth and I met up to discuss the latest results from the Hadron Collider Physics (HCP) Symposium and what they mean for the Higgs searches. We have moved past discovery and now we are starting to perform precision measurements. Is this the Standard Model Higgs boson, or some other Higgs boson? Should we look forward to a whole new set of discoveries around the corner, or is the Higgs boson the final word for new physics that the LHC has to offer? We’ll find out more in the coming months!