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Posts Tagged ‘susy’

Zooming in on new particles

Friday, September 20th, 2013

The Large Hadron Collider (LHC) at CERN has stopped in the spring to undergo a major consolidation program but this has not stopped the search for new physics. On the contrary, physicists are taking advantage of the interruption to finalise all analyses with the whole data collected so far.

Dozens of new results have been presented by the four LHC experiments at several conferences since the end of operation. While only a handful of these results have made the headlines, a wealth of new information is now available, allowing theorists to refine their models.

Even with the discovery of a Higgs boson, physicists know that the Standard Model of particle physics cannot be the final answer since it has known shortcomings. For example, it fails to provide an explanation for dark matter or why the masses of fundamental particles such as electrons and muons are so different. Another theory called supersymmetry (or SUSY for short) is one of the most popular and most promising ways to extend the Standard Model, but it has yet to manifest itself.

One major difficulty when testing this new theory is the large number of parameters it introduces. To find the new particles predicted by SUSY, we must explore a vast territory spanned by 105 dimensions, corresponding to its 105 free parameters. Finding these new particles is like trying to spot a stranger in a crowd of millions.

Fortunately, theorists have attempted to give us experimentalists some guidance to constrain these parameters using theoretical or experimental considerations. One model that has gained popularity lately is called the phenomenological Minimal Supersymmetric Model or pMSSM and uses only 19 parameters. It takes into account information from all aspects of particle physics, incorporating constraints from the measured characteristics of the Z and Higgs bosons, b-quark physics, astrophysics as well as direct searches for dark matter at underground facilities and supersymmetric particles at the LHC.

Several groups of theorists and experimentalists have combined all these recent results to see which areas of the reduced but still huge parameter space of the pMSSM model are still allowed.

Their approach consists in generating millions of possible values corresponding to the masses and couplings of the hypothesised SUSY particles. The couplings are quantities related to the probability to produce these particles at the LHC.

Then they impose various constraints obtained from the many quantities measured by past and current experiments to see which points among all possibilities are still allowed.

Two theorists, Alex Arbey and Nazila Mahmoudi, and experimentalist Marco Battaglia, contrary to their earlier work, performed their latest scan assuming the four positive results reported by direct dark matter experiments were true dark matter signals to see if these results could be explained within SUSY.

While attempts by other groups were not able to find SUSY scenarios in agreement with the parameters of the possible dark matter signal, their results were rather surprising: they found surviving scenarios pointing to a light neutralino, with a mass of only 10 GeV, twelve times lighter than the Higgs boson. The second lightest particle is the super partner of the bottom quark, called sbottom, at around 20 GeV.

SUSY-predictions

The mass ranges predicted for different SUSY particles coming out of this study. The Higgs boson discovered last summer, h0, is assumed to be the lightest of the five Higgs bosons predicted by SUSY and the lightest SUSY particle is the neutralino, χ0.

If this scenario were correct, why would such a light particle have escaped detection? The reason is that most searches led by the CMS and ATLAS experiments have focused so far on events where a large amount of energy is missing.

This would be the case when some heavy but invisible SUSY particle escapes from our detectors. Such criteria are needed to reduce the overwhelming background and isolate the few events containing traces of SUSY particles. But a light neutralino would only carry a small quantity of energy and would have gone undetected.

While theorists are assessing which corners of the parameter space are still allowed, experimentalists are evaluating the impact of their selection criteria on detecting particles having the characteristics of the remaining allowed regions. New strategies are now being sought to explore this possibility.

Operating the LHC at higher energy and collecting larger datasets starting in 2015 should give definite answers to these questions. These combined efforts may soon pave the way to new discoveries.

Pauline Gagnon

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Le Grand collisionneur de hadrons (LHC) du CERN a cessé d’opérer au printemps pour entreprendre un programme de consolidation majeure, mais la quête pour une « nouvelle physique » se poursuit. Les physicien-ne-s profitent de cette pause pour finaliser toutes leurs analyses avec l’ensemble des données recueillies jusqu’à présent.

Des dizaines de nouveaux résultats ont déjà été présentés par les quatre expériences du LHC lors de diverses conférences depuis la fin des opérations. Bien qu’une poignée seulement de ces résultats ait fait les manchettes, l’information nouvellement disponible permet aux théoricien-ne-s d’affiner leurs modèles.

Même avec la découverte d’un boson de Higgs, les physicien-ne-s savent bien que le Modèle Standard de la physique des particules ne peut pas être la réponse finale car il comporte plusieurs lacunes. Par exemple, il ne fournit aucune explication sur la nature de la matière sombre ou pourquoi les masses des particules fondamentales comme celles des électrons et des muons diffèrent autant.

Une autre théorie appelée supersymétrie (SUSY pour les intimes) offre la possibilité d’étendre le Modèle standard. C’est l’alternative la plus populaire mais encore faut-il arriver à prouver son existence en trouvant les nouvelles particules qu’elle prédit.

La difficulté majeure pour tester cette théorie vient du fait qu’elle introduit de nombreux paramètres. Pour trouver les

nouvelles particules supersymétriques qu’elle prédit, il faut donc explorer un vaste territoire à 105 dimensions, correspondant aux 105 paramètres libres. Trouver ces particules est comme essayer de repérer un visage inconnu dans une foule de millions de personnes.

Heureusement, les théoricien-ne-s tentent d’orienter les expérimentateurs et expérimentatrices en réduisant cet espace autant que possible à l’aide de considérations théoriques et expérimentales. Un modèle qui a gagné en popularité ces derniers temps est appelé le modèle phénoménologique supersymétrique minimal ou pMSSM. Il utilise seulement 19 paramètres.

Ce modèle incorpore l’information provenant de tous les aspects de la physique des particules. Il intègre les contraintes obtenues à partir des mesures des caractéristiques des bosons Z et bosons de Higgs, de la physique du quark b, de l’astrophysique, ainsi que les recherches directes de matière sombre venant des installations sous-terraines et de particules supersymétriques au LHC.

Plusieurs groupes comprenant des théoricien-ne-s et des expérimentateurs et expérimentatrices ont combiné tous ces résultats récents et passés pour déterminer quelles zones de l’espace des paramètres réduit mais toujours considérable du modèle de pMSSM sont toujours permis.

Ils et elles génèrent d’abord des millions de valeurs possibles correspondant aux valeurs de masses et couplages des particules supersymétriques hypothétiques. Les couplages sont en gros des quantités reliées à la probabilité de produire ces particules au LHC.

Ensuite, ils et elles imposent les diverses contraintes obtenues à partir des nombreuses quantités mesurées par les expériences passées et actuelles pour voir quels points parmi toutes ces possibilités demeurent encore autorisés.

Deux théoriciens, Alex Arbey et Nazila Mahmoudi, et un expérimentateur, Marco Battaglia, contrairement à leurs travaux antérieurs, ont inclus dans leur dernière analyse les résultats positifs rapportés par quatre expériences de recherche directe de matière sombre en supposant qu’ils viennent bien de la matière sombre.

Alors que les tentatives d’autres groupes n’avaient pu trouver de scénarios de SUSY en accord avec les possibles signaux de matière sombre,  leurs résultats sont plutôt surprenants: ils ont trouvé des scénarios suggérant la possibilité d’une particule supersymmétrique appelée neutralino qui serait très légère, avec une masse d’à peine 10 GeV, soit douze fois moins que le boson de Higgs. La seconde particule la plus légère serait la super-partenaire du quark b, appelée sbottom, avec une masse d’environ 20 GeV.

SUSY-predictions

La gamme des masses prévues pour les différentes particules supersymétriques ressortant de cette étude. Le boson de Higgs découvert l’été dernier, h0, correspondrait au plus léger des cinq bosons de Higgs prédits par SUSY et la particule de SUSY la plus légère serait le neutralino χ0.

Si ce scénario est correct, comment une particule aussi légère aurait-elle pu échappé à la détection? La raison est simple: la plupart des recherches menées par les expériences CMS et ATLAS ont tenté jusqu’ici de détecter des événements contenant une grande quantité d’énergie manquante.

C’est le cas pour les événements où une particule supersymmetrique lourde et invisible à nos détecteurs s’échappe. De tels critères de sélection sont nécessaires afin de réduire la quantité écrasante de bruit de fond et isoler les rares événements contenant des particules supersymétriques. Mais des neutralinos légers n’emporteraient qu’une petite partie d’énergie et serait donc passée inaperçue.

Pendant que les théoricien-ne-s déterminent quelles régions de l’espace des paramètres sont encore autorisées, les expérimentateurs et expérimentatrices évaluent l’impact de leurs critères de sélection sur la détection des particules ayant les caractéristiques des régions restantes. De nouvelles stratégies sont actuellement recherchées pour explorer cette possibilité.

En opérant le LHC à plus haute énergie en 2015 et en produisant encore plus de données, on pourra obtenir des réponses définitives à ces questions. Ces efforts combinés ouvriront peut-être bientôt la voie à de nouvelles découvertes.

Pauline Gagnon

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Oh what a beautiful day

Tuesday, July 23rd, 2013

In case you hadn’t heard, the past few days have been big days for B physics, i.e. particle physics involving a b quark. On the 18th and 19th, there were three results released in particular, two by LHCb and one by CMS. Specifically, on the 18th LHCb released their analysis of \( B_{(s)}\to\mu\mu\) using the full 3 fb\(^{-1}\) dataset, corresponding to 1 fb\(^{-1}\) of 2011 data at 7 TeVand 2 fb\(^{-1}\) of 2012 data at 8 TeV. Additionally, CMS also released their result using 5 fb\(^{-1}\) of 7 TeV and 30 fb\(^{-1}\) of 8 TeV data.

no FCNC

The decay \(B_{(s)}\to\mu\mu\) cannot decay via tree-level processes, and must proceed by higher level processes ( shown below)

These analyses have huge implications for SUSY. The decay \( B_{(s)}\to\mu\mu\) cannot proceed via tree-level processes, as they would involve flavor changing neutral currents which are not seen in the Standard Model (picture to the right). Therefore, the process must proceed at a higher order than tree level. In the language of Feynman Diagrams, the decay must proceed by either loop or penguin diagrams, show in the diagrams below. However, the corresponding decay rates are then extremely small, about \(3\times10^{-9}\). Any deviation from this extremely small rate, however, could therefore be New Physics, and many SUSY models are strongly constrained by these branching fractions.

The results reported are:

Experiment    \(\mathcal{B}(B_{s}\to\mu\mu)\) Significance \(\mathcal{B}(B\to\mu\mu)\)
LHCb \( 2.9^{+1.1}_{-1.0} \times 10^{-9}\) 4.0\(\sigma\) \(<7.4\times 10^{-10}(95\% CL) \)
CMS \(3.0^{+1.0}_{-0.9}\times 10^{-9}\) 4.3 \(\sigma\) \(< 1.1\times 10^{-9} (95\% CL)\)
bs_loop_penguin

Higher order diagrams

Both experiments saw an excess of events events for the \(B_{s}\to\mu\mu)\) channel, corresponding to \(4.o\sigma\) for LHCb (updated from \(3.5 \sigma\) of last year), and \(4.3\sigma\) for CMS. The combined results will, no doubt, be out very soon. Regardless, as tends to happen with standard model results, SUSY parameter space has continued to be squeezed. Just to get a feel of what’s happening, I’ve made a cartoon of the new results overlaid onto an older picture from D. Straub to see what the effect of the new result would be. SUSY parameter space is not necessarily looking so huge. The dashed line in the figure represents the old result. Anything shaded in was therefore excluded. By adding the largest error on the branching fraction of \(B_s\to\mu\mu\), I get the purple boundary, which moves in quite a bit. Additionally, I overlay the new boundary for \(B\to\mu\mu\) from CMS in orange and from LHCb in green. An interesting observation is that if you take the lower error for LHCb, the result almost hugs the SM result. I won’t go into speculation, but it is interesting.

Cartoon of updated limits

Cartoon of Updated Limits on SUSY from \(B\to\mu\mu\) and \(B_s\to\mu\mu\). Orange Represents the CMS results and green represents LHCb results for \(B_s\to\mu\mu\) . Purple is the shared observed upper limit on \(B\to\mu\mu\). Dashed line is the old limit. Everything outside the box on the bottom left is excluded. Updated from D. Straub (http://arxiv.org/pdf/1205.6094v1.pdf)

 

Additionally, for a bit more perspective, see Ken Bloom’s Quantum Diaries post.

As for the third result, stay tuned and I’ll write about that this weekend!

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Everything must fit nicely together

Tuesday, July 10th, 2012

Yesterday, at the International Conference on High Energy Physics in Melbourne we heard three presentations from ATLAS, CMS and the Tevatron experiments, (D0 and CDF) on the Higgs boson searches. It was great to see how all these results are consistent with each other: between the four experiments, the two accelerators operating at different energies and the six independent decay channels. All give the same picture: we have really found a new boson.

Everybody’s attention is now turned towards establishing the exact identity of this boson. Is it the one predicted by the Standard Model or one of the five Higgs bosons associated with supersymmetry, another theory that attempts to fix the few remaining problems of the Standard Model.

Although the theory was unable to predict the exact mass of the Higgs boson, it provided strong constraints on where it could be found. Many quantities are interconnected by the equations of the Standard Model. This is why we keep improving the uncertainty margin on these quantities. Putting all this information together allows us to check the consistency of the model.

This has been the highlight of many conferences for a decade or two. Each new update showed how much progress had been accomplished when all the measurements were combined in a complex algorithm designed to test the so-called “electroweak” part of the Standard Model all in one go. This is very similar to checking the stability of a very elaborate mobile after modifying each of its components slightly.

Yesterday, for the first time, we saw what the newly measured mass of what is most-likely a Higgs boson adds to this global picture.

The vertical axis shows the measured mass of the W boson and the horizontal axis, the mass of the heaviest quark, the top quark. The blue ellipse is centered on the measured values of these two masses. The ellipse gives the error margin. There is a narrow blue band below the large green band. This represents the actual measured mass of the Higgs boson announced on July 4th, the width being its uncertainty. So as it stands, given the overlap, there is agreement, at least within errors.

The black ellipse is a projection of what this picture will look like once the LHC experiments reduce the uncertainty on the W mass from the current 15 MeV to only 5 MeV. If all is consistent within the Standard Model, the black ellipse will have to overlap with the narrow blue band indicating the Higgs boson mass. If the central value of the W mass does not change, then there will be some inconsistency with the Standard Model (the very narrow blue strip). On the other hand, if supersymmetry or SUSY is the real, more global theory of nature, the green area gives the mass values allowed for the W and top quark. MSSM stands for Minimal Supersymmetric Model and is just one specific model within the vast SUSY space.

This plot might reassure a few: there is still plenty of room for supersymmetry. This theory is far from being dead. But as someone commented: “The huge number of SUSY presentations at this conference was inversely proportional to the number of evidence for it!”

The bets are still open on what will come next. Is this Higgs boson the one predicted by the Standard Model, supersymmetry or some other version? Patience is in order but the answer will eventually come.

Pauline Gagnon

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Hier, lors de la Conférence Internationale de la Physique des Hautes Énergies à Melbourne, les expériences ATLAS, CMS et celles du Tevatron (D0 et CDF) ont fait le point sur le boson de Higgs. C’était super de constater comment tous les résultats sont consistants entre eux: ceux des quatre expériences, des deux accélérateurs opérant à des énergies différentes et des six canaux de désintégration indépendants. Tous donnent le même message: on a bel et bien trouver un nouveau boson.

Prochaine étape: établir s’il s’agit du boson de Higgs prédit par le modèle standard de la physiqe des particules ou d’un des cinq bosons de Higgs venant de la supersymmétrie, une autre théorie qui tente de remédier aux dernières lacunes du modèle standard.

Bien que la théorie soit incapable de prédire exactement la masse du boson de Higgs, elle impose des contraintes strictes sur sa valeur. Ses équations interconnectent plusieurs paramètres. On améliorant la précision des mesures de masses et autres paramètres du modèle et en combinant toute l’information, on peut voir si tout se tient.

Cette vérification a constitué un des points forts des grandes conférences durant les vingt dernières années. A chaque nouvelle mise à jour, on pouvait se rendre compte des progrès accomplis lorsque les mesures les plus récentes étaient incorporées dans un programme complexe conçu pour vérifier la théorie dite « électrofaible » d’un seul coup. Un peu comme si on vérifiait la stabilité d’un immense mobile après avoir modifié chacun de ses éléments.

Hier, pour la première fois, la masse du boson mesurée la semaine dernière était incorporée aux équations pour voir son impact global.

La verticale représente la masse mesurée expérimentalement pour le boson W et à l’horizontale, la valeur de la masse du quark top, le plus lourd de tous les quarks. L’ellipse en bleu est centrée sur la mesure de ces deux quantités. Sa hauteur et largeur correspondent à la marge d’erreur sur ces deux mesures. L’ étroite bande en bleue représente la masse obtenue la semaine dernière pour ce qui pourrait être le boson de Higgs. Le fait que l’ellipse et cette étroite bande se chevauche en partie indique que tout est cohérent avec le modèle standard, du moins à l’intérieur de la marge d’erreur actuelle.

Mais si les expériences du Grand Collisionneur de Hadrons ou LHC réussissent à réduire la marge d’erreur sur la masse du boson W de 15 MeV (valeur actuelle) à 5 MeV (ellipse en noir), et si la valeur centrale des masses du top et du W ne change pas, il y aura une certaine tension i.e. la bande bleue représentant la masse du Higgs ne coïncidera plus avec l’ellipse en noir.

Par contre, la bande verte indique les valeurs encore possible pour les masses du W et du top si le boson de Higgs correspond non pas à celui prédit par le modèle standard, mais plutôt à un des cinq bosons de Higgs postulés par un des modèles de supersymmétrie connu sous le nom de MSSM ou Minimum Supersymmetric Model.

La figure ci-dessus réconfortera quelques personnes: il y a encore beaucoup de place pour la supersymmétrie même si toutes les tentatives actuelles n’ont toujours pas révélées sa présence. Comme un conférencier l’a exprimé hier: « La quantité de présentations à cette conférence sur la supersymmétrie est inversement proportionnelle à son évidence ». Entre temps, le modèle standard demeure toujours valide.

Tous les paris sont ouverts sur ce que l’on va maintenant découvrir. Est-ce le boson de Higgs du modèle standard, de la supersymétrie ou d’une autre théorie? Il faudra encore un peu de patience avant d’en avoir le cœur net mais la réponse viendra éventuellement.

Pauline Gagnon

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Je suis présentement à la  Conférence Internationale de la Physique des Hautes Énergies à Melbourne et les deux dernières journées semblent avoir été une revue des innombrables tentatives infructueuses à briser le Modèle Standard de la physique des particules. Pourquoi tant d’acharnement de la part des physiciens et physiciennes? Ne pourrait-on pas simplement se reposer après avoir enfin trouvé ce qui pourrait bien être le boson de Higgs, le chainon manquant à une théorie si fructueuse?

Bien sûr, nous sommes encore tous fiers ce cet accomplissement mais aussi déjà impatients de passer à l’étape suivante: découvrir quelle théorie plus globale se cache derrière celle qu’on connaît. La moindre déviation dans les prédictions théoriques actuelles pourrait ouvrir la voie vers de nouvelles découvertes. Toutes les expériences scrutent donc ce modèle dans les moindres détails, à la recherche de la moindre faille.

L’expérience LHCb du Grand Collisionneur de Hadrons (LHC) au CERN a montré deux résultats fort intéressants aujourd’hui. Le premier diffère avec un résultat de D0, une expérience menée à Fermilab, où une déviation par rapport à la prédiction du modèle standard avait été rapportée. La mesure faite par LHCb est en accord avec la prédiction du modèle standard et ne peut donc confirmer le résultat de l’expérience D0.

Le second résultat de LHCb établi pour la première fois qu’il existe une petite asymétrie dans certaines désintégrations de mésons B. Les mésons B sont des particules composées d’un quark u et d’un antiquark b. LHCb a observé que ces mésons B se désintègrent plus souvent en un kaon et deux pions, ou en trois kaons, que leur contrepartie d’antimatière, les antimésons B.

De telles différences entre le comportement de la matière et de l’antimatière sont étudiées afin de comprendre pourquoi l’univers a apparemment évolué vers un monde fait entièrement de matière? C’est une des questions fondamentales que la collaboration LHCb cherche à élucider. Chaque petite asymétrie comme celle dévoilée aujourd’hui éclaire un peu la question. En laboratoire, comme dans les collisions produites par le LHC, on crée toujours matière et antimatière en quantités égales. On suppose donc qu’il en fut de même lors du Big Bang.

En parallèle, les expériences CMS et ATLAS qui opèrent elles aussi au LHC, ont montré un nombre impressionnant de résultats portant sur la recherche de nouveaux phénomènes allant au-delà du modèle standard, quelque chose qui révèlerait l’existence de ce que l’on appelle « la nouvelle physique ».

Les deux approches pourraient nous faire avancer d’un pas: soit en détectant directement de nouvelles particules non prédites par la théorie actuelle, soit en décelant une toute petite faille dans le modèle standard. Mais toutes les tentatives à ce jour ont échouées. Ce sera probablement comme pour le boson de Higgs: il nous faudra beaucoup de patience. Et comme disait ma mère: « Cent fois sur le métier, remettez votre ouvrage ». A force de raffiner nos recherches et en éliminant une à une toutes les fausses pistes, la persévérance nous mettra bien sur la bonne piste.

Pauline Gagnon

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So many attempts, so little luck

Sunday, July 8th, 2012

I am attending the International Conference on High Energy Physics in Melbourne and for the last two days, it seems the main theme has been reviewing the many unsuccessful attempts at breaking the Standard Model of particle physics. But why would physicists try to do that? Can’t we just be happy about having found what could be the Higgs boson, the last major missing piece of an extremely successful theory?

Of course, we are still extremely proud of this achievement but finding the secret passage to the next layer of the theory, which every theorist believes exists, is the next step on our agenda. Any deviation from a prediction of the Standard Model would open the door to new discoveries. So every experiment is scrutinizing the model to the minutest detail, trying to find the slightest flaw.

The LHCb experiment at CERN’s Large Hadron Collider (LHC) showed two interesting results today. First they presented a measurement that is different from one reported by D0 from Fermilab two years ago, which showed a deviation from what the Standard Model predits. The LHCb result is consistent with the Standard Model prediction and does not confirm the deviation reported by the D0 experiment.

The second LHCb result established for the first time that there is a slight asymmetry in some specific decays of B mesons. B mesons are composite particles made of a u quark and an anti-b quark. They observed that more B mesons than their antimatter counterparts, anti-B mesons, decay into one kaon and two pions, or into three kaons.

Such asymmetries are studied in the hope of explaining why the universe apparently evolved to be made entirely of matter. When matter is created out of pure energy (like at the time of the Big Bang or out of the energy released in proton collisions in the LHC), matter and antimatter are created in equal amounts. Why did the universe evolve into a place where matter clearly dominates? This is one of the key questions the LHCb collaboration is trying to answer and every small asymmetry, such as the one reported today, sheds a bit of light on this question.

In parallel, both CMS and ATLAS, two multi-purpose experiments operating also at the LHC, showed an impressive number of searches for new phenomena going beyond the Standard Model, something that would reveal the existence of what is referred to as “New Physics”.

Either way will take us ahead: directly, by finding new particles not postulated by the current theory or indirectly, by discovering a flaw in the Standard Model. So far, nothing has emerged. Just as with the quest for the Higgs boson, we have to be patient as many theorists have reminded us already. In the mean time, every new limit, every new measurement steers us in the right direction. As my mother liked to say: “Go over your work a hundred times until it is perfect”. With enough perseverance, by eliminating one by one all the wrong models, we will eventually find the right one.

Pauline Gagnon

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Après les résultats spectaculaires annoncés hier au CERN sur la découverte d’un nouveau boson, la plus grande conférence en physique des particules de l’année a débuté aujourd’hui à Melbourne. Mais cette première présentation sera dure à battre.

Comme plusieurs personnes l’ont mentionné, il est encore tôt pour dire si ce boson est bien le boson de Higgs bien que toutes les chances soient de ce côté. Il faut d’abord établir s’il se comporte exactement comme le boson de Higgs du Modèle Standard. Se désintègre-t-il dans les proportions prescrites par la théorie? Il nous faut donc vérifier tout ça avec la plus grande précision possible, pas que nous soyons compulsifs mais la moindre petite variation pourrait révéler l’entrée du « passage secret ».

Des théoriciens comme Peter Higgs, François Englert et Robert Brout, ont permis cette avancée en postulant en 1964 l’existence du mécanisme de Higgs et du boson de Higgs. Encore aujourd’hui, ce sont souvent les théoriciennes et théoriciens qui nous orientent dans la bonne direction.

Tous et toutes s’entendent à dire que le modèle théorique actuel a ses limites. Le Modèle Standard serait à la physique des particules ce que les quatre opérations de base (addition, soustraction, multiplication et division) sont aux mathématiques. Bien qu’elles suffisent à accomplir la plupart des tâches quotidiennes, on doit à l’occasion faire appel à la géométrie ou au calcul différentiel.

Tout ça pour dire qu’il existe des signes indiquant que le Modèle Standard n’est que la première couche d’une théorie plus complexe. Plusieurs pensent que la couche supérieure est une théorie appelée supersymétrie ou SUSY.

Une des difficultés majeures de cette théorie, c’est qu’elle comporte une centaine de paramètres non définis, ce qui la rend incapable de faire des prédictions concrètes. Sauf si on fixe la valeur de plusieurs de ces paramètres. On a alors des modèles plus gérables, comme par exemple le CMSSM ou Constrained Minimal Supersymmetric Model.

Aujourd’hui, à la Conférence Internationale de Physiques des Hautes Énergies, plusieurs théoricien-ne-s ont discuté de l’impact sur ces modèles de savoir maintenant que la masse du Higgs est 126 GeV. Par exemple, Dmitri Kanikov a montré qu’on peut mettre à profit les différentes interconnections au sein de la théorie pour voir comment les plus récentes limites établies expérimentalement peuvent substantiellement contraindre les paramètres du CMSSM.

Nazila Mahmoudi a quant à elle pousser cette approche un peu plus loin en démontrant qu’on peut non seulement circonscrire les paramètres de modèles tels que ceux du CMSSM, mais aussi ceux de SUSY. Ceci l’a conduite avec ses collègues à réaliser que la toute nouvelle valeur de la masse du boson de Higgs permet déjà d’éliminer certains de ces modèles réduits.

L’axe vertical montre la valeur de la masse du boson de Higgs et les deux traits horizontaux, la marge d’erreur sur cette valeur. Tous les modèles qui tombent en dehors de cette marge comme le « minimal Gauge Mediated SUSY Breaking Model » et le « no-scale » (en gris et en rose sur le graphe) sont éliminés.

Elle s’est montrée très optimiste même si les recherches actuelles au LHC n’ont toujours pas révélé la présence de particules supersymétriques. Elle a démontré qu’en fait il reste encre bien des valeurs permises pour les paramètres de SUSY. Si on ne les a toujours pas observées, ce n’est pas parce qu’elles n’existent pas mais peut-être simplement parce qu’elle sont plus lourdes ou appartiennent à des configurations plus complexes, les rendant plus difficiles à débusquer. En éliminant un à un les modèles erronés, on progresse dans la bonne direction.

Rien de tel qu’une note d’optimisme pour clore cette première journée d’une conférence qui promet.

Pauline Gagnon

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After the spectacular results reported yesterday at CERN on the discovery of a new boson, the largest particle physics conference of the year started today in Melbourne. Such announcement put the bar high for all the speakers.

As many people have pointed out already, it is still early to call the new boson a “Higgs boson” although the odds are really high. First we must check that it behaves exactly like the Higgs boson. Is it produced as often as the Standard Model predicts, and does it decay in the same proportions as expected? Verifying these properties with the highest accuracy will be the main task in the coming months and years. It’s not that physicists are compulsive about precision, but this is exactly where we might find the opening to the “secret passage”.

Theorists like Peter Higgs, François Englert and Robert Brout in 1964 showed us the way when they postulated the existence of the Higgs boson and Higgs mechanism. Today still, theorists are trying to guide the experimentalists in the right direction.

All theorists today agree that our current theoretical model has its limits. The Standard Model appears to be to the world of particle physics what the four basic operations are to mathematics. Most daily tasks are achieved using only additions, subtractions, multiplications and divisions. But we all know that there is more to mathematics: geometry and trigonometry for example are needed to solve more complex problems.

All this to say that there are clear signs that the Standard Model is only the first layer of a more complex theory. Many believe the next layer is a theory called supersymmetry or SUSY.

One major difficulty with this theory is that is has more than 100 free parameters, making it impossible to obtain predictions without assigning fixed values to some of these parameters. This lead to more manageable models, like the Constrained Minimal Supersymmetric Model or CMSSM.

Today, at the International Conference on High Energy Physics, several theorists discussed the impact of the recently revealed mass of the Higgs boson on the CMSSM model. For example, Dmitri Kanikov showed that one can use the intrinsic interconnections within the theory to see how the current limits obtained from the most recent experiments substantially constrain the parameters of the CMSSM.

Nazila Mahmoudi took this approach one step further by imposing constraints not to the CMSSM model but rather to the whole set of free SUSY parameters. This lead her and her colleagues to realize that with the actual searches and mostly, the stringent constraint coming from the Higgs mass at about 126 GeV, many of the constrained models are nearly ruled out.

The vertical axis shows the Higgs boson mass. If one assumes a Higgs mass between 123-129 GeV, scenarios such a minimal Gauge Mediated SUSY Breaking Model and no-scale (shown in gray and magenta) are excluded.

She was very optimistic even though the current searches at the LHC have not yet revealed any new SUSY particles. She showed that in fact there are plenty of values still allowed for the many parameters of SUSY. As she stated, if we have not found SUSY particles yet, it does not mean they are not there but simply that they must be much heavier or belong to more complex configurations, making them harder to find. By eliminating models like that, it helps zoom on the right one.

Nice optimistic way to close this first day of the conference.

Pauline Gagnon

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Update: Section added to include LEP11 Results on Higgs Boson Exclusion (01 Sept 2011)

Expect bold claims at this week’s SUSY 2011 (#SUSY11 on Twitter, maybe) Conference at Fermilab, in Batavia, Illinois. No, I do not have any secret information about some analysis that undoubtedly proves Supersymmetry‘s existence; though, it would be pretty cool if such an analysis does exist. I say this because I came back from a short summer school/pre-conference that gave a very thorough introduction to the mathematical framework behind a theory that supposes that there exists a new and very powerful relationship between particles that make up matter, like electrons & quarks (fermions), and particles that mediate the forces in our universe, like photons & gluons (bosons). This theory is called “Supersymmetry”, or “SUSY” for short, and might explain many of the shortcomings of our current description of how Nature works.

At this summer school, appropriately called PreSUSY 2011, we were additionally shown the amount of data that the Large Hadron Collider is expected to collect before the end of this year and at the end of 2012. This is where the game changer appeared. Back in June 2011, CERN announced that it had collected 1 fb-1 (1 inverse femtobarn) worth of data – the equivalent of 70,000 billion proton-proton collisions – a whole six months ahead of schedule. Yes, the Large Hadron Collider generated a year’s worth of data in half a year’s time. What is more impressive is that the ATLAS and CMS experiments may each end up collecting upwards of 5 fb-1 before the end of this year, a benchmark number a large number of people said would be a “highly optimistic goal” for 2012. I cannot emphasize how crazy & surreal it is to be seriously discussing the possibility of having 10 fb-1, or even 15 fb-1, by the end of 2012.

Figure 1: Up-to-date record of the total number of protons collisions delivered to each of the Large Hadron Collider Detector Experiments. (Image: CERN)

What this means is that by the end of this year, not next year, we will definitely know whether or not the higgs boson, as predicted by the Standard Model, exists. It also means that by next year, experimentalists will be able to rule out the most basic versions of Supersymmetry which were already ruled out by previous, high-precision measurements of previously known (electroweak) physics. Were we to find Supersymmetry at the LHC now and not when the LHC is at designed specifications, which are expected to be reached in 2014, then many physicists would be at a loss trying to rectify why one set of measurements rule out SUSY but another set of measurements support its existence.

What we can expect this week, aside from the usual higgs boson and SUSY exclusion plots, are a set of updated predictions as to where we expect to be this time next year. Now that the LHC has given us more data than we had anticipated we can truly explore the unknown, so trust me when I say that the death of SUSY has been greatly exaggerated.

More on Higgs Boson Exclusion (Added 01 Sept 2011)

This morning a new BBC article came out on the possibility of the higgs being found by Christmas. So why not add some plots, shown at August’s Lepton-Photon 2011 Conference, that show this? These plots were taken from Vivek Sharma’s Higgs Searches at CMS talk.

If there is no Standard Model higgs boson, then the Compact Muon Solenoid Detector, one of the two general purpose LHC detectors, should be able to exclude the boson, singlehandedly, with a 95% Confidence Level. ATLAS, the second of the two general purpose detectors, is similarly capable of such an exclusion.

Figure A: The CMS Collaboration projected sensitivity to excluding the higgs boson with 5 fb-1 at √s = 7 TeV; the black line gives combined (total) sensitivity.

Things get less clear if there is a higgs boson because physical & statistical fluctuations adds to our uncertainty. If CMS does collect 5 fb-1 before the winter shutdown, then it is capable of claiming at least a 3σ (three-sigma) discovery for a higgs boson with a mass anywhere between mH≈ 120 GeV/c2 and mH ≈ 550 GeV/c2 . For a number of (statistical/systematic) reasons, the range might shrink or expand with 5 fb-1 worth of data but only by a few GeV/c2. In statistics, “σ” (sigma) is the Greek letter that represents a standard deviation; a “3σ result” implies that there is only a 0.3% chance of being a fluke. The threshold for discovery is set at 5σ, or a 0.000 06% of being a random fluke.

Figure B: The CMS Collaboration projected sensitivity to discovering the higgs boson with 1 (black), 2 (brown?), 5 (blue), and 10 (pink)  fb-1 at √s = 7 TeV.

By itself, the CMS detector is no longer sensitive. By combing their results, however, a joint ATLAS-CMS combined analysis can do the full 3σ discovery and a 5σ job down to 128 GeV/c2. The 114 GeV/c2 benchmark that physicists like to throw around is lower bound on the higgs boson mass set by CERN’s LEP Collider, which shutdown in 2000 to make room for the LHC.

Figure C: The projected sensitivity of a joint ATLAS-CMS analysis for SM higgs exclusion & discovery for various benchmark data sets.

However, there are two caveat in all of this. The smaller one is that these results depend on another 2.5 fb-1 being delivered by the upcoming winter shutdown; if there are any more major halts in data collection, then the mark will be missed. The second, and more serious, caveat is that this whole time I have been talking about the Standard Model higgs boson, which has a pretty rigid set of assumptions. If there is new physics, then all these discovery/exclusion bets are off. :)

Nature’s Little Secrets

On my way to PreSUSY, a good colleague of mine & I decided to stop by Fermilab to visit a friend and explore the little secret nooks that makes Fermilab, in my opinion, one of the most beautiful places in the world (keep in mind, I really love the Musée d’Orsay). What makes Fermilab such an gorgeous place is that is doubles as a federally sanctioned nature preserve! From bison to butterflies, the lab protects endangered or near-endangered habitats while simultaneously reaching back to the dawn of the Universe. Here is a little photographic tour of some of Nature’s best kept secrets. All the photos can be enlarged by clicking on them. Enjoy!

Figure 2: The main entrance to the Enrico Fermi National Accelerator Laboratory, U.S. Dept. of Energy Laboratory Designation: FNAL, nicknamed Fermilab. The three-way arch that does not connect evenly at the top is called Broken Symmetry and appropriately represents the a huge triumph of Theoretical (Solid State & High Energy) Physics: Spontaneous Symmetry Breaking. Wilson Hall, nicknamed “The High-Rise” can be see in the background. (Image: Mine).

Figure 3: Wilson Hall, named after FNAL’s first director and Manhattan Project Scientist Robert Wilson, is where half of Fermilab’s magic happens. Aside from housing all the theorists & being attached to the Tevatron Control Room, it also houses a second control room for the CMS Detector called the Remote Operations Center. Yes, the CMS Detector can be fully controlled from Fermilab. The photo was taken from the center of the Tevatron ring. (Image: Mine)

Figure 4: A wetlands preserve located at the center of the Tevatron accelerator ring. The preservation has been so successful at restoring local fish that people with an Illinois fishing license (See FAQ) are actually allowed to fish. From what I have been told, the fish are exceptionally delicious the closer you get to the Main Ring. I wonder if it has anything to do with all that background neutrino rad… never mind. :)
Disclaimer: The previous line was a joke; the radiation levels at Fermilab are well within safety limits! (Image: Mine)

Figure 5: The Feynman Computing Center (left) and BZero (right), a.k.a., The CDF Detector Collision Hall. The Computing Center, named after the late Prof. Richard Feynman, cannot be justly compared to any other data center, except with maybe CERN‘s computing center. Really, there is so much experimental computer research, custom built electronics, and such huge processing power that there are no benchmarks that allows for it to be compared. Places like Fermilab and CERN set the benchmarks. The Collider Detector at Fermilab, or CDF for short, is one of two general purpose detectors at Fermilab that collects and analyzes the decay products of proton & anti-proton collisions. Magic really does happen in that collision hall. (Image: Mine)

Figure 6: The DZero Detector Collision Hall (blue building, back), Tevatron Colling River (center) , and Collision Hall Access Road (foreground). Like CDF (Figure 5), DZero is one of two general-purpose detectors at Fermilab that collects and analyzes the decay products of proton & anti-proton collisions. There is no question that the Tevatron generates a lot of heat. It was determined long ago that by taking advantage of the area’s annual rainfall and temperature the operating costs of running the collider could be drastically cut by using naturally replenishable source of water to cool the collider. If there were ever a reason to invest in a renewable energy source, this would be it. The access road doubles as a running/biking track for employees and site visitors. If you run, one question that is often asked by other scientists is if you are a proton or anti-proton. The anti-protons travel clockwise in the Main Ring and hence you are called an anti-proton if you bike/run with the anti-protons; the protons travel counter-clockwise. FYI: I am an anti-proton. (Image: Mine)

Figure 7: The Barn (red barn, right) and American bison pen (fence, foreground). Fermilab was built on prairie land and so I find it every bit appropriate that the laboratory does all it can to preserve an important part of America’s history, i.e., forging the Great American Frontier. Such a legacy of expanding to the unknown drives Fermilab’s mantra of being an “Ongoing Pioneer of Exploring the Frontier of Discovery.” (Image: Mine)

Figure 8: American bison (bison bison) in the far background (click to enlarge). At the time of the photo, a few calves had just recently been born. (Image: Mine)

 

Happy Colliding.

 

- richard (@bravelittlemuon)

 

 

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