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Posts Tagged ‘ICHEP2012’

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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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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ICHEP 2012: Day Three

Friday, July 6th, 2012

Today was another day of parallel talks. There was again six streams of talks where again the topics in each stream varied throughout the day. I would have loved to spend the day in the Higgs stream, listening to the new ATLAS, CMS, CDF and D0 results, however, I spent the day in the heavy flavour physics stream. This was because my talk was scheduled in this stream.


Today, on behalf of the LHCb collaboration, I presented measurements of \(B_s^0\) meson lifetimes. And because I can, I’ll summarise the results I presented for you all. *winks*

First off, I need to give you a bit of background regarding the \(B_s^0\) meson. As the \(B_s^0\) meson is neutral, it can transform, via the box Feynman diagrams on the left to its antimatter partner, the \(\overline{B}_s^0\) meson, and back again. If we look at and manipulate the equations governing the mixing and decay of the \(B_s^0-\overline{B}_s^0\) meson system we find that there are two \(B_s^0\) mass eigenstates, \(B_{s,H}^0\) and \(B_{s,L}^0\), with two different lifetimes, \(\tau_H = 1 / \Gamma_H\) and \(\tau_L = \Gamma_l\) with \(\Delta\Gamma_s = \Gamma_L – \Gamma_H\) and \(\Gamma_s = (\Gamma_L+\Gamma_H)/2\).

We can measure \(\Delta\Gamma_s\) and \(\Gamma_s\) through the analysis of the decay \(B_s^0 \to J/\psi \phi\) and access \(\tau_H\) and \(\tau_L\) from the measurement of the \(B_s^0\) lifetime in \(B_s^0 \to J/\psi f_0(980)\) and \(B_s^0 \to K^+K^-\) decays:

\(\Gamma_s = 0.6580 \pm 0.0054 \pm 0.0066\, {\rm ps}^{−1}\)
\(\Delta\Gamma_s = 0.116 \pm 0.018 \pm 0.006 \,{\rm ps}^{−1}\)
\(\tau_H \simeq \tau_{J/\psi f_0} = 1.700 \pm 0.040 \pm 0.026 \,{\rm ps}\)
\(\tau_L \simeq \tau_{KK} = 1.468 \pm 0.046 \pm 0.006 \,{\rm ps}\)

These results can all be shown as a function of \(\Delta\Gamma_s\) and \(\Gamma_s\) like below. You can see that all the results are fairly consistent, and the experimental combination overlaps all three individual experimental results. It is also consistent with the theoretical prediction of \(\Delta\Gamma_s\).

The measurement of \(B_s^0\) lifetimes and the information they provide regarding \(\Delta\Gamma_s\) is interesting as the value of \(\Delta\Gamma_s\) can be affected by physics beyond the Standard Model…

And that’s it for Day Three of ICHEP 2012 for me. Until Monday everybody!

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ICHEP 2012: Day Two

Friday, July 6th, 2012

Today was another day of parallel talks. There was again six streams of talks, though the topics in each stream varied throughout the day. Additionally, a high school masterclass was run for high school students and a development day for high school teachers.

I mention these because I was asked to give a talk during the development day, introducing the teachers to the main concepts of particle physics. I was really worried about giving this talk as I haven’t had any interaction with high school teachers or students since I was a high school student myself. I also didn’t have time to really practice and refine my presentation. I did one practice talk last night at home, and received so many comments that I ended up completely rewriting the talk overnight. Unfortunately I didn’t have time to practice the final version of the talk, and due to the lack of sleep, got lost a few times during the presentation as well as during the questions. Hopefully the teachers got something out of the presentation, I definitely learnt a lot about presenting to non-experts. A skill that I’m going to be putting into practice very soon, as one of the teachers attending works at my old high school and asked me to go and talk to the students there sometime.

I spent the rest of the day in the heavy flavour physics sessions, which were mostly about searches for rare B, D and Kaon decays. I think I can probably summarise all the talks by saying that nothing was found to be significantly in disagreement of the Standard Model and/or results from other experiments…

And that’s it for Day Two of ICHEP 2012 for me. Until tomorrow everybody!

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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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It’s the day after finding a new particle, which looks an awful lot like the Standard Model Higgs boson. CERN is relatively quiet. Many of us are very, very tired. But new collisions are happening in the LHC, new data are pouring in, and there are new questions now for us to answer. Is this really the Higgs? Does it look exactly the way the Standard Model says it does? Or is it different in some way that will give us clues to new directions in physics? What other particles are still out there for us to discover?

Join me, ATLAS physicist Zach Marshall, and theorist Matt Strassler, as we take a look at some of these questions in a live online Q&A with Nature News. It starts in only an hour and fifteen minutes, at 3:00 PM in Switzerland, 2:00 PM in Britain, and 9:00 AM on the east coast of the United States.

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ICHEP 2012: Day One

Thursday, July 5th, 2012

Today was a jam packed day of parallel talks. There were six streams of talks: theory, heavy flavour (b, c, and s quark) physics, top quark measurements, supersymmetry searches, neutrino physics, and jet physics. I split my time between the various streams, starting the day in the heavy flavour physics stream, and moving to the supersymmetry, then the top quark physics and ending the day back in the heavy flavour physics stream. I thought I would (very briefly) highlight one talk in each session…

Firstly, Leptonic and semileptonic B decays with taus at BaBar by Guglielmo De Nardo of The University of Napoli and INFN. While he didn’t present any new results (they were presented earlier at FPCP), they are still worth highlighting, as they are in tension with the Standard Model. So what results did he present? The measurements of various branching ratios of leptonic and semileptonic B decays with taus, and their ratios. Specifically, R(D) = BF(B ->D τ ν) / BF(B -> D l ν) = and R(D(*)) = BF(B ->D(*) τ ν) / BF(B -> D(*) l ν) = 0.332 ± 0.024 ± 0.018 which in combination exceed the Standard Model predicted values by 3.4σ. Since the results had been presented before, instead of going into the analysis in detail, he presented the analysis within the 2HDM type II theory and concluded that that particular theory couldn’t account for the results.

Secondly, Searches for direct pair production of third generation squarks with the ATLAS detector by Martin White of The University of Melbourne. I’m highlighting this result because of personal reasons. I worked on one of the results presented in this talk during my PhD. Below is a nice summary of the supersymmetric top exclusions he presented. I don’t really want to go into detail about the plot, except to say that no supersymmetric tops have been seen by ATLAS in the six analyses summarised within it.

Thirdly, Tevatron and LHC top mass combinations by Frederic Deliot of The Centre d’Etudes de Saclay. I’m highlighting this talk to be fair to all the experiments. I wouldn’t want to select a CDF result in preference to a D0, ATLAS or CMS one for example. The top quark mass is an interesting measurement as it can be used, in combination with the W boson mass, to predict the Standard Model Higgs boson mass. And so the more precise the measurement of the top quark mass is, the more precise the prediction is. Also, now that we have found a Higgs-like particle at 125 GeV, we could see whether the three masses are consistent within the Standard Model or not. The Tevatron combination is m = 173.18 ± 0.56(stat) ± 0.75(syst) GeV and the LHC combination is m = 173.3 ± 0.5 (stat) ± 1.3 (syst) GeV. It is interesting to note that both values are systematically limited (the systematic error is larger than the statistical one) so it won’t be easy to improve the results.

And finally, Direct CP violation in charm at Belle by Byeong Rok Ko of Korea University. In doing this, I apologise to the BaBar, CDF and LHCb presentations on the same topic, but I choose the Belle presentation as it contained the HFAG combination of all the charm CP violation results.

I blogged about the observation of CP violation in the charm meson system by LHCb when the result was released last year. Since then, BaBar, Belle and CDF have all measured the same quantities and the plot above is the combination of all the results. Numerically, the average \(\Delta A_{CP} = (-0.74±0.15)\%\) and is ~4.9σ away from zero. Which means the LHCb result has been confirmed and we need a theoretical explanation and complementary measurements from other decays…

And that’s it for Day One of ICHEP 2012 for me. Until tomorrow everybody!

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Plus Two

Wednesday, July 4th, 2012

“+2”, those two tiny characters behind the arrival time on my booking confirmation nicely summarize the shape I’m in… Two back to back overnight flights, 25 hours from first take-off to final landing, got me from Munich to Melbourne. A long trip, but that is not the most remarkable thing about my first trip down-under. Usually, when you get off the plane the world is about the same it was when you got on. Not this time though. Particle physics is a lot different: A new particle has been discovered at the LHC, which very much looks like the Higgs boson. This means the ICHEP conference will prove to be particularly exciting, also in view of future projects in particle physics. There will be three talks on physics at linear colliders in the parallel ICHEP tracks, on Top and SUSY, as well as on Higgs physics, with the Higgs talk to be given by myself. Luckily I still have a bit of time to adapt my introduction to the new situation.

So, right now, straight from the airport, I’m getting ready to dive into the latest results… For me mostly SUSY and Top today, I expect. I’m looking forward to an exciting conference!

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