## Posts Tagged ‘LHCb’

### Back From Hibernation, and a Puzzling Asymmetry

Monday, March 4th, 2013

I know in my life at least, there are periods when all I want to do is talk to the public about physics, and then periods where all I would like to do is focus on my work and not talk to anyone. Unfortunately, the last 4 or so months falls into the latter category. Thank goodness, however, I am now able to take some time and write about some interesting physics which had been presented both this year and last. And while polar bears don’t really hibernate, I share the sentiments of this one.

Okay, I swear I'm up this time! Photo by Andy Rouse, 2011.

A little while ago, I posted on Dalitz Plots, with the intention of listing a result. Well, now is the time.

At the 7th International Workshop on the CKM Unitarity Triangle, LHCb presented preliminary results

Asymmetry of $$B^{\pm}\to\pi^{\pm}\pi^+\pi^-$$ as a function of position in the Dalitz Plot. Asymmetry is mapped to the z-axis. From LHCb-CONF-2012-028

for CP asymmetry in the channels $$B\to hhh$$, where $$h$$ is either a $$K$$ or $$\pi$$. Specifically, the presentation was to report on searches for direct CP violation in the decays $$B^{\pm}\to \pi^{\pm} \pi^{+} \pi^{-}$$ and $$B^{\pm}\to\pi^{\pm}K^{+}K^{-}$$.  If CP was conserved in this decay, we would expect decays from $$B^+$$ and $$B^-$$ to occur in equal amounts. If, however, CP is violated, then we expect a difference in the number of times the final state comes from a $$B^+$$ versus a $$B^-$$. Searches of this type are effectively “direct” probes of the matter-antimatter asymmetry in the universe.

Asymmetry of $$B^\pm\to\pi^\pm K K$$ as a function position in the Dalitz plot. Asymmetry is mapped onto the z-axis.From LHCb-CONF-2012-028

By performing a sophisticated counting of signal events, CP violation is found with a statistical significance of $$4.2\sigma$$ for $$B^\pm\to\pi^\pm\pi^+\pi^-$$ and $$3.0\sigma$$ for $$B^\pm\to\pi^\pm K^+K^-$$. This is indeed evidence for CP violation, which requires a statistical significance >3$$\sigma$$.The puzzling part, however, comes when the Dalitz plot of the 3-body state is considered. It is possible to map the CP asymmetry as a function of position in the Dalitz plot, which is shown on the right. It’s important to note that these asymmetries are for both signal and background. Also, the binning looks funny in this plot because all bins are of approximately equal populations. In particular, notice red bins on the top left of the $$\pi\pi\pi$$ Dalitz plot and the dark blue and purple section on the left of the $$\pi K K$$ Dalitz plot. By zooming in on these regions, specifically $$m^2(\pi\pi_{high})>$$15 GeV/c$$^2$$ and $$m^2(K K)<$$3 GeV/c$$^2$$, and separating by $$B^+$$ and $$B^-$$, a clear and large asymmetry is shown (see plots below).

Now, I’d like to put these asymmetries in a little bit of perspective. Integrated over the Dalitz Plot, the resulting asymmetries are

$$A_{CP}(B^\pm\to\pi^\pm\pi^+\pi^-) = +0.120\pm 0.020(stat)\pm 0.019(syst)\pm 0.007(J/\psi K^\pm)$$

and

$$A_{CP}(B^\pm\to\pi^\pm K^+K^-) = -0.153\pm 0.046(stat)\pm 0.019(syst)\pm 0.007(J/\psi K^\pm)$$.

Whereas, in the regions which stick out, we find:

$$A_{CP}(B^\pm\to\pi^\pm\pi^+\pi^-\text{region}) = +0.622\pm 0.075(stat)\pm 0.032(syst)\pm 0.007(J/\psi K^\pm)$$

and

$$A_{CP}(B^\pm\to\pi^\pm K^+K^-\text{region}) = -0.671\pm 0.067(stat)\pm 0.028(syst)\pm 0.007(J/\psi K^\pm)$$.

These latter regions correspond to a statistical significance of >7$$\sigma$$ and >9$$\sigma$$, respectively. The interpretation of these results is a bit difficult: the asymmetries are four to five times that of the integrated asymmetries, and are not necessarily associated with a single resonance. We would expect in the $$\rho^0$$ and $$f_0$$ resonances to appear in the lowest region of $$\pi\pi\pi$$ Dalitz plot, in the asymmetry. In the $$K K\pi$$ Dalitz plot, there are really no scalar particles which we expect to give us an asymmetry of the kind we see. One possible answer to both these problems is that the quantum mechanical amplitudes are only partially interfering and giving the structure that we see. The only way to check this would be to do a more detailed analysis involving a fit to all of the possible resonances in these Dalitz plots. All I can say is that this result is certainly puzzling, and the explanation is not necessarily clear.

Zoom onto $$m^2(\pi\pi)$$ lower axis (left) and $$m^2(K K)$$ axis (right) . Up triangles are $$B^+$$, down are $$B^-$$

### Mixing it up

Wednesday, November 14th, 2012

One of the other results presented at the Hadron Collider Physics Symposium this week was the result of a search for $$D^{0}–\bar{D}^{0}$$ mixing at LHCb.

Cartoon: If a $$D^0$$ is produced, at some time t later, it is possible that the system has "oscillated" into a $$\bar{D}^0$$. This is because the mass eigenstates are not the same as the flavor eigenstates.

Neutral meson mixing is predicted for any neutral meson system, and has been verified for the $$K^0–\bar{ K}^0$$, $$B^0–\bar{B}^0$$ and $$B_s^0–\bar{B_s}^0$$ systems. However, for the $$D^0–\bar{D}^0$$ system, no one measurement has provided a result with greater than $$5\sigma$$ significance that mixing actually occurs, until now.

The actual measurement is of $$R(t)=R$$, which is effectively the Taylor expansion of the time dependent ratio of $$D^0 \rightarrow K^+ \pi^-$$ (“Wrong Sign” decay) to $$D^0\rightarrow K^- \pi^+$$ (“Right Sign” decay). Charge conjugates of these decays are also included. There is a “Wrong Sign” and a “Right Sign” because the Right Sign decays are much more probable, according to the standard model.

The mixing of the $$D^0–\bar{D}^0$$ system is described by the parameters $$x = \Delta m /\Gamma$$ and $$y = \Delta \Gamma / 2\Gamma$$, where $$\Delta m$$ is the mass difference between the $$D^0$$ and $$\bar{D}^0$$, $$\Delta \Gamma$$ is the difference of widths of the mass peaks, and $$\Gamma$$ is the average width. What appears in the description of $$R$$, however, is $$x’$$ and $$y’$$, which give the relations between the $$x$$ and $$y$$ with added information about the strong phase difference between the Right Sign and Wrong Sign decays. The important part about $$x’$$ and $$y’$$ are that they appear in the time dependent terms of the Taylor expansion of $$R$$. If there were no mixing at all, then we would expect the ratio to remain constant, and the higher order time dependence to vanish. If mixing does occur, however, then a clear, non-flat trend should be seen, and hence a measurement of $$x’$$ and $$y’$$. That is why the time dependent analysis is so important.

Fit of ratio of WS and RS decays as a function of decay time of the D meson. Flat line would be no mixing, sloped line indicates mixing. From http://arxiv.org/pdf/1211.1230.pdf

Result of the mixing parameter fit of the neutral D meson system. 1,3 and 5 standard deviation contours are shown, and the + represents no mixing. From http://arxiv.org/pdf/1211.1230.pdf

The result is a 9.1 $$\sigma$$ evidence for mixing, which is also in agreement with previous results from BaBar, Belle and CDF. On top of confirming that the neutral D meson system does mix, this result is of particular importance because, coupled with the result of CP violation in the charm system, it begs the question whether or not there is much more interesting physics beyond the standard model involving charm just waiting to be seen. Stay tuned!

### Huge impact from a tiny decay

Wednesday, November 14th, 2012

The Hadron Collider Physics Symposium opened on November 12 in Kyoto on a grand note. For the first time, the LHCb collaboration operating at the Large Hadron Collider (LHC) at CERN showed evidence for an extremely rare type of events, namely the decay of a Bs meson into a pair of muons (a particle very similar to the electron but 200 times heavier). A meson is a composite class of particles formed from a quark and an antiquark. The Bs meson is made of a bottom quark b and a strange quark s. This particle is very unstable and decays in about a picosecond (a millionth of a millionth of a second) into lighter particles.

Decays into two muons are predicted by the theory, the Standard Model of particle physics, that states it should occur only about 3 times in a billionth of decays. In scientific notation, we write (3.54±0.30)x10-9 where the value of 0.30 represents the error margin on this theoretical calculation. Now, the LHCb collaboration proudly announced that they observed it at a rate of (3.2+1.5-1.2)x10-9 , a value very close to the theoretically predicted value, at least within the experimental error.

Here is the plot shown by the LHCb collaboration for the number of events found in data as a function of the combined mass of the two muons. The solid blue line represents the sum of all types of events from known phenomena containing two muons. The dashed curve in red shows the number of events coming from a Bs meson. With the current error margin on the measurement (shown by the

vertical and horizontal bars on the data points), the data seem to agree with all expected contributions from known sources, leaving little room for new phenomena.

This represents a great achievement, not only because this is the rarest process ever observed, but because it puts stringent limits on new theories. Here is why.

Theorists are convinced that a theory far more encompassing than the Standard Model exists even though we have not detected its presence yet. As if the Standard Model is to particle physics what the four basic operations (addition, multiplication, division and subtraction) are to mathematics. They are sufficient to tackle daily problems but one needs algebra, geometry and calculus to solve more complex problems. And in particle physics, we do have problems we cannot solve with the Standard Model, such as explaining the nature of dark matter and dark energy.

A good place to catch the first signs of “new physics” is where the Standard Model predicts very faint signals such as in Bs mesons decaying into two muons. These decays occur extremely rarely because the Standard Model only has limited ways to produce them. But if an additional mechanism comes into play due to some new theory, we would observe these decays at a rate different from what is expected within the Standard Model.

This is a bit like using the surface of a lake to detect the presence of an invisible creature, hoping its breath would create a ripple on the water surface. It would only work if the lake were extremely calm or disturbed only by an occasional tiny fish.  Here the Standard Model acts like all known little animals creating ripples on the water surface.  The hope was to detect other ripples in the absence of known causes (fish, frogs or mosquitoes). The LHCb result reveals no extra ripples yet. So either the new creature does not breathe as expected or we need to find another method to see it. It will be easier to know once the error margin is reduced with more data.

This new result pushes the reach for new physics even further. Nevertheless, it will help theorists eliminate faulty models like on the plot below and eventually zoom on the right solution. Meanwhile, experimentalists will have to devise yet more stringent tests to be able to discover the way to this new physics.

This plot shows how this measurement (horizontal axis) shown earlier this year reduced the space where new physics could be seen. With this new result, the constraints will even be stronger.

(For more details, see LHCb website)

Pauline Gagnon

### Impact majeur pour une toute petite mesure

Wednesday, November 14th, 2012

Le « Symposium des collisionneurs d’hadrons » s’est ouvert lundi  à Kyoto sur une bonne note.  Pour la première fois, la collaboration LHCb qui opère au Grand Collisionneur de Hadrons (LHC) au CERN a dévoilé la toute première évidence de désintégrations rarissimes, celles de mésons Bs en deux muons (une particule semblable à l’électron mais 200 fois plus lourde). Les mésons forment une classe de particules faites d’un quark et d’un antiquark, ici un quark b et un antiquark s.

Ces mésons sont très instables et se désintègrent en particules plus légères en une picoseconde, soit un millionième de millionième de seconde.

Ces désintégrations en deux muons sont prédites par la théorie, le modèle standard de la physique des particules. Cela devrait se produire environ 3 fois par milliards de désintégrations. En notation scientifique, on écrit (3.54±0.30)x10-9. La valeur de 0.30 représente la marge d’erreur théorique. La collaboration LHCb a donc annoncé avec beaucoup de fierté avoir mesuré (3.2+1.5-1.2)x10-9 ,  soit une valeur très proche de la valeur théorique en tenant compte de la marge d’erreur expérimentale.

Voici la distribution des valeurs obtenues par LHCb pour la masse combinée des deux muons. La courbe continue en bleu représente la somme de toutes les contributions de sources connues contenant deux muons. La courbe rouge en pointillés montre le nombre d’évènements venant de la désintégration de mésons Bs. Les points en noir représentent les données avec leur marge d’erreur (barres verticales et horizontales). Les données semblent correspondre à la somme de toutes les sources connues, laissant peu de place pour de nouveaux phénomènes.

Ce résultat constitue un véritable exploit, non seulement parce que c’est le plus petit taux de désintégration jamais mesuré, mais surtout parce qu’il impose de fortes contraintes sur tous les nouveaux modèles théoriques. Voici pourquoi.

Les théoriciens et théoriciennes sont convaincus qu’il existe une théorie plus complète que le modèle standard actuel, même si on n’a pas encore réussi à en détecter le moindre effet. Un peu comme si le modèle standard était à la physique des particules ce que l’arithmétique (addition, soustraction etc.) est aux mathématiques. Cela suffit amplement pour les opérations quotidiennes mais pour les tâches plus complexes, il nous faut l’algèbre ou le calcul intégral et différentiel.  En physique des particules, nous avons des problèmes que l’on ne peut résoudre avec le modèle standard, comme par exemple il n’explique pas la nature de la matière noire. On cherche donc les premiers signes de la « nouvelle physique ».

Un bon endroit où regarder pour détecter cette nouvelle physique est justement là où la théorie actuelle prédit des phénomènes très rares comme ces désintégrations de mésons Bs. Elles sont justement rarissimes car la théorie n’a que très peu de façons de les produire. Si cette nouvelle physique existe, on soupçonne qu’elle contribuera par de nouveaux mécanismes. On verrait peut-être alors ce genre de désintégrations se produire un peu plus souvent.

C’est un peu comme si on voulait utiliser la surface d’un lac pour déceler la présence d’une créature invisible en espérant voir les rides que son souffle produirait sur l’eau. Bien sûr, cela ne pourrait fonctionner que pour un lac très calme, à peine perturbé par un tout petit poisson ou un insecte dans l’espoir de voir apparaitre des vaguelettes venant d’une autre source. Le résultat actuel de LHCb montre qu’en fait aucune ride n’est visible sur le lac qu’on ne puisse imputer à une cause déjà connue. Si la situation demeure inchangée lorsque la marge d’erreur aura diminuée (en analysant dans les mois à venir les données en cours d’acquisition), on devra conclure que soit la nouvelle créature ne respire pas comme on le pensait, soit qu’il faudra élaborer une méthode plus efficace.

Ce nouveau résultat semble indiquer que la nouvelle physique sera plus difficile à révéler qu’on ne l’espérait. En attendant, cela permettra aux théoriciennes et théoriciens d’éliminer les nouveaux modèles inadéquats, ce qui finira éventuellement par les orienter dans la bonne direction. Entre temps, les expérimentateurs et expérimentatrices devront élaborer des techniques plus raffinées pour enfin mettre le doigt sur cette nouvelle physique.

Ce graphe démontre comment cette mesure (représentée par l’axe horizontal) effectuée plus tôt cette année avait déjà grandement contraint les valeurs permises pour différents modèles. Le nouveau résultat les renforcera encore davantage.

(Pour plus de détails, voir la page publique de LHCb (en anglais seulement))

Pauline Gagnon

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### How is new physics discovered?

Friday, September 28th, 2012

Finding an experimental anomaly is a great way to open the door to a new theory. It is such a good trick that many of us physicists are bending over backward trying to uncover the smallest deviation from what the current theory, the Standard Model of particle physics, predicts.

This is the approach the LHCb collaboration at CERN is pursuing when looking at very rare decays. A minute deviation can be more easily spotted for rare processes. One good place to look is in the rate of K meson decays, a particle made of one strange quark s and one anti-down quark d.

There are in fact two sorts of K mesons: short-lived ones, K0S (called K-short) and long-lived ones, K0L (“K-long”). In the early 1970’s, scientists discovered that the K0L were decaying into a pair of muons 10 000 times less often than the theory predicted. At the time, the theory knew of only three quarks: u, d and s. This hinted three theorists, Sheldon Glashow, John Iliopoulos and Luciano Maiani to propose a mechanism that required the existence of a new, unknown quark, the charm quark c, to explain how this rate could be so suppressed. This explanation is now called the GIM mechanism, an acronym based on their last names.

This major breakthrough on a theoretical level was soon confirmed by the discovery of the charm quark in 1974.

Recently, the LHCb collaboration has turned its attention to measuring the decay rate of the short-lived kaons K0S, the only K mesons decaying fast enough to be seen with precision in their detector.

To make this measurement, they had to select billions of muon pairs and see if any was coming from the decay of a K0S. One can reconstruct the mass of a decaying particle by adding together the mass and momentun of all its fragments. If these muons were coming from the decays of K0S, the reconstructed mass would be the K0S mass. An accumulation of events would appear near this value in the distribution of all the recombined masses.

But as can be seen in the figure below, no such accumulation appears in the region around 500 MeV, the K0S mass value. This allowed the LHCb collaboration to estimate how often a K0S can decay into two muons, a quantity called the branching ratio. They placed a limit at less than 9 times in a billion, or in scientific notation, BR(K0S →μμ ) < 9 x 10-9 with 90% confidence level using all of the 2011 data. Since no peak appears anywhere on this curve, it means the muon pairs were produced in a variety of decays where other particles were also produced.

They have a long way to go since it is still about 2000 times larger than what the Standard Model predicts, namely a branching ratio of 5×10-12. Nevertheless, LHCb is getting closer to the theoretical prediction and eventually, given enough data, they might be able to test it.

Not easy to get to the next layer of the theory when the current one makes predictions requiring thousands of billions of events to be tested.

Pauline Gagnon

### Comment découvrir de nouvelles théories?

Friday, September 28th, 2012

La découverte d’une anomalie expérimentale est une bonne façon d’ouvrir la voie vers de nouvelles théories. Ça marche tellement bien que plusieurs physiciens et physiciennes se consacrent à déceler la moindre déviation par rapport aux prédictions de la théorie actuelle, le Modèle Standard de la physique des particules.

C’est l’approche adoptée par la collaboration LHCb du CERN dans leur recherche de désintégrations extrêmement rares. Dans ce cas, même une minuscule déviation devient visible. Une possibilité consiste à mesurer le taux de désintégration de mésons K, des particules formées d’un quark s et d’un anti- quark d.

Il existe en fait deux versions de ces mésons K: les premiers ont une durée de vie courte, les K0S (pour K-short en anglais) et les autres ont une longue vie, K0L (pour K-long).  Au début des années 70, certain-e-s scientifiques ont découvert que les K0L se désintégraient en une paire de muons environ 10 000 fois moins souvent que ce que prédisait la théorie. A l’époque, la théorie ne comptait que trois quarks: u, d et s.

Cette observation inspira trois théoriciens, Sheldon Glashow, John Iliopoulos et Luciano Maiani. Ils ont proposé une solution qui impliquait l’existence d’un nouveau quark encore inconnu, le quark charmé c. Cette explication porte aujourd’hui le nom de mécanisme de GIM (pour Glashow, Iliopoulos et Maiani). Cette percée au niveau théorique fut confirmée peu de temps après par la découverte du quark charmé en 1974.

Récemment, la collaboration LHCb a décidé de mesurer le taux de désintégration en paires de muons des mésons K de courte durée de vie, les K0S. Les K0S sont les seuls mésons K qui se désintègrent assez vite pour être captés avec précision dans leur détecteur.

Pour ce faire, on a dû sélectionné des milliards d’évènements contenant une paire de muons et voir si certaines paires provenaient de la désintégration d’un K0S. On peut déterminer la masse d’une particule en additionnant la masse et la quantité de mouvement de tous ses fragments après sa désintégration. Si ces paires de muons provenaient d’un K0S, la masse reconstituée aurait la masse du K0S. Une accumulation d’évènements apparaitrait alors tout près de cette valeur dans la distribution de toutes les valeurs de masses reconstituées.

Mais dans le graphe ci-dessous, il n’y a aucune accumulation d’évènements dans la région autour de 500 MeV, la valeur de la masse du K0S. La collaboration LHCb a ainsi pu conclure qu’un K0S ne peut se désintégrer en deux muons que moins de 9 fois sur un milliard.  En termes scientifiques, on écrirait que ce taux, dénoté BR, est BR(K0S →μμ ) < 9 x 10-9 avec un niveau de confiance de 90% et ce, après avoir analyser toutes les donnes récoltées en 2011. Puisqu’aucun pic n’apparaît le long de la courbe, on peut conclure que ces paires de muons venaient d’une variété de désintégrations où d’autres particules furent aussi produites en plus de la paire de muons.

Il reste du chemin à faire puisque la limite mesurée est encore 2000 fois plus élevée que ce que prédit le modèle standard, soit 5×10-12. Néanmoins, LHCb se rapproche de la prédiction théorique et pourra peut-être, avec suffisamment de données, arriver à la tester un jour.

Pas facile de découvrir le prochain pallier théorique quand on doit analyser des milliards d’évènements juste pour tester la théorie actuelle.

Pauline Gagnon

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### Measuring matter-antimatter asymmetries…

Thursday, August 2nd, 2012

I’ve mentioned before that measuring CP violation is important in understanding why we have a matter dominated universe. So far, CP violation has been observed in the decay and mixing of neutral mesons containing strange, charm and bottom quarks and most measurements have been consistent with theory.

However, there is one measurement which has found evidence for significant CP violation in the decays of neutral B mesons, beyond what is expected from theory. In 2010, with an update in 2011, reported an interesting observation: that the number of events containing two positively charged muons is lower than the number of events containing two negatively charged muons. Like-sign dimuons can be produced from the decays of pairs of neutral B mesons, since they can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons is an indication of CP violation. The observed difference was close to 1% and 3.9σ away from the theory prediction. The analysis could not distinguish between the two different neutral B mesons, $$B^0_d$$ and $$B^0_s$$, so the difference had to be expressed in terms of two asymmetries: $$a^d_{sl}$$, the semileptonic asymmetry of $$B^0_d$$ mesons, and $$a^s_{sl}$$, the semileptonic asymmetry of $$B^0_s$$ mesons.

At ICHEP, DØ presented direct measurements of $$a^d_{sl}$$ and $$a^s_{sl}$$, by looking at the decays, $$B^0_d \rightarrow D^{(*)\pm}\mu^\mp X$$ and $$B^0_s \rightarrow D_s^\pm\mu^\mp X$$.

On the left, I have made a plot of these three results, comparing them to the Standard Model predictions. You can see that all three results are somewhat inconsistent with the prediction, which could indicate a contribution from new physics.

But of course, DØ isn’t the only experiment that is able to measure these asymmetries…

$$a^d_{sl}$$ has been previously measured by both Belle and BaBar using $$B^0_d$$ meson pairs produced by the decay of the $$\Upsilon(4S)$$ meson and the results combined by the Heavy Flavour Averaging Group (HFAG).

And… LHCb released a preliminary result for ICHEP, measuring $$a^s_{sl}$$ using $$B^0_s \rightarrow D_s^\pm\mu^\mp X$$ decays.

On the right, I’ve added these results to the DØ ones, and now you can see that the situation now isn’t as compelling for new physics, with the BaBar, Belle and LHCb results all being compatible with the theory.

However, all experimental results are still compatible within two standard deviations, so new results are needed to definitively resolve the issue… Stay tuned to see if this is where evidence of new physics is found!

### Bien plus de tentatives que de chance!

Sunday, July 8th, 2012

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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### Needle in a haystack

Thursday, May 10th, 2012

We are back to discussing B physics today, with the observation of the rare decay: $$B^- \rightarrow \pi^- \mu^+ \mu^-$$. So what is this decay? It’s a $$B^-$$ meson (made of a b and an anti-u quark) decaying into a $$\pi^-$$ meson (made of a d and an anti-u quark) and two muons. And why is it so rare? Well, it’s a flavour changing neutral current decay. Which means that there’s a change in quark flavour in the decay, but not charge. This type of decay is forbidden at tree level in the Standard Model and so has to proceed via a loop, which can be seen in the centre of the Feynman diagram below.

If you look closer at the loop, you can see that for the decay to occur, a b quark needs to change flavour to a t or c quark, which then needs to change to a d quark. This is another reason why this decay is so rare. Transitions in quark flavour are governed by the CKM matrix, which I illustrate on the right, where the larger squares indicate more likely transitions. So while the transition from b to t is likely, the transition from t to d is very unlikely, and the b to c and c to d transitions are both fairly unlikely. This means, that whichever path is taken, the b to d quark transition is very very unlikely.

Okay, now to the LHCb result. Below I have a plot of the fitted invariant mass for selected $$\pi^-\mu^+ \mu^-$$ candidates, showing a clear peak for $$B-$$ decays (green long dashed line). Also shown are the backgrounds from partially reconstructed decays (red dotted line) and misidentified $$K^-\mu^+ \mu^-$$ decays (black dashed line). Candidates for which the $$\mu^+ \mu^-$$ pair is consistent with coming from a $$J/\psi$$ or $$\psi(2S)$$ are excluded.

We see around 25 $$B^- \rightarrow \pi^- \mu^+ \mu^-$$ events and measure a branching ratio of approximately 2 per 100 million decays. This result makes this decay the rarest $$B$$ decay ever observed!