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Posts Tagged ‘CPV’

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

Dalitz Plot Asymmetry for \(B^\pm\to\pi^\pm\pi\pi\)

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 \(B^\pm\to\pi^\pm K K\). From LHCb-CONF-2012-028

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

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


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

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

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


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

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.Zoom of \(m^2(K K)\)

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


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!


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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What does CP violation look like?

Sunday, December 4th, 2011

As promised in my last post, today I’ll be talking about one way we measure CP violation. In particular, I’ll be reporting a direct CP asymmetry result which we released for the EPS conference back in July.

Direct CP violation is conceptually the easiest type of CP violation to understand. It is simply when the amplitude of a process differs from its CP conjugate. For instance, it would be when the branching ratio for the process \(B_{d} \rightarrow K^{+} + \pi^{-}\) differs from \(\bar{B}_{d} \rightarrow K^{-} + \pi^{+}\) or the branching ratio for \(B_{s} \rightarrow K^{-} + \pi^{+}\) differs from \(\bar{B}_{s} \rightarrow K^{+} + \pi^{-}\). The diagram below shows the full \(B_{s}\) and \(\bar{B}_{s}\) decays, including their quark components.

And below are the results… The left two plots show the \(K^{+} + \pi^{-}\) final state, while the right two plots show the \(K^{-} + \pi^{+}\) final state. The bottom two plots are the same as the upper two, just with a different \(y\) scale. The dark blue line shows the full fit, the red line is \(B_{d} \rightarrow K + \pi\), the dark red is wrong sign \(B_{d} \rightarrow K + \pi\), the light blue is misidentified \(B_{d} \rightarrow \pi + \pi\), the yellow line is misidentified \(B_{s} \rightarrow K + K\), the green line is \(B_{s} \rightarrow K + \pi\), the grey line is combinatorial background and orange is three-body partially reconstructed decays.

The extra dashed red line and arrow shows the difference between \(B_{d}\) and \(\bar{B}_{d}\) amplitudes while the dashed green line and arrow shows the difference between the \(B_{s}\) and \(\bar{B}_{s}\) amplitudes.

There you have it. The amplitudes are different. Direct CP violation. Nice, isn’t it?


For the more technically minded out there, we measured the direct CP asymmetries to be:

\(A_{CP}(B_{d} \rightarrow K \pi)=−0.088\pm0.011(stat)\pm0.008(syst)\)
\(A_{CP}(B_{s} \rightarrow \pi K)=0.27\pm0.08(stat)\pm0.02(syst)\)

where the former is the best measurement in the world of that quantity, while the latter is the first evidence of CP violation in that decay.


What exactly is CP violation?

Monday, November 14th, 2011

When we look around ourselves, everything is made up of matter – protons, neutrons and electrons. Even looking out into space, all the planets, stars and gas that we can observe is made up of these particles. There is a cosmological excess of matter over antimatter which is at odds with the theoretical symmetry between them.

The theoretical symmetry between matter and antimatter is more commonly known to particle physicists as CP. If nature treated matter and antimatter alike, then nature would be CP-symmetric. If not, CP is violated.

CP is the combination of two other more fundamental symmetries, Charge conjugation and Parity. C is the symmetry between positive and negative charge while the P is the symmetry of spatial coordinates.

If we take a particle with positive charge, C reverses the charge, meaning the particle will now have negative charge, and vice versa.

Note that if we start with a neutral particle, C will have no effect, since it has no charge.


P is a little harder to explain, though more intuitive, as we encounter a symmetry of spatial coordinates every time we look into a mirror. I am right-handed, but when I look into a mirror, my reflection is left-handed. This almost a perfect analogy to the P symmetry in particle physics, which transforms left-handed particles to right-handed ones.


So the combination of CP on a left-handed, negatively-charged particle would transform it into a right-handed, positively-charged particle.


You may be a little confused as to why I’m describing particles as having a handedness, they obviously don’t have hands or a preference for one over another! It has to do with the fact that all particles have a property called spin, which for simplicity, we can visualise as rotation around an axis. The direction that the particle spins with respect to its direction of motion determines whether it is left-handed or right-handed.

So there you have it. What C, and P and CP are and why we are interested in CP violation. Tune in to my next post on one of the ways we can measure it… And maybe the next next post on another way… And maybe the next next next post on yet another way… Yes, we particle physicists are that interested in CP violation!


Image credits in this post go to Symmetry Magazine and Flip Tanedo.


A New Surprising Result!

Monday, November 14th, 2011

Today is the start of the Hadron Collider Physics conference, which is being held in Paris this year, hosted by IN2P3. If you understood the post from the organisers last week, you would have learnt that a collaboration requested an extra talk to present a new surprising result.

I’m here to tell you that that collaboration was LHCb and, yes we do have a new surprising result:

The first evidence of CP violation in the charm system.
\(\Delta A_{CP} = −0.82 \pm 0.21 (stat) \pm 0.11 (sys) \% \)

This is pretty exciting as the current Standard Model prediction of this effect is 1‰ or less.

Congratulations to everybody involved in the measurement. I know that the team has been working flat out for past few months double testing and triple testing their analysis in preparation for the public release. Unfortunately there is never any rest for the weary as both the LHCb collaboration and theorists have a lot of work ahead of them in the upcoming months. We experimentalists need to include the rest of the 2011 data and there are plans to perform a completely independent measurement of the same quantity using a different analysis strategy. The theorists need to go back and see whether the Standard Model can accommodate such a large amount of CP violation in the charm system or whether the observation needs a new physics explanation.


I sort of embellished for dramatic effect in my last post when I concluded with the statement that: Only more data will tell us the answer to the million dollar question: Is it Standard Model or New Physics?

In the case of the \(B_s\) mixing phase, \(\phi_s\) this isn’t strictly true. More data will improve the measurement of this quantity using the \(B_s \rightarrow J/\psi + \phi\) decay. However, we can measure \(\phi_s\) using other decay modes of the \(B_s\) meson. One such decay is \(B_s \rightarrow J/\psi + f_0\).
As you can see from the Feynmann diagram, this \(J/\psi + f_0\) decay is very similar to the \(J/\psi + \phi\) decay. However, it occurs at a much lower rate. So low in fact, that LHCb was the first experiment to observe it!

This time I’ll present the numerical results for \(\phi_s\) instead of the graphical ones:

The Standard Model prediction is: \(-0.036 \pm 0.002\) rad
The \(J/\psi + \phi\) result is: \(0.13 \pm 0.18 \pm 0.07\) rad
The \(J/\psi + f_0\) result is: \(−0.29 \pm 0.35 \pm 0.02\) rad

The first error value quoted for the experimental results is from statistics, while the second error value from systematic uncertainties.

Looking closely at the numbers, you can see that both experimental results are consistent with each other and with the Standard Model, and both experimental results have larger statistical errors compared to systematic errors.

Comparing the two experimental results now, the \(J/\psi + \phi\) result has a smaller statistical error compared to the \(J/\psi + f_0\) result due to the lower rate of \(J/\psi + f_0\) decays. The \(J/\psi + \phi\) analysis used \(8276 \pm 94\) signal events, while the \(J/\psi + f_0\) analysis only found \(1428 \pm 47\). On the other hand, the \(J/\psi + f_0\) analysis is simpler than the \(J/\psi + \phi\) one, resulting in the smaller systematic error.

Okay, now I can say with a clear conscience: only more data will allow us to measure the \(B_s\) mixing phase, \(\phi_s\) to greater accuracy and determine whether this will be where nature deviates from prediction…


Why B physics? Why not A Physics?

Sunday, August 28th, 2011

In my last post, I showed that LHCb is the best LHC detector for B physics, using the decay of the \(B_s\) meson into a \(J/\psi\) meson and \(\phi\) meson as an example. Today I’m going to try and explain why we want to study this particular decay and show you our latest result.

The reason we are interested in studying the decays of B mesons is that they may shed light on one of the major mysteries of the universe, namely the source of the observed matter-antimatter symmetry. Matter and antimatter are assumed to exist in equal amounts at the beginning of the universe, but as the universe expanded and cooled, an asymmetry developed between them, leaving a universe completely dominated by matter.

The Standard Model predicts an asymmetry between matter and antimatter, but at a level that is too small to explain the observed asymmetry in the Universe. Deviations from the predictions would indicate new physics.

As an aside, the difference between the properties of matter and antimatter is called CP violation. I bring up this factoid as it makes up part of the LHCb logo, which I thought was quite clever when I first saw it.

Anyway, one area in which the Standard Model predicts an asymmetry is the \(B_s\) meson system, that is, anti-\(B_s\) mesons are not exact mirror images[*] of \(B_s\) mesons. This difference is encapsulated in the \(B_s\) mixing[***] phase \(\phi_s\). This phase is what can be measured from the decay, \(B_s \rightarrow J/\psi + \phi\), which we just presented at the Lepton Photon conference in Mumbai.

I’ll spare you all the technical details of the analysis (the details of which should be appearing here soon) and skip to the result…

Okay, I know there’s a lot of information on this graph, so let’s go through it piece by piece. Firstly, the x-axis represents the \(B_s\) mixing phase, \(\phi_s\), while the y-axis represents the \(B_s\) decay width[****] difference \(\Gamma_s\). Both of these properties are shown as it is not possible to measure them independently. The Standard Model prediction for both of these variables is shown as the black point, while the CDF, D0 and LHCb results are shown as coloured contours, with the solid line representing the 68% confidence limit and the dashed line showing the 95% confidence limit.

The results of the measurements favour two regions, one of which is located around the Standard Model prediction, though not centered on it, indicating the possibility for new physics. The LHCb result however, is disappointingly much closer to the prediction than the CDF and D0 results.

Only more data will tell us the answer to the million dollar question: Is it Standard Model or New Physics?

[*] I’m assuming here that you all know what antimatter is. If not, a common analogy is that antimatter is the mirror image of matter. More technically, antimatter has all the same properties of matter, apart from opposite charge and parity[**]. For example, the antimatter particle of a negatively charged left-handed electron is a positively charged right-handed positron.

[**] Parity is another name for chirality, which Flip explains very well in this post.

[***] A very interesting property of neutral mesons, such as the \(B_s\), is that they can spontaneously transform themselves into their own antiparticles (and visa versa). This phenomenon, known as flavor oscillation or mixing and I’ll definitely be discussing it in a future post.

[****] It turns out that one of the possible differences between \(B_s\) mesons and anti-\(B_s\) mesons is a property called decay width.

Oh, if anybody was wondering, there is no such thing as A Physics in particle physics, which is why we don’t study it…


Matter and anti-matter

Tuesday, May 18th, 2010

Recently the D0 collaboration at the Tevatron announced an interesting result.  Having come from the BaBar experiment and worked on CP violation, I found it exciting.  Our universe is dominated by matter.  It’s everywhere and there is almost no anti-matter to be found.  This is one of the principle questions in our sub-atomic understanding of the universe.  The answer to this was put forward many years ago by Sakharov; there has to be CP violation, meaning that the swapping a particle with its anti-particle and looking at the interaction in a mirror can’t be the same as the original.

CP violation was discovered several years ago and has already won Nobel prizes.  The B-factories have measured many of the CP violating parameters of the Standard Model and come up with a rather coherent picture.  These measurements and constraints are embodied in the CKM triangle, where the height of the triangle is a measure of the amount of CP violation.

CKM Triangle

Recent CKM fitter result

It’s beautiful really.  Like a piece of fine art.

Kandinsky composition VIII

Kandinsky Composition VIII

There is just one problem; the amount of CP violation is insufficient by about 10 orders of magnitude. This means that there has to be more CP violation out there that we don’t know anything about.

This is where D0 comes in. They have been looking at CP violation in decays that aren’t accessible at the B-factories, and they found something. The Standard Model says they shouldn’t find much at all, but they did. I think it’s exciting. This isn’t the first sign of stress on the Standard Model, and there will undoubtedly be more coming in the next few years from LHC and other experiments. I think it is an exciting time to be involved in fundamental science research since we will be revising and rewriting many long held theories in the coming decades.

D0 result

D0 asymmetry result is separated from the Standard Model representing the blue point.