## Archive for November, 2011

### New measurement by OPERA…same strange result?!?!

Friday, November 18th, 2011

So the BBC is reporting that the results of the neutrino run done most recently by the OPERA experiment is confirming their previous result and continuing to find superluminal neutrino speeds.

http://www.bbc.co.uk/news/science-environment-15791236

In this iteration of the experiment OPERA attempted to address, amongst many points, one of their largest sources of uncertainty. Namely, the bunch length of the protons that were being sent from CERN and were producing the neutrinos that they were measuring.

By shortening the bunch widths you have a greater certainty about where the neutrinos are being created and thus you know your initial time to a much higher degree of accuracy.

Needless to say this is a big deal if it is true.

They have updated their paper to include this systematic “fix” as well as complete discussion of various other effects taken into account can be found here on the arxiv.

This is a very exciting find in physics and with the reported plan to submit this paper for review to a journal a final vetting is in due course.

Now we must wait for this experiment to be repeated by the many other long baseline experiments, such as MINOS here at Fermilab and T2K in Japan!

### Un samedi de science: comment susciter l’intérêt des jeunes filles pour les carrières scientifiques ?

Friday, November 18th, 2011

Le 12 novembre, plus de 240 jeunes filles âgées de 9 à 14 ans ont envahi la faculté des sciences de l’université de Genève pour la  seconde édition genevoise d’Élargis tes horizons. Cette initiative, qui a débuté en 1974 aux États-Unis, vise à inciter les jeunes filles à envisager une carrière en science en leur donnant la chance de participer à des ateliers ludiques et interactifs dans divers domaines techniques et scientifiques.

La philosophie d’Élargis tes horizons est de donner l’exemple : tous les ateliers et les stands sont animés par des femmes scientifiques afin que les jeunes filles puissent vraiment s’identifier à elles et s’imaginer à leur place.

Certaines filles ont construit leur propre chambre à brouillard pour « voir l’invisible » : les rayons cosmiques. (photo de Doris Chromek-Burckhart)

Cette année, les participantes, recrutées dans des écoles locales publiques et privées françaises et suisses, ont eu le choix entre 11 ateliers différents où elles pouvaient, par exemple, construire une voiture solaire, programmer un robot, découvrir la chimie à la base de la cuisine ou mouler leur propre médaille.

Les animatrices étaient issues d’organisations et de compagnies telles que Novartis, Merck-Serono, l’EPFL, mais aussi des universités de Lisbonne et de Liverpool. Douze physiciennes du CERN ont animé trois ateliers dans lesquels les jeunes filles ont pu construire une chambre à brouillard pour voir les rayons cosmiques, manipuler des instruments interactifs abordant les questions sans réponses sur lesquelles travaille le CERN ou encore, s’amuser avec de l’azote liquide. Il y avait aussi un stand où les jeunes filles ont pu découvrir le Zoo complet des particules, des posters représentant les femmes des expériences du CERN ainsi que le livre animé en 3D de l’expérience ATLAS. Elles ont pu discuter avec des physiciennes et en apprendre un peu plus sur ce qu’on fait au CERN.

Monica Dunford, qui coordonnait la participation des femmes du CERN à cet événement, captive son auditoire au stand du CERN. (photo de Doris Chromek-Burckhart)

Certaines avaient déjà une bonne idée. Quand j’ai demandé à un groupe de petites filles ce que font les physiciennes, plusieurs mains se sont levées. Une d’elles m’a répondu, toute fière : « Elles font de la physique! »

Difficile de dire qui a eu le plus de plaisir: les animatrices ou les participantes! Dans notre atelier, on a pu plonger des ballons et des bonbons en gélatine dans l’azote liquide ou encore créer un jet d’eau comme celui de Genève en éjectant l’eau d’un contenant scellé sous la pression exercée par l’expension de l’azote liquide.

Création d’un champ de Higgs tout en observant quelles particules (les peluches du Zoo de particules) sont influencées et acquièrent une masse. (photo de Doris Chromek-Burckhart)

En partant, les jeunes filles, ravies par leur journée de science, ont pu emporter plein de souvenirs, y compris des clés USB fournies par le projet Marie Curie – ACEOLE, et des jeux de cartes ou blocs magiques expliquant la physique des particules offerts par le service d’éducation du CERN.

Les projets comme Élargis tes horizons commencent à porter leurs fruits et le nombre de femmes en science est en augmentation. Au CERN, où travaillent près de 10 000 chercheurs et chercheuses venant de centaines d’instituts d’environ 70 pays différents, les femmes comptent pour 18% des physiciens et ingénieurs. Cette fraction est encore plus élevée parmi les jeunes chercheurs et donne une idée de la place qu’occupent les physiciennes dans ces pays.

Pauline Gagnon

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### A Saturday of science: inspiring young girls’ careers

Friday, November 18th, 2011

On 12 November, more than 240 girls aged 9-14 descended upon the Geneva University Science building for the second Geneva edition of Expand Your Horizons. This initiative, started in the United States in 1974, aims to inspire young girls to consider scientific careers by giving them a chance to do fun, hands-on experiments in all sorts of technical and scientific fields.

The philosophy of Expand Your Horizons is to set the example: all workshops and career booths are staffed by women scientists, so that the girls can really identify with the scientists and feel that they could be them.

For the 2011 Geneva event, the participants, recruited from nearby public and private schools, both from France and Switzerland, could choose from 11 different workshops, including building a solar car, programming a robot, discovering the underlying chemistry in a kitchen or casting their own medal.

Some of the girls got to build their own cloud chamber to “see the invisible” cosmic rays.  (photo credit: Doris Chromek-Burckhart)

The workshop leaders came not only from organizations and companies like Novartis, Merck Serono, and EPFL, but also from universities in Lisbon and Liverpool. Twelve female physicists from CERN lead three different workshops where the kids got to build their own cloud chamber to see cosmic rays, play with interactive setups about the unanswered questions we are trying to tackle at CERN, and have cool fun with liquid nitrogen. There was also a booth where the girls had the whole Particle Zoo to play with, posters showing women from CERN experiments, and a pop-up book of the ATLAS detector. They could talk with the physicists, asking all sorts of questions, finding more about CERN and what physicists do.

Monica Dunford, who coordinated the CERN women’s participation in this workshop, enthralls her audience at the CERN booth.  (photo credit: Doris Chromek-Burckhart)

Some already knew though. When I asked the kids attending our workshop: “What does a physicist do?” several hands eagerly shot up in the air. One little girl sitting in the front row proudly answered: “She does physics!”

It was hard to tell who had more fun, the scientists or the kids. In our workshop, we dipped balloons and gummy bears in liquid nitrogen and made a water jet similar to Geneva’s famous landmark, using expanding liquid nitrogen to push water out of a sealed container.

Creating a Higgs field and watching which particles (from the Particle Zoo) are influenced by it and acquire mass. (photo credit: Doris Chromek-Burckhart)

The girls still had wonder in their eyes as they left, taking not only lasting memories of their experience but also goodies like memory sticks – thanks to the Marie Curie – ACEOLE project, playing cards or magic blocks explaining particle physics, courtesy of the CERN education program.

Initiatives like Expand Your Horizons have been paying off and the number of women in scientific fields is increasing. At CERN, where about 10,000 scientists are employed by hundreds of institutes from roughly 70 countries, women scientists now account for about 18% of all physicists and engineers. This percentage is much higher among young scientists and gives the pulse for how women are doing in physics in these countries.

Pauline Gagnon

### An Update from OPERA!

Friday, November 18th, 2011

CERN provided an update regarding the muon-neutrino time of flight measurement performed by the OPERA Collaboration this morning, which I shall now quote:

“Following the OPERA collaboration’s presentation at CERN on 23 September, inviting scrutiny of their neutrino time-of-flight measurement from the broader particle physics community, the collaboration has rechecked many aspects of its analysis and taken into account valuable suggestions from a wide range of sources. One key test was to repeat the measurement with very short beam pulses from CERN. This allowed the extraction time of the protons, that ultimately lead to the neutrino beam, to be measured more precisely.

The beam sent from CERN consisted of pulses three nanoseconds long separated by up to 524 nanoseconds. Some 20 clean neutrino events were measured at the Gran Sasso Laboratory, and precisely associated with the pulse leaving CERN. This test confirms the accuracy of OPERA’s timing measurement, ruling out one potential source of systematic error. The new measurements do not change the initial conclusion. Nevertheless, the observed anomaly in the neutrinos’ time of flight from CERN to Gran Sasso still needs further scrutiny and independent measurement before it can be refuted or confirmed.

On 17 November, the collaboration submitted a paper on this measurement to the peer reviewed Journal of High Energy Physics (JHEP). This paper is also available on the ArXiv preprint server.” [1]

# Comparison with the Original OPERA Measurement

If you’ll recall, the original OPERA experiment measured the time of flight, or how long it takes to go from point A to B, for a muon-neutrino beam traveling from CERN to Gran Sasso (this distance is ~730km).  The muon-neutrino beam under study was created using a proton beam taken from CERN’s Super Proton Synchrotron (SPS).  In this original measurement, the proton beam taken from the SPS was 10.5 microseconds long in time.  The OPERA Collaboration originally reported that the muon-neutrinos traveled 60.7 ± 10.1 nanoseconds faster than the speed of light (FTL)!

But one obvious criticism of the original OPERA Measurement was that they were unable to determine exactly which proton gave rise to a muon-neutrino that struck the OPERA detector.  This is a problem since the proton beam time was several orders of magnitude larger than the originally quoted FTL observation.  e.g. If a muon neutrino strikes the OPERA Detector and was thought to come from the start of the proton beam, but actually came from the end of the proton beam there would be no observed FTL behavior.

Now in the OPERA Collaboration’s newest measurement, they are still unable to determine which proton created a muon-neutrino that struck the OPERA detector. However, the quote above shows that the OPERA Collaboration repeated their measurement using a proton beam that is only three nanoseconds long and this change in proton beam length has not affected their results! So the fact that you don’t know which proton creates the muon neutrino that strikes the your detector no longer matters! This is because the reported excess in the time of flight of the neutrinos is twenty times the proton beam’s time in this new measurement; as opposed to being three orders of magnitude smaller in the original measurement.

# The Future

Whether or not the OPERA publication will be accepted by JHEP, a peer-review journal, remains to be seen.  The OPERA measurement also needs to be confirmed by another experiment, to ensure the phenomenon is actually real.  But, rumor has it that the MINOS and T2K Experiments are gearing up to repeat the OPERA Measurement.  So stay tuned on this rapidly developing phenomenon!

For those of you interested the updated OPERA manuscript can be found on arXiv.org.

Until Next Time,

-Brian

# References

[1] CERN, “OPERA experiment reports anomaly in flight time of neutrinos from CERN to Gran Sasso,” http://press.web.cern.ch/press/pressreleases/releases2011/pr19.11e.html, November 18th, 2011.

### Retreating to the Mountains

Thursday, November 17th, 2011

By now, I can consider myself more or less settled down in Geneva and at CERN. And so far, I’m having a great time.
Two weeks ago, the whole Theory Division went for three days into retreat in the French Alps. Finally, I made it to the Ecole de Physique in Les Houches! The place is very secluded (as is fitting for a retreat), but the view is spectacular. Les Houches, even though a tiny place up in the mountains, is on the map of most theoretical physicists. The school is famous for its several weeks long programs and schools, but for some reason I just never made it there, definitely a gap that had to be filled. The school has one larger building housing the lecture hall and library, the restaurant, and a number of smaller chalet-type houses in which the participants are sleeping. Going “to town” necessitates a 4 km walk steeply downhill (and climbing back up afterwards).
During the Theory Retreat, every fellow had to give a 10-minute presentation, so everyone of the 70-odd people could get to know each other. It was essentially organized for the newly arrived fellows, and I thought it was a great idea. Even though three days were not enough to meet everyone in person, it feels nice to recognize the faces of the people I meet in the hallway every day.

### Partonic matter and perfect fluidity: it’s heavy ions time!

Tuesday, November 15th, 2011

This is becoming a tradition: at the end of the year, the Large Hadron Collider (LHC) replaces the protons in the accelerator with lead nuclei. These are lead atoms stripped of their electrons, with “only” the 208 nucleons (that is protons or neutrons, the particles found inside the nucleus) left in the nucleus. So instead of having single protons colliding, they will soon bring 208 nucleons into collision. This is a bit like playing billiard not with single balls but with 208 agglomerated-balls. It’s bound to bring a few surprises!

Just like regular matter has many phases (water is found in solid, liquid and gaseous forms), nuclear matter, that is, the matter within the atomic nucleus, comes in hadronic or partonic phases.

The hadronic phase could be seen as having in fact a liquid form, when protons and neutrons are bound inside the nucleus just like water molecules, and a gaseous form, when the nuclear bounds are broken, leaving the protons and neutrons free to float around.

Then there is the partonic phase that occurs when enough energy is available to break the nucleons, leaving their constituents called partons, the quarks and gluons, in a free form. This is what is called the quark-gluon plasma, the most energetic soup ever cooked in the whole Universe, and more recently on Earth, thanks to the LHC.

It is believed that the quark-gluon plasma was the state of matter just after the Big Bang. The heat bath at the existing temperature provided enough energy for free quarks to overcome the powerful attraction created by the strong nuclear force, the strongest of all known forces.

This phase of matter is really not well known, having been studied only recently at high-energy accelerators and, of course, when the LHC brought heavy lead ions into collision around this time last year. Amazingly though, the theory describing the strong force called Quantum Chromodynamics (QCD) had predicted that nuclear matter would undergo a phase transition at a given but extremely high temperature.

Last December, under the energy released by lead-lead collisions, we could see a quark-gluon plasma had formed and that these seemingly disconnected particles exhibited a strong common behavior. But exactly how the transition from the hadronic to the partonic phase took place remains to be understood. The theory predicts this transition happens suddenly, like sublimation when ice evaporates, and not progressively like going from ice to water, where the two phases coexist at the liquefaction point. All this needs to be checked experimentally.

This year, having gained much more experience, the LHC team hopes to provide 20 to 40 times more collisions of these heavy ions, enabling the physicists from the ALICE experiment, which is dedicated to this type of research, but also from the ATLAS and CMS collaborations to study the quark-gluon plasma properties.

One spectacular behavior of this matter derived from basic principles by various theoretical methods (QCD, lattice or string theory calculations) all predict that the quark-gluon plasma is a perfect fluid, that is, its viscosity drops to near zero. Its viscosity is even smaller than superfluid helium, which is a mixture of two components, only one of which has zero viscosity. A superfluid does not stay in its container but climbs its walls and spreads out. Only two superfluids are known: liquid helium and Bose-Einstein condensates, when matter lies in the least energetic state near the absolute zero. Surprisingly, this perfect fluidity occurs at thousands of billions of degrees for the quark-gluon plasma while the two superfluids are found at the other extreme temperature near the absolute zero, namely  -273.15 degrees Celsius.

One way to study the quark-gluon plasma is to observe what happens in events where two jets of particles are created from the lead ion collisions. An event is just a snapshot taken when some heavy ions collide, showing all particles coming out of these collisions, very much like watching mini-firework. To conserve energy and momentum, the fragments must fly out in all directions. So when two jets are produced, they should proceed? back-to-back. What was observed last year is that the jet produced at the surface of the quark-gluon plasma could escape whereas the other one, the one pointing towards the hot, dense plasma, was absorbed and scattered by this very dense medium, as shown in the figure below.

Better still is to select events where one jet made of several light particles recoils against a photon. The photon can cross the quark-gluon plasma unaffected, acting as a much better indicator of its original direction than another jet that can also be affected by the plasma. Unfortunately, these events are much more rare but thanks to a new trigger, the ALICE collaboration hopes to find some of these events, even though it is not trivial to distinguish primary photons from secondary photons coming from background sources. The best probe of all is when a Z boson is created with a jet since, like the photon, it is unaffected by the plasma, but unlike the photon, it cannot be coming from a different source.

Despite all appearances, this can be regarded as applied research. The goal is to understand what happened right after the Big Bang, during the phase change from partonic to hadronic phase, all the way to getting clues to matter creation and star formation, to better understand how matter took shape.

Before moving to the ion-ion program, the LHC team will attempt to collide protons with heavy ions in preparation for a new program for next year. Endless fun ahead!

Pauline Gagnon

Collisions of heavy ions where two energetic jets are produced as seen in the ALICE detector, represented here like a cylinder that has been unrolled. In the top figure, a event where the ions grazed each other (peripheral collision) and where both jets are clearly visible. In the bottom figure, a head-on (or central) collision. One jets is absorbed and scattered by the quark gluon plasma while traversing it, while the other, moving away from the plasma, can escape.

### Matière partonique et fluidité parfaite: c’est la saison des ions lourds au LHC

Tuesday, November 15th, 2011

Cela devient une tradition au CERN: à chaque automne, le Grand collisionneur de hadrons (LHC) remplace les protons dans l’accélérateur par des ions de plomb. Il s’agit d’atomes de plomb auxquels on a retiré les électrons, ne laissant « que » les 208 nucléons, c’est-à-dire 208 protons et neutrons, les particules qui se trouvent dans le noyau atomique. Donc au lieu d’amener en collision des protons contre d’autres protons, on génère des collisions d’agglomérats de 208 nucléons. C’est un peu comme jouer au billard, non pas avec une seule boule, mais avec des paquets de 208 boules… De quoi s’attendre à des phénomènes surprenants!

Tout comme la matière ordinaire possède plusieurs phases (on retrouve l’eau sous forme solide, liquide ou gazeuse), la matière nucléaire (celle à l’intérieur des noyaux) peut elle se manifester sous phase hadronique ou partonique.

La phase hadronique prend elle même deux formes: « liquide » lorsque les protons et neutrons sont liés à l’intérieur des noyaux, et « gazeuse », lorsque ceux-ci ont assez d’énergie pour sortir du noyau et coexister librement.

Et puis, il y a la phase partonique, quand les constituants des nucléons eux-mêmes se libèrent. Avec une énergie suffisante, les quarks et les gluons sont libérés : ils flottent côte-à-côte, ignorant l’attraction de la force nucléaire. C’est ce qu’on appelle un plasma de quarks et gluons, la soupe la plus énergétique jamais cuisinée dans l’Univers, et plus récemment, ici même sur terre, grâce au LHC.

On pense que la matière se trouvait dans cet état de plasma de quarks et gluons  juste après le Big Bang. La température régnant à ce moment était telle que  l’énergie était suffisante pour que les quarks et les gluons évoluent librement. L’attraction exercée par la force nucléaire, la plus puissante de toutes les forces connues, était insuffisante pour emprisonner quarks et gluons dans les nucléons.

Cet état de la matière est pratiquement inconnu car ce n’est que récemment qu’on a commencé à en produire avec les accélérateurs de hautes énergies, comme l’an dernier avec les premières collisions d’ions lourds du LHC. Mais la théorie décrivant la force nucléaire, la chromodynamique quantique (QCD), avait déjà prédit que la matière nucléaire passerait de la phase hadronique à la phase partonique à très haute température.

En décembre dernier, sous l’effet de l’énergie dégagée par les collisions d’ions lourds, on a pu observer qu’un plasma de quarks et gluons s’était formé et qu’il se comportait comme un fluide, avec des propriétés communes à l’ensemble. La théorie prévoit que cette transition de phase est soudaine comme la sublimation (lorsque la glace s’évapore) et non pas continue, comme lorsque la glace fond et que les phases liquides et solides coexistent.

Cette année, grâce à toute l’expérience acquise, l’équipe du LHC espère augmenter par un facteur de 20 à 40 la quantité de collisions d’ions lourds produites.  Ceci permettrait aux physiciens et physiciennes de la collaboration ALICE, une expérience dédiée à ce genre de recherches, mais aussi ATLAS et CMS, deux grandes collaborations aux intérêts pluridisciplinaires, d’étudier les propriétés du plasma de quarks et gluons.

Différentes approches théoriques (QCD, réseau ou théorie des cordes) prédisent toutes que ce plasma sera un fluide parfait, avec une viscosité inférieure à celle de l’hélium superfluide, qui est un mélange de deux composantes, une seule ayant une viscosité nulle. Un superfluide ne reste pas dans son contenant mais se met à grimper ses parois pour s’étaler librement.  Or, seules deux substances superfluides sont connues : une forme d’hélium liquide et un condensat de Bose-Einstein, deux types de matières qui n’existent que proche du zéro absolu de température, soit -273.15 degrés Celsius, tandis que le plasma de quarks et gluons existe à des milliers de milliards de degrés…

Une des façons d’étudier ce plasma est d’analyser ce qui se produit lorsqu’un évènement possède deux gerbes de particules émergeant des collisions d’ions lourds. Un évènement est une photo de ce qui se produit immédiatement après une collision et qui montre comment les fragments de ces collisions se dispersent, un peu comme dans de mini feux d’artifice. Pour conserver à la fois énergie et quantité de mouvement, les particules doivent être émises dans toutes les directions. Donc, si seulement deux gerbes de particules sont émises, elles doivent l’être dos-à-dos. Ce qu’on a observé l’an dernier, c’est que si une de ces gerbes était émise près de la surface, elle pouvait s’en échapper tandis que si elle devait traverser le plasma, elle y était absorbée et dispersée. Ce dispersement prouve l’existence d’une substance assez dense pour absorber la gerbe de particules, un phénomène qui n’avait jamais pu être observé.

Cette technique fonctionne encore mieux lorsqu’une gerbe est produite avec un photon ou un boson Z car ces derniers ne sont pas affectés par le plasma, révélant encore plus précisément d’où ils émergent. Malheureusement, ces évènements sont beaucoup plus rares mais grâce à un système de déclenchement amélioré, ALICE espère cette année récolter quelques évènements contenant un photon.

Malgré toutes les apparences, ce genre de recherche peut être qualifié de recherche appliquée. On veut élucider les mécanismes de formation de la matière juste après le Big Bang, lorsqu’elle est passée de la phase partonique à la phase hadronique, ingrédient essentiel pour comprendre comment la matière s’est formée.

Avant de passer au programme de collisions d’ions lourds, l’équipe du LHC tentera demain de faire entrer en collision pour la première fois des protons contre des ions lourds. De quoi s’amuser encore longtemps!

Pauline Gagnon

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Collision d’ions lourds où deux gerbes de particules sont produites dos-à-dos. La grille en bleu représente le détecteur ALICE, mais comme une cylindre mis à plat. En haut, après une collision « périphérique » (où les ions se frôlent) les deux gerbes sont bien visibles, tandis qu’en bas, dans une collision « centrale » (de plein fouet), la gerbe qui pointait vers l’intérieur du plasma de quarks et gluons a été absorbée et diffusée par le plasma.

### 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.

### LHCb reports observation of CP-violation in charm. Welcome New Physics? Or not?

Monday, November 14th, 2011

One of CERN’s collaborations, the LHCb, has reported observation of direct CP-violation in the decays of charmed mesons at the Hadronic Collider Physics Symposium 2011 (HCP 2011) in Paris today. This is a fantastic news! While I am not at HCP 2011, kind folks at LHCb let me know about this fantastic measurement — since charm physics is my specialty.

So, what are we talking about here?

First things first. CP (or Charge Parity) is a set of (discrete) transformations performed on a theory’s Lagrangian — a function that describes what particles we have in a theory and how they interact. If your Lagrangian is symmetric  under this transformation, then particles and antiparticles — matter and antimatter — have the same properties. If not — interactions of matter particles are different from interactions of antimatter particles.  This possible difference is a crucial property of a theory because, according to three Sakharov criteria, the Universe could evolve in what we see around us only if matter and antimatter have different interaction properties. Otherwise, at best, we’d have big chanks of antimatter floating around — or at worst would not not exist at all.

This is why many huge experiments built to study CP violation. Big national labs’ flagship experiments were designed to search and study CP-violation (BaBar at SLAC, Belle at KEK, LHCb at CERN), with hopes to see glimpses of New Physics that could explain matter-antimatter asymmetry in the Universe. This new result from LHCb can in principle provide one.

LHCb experiment

So, what did LHCb see? The reported analysis looks at the difference of a difference — i.e. a difference of CP-violating asymmetries in kaons and pions. The CP-violating asymmetry is defined as the difference between decay widths (roughly speaking, decay probabilities) of a neutral D-meson to decay into a final state, say positive K-meson and a negative K-meson and the same quantity for the D-anti-particle to decay to the same final state. This quantity is also defined for the final state of two pions — and it is CP-violating!

The structure of this CP-violating asymmetry, aCP, is not that simple. Because D0 is a neutral particle it can, in principle, mix with its antiparticle (see here) — and this antiparticle can also decay into the same final state! This process can be also CP-violating (this type of CP-violation is called indirect CP-violation). So the result would depend on both types of CP-violation!

Moreover, experimentally, the asymmetries like this are not easy to measure — there are experimental systematics associated with D-production asymmetries, difference of interactions of positive and negative kaons with matter, etc. For this reason, experimentalists at LHCb decided to report the difference of CP-violating asymmetries, in which many of those effects, like productions asymmetries, would cancel. So, here is the result:

ΔaCP = -0.82 ± 0.21 (stat) ±0.11 (syst)%

In other words, this quantity is 3.5 sigmas away from being zero. The first question that one should ask is whether this quantity is consistent with previous measurements. The biggest question, however, is whether this quantity is consistent with Standard Model expectations.

There is a bunch of previous measurements available for aCP (KK) and aCP (ππ) separately. The thing is that

aCP (KK) = – aCP (ππ)

or approximately so. So by subtracting those quantities we not only subtract the experimental uncertainties, but also enhance the signal! However, looking at the table on page 6 of the talk, one can immediately realize that this measurement is at least consistent with the previous ones.

Is it a sign of something beyond the Standard Model? This one is hard to answer. I usually put an upper bound on the SM value (that is, absolute value) of asymmetries like aCP (KK) at 0.1% — which would make ΔaCP to be about 0.2%. Is it consistent with LHCb findings? Maybe. The size of this asymmetry is notoriously difficult to estimate due to hadronic effects. Maybe it is a sign of New Physics — this could be an exciting conclusion, as we have never seen CP-violation in up-quark sector.

It is interesting that the first “big” result from LHC comes in the realm of charm physics, not Higgs searches. Moreover, all “big” results in the last decade were from the experiments searching for New Physics indirectly, in the “intensity frontier” (this is lingo of US Department of Energy) — with most of them coming from charm physics. Maybe at the very least LHC-b should be renamed as LHC-c?

### 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.