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Posts Tagged ‘Pentaquarks’

Hauptgebäude1

“Nous posons des questions depuis 1365”. Cette déclaration inspirante accrochée près de son imposant portail d’entrée, marque le 650ième anniversaire de l’Université de Vienne. Cette bannière accueillait ce matin les 730 physiciens et physiciennes venu assister à la principale conférence de physique des particules de l’année organisée par la Société Européenne de Physique. Pendant une semaine, les participants et participantes devront choisir parmi des centaines de présentations où on fera le tour des connaissances actuelles en physique des particules et des nouvelles avenues avancées. Première énorme surprise : le thermomètre affichait 39˚C hier, la plus haute température jamais enregistrée à Vienne.

Déjà, la première journée comportait son lot de résultats récents et excitants. Tel qu’annoncé la semaine dernière, la collaboration LHCb du CERN a découvert les tous premiers pentaquarks, des particules composées de cinq quarks. Les quarks sont quelques uns des grains de matière fondamentaux. Les physiciens et physiciennes observent déjà depuis des décennies des dizaines de particules faites de deux ou trois quarks. Par exemple, plusieurs particules sont faites d’une paire de quark et d’antiquark. D’autres particules, comme les protons et les neutrons, contiennent trois quarks. Tout récemment, quelques groupes expérimentaux avaient aussi rapporté la découverte de tétraquarks, des objets composés de quatre quarks. Et finalement, la semaine dernière, grâce à l’énorme quantité de données rendues disponibles par le Grand Collisionneur de Hadrons, ou LHC, les scientifiques de l’expérience LHCb ont fièrement annoncé la découverte de pentaquarks. Ils et elles ont ainsi pu réaliser ce que beaucoup d’autres groupes avaient en vain essayé de faire pendant des décennies. On s’attendait à leur existence, mais ils n’avaient jamais été observés auparavant. Ce qui prouve bien qu’il nous reste encore beaucoup à découvrir et à comprendre.

Autre belle nouvelle: l’expérience de neutrinos T2K, qui se déroule au Japon, a peut-être détecté les premiers signes d’oscillations d’antineutrinos. On connaît à ce jour trois types de neutrinos, chacun accompagnant sa propre particule, soit l’électron, le muon ou le tau. Le processus d’oscillation décrit comment des neutrinos d’un type particulier peuvent se changer en un autre type de neutrinos.  Ce phénomène a déjà été observé pour les neutrinos, mais ce serait une première avec les antineutrinos. Mais tout est loin d’être clair, au contraire. D’abord, l’équipe de T2K n’a que trois petits évènements à se mettre sous la dent et il n’y a encore aucune certitude qu’on ait bel et bien affaire à des antineutrinos et non pas des neutrinos. Il faudra encore attendre une année ou deux avant que suffisamment d’évènements soient accumulés pour qu’on puisse en avoir le cœur net. Mais si c’était le cas, cela nous en apprendrait davantage sur les similitudes ou différences entre matière et antimatière.

Plusieurs expériences essaient aussi d’établir s’il n’existerait pas un autre type de neutrinos, appelés neutrinos stériles, dont le spin serait l’inverse des autres neutrinos, c’est-à-dire qu’ils tourneraient sur eux-mêmes dans le sens inverse des neutrinos habituels. Bien sûr, toute découverte de nouvelles particule est à suivre. Mais la confirmation de l’existence de neutrinos stériles tout particulièrement. Cela enverrait une onde de choc en physique des particules car ce serait une observation directe d’une physique nouvelle bien plus vaste que le Modèle Standard actuel. Il faudrait alors tout revoir. Et qui sait? Les physiciens et physiciennes pourraient bien avoir de quoi continuer à se poser des questions pendant les 650 années à venir…

Pauline Gagnon

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution ou consultez mon site web

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Hauptgebäude1« We have been asking questions since 1365 ». This inspiring statement marking the 650th anniversary of the University of Vienna, is hanging near the imposing entrance of its main building. This sign welcomed this morning the 730 physicists who came to Vienna to participate to the main particle physics conference this year organised by the European Physics Society. For a week, the participants will have to choose among hundreds of presentations where the current status of knowledge in particle physics will be presented along with the newest avenues. and we are already making history: Vienna recorded yesterday its highest ever temperature with 39 ˚C.

And this first day brought recent and exciting results. As announced last week, the LHCb collaboration at CERN has discovered the first pentaquarks, composite objects made of five quarks. Quarks are some of the building blocks of matter. Physicists have observed for decades dozens of different particles made of two or three quarks. For example, many particles are made of a pairs of quark and antiquark, while others, like protons and neutrons, contain three quarks. However, in recent years, a few experimental groups also reported the discovery of tetraquarks, objects composed of four quarks. Finally, last week, thanks to the huge dataset made available by the Large Hadron Collider, scientists from the LHCb experiment achieved what many other groups had tried to do for decades without success, and proudly announced the discovery of pentaquarks. Such composite objects were expected but never observed before. It goes to show how much we still have to discover and understand.

Another nice piece of news: the T2K neutrino experiment, which takes place in Japan, may have detected the first signs for oscillations of antineutrinos. To this day, there are three known types of neutrinos, each one accompanying its own particle, namely the electron, the muon and the tau. The oscillation process describes how one type of neutrinos can change into another type. This phenomenon has already been observed for neutrinos, but it would be the first observation for antineutrinos. However, all is far from being set in concrete yet, quite the contrary. With only three events at hands, the T2K team still needs to verify if these events really involve antineutrinos and not just neutrinos. They therefore need to collect more date for another year or two before this issue can be settled. If it turns out to be indeed antineutrinos, we would learn more on the similarities or the differences between matter and antimatter.

Several experiments are also trying to establish if there could also be another type of neutrinos, called sterile neutrinos. Their spin would be the opposite of known neutrinos, meaning they would be spinning on themselves in the opposite direction. Clearly, any new type of particle is something worth watching for. The confirmation of the existence of sterile neutrinos would send a shock wave in particle physics since it would constitute an indisputable proof for the existence of a theory more encompassing than the current theoretical model, called the Standard Model. Everything would then have to be rethought. And who knows? Physicists could very well have enough to keep asking difficult questions for another 650 years…

Pauline Gagnon

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Hi All,

Exciting news came out the Japanese physics lab KEK (@KEK_jp, @KEK_en) last week about some pretty exotic combinations of quarks and anti-quarks. And yes, “exotic” is the new “tantalizing.” At any rate, I generally like assuming that people do not know much about hadrons so here is a quick explanation of what they are. On the other hand, click to jump pass “Hadrons 101” and straight to the news.

Hadrons 101: Meeting the Folks: The Baryons & Mesons

Hadrons are pretty cool stuff and are magnitudes more quirky than those quarky quarks. The two most famous hadrons, the name for any stable combination of quarks and anti-quarks, are undoubtedly the proton and the neutron:

According to our best description of hadrons (Quantum Chromodynamics), the proton is effectively* made up two up-type quarks, each with an electric charge of +2/3 elementary charges**; one down-type quark, which has an electric charge of -1/3 elementary charges; and all three quarks are held together by gluons, which are electrically neutral. Similarly, the neutron is effectively composed of two down-type quarks, one up-type quark, and all the quarks are held strongly together by gluons. Specifically, any combination of three quarks or anti-quarks is called a baryon. Now just toss an electron around the proton and you have hydrogen, the most abundant element in the Universe! Bringing together two protons, two neutrons, and two electrons makes helium. As they say, the rest is Chemistry.

However, as the name implies, baryons are not the only type of hadrons in town. There also exists mesons, combinations of exactly one quark and one anti-quark. As an example, we have the pions (pronounced: pie-ons). The π+ (pronounced: pie-plus) has an electric charge of +1 elementary charges, and consists of an up-type quark & an anti-down-type quark. Its anti-particle partner, the π (pronounced: pie-minus), has a charge of -1, and is made up of an anti-up-type quark & a down-type quark.

 

If we now include heavier quarks, like strange-type quarks and bottom-type quarks, then we can construct all kinds of baryons, mesons, anti-baryons, and anti-mesons. Interactive lists of all known mesons and all known baryons are available from the Particle Data Group (PDG)***. That is it. There is nothing more to know about hadrons, nor has there been any recent discovery of additional types of hadrons. Thanks for reading and have a great day!

 

* By “effectively,” I mean to ignore and gloss over the fact that there are tons more things in a proton, like photons and heavier quarks, but their aggregate influences cancel out.

** Here, an elementary charge is the magnitude of an electron’s electron charge. In other words, the electric charge of an electron is (-1) elementary charges (that is, “negative one elementary charges”). Sometimes an elementary charge is defined as the electric charge of a proton, but that is entirely tautological for our present purpose.

*** If you are unfamiliar with the PDG, it is arguably the most useful site to high energy physicists aside from CERN’s ROOT user guides and Wikipedia’s Standard Model articles.

The News: That’s Belle with an e

So KEK operates a super-high intensity electron-positron collider in order to study super-rare physics phenomena. It’s kind of super. Well, guess what. While analyzing collisions with the Belle detector experiment, researchers discovered the existence of two new hadrons, each made of four quarks! That’s right, count them: 1, 2, 3, 4 quarks! In each case, one of the four quarks is a bottom-type quark and another is an anti-bottom quark. (Cool bottom-quark stuff.) The remaining two quarks are believed to be an up-type quark and an anti-down type quark.

The two exotic hadrons have been named Zb(10610) and Zb(10650). Here, the “Z” implies that our hadrons are “exotic,” i.e., not a baryon or meson, the subscript “b” indicates that it contains a bottom-quark, and the 10610/10650 tell us that our hadrons weigh 10,610 MeV/c2 and 10,650 MeV/c2, respectively. A proton’s mass is about 938 MeV/c2, so both hadrons are about 11 times heavier than the proton (that is pretty heavy). The Belle Collaboration presser is really great, so I will not add much more.

Other Exotic Hadrons: When Barry met Sally.

For those keeping track, the Belle Collaboration’s recent finding of two new 4-quark hadrons makes it the twelfth-or-so “tetra-quark” discovery. What makes this so special, however, is that all previous tetra-quarks have been limited to include a charm-type quark and an anti-charm-type quark. This is definitely the first case to include bottom-type quarks, and therefore offer more evidence that the formation of such states is not a unique property of particularly charming quarks but rather a naturally occurring phenomenon affecting all quarks.

Furthermore, it suggests the possibility of 5-quark hadrons, called penta-quarks. Now these things take the cake. They are a sort of grand link between elementary particle physics and nuclear physics. To be exact, we know 6-quark systems exist: it is called deuterium, a radioactive stable isotope of hydrogen (Thanks to @incognitoman for pointing out that deuterium is, in fact, stable.). 9-quark systems definitely exist too, e.g., He-3 and tritium. Etc. You get the idea. Discovering the existence of five-quark hadrons empirically establishes a very elegant and fundamental principle: That in order to produce a new nuclear isotope, so long as all Standard Model symmetries are conserved, one must simply tack on quarks and anti-quarks. Surprisingly straightforward, right? Though sadly, history is not on the side of 5-quark systems.

Now go discuss and ask questions! 🙂

Run-of-the-mill hadrons that are common to everyday interactions involving the Strong Nuclear Force (QCD) are colloquially called “standard hadrons.” They include mesons (quark-anti-quark pairs) and baryons (three-quark/anti-quark combinations). Quark combinations consisting of more than three quarks are called “exotic hadrons.”

 

 

 

 

Happy Colliding.

– richard (@bravelittlemuon)

 

PS, I am always happy to write about topics upon request. You know, QED, QCD, OED, etc.

http://en.wikipedia.org/wiki/Neutron
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