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Four-quark particles

Last week, the LHCb collaboration announced the confirmation of a new particle that seems to be made of four quarks. This result builds upon years of work by collaborations around the world including BELLE and BESIII.  

Major Harvest of Four-Leaf Clover

By Pauline Gagnon (CERN) | April 9, 2014
The LHCb Collaboration at CERN has just confirmed the unambiguous observation of a very exotic state, something that looks strangely like a particle being made of four quarks.  

That’s Right, Count Them: 4 Quarks

By Richard Ruiz | January 20, 2012
Exciting news came out the Japanese physics lab KEK last week about some pretty exotic combinations of quarks and anti-quarks.

Who ordered that?! An X-traordinary particle

By Aidan Randle-Conde | October 10, 2011
The quark model of matter was going so well until the X(3872) came along and messed everything up.

Latest Posts

A version of this article appeared in symmetry on April 14, 2014.

From accelerators unexpectedly beneath your feet to a ferret that once cleaned accelerator components, symmetry shares some lesser-known facts about particle accelerators. Image: Sandbox Studio, Chicago

From accelerators unexpectedly beneath your feet to a ferret that once cleaned accelerator components, symmetry shares some lesser-known facts about particle accelerators. Image: Sandbox Studio, Chicago

The Large Hadron Collider at CERN laboratory has made its way into popular culture: Comedian John Stewart jokes about it on The Daily Show, character Sheldon Cooper dreams about it on The Big Bang Theory and fictional villains steal fictional antimatter from it in Angels & Demons.

Despite their uptick in popularity, particle accelerators still have secrets to share. With input from scientists at laboratories and institutions worldwide, symmetry has compiled a list of 10 things you might not know about particle accelerators.

There are more than 30,000 accelerators in operation around the world.

Accelerators are all over the place, doing a variety of jobs. They may be best known for their role in particle physics research, but their other talents include: creating tumor-destroying beams to fight cancer; killing bacteria to prevent food-borne illnesses; developing better materials to produce more effective diapers and shrink wrap; and helping scientists improve fuel injection to make more efficient vehicles.

One of the longest modern buildings in the world was built for a particle accelerator.

Linear accelerators, or linacs for short, are designed to hurl a beam of particles in a straight line. In general, the longer the linac, the more powerful the particle punch. The linear accelerator at SLAC National Accelerator Laboratory, near San Francisco, is the largest on the planet.

SLAC’s klystron gallery, a building that houses components that power the accelerator, sits atop the accelerator. It’s one of the world’s longest modern buildings. Overall, it’s a little less than 2 miles long, a feature that prompts laboratory employees to hold an annual footrace around its perimeter.

Particle accelerators are the closest things we have to time machines, according to Stephen Hawking.

In 2010, physicist Stephen Hawking wrote an article for the UK paper the Daily Mail explaining how it might be possible to travel through time. We would just need a particle accelerator large enough to accelerate humans the way we accelerate particles, he said.

A person-accelerator with the capabilities of the Large Hadron Collider would move its passengers at close to the speed of light. Because of the effects of special relativity, a period of time that would appear to someone outside the machine to last several years would seem to the accelerating passengers to last only a few days. By the time they stepped off the LHC ride, they would be younger than the rest of us.

Hawking wasn’t actually proposing we try to build such a machine. But he was pointing out a way that time travel already happens today. For example, particles called pi mesons are normally short-lived; they disintegrate after mere millionths of a second. But when they are accelerated to nearly the speed of light, their lifetimes expand dramatically. It seems that these particles are traveling in time, or at least experiencing time more slowly relative to other particles.

The highest temperature recorded by a manmade device was achieved in a particle accelerator.

In 2012, Brookhaven National Laboratory’s Relativistic Heavy Ion Collider achieved a Guinness World Record for producing the world’s hottest manmade temperature, a blazing 7.2 trillion degrees Fahrenheit. But the Long Island-based lab did more than heat things up. It created a small amount of quark-gluon plasma, a state of matter thought to have dominated the universe’s earliest moments. This plasma is so hot that it causes elementary particles called quarks, which generally exist in nature only bound to other quarks, to break apart from one another.

Scientists at CERN have since also created quark-gluon plasma, at an even higher temperature, in the Large Hadron Collider.

The inside of the Large Hadron Collider is colder than outer space.

In order to conduct electricity without resistance, the Large Hadron Collider’s electromagnets are cooled down to cryogenic temperatures. The LHC is the largest cryogenic system in the world, and it operates at a frosty minus 456.3 degrees Fahrenheit. It is one of the coldest places on Earth, and it’s even a few degrees colder than outer space, which tends to rest at about minus 454.9 degrees Fahrenheit.

Nature produces particle accelerators much more powerful than anything made on Earth.

We can build some pretty impressive particle accelerators on Earth, but when it comes to achieving high energies, we’ve got nothing on particle accelerators that exist naturally in space.

The most energetic cosmic ray ever observed was a proton accelerated to an energy of 300 million trillion electronvolts. No known source within our galaxy is powerful enough to have caused such an acceleration. Even the shockwave from the explosion of a star, which can send particles flying much more forcefully than a manmade accelerator, doesn’t quite have enough oomph. Scientists are still investigating the source of such ultra-high-energy cosmic rays.

Particle accelerators don’t just accelerate particles; they also make them more massive.

As Einstein predicted in his theory of relativity, no particle that has mass can travel as fast as the speed of light—about 186,000 miles per second. No matter how much energy one adds to an object with mass, its speed cannot reach that limit.

In modern accelerators, particles are sped up to very nearly the speed of light. For example, the main injector at Fermi National Accelerator Laboratory accelerates protons to 0.99997 times the speed of light. As the speed of a particle gets closer and closer to the speed of light, an accelerator gives more and more of its boost to the particle’s kinetic energy.

Since, as Einstein told us, an object’s energy is equal to its mass times the speed of light squared (E=mc2), adding energy is, in effect, also increasing the particles’ mass. Said another way: Where there is more “E,” there must be more “m.” As an object with mass approaches, but never reaches, the speed of light, its effective mass gets larger and larger.

The diameter of the first circular accelerator was shorter than 5 inches; the diameter of the Large Hadron Collider is more than 5 miles.

In 1930, inspired by the ideas of Norwegian engineer Rolf Widerøe, 27-year-old physicist Ernest Lawrence created the first circular particle accelerator at the University of California, Berkeley, with graduate student M. Stanley Livingston. It accelerated hydrogen ions up to energies of 80,000 electronvolts within a chamber less than 5 inches across.

In 1931, Lawrence and Livingston set to work on an 11-inch accelerator. The machine managed to accelerate protons to just over 1 million electronvolts, a fact that Livingston reported to Lawrence by telegram with the added comment, “Whoopee!” Lawrence went on to build even larger accelerators—and to found Lawrence Berkeley and Lawrence Livermore laboratories.

Particle accelerators have come a long way since then, creating brighter beams of particles with greater energies than previously imagined possible. The Large Hadron Collider at CERN is more than 5 miles in diameter (17 miles in circumference). After this year’s upgrades, the LHC will be able to accelerate protons to 6.5 trillion electronvolts.

In the 1970s, scientists at Fermi National Accelerator Laboratory employed a ferret named Felicia to clean accelerator parts.

From 1971 until 1999, Fermilab’s Meson Laboratory was a key part of high-energy physics experiments at the laboratory. To learn more about the forces that hold our universe together, scientists there studied subatomic particles called mesons and protons. Operators would send beams of particles from an accelerator to the Meson Lab via a miles-long underground beam line.

To ensure hundreds of feet of vacuum piping were clear of debris before connecting them and turning on the particle beam, the laboratory enlisted the help of one Felicia the ferret.

Ferrets have an affinity for burrowing and clambering through holes, making them the perfect species for this job. Felicia’s task was to pull a rag dipped in cleaning solution on a string through long sections of pipe.

Although Felicia’s work was eventually taken over by a specially designed robot, she played a unique and vital role in the construction process—and in return asked only for a steady diet of chicken livers, fish heads and hamburger meat.

Particle accelerators show up in unlikely places.

Scientists tend to construct large particle accelerators underground. This protects them from being bumped and destabilized, but can also make them a little harder to find.

For example, motorists driving down Interstate 280 in northern California may not notice it, but the main accelerator at SLAC National Accelerator Laboratory runs underground just beneath their wheels.

Residents in villages in the Swiss-French countryside live atop the highest-energy particle collider in the world, the Large Hadron Collider.

And for decades, teams at Cornell University have played soccer, football and lacrosse on Robison Alumni Fields 40 feet above the Cornell Electron Storage Ring, or CESR. Scientists use the circular particle accelerator to study compact particle beams and to produce X-ray light for experiments in biology, materials science and physics.

Sarah Witman


Même avant mon départ pour La Thuile (Italie), les résultats des Rencontres de Moriond remplissaient déjà les fils d’actualités. La session de cette année sur l’interaction électrofaible, du 15 au 22 mars, a débuté avec la première « mesure mondiale » de la masse du quark top, basée sur la combinaison des mesures publiées jusqu’à présent par les expériences Tevatron et LHC. La semaine s’est poursuivie avec un résultat spectaculaire de CMS sur la largeur du Higgs.

Même si elle approche de son 50e anniversaire, la conférence de Moriond est restée à l’avant-garde. Malgré le nombre croissant de conférences incontournables en physique des hautes énergies, Moriond garde une place de choix dans la communauté, pour des raisons en partie historiques : cette conférence existe depuis 1966 et elle s’est imposée comme l’endroit où les théoriciens et les expérimentateurs viennent pour voir et être vus. Regardons maintenant ce que les expériences du LHC nous ont réservé cette année…

Nouveaux résultats­­­

Cette année, le clou du spectacle à Moriond a bien entendu été l’annonce de la meilleure limite à ce jour pour la largeur du Higgs, à < 17 MeV avec 95 % de confiance, présentée aux deux sessions de Moriond par l’expérience CMS. La nouvelle mesure, obtenue par une nouvelle méthode d’analyse basée sur les désintégrations du Higgs en deux particules Z, est environ 200 fois plus précise que les précédentes. Les discussions sur cette limite ont porté principalement sur la nouvelle méthode utilisée pour l’analyse. Quelles hypothèses étaient nécessaires ? La même technique pouvait-elle être appliquée à un Higgs se désintégrant en deux bosons W ? Comment cette nouvelle largeur allait-elle influencer les modèles théoriques pour la nouvelle physique ? Nous le découvrirons sans doute à Moriond l’année prochaine…

L’annonce du premier résultat mondial conjoint pour la masse du quark top a aussi suscité un grand enthousiasme. Ce résultat, qui met en commun les données du Tevatron et du LHC, constitue la meilleure valeur jusqu’ici, au niveau mondial, à 173,34 ± 0,76 GeV/c2. Avant que l’effervescence ne soit retombée à la session de QCD de Moriond, CMS a annoncé un nouveau résultat préliminaire fondé sur l’ensemble des données collectées à 7 et 8 TeV. Ce résultat est à lui seul d’une précision qui rivalise avec celle de la moyenne mondiale, ce qui démontre clairement que nous n’avons pas encore atteint la plus grande précision possible pour la masse du quark top.

ot0172hCe graphique montre les quatre mesures de la masse du quark top publiées respectivement par les collaborations ATLAS, CDF, CMS et D0, ainsi que la mesure la plus précise à ce jour obtenue grâce à l’analyse conjointe.

D’autres nouveautés concernant le quark top, entre autres les nouvelles mesures précises de son spin et de sa polarisation issues du LHC, ainsi que les nouveaux résultats d’ATLAS pour la section efficace du quark top isolé dans le canal de désintégration t, ont été présentés par Kate Shaw le mardi 25 mars. La période II du LHC permettra d’approfondir encore notre compréhension du sujet.

Une mesure fondamentale et délicate permettant d’explorer la nature de la brisure de la symétrie électrofaible portée par le mécanisme de Brout-Englert-Higgs est celle de la diffusion de deux bosons vecteurs massifs. Cet événement est rare, mais en l’absence du boson de Higgs sa fréquence augmenterait fortement avec l’énergie de la collision, jusqu’à enfreindre les lois de la physique. Un indice de la collision d’un boson vecteur de force électrofaible a été détecté pour la première fois par ATLAS dans des événements impliquant deux leptons de même charge et deux jets présentant une grande différence de rapidité.

S’appuyant sur l’augmentation du volume de données et une meilleure analyse de celles-ci, les expériences du LHC s’attaquent à des états finaux multi-particules rares et difficiles qui font intervenir le boson de Higgs. ATLAS en a présenté un excellent exemple, avec un nouveau résultat dans la recherche de la production d’un Higgs associé à deux quarks top et se désintégrant en une paire de quarks b. Avec une limite prévue de 2,6 fois la prédiction du Modèle standard pour ce seul canal et une intensité de signal relative observée de 1,7 ± 1,4, la future exploitation à haute énergie du LHC, avec laquelle la fréquence de cet événement augmentera, suscite de grands espoirs.

Dans le même temps, dans le monde des saveurs lourdes, l’expérience LHCb a présenté des analyses supplémentaires de l’état exotique X(3872). L’expérience a confirmé de manière non ambiguë que ses nombres quantiques Jpc sont 1++ et a mis en évidence sa désintégration en ψ(2S)γ.

L’étude du plasma de quarks et de gluons se poursuit dans l’expérience ALICE, et les discussions ont porté surtout sur les résultats de l’exploitation du LHC en mode proton-plomb (p-Pb). En particulier, la « double crête » nouvellement observée dans les collisions p-Pb est étudiée en détail, et des analyses du pic de ses jets, de sa distribution de masse et de sa dépendance à la charge ont été présentées.

Nouvelles explorations

Grâce à notre nouvelle compréhension du boson de Higgs, le LHC est entré dans l’ère de la physique du Higgs de précision. Notre connaissance des propriétés du Higgs – par exemple les mesures de son spin et de sa largeur – s’est améliorée, et les mesures précises des interactions et des désintégrations du Higgs ont elles aussi bien progressé. Des résultats relatifs à la recherche d’une physique au-delà du Modèle standard ont également été présentés, et les expériences du LHC continuent de s’investir intensément dans la recherche de la supersymétrie.

En ce qui concerne le secteur de Higgs, de nombreux chercheurs espèrent trouver les cousins supersymétriques du Higgs et des bosons électrofaibles, appelés neutralinos et charginos, par l’intermédiaire de processus électrofaibles. ATLAS a présenté deux nouveaux articles résumant de multiples recherches en quête de ces particules. L’absence d’un signal significatif a été utilisée pour définir des limites d’exclusion pour les charginos et les neutralinos, soit 700 GeV – s’ils se désintègrent via des partenaires supersymétriques intermédiaires de leptons – et 420 GeV – quand ils se désintègrent seulement via des bosons du Modèle standard.

Par ailleurs, pour la première fois, une recherche du mode électrofaible le plus difficile à observer, produisant une paire de charginos qui se désintègrent en bosons W, a été entreprise par ATLAS. Ce mode ressemble à celui de la production de paires de W du Modèle standard, dont le taux mesuré actuellement paraît légèrement plus élevé que prévu.

Dans ce contexte, CMS a présenté de nouveaux résultats dans la recherche de la production d’une paire électrofaible de higgsinos via leur désintégration en un Higgs (à 125 GeV) et un gravitino de masse presque nulle. L’état final montre une signature caractéristique de jets de quatre quarks b, compatible avec une cinématique de double désintégration du Higgs. Un léger excès du nombre d’événements candidats signifie que l’expérience ne peut pas exclure un signal de higgsino. On établit des limites supérieures de l’intensité du signal d’environ deux fois la prédiction théorique pour des masses du higgsino comprises entre 350 et 450 GeV.

Dans plusieurs scénarios de supersymétrie, les charginos peuvent être métastables et ils pourraient potentiellement être détectés sous la forme de particules à durée de vie longue. CMS a présenté une recherche innovante de particules génériques chargées à durée de vie longue, effectuées en cartographiant l’efficacité de détection en fonction de la cinématique de la particule et de la perte d’énergie dans le trajectographe. Cette étude permet non seulement d’établir des limites strictes pour divers modèles supersymétriques qui prédisent une durée de vie du chargino (c*tau) supérieure à 50 cm mais elle fournit également un puissant outil à la communauté des théoriciens pour tester de manière indépendante les nouveaux modèles prédisant des particules chargées à durée de vie longue.

Afin d’être aussi général que possible dans la recherche de la supersymétrie, CMS a également présenté les résultats de nouvelles recherches, dans lesquelles un grand sous-ensemble des paramètres de la supersymétrie, tels que les masses du gluino et du squark, sont testés pour vérifier leur compatibilité statistique avec différentes mesures expérimentales. Cela a permis d’établir une carte des probabilités dans un espace à 19 dimensions. Cette carte montre notamment que les modèles prédisant des masses inférieures à 1,2 TeV pour le gluino et inférieures à 700 GeV pour le sbottom et le stop sont fortement défavorisés.

mais pas de nouvelle physique

Malgré toute ces recherches minutieuses, ce qu’on a le plus entendu à Moriond, c’était: « pas d’excès observé » – « cohérent avec le Modèle standard ». Tous les espoirs reposent maintenant sur la prochaine exploitation du LHC, à 13 TeV. Si vous souhaitez en savoir davantage sur les perspectives ouvertes par la deuxième exploitation du LHC, consultez l’article suivant du Bulletin du CERN: “La vie est belle à 13 TeV“.

En plus des divers résultats des expériences du LHC qui ont été présentés, des nouvelles ont aussi été rapportées à Moriond par les expériences du Tevatron, de BICEP, de RHIC et d’autres expériences. Pour en savoir plus, consultez les sites internet de la conférence, Moriond EW et Moriond QCD.


On the Shoulders of…

Monday, April 14th, 2014

My first physics class wasn’t really a class at all. One of my 8th grade teachers noticed me carrying a copy of Kip Thorne’s Black Holes and Time Warps, and invited me to join a free-form book discussion group on physics and math that he was holding with a few older students. His name was Art — and we called him by his first name because I was attending, for want of a concise term that’s more precise, a “hippie” school. It had written evaluations instead of grades and as few tests as possible; it spent class time on student governance; and teachers could spend time on things like, well, discussing books with a few students without worrying about whether it was in the curriculum or on the tests. Art, who sadly passed some years ago, was perhaps best known for organizing the student cafe and its end-of-year trip, but he gave me a really great opportunity. I don’t remember learning anything too specific about physics from the book, or from the discussion group, but I remember being inspired by how wonderful and crazy the universe is.

My second physics class was combined physics and math, with Dan and Lewis. The idea was to put both subjects in context, and we spent a lot of time on working through how to approach problems that we didn’t know an equation for. The price of this was less time to learn the full breadth subjects; I didn’t really learn any electromagnetism in high school, for example.

When I switched to a new high school in 11th grade, the pace changed. There were a lot more things to learn, and a lot more tests. I memorized elements and compounds and reactions for chemistry. I learned calculus and studied a bit more physics on the side. In college, where the physics classes were broad and in depth at the same time, I needed to learn things fast and solve tricky problems too. By now, of course, I’ve learned all the physics I need to know — which is largely knowing who to ask or which books to look in for the things I need but don’t remember.

There are a lot of ways to run schools and to run classes. I really value knowledge, and I think it’s crucial in certain parts of your education to really buckle down and learn the facts and details. I’ve also seen the tremendous worth of taking the time to think about how you solve problems and why they’re interesting to solve in the first place. I’m not a high school teacher, so I don’t think I can tell the professionals how to balance all of those goods, which do sometimes conflict. What I’m sure of, though, is that enthusiasm, attention, and hard work from teachers is a key to success no matter what is being taught. The success of every physicist you will ever see on Quantum Diaries is built on the shoulders of the many people who took the time to teach and inspire them when they were young.


Major harvest of four-leaf clover

Wednesday, April 9th, 2014

The LHCb Collaboration at CERN has just confirmed the unambiguous observation of a very exotic state, something that looks strangely like a particle being made of four quarks. As exotic as it might be, this particle is sternly called Z(4430)-, which gives its mass at 4430 MeV, roughly four times heavier than a proton, and indicates it is has a negative electric charge. The letter Z shows that it belongs to a strange series of particles that are referred to as XYZ states.

So what’s so special about this state? The conventional and simple quark model states that there are six different quarks, each quark coming with its antiparticle.  All these particles form bound states by either combining two or three of them. Protons and neutrons for example are made of three quarks. All states made of three quarks are called baryons. Other particles like pions and kaons, which are often found in the decays of heavier particles, are made of one quark and one antiquark. These form the mesons category. Until 2003, the hundreds of particles observed were classified either as mesons or baryons.

And then came the big surprise: in 2003, the BELLE experiment found a state that looked like a bound state of four quarks. Many other exotic states have been observed since. These states often look like charmonium or bottomonium states, which contain a charm quark and a charm antiquark, or a bottom and antibottom quarks. Last spring, the BESIII collaboration from Beijing confirmed the observation of the Zc(3900)+ state also seen by BELLE.

On April 8, the LHCb collaboration reported having found the Z(4430)- with ten times more events than all other groups before. The data sample is so large that it enabled LHCb to measure some of its properties unambiguously. Determining the exact quantum numbers of a particle is like getting its fingerprints: it allows physicists to find out exactly what kind of particle it is. Hence, the Z(4430)- state appears to be made of a charm, an anti-charm, a down and an anti up quarks. Their measurement rules out several other possibilities.


The squared mass distribution for the 25,200 B meson decays to ψ’ π- found by LHCb in their entire data set. The black points represent the data, the red curve the result of the simulation when including the presence of the Z(4430)- state. The dashed light brown curve below shows that the simulation fails to reproduce the data if no contribution from Z(4430)- is included, establishing the clear presence of this particle with 13.9σ (that is, the signal is 13.9 times stronger than all possible combined statistical fluctuations. These are the error bars represented by the small vertical line attached to each point).

Theorists are hard at work now trying to come up with a model to describe these new states. Is this a completely new tetraquark, a bound state of four quarks, or some strange combination of two charmed mesons (mesons containing at least one charm quark)? The question is still open.

Pauline Gagnon

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For more information, see the LHCb website


La collaboration LHCb du CERN vient de confirmer hors de tout doute l’existence d’un état très exotique, quelque chose qui ressemble étrangement à une particule formée de quatre quarks. Aussi exotique qu’elle puisse paraître, cette particule porte le nom très pragmatique de Z(4430)-. Ce nom indique sa masse à 4430 MeV, soit  environ quatre fois celle d’un proton, et signale qu’elle a une charge électrique négative. La lettre Z montre qu’elle appartient à une étrange série de particules communément regroupées sous l’appellation d’états XYZ.

Mais qu’est-ce que cet état a donc de si spécial? Le modèle conventionnel des quark est tout simple: il existe six quarks différents, chacun venant avec son antiparticule. Ces douze particules peuvent se combiner pour former des états liés en regroupant deux ou trois d’entre eux. Par exemple, les protons et des neutrons sont composés de trois quarks. Tous les états faits de trois quarks sont appelés baryons. D’autres particules comme les pions et les kaons, qu’on retrouve souvent dans les désintégrations de particules plus lourdes, sont formées d’un quark et d’un antiquark. Elles appartiennent à la catégorie des mésons. Les centaines de particules observées jusqu’en 2003 étaient toutes classifiées soit comme mésons, soit comme baryons.

Puis vint la grande surprise: en 2003, l’expérience BELLE trouva le premier état lié fait en apparence de quatre quarks. Beaucoup d’autres états exotiques similaires ont été observés depuis. Ces états ressemblent souvent à des états de charmonium ou de bottomonium, des particules qui contiennent respectivement un quark charmé et un antiquark charmé, ou un quark bottom et un anti-bottom. Au printemps dernier, la collaboration BESIII de Beijing a confirmé l’observation du Zc(3900)+, un état aussi détecté par BELLE.

Le 8 avril, la collaboration LHCb a rapporté avoir trouvé l’état Z(4430)- avec dix fois plus d’événements que tous les autres groupes précédents. Leur échantillon de données est si grand qu’il a permis à LHCb de mesurer certaines de ses propriétés sans équivoque. La détermination des nombres quantiques exacts d’une particule équivaut à l’obtention de ses empreintes digitales: cela permet aux physicien-ne-s de cerner plus exactement à quelle particule on a affaire. Il en ressort que l’état Z(4430)- serait formé d’un quark charmé, d’un antiquark charmé, d’un quark bottom et un anti-bottom. Leur mesure exclut toutes autres possibilités.


La distribution de la masse (au carré) des 25200 mésons B se désintégrant en paires de ψ’ π- trouvés par LHCb dans l’ensemble de leurs données. Les points noirs représentent les données expérimentales et la courbe en rouge, le résultat de la simulation lorsqu’on inclut la présence du Z(4430)-. La courbe en pointillés juste en dessous en brun clair montre que la simulation ne peut reproduire les données si on supprime la contribution du Z(4430)-. Ceci établit clairement la présence de cette particule avec 13.9σ (c’est-à-dire le signal est 13.9 fois plus fort que toutes les fluctuations statistiques combinées possible. La fluctuation de chaque point est représentée par la petite ligne verticale qui lui est attachée).

Les théoricien-ne-s sont à pied d’oeuvre pour essayer d’imaginer un modèle pouvant décrire ces nouveaux états. S’agit-il d’états complètement nouveaux faits de quatre quarks liés ensemble, des tétraquarks, ou est-ce une étrange combinaison de deux mésons charmés (des mésons contenant au moins un quark charmé)? La question est toujours ouverte.

Pauline Gagnon

Pour être averti-e 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

Pour plus de tails (en anglais) voir le site de l’expérience LHCb



A version of this article appeared in symmetry on April 8, 2014.

Physicist Aaron Chou keeps the Holometer experiment—which looks for a phenomenon whose implications border on the unreal—grounded in the realities of day-to-day operations. Photo: Reidar Hahn

Physicist Aaron Chou keeps the Holometer experiment—which looks for a phenomenon whose implications border on the unreal—grounded in the realities of day-to-day operations. Photo: Reidar Hahn

The beauty of the small operation—the mom-and-pop restaurant or the do-it-yourself home repair—is that pragmatism begets creativity. The industrious individual who makes do with limited resources is compelled onto paths of ingenuity, inventing rather than following rules to address the project’s peculiarities.

As project manager for the Holometer experiment at Fermilab, physicist Aaron Chou runs a show that, though grandiose in goal, is remarkably humble in setup. Operated out of a trailer by a small team with a small budget, it has the feel more of a scrappy startup than of an undertaking that could make humanity completely rethink our universe.

The experiment is based on the proposition that our familiar, three-dimensional universe is a manifestation of a two-dimensional, digitized space-time. In other words, all that we see around us is no more than a hologram of a more fundamental, lower-dimensional reality.

If this were the case, then space-time would not be smooth; instead, if you zoomed in on it far enough, you would begin to see the smallest quantum bits—much as a digital photo eventually reveals its fundamental pixels.

In 2009, the GEO600 experiment, which searches for gravitational waves emanating from black holes, was plagued by unaccountable noise. This noise could, in theory, be a telltale sign of the universe’s smallest quantum bits. The Holometer experiment seeks to measure space-time with far more precision than any experiment before—and potentially observe effects from those fundamental bits.

Such an endeavor is thrilling—but also risky. Discovery would change the most basic assumptions we make about the universe. But there also might not be any holographic noise to find. So for Chou, managing the Holometer means building and operating the apparatus on the cheap—not shoddily, but with utmost economy.

Thus Chou and his team take every opportunity to make rather than purchase, to pick up rather than wait for delivery, to seize the opportunity and take that measurement when all the right people are available.

“It’s kind of like solving a Rubik’s cube,” Chou says. “You have an overview of every aspect of the measurement that you’re trying to make. You have to be able to tell the instant something doesn’t look right, and tell that it conflicts with some other assumption you had. And the instant you have a conflict, you have to figure out a way to resolve it. It’s a lot of fun.”

Chou is one of the experiment’s 1.5 full-time staff members; a complement of students rounds out a team of 10. Although Chou is essentially the overseer, he runs the experiment from down in the trenches.

Aaron Chou, project manager 
for Fermilab’s Holometer, tests the experiment’s instrumentation. Photo: Reidar Hahn

Aaron Chou, project manager 
for Fermilab’s Holometer, tests the experiment’s instrumentation. Photo: Reidar Hahn

The Holometer experimental area, for example, is a couple of aboveground, dirt-covered tunnels whose walls don’t altogether keep out the water after a heavy rain. So any time the area needs the attention of a wet-dry vacuum, he and his team are down on the ground, cheerfully squeegeeing, mopping and vacuuming away.

“That’s why I wear such shabby clothes,” he says. “This is not the type of experiment where you sit behind the computer and analyze data or control things remotely all day long. It’s really crawling-around-on-the-floor kind of work, which I actually find to be kind of a relief, because I spent more than a decade sitting in front of a computer for more well-established experiments where the installation took 10 years and most of the resulting experiment is done from behind a keyboard.”

As a graduate student at Stanford University, Chou worked on the SLD experiment at SLAC National Accelerator Laboratory, writing software to help look for parity violation in Z bosons. As a Fermilab postdoc on the Pierre Auger experiment, he analyzed data on ultra-high-energy cosmic rays.

Now Chou and his team are down in the dirt, hunting for the universe’s quantum bits. In length terms, these bits are expected to be on the smallest scale of the universe, the Planck scale: 1.6 x 10-35 meters. That’s roughly 10 trillion trillion times smaller than an atom; no existing instrument can directly probe objects that small. If humanity could build a particle collider the size of the Milky Way, we might be able to investigate Planck-scale bits directly.

The Holometer instead will look for a jitter arising from the cosmos’ minuscule quanta. In the experiment’s dimly lit tunnels, the team built two interferometers, L-shaped configurations of tubes. Beginning at the L’s vertex, a laser beam travels down each of the L’s 40-meter arms simultaneously, bounces off the mirrors at the ends and recombines at the starting point. Since the laser beam’s paths down each arm of the L are the same length, absent a holographic jitter, the beam should cancel itself out as it recombines. If it doesn’t, it could be evidence of the jitter, a disruption in the laser beam’s flight.

And why are there two interferometers? The two beam spots’ particular brightening and dimming will match if it’s the looked-for signal.

“Real signals have to be in sync,” Chou says. “Random fluctuations won’t be heard by both instruments.”

Should the humble Holometer find a jitter when it looks for the signal—researchers will soon begin the initial search and expect results by 2015—the reward to physics would be extraordinarily high, especially given the scrimping behind the experiment and the fact that no one had to build an impossibly high-energy, Milky Way-sized collider. The data would support the idea that the universe we see around us is only a hologram. It would also help bring together the two thus-far-irreconcilable principles of quantum mechanics and relativity.

“Right now, so little experimental data exists about this high-energy scale that theorists are unable to construct any meaningful models other than those based on speculation,” Chou says. “Our experiment is really a mission of exploration—to obtain data about an extremely high-energy scale that is otherwise inaccessible.”

What’s more, when the Holometer is up and running, it will be able to look for other phenomena that manifest themselves in the form of high-frequency gravitational waves, including topological defects in our cosmos—areas of tension between large regions in space-time that were formed by the big bang.

“Whenever you design a new apparatus, what you’re doing is building something that’s more sensitive to some aspect of nature than anything that has ever been built before,” Chou says. “We may discover evidence of holographic jitter. But even if we don’t, if we’re smart about how we use our newly built apparatus, we may still be able to discover new aspects of our universe.”


I’ve just been watching the first couple of episodes of the new, reborn, perhaps rebooted, Cosmos. About 4 million people have been watching each of the episodes when broadcast. Out of a US population of about 300 million. Said that way, it doesn’t sound like a huge success, but science has much less of a grip on the American public than science fiction (or at least folks in spandex hitting each other over the head) or comedy about scientists. Over the years, it’s said that Sagan’s Cosmos has been the most watched PBS series world-wide, ever, and I have confidence that the new one, with current special effects, and its hooks to the 2010s rather than the late 1970s, will be watched for many years to come.

Different times and different shows. It’s worth thinking about why this isn’t a PBS show today. Why is that? And why are there still creationists around to poke holes in our schools?

Anyway, what I’ve seen so far, I’ve liked quite a bit. There are plenty of eloquent positive reviews out there, so let me highlight one thing of which I am not a fan. With the excellent special effects, along with the excellent astronomical images available, it’s not always clear in the show what is a real image and what is artwork. In Sagan’s Cosmos, we see visualizations and we see telescopic views, and we can know which is which. With the current Cosmos, it’s a lot harder to tell. And a third category, simulations also poke in somewhere between the models and true imaging. Simulations based on the physics, so therefore “true” and “correct,” but not real images of objects in the sky. I’ve seen NASA artist renditions clearly marked in the corner. It would be a nice addition to the show, not to justify the scientific validity but to clarify, to mark the boundaries of what we see, what we know, and what we conjecture. Three different parts of the science.


Even before my departure to La Thuile in Italy, results from the Rencontres de Moriond conference were already flooding the news feeds. This year’s Electroweak session from 15 to 22 March, started with the first “world measurement” of the top quark mass, from a combination of the measurements published by the Tevatron and LHC experiments so far. The week went on to include a spectacular CMS result on the Higgs width.

Although nearing its 50th anniversary, Moriond has kept its edge. Despite the growing numbers of must-attend HEP conferences, Moriond retains a prime spot in the community. This is in part due to historic reasons: it’s been around since 1966, making a name for itself as the place where theorists and experimentalists come to see and be seen. Let’s take a look at what the LHC experiments had in store for us this year…

New Results­­­

Stealing the show at this year’s Moriond was, of course, the announcement of the best constraint yet of the Higgs width at < 17 MeV with 95% confidence reported in both Moriond sessions by the CMS experiment. Using a new analysis method based on Higgs decays into two Z particles, the new measurement is some 200 times better than previous results. Discussions surrounding the constraint focussed heavily on the new methodology used in the analysis. What assumptions were needed? Could the same technique be applied to Higgs to WW bosons? How would this new width influence theoretical models for New Physics? We’ll be sure to find out at next year’s Moriond…

The announcement of the first global combination of the top quark mass also generated a lot of buzz. Bringing together Tevatron and LHC data, the result is the world’s best value yet at 173.34 ± 0.76 GeV/c2.  Before the dust had settled, at the Moriond QCD session, CMS announced a new preliminary result based on the full data set collected at 7 and 8 TeV. The precision of this result alone rivals the world average, clearly demonstrating that we have yet to see the ultimate attainable precision on the top mass.

ot0172hThis graphic shows the four individual top quark mass measurements published by the ATLAS, CDF, CMS and DZero collaborations, together with the most precise measurement obtained in a joint analysis.

Other news of the top quark included new LHC precision measurements of its spin and polarisation, as well as new ATLAS results of the single top-quark cross section in the t-channel presented by Kate Shaw on Tuesday 25 March. Run II of the LHC is set to further improve our understanding of this

A fundamental and challenging measurement that probes the nature of electroweak symmetry breaking mediated by the Brout–Englert–Higgs mechanism is the scattering of two massive vector bosons against each other. Although rare, in the absence of the Higgs boson, the rate of this process would strongly rise with the collision energy, eventually breaking physical law. Evidence for electroweak vector boson scattering was detected for the first time by ATLAS in events with two leptons of the same charge and two jets exhibiting large difference in rapidity.

With the rise of statistics and increasing understanding of their data, the LHC experiments are attacking rare and difficult multi-body final states involving the Higgs boson. ATLAS presented a prime example of this, with a new result in the search for Higgs production in association with two top quarks, and decaying into a pair of b-quarks. With an expected limit of 2.6 times the Standard Model expectation in this channel alone, and an observed relative signal strength of 1.7 ± 1.4, the expectations are high for the forthcoming high-energy run of the LHC, where the rate of this process is enhanced.

Meanwhile, over in the heavy flavour world, the LHCb experiment presented further analyses of the unique exotic state X(3872). The experiment provided unambiguous confirmation of its quantum numbers JPC to be 1++, as well as evidence for its decay into ψ(2S)γ.

Explorations of the Quark-Gluon Plasma continue in the ALICE experiment, with results from the LHC’s lead-proton (p-Pb) run dominating discussions. In particular, the newly observed “double-ridge” in p-Pb is being studied in depth, with explorations of its jet peak, mass distribution and charge dependence presented.

New explorations

Taking advantage of our new understanding of the Higgs boson, the era of precision Higgs physics is now in full swing at the LHC. As well as improving our knowledge of Higgs properties – for example, measuring its spin and width – precise measurements of the Higgs’ interactions and decays are well underway. Results for searches for Beyond Standard Model (BSM) physics were also presented, as the LHC experiments continue to strongly invest in searches for Supersymmetry.

In the Higgs sector, many researchers hope to detect the supersymmetric cousins of the Higgs and electroweak bosons, so-called neutralinos and charginos, via electroweak processes. ATLAS presented two new papers summarising extensive searches for these particles. The absence of a significant signal was used to set limits excluding charginos and neutralinos up to a mass of 700 GeV – if they decay through intermediate supersymmetric partners of leptons – and up to a mass of 420 GeV – when decaying through Standard Model bosons only.

Furthermore, for the first time, a sensitive search for the most challenging electroweak mode producing pairs of charginos that decay through W bosons was conducted by ATLAS. Such a mode resembles that of Standard Model pair production of Ws, for which the currently measured rates appear a bit higher than expected.

In this context, CMS has presented new results on the search for the electroweak pair production of higgsinos through their decay into a Higgs (at 125 GeV) and a nearly massless gravitino. The final state sports a distinctive signature of 4 b-quark jets compatible with a double Higgs decay kinematics. A slight excess of candidate events means the experiment cannot exclude a higgsino signal. Upper limits on the signal strength at the level of twice the theoretical prediction are set for higgsino masses between 350 and 450 GeV.

In several Supersymmetry scenarios, charginos can be metastable and could potentially be detected as a long-lived particle. CMS has presented an innovative search for generic long-lived charged particles by mapping their detection efficiency in function of the particle kinematics and energy loss in the tracking system. This study not only allows to set stringent limits for a variety of Supersymmetric models predicting chargino proper lifetime (c*tau) greater than 50cm, but also gives a powerful tool to the theory community to independently test new models foreseeing long lived charged particles.

In the quest to be as general as possible in the search for Supersymmetry, CMS has also presented new results where a large subset of the Supersymmetry parameters, such as the gluino and squark masses, are tested for their statistical compatibility with different experimental measurements. The outcome is a probability map in a 19-dimension space. Notable observations in this map are that models predicting gluino masses below 1.2 TeV and sbottom and stop masses below 700 GeV are strongly disfavoured.

… but no New Physics

Despite careful searches, the most heard phrase at Moriond was unquestionably: “No excess observed – consistent with the Standard Model”. Hope now lies with the next run of the LHC at 13 TeV. If you want to find out more about the possibilities of the LHC’s second run, check out the CERN Bulletin article: “Life is good at 13 TeV“.

In addition to the diverse LHC experiment results presented, Tevatron experiments, BICEP, RHIC and other experiments also reported their breaking news at Moriond. Visit the Moriond EW and Moriond QCD conference websites to find out more.

Katarina Anthony-Kittelsen


This article originally appeared in symmetry on March 31, 2014.

Three decades ago in March, scientists from Latin America came to do research at Fermilab, forming the ties of a lasting collaboration.

Three decades ago in March, scientists from Latin America came to do research at Fermilab, forming the ties of a lasting collaboration.

In 1983, Fermilab Director Leon Lederman put his money on the table at the second Pan American Symposium on Elementary Particles and Technology in Rio de Janeiro. His daring proposition: If the Brazilian Research Council would not at the time fund that nation’s physicists to do research at Fermilab, he would pay the salaries himself.

His parlay worked. A year later, 30 years ago this month, four physicists from Brazil took paid leave to work on the E691 fixed-target experiment at Fermilab. They were Fermilab’s first Latin American scientists and the beginning of its relationship with the region.

“Lederman made the bold offer in that meeting,” says Carlos Escobar, one of the four trailblazing Brazilians who crossed over the Equator to Fermilab. “That was the deciding factor.”

Mexico soon followed, spearheaded by then Universidad Nacional Autónoma de México professor Clicerio Avilez. The university sent two scientists and a graduate student, the first Latin American student to get his PhD for work done at Fermilab.

Since then, the collaboration between Fermilab and Latin American institutions has grown to also include Argentina, Chile, Colombia, Ecuador and Peru. Twenty-one Latin American institutions participate in the collaboration, which consists of theorists and members of eight experiments: CMS, DAMIC, DZero, LBNE, MINERvA and MINOS, as well as on the Dark Energy Survey and the Pierre Auger Observatory—both of which reside in South America. That’s in addition to the nine fixed-target experiments that completed their runs in the 1990s.

Lederman began planting the seeds of collaboration in 1979, noting that Latin American nations boasted strong scientific groups and an impressive history of innovation.

“Latin America represented a huge potential treasure of human resources which would, I was sure, eventually be devoted to scientific research to the benefit of the nations of South and Central America and, indeed, the world,” he wrote in a 2006 paper.

Since those days, the collaboration with Fermilab, as well as steadily gaining economic strength and higher publicity for science, have placed particle physics research south of the Rio Grande on firmer ground. Fermilab not only provided scientists with particle physics experiments to work on, it also hosted workshops that were attended by Latin American engineers, physicists, technicians and students.

“When I first started, there were only two groups in Mexico cultivating theoretical high-energy physics, and none tilling the field of experimental high-energy physics,” says Julian Felix Valdez, a University of Guanajuato professor whose connection with Fermilab began in 1990, when he was a graduate student. Then, he says, things changed as Universidad Nacional Autónoma de México and Instituto Politécnico Nacional began sending students to Fermilab.

“Thirty years later, there are groups in experimental high-energy physics at eight Mexican universities, as well as other groups emerging at other Mexican universities,” Felix Valdez says. He estimates about 100 Mexican scientists work on particle physics at home and an additional 30 abroad.

The flow of students hasn’t abated, and most now come to Fermilab to work on neutrino research. For future generations, it could mean working on Fermilab’s Long-Baseline Neutrino Experiment.

“There’s a good stream of people. Once the connection’s established, it doesn’t sever. It keeps flowing,” says Pontificia Universidad Católica del Perú master’s student Maria Jose Bustamante, who is on the MINERvA neutrino experiment. “Of course you need an institution to do that.”

Enlisting more institutions to invigorate the flow is perhaps still the biggest challenge facing the collaboration today. To that end, Fermilab’s fifth director, Pier Oddone, and his deputy, Young-Kee Kim, picked up where Lederman left off, says MINERvA scientist Jorge Morfin, one of the founding members of the Latin American collaboration. Oddone and Kim helped formalize the Latin American Initiative in 2010, suggesting more written agreements between Fermilab and Latin American institutions and funding agencies.

“No one on MINERvA would doubt that the contribution of these Latin American students has been significant. This has been a real working benefit for the experiment here at Fermilab,” Morfin says. The number of students that work or have worked on MINERvA totals 24 master’s students, nine doctoral students and two postdocs. “Now they can work on experiments throughout the world. It’s been a nice return, a give and take,” he says.

Collaboration also provides opportunities for visiting scientists to bring technologies from their home countries to Fermilab. Escobar notes that Brazilian companies provided several pieces of instrumentation for Fermilab experiments, including drift chambers and detectors for DZero. It goes the other way, too: Scientists take new technologies developed at Fermilab back to industries at home.

“People see the local industries benefit from this kind of collaboration with a place that does fundamental research,” Morfin says. “It translates into actual progress for local industries and local technology.”

To see another 30 years of flourishing high-energy physics in the western hemisphere requires an investment in physics from both sides of the Equator, Felix Valdez says.

“Physics—especially high-energy physics—is an international task,” he says.

Leah Hesla


April fool’s lands on CERN

Wednesday, April 2nd, 2014

Apart from the usual jokes on Quantum Diaries (See for example the blog posts of Kyle, Byron, Aidan, Alexey), this year’s April fool’s had quite some remarkable ‘fish’ worth mentioning:

On the official website CERN announced they were going to switch to comic sans, featuring a video of ATLAS spokesperson Fabiola Gianotti. The use of comic sans in the slides of the Higgs discovery in 2012 caused quite a commotion.

Also on the CERN website, it was announced that new parking rules will be enforced at all entrance gates, allowing only cars whose digits on the number plate is odd (even) on odd (even) days, respectively. With these new rules it seems to be more advantageous to have an ‘odd’ licence plate (Some months end on 31 which is odd, followed by the first day of the next month, which is odd again).

There is a vacant position for Director General of CERN coming up, you can apply here. In fact, I am not even completely sure whether this is an April fool’s joke or not, is it?

Then there was google, launching an app to catch wild pokemon. Of course, CERN is indicated as a pokelab on the map.

My friend Andri seemed to have written a paper together with Peter Higgs. I wonder how I could have ever overlooked the paper with A. Turing in the references.

Finally, building 27 seemed to have suffered some damage and the coffee will be more expensive as of April 1st (which unfortunately seems not to be a April fool’s joke).

Have I missed any? Please put them in the comments or tweet to @KnoopsRob.

CERN as a pokelab on google's pokemon app

CERN as a pokelab on google’s pokemon app

The coffee will be more expensive as of April 1st (note a joke), thanks to Alex Brown for pointing this out.

The coffee will be more expensive as of April 1st (not a joke), thanks to Alex Brown for pointing this out.

April fool's arrives in Building 27

April fool’s arrives in Building 27