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Archive for March, 2014

This article appeared in Fermilab Today on March 26, 2014.

Tom Wicks, rigging superintendent at Joliet Steel & Construction, stands next to the stripped-down CDF detector at Fermilab. His mother, Lois Anderson, helped build the detector as an ironworker nearly 30 years ago. Photo: Amanda Solliday

Tom Wicks, rigging superintendent at Joliet Steel & Construction, stands next to the stripped-down CDF detector at Fermilab. His mother, Lois Anderson, helped build the detector as an ironworker nearly 30 years ago. Photo: Amanda Solliday

One day when Tom Wicks was a child, he biked over to see his mom, Lois Anderson, working at an office in Aurora, Ill. She was at the top of the building, welding and torching as ironworkers do.

“That’s when my son told me, ‘I want to do that,'” Anderson said.

Both mother and son have worked as ironworkers on Fermilab experiments throughout their careers. Anderson, known as “Sarge” during business hours and the only female on her crew for decades, began ironworking at CDF — one of two detectors located on the Tevatron ring — when it was “a hole in the ground” in the early 1980s. Anderson and Wicks, rigging superintendent at Joliet Steel & Construction, worked together on the last upgrade of the detector in 2001.

Now Wicks is dismantling much of the roughly 4,000-ton particle detector that he, his mother and his stepfather helped build.

“She likes to tease me about it. ‘All that work we’ve put into it, and now you’re tearing it apart?'” Wicks said.

CDF ran for more than two decades, collecting data from proton-antiproton collisions from 1985 until the Tevatron shut down in 2011. Scientists at CDF and its sister detector DZero discovered the last quark predicted by the Standard Model, the top quark. Both collaborations still analyze valuable data collected from the detectors.

In its heyday, the large orange and blue CDF detector drew crowds when upgrades required rolling the machine from the collision tunnel to an open assembly hall.

“During the last upgrade, it was like a football game,” Wicks said. “There were so many people watching, you couldn’t get a space along the rail to watch us do it.”

Wicks and his crew began working with Fermilab staff to remove equipment from the CDF detector in March 2013. They will likely finish next month, leaving intact the multmillion-dollar solenoid magnet at the core of the detector.

John Wackerlin, a fellow ironworker and foreman at Walbridge, led one of the teams tasked with decommissioning the experiment. Like Wicks, he’s laying to rest something his family helped build. His father, Bob Wackerlin, welded together the structure that houses the 30-foot-tall detector.

The elder Wackerlin’s work at Fermilab started even before CDF. When his wife was pregnant with John, Bob Wackerlin worked underground in the 4-mile Tevatron tunnel while it was still being dug. He retired after 42 years as an ironworker and said he’s proud of his family’s connection to the laboratory.

“I’ve worked in just about every building on this site,” Bob Wackerlin said. “Fermilab projects are some of the best jobs that come across our ironworkers union. It’s employed a lot of people over the years.”

His son added, “Working with physicists and the talent and brainpower here — it’s unreal.”

Although CDF is turned off and its many wires and cables scrapped, much of the detector will find a home in future experiments. The solenoid magnet, for example, could be reused in another particle experiment, said Fermilab scientist Jonathan Lewis. Scientists are recycling parts of the detector for other high-energy physics projects at Fermilab, and electronics, phototubes and assorted pieces of CDF have also been shipped to other labs and universities in the United States, Europe and Japan.

Both families see this as progress.

“Once you’ve learned something from one experiment, it makes way for new experiments,” John Wackerlin said. “So now we can go on to even bigger and better things. I’m excited about it.”

Amanda Solliday


A quick ski through history

Sunday, March 23rd, 2014

This past week about 175 lucky particle physicists gathered in La Thuile, a mountain town in the Italian Alps, for one of the annual Rencontres de Moriond conferences. This is one of the highlights of the particle-physics calendar, perhaps the most important gathering of particle physicists between the summer-time Lepton-Photon and ICHEP conferences for the presentation of new results. The major experimental collaborations of the world have been wrapping up a flurry of activity in preparation for the high-profile meetings taking place over the next few weeks. The atmosphere on the LHC experiments has been a bit less intense this year than last year, as the flashiest results from the 2010-12 data sample have already been released, but there was still a push to complete as many measurements as possible for presentation at this conference in particular.

I’ve only been to a Moriond conference once, but it was quite an experience. The conference is held at a ski resort to encourage cameraderie and scientific exchanges outside the conference room, and that leads to an action-packed week. Each morning of the week opens with about three hours of scientific presentations. The mid-morning finish allows for an almost-full day of skiing for those who chose to go (and as you might imagine, many do). This is a great opportunity to spend leisure time with colleagues, meet new people and discuss what had been learned that morning. After the lifts have closed, everyone returns to the hotel for another three hours of presentations. This is followed by a group dinner to continue the conversation. Everyone who has the chance to go realizes that they are very lucky to be there, but at the same time it is a rather exhausting experience! Or, as Henry Frisch, my undergraduate mentor and a regular Moriond attendee, once told me, “There are three things going on at Moriond — the physics, the skiing, and the food — and you can only do two out of the three.” (I skipped lunch on most days.)

As friends were getting ready to head south from CERN through the Mont Blanc tunnel to Italy (and as I was getting ready for my first visit to the United States in more than seven months, for the annual external review of the US LHC operations programs), I realized that it has in fact been ten years since the Moriond conference I went to. Thankfully, the conference organizers have maintained the conference website from 2004, allowing me to relive my presentation from that time. It is a relief to observe that our understanding of particle physics has advanced quite a bit since then! At that Moriond, the Tevatron was just starting to kick into gear for its “Run 2,” and during the previous year we had re-established the signal for the top quark that had first been observed in the mid-1990s. We were just starting to explore the properties of the top quark, but we were hampered by the size of the data sample at that point. It is amusing to look back and see that we were trying to measure the mass of the top quark with a mere six dilepton decay events! Over the coming years, the Tevatron would produce hundreds more such events, and the CDF and D0 experiments would complete the first thorough explorations of the top quark, demonstrating that its properties are totally in line with the predictions of the standard model. And since then, the LHC has done the Tevatron one better, thanks to both an increase in the top-quark production rate at the higher LHC energy and the larger LHC collision rate. The CMS top-quark sample now boasts about 70,000 dilepton candidate events, and the CMS measurement of the top-quark mass is now the best in the world.

Top-quark physics is one of the topics I’m most familiar with, so it is easy for me to mark progress there, but of course it has been a remarkable decade of advances for particle physics, with the discovery of the Higgs boson, a more thorough understanding of neutrino masses and mixing, and constraints on the properties of dark matter. Next year, the LHC will resume operations in its own “Run 2″, with an even higher collision energy and higher collision rates than we had in 2012. It is a change almost as great as that we experienced in moving from the Tevatron to the first run of the LHC. I cannot wait to see how the LHC will be advancing our knowledge of particle physics, possibly through the discovery of new particles that will help explain the puzzles presented by the Higgs boson. You can be sure that there will be a lot of excited chatter on the chair lifts around the dinner table at the 2016 Moriond conferences!


This is the last part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explained what is the Standard Model and show its limitations. Then I introduced supersymmetry and explained how it would fix the main flaws of the Standard Model. I now review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider (LHC) at CERN.

If Supersymmetry (or SUSY for short) is as good as it looks, why has none of the new SUSY particles been found yet? There could be many reasons, the simplest being that this theory is wrong and supersymmetric particles do not exist. If that were the case, one would still need another way to fix the Standard Model.

SUSY can still be the right solution if supersymmetric particles have eluded us for some reasons: we might have been looking in the wrong place, or in the wrong way or they could still be out of the reach of current accelerators.

So how does one go looking for supersymmetric particles? One good place to start is at CERN with the Large Hadron Collider or LHC. The 27-km long accelerator is the most powerful in the world. It brings protons into collisions at nearly the speed of light, generating huge amounts of energy in the tiniest points in space.  Since energy and matter are two forms of the same essence, like water and ice, the released energy materializes in the form of fundamental particles. The hope is to create some of the SUSY particles.

One major problem is that nobody knows the mass of all these new particles. And without the mass, it is very much like looking for someone in a large city without knowing the person’s address. All one can do then is comb the city trying to spot that person. But imagine the task if you don’t even know what the person looks like, how she behaves or even in which city, let alone which country she lives in.

Supersymmetry is in fact a very loosely defined theory with a huge number of free parameters. These free parameters are quantities like the masses of the supersymmetric particles, or their couplings, i.e. quantities defining how often they will decay into other particles. Supersymmetry does not specify which value all these quantities can take.

Hence, theorists have to make educated guesses to reduce the zone where one should search for SUSY particles. This is how various models of supersymmetry have appeared. Each one is an attempt at circumscribing the search zone based on different assumptions.

One common starting point is to assume that a certain property called R-parity is conserved. This leads to a model called Minimal SUSY but this model still has 105 free parameters. But with this simple assumption, one SUSY particle ends up having the characteristics of dark matter. Here is how it works: R-parity conservation states that all supersymmetric particles must decay into other, lighter supersymmetric particles. Therefore, the lightest supersymmetric particle or LSP cannot decay into anything else. It remains stable and lives forever, just like dark matter particles do. Hence the LSP could be the much sought-after dark matter particle.

SUSY-cascade-Fermilab Credit: Fermilab

How can the Large Hadron Collider help? Around the accelerator, large detectors act like giant cameras, recording how the newly created and highly unstable particles break apart like miniature fireworks. By taking a snapshot of it, one can record the origin, direction and energy of each fragment and reconstruct the initial particle.

Heavy and unstable SUSY particles would decay in cascade, producing various Standard Model particles along the way. The LSP would be the last possible step for any decay chain. Generally, the LSP is one of the mixed SUSY states with no electric charge called neutralino. Hence, each of these events contains a particle that is stable but does not interact with our detectors. In the end, there would be a certain amount of energy imbalance in all these events, indicating that a particle has escaped the detector without leaving any signal.

At the LHC, both the CMS and ATLAS experiments have searched billions of events looking for such events but to no avail. Dozens of different approaches have been tested and new possibilities are constantly being explored. Each one corresponds to a different hypothesis, but nothing has been found so far.

dijet-monjet Two events with jets as seen in the ATLAS detector. (Left) A very common event containing two jets of particles. The event is balanced, all fragments were recorded, no energy is missing. (Right) A simulation of a mono-jet event where a single jet recoils against something unrecorded by the detector. The imbalance in energy could be the signature of a dark matter particle like the lightest supersymmetric particle (LSP), something that carries energy away but does not interact with the detector, i.e. something we would not see.

One reason might be that all supersymmetric particles are too heavy to have been produced by the LHC. A particle can be created only if enough energy is available. You cannot buy something that costs more money than you have in your pocket. To create heavy particles, one needs more energy. It is still possible all SUSY particles exist but were out of the current accelerator reach. This point will be settled in 2015 when the LHC resumes operation at higher energy, going from 8 TeV to at least 13 TeV.

If the SUSY particles are light enough to be created at 13 TeV, the chances of producing them will also be decupled, making them even easier to find. And if we still do not find them, new limits will be reached, which will also greatly help focus on the remaining possible models.

SUSY has not said its last word yet. The chances are good supersymmetric particles will show up when the LHC resumes. And that would be like discovering a whole new continent.

Pauline Gagnon

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Voici le dernier d’une série de trois volets sur la supersymétrie, la théorie qui pourrait aller au-delà du Modèle Standard. J’ai expliqué dans un premier temps ce qu’est le Modèle standard et montré ses limites. Puis dans un deuxième volet, j’ai présenté la supersymétrie et expliqué comment elle pourrait résoudre plusieurs lacunes du Modèle standard. Finalement, je passe ici en revue comment les physicien-ne-s essaient de découvrir des « superparticules » au Grand collisionneur de hadrons (LHC) du CERN.

Si la supersymétrie (ou SUSY pour les intimes) est aussi miraculeuse que prédite, pourquoi aucune nouvelle particule supersymétrique n’a t-elle été trouvée à ce jour ? Il pourrait y avoir beaucoup de raisons, la plus simple étant que cette théorie soit fausse et les particules supersymétriques n’existent tout simplement pas. Si c’était le cas, on devrait alors trouver une alternative pour parer aux lacunes du Modèle standard.

Mais SUSY est toujours une solution plausible et ses particules supersymétriques ont pu nous échapper pour d’autres raisons. Peut-être avons nous regardé au mauvais endroit ou de la mauvaise façon. Ou encore elles pourraient être hors de portée de nos accélérateurs.

Mais au fait où et comment cherche-t-on des particules supersymétriques ? Le Grand collisionneur de hadrons (LHC) CERN est l’endroit idéal. Cet accélérateur de 27 km de longueur est le plus puissant au monde. Il provoque des collisions entre des protons lancés à une vitesse proche de celle de la lumière. Ces collisions produisent des quantités d’énergie énormes concentrés en de minuscules points de l’espace. Puisque l’énergie et la matière sont deux formes d’une même essence, comme l’eau et la glace, l’énergie libérée se matérialise sous forme de particules fondamentales. Il est donc possible de créer certaines de ces particules supersymétriques au LHC.

Malheureusement, personne ne connaît la masse de toutes ces nouvelles particules. Et sans la masse, c’est un peu comme chercher quelqu’un dans une grande ville sans connaître son adresse. Il faudrait alors ratisser la ville pour découvrir cette personne. Mais imaginez la tâche si vous ne savez même pas à quoi la personne ressemble, comment elle se comporte, ni même la ville ou le pays elle habite.

La supersymétrie est en fait une théorie comportant de nombreux paramètres libres. Ces paramètres représentent des quantités comme les masses des particules supersymétriques ou leurs couplages, c’est-à-dire la probabilité qu’elles se désintègrent en d’autres particules. La supersymétrie ne spécifie pas quelles valeurs ces quantités peuvent prendre.

Les théoricien-ne-s doivent donc faire des suppositions pour réduire la zone de recherches. C’est ainsi que divers modèles de supersymétrie sont apparus. Chaque modèle représente une tentative pour circonscrire la zone de recherche basée sur des suppositions différentes.

Une hypothèse populaire consiste à supposer qu’une certaine propriété appelée la parité R est conservée. C’est le cas pour le modèle minimal de SUSY mais il conserve tout de même 105 paramètres libres. Mais de cette simple supposition surgit une particule de SUSY ayant les caractéristiques de la matière sombre.

Voici comment ça marche : la conservation de R-parité stipule que toutes les particules supersymétriques doivent se désintégrer en d’autres particules supersymétriques. Par conséquent, la particule supersymétrique la plus légère, le LSP (de l’acronyme anglais Lightest Supersymmetric Particle) ne peut se désintégrer en rien d’autre et reste stable. Elle existe pour toujours, comme les particules de matière sombre. Le LSP pourrait donc être la particule de matière sombre tant recherchée.

Comment le Grand collisionneur de hadrons peut-il aider? Autour de l’accélérateur, de grands détecteurs agissent comme des appareils photo géants, enregistrant comment les particules nouvellement créées et fortement instables se brisent, créant de mini feux d’artifice. Ces clichés permettent d’enregistrer l’origine, la direction et l’énergie de chaque fragment et ainsi reconstruire la particule initiale.SUSY-decay-fr

Des particules de SUSY lourdes et instables se désintégreraient en cascade, produisant diverses particules du Modèle standard en chemin. Le LSP serait la dernière étape possible pour n’importe quelle chaîne de désintégration. Généralement, le LSP est un des états de SUSY mélangés sans charge électrique appelée neutralino. Au final, chaque événement supersymétrique contiendrait une particule stable, qui n’interagirait pas avec nos détecteurs. On observerait donc un déséquilibre dans la quantité d’énergie de tous ces événements, indiquant qu’une particule s’est échappée du détecteur sans laisser de signaux dans les diverses couches du détecteur.

Au LHC, les physicien-ne-s des expériences CMS et ATLAS ont trié des milliards d’événements à la recherche de tels événements, mais en vain. Des douzaines d’approches différentes ont été testées et de nouvelles possibilités sont constamment explorées. Chacune correspond à une hypothèse différente, mais rien n’a encore été trouvé.

Deux événements contenant des gerbes de particules captés par le détecteur ATLAS. (A gauche) un événement très courant contenant deux gerbes de particules. L’événement est équilibré en énergie, tous les fragments ont été enregistrés, aucune énergie ne manque. (A droite) une simulation d’un événement contenant une seule gerbe reculant contre quelque chose qui échappe au détecteur. Le déséquilibre dans l’énergie serait la signature d’une particule de matière sombre comme la particule supersymétrique la plus légère (LSP), une particule qui emporterait une certaine quantité d’énergie, mais n’interagirait pas avec le détecteur et que l’on ne verrait donc pas.

Il se peut aussi que toutes les particules supersymétriques soient trop lourdes pour avoir été produites par le LHC. Une particule peut être créée seulement si suffisamment d’énergie est disponible. On ne peut pas acheter quelque chose qui coûte plus que ce que l’on a dans sa poche. Pour créer des particules lourdes, il faut plus d’énergie. Il est donc toujours possible que toutes les particules de SUSY existent, mais qu’elles soient hors de la portée actuelle de l’accélérateur du LHC. Mais on en saura plus en 2015 quand le LHC reprendra du service à plus haute énergie, passant de 8 TeV à au moins 13 TeV.

Si les particules de SUSY sont assez légères pour être créé à 13 TeV, leurs chances de production seront aussi décuplées, les rendant encore plus facile à trouver. Et si nous ne les trouvons toujours pas, de nouvelles limites seront atteintes, ce qui permettra de se concentrer sur les modèles possibles restants.

SUSY n’a pas encore dit son dernier mot. Il reste de bonnes chances pour que des particules supersymétriques apparaissent quand le LHC redémarrera. Et si c’était le cas, ce serait aussi extraordinaire que la découverte d’un tout nouveau continent.

Pauline Gagnon

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This article appeared in symmetry on March 19, 2014.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

Scientists working on the world’s leading particle collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron and CERN’s Large Hadron Collider. These machines are the past and current holders of the record for most powerful particle collider on Earth.

Scientists from the four experiments involved—ATLAS, CDF, CMS and DZero—announced their joint findings on the mass of the top quark today at the Rencontres de Moriond international physics conference in Italy.

Together the four experiments pooled their data analysis power to arrive at a new world’s best value for the mass of the top quark of 173.34 ± 0.76 GeV/c2.

Experiments at the LHC at the CERN laboratory in Geneva, Switzerland and the Tevatron collider at Fermilab in Illinois, USA are the only ones that have ever seen top quarks—the heaviest elementary particles ever observed. The top quark’s huge mass (more than 100 times that of the proton) makes it one of the most important tools in the physicists’ quest to understand the nature of the universe.

The new precise value of the top-quark mass will allow scientists to test further the mathematical framework that describes the quantum connections between the top quark, the Higgs particle and the carrier of the electroweak force, the W boson. Theorists will explore how the new, more precise value will change predictions regarding the stability of the Higgs field and its effects on the evolution of the universe. It will also allow scientists to look for inconsistencies in the Standard Model of particle physics—searching for hints of new physics that will lead to a better understanding of the nature of the universe.

“The combining together of data from CERN and Fermilab to make a precision top quark mass result is a strong indication of its importance to understanding nature,” says Fermilab director Nigel Lockyer. “It’s a great example of the international collaboration in our field.”

Courtesy of: Fermilab and CERN

Courtesy of: Fermilab and CERN

A total of more than six thousand scientists from more than 50 countries participate in the four experimental collaborations. The CDF and DZero experiments discovered the top quark in 1995, and the Tevatron produced about 300,000 top quark events during its 25-year lifetime, completed in 2011. Since it started collider physics operations in 2009, the LHC has produced close to 18 million events with top quarks, making it the world’s leading top quark factory.

“Collaborative competition is the name of the game,” says CERN’s Director General Rolf Heuer. “Competition between experimental collaborations and labs spurs us on, but collaboration such as this underpins the global particle physics endeavor and is essential in advancing our knowledge of the universe we live in.”

Each of the four collaborations previously released their individual top-quark mass measurements. Combining them together required close collaboration between the four experiments, understanding in detail each other’s techniques and uncertainties. Each experiment measured the top-quark mass using several different methods by analyzing different top quark decay channels, using sophisticated analysis techniques developed and improved over more than 20 years of top quark research beginning at the Tevatron and continuing at the LHC. The joint measurement has been submitted to the arXiv.

A version of this article was originally issued by Fermilab and CERN as a press release.


Voici la deuxième partie d’une série de trois sur la supersymétrie, la théorie qui pourrait aller au-delà du Modèle standard. J’ai expliqué dans un premier temps ce qu’est le Modèle standard et montré ses limites. Je présenterai ici la supersymétrie et expliquerai comment elle pourrait résoudre plusieurs lacunes du Modèle standard. Finalement, je passerai en revue comment les physicien-ne-s essaient de découvrir des « superparticules » au Grand collisionneur de hadrons (LHC) du CERN.

Les théoricien-ne-s doivent souvent attendre pendant des décennies pour voir leurs idées confirmées par des découvertes expérimentales. Ce fut le cas pour François Englert, Robert Brout et Peter Higgs  dont la théorie, élaborée en 1964, ne fut confirmée qu’en 2012 avec la découverte du boson de Higgs  par les expériences du Grand collisionneur de hadrons (LHC).

Aujourd’hui, beaucoup de théoricien-ne-s ayant participé à l’élaboration de ce que l’on connaît maintenant comme la supersymétrie, attendent de voir ce que le LHC révélera.

La supersymétrie est une théorie qui est d’abord apparue comme une symétrie mathématique dans la théorie des cordes au début des années 1970. Au fil du temps, plusieurs personnes y ont apporté de nouveaux éléments, pour finalement aboutir aujourd’hui avec la théorie la plus prometteuse pour aller au-delà du Modèle standard. Parmi les pionniers, il faut d’abord citer deux théoriciens russes, D. V. Volkov et V. P Akulov. Puis en 1973, Julius Wess et Bruno Zumino ont écrit le premier modèle supersymétrique à quatre dimensions, pavant la voie aux développements futurs. L’année suivante, Pierre Fayet a généralisé le mécanisme de Brout-Englert-Higgs  à la supersymétrie et a introduit pour la première fois des superpartenaires pour les particules du Modèle standard.

Tout ce travail ne serait resté qu’un pur exercice mathématique si on n’avait remarqué que la supersymétrie pouvait résoudre certains problèmes fondamentaux du Modèle standard.

Comme nous avons vu, le Modèle standard contient deux types de particules fondamentales : les grains de matière, les fermions avec des valeurs de spin de ½, et les porteurs de force, les bosons avec des valeurs entières de spin.

Le simple fait que les bosons et les fermions n’aient pas les mêmes valeurs de spin les fait se comporter différemment. Chaque groupe obéit à des lois statistiques différentes. Par exemple, deux fermions identiques ne peuvent pas exister dans le même état quantique. Un de leurs nombres quantiques doit être différent. Ces nombres quantiques caractérisent diverses propriétés : leur position, leur charge, leur spin ou leur charge “de couleur” pour les quarks. Puisque tout le reste est identique, deux électrons sur une même orbite atomique doivent avoir deux orientations différentes de spin, une pointant vers le haut, l’autre vers le bas. Cela implique qu’au plus deux électrons peuvent cohabiter sur une même orbite atomique puisqu’il n’y a que deux orientations possibles pour leur spin. Les atomes ont donc plusieurs orbites atomiques pour accommoder tous leurs électrons.

Au contraire, il n’y a aucune restriction imposée au nombre de bosons autorisés à exister dans le même état. Cette propriété explique le phénomène de supraconductivité. Une paire d’électrons forme un boson puisque deux spins de une demie donnent un spin de 0 ou 1 suivant s’ils sont alignés ou non. Dans un supraconducteur, toutes les paires d’électrons peuvent être identiques, chaque paire possédant exactement les mêmes nombres quantiques, ceci étant permis pour les bosons. On peut donc échanger deux paires librement, comme pour du sable mouvant. Tous ses grains de sable sont de taille identique et peuvent changer de position librement, d’où son instabilité. De même, dans un supraconducteur, toutes les paires d’électrons peuvent changer de position, sans aucune friction et donc sans aucune résistance électrique.

La supersymétrie se fonde sur le Modèle standard et associe un « superpartenaire » à chaque particule fondamentale. Les fermions obtiennent des bosons comme superpartenaires et les bosons sont associés à des fermions. Ceci unifie les composantes fondamentales de la matière avec les porteurs de force. Tout devient plus harmonieux et plus symétrique.


La supersymétrie se fonde sur le Modèle standard et vient avec plusieurs nouvelles particules supersymétriques, représentées ici avec un tilde (~) au-dessus de leur symbole. (Diagramme tiré du film « Particle Fever » et reproduit avec la permission de Mark Levinson).

Mais il y a d’autres conséquences importantes. Le nombre de particules fondamentales double. La supersymétrie associe un superpartenaire à chaque particule du Modèle standard. De plus, plusieurs de ces partenaires peuvent se mélanger, donnant des états combinés comme les charginos et les neutralinos.

Les implications sont nombreuses. Première conséquence majeure : les deux superpartenaires du quark top, appelés stops, peuvent neutraliser la grande correction du quark top à la masse du boson de Higgs. Deuxième implication: la particule supersymétrique la plus légère (en général un des états mélangés sans charge électrique appelée neutralino) a justement les propriétés que la matière sombre devrait avoir.

Non seulement la supersymétrie réparerait plusieurs gros défauts du Modèle standard, mais elle résoudrait aussi le problème de la matière sombre. On ferait d’une pierre deux coups. Seul minuscule petit problème : si ces particules supersymétriques existent, pourquoi ne les a t’on pas encore trouvées? J’aborderai cette question dans la troisième et dernière partie de cette série.

Pauline Gagnon

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Supersymmetry: a tantalising theory

Wednesday, March 19th, 2014

This is the second part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explained what is the Standard Model and showed its limitations. I now introduce supersymmetry and explain how it would fix the main flaws of the Standard Model. Finally, I will review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider (LHC) at CERN.

Theorists often have to wait for decades to see their ideas confirmed by experimental findings. This was the case for François Englert, Robert Brout and Peter Higgs whose theory, elaborated in 1964, only got confirmed in 2012 with the discovery of the Higgs boson by the LHC experiments.

Today, many theorists who participated in the elaboration of what is now known as supersymmetry, are waiting to see what the LHC will reveal.

Supersymmetry is a theory that first appeared as a mathematical symmetry in string theory in early 1970s. Over time, several people contributed new elements that eventually led to a theory that is now one of the most promising successors to the Standard Model. Among the pioneers, the names of two Russian theorists, D. V. Volkov and V. P Akulov, stand out. In 1973, Julius Wess and Bruno Zumino wrote the first supersymmetric model in four dimensions, paving the way to future developments. The following year, Pierre Fayet generalized the Brout-Englert-Higgs mechanism to supersymmetry and introduced superpartners of Standard Model particles for the first time.

All this work would have remained a pure mathematical exercise unless people had noticed that supersymmetry could help fix some of the flaws of the Standard Model.

As we saw, the Standard Model has two types of fundamental particles: the grains of matter, the fermions with spin ½, and the force carriers, the bosons with integer values of spin.

The mere fact that bosons and fermions have different values of spin makes them behave differently. Each class follows different statistical laws. For example, two identical fermions cannot exist in the same quantum state, that is, something -one of their quantum numbers – must be different. Quantum numbers refer to various properties: their position, their charge, their spin or their “colour” charge for quarks. Since everything else is identical, two electrons orbiting on the same atomic shell must have different direction for their spin. One must point up, the other down. This means at most two electrons can cohabit on an atomic shell since there are only two possible orientations for their spins. Hence, atoms have several atomic shells to accommodate all their electrons.

On the contrary, there are no limitations on the number of bosons allowed in the same state. This property is behind the phenomenon called superconductivity. A pair of electrons forms a boson since adding two half spins gives a combined state with a spin of 0 or 1, depending if they are aligned or not. In a superconductor, all pairs of electrons can be identical, with exactly the same quantum numbers since this is allowed for combined spin values of 0 or 1. Hence, one can interchange two pairs freely, just like two grains of sand of identical size can swap position in quick sand, which makes it so unstable. Likewise, in a superconductor, all pairs of electrons can swap position with others, leaving no friction. An electric current can then flow without encountering any resistance.

Supersymmetry builds on the Standard Model and associates a “superpartner” to each fundamental particle. Fermions get bosons as superpartners, and bosons get associated with fermions. This unifies the building blocks of matter with the force carriers. Everything becomes more harmonious and symmetric.


Supersymmetry builds on the Standard Model and comes with many new supersymmetric particles, represented here with a tilde (~) on them. (Diagram taken from the movie “Particle fever” reproduced with permission from Mark Levinson)

But there are other important consequences. The number of existing fundamental particles doubles. Supersymmetry gives a superpartner to each Standard Model particle. In addition, many of these partners can mix, giving combined states such as charginos and neutralinos

This fact has many implications. First major consequence: the two superpartners to the top quark, called the stops, can cancel out the large contribution from the top quark to the mass of the Higgs boson. Second implication: the lightest supersymmetric particle (in general one of the mixed states with no electric charge called neutralino) has just the properties one thinks dark matter should have.

Not only supersymmetry would fix the flaws of the Standard Model, but it would also solve the dark matter problem. Killing two huge birds with one simple stone. There is just one tiny problem: if these supersymmetric particles exist, why have we not found any yet? I will address this question in the next part in this series.

Pauline Gagnon

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I previously blogged about how CERN is embracing the power of citizen science to assist with it’s research (see here). [email protected] also allows non-scientists to get involved with particle physics at CERN. To learn a bit more about citizen science, I recently attended the third Citizen Cyberscience Summit in London, and was asked by online publication ‘International Science Grid This Week’ to write an article on the event. The article was published here on 12 March 2014, and I replicate it below.

February saw London host the Third Citizen Cyberscience Summit, a three-day event dedicated to the expanding field of citizen science. More than 300 delegates from around the world assembled to network, share ideas, and get creative. The event provided fascinating insight into how computers, mobile phones, and other devices are helping to mobilize the citizen science community. Attendees were left with the distinct impression that citizen science is no passing fad but a movement on the forefront of a fundamental shift in how we approach science and education.

Citizen Science?

For the uninitiated, citizen science is scientific research conducted in whole or part by amateur or non-professional scientists. There is a spectrum of different kinds of citizen science from ‘crowdsourcing’, in which citizens analyze data, to ‘extreme citizen science’, where scientists collaborate with citizens in problem definition, data collection and analysis.

One of the youngest delegates gets the chance to hack a drone. Image courtesy James Doherty.

One of the youngest delegates gets the chance to hack a drone. Image courtesy James Doherty.

Citizen science stars

On day one of the summit, keynote speeches from the heavyweights of citizen science left delegates with no doubt that citizen science is a big deal. The crown jewel of citizen science remains the Zooniverse, a top-down crowdsourcing platform where citizens analyze large sets of data, such as pictures of galaxies that need to be classified as elliptical or spiral. The Zooniverse has reached the significant milestone of one million contributors and plans to use human input to program computers for data analysis.

Other leaders in the field include Eyewire, an addictive game in which participants help scientists map neural connections in the brain. Erinma Ochu delivered a heart-warming account of her sunflower project, in which participants grow and help to analyze sunflowers, and Daniel Lombraña González reported on how CrowdCrafting has helped interest groups in the US monitor fracking activities.

World Community Grid, which uses spare capacity on computers and mobile devices to power scientific research on health, poverty and sustainability, was also featured at the summit. Sophia Tu and Juan Hindo of IBM, which sponsors and supports World Community Grid, announced a major scientific breakthrough in childhood cancer. Donated computing capacity has enabled researchers of childhood cancer to discover seven drug candidates that are highly effective at destroying tumors without any apparent side effects. This may also have applications for adult cancers, including breast and lung cancer.

The atomic force microscope hacked by Lego2nano students. Image courtesy James Doherty.

The atomic force microscope hacked by Lego2nano students. Image courtesy James Doherty.

Tu and Hindo also talked at the summit about their experience launching an Android mobile app in partnership with BOINC of the University of California, Berkley, US, last July, becoming one of the first volunteer computing initiatives to go mobile. They found that many citizen scientists are more comfortable downloading a mobile app than installing software on their computer: signups jumped ten times the week of launch and the app quickly reached ‘Top 5 Trending’ status in the Google Play store.

Citizen science’s expanding influence was evidenced by a series of talks on policy and engagement. Jacquie McGlade’s video presentation highlighted the importance of inclusiveness and keeping the gates of citizen science open to all. A recurring theme emerged that citizens are enjoying more autonomy in defining the projects to which they contribute. And Kaitlin Thaney of the Mozilla Science lab argued that the wider researcher community should draw inspiration from citizen science and the web’s open-source revolution to itself become more open and collaborative.

Engaging and empowering citizens

On the second day, the summit moved to less formal surroundings in University College London (UCL), UK, for workshops, panel discussions, and short presentations. A panel debate with five female citizen scientists highlighted the commitment of people engaged with citizen science and the empowerment they experience from contributing to projects.

At a series of talks on DIY citizen science, Francois Grey, coordinator of the Citizen Cyberscience Centre, described how the Lego2nano summer school program mobilized a group of students to construct an atomic force microscope for a fraction of the price of those found in modern labs. Grey argued that because so much can be learned from making and programming technology, young people should be encouraged to become makers, engineers, and programmers.

In the evening, the Citizen Science Cafe also afforded those not attending the conference the opportunity to pop-in after work to mingle with others passionate about citizen science.

What can you do for your community?

By day three of the summit, delegates were chomping at the bit to start doing some citizen science themselves. Saturday’s ‘hackdaychallenge’ provided an opportunity for collaboration on a number of different projects, including: constructing an application that enables citizens to analyze photographs from the International Space Stationmaking the most of solar panels installed in Ghanian schools; and an application developed by child psychologist Caspar Addyman for analyzing baby laughter. The winning project was the Pulse Sensor Textile Challenge, which aims to measure the impact of textiles on emotions.

The closing keynote presentation was delivered by journalist-turned-academic Jeff Howe, who first coined the term ‘crowdsourcing’. In this entertaining and insightful talk, Howe noted that the most successful crowdsourcing projects are often a gift from an individual to the community, and providediStockphoto.com as an illustrative example. He argued that the cardinal rule of crowdsourcing is that one should ask ‘what can you do for your community, not what your community can do for you’. Like Grey, Howe suggested that citizen science has the potential to bring about a fundamental change in how young people are educated.

Hackday team discusses how photographs taken on mobile phones may be used in disaster response. Image courtesy James Doherty.

Hackday team discusses how photographs taken on mobile phones may be used in disaster response. Image courtesy James Doherty.

More than a passing fad

A recent article in Nature hinted at a certain decline in citizen science, but little evidence of this trend was on display at the summit, which was saturated with energy, enthusiasm and love for citizen science. Grey described the event not as a “thin broth with just one intellectual ingredient but a rich stew of ideas with spices from far-away fields”. This is, indeed, reflective of the pervasive nature of citizen science: there is a vibrant community across the globe analyzing data, playing games, growing sunflowers, posting pictures, monitoring pollution, and more.

So, citizen science seems to be much more than a passing fad that is now in decline. Rather, it is a movement that empowers its participants. That demands openness, collaboration and accessibility. That has the potential to bring about change. And most importantly that recognizes, as Howe puts it, that “everyone has something to offer”.


— by T.I. Meyer, Head of Strategic Planning & Communication

This past Saturday, I attended a “celebration of life” for Erich W. Vogt, one of the founders of TRIUMF and perhaps the last of the generation of “Renaissance-man” style leaders who helped shape the modern era of particle and nuclear physics.

“Celebration of life” is North American politeness for memorial service. Erich passed away on February 19, 2014, at the age of 84. He was with family and friends until the very end, and each day he would tell us a new historical anecdote, hilarious and penetrating as always, and then comment on his intentions to return to work at TRIUMF the next morning.

The service itself was spectacular with about 400 people packed into the former faculty club on the UBC campus. We were regaled with a litany of precise, powerful speeches that mirrored Erich’s personality in so many ways: witty, thoughtful, provocative, and unabashed. The collected wisdom and life experience in the room was stupefying, perhaps an even larger testament to the impact that Erich had on all of us—and the entire world.

I went with my wife and our three-month old daughter. I told people that I was hoping she’d be inspired by the legacy and soak up some of the aura of longevity and greatness.

But that got me to thinking. Erich was one of “those” scientists, the ones who were shrewd, sharp-witted, and educated in everything from particle physics and international politics to porcelain plateware and the development of the modern piano. In his spare time, he met Einstein, befriended prime ministers, raised money for and founded a laboratory in Israel, wrote an authoritative history of his family and its origins, and helped articulate and lead the vision for a national subatomic-physics laboratory in Canada that became TRIUMF.

We can look through the records and the recollections of those who knew Erich to trace out how he became who he was. But I often wonder where the next generation of Erichs is coming from. Are they here and I just don’t see them? Is our society still inspiring and retaining people like this? Is there still a valuable role for these types of “Renaissance” people? Moreover, are they needed, or is there even a place for them in our 21st century culture?

It does seem that the best and brightest of any generation tend to seek their personal, financial, and intellectual fortunes at the edgy frontiers. Some people argue that science has faded from the position of being The Most Exciting and Challenging Frontier and is now replaced by entrepreneurship, social expression, and so on. These people would argue that the next generation of “Renaissance” types are still there, but they are no longer flocking to science, or even more specifically, to physics. They are simply going elsewhere.

Others will argue that the modern system of measuring achievement works against the Renaissance individual. In the 20th century, the ambitious intellectual was able to develop mastery in multiple fields and to pursue vigourously multiple interests in an environment that placed fewer burdens on them. The culture allowed—and even encouraged—such a person to seek greatness. But in today’s landscape, to be successful, one needs to be increasingly specialized and spend more time writing grants, reviewing articles, and attending soft-skills training classes. It is said that we’ve moved into the era where “Jack of all trades, master of none” holds true, and that is how we dismiss the Renaissance person.

But are we in a society that no longer allows these broad-minded, passionate individuals to blossom and flourish? Has there been a recalibration of culture where these types are now as important as the focused specialist? Or perhaps the world is so complicated and fractured that a classical approach to mastery is simply ineffective?

In my view, the truth is somewhere in the middle. The 21st century is going to require a new type of individual to make pivotal contributions. The qualities of leadership and greatness do last more than one generation, but they evolve perhaps every three or four generations. Instead of wishing for the leaders of the last era, our task is to look at the world today: who is making an impact, what are they bringing to the table, and how can we make more of that happen?

And in our world of networks (virtual and social) and complexities, greatness can emerge more easily from the combined contributions of dozens or even hundreds of people. For instance, a select few physicists won the Nobel Prize for the experimental work that discovered the electron, the neutrino, and so on. For the Higgs boson, however, the Nobel Prize went to the two surviving theorists who posited its existence, in part because the discovery-in-reality was the product of a cast of 10,000 people. It would be silly to try and select just two or three people that made it happen. It took everyone! Now, and perhaps for the 21st century, that is greatness.

Looking across the frontiers of science, who are the leaders today? Are there common characteristics? How do they distinguish themselves?

Tell me what you see!


This is the first part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explain what is the Standard Model and show its limitations. Then I introduce supersymmetry and explain how it would fix the main flaws of the Standard Model. Finally, I will review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider at CERN.

The Standard Model describes what matter is made of and how it holds together. It rests on two basic ideas: all matter is made of particles, and these particles interact with each other by exchanging other particles associated with the fundamental forces.

The basic grains of matter are fermions and the force carriers are bosons. The names of these two classes refer to their spin – or angular momentum. Fermions have half-integer values of spin whereas bosons have integer values as shown in the diagram below.


Fermions come in two families. The leptons family has six members, with the electron being the best known of them. The quarks family contains six quarks. The up and down quarks are found inside protons and neutrons. The twelve fermions are the building blocks of matter and each one has a spin value of ½.

These particles interact with each other through fundamental forces. Each force comes with one or more force carriers. The nuclear force comes with the gluon and binds the quarks within the proton and neutrons. The photon is associated with the electromagnetic force. The weak interaction is responsible for radioactivity. It comes with the Z and W bosons. All have a spin of 1.

The main point is: there are grains of matter, the fermions with spin ½, and force carriers, the bosons with integer values of spin.

The Standard Model is both remarkably simple and very powerful. There are complex equations expressing all this in a mathematical way. These equations allow theorists to make very precise predictions. Nearly every quantity that has been measured in particle physics laboratories over the past five decades falls right on the predicted value, within experimental error margins.

So what’s wrong with the Standard Model? Essentially, one could say that the whole model lacks robustness at higher energy. As long as we observe various phenomena at low energy, as we have done so far, things behave properly. But as accelerators are getting more and more powerful, we are about to reach a level of energy which existed only shortly after the Big Bang where the equations of the Standard Model start getting shaky.

This is a bit like with the laws of physics at low and high speed. A particle moving at near the speed of light cannot be described with the simple laws of mechanics derived by Newton. One needs special relativity to describe its motion.

One major problem of the Standard Model is that it does not include gravity, one of the four fundamental forces. The model also fails to explain why gravity is so much weaker than the electromagnetic or nuclear forces. For example, a simple fridge magnet can counteract the gravitational attraction of a whole planet on a small object.

This huge difference in the strength of fundamental forces is one aspect of the “hierarchy problem”. It also refers to the wide range in mass for the elementary particles. In the table shown above, we see the electron is about 200 times lighter than the muon and 3500 times lighter than the tau. Same thing for the quarks: the top quark is 75 000 times heavier than the up quark. Why is there such a wide spectrum of masses among the building blocks of matter? Imagine having a Lego set containing bricks as disparate in size as that!

The hierarchy problem is also related to the Higgs boson mass. The equations of the Standard Model establish relations between the fundamental particles. For example, in the equations, the Higgs boson has a basic mass to which theorists add a correction for each particle that interact with it. The heavier the particle, the larger the correction. The top quark being the heaviest particle, it adds such a large correction to the theoretical Higgs boson mass that theorists wonder how the measured Higgs boson mass can be as small as it was found.

This seems to indicate that other yet undiscovered particles exist and change the picture. In that case, the corrections to the Higgs mass from the top quark could be cancelled out by some other hypothetical particle and lead to the observed low Higgs boson mass. Supersymmetry just happens to predict the existence of such particles, hence its appeal.

Last but not least, the Standard Model only describes visible matter, that is all matter we see around us on Earth as well as in stars and galaxies. But proofs abound telling us the Universe contains about five times more “dark matter”, a type of matter completely different from the one we know, than ordinary matter. Dark matter does not emit any light but manifests itself through its gravitational effects.  Among all the particles contained in the Standard Model, none has the properties of dark matter. Hence it is clear the Standard Model gives an incomplete picture of the content of the Universe but supersymmetry could solve this problem.

Pauline Gagnon

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