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Archive for 2012

11 rhymes with 1927, so let’s talk about the fifth Solvay Conference!

See some of video footage of the conference itself: http://www.youtube.com/watch?v=8GZdZUouzBY

Although the Heisenberg matrix method and Schrodinger wave mechanics had already been reconciled in 1926, and most physicists had come to a consensus shortly after the Solvay Conference, Bohr and Einstein continued to disagree. They discussed their disagreements extensively in the famous Einstein-Bohr debates, and this culminated in the famous EPR paradox in 1935. The resolution of the paradox would not come until long after Einstein had died. You can read Einstein’s infamous paper here: http://prola.aps.org/abstract/PR/v47/i10/p777_1

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10 is for the missing 10th particle. It’s no longer missing! A single event completed our view of the light spin-3/2 baryons.

See the movie Decay: http://www.decayfilm.com/
Event image taken from: http://teachers.web.cern.ch/teachers/archiv/HST2001/bubblechambers/omegaminus.pdf

Apologies for the lateness, I’m playing catch-up following a busy week and many technical issues!

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Gluino, Higgsino, bingo!

Monday, December 10th, 2012

Gluinos and Higgsinos are some of the many undiscovered particles we may find at the Large Hadron Collider (LHC) if a theory called supersymmetry, also known as SUSY, turns out to be true. This theory is built on the Standard Model, the current theoretical model of particle physics.

The Standard Model relies on the Higgs boson to hold true. But even with this boson, physicists know that this model cannot be the final answer as it has a few shortcomings. For example, it fails to provide an explanation for dark matter or why the masses of fundamental particles such as electrons and muons are so different. This theory of supersymmetry is one of the most popular and most promising ways to extend the Standard Model, but it has yet to manifest itself.

SUSY is very popular since it brings lots of harmony in the world of sub-atomic particles. In the Standard Model, there are two types of particles: fermions and bosons. The fermions include quarks and leptons and are the building blocks of matter.  These particles have “spin” values of ½. The force carriers are bosons, the other family of particles. They have integer values of spin, that is, 0 or 1.

Supersymmetry would blend fermions and bosons together by associating partners to each particle: a fermion would be paired with a boson, and vice-versa. For example, each quark would come with a “squark,” the name given to the supersymmetric partners of quarks. The squarks would be bosons rather than fermions and would carry spin 0. The same thing goes for leptons. Likewise, the known bosons (gluons, Higgs, W, Z and photons) would come with fermion superpartners with half spin values. These would be the gluinos, Higgsinos, winos, zinos and photinos. A mixture of the force carrier superpartners (all except the gluinos) gives charginos and neutralinos, the latter being particles that would be the perfect candidates for dark matter.

But it is difficult to work with SUSY (nothing personal of course!). Even in its minimal version called the Minimal Supersymmetric Model or MSSM, the theory comes with 105 free parameters. This means each parameter, like the masses of all these particles, is free to take any value it likes.

Think of a parameter as a dimension. Say we need a search party to locate hikers lost anywhere in the Alps. We would have to check every 10 m or so within that huge area. So even when trying to select one single point in a two-dimensional space, the task is daunting. The exact location can be any of a multitude of points within a huge two-dimensional space. Now try to imagine the same situation with a 105-dimensional object! It becomes hopeless.

Adding some reasonable constraints helps such as saying the location can only be on firm ground and not in a lake. This is why theorists have been trying to limit the range of each parameter to reduce the space that would need to be searched to find all these new SUSY particles. A subset of the MSSM model called the Constrained MSSM (CMSSM) model was built leaving only a handful of free parameters. This was achieved by picking somehow arbitrary values for some of these parameters, often guided by taste or guesses for lack of experimental constraints. This is a bit as if in our search for the lost hikers, we decided to ignore Switzerland because we did not like cheese, instead of taking into account the hikers’ interests or habits. But despite all its shortcomings, this model is still largely used.

Since every new theory can only be valid if it can reproduce all known observations, the phenomenological MSSM or pMSSM model was developed using all sorts of measurements done over the past decades in particle physics to constrain the original set of 105 free parameters of the MSSM. With these experimental assumptions, the pMSSM model is reduced to 19 free parameters.  There is progress.

Three theorists Alex Arbey, Abdelhak Djouadi and Nazila Mahmoudi, and one experimentalist, Marco Battaglia, form one of the teams who are now going one step further. They are using all available experimental information to see which values of each parameter are still allowed for the different models. This technique requires lots of computing power to test each point of the multi-dimensional space but in the end, one can really see where supersymmetry can still hide.

This method had already revealed that very constrained and specific versions of SUSY like the CMSSM model were getting squeezed into small corners. On the other hand, the pMSSM model has been reduced considerably but still has plenty of space available. Taking into account the recent experimental constraints from the LHC and astrophysics results on dark matter searches, including the recent value obtained by LHCb on Bs mesons decaying into two muons, about 10% of the hundred million possibilities these scientists studied remain valid. And when the measurements on the Higgs-like boson mass and decay rates are taken into account, only 2.5% of the original scenarios in their study survive.

Thanks to this technique of weaving together experimental facts and theoretical knowledge, this team of scientists has been able to reduce a near infinite number of possibilities to only a few percent of what it originally was, making it easier to narrow down the search. Nevertheless, this still leaves plenty of space where one form or another of supersymmetry can exist. We might not be lucky enough to discover SUSY this year but will surely have a good chance at it once the LHC comes back to full power in 2015 after extensive work aimed at increasing the capacity of the accelerator in the coming two years.

Meanwhile, SUSY is still alive and might be kicking around in one point of its now much more confined 105-dimensional space.

Pauline Gagnon

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Gluino, Higgsino, bingo!

Monday, December 10th, 2012

Gluinos et Higgsinos sont quelques-unes des nouvelles particules jamais observées mais que l’on pourrait bien découvrir avec le Grand Collisionneur de Hadrons (LHC) si une théorie appelée supersymétrie ou SUSY s’avère exacte. Cette théorie est bâtie sur le Modèle Standard de la physique des particules.

Même avec le boson de Higgs, le Modèle Standard aura encore quelques lacunes. Par exemple, il n’arrive pas à expliquer la nature de la matière noire ou encore pourquoi les masses des particules fondamentales comme les électrons et les muons diffèrent autant. La théorie de supersymétrie est l’une des plus populaires et prometteuses pour dépasser le Modèle Standard mais encore faut-il qu’on arrive à la détecter.

Sa popularité tient à l’harmonie qu’elle apporte au monde des particules sub-atomiques. On y trouve deux grandes catégories de particules : les fermions et les bosons. Les fermions comprennent les quarks et les leptons, les briques de base pour construire toute la matière. Ces particules portent une valeur de « spin » de ½. Les porteurs de force appartiennent aux bosons, l’autre famille de particules. Ils ont des valeurs entières de spin, soit 0 ou 1.

La supersymétrie unifierait ces deux mondes en associant un boson à chaque fermion, et vice versa. Chaque quark serait accompagné de son « squark », le nom donné aux partenaires supersymétriques des quarks. Les squarks seraient des bosons au lieu de fermions et auraient un spin de 0 ou 1. La même chose irait pour les leptons. Pareillement, tous les bosons connus (gluons, Higgs, W, Z et photons) viendraient avec leur superpartenaire, des fermions de spin ½. En mélangeant ces fermions (tous sauf les gluinos), on obtient les charginos et neutralinos. Ces derniers pourraient constituer la fameuse matière noire.

Mais travailler avec SUSY n’est pas facile (et je ne pense à personne en particulier…) Même dans sa version minimale, le modèle dit MSSM (pour Minimal Supersymmetric Model) compte 105 paramètres libres. En d’autres mots, chacun de ces 105 paramètres (dont les masses de toutes ces particules) peut prendre n’importe laquelle valeur.

Un paramètre est un peu comme une dimension. Imaginez qu’on cherche à localiser un groupe de randonneuses perdues quelque part dans les Alpes. Il faudrait vérifier chaque point sur une carte tous les 10 mètres dans cet immense superficie. Donc même dans un espace bi-dimensionnel (i.e. avec seulement deux paramètres), il existe une quantité effroyable de lieux potentiels à vérifier. Alors essayez d’imaginer la situation avec 105 paramètres libres. Cela devient impossible.

On peut se faciliter la tâche en ajoutant des contraintes raisonnables, comme par exemple, limiter nos recherches à la terre ferme et éliminer tous les lacs. C’est précisément l’approche adoptée : limiter le périmètre de recherches pour ces particules supersymétriques en éliminant les endroits improbables.

Un sous-ensemble du modèle MSSM appelé CMSSM pour Constrained MSSM a été élaboré afin de ne laisser qu’une poignée de paramètres libres. On y est parvenu en fixant plus ou moins arbitrairement les valeurs de plusieurs paramètres, guidé par le goût personnel ou l’intuition. Un peu comme si on décidait d’abandonner les recherches dans toute la Suisse parce qu’on n’aime pas le fromage, sans tenir compte des habitudes ou intérêts des randonneuses égarées.  Malgré ces origines douteuses, ce modèle demeure encore très utilisé.

Puisque tout modèle se doit de reproduire toutes les observations expérimentales faites à ce jour, un nouveau modèle dit phénoménologique (dénoté pMSSM) a été élaboré en utilisant des contraintes extraites de toutes sortes d’observations expérimentales effectuées au cours des dernières décennies. Avec ces contraintes expérimentales, le modèle pMSSM se réduit à 19 paramètres libres. On avance !

Trois théoricien-ne-s, Alex Arbey, Abdelhak Djouadi et Nazila Mahmoudi, ainsi qu’un expérimentateur Marco Battaglia, forment une des équipes qui ont poussé cette logique un cran plus haut. Ils incorporent l’information expérimentale récente pour voir quelles valeurs de ces 19 paramètres sont encore valides. Cette technique a le défaut d’exiger des quantités énormes de calculs pour tester chaque point de cet espace à 19 dimensions mais au final, on peut vraiment voir où les particules supersymétriques peuvent encore se cacher.

Cette méthode avait déjà révélé que les versions de SUSY très contraintes et spécifiques comme le CMSSM sont désormais confinées à très peu de valeurs de paramètres encore possibles. Les plus fortes contraintes sont imposées par l’absence de découverte par ATLAS et CMS de squarks de masses allant jusqu’à 1 ou 1.5 TeV, soit dix fois la masse du boson trouvé en juillet.

La même technique appliquée au pMSSM montre que ce modèle est désormais plus limité mais a encore beaucoup de valeurs de paramètres permises. Si en plus on ajoute les résultats d’astrophysique sur la matière noire ainsi que les mesures récentes du LHC dont celle de LHCb sur les désintégrations de Bs en deux muons, seules 10% des millions de possibilités initiales étudiées par ces chercheur-e-s subsistent. Les résultats sur le potentiel boson de Higgs, eux ne laissent que 2.5% des possibilités de leur scénario initial.

Grâce à cette technique qui entremêle résultats expérimentaux et connaissances théoriques, cette équipe de scientifiques a pu réduire un nombre quasi infini de possibilités à quelques pourcents, permettant ainsi de mieux cibler les recherches.  Malgré tout, il reste encore plein d’endroits dans ce vaste espace multi-dimensionnel où une forme ou une autre de supersymétrie peut encore exister. On n’aura peut-être pas la chance de la découvrir cette année mais nos chances seront accrues lorsque le LHC reprendra du service à plus haute énergie  après la longue période d’entretien et d’améliorations prévue pour 2013-2014.

Entre temps, SUSY est toujours en pleine santé et s’ébat peut-être dans un petit recoin de ce désormais plus restreint espace à 105 dimensions.

Pauline Gagnon

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9 is for the 9 elements of the neutrino mixing matrix. Why do they mix? What does it tell us? Does this give us even more matter-antimatter asymmetry?

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8 is for gluons! They’re strong, they’re coloured, and you can never catch one all by itself!

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In any genealogy there are always things one wants to hide; the misfit relative, the children born on the wrong side of the sheet, or the relative Aunt Martha just does not like. As a genealogist it takes a lot of effort to find these things out. Genealogies tend to be sanitized: the illegitimate grandchild becomes a legitimate child, the misfit relative somehow missed being included, and Uncle Ben aged 15 years during the 10 years between censuses and died at age one hundred although he was only eighty[1] according to the earliest records. The roots of science, like all family histories, have undergone a similar sanitization process. In a previous essay, I gave the sanitized version. In this essay, I give the unauthorized version. Aunt Martha would not be happy.

The Authorized Version has the origins of science tied closely to the Greek philosophical tradition with science arising within and from that tradition. While that has some truth to it, it is not the whole truth. It was two millennia from Aristotle to the rise of science and there have been many rationalizations for this delay, many starring Christianity as the culprit. However, Christianity did not gain political strength for a few centuries after Aristotle’s death. So, if the cause was Christianity, it must have had a miraculous non-causal effect.

A lot happened between Aristotle and the rise of science: the rise and fall of the Roman Empire, the rise of Christianity and Islam, the Renaissance, the reformation, and the printing press.  A lot of what could be called knowledge was developed but did not provide major gains in philosophy. The Romans were for the most part engineers not philosophers, but to do engineering takes real knowledge. Let us pass on to the Arabs. They are generally considered as a mere repository for Greek knowledge which was then passed on to the West largely intact but with some added commentary. I suspect that is not correct, as I will argue shortly.

There are two contributions to the development of science that are frequently downplayed: astrology and alchemy. These are the ancestors that science wants to hide. We all know the story of the Ptolemy and Copernicus, but the motivation for the development of astronomy was astrological and religious. From the ancient Babylonians to the present day, people have tried to divine the future by studying the stars. It is no accident that astronomy was one of the first sciences. It had practical applications: astrology (Kepler was a noted astrologer) and the calculation of religious holidays, most notably Easter.  One of the reasons Copernicus’s book was not banned was because the church found it useful for calculating the date of Easter. The motion of the planets is also sufficiently complicated that they could not be predicted trivially, yet they were sufficiently simple to be amenable to treatment by the mathematics of the day. Hence, it became the gold standard of science.  Essentially we dropped the motivation but kept the calculations.

Alchemy, the other problem ancestor, is even more interesting.  The Arabs, those people who are considered to have produced nothing new, had within their ranks Jābir ibn Hayyān (721 – 815),  chemist and alchemist, astronomer and astrologer, engineer, geographer, philosopher, and physicist, pharmacist and physician—in general, an all-around genius. He, along with Robert Boyle (1627 – 1691), is regarded as the founder of modern chemistry, but note how far in advance Jābir ibn Hayyān was—900 years.  Certainly he took alchemy beyond the occult to the practical. Although the alchemists never succeeded in turning lead into gold, they did produce a lot of useful metallurgy and chemistry. It is indeed possible that along with his chemical pursuits, Jābir ibn Hayyān forged the foundation of science by going down to the laboratory and seeing how things actually worked.  It is no surprise that the first two people to introduce something like science into Western Europe, Frederick II (1194 – 1250) and Roger Bacon (c. 1214–1294), were both very familiar with Arab scholarship and presumably with Jābir ibn Hayyān’s work.  In addition, both Isaac Newton (1642 – 1727) and Robert Boyle (1627 – 1691) were alchemists. A major role for alchemy in the development of science cannot be creditably denied.

It is perhaps wrong to think of astrology and alchemy as separate. To turn cabbage into sauerkraut, you need to know the phase of the moon[2] and the same probably holds true for turning lead into gold. Hence, most alchemists were also astrologers.  But alchemy and astrology have always had a dark side of occult, showmanship, and outright fraud. A typical, perhaps apocryphal, example would be Dr. Johann Georg Faust (c. 1480 – c. 1540). He was killed around 1540 when his laboratory exploded. Or his laboratory exploded when the devil came to collect his soul.  This person is presumably the origin of the Dr. Faust legend of the man who sold his soul to the devil for knowledge.  It interesting that this legend arose in the late sixteenth century just as science was beginning to rise from obscurity. The general population’s suspicion of learning, there from the beginning, has perhaps never really gone away.

The philosophers and theologians, the beautiful people, had their jobs in the monasteries and universities but science owes more to the people who sold their soul, or at least their health, to the devil for knowledge. These were the people who actually went down to the laboratories and did the dirty work to see how the world actually works.

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[1] My genealogy of the Musquodoboit Valley has all these and more (http://www.rootsweb.ancestry.com/~canns/mus.pdf).

[2] Or so my uncle claimed.

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Why is spin called spin? After all, nothing is spinning! To find out we need to go back about 100 years…

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“Who ordered that?!” Those infamous words, uttered by Rabi, were the first sign that there was more to the universe than just protons, neutrons and electrons. Meet the muon!
ATLAS event display taken from http://www.atlas.ch/photos/ , where ATLAS shows off pretty images.

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5 is for pentaquarks, the exotic baryons that got discovered and then undiscovered!
Slides taken from Pat Burchat’s excellent talk on pentaquarks: https://www.stanford.edu/dept/physics/people/faculty/docs/burchatUCIseminar.pdf
Fake peak plot from paper by Torres and Oset: http://arxiv.org/abs/1012.2967
The Particle Data Group (PDG) review on pentaquarks (PDF documents) 2008, 2006, 2004.

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