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Posts Tagged ‘Nobel Prize’

Nobel Prize in Physics 2015

Tuesday, October 6th, 2015

So, the Nobel Prize in Physics 2015 has been announced. To much surprise of many (including the author), it was awarded jointly to Takaaki Kajita and Arthur B. McDonald “for the discovery of neutrino oscillations, which shows that neutrinos have mass.” Well deserved Nobel Prize for a fantastic discovery.

What is this Nobel prize all about? Some years ago (circa 1997) there were a couple of “deficit” problems in physics. First, it appeared that the detected number of (electron) neutrinos coming form the Sun was measured to be less than expected. This could be explained in a number of ways. First, neutrino could oscillate — that is, neutrinos produced as electron neutrinos in nuclear reactions in the Sun could turn into muon or tau neutrinos and thus not be detected by existing experiments, which were sensitive to electron neutrinos. This was the most exciting possibility that ultimately turned out to be correct! But it was by far not the only one! For example, one could say that the Standard Solar Model (SSM) predicted the fluxes wrong — after all, the flux of solar neutrinos is proportional to core temperature to a very high power (~T25 for 8B neutrinos, for example). So it is reasonable to say that neutrino flux is not so well known because the temperature is not well measured (this might be disputed by solar physicists). Or something more exotic could happen — like the fact that neutrinos could have large magnetic moment and thus change its helicity while propagating in the Sun to turn into a right-handed neutrino that is sterile.

The solution to this is rather ingenious — measure neutrino flux in two ways — sensitive to neutrino flavor (using “charged current (CC) interactions”) and insensitive to neutrino flavor (using “neutral current (NC) interactions”)! Choosing heavy water — which contains deuterium — is then ideal for this detection. This is exactly what SNO collaboration, led by A. McDonald did

Screen Shot 2015-10-06 at 2.51.27 PM

As it turned out, the NC flux was exactly what SSM predicted, while the CC flux was smaller. Hence the conclusion that electron neutrinos would oscillate into other types of neutrinos!

Another “deficit problem” was associated with the ratio of “atmospheric” muon and electron neutrinos. Cosmic rays hit Earth’s atmosphere and create pions that subsequently decay into muons and muon neutrinos. Muons would also eventually decay, mainly into an electron, muon (anti)neutrino and an electron neutrino, as

Screen Shot 2015-10-06 at 2.57.37 PM

As can be seen from the above figure, one would expect to have 2 muon-flavored neutrinos per one electron-flavored one.

This is not what Super K experiment (T. Kajita) saw — the ratio really changed with angle — that is, the ratio of neutrino fluxes from above would differ substantially from the ratio from below (this would describe neutrinos that went through the Earth and then got into the detector). The solution was again neutrino oscillations – this time, muon neutrinos oscillated into the tau ones.

The presence of neutrino oscillations imply that they have (tiny) masses — something that is not predicted by minimal Standard Model. So one can say that this is the first indication of physics beyond the Standard Model. And this is very exciting.

I think it is interesting to note that this Nobel prize might help the situation with funding of US particle physics research (if anything can help…). It shows that physics has not ended with the discovery of the Higgs boson — and Fermilab might be on the right track to uncover other secrets of the Universe.

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Nobel Week 2015

Monday, October 5th, 2015

So, once again, the Nobel week is upon us. And one of the topics of conversations for the “water cooler chat” in physics departments around the world is speculations on who (besides the infamous Hungarian “physicist” — sorry for the insider joke, I can elaborate on that if asked) would get the Nobel Prize in physics this year. What is your prediction?

With invention of various metrics for “measuring scientific performance” one can make some educated guesses — and even put the predictions on the industrial footage — see Thomson Reuters predictions based on a number of citations (they did get the Englert-Higgs prize right, but are almost always off). Or even try your luck with on-line betting (sorry, no link here — I don’t encourage this). So there is a variety of ways to make you interested.

My predictions for 2015: Vera Rubin for Dark Matter or Deborah Jin for fermionic condensates. But you must remember that my record is no better than that of Thomson Reuters.

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Jacques Martino, Directeur de l’Institut national de physique nucléaire et des particules du CNRS, adresse ses félicitations à François Englert et Peter Higgs pour le Prix Nobel de physique 2013, et rappelle la contribution en France du CNRS à la découverte du fameux boson.

Enthousiasme général des physiciens et ingénieurs des expériences Atlas et CMS lors de l'annonce du Prix Nobel de Physique 2013. © CERN

Enthousiasme général des physiciens et ingénieurs des expériences Atlas et CMS lors de l’annonce du Prix Nobel de Physique 2013. © CERN


« Au nom du CNRS, je veux féliciter François Englert et Peter Higgs pour l’intuition extraordinaire dont ils ont fait preuve il y a presque 50 ans, en “inventant” le “boson de Higgs”. Le boson de Higgs a été théorisé dans les années 1960, notamment pour expliquer pourquoi certaines particules ont une masse alors que d’autres n’en ont pas. Il est alors devenu un véritable Graal pour nos physiciens. Il est en effet la clé de voûte du Modèle standard de la physique des particules, un ensemble théorique cohérent permettant de décrire le monde des particules subatomiques. Sans nul doute, la découverte d’un boson de Higgs vient donc de manière éclatante conforter ce modèle standard !

Il est indéniable que cette prédiction a animé des milliers de chercheurs durant toutes ces années, et je veux saluer aussi le travail titanesque accompli par les chercheurs,  ingénieurs et techniciens qui ont construit le LHC au CERN ainsi que les détecteurs Atlas et CMS. Ce prix Nobel célébré aujourd’hui, il nous appartient un peu aussi, car nos chercheurs français ont participé de manière très importante à cette grande quête collective qu’a été la traque du boson de Higgs.

Il aura fallu relever des défis technologiques colossaux qu’il s’agisse de l’accélérateur, des détecteurs ou bien encore des infrastructures de calcul permettant de traiter l’énorme quantité de données produites. Car rechercher le boson de Higgs revient véritablement à chercher une aiguille dans une botte de foin !

Plusieurs centaines de personnes du CNRS ont apporté leur pierre à la construction des  expériences du LHC et joué un rôle décisif dans l’exploitation scientifique des données. L’action déterminante du CNRS dans ce domaine serait sans aucun doute impossible sans l’expertise reconnue de l’IN2P3 qui fédère l’ensemble de ces activités et qui participe ainsi avec force au rayonnement national et international du CNRS. Ces recherches rappellent aussi de manière remarquable combien la collaboration internationale peut être porteuse de réussite.

Cette découverte majeure est le premier succès du LHC et vient ainsi couronner le succès de toute une communauté. Pour toute cette communauté, aujourd’hui est un jour de fête. Et pour le CNRS, cette découverte récompense 20 années d’investissements technologiques et humains dans lesquels une douzaine de laboratoires de CNRS, ont joué un rôle majeur aux côtés du CERN, ainsi que 200 chercheurs français.

La vie du LHC ne fait que commencer et cette réussite est certainement porteuse d’un avenir riche de nouvelles découvertes qui mobiliseront nos équipes dans les années qui viennent. Le Higgs a encore bien des secrets à nous livrer, nous l’avons pour l’instant seulement “aperçu”, et il convient de préciser sa nature et ses caractéristiques. Il s’agit là d’un énorme chantier à venir. Mais le programme de recherche du LHC dépasse largement ce cadre !  Le Modèle standard de la physique des particules s’il se voit conforté, laisse de nombreuses questions en suspens. Matière noire, supersymétrie… La recherche d’une nouvelle physique au-delà du Modèle standard va ainsi se poursuivre dans les années pour repousser toujours les frontières de notre compréhension de la matière et de l’Univers. »

À voir également :

Jacques Martino réagit à l’annonce du Prix Nobel de Physique 2013


François Englert et Peter W. Higgs, Prix Nobel… par CNRS

Comment chasse-t-on le boson ?


La chasse au boson de Higgs par CNRS

et pour tout savoir sur le LHC et le boson de Higgs (actus, BDs, vidéos): http://lhc-france.fr/higgs

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Today the 2013 Nobel Prize in Physics was awarded to François Englert (Université Libre de Bruxelles, Belgium) and Peter W. Higgs (University of Edinburgh, UK). The official citation is “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.” What did they do almost 50 years ago that warranted their Nobel Prize today? Let’s see (for the simple analogy see my previous post from yesterday).

The overriding principle of building a theory of elementary particle interactions is symmetry. A theory must be invariant under a set of space-time symmetries (such as rotations, boosts), as well as under a set of “internal” symmetries, the ones that are specified by the model builder. This set of symmetries restrict how particles interact and also puts constraints on the properties of those particles. In particular, the symmetries of the Standard Model of particle physics require that W and Z bosons (particles that mediate weak interactions) must be massless. Since we know they must be massive, a new mechanism that generates those masses (i.e. breaks the symmetry) must be put in place. Note that a theory with massive W’s and Z that are “put in theory by hand” is not consistent (renormalizable).

The appropriate mechanism was known in the beginning of the 1960’s. It goes under the name of spontaneous symmetry breaking. In one variant it involves a spin-zero field whose self-interactions are governed by a “Mexican hat”-shaped potential

MexicanHat

It is postulated that the theory ends up in vacuum state that “breaks” the original symmetries of the model (like the valley in the picture above). One problem with this idea was that a theorem by G. Goldstone required a presence of a massless spin-zero particle, which was not experimentally observed. It was Robert Brout, François Englert, Peter Higgs, and somewhat later (but independently), by Gerry Guralnik, C. R. Hagen, Tom Kibble who showed a loophole in a version of Goldstone theorem when it is applied to relativistic gauge theories. In the proposed mechanism massless spin-zero particle does not show up, but gets “eaten” by the massless vector bosons giving them a mass. Precisely as needed for the electroweak bosons W and Z to get their masses!  A massive particle, the Higgs boson, is a consequence of this (BEH or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism and represents excitation of the Higgs field about its new vacuum state.

It took about 50 years to experimentally confirm the idea by finding the Higgs boson! Tracking the historic timeline, the first paper by Englert and Brout, was sent to Physical Review Letter on 26 June 1964 and published in the issue dated 31 August 1964. Higgs’ paper, received by Physical Review Letters on 31 August 1964 (on the same day Englert and Brout’s paper was published)  and published in the issue dated 19 October 1964. What is interesting is that the original version of the paper by Higgs, submitted to the journal Physics Letters, was rejected (on the grounds that it did not warrant rapid publication). Higgs revised the paper and resubmitted it to Physical Review Letters, where it was published after another revision in which he actually pointed out the possibility of the spin-zero particle — the one that now carries his name. CERN’s announcement of Higgs boson discovery came 4 July 2012.

Is this the last Nobel Prize for particle physics? I think not. There are still many unanswered questions — and the answers would warrant Nobel Prizes. Theory of strong interactions (which ARE responsible for masses of all luminous matter in the Universe) is not yet solved analytically, the nature of dark matter is not known, the picture of how the Universe came to have baryon asymmetry is not cleared. Is there new physics beyond what we already know? And if yes, what is it? These are very interesting questions that need answers.

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