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Archive for July, 2012

The contentious relation between science and religion is the topic of this, the penultimate[1] post in the current series.  Ever since science has gone mainstream, there have been futile attempts to erect a firewall between science and religion. Galileo got in trouble with the Catholic Church, not so much for saying the earth moved as for suggesting the church steer clear of scientific controversies.  More recently, we have methodological naturalism (discussed in a previous post), a misidentification of why the supernatural is absent from science. Then there is the: science cannot answer the why question—but it can when it helps make better models (also discussed in a previous post). For example, why do beavers build dams? This can be answered by science. And there is the ever popular non-overlapping magisteria (NOMA) of Stephen J. Gould (1941 – 2002).  NOMA claims that “the magisterium of science covers the empirical realm: … The magisterium of religion extends over questions of ultimate meaning and moral value.”

The empirical realm covers not just what can be directly observed but what can be implied from what is observed. For example, quarks, and even something as well-known as electrons, are not directly observed but are implied to exist. That would also be true for citizens of the spirit or netherworld. If they exist, they presumably have observable effects. If they have no observable effect, does it matter if they exist or not? Similarly, a religion with no empirical content would be quite sterile, i.e. would prayer be meaningful if it had absolutely no observable effects?

Moral issues cannot be assigned purely to the religious sphere. The study of brain function impacts questions of free will and moral responsibility. Disease and brain injury can have very specific effects on behaviour, for example, a brain injury led to excessive swearing in one person. What about homosexuality? Is it biological or a lifestyle choice? Recent research has indicated a genetic component in homosexuality, thus mixing science with what some regard as a moral issue. Finally, what about when life begins and ends? Who decides who is dead and who is alive? And by what criteria?  Scientific or religious? This has huge implications for when to remove life support. The bigger fight is over abortion and the question of when independent life begins. Is it when the sperm fertilizes the egg? That is a scientific concept developed with the use of the microscope. That simple definition has problems when there are identical twins where the proto-fetus splits in two much later than at conception. In the other direction, both the sperm and the egg can be considered independent life. After all, the sperm has the ability to leave the donor’s body and survive for a period of time. The arguments one hears regarding when independent life begins are frequently an ungodly combination of scientific and theological arguments.

In the end, there is only one reality, however we choose to study or approach it.  Thus, any attempt to put a firewall between different approaches to reality will ultimately fail, be they based on science, religion, or philosophy.  At least the various religious fundamentalists recognize this, but their solution would take us back to the dark ages by subjugating science to particular religious dogmas. However, it does not follow that religion and science have to be in conflict. Since there is so much variation in religions, some are and some are not in conflict with any particular model developed by science. Still, it should be a major concern for theology that something like religion has not arisen naturally from scientific investigations.  While there are places God can hide in the models science produces, there is no place where He is made manifest. And it is not because He is excluded by fiat either (see the essay on methodological naturalism referenced above).

One should not make the same mistake as Andrew Dickson White (1832 –1918) in setting science and religion in perpetual hostility. He was a co-founder of Cornell University and its first president. He was also embittered by the opposition from the church to the establishment of Cornell as a secular institute. The result was the book: History of the Warfare of Science with Theology in Christendom (1896); a polemic against Christianity masquerading as a scholarly publication. This book, along with History of the Conflict between Religion and Science by John William Draper (1811 – 1882), introduced the conflict thesis regarding the relation between science and religion and said it is perpetual hostility. Against that, we note Newton, Galileo, and Kepler were all very religious and much science was done by clergymen in nineteenth century England. White’s book, in particular, has many problems. One is that the very opposition to change is cast as science versus religion rather than recognizing a lot of it as simple resistance to change. Even science is not immune to that—witness the fifty year delay in the acceptance of continental drift. The historical interplay between science and religion is now recognized to be very complex with them sometimes in conflict, sometimes in concord, and most commonly, indifferent.

If we take a step back from the results of science and its relation to particular religious dogmas, and look instead at the relation between the scientific method and theology, we see a different picture. Like science and western philosophy, science and theology represent competing paradigms for the nature of knowledge.   Science is based on observation and observationally constrained models; Western philosophy on rational arguments; while theology is based more on spirituality, divine revelation, and spiritual insight. This is, in many ways, a more serious conflict than between scientific results and particular religions. Particular religions can change, and frequently have changed, in response to new scientific orthodoxy, but it is much more difficult to change one’s conceptual framework or paradigm. Also, as Thomas Kuhn (1922 – 1996) and Paul Feyerabend (1924 – 1994) pointed out, different paradigms tend to be incommensurate. They provide different frameworks that make communication difficult. They also have conflicting methods for deciding questions, making cross-paradigm conflict resolution difficult, if not impossible. Hence, there will be tension between science and theology forever, with neither dominating.

To receive a notice of future posts follow me on Twitter: @musquod.

[1] NLP in the notation of effective field theorists.


Don Lincoln auditions for TED2013

A month ago in Quantum Diaries, Fermilab scientist Don Lincoln wrote about his experience auditioning for TED, the venerable series of just-this-side-of-scholarly talks that teaches curious audiences about spotting a liar, Legos for grownups and ultrasound surgery.

On a topic equally compelling, Lincoln discusses in his audition how particle physicists recreate the birth of the universe.

The live audition phase of TED2013 is now finished, and the TED talent search folks recently posted videos of the auditions on the web, so now you too can view Lincoln’s audition.

More importantly, you can rate it.

With enough positive feedback for his talk, Lincoln could join the TED2013 slate of speakers, sharing with the world the fascinating workings of the subatomic realm. And wouldn’t it be fun to see an experimental particle physicist in front of the big screen, wearing the familiar TED headset, expounding on particle collisions?

So rate it and enjoy!

Leah Hesla


Shameless Plugging

Thursday, July 12th, 2012

Homer Wolfe (CDF)

So a good friend of mine, former Quantum Diaries blogger, and exceptional scientist, Homer Wolfe from The Ohio State University, is doing a live chat today about what the world of physics looks like now that we (may have) found the Higgs. The chat is taking place at 2pm (CT) 3pm (EST)

You can find a link to the live chat here:

Homer has an exceptional ability to provide a wonderful understanding of very complex physics that is accessible to even those “not in the know”. Even more, his broad knowledge of science and physics in particular should make this discussion very interesting and entertaining. (He is also an exceptionally funny person…so expect some good humour too)

Following the live chat, Homer is also giving a special Joint Experimental-Theoretical physics seminar at Fermilab on the results from CDF (Collider Detection at Fermilab) that was presented at ICHEP (physics conference) this last week. I highly encourage anyone at Fermilab to take this opportunity to go see his talk. He is an exceptional speaker and will be sure to add a great flair to the results from CDF.


So that is my shameless plugging of a very good friend who is one of the most talented physicists I have ever known.




Dark matter: No model, just guesses

Wednesday, July 11th, 2012

On the last day of the International Conference on High Energy Physics dark matter took a central seat.

As many of you know, ourselves, the earth, all stars and galaxies are made of atoms. These atoms emit light when they are excited and that is how astronomers can explore the vast universe. But this matter only accounts for 4% of the content of the universe while dark matter makes up 24% of it. An unknown type of energy dubbed “dark energy” makes up the remaining 76%.

Dark matter was discovered in 1933 by Swiss physicist Fritz Zwicky. But to this day, scientists still don’t know what it is made of. This matter emits no light, which is why it was called “dark matter”.

Dark matter seems to react only to gravitational force and this is how it was discovered. Zwicky realized there was more matter in the universe than what was visible from the light emitted by stars and galaxies. This matter creates a much stronger gravitational field than what can be accounted for if you only rely on visible matter.

Neal Weiner, a theorist from New York University, started his lecture saying that contrary to the Higgs boson, for dark matter “we have no model, only guesses”. There is nothing within the Standard Model of particle physics to account for dark matter. This is one key reason we physicists are all convinced there is a bigger theory hiding behind the current known one.

So theorists and experimentalists are in the dark… As Neal stressed, there are many manifestations of dark matter. Different experiments observe strange signals where dark matter could be the explanation. But formulating an explanation is far from being trivial.

For example, several experiments have reported seeing more positrons than electrons coming from outer space. Positrons are the antimatter for electrons. Recently, the Pamela and the Fermi experiments both saw an excess of positrons, particularly at high energy. Given that the universe is made of matter, one needs to explain where these anti-electrons come from.

Some astronomers think it could be produced by pulsars but the jury is still out on this. Others argue that dark matter could annihilate into a pair of electron and positron, creating more positrons than expected. But it is not easy to cook up a theory that would do that. Hopefully, new data will come in 2013 from the Planck satellite to resolve this issue.

The DAMA/Libra experiment has been reporting a loud and clear signal (8.7 sigma) from dark matter for years. Unfortunately, nobody else can detect this signal as Lauren Hsu from Fermilab explained in her review of dark matter experiments. One possibility is that their detector, which is made of iodine, is sensitive to dark matter particles but other chemical elements used by the other experiments were not. Two new experiments were built using iodine, COUPP and KIMS, and should soon have enough data to get the final word on this long-standing anomaly.

Dark matter might interact with the Higgs boson. If that’s the case, now that we have a mass for it, we can test specific hypotheses. The XENON100 experiment is just at the limit of sensitivity for this and new results will come soon.

This is a huge, open question in particle physics. Let’s hope the new (Higgs?) boson discovery will soon be followed by some clues on the nature of dark matter. Exciting times ahead.

Pauline Gagnon

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Lors de la dernière journée de la Conférence Internationale sur la Physique des Hautes Energies, on a fait le point sur la matière noire. Comme plusieurs d’entre vous le savent, nous sommes tous: nous-mêmes, la terre, les étoiles et les galaxies faits d’atomes. Ces atomes émettent de la lumière lorsqu’ils sont excités, ce qui permet aux astronomes d’étudier l’univers. Mais toute cette matière ne compte que pour 4% du contenu total de l’univers alors que la matière noire en fait 24%. Les 76% restant viennent d’énergie d’un type inconnu surnommée « énergie noire. »

La matière noire fut découverte en 1933 par le physicien suisse Fritz Zwicky. Mais on ignore toujours de quoi il s’agit. Cette matière n’émet aucune lumière, d’où son nom.

La matière noire semble jusqu’à maintenant ne réagir qu’à la force gravitationnelle et c’est ce qui a permis de la déceler. Zwicky constata qu’il y avait plus de matière dans l’univers que ce qu’il voyait basé sur la lumière émise par les étoiles et les galaxies. Cette matière crée un champ gravitationnel bien plus fort que ce que la matière visible peut engendrer.

Neal Weiner, un théoricien de l’université de New York, a ouvert sa présentation en disant que, contrairement au boson de Higgs, pour la matière noire « on n’a aucun modèle, que des hypothèses ». Il n’y a rien dans le Modèle Standard de la physique des particules qui la décrive. C’est d’ailleurs un point clé indiquant clairement que le modèle standard a ses limites, et qu’une autre théorie plus globale devra le remplacer.

Les théoriciens et les expérimentatrices sont donc tous dans le noir… Come Neal l’a souligné, il y a déjà plusieurs manifestations de cette matière noire. Plusieurs expériences observent d’étranges signaux qui pourraient s’expliquer en termes de particules de matière noire. Mais formuler la bonne explication s’avère compliqué.

Par exemple, plusieurs expériences mesurent un excès de positons par rapport au nombre d’électrons observés venant du cosmos. Les positons sont l’antimatière des électrons. Récemment, les satellites Pamela et Fermi ont mesuré que cet excès est plus prononcé à haute énergie. Mais comme l’univers est fait de matière, d’où viennent ces positons?

Certains astronomes pensent qu’ils pourraient provenir de pulsars mais cela reste à prouver, ce qui est difficile. D’autres proposent plutôt qu’ils émanent de l’annihilation de particules de matière noire en une paire d’électron et positon.

Mais encore là, ce n’est pas facile à justifier théoriquement. Espérons que les nouvelles données attendues en 2013 par le satellite Planck aidera à résoudre ce problème.

Et puis il y a l’expérience DAMA/Libra qui clame depuis des années avoir obtenu un signal très fort (8.7 sigma). Le seul hic est que personne d’autre ne le capte comme l’a expliqué Lauren Hsu de Fermilab dans sa revue des résultats expérimentaux. Il est possible que les autres détecteurs n’y soient pas sensibles puisque seul DAMA/Libra utilisait un détecteur à l’iode. Deux nouvelles expériences COUPP et KIMS sont maintenant en cours ayant elles aussi de l’iode comme détecteur. Elles devraient avoir bientôt suffisamment de données pour trancher la question.

Autre possibilité: la matière noire interagit peut-être avec le boson de Higgs. Maintenant qu’on en connaît la masse, il se pourrait que l’expérience XENON100 puisse bientôt atteindre la sensibilité nécessaire pour tester cette hypothèse.

C’est donc une énorme question encore ouverte en physique des particules.

Peut-être que le nouveau boson (de Higgs?) apportera quelques indices qui nous permettront d’en apprendre plus.  Ça promet.

Pauline Gagnon

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Wednesday, July 11th, 2012

landmark (n): 1. An object or feature of a landscape or town that enables someone to establish their location. 2. An event, discovery, or change marking an important stage or turning point in something.

Several times in his ICHEP 2012 closing presentation today, CERN Director General Rolf Heuer referred to ICHEP 2012 as a “landmark” conference. This inspired me to take a look at the dictionary definition of “landmark.”

Indeed, the conference seems to fulfill both definitions of the word. First, we have very much defined the location of the field of particle physics. The new particle observed this week, which at the moment appears to have the properties of the Higgs boson, now establishes that the standard model of electroweak symmetry breaking, the main working theory of particle physics for forty years, is correct. Many speakers at the conference who were not explicitly talking about the Higgs still needed to talk about the model, and they all noted with some relief that what they wrote down about the Higgs potential and so forth is now known to be true. (Implicit in all this is the usual scientific caveat that “true” only means “not yet shown to be false,” but please bear with me.) Of course we have long assumed and hoped that it was true, given how much supporting evidence there was for the model all along. But now we know our location: we have a theory that works.

(I should note that while all the attention lately has been on the Higgs, there has been another major advance this year, and that is the measurement of the neutrino mixing angle θ13 at the Daya Bay nuclear reactor experiment in China, with confirmation in other experiments. This establishes that all elements of the neutrino-mixing matrix are non-zero, allowing for the possibility of CP violation in that system. So our colleagues in the neutrino sector also have a landmark measurement to show for themselves.)

And most definitely this observation, and the conference where we first learned about it, is also a landmark in the second sense. We now turn our attention to what this discovery means. What are the next set of questions to ask, and how do we go about asking them? Our first job is to do our very best to characterize the “Higgs boson.” Once its mass is established, the standard model fully specifies all of its other properties. Does it decay to the right final states as often as it is supposed to? Various extensions to the standard model predict different values for these “branching ratios.” Does it have the right spin? We know that the spin must be an even integer, but we don’t know if it is zero as predicted. The LHC will continue taking data into February of next year, before a two-year long shutdown. We’re going to need every ounce of data we can get, and all the cleverness we can muster, to answer these questions as accurately as we can. It will keep us busy during the data drought that is about to come.

I have been spending a lot of time this week thinking about what comes next, especially in terms of future facilities. The LHC is going to be the workhorse of Higgs physics for some time, but it will have its limits. Alas, there are some bad-news landmarks from this conference too: so far, there is no evidence at all for any other new particles beyond the Higgs at this mass scale — no supersymmetry, no exotic fermions or bosons, nothing. We can’t exclude the possibility that even the 13 TeV LHC that will start to run at the end of 2014 might not have enough energy to produce any other new particles. Heuer talked a little about the “high-energy LHC,” which he also noted is really a completely new machine in the same LHC tunnel, but it’s hard to imagine that this happens before 2030. Can we do something else in the meantime? Are there revolutionary, game-changing ideas that we can bring to the table? And, as a writer from the United States, I have to ask — can we build this machine at home?

Big questions for what we hope is a big new era of particle physics! This ICHEP, the first in the Southern Hemisphere, will truly be remembered as a landmark. I would like to take this opportunity to thank our hosts for all of their work in hosting this remarkable conference in an equally remarkable location.


Versão em Português abaixo…

Passed the crazy week of the Higgs finding, or, if we are to keep the complete scientific correctness, the weird different particle which is most likely the one that we have been searching for “just” 30 years, well, after that I feel like it is time to explain the different pieces that contributed to such historic finding. The whole thing depends, as is often said, in a number of different factors which we will never be able to put in a few pages of a blog or anything of the sort. Still, I’d like to urge you to hang on a bit and hopefully, you will find as I do, lots of interesting little details. I will try to make a little weekly series that should tell the history of the parts of the ATLAS detector which I happen to be closer to : The ATLAS Calorimeter and Trigger systems.

Let’s start right at the moment when the huge energy accumulated by a proton (this little system of 3 massive particles called quarks and nobody knows how many gluons) is concentrated in a very small volume of space. This energy, following the famous E=mc2, crystalizes in the form of different types of particles. Many of the collisions happen at the so-called parton (a quark or a gluon) level. That means that most likely a shower of particles called a jet will come out of the collision. Well, actually, two jets will usually be produced (things must always balance!). Very rarely, however, other processes take the role and make something completely different. For instance, sometimes, they will produce a Z or W boson. Both are, in the kingdom of particles that we call the Standard Model, what we could call the heavy weights, having lots of mass (one can say more than 80 GeV – Giga-electron-Volts). These guys, when formed, have a very short life (around 3×10-25s), but don’t waste your time thinking on how many zeros do you have to write, just keep in mind that no Z’s or W’s will ever leave the small pipes that bring the protons into collision. Much before that happens, a Z, for instance, will decay, producing a pair of particles that take the energy unfrozen (if you want) in their mass as speed. So, we talk about Z->ee. The Z particle has a mass around 91 GeV and the electrons will have, on average, half of that in “speed energy”. One interesting thing that is always present in physics (check my other colleagues in this blog) is that many properties must be conserved. For instance, the Z particle has no electric charge, but the electron has a negative charge. So, actually, what we get is not a pair of electrons, but, rather, an electron-positron pair, the positron being the positive charged version of the electron, or as we call the electron antiparticle. So, if I wanted to be more rigorous, I should have written Z->e+ e, meaning that a zero charged particle results in a positive and negative charges : the sum is zero again!

The particles (and the anti-particles!) will invade the detector coming from the center (the beam pipe) crossing layers of detectors in the way and will finish their journey in the calorimeters. These devices were developed during many years and now, only in one of the ATLAS calorimeters, we are around 300 people working together!!. For now I will stop here. In the next week, I will explain what happens when each of the electrons enter in the calorimeter and how we use this information to detect the electron and make physics out of it!

To give you a quick taste of what is to come, I call your attention to two videos available in youtube. In the first one, you see the chain of accelerators with increasing size and proton energy. When we get to the LHC, the image zooms inside the tunnel and you will see the equations of the Standard Model of particles in the walls (like we would do that..) The proton will cross the French/Switzerland border in a complete illegal form (no passports!!!) and you will see the colored quarks inside the proton until they meet inside the detector. In the second collision, you will see the Z->ee event. After the collision, the software marks the two blue tracks left be the electron-position pair in the tracking detector and “illuminates” a few of the calorimeter cells represented in green in the movie. We will discuss what happens and how we can see these cells in the next postings. And, later on, you will understand the relation between detecting a Z and detecting a Higgs…
First video : http://www.youtube.com/watch?v=NhXMXiXOWAA
Second video : http://www.youtube.com/watch?v=RdYvtm4CIAE

Portugaise version :

Como Funciona um Detetor de Partículas!!

Passada a semana louca da descoberta do Higgs, ou se quisermos manter a imparcialidade e a retidão científica, a estranha e diferente partícula que muito provavelmente é aquela que estivemos procurando por “apenas” 30 anos, penso que é hora de explicar um pouco todas as peças que contribuíram para essa descoberta histórica. A coisa toda depende numa multitude de fatores os quais nunca poderemos colocar em algumas páginas de um blog. Ainda assim, peço que vocês agüentem firme e, quem sabe, vocês encontraram o mesmo prazer que eu em compreender os pequenos detalhes que fazem o sucesso dessa incrível experiência. Tentarei manter um fluxo de episódios semanais explicando como funciona a parte do ATLAS que conheço mais de perto : O Calorímetro e o Sistema de seleção do ATLAS.

Comecemos exatamente no momento em que a enorme energia acumulada por um próton (esse pequeno sistema de três partículas massivas e não sabemos quantos glúons) se concentra num pequeno volume de espaço. Seguindo o famoso E=mc2, essa energia “se cristaliza” na forma de diferentes tipos de partícula. A maior parte das colisões ocorre entre partons (quarks ou glúons), resultando numa cascata de diferentes partículas, à qual damos o nome de “jato”. Normalmente, como a experiência tem um certo balanço a respeitar, temos dois jatos sendo produzidos com energias bastante similares. Muito raramente, entretanto, outros processos acontecem e algo completamente diferente pode surgir. Por exemplo, algumas vezes, tais processos podem produzir um bóson Z ou W. Ambos são, no reinado das partículas que chamamos de Modelo Padrão, o que podemos chamar de Pesos Pesados (pode-se dizer falar de uma massa maior que 80 GeV – Giga-elétron-Volts). Tais partículas têm uma vida muito curta de 3×10-25s, mas nem perca tempo pensando em quantos zeros se deve colocar depois da virgula. Saiba apenas que um Z formado não chega jamais a tocar o tubo que traz os prótons até o ponto de colisão. Um Z decai, produzindo, um par de partículas que levam a energia contida na massa do Z. Assim, falamos de Z->ee. Como o Z tem uma massa próxima a 91 GeV, os elétrons vão carregar média metade desse valor em “energia do movimento”. Outra coisa interessante (pesquise um pouco os artigos de meus colegas nesse blog) e que é sempre importante em física é que muitas quantidades devem ser conservadas. Assim, como o Z não tem carga elétrica e o elétron tem uma carga negativa, um dos elétrons é, na verdade, um pósitron, a anti-partícula do elétron com carga positiva. Assim, o Z sem carga resulta em uma carga positiva e uma negativa : a soma é zero! Se eu quiser ser realmente rigoroso, tenho que escrever Z->e+ e.

As partículas (e as anti-partículas!) invadem o detetor vindo do centro (onde está o tubo com os feixes) atravessando camadas de detetores e terminando sua viagem nos calorímetros. Esses aparelhos foram desenvolvidos em muitos anos de estudo e, hoje em dia, apenas um dos calorímetros do ATLAS ainda precisa de 300 pessoas trabalhando continuamente!! Por agora, eu vou parar por aqui. Na próxima semana, vou explicar o que acontece quando cada um dos elétrons entra no calorímetro e como usamos essa informação para detectar o elétron e “fazer física”!

Para dar um gostinho do que está por vir, gostaria de chamar atenção de vocês pra dois vídeos disponíveis no youtube. No primeiro, vocês podem ver toda a seqüência de aceleradores com tamanho e energia cada vez maiores. Quando chegamos no LHC, a imagem entra no túnel, em cuja parede, podemos ver as equações do Modelo Padrão de partículas (como se fosse verdade!). O próton que seguimos atravessa “ilegalmente” (alguém já viu um próton com passaporte?!) a fronteira da França com a Suíça e vocês podem ver os quarks viajando dentro do próton até a colisão dentro do detetor. Na segunda colisão, vocês podem ver um evento Z->ee se formando. Depois da colisão, o programa identifica os traços deixados pelo par elétron-pósitron no detetor de traços com linhas azuis. O par também “ilumina” algumas células do calorímetro representadas em verde no filme. Vamos discutir na semana que vem o que acontece e como podemos ver essas células no próximo blog… E, mais tarde, vamos entender qual a diferença entre detectar um Z e um Higgs…

Primeiro vídeo : http://www.youtube.com/watch?v=NhXMXiXOWAA
Segundo vídeo : http://www.youtube.com/watch?v=RdYvtm4CIAE
Canal ATLAS/Brasil : http://webcast.web.cern.ch/webcast/play.php?type=permanent&event=12


Suite aux récents résultats du LHC concernant le boson de Higgs , Jacques Martino, Directeur de l’Institut national de physique nucléaire et de physique des particules du CNRS, adresse ses félicitations aux personnels de l’Institut.

Depuis le CERN ou par webcast depuis les laboratoires, les personnels de l'Institut national de physique nucléaire et de physique des particules du CNRS ont été nombreux à suivre en direct le séminaire LHC du 4 juillet 2012. Image : CERN

« Nous avons vécu, mercredi dernier, avec l’annonce de la découverte d’un boson à très forte saveur Higgs, une “folle” journée où l’ensemble de l’Institut a été récompensé d’un effort de recherche qui s’est étalé sur une bonne vingtaine d’années, et qui je l’espère va continuer et nous apporter d’autres découvertes.
Je souhaite que tout l’Institut se sente félicité et honoré par cette découverte ; bien sûr tous ceux qui ont travaillé directement sur cette recherche, mais aussi tous ceux qui ont rendu possible la participation de l’IN2P3 à cette découverte : les chercheurs, mais aussi les ingénieurs, techniciens et administratifs impliqués sur ou autour d’Atlas, CMS, du LHC et ses accélérateurs, du centre de calcul… Mais aussi tous les agents, dans tous nos labos, qui ont contribué à rendre cet effort possible, et fructueux.
Je souhaite ici associer toutes les autres disciplines de l’Institut : nous sommes dans un même bateau, notre recherche est avant tout “subatomique”, et le résultat majeur obtenu aujourd’hui par la physique des particules doit et va tous nous “booster”.
Nous sommes tous heureux et fiers que l’IN2P3 y ait participé, et que notre organisation ait permis d’y apporter une contribution très significative. Faut-il ici rappeler que cette organisation en réseau est, entre autre, celle qui a permis de coordonner nos efforts ? C’est une plus-value significative sans laquelle ni le CNRS, ni les Universités n’auraient pu avoir une place si visible. C’est aussi notre organisation qui nous a permis une excellente coordination avec nos collègues de l’Irfu, que j’associe à ce message.

Les résultats dévoilés mercredi dernier constituent un moment historique de la physique des particules. L’IN2P3, grâce à ses chercheurs, ingénieurs, techniciens et administratifs, a su être présent dès le début et faire les choix pertinents lors de la conception, de la construction, des analyses tant dans Atlas que CMS, choix qui nous ont donné une position très visible et reconnue.
Nous sommes tous fiers que ces investissements humains et financiers aient permis à nos chercheurs de jouer un rôle leader dans la découverte du “Higgs”. Le LHC n’est clairement qu’au début de son histoire, et nous ne doutons pas que d’autres résultats de grande qualité sont encore à venir dans Atlas et CMS, mais aussi LHCb et Alice.
Je voudrais terminer en vous rapportant un mail de félicitations que j’ai reçu de la part d’un ami médecin : il nous remercie pour cette journée où l’IN2P3 lui a permis de rêver. Oui, en effet, le progrès des connaissances, dans chacune de nos disciplines, porte une part de rêve, d’enchantement qui sont aussi un fort soutien, voire un moteur, à nos actions. Et si en plus c’est partagé au-delà de notre discipline, je crois que notre raison d’être et de travailler en est confortée. Continuons à faire progresser les connaissances de notre domaine, et poursuivons les activités de recherches plus appliquées d’ores et déjà entreprises : ceci doit être notre double leitmotiv.
Je terminerai en vous rappelant que lors de notre conférence de presse, avec le CEA, à Paris, nous avons reçu un appel téléphonique de notre Ministre, Madame Fioraso, dont les mots de félicitations sont sur notre page Web IN2P3. S’il est aujourd’hui encore bien tôt pour en tirer quelques certitudes quant à nos budgets à venir, il va sans dire qu’un tel intérêt ne peut être lu que positivement.

Bravo encore et félicitations à tous. »


Everything must fit nicely together

Tuesday, July 10th, 2012

Yesterday, at the International Conference on High Energy Physics in Melbourne we heard three presentations from ATLAS, CMS and the Tevatron experiments, (D0 and CDF) on the Higgs boson searches. It was great to see how all these results are consistent with each other: between the four experiments, the two accelerators operating at different energies and the six independent decay channels. All give the same picture: we have really found a new boson.

Everybody’s attention is now turned towards establishing the exact identity of this boson. Is it the one predicted by the Standard Model or one of the five Higgs bosons associated with supersymmetry, another theory that attempts to fix the few remaining problems of the Standard Model.

Although the theory was unable to predict the exact mass of the Higgs boson, it provided strong constraints on where it could be found. Many quantities are interconnected by the equations of the Standard Model. This is why we keep improving the uncertainty margin on these quantities. Putting all this information together allows us to check the consistency of the model.

This has been the highlight of many conferences for a decade or two. Each new update showed how much progress had been accomplished when all the measurements were combined in a complex algorithm designed to test the so-called “electroweak” part of the Standard Model all in one go. This is very similar to checking the stability of a very elaborate mobile after modifying each of its components slightly.

Yesterday, for the first time, we saw what the newly measured mass of what is most-likely a Higgs boson adds to this global picture.

The vertical axis shows the measured mass of the W boson and the horizontal axis, the mass of the heaviest quark, the top quark. The blue ellipse is centered on the measured values of these two masses. The ellipse gives the error margin. There is a narrow blue band below the large green band. This represents the actual measured mass of the Higgs boson announced on July 4th, the width being its uncertainty. So as it stands, given the overlap, there is agreement, at least within errors.

The black ellipse is a projection of what this picture will look like once the LHC experiments reduce the uncertainty on the W mass from the current 15 MeV to only 5 MeV. If all is consistent within the Standard Model, the black ellipse will have to overlap with the narrow blue band indicating the Higgs boson mass. If the central value of the W mass does not change, then there will be some inconsistency with the Standard Model (the very narrow blue strip). On the other hand, if supersymmetry or SUSY is the real, more global theory of nature, the green area gives the mass values allowed for the W and top quark. MSSM stands for Minimal Supersymmetric Model and is just one specific model within the vast SUSY space.

This plot might reassure a few: there is still plenty of room for supersymmetry. This theory is far from being dead. But as someone commented: “The huge number of SUSY presentations at this conference was inversely proportional to the number of evidence for it!”

The bets are still open on what will come next. Is this Higgs boson the one predicted by the Standard Model, supersymmetry or some other version? Patience is in order but the answer will eventually come.

Pauline Gagnon

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Hier, lors de la Conférence Internationale de la Physique des Hautes Énergies à Melbourne, les expériences ATLAS, CMS et celles du Tevatron (D0 et CDF) ont fait le point sur le boson de Higgs. C’était super de constater comment tous les résultats sont consistants entre eux: ceux des quatre expériences, des deux accélérateurs opérant à des énergies différentes et des six canaux de désintégration indépendants. Tous donnent le même message: on a bel et bien trouver un nouveau boson.

Prochaine étape: établir s’il s’agit du boson de Higgs prédit par le modèle standard de la physiqe des particules ou d’un des cinq bosons de Higgs venant de la supersymmétrie, une autre théorie qui tente de remédier aux dernières lacunes du modèle standard.

Bien que la théorie soit incapable de prédire exactement la masse du boson de Higgs, elle impose des contraintes strictes sur sa valeur. Ses équations interconnectent plusieurs paramètres. On améliorant la précision des mesures de masses et autres paramètres du modèle et en combinant toute l’information, on peut voir si tout se tient.

Cette vérification a constitué un des points forts des grandes conférences durant les vingt dernières années. A chaque nouvelle mise à jour, on pouvait se rendre compte des progrès accomplis lorsque les mesures les plus récentes étaient incorporées dans un programme complexe conçu pour vérifier la théorie dite « électrofaible » d’un seul coup. Un peu comme si on vérifiait la stabilité d’un immense mobile après avoir modifié chacun de ses éléments.

Hier, pour la première fois, la masse du boson mesurée la semaine dernière était incorporée aux équations pour voir son impact global.

La verticale représente la masse mesurée expérimentalement pour le boson W et à l’horizontale, la valeur de la masse du quark top, le plus lourd de tous les quarks. L’ellipse en bleu est centrée sur la mesure de ces deux quantités. Sa hauteur et largeur correspondent à la marge d’erreur sur ces deux mesures. L’ étroite bande en bleue représente la masse obtenue la semaine dernière pour ce qui pourrait être le boson de Higgs. Le fait que l’ellipse et cette étroite bande se chevauche en partie indique que tout est cohérent avec le modèle standard, du moins à l’intérieur de la marge d’erreur actuelle.

Mais si les expériences du Grand Collisionneur de Hadrons ou LHC réussissent à réduire la marge d’erreur sur la masse du boson W de 15 MeV (valeur actuelle) à 5 MeV (ellipse en noir), et si la valeur centrale des masses du top et du W ne change pas, il y aura une certaine tension i.e. la bande bleue représentant la masse du Higgs ne coïncidera plus avec l’ellipse en noir.

Par contre, la bande verte indique les valeurs encore possible pour les masses du W et du top si le boson de Higgs correspond non pas à celui prédit par le modèle standard, mais plutôt à un des cinq bosons de Higgs postulés par un des modèles de supersymmétrie connu sous le nom de MSSM ou Minimum Supersymmetric Model.

La figure ci-dessus réconfortera quelques personnes: il y a encore beaucoup de place pour la supersymmétrie même si toutes les tentatives actuelles n’ont toujours pas révélées sa présence. Comme un conférencier l’a exprimé hier: « La quantité de présentations à cette conférence sur la supersymmétrie est inversement proportionnelle à son évidence ». Entre temps, le modèle standard demeure toujours valide.

Tous les paris sont ouverts sur ce que l’on va maintenant découvrir. Est-ce le boson de Higgs du modèle standard, de la supersymétrie ou d’une autre théorie? Il faudra encore un peu de patience avant d’en avoir le cœur net mais la réponse viendra éventuellement.

Pauline Gagnon

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