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Archive for July, 2013

Long-standing discrepancy put to rest

Saturday, July 20th, 2013

This morning at the European Physics Society conference in Stockholm, the LHCb experiment operating at the Large Hadron Collider (LHC) CERN brought one more argument to put to rest a long-standing discrepancy that had kept theorists puzzled for nearly two decades.

LHCb presented the most precise measurement to date of the b baryon lifetime. A baryon is a family of composite particles made of three quarks.  For example, protons and neutrons are made of a combination of u and d quarks.  What makes b baryons so special is that they contain a b quark, a much heavier type of quark. Composite particles containing b quarks like B mesons (made of a b and either a u or d quarks) and b baryons are unstable, meaning they have a short lifetime. About one picosecond after being created, they break down into smaller particles.

In theory, both B mesons and b baryons should have approximately the same lifetime. But in the 1990’s, when CERN operated with its previous accelerator called LEP (Large Electron Positron collider), all experiments measured a systematically shorter lifetime for b baryons than B mesons as can be seen on the plot below. Although the LEP experimental errors were quite large, the general trend of lower values was very puzzling since all four experiments (ALEPH, DELPHI, OPAL and L3) were working independently. Lb_lifetime_comparison

The various b baryon lifetime measurements over time from the oldest results at the bottom to the three latest results from the LHC experiments at the top. The measured value has now shifted toward a value of 1.5 picoseconds, as measured for the B mesons.

This prompted theorists to re-examine their calculations and to look for overlooked effects that could explain the difference. Despite all efforts, it was nearly impossible to reconcile the measured b baryon lifetime (somewhere between 1.1 to 1.3 picosecond) with the B meson lifetime at around 1.5 ps.

Nearly a decade later, D0 and CDF, the two experiments from another accelerator, the Tevatron near Chicago, started closing the gap. It took another decade for the LHC experiments to show that in fact, there is no large difference between b baryon and B meson lifetimes.

Already, earlier this year, ATLAS and CMS both reported values in line with the B meson lifetime. With this latest and most precise result from the LHCb experiment, there is now enough evidence to close the case on this two-decade-old discrepancy. LHCb measured the b baryon lifetime to be 1.482 ± 0.018 ± 0.012 ps. Hence, both lifetimes are now measured close to 1.5 picosecond and LHCb calculated their ratio to be 0.976 ± 0.012 ± 0.006, very close to unity as theoretically expected.

One possible explanation is that all LEP experiments were affected by a common but unknown systematic shift or simply, some statistical fluctuation (i.e. bad luck). The exact cause might never be found but at least, the problem is solved. This is a great achievement for theorists who can now rest assured that their calculations were right after all.

Pauline Gagnon

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The Illusion of Purpose

Friday, July 19th, 2013

From whence does purpose arise?  People always want to know why: What is the purpose? Why did hurricane Sandy hit New York? Was it punishment for past sins? The idea of purpose is so central to people’s thinking that we want a purpose for every happening. This was engrained in philosophy as Aristotle’s final cause. Aristotle regarded the final cause as the most important of his four causes and it became central to medieval philosophy. Understanding the final cause has, indeed, been important to our survival. It was important to know the reason for the lioness taking a stroll. Was she just doing it for exercise or was she looking for a meal? If the latter, it was very important to give her a wide berth. Similarly in social interactions, it is important to know the purpose behind a person’s behaviour. Are they just being nice or do they have ulterior motives? And if so, what?

Purpose or Aristotle’s final cause[1] is entirely from the physical sciences and downplayed in science generally. This leads to an argument against evolution.  Evolution by natural selection has no purpose but is a stochastic process with the direction of each step being independent of the previous step and depending only on the local conditions at that moment. Apes did not evolve to form the stock from which humans later arose but rather humans arose as a result of local environmental pressures on the ape. The precise argument against evolution by natural selection is that since natural processes have no purpose, purpose could not have arisen unless there was an outside agency to give purpose. Since purpose is seen, for example in animal and human behaviour, such an outside agency must exist. If evolution produced this purpose, it must have been guided by an external purpose and not be due entirely to natural selection.

This argument is a prime example of proof by lack of imagination. It relies on not having enough imagination to find a method for purpose to arise from natural selection.  Hence, the precise argument against evolution can be stated as: I can imagine no way that purpose can arise except by an external agency, therefore evolution must be caused by an external agency. The counter to proof by lack of imagination is the just so story. That is a story made up to explain a given occurrence without any evidence of wide spread validity. Generally, I regard just so stories as uninteresting and certainly not science[2]. To make the just so story science, one would have to use it to make testable predictions. But as a counter to proof by lack of imagination, that is not necessary. All that is necessary is that to provide one plausible counter example.  I will now give a just so story to counter the argument in the last paragraph.

So, let’s see how the illusion of purpose, if not purpose itself, could arise. Consider some bacteria in a solution with a gradient for food.  The bacterium that moves towards more food will on average produce more offspring and therefore the population will eventually be dominated by those that move up the gradient. The resulting behaviour appears to have a purpose: namely to get more food.  However, it is just the response to the local conditions, conditioned by evolution’s feedback loop.

One can apply the same type of reasoning to more complex situations and in every case evolution favours those individuals whose behavior appears to have purpose. Consider the case of a bird building a nest. Birds that build nests that do a better job of protecting their young will have more offspring (balanced somewhat by the cost of building the nest).  Similarly with the behaviour of young men courting young women (and vice versa). Those that are successful produce offspring while those that aren’t, don’t reproduce.  Thus, the behaviour seems to have a purpose but in fact, it is only that those who behave in a certain way leave offspring and hence, the behaviour is all that survives. Incidentally, this also explains why there are so few geeks.

Thus, we see that purpose, or to be more precise, the illusion of purpose, can arise from the feedback loop in evolution. Evolution favours those behaviours that work towards the end of producing more offspring, which is a purely mechanical process. But saying purpose is an illusion is perhaps going too far. In building models of animal and even plant behaviour, purpose is a useful concept that makes the job easier. Models that include purpose are simpler and make better predictions than those without and even if they didn’t, we are human after all, and do enjoy a just so story. Purpose, like the nucleon, is an emergent property[3] that arises from the underlying dynamics. So the next time you are pursuing a member of opposite sex with a definite purpose in mind, remember that purpose is, if not an illusion, just an emergent property.

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


[2] They can however be entertaining.

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IceCube High Energy Events

Wednesday, July 17th, 2013

prl

The IceCube TeV events are now in print in Physical Review Letters. With a nice cover image… The paper is available on arxiv. There has been a lot of speculation among theorists, and there are many more neutrino events in newer analyses which have been shown in meetings. So, just a quick update here…

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Welcome to CERN baby!

Sunday, July 14th, 2013

I arrive into Geneva airport laden with a huge backpack (mostly full of pants), a guitar and enough technology to launch a small mission to Mars. I hop onto the Y-bus to CERN.

CERN’s main Meyrin site spans the Swiss/French border and is plonked in the midst of beautiful agricultural estates which nestle in the shadows of the Jura mountain range. The largest accelerators, such as the Large Hadron Collider, are buried deep beneath French farmland and so there are several CERN outposts dotted around in France too.

The Meyrin site is massive and appears haphazardly distributed on arrival. Buildings are numbered in the order in which they were built and Building 41 will be my home for the next two months.

CERN's Meyrin site.

CERN’s Meyrin site.

I  settle into my nice little hostel room and go exploring. The weather is beautiful, there’s the chance of a swim in the lake and I’m feeling relaxed.

Until Day 1, when I loiter along with a multitude of nervous-looking students outside the CERN hostel.

After the initial welcome and introductions from HR we head en masse to Building 55 to pick up our security cards.

The two hour wait in glorious sunshine affords an excellent opportunity to bond with my fellow students and I meet a lovely little crew from Denmark, Spain, Italy, Germany, Romania and Brazil – which gives you a sense of the diversity of nationalities at CERN.

I track down my Aussie summie office mate Josh and we cycle across the border on our standard issue CERN bikes to the Prévessin site, or the ‘North Area’, in France where we will be based for our research project.

CERN bikes.

CERN bikes.

On meeting our supervisor Ralph, Josh and I are immediately accompanied to the lab to commence soldering. Stay tuned for creative electronics…

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An unexpected collision

Sunday, July 14th, 2013

So here we go! I’ve finished my exams and after a week of camping and getting soggy in Wales I turn my attention to CERN.

I received an e-mail from my supervisor Ralph, an extremely clever and even more endearing German whom I met for lunch in London earlier in the year. The e-mail goes something like this:

“Hi James,

I’m sure you are already au fait with the following but just in case you should brush up on…”

Ralph then goes on to list several text books, loads of software and a few programming languages most of which I have never heard of. Reality hits that this summer is going to involve a lot of hard work and not just chillaxing on the banks of Lake Geneva.

So besides learning how to code, how accelerators work and what the heck a Fast Fourier Transform is, I will be packing pants. Why I hear you say?

Summies generally live in hostel accommodation on the CERN site. To avoid the trials and tribulations of communal kitchens and washing facilities I plan to eat big lunches in R1 (CERN’s really good canteen which serves cold beer) and avoid doing any clothes washing for as long a possible.

From university days I know the trick to the latter is to have many pairs of pants – so its going to be a good trading month for Marks and Spencer.

Science pants

Science pants

My other, less than ideal, bit of preparation this week was getting knocked off my bike at a big roundabout in Oxford. I am fine but my bike is a bit mangled. I was expecting lots of collisions this summer but not this sort!

The next post will be from CERN – wish me ‘bonne chance’.

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You may be wondering what the CERN Student Summer Programme is all about.

Each year CERN invites around 300 student physicists, engineers and computer scientists from across the globe to participate in its summer programme. The students attend a lecture series delivered by some of the world’s leading particle physicists and carry out a research project.

The programme offers students a taste of life at CERN and offers CERN the opportunity to earmark future talent.

The summer students, collectively known as “summies”, also enjoy an active social life with extensive exploration of Geneva’s night life  – I feel I can make a particularly valuable contribution to the programme on this front.

My research project will involve developing a system to analyse the beam in the Large Hadron Collider and, potentially, making a mini-accelerator. I’m well excited!

summer-students

A previous generation of summer students pose in front of the Globe – CERN’s showcase visitor centre.

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Last part in a series of four on Dark Matter

After reviewing how dark matter reveals its presence through gravitational effects, the lack of direct evidence of interaction with regular matter and the cosmological evidence supporting its existence, here is what the Large Hadron Collider (LHC) at CERN can do.

We can find dark matter with the LHC but only if dark matter interacts with regular matter. Since we do not know how this may happen, we design traps suited for as many beasts as there are theories. Here are a few.

Supersymmetry

The current theory describing particle physics is the Standard Model. It has been extremely successful, explaining just about everything observed so far. Unfortunately, at higher energy, its equations start to break down.

This is why theorists developed Supersymmetry (or SUSY), building on the Standard Model and extending it further. What is truly remarkable is that this new theory invented to fix the flaws of the Standard Model predicts the existence of particles with the properties expected from dark matter, hence its great popularity.

All would be perfect except that no one has detected any of the many expected supersymmetric particles. This might simply mean that these particles are heavier than the current LHC reach. We will have more chances of discovering them once the LHC resumes in 2015 at much higher energy.

The lightest supersymmetric particle

In the LHC, protons collide, producing large amounts of energy. Since energy, E, and mass, m, are two forms of the same essence as stated by the famous E = mc2, energy can materialise into new particles.  Heavy particles are unstable and quickly decay into lighter ones.

Some variants of SUSY predict that all supersymmetric particles must decay into other supersymmetric particles. Under this assumption, the lightest SUSY particle cannot decay into anything else and remains stable, not interacting with anything else just like dark matter is expected to be.

sqark-cascade

A typical decay chain is shown above. A supersymmetric quark decays into another SUSY particle, χ2, and a normal quark, q. At the two subsequent stages, an electron or muon (denoted l+ and l) and lighter SUSY particles are produced. The lightest one, in this case a particle called neutralino χ1, cannot decay into anything else and escapes the detector leaving no signal behind.

Seeing the invisible

An event is a snapshot capturing all lighter particles emitted when an unstable particle decays. And within each event, the energy needs to be balanced. So even when a particle flies across the detector leaving no signal, it can still be detected through the energy imbalance in the event. Invisible particles such as the lightest supersymmetric particles can be detected this way.

Both the CMS and ATLAS collaborations have been looking for events containing large amounts of unbalanced energy accompanying a single photon or a single jet (a jet is a bundle of particles made of quarks).

MET-photon-ATLAS

This figure displays an event from the ATLAS experiment containing a single photon (the energy deposit is shown in yellow around 4 o’clock on the left picture) and the missing energy represented by the pink dashed line around 10 o’clock.

This is exactly what an event containing the lightest supersymmetric particle and a photon would look like. But an event containing a Z boson and a photon would look just the same if the Z boson decayed into two neutrinos (other particles that do not interact with the detector).

Unfortunately, nothing has been observed in any of the channels studied so far that is in excess of what is expected from the background, i.e. other known types of events giving similar signatures.

Unlike the direct dark matter searches, the LHC analyses are sensitive to light dark matter particles. Remember the messy plot I showed about direct searches for dark matter? CMS and ATLAS can help clarify the situation, although their results depend on theoretical assumptions when the direct searches don’t.

Below are the CMS results for a search of events containing a single jet and missing energy.  The horizontal axis gives the mass of the dark matter candidate and the vertical axis, the allowed interaction rate with ordinary matter. Everything above the various lines is excluded. CMS (solid red line) exclude light dark matter particles for large interaction rates, a region inaccessible to XENON100, (solid blue curve) the most powerful experiment for direct dark matter searches.

SpinIndependent_woScalar-CMS

The Higgs boson and dark matter

Another approach to find dark matter relies on some theories that predict that the Higgs boson could decay into dark matter particles. Higgs bosons can be produced with another boson, such as with a Z boson. If the Higgs boson decays to any type of dark matter, we would only see the decay products of the Z and missing energy for the Higgs boson. Searches for such decays have so far not revealed anything above the expected background level. inv-Higgs

A dark parallel world

A group of theorists developed an amazing Theory of Dark Matter incorporating ideas of a Hidden Valley where two worlds would evolve in parallel: our world with Standard Model and the yet undiscovered supersymmetric particles, and a dark world populated with dark particles as depicted below, where each horizontal line represents a particle of a given mass.

HiddenValley

The idea is that the LHC could produce heavy supersymmetric particles. These particles would decay in a cascade into lighter ones down to the lightest SUSY one. That particle would be a “messenger” capable of crossing over the Hidden Valley, escaping into the dark sector and becoming invisible to us.

In the dark sector, this particle could decay in a cascade into lighter dark particles until it reaches the lighest supersymmetric dark particle, another messenger capable of tunnelling back to our world where it would reappear into many pairs of electrons or muons.

This may sound like pure science fiction but it is all rooted in sound, but still unproven, physics as a quick check with the original papers cited above will demonstrate.

I was until recently one of the experimentalists looking for signs of this Hidden Valley, selecting events containing regrouped pairs of electrons and muons but so far, nothing has been found.

Experimentalists are still looking, there and in many other places, constantly refining their searches and trying new strategies. If dark matter interacts with matter, we ought to find it.

First part in a Dark Matter series:        How do we know Dark Matter exists?

Second part in a Dark Matter series:   Getting our hands on dark matter

Third part in a Dark Matter series:      Cosmology and dark matter

Pauline Gagnon

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Dernier volet d’une série de quatre sur la matière sombre

Après avoir examiné comment la matière sombre révèle sa présence à travers des effets gravitationnels, l’absence de preuves directes d’interaction avec la matière ordinaire et comment la cosmologie soutient aussi son existence, voici ce que le Grand collisionneur de hadrons (LHC) du CERN peut accomplir.

Nous pourrons peut-être trouver la matière sombre avec le LHC mais seulement si la matière sombre interagit avec la matière ordinaire. Comme nous ne connaissons pas le processus exact, nous élaborons des pièges adaptés à autant de bestioles qu’il y a de théories. En voici quelques-unes.

La supersymétrie
Le Modèle standard, la théorie actuelle décrivant la physique des particules, réussi à expliquer presque tout ce qui a été observé jusqu’à présent. Malheureusement, à plus haute énergie, ses équations ne tiennent plus la route.

C’est pourquoi des théoricien-ne-s ont développé la  supersymétrie  (ou SUSY pour les intimes) qui englobe le modèle standard mais va plus loin. Ce qui est vraiment remarquable, c’est que cette nouvelle théorie élaborée pour corriger les défauts du modèle standard prédit l’existence de particules ayant les caractéristiques de la matière sombre, d’où sa grande popularité.

Tout serait parfait, sauf qu’aucune des nombreuses particules supersymétriques postulées n’a encore été détectée. Est-ce simplement parce que ces particules sont hors de la portée actuelle du LHC ? Nous aurons plus de chances de les découvrir après son redémarrage en 2015 à bien plus haute énergie.

La plus légère des particules supersymétriques
Dans le LHC, les protons entrent en collision, produisant de grandes quantités d’énergie. Puisque l’énergie, E, et la masse, m, sont deux formes d’une même essence comme le stipule la célèbre E = mc2, l’énergie peut se matérialiser en nouvelles particules. Les particules lourdes sont instables et se désintègrent rapidement en plus légères.

Certaines variantes de SUSY prédisent que toutes les particules supersymétriques doivent se désintégrer en d’autres particules supersymétriques. Suivant cette assomption, la particule supersymétrique la plus légère ne peut pas se désintégrer et reste stable, incapable d’interagir avec quoi que ce soit d’autre, exactement comme on s’y attend pour la matière sombre.
sqark-cascade

Voici une chaîne de désintégration typique. Un quark supersymétrique se désintègre en une autre particule supersymétrique, χ2, et en un quark ordinaire, q. Lors des deux étapes suivantes, un électron ou muon (notés l+ and l) et des particules supersymétriques plus légères sont produites. La plus légère, dans ce cas particulier une particule appelée neutralino, χ1 ne peut se désintégrer en quoi que ce soit d’autre et s’échappe du détecteur sans laisser de trace.

Voir l’invisible
Un événement est un cliché révélant toutes les particules plus légères émises lors des désintégrations de particules instables. Pour chaque évènement, l’énergie doit être balancée. Ainsi, même lorsqu’une particule traverse le détecteur en ne laissant aucun signal, elle peut être détectée grâce au déséquilibre de l’énergie de cet événement. On détecte donc les particules supersymétriques les plus légères et invisibles de cette façon.

Les collaborations CMS et ATLAS cherchent donc des événements ayant un fort déséquilibre en énergie accompagné soit d’un unique photon soit d’un jet (une gerbe de particules constituées de quarks).

MET-photon-ATLAS

Ci-dessus, on voit un événement capté par l’expérience ATLAS contenant un seul photon (le dépôt d’énergie indiqué en jaune vers 4 heures à gauche et aussi à droite) et l’énergie manquante représentée par la ligne pointillée rose vers 10 heures.

C’est exactement ce à quoi un événement contenant la particule supersymétrique la plus légère et un photon ressemblerait. Mais un événement contenant un boson Z et un photon a la même allure quand le boson Z se désintègre en deux neutrinos (autres particules qui n’interagissent pas avec le détecteur).

Malheureusement, jusqu’à présent, pour les multiples scénarios étudiés, rien n’a été trouvé sauf le bruit de fond attendu, c’est à dire tous les autres types d’événements connus ayant la même signature.

Contrairement aux recherches directes de matière sombre, les analyses du LHC sont sensibles aux particules de matière sombre même légères. Rappelez-vous le diagramme très fouillis que j’ai montré sur les recherches directes de matière sombre? CMS et ATLAS peuvent aider à clarifier la situation, même si leurs résultats dépendent d’hypothèses théoriques contrairement aux recherches directes.

Voici les résultats de l’expérience CMS pour les recherches d’évènements contenant un seul jet et de l’énergie manquante. L’axe horizontal donne la masse du candidat de matière sombre et l’axe vertical, le taux d’interaction avec la matière ordinaire. Toutes les valeurs au-dessus des différentes courbes sont exclues. CMS (ligne rouge) exclue les particules de matière sombre légère ayant un taux d’interaction élevé, une région inaccessible à XENON100 (courbe bleue), l’expérience la plus puissante pour la recherche directe de la matière sombre.

SpinIndependent_woScalar-CMS

Boson de Higgs et matière sombre
Une autre approche visant à trouver la matière sombre repose sur certaines théories prédisant que le boson de Higgs pourrait se désintégrer en particules de matière sombre.

inv-HiggsLes bosons de Higgs sont parfois produits avec un boson Z. Si le boson de Higgs se désintègre en matière sombre, nous verrions seulement les débris du boson Z et de l’énergie manquante pour le boson de Higgs. Les recherches en ce sens ont jusqu’ici rien révélé de plus que le bruit de fond attendu.

Des mondes parallèles
Des théoricien-ne-s ont développé une étonnante théorie de la matière sombre incorporant les idées d’une vallée cachée où deux mondes évolueraient en parallèle: notre monde avec les  particules du modèle standard et celles de la supersymétrie (bien qu’encore inconnues), et un monde complètement séparé peuplé de particules sombres comme illustré ci-dessous. Ici, chaque ligne horizontale représente une particule d’une masse donnée.

ValleeCachee

L’idée est que le LHC pourrait produire des particules supersymétriques lourdes. Ces particules se désintégreraient en cascade. La plus légère des particules de SUSY serait un “messager” capable de traverser la vallée cachée et de s’échapper dans le secteur sombre, devenant invisible pour nous.

Dans le secteur sombre, cette particule se désintégrerait en une cascade de particules sombres jusqu’à ce qu’elle atteigne la plus légère des particules supersymétriques sombres, un autre messager capable de réapparaître dans notre monde en émettant de nombreuses paires d’électrons ou de muons.

Même si cela ressemble à de la science-fiction, il s’agit bien de physique non vérifiée mais très sérieuse comme en attestent les articles cités ci-dessus.

J’étais jusqu’à tout récemment l’une des expérimentatrices et expérimentateurs à la recherche de signes de cette vallée cachée. Nous sélectionnions des événements contenant des paires regroupées d’électrons et de muons, mais n’avons rien trouvé de plus que le bruit de fond.

Les recherches continuent, là et dans de nombreux autres endroits, tout en raffinant constamment les méthodes et en essayant de nouvelles stratégies. Si la matière sombre interagit avec la matière, nous devrions la trouver.

Premier volet:     Comment sait-on que la matière sombre existe?

Deuxième volet: Comment mettre la main sur la matière sombre

Troisième volet: Cosmologie et matière sombre

Pauline Gagnon

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With Snowmass on the Mississippi less than two weeks away the Young Physicist Movement (YPM) will be closing the survey on July 15th 2013. This means if you haven’t taken the survey you have less than one week to take it and let your voice be heard!

http://tinyurl.com/snowmassyoung

We are busy starting to compile the results, verify our data, and dreaming of the plots that we think will be of the most interest to the organizers of Snowmass.

Ever wonder what others think their odds of finding a permanent position in High Energy Physics is? Ever wonder if people working in the Intensity Frontier have a different outlook than those in the Energy Frontier?  Have you thought about which experiments that are planned seem most exciting to graduate students, post-docs, tenured faculty?

Hopefully we will have answers to these and many other interesting questions using the data in the survey. But none of this is possible if we don’t get as many people in HEP to take this survey. So please, encourage your fellow collaborators, office mates, professors, grad students, undergrads, people who have left the field….EVERYONE who has touched High Energy Physics to take the survey!

Snowmass on the Mississippi takes place July 29th in Minneapolis

Snowmass on the Mississippi takes place July 29th in Minneapolis

This brings me to the next important point. Just as it has been important that physicists (young and not so young) have been involved in the planning process up till now, it is even more critical that we get as many people to attend Snowmass as possible. Speaking as a young person in this field I am specifically stressing this point to anyone who is in this field and intends to make a go at making this their career. The topics that will be discussed, decisions that will be made, and recommendations that will be passed on to the funding agencies all come out of the Snowmass planning process. Thus if you are not part of that conversation, you risk having someone else, someone who will likely no longer be in High Energy Physics, making the long term planning decision for you.

However, it is definitely not to late to have your voice heard! But first you have to get to the conference. Registration details can be found at http://www.hep.umn.edu/css2013/
You should be reaching out to your advisors, bosses, and PI’s and finding out if you can go to the conference. Rooms in the dorms are relatively cheap, carpools are being arranged, finding roommates to split the cost of a hotel are being sought. Snowmass YPM is trying to help as many people as possible. If you are interested but just need help connecting some of the pieces please feel free to email any of the conveners in the YPM, our emails can be found http://snowmassyoung.hep.net/about.html

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Troisième volet d’une série de quatre sur la matière sombre

Voici le troisième volet d’une série sur la matière sombre. J’ai déjà examiné comment elle se révèle à travers des effets gravitationnels et l‘absence de preuves directes d’interaction avec la matière visible. Voyons maintenant comment la cosmologie soutient également l’existence de la matière sombre.

Graines de galaxies
Il est maintenant largement admis que toute la matière (sombre et visible) était distribuée uniformément juste après le Big Bang. En résumé, une expansion rapide a suivi pendant laquelle l’Univers s’est refroidi et les particules ont ralentit suffisamment pour former des noyaux, et ce trois minutes après le Big Bang. Les premiers atomes sont apparus 300 000 ans plus tard, et les galaxies se sont formées entre une centaine et un millier de millions d’années plus tard. Bon.

BigBang-fr

Comment l’Univers est-il passé d’un gigantesque nuage de matière uniformément répartie à la formation de grandes structures comme les galaxies? La matière sombre est probablement à blâmer.

Etant plus lourde que la matière ordinaire, elle s’est ralentie plus tôt. De petites fluctuations quantiques ont évolué en minuscules grumeaux de matière sombre. Ces grumeaux ont grossi en attirant plus de matière sombre sous l’effet de l’attraction gravitationnelle et cela a fait boule de neige. Puisque la matière sombre n’interagit que très faiblement, cette petites graines ont aussi pu résister à la tempête des débuts de l’Univers.

Une fois la matière visible refroidie après l’expansion de l’Univers, elle a commencé à s’accumuler sur les grumeaux de matière sombre. Par conséquent, la matière sombre a semé les graines des galaxies. «Tout cela aurait aussi été possible sans la matière sombre, mais il aurait fallu beaucoup plus de temps», explique Alexandre Arbey, théoricien au CERN.

Simuler la formation de l’Univers
Pas convaincu-e ? Aujourd’hui, les cosmologistes peuvent reproduire ce processus à l’aide de simulations informatiques. Comme point de départ, ils et elles injectent dans leurs modèles la quantité de matière ordinaire et sombre présente juste après le Big Bang. Les observations du fond diffus cosmologique fournissent ces estimations. Puis on laisse évoluer le tout sous l’effet attractif de la gravité et répulsif de l’expansion de l’Univers.

Toutes ces suppositions doivent converger pour reproduire la quantité de matière sombre observée aujourd’hui, une quantité appelée «l’abondance relique”. Si tout est bien réglé, les scientifiques peuvent reconstituer toute l’évolution de l’Univers en accéléré depuis le Big Bang jusqu’à nos jours.

computer-simulation
Les résultats sont frappants comme le montrent les trois photos ci-dessus. Ces images générées par ordinateur montrent la distribution de la matière sombre 470.000.000 années après le Big Bang, puis 2,1 et 13,4 milliards d’années plus tard (aujourd’hui). La matière sombre forme d’abord de petits grumeaux, puis de longs filaments et enfin des structures à grande échelle apparaissent.

Des scientifiques du CNRS viennent de publier une vidéo étonnante montrant comment ils et elles utilisent maintenant ces méga simulations dans l’espoir de sélectionner parmi les différents modèles de matière et d’énergie sombres celui qui correspond aux observations actuelles.

Matière sombre froide
Une autre approche permettant de sélectionner quelle théorie de la matière sombre correspond le mieux à la réalité a été démontrée le mois dernier par un groupe de scientifiques travaillant avec le télescope Subaru. L’équipe a étudié la distribution de matière sombre dans cinquante amas de galaxies. La moyenne de toutes les données montre que la densité de matière sombre diminue progressivement à partir du centre des amas galactiques jusqu’à leurs contours diffus.

Cette mesure correspond aux prédictions de la théorie de la matière sombre froide (CDM), qui stipule que celle-ci est composée de particules se déplaçant lentement. Pour de la matière sombre chaude, les particules se déplaceraient presque à la vitesse de la lumière, comme les neutrinos.
Galaxy-cluster-density-Subaru
La théorie de la matière sombre froide prédit que les régions centrales des amas de galaxies ont une densité de matière sombre inférieure tandis que les galaxies individuelles ont un paramètre de concentration élevé.

Signaux inexpliqués venus de l’espace
Les astronomes n’apportent pas que des éléments de réponse au mystère de la matière sombre mais soulèvent aussi des questions. Par exemple, il y a une dizaine d’années, l’expérience INTEGRAL-SPI a trouvé une source de rayons gamma intense à 511 keV en provenance du centre galactique, là où la matière sombre est la plus concentrée. Cette valeur de 511 keV correspond exactement à l’équivalent en énergie de la masse d’un électron ou positron.

diagramme

Cela avait donc toutes les allures de particules de matière sombre s’annihilant ou se désintégrant en une paire d’électron et positron, puis ceux-ci s’annihilant en rayons gamma comme dans le diagramme ci-dessus. Malheureusement, aujourd’hui l’intérêt s’estompe car les théoricien-ne-s peinent à définir un mécanisme expliquant cet effet tout en respectant les nombreuses contraintes imposées par d’autres observations.

Plusieurs expériences à bord de satellites (HEAT, Pamela et FERMI) ont observé un excès de positrons dans les rayons cosmiques. Un positron est l’antimatière de l’électron. Puisque la matière l’emporte sur l’antimatière dans l’Univers (autrement, nous et les galaxies ne serions pas là), difficile d’expliquer l’origine de ces positrons.

Plusieurs théoricien-ne-s ont invoqué des sources astronomiques comme les pulsars, mais le débat est loin d’être clos. Seraient-ce les premiers signes concrets de matière sombre interagissant avec la matière visible? L’expérience AMS à bord de la Station spatiale internationale a déjà démontré la haute qualité de ses données et pourrait bien trancher d’ici peu.

La matière sombre conserve son mystère, mais tout évolue rapidement. Dans mon prochain blog, j’aborderai comment le Grand collisionneur de hadrons (LHC) du CERN pourra contribuer après son redémarrage en 2015.

Premier volet: Comment sait-on que la matière sombre existe?

Deuxième volet: Comment mettre la main sur la matière sombre

Quatrième volet: Le LHC résoudra-t-il l’énigme de la matière sombre?

Pauline Gagnon

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