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Posts Tagged ‘dark matter’

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.


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.


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).


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.


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.


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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Cosmology and dark matter

Monday, July 8th, 2013

Third part in a series of four on Dark Matter

I have already reviewed how it reveals its presence through gravitational effects and the lack of direct evidence of interaction with regular matter. Let’s now look how cosmological evidence also supports the existence of dark matter.

Galaxy seeds

It is now widely accepted that all matter (dark and visible) started out being uniformly distributed just after the Big Bang. To make a long story short, a rapid expansion followed where the Universe cooled down and particles slowed down enough to form nuclei three minutes after the Big Bang. The first atoms appeared 300 000 years later while galaxies formed between a hundred and a thousand million years later.


How did the Universe change from being a gigantic cloud of uniformly distributed matter to containing large structures?  Dark matter is probably the one to be blamed.

Dark matter is heavier than regular matter and slowed down earlier. Small quantum fluctuations eventually turned into small lumps of dark matter. These lumps attracted more dark matter under the effect of the gravitational attraction, in a very slow snowball effect. Since dark matter also interacts very weakly, these planted seeds survived well through the stormy moments of the early Universe.

Once matter cooled off as the Universe expanded, it started accumulating on the lumps of dark matter. Hence, dark matter planted the seeds for galaxies. “All this could have happened without dark matter, although it would have taken much more time,” explains Alexandre Arbey, theorist at CERN.

Simulating the formation of the Universe

Not convinced? Nowadays, scientists can reproduce this process using computer simulations. As a starting point, they inject into their models how much matter and dark matter there was right after the Big Bang. The observations of the cosmic microwave background provide these estimates. Then they let it evolve under the attractive effect of gravity and the repulsive effect of the Universe expansion.

All these guesses must converge to reproduce the amount of dark matter leftover today, a quantity called the “relic abundance”. If all is properly tuned, scientists can recreate the whole evolution of the Universe in fast motion from the moment of the Big Bang until today.computer-simulation

The results are striking as can be seen on the three pictures above. These computer-generated images show the distribution of dark matter 470 million years after the Big Bang, then 2.1 and 13.4 billion years later (today). Dark matter first formed small lumps, then long filaments and finally large-scale structures appeared.

Scientists from the French National Centre for Scientific Research (CNRS) just released an amazing video showing how they are now using these mega simulations in the hope to discriminate against different dark matter and dark energy models by comparing these images with current observations.

Cold dark matter

Another way to figure out which theory of dark matter best fits the reality was provided last month by a group of scientists working with the Subaru telescope. They studied the distribution of dark matter in fifty galaxy clusters. Averaging all the data, they found that the dark matter density gradually decreases from the centre of the clusters to their diffuse outskirts.

This new evidence conforms to the predictions of cold dark matter theory (CDM), which states that dark matter is made of slow moving particles. Hot dark matter candidates like neutrinos would be made of particles moving close to the speed of light.


Cold dark matter theory predicts that central regions of galaxy clusters have a lower dark matter density while individual galaxies have a higher concentration parameter.

Unexplained signals from outer space

Astronomers are not just providing clues to the mystery of dark matter but also raising questions.  For example, a decade ago, the INTEGRAL-SPI experiment found an intense gamma ray source at 511 keV coming from the galactic centre, exactly where dark matter is most concentrated. This value of 511 keV is precisely the energy corresponding to the electron or positron mass.


This smelled incredibly like dark matter particles annihilating or decaying into pairs of electron and positron, which in turn can annihilate into gamma rays as depicted on the diagrams above. Unfortunately, nowadays the excitement has somewhat wound down since theorists have a hard time reconciling its characteristics with numerous other observations.

Several satellite experiments (HEAT, PAMELA and FERMI) have observed an excess of positrons in cosmic rays. A positron is the antimatter counterpart of the electron. Given matter prevails over antimatter in the Universe (otherwise, we and the galaxies would not be there), astrophysicists have to figure out where these positrons come from.

Many theorists have attempted to explain this in terms of astronomical phenomena but the jury is still out. Could this be the first concrete sign of dark matter? The AMS experiment on-board the International Space Station has already shown that they have high quality data and could provide a definitive answer very soon.

Dark matter remains a mystery but this field is fast evolving. In my next blog, I will look at what the Large Hadron Collider (LHC) at CERN could do after restart in 2015.

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

Fourth part in a Dark Matter series:  Can the LHC solve the Dark Matter mystery?

Pauline Gagnon

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Getting our hands on dark matter

Monday, July 1st, 2013

Second part in a series of four on Dark Matter

In a previous blog, I reviewed the many ways dark matter manifests itself through gravitational effects. But to this day, nobody has managed an unambiguous direct observation of dark matter.

This is not surprising given we are talking about a completely different and totally unknown type of matter, something not made of quarks and leptons like all visible matter (humans, planets, stars and galaxies).

Nevertheless, just as the quarks and leptons are the building blocks of visible matter, physicists expect dark matter is also made of fundamental particles, albeit dark particles. So we need to catch dark matter particles interacting in some way with particles of regular matter.

So far, all we know is that dark matter reacts to gravitation but not to electromagnetism since it does not emit any light. Maybe it interacts with ordinary matter through the weak nuclear force, the one responsible for radioactive decays. Dark matter would then be made of weakly interacting particles.

Weakly Interacting Massive Particles

One popular hypothesis is that dark matter particles would be WIMPs, which stands for Weakly Interacting Massive Particles. How often can WIMPs interact with matter? It should be less than 0.1 times per year per kilogram of sensitive material in the detector, depending on the WIMP mass.

The detection principle is simple: once in a while, a WIMP will strike a nucleus in one of the detector’s atoms, which will recoil and induce a small recordable vibration.


From Lauren Hsu’s review talk at ICHEP 2012.

The vertical axis shows the number of times a dark particle transfer a given amount of energy to a nucleus. The more massive the detector and the longer you operate it, the higher are the chances of recording a collision.

The detector material also matters as seen on the plot above: collisions are more energetic, hence easier to detect,  with Germanium (Ge) than with heavier nuclei like Xenon (Xe), but the total number of collisions is higher with the latter material.

These detectors are placed deep in mines or tunnels to block cosmic rays that would induce false signals in the detector. Eliminating all sources of background is the biggest challenge facing these experiments.

Dark matter wind

If the Universe is full of dark matter, we on Earth should feel a wind of dark particles as we travel around the Sun. This rate is evaluated to be of the order of a million particles per square centimetre per second for a WIMP ten times heavier than a proton.

And just like a cyclist riding on a circular track on a windy day, we should feel a head wind of dark matter particles in June and a tail wind in December since there is a greater concentration of dark matter in the centre of the galaxy.



Imagine now a detector operating on Earth and sensitive to WIMPs. The variations in the wind intensity would be detected as an annual modulation in the number of dark matter particles hitting the detector throughout the year.

This is exactly what the DAMA/LIBRA experiment claims to observe for more than a decade now as shown on the plot below. Their signal is loud and clear (8.9 sigma) but unfortunately, refuted by several experiments.

DAMA-LIBRAThe number of events recorded by DAMA/LIBRA as a function of time (more than 10 years) shows a clear annual modulation.

Three other experiments have also reported signals: CoGent sees a faint modulation while both CRESST and CDMS observed a few events in excess of the expected background.

All would be great if these four experiments would all agree on the characteristics of the dark matter particle but that is unfortunately not the case.

Many theorists have deployed heroic efforts to devise new models to explain why some experiments see a signal while others do not, but no model is widely accepted yet. The situation remains terribly confusing as can be appreciated from the plot below.


The vertical axis represents the possible rate at which a dark matter particle could interact with regular matter while the horizontal axis gives the mass of the hypothetical dark particle. The areas in solid colours delimit the possible values obtained by the four experiments having a signal. Only CoGent and CDMS agree.

The lines show the limits placed on the allowed dark matter interaction rate and mass by some of the experiments that reported no signal. All values above those lines are excluded, meaning the null experiments are in direct contradiction with the four groups that reported a signal.

As frustrating as this might seen, it is in fact not surprising given the complexity of these experiments. It could be due to experimental flaws or there might be a theoretical explanation.

Many experiments are collecting more data and new ones are being built. With theorists and experimentalists being hard at work, hopefully there will soon be a breakthrough.

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

Fourth part in a Dark Matter series:  Can the LHC solve the Dark Matter mystery?

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline
 or sign-up on this mailing list to receive and e-mail notification.

Further information:

Hangout with CERN: The Dark Side of the Universe

TED Ed clip: Dark matter: The matter we can’t see

TED talk by Pat Burchat: Shedding light on dark matter



Grey matter confronted to dark matter

Thursday, April 4th, 2013

After 18 years spent building the experiment and nearly two years taking data from the International Space Station, the Alpha Magnetic Spectrometer or AMS-02 collaboration showed its first results on Wednesday to a packed audience at CERN. But Prof. Sam Ting, one of the 1976 Nobel laureates and spokesperson of the experiment, only revealed part of the positron energy spectrum measured so far by AMS-02.

Positrons are the antimatter of electrons. Given we live in a world where matter dominates, it is not easy to explain where this excess of positrons comes from. There are currently two popular hypotheses: either the positrons come from pulsars or they originate from the annihilation of dark matter particles into a pair of electron and positron.  To tell these two hypotheses apart, one needs to see exactly what happens at the high-energy end of the spectrum. But this is where fewer positrons are found, making it extremely difficult to achieve the needed precision. Yesterday, we learned that AMS-02 might indeed be able to reach the needed accuracy.

The fraction of positrons (measured with respect to the sum of electrons and positrons) captured by AMS-02 as a function of their energy is shown in red. The vertical bars indicate the size of the uncertainty. The most important part of this spectrum is the high-energy part (above 100 GeV or 102) where the results of two previous experiments are also shown: Fermi in green and PAMELA in blue. Note that the AMS-02 precision exceeds the one obtained by the other experiments. The spectrum also extends to higher energy. The big question now is to see if the red curve will drop sharply at higher energy or not. More data is needed before the AMS-02 can get a definitive answer.

Only the first part of the story was revealed yesterday. The data shown clearly demonstrated the power of AMS-02. That was the excellent news delivered at the seminar: AMS-02 will be able to measure the energy spectrum accurately enough to eventually be able to tell where the positrons come from.

But the second part of the story, the punch line everyone was waiting for, will only be delivered at a later time. The data at very high energy will reveal if the observed excess in positrons comes from dark matter annihilation or from “simple” pulsars.  How long will it take before the world gets this crucial answer from AMS-02? Prof. Ting would not tell. No matter how long, the whole scientific community will be waiting with great anticipation until the collaboration is confident their measurement is precise enough. And then we will know.

If AMS-02 does manage to show that the positron excess has a dark matter origin, the consequences would be equivalent to discovering a whole new continent. As it stands, we observe that 26.8% of the content of the Universe comes in the form of a completely unknown type of matter called dark matter but have never been able to catch any of it. We only detect its presence through its gravitational effects. If AMS-02 can prove dark matter particles can annihilate and produce pairs of electrons and positrons, it would be a complete revolution.


Here are two plots to show how different the positron fraction spectrum (i.e. the curve showing the fraction of positrons as a function of energy) would differ at high energy (the rightmost part of the plot) if the positrons come from the sum of all pulsars around or if it comes from dark matter annihilation. Note they are not on the same scale and difficult to compare, but they still give some idea. It will be easier once theorists update their plots with the new AMS-02 data points on them and of course, once AMS-02 releases further information at high energy.

This is one theoretical prediction of what the positron fraction spectrum should look like if the positrons come from dark matter particles like neutralinos (represented by the symbol χ). Two curves are shown, depending on the hypothetical mass of the neutralino (mχ) at 400 GeV or 800 GeV. In each case, the maximum energy the positrons can get is roughly equal to the the mass of the neutralino, such that the curve ends close to the neutralino mass. Note the logarithmic scale on both axes.

Here is the expected spectrum if the positrons come from the sum of all pulsars. Three hypotheses were shown but only the middle one seemed to fit the PAMELA experimental results. The important feature is that this curve comes down smoothly, and not sharply at neutralino mass as with the dark matter hypothesis. Again, this curve only represents one theoretical prediction as done by Dan Hooper and his colleagues. The data point in red are from the PAMELA experiment and stop around 100 GeV. The hope is that AMS-02 will be able to provide accurate measurements at higher energies, up to several hundred GeV.

Pauline Gagnon

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Après 18 années passées à bâtir l’expérience et presque deux autres à accumuler des données à bord le la Station Spatiale Internationale, la collaboration du Spectromètre Magnétique Alpha ou AMS-02 a révélé hier au CERN ses tout premiers résultats devant un auditorium plein à craquer. Le Prof. Sam Ting, prix Nobel de 1976 et porte-parole de l’expérience, n’a cependant dévoilé qu’une partie du spectre de l’énergie des positrons mesurés par AMS-02.

Les positrons sont l’antimatière des électrons. Comme on habite dans un monde où la matière domine, il n’est pas facile d’expliquer l’origine de cet excès de positrons venus de l’espace. Il existe deux hypothèses privilégiées : soit ces positrons sont émis par des pulsars, soit ils proviennent de particules de matière sombre qui s’annihilent en produisant un positron et un électron.

Pour distinguer ces deux hypothèses, il faut connaitre très exactement ce qui arrive au spectre de positrons à haute énergie. Mais comme il y en a très peu à haute énergie, il est très difficile d’obtenir un résultat précis. Or voilà la bonne nouvelle annoncée hier par la collaboration AMS : leurs données atteindront le niveau de précision requis.


La fraction de positrons (mesurés par rapport au nombre total d’électrons et de positrons) capturée par AMS-02 en fonction de l’énergie des positrons est indiquée en rouge. Les barres verticales représentent la marge d’incertitude. La partie la plus importante de ce spectre se trouve à haute énergie, au-delà de 100 (ou 102) GeV. Là où les résultats de deux expériences précédentes sont aussi indiqués : en vert, ceux de Fermi et en bleu, ceux de PAMELA. Remarquez que la précision d’AMS-02 dépasse largement celle des expériences précédentes. Le spectre va aussi beaucoup plus haut en énergie. Reste à savoir si cette courbe chutera abruptement à plus haute énergie (signe que les positrons viendraient de matière sombre) ou pas (si les pulsars en sont la source). La collaboration attend d’avoir accumulé plus de données avant de se prononcer.

Seule la première partie de l’histoire a été dévoilée hier. Les données actuelles laissent déjà présager de ce qu’AMS-02 pourra accomplir. C’était la bonne nouvelle communiquée hier: AMS-02 devrait pouvoir mesurer le spectre des positrons à haute énergie avec suffisamment de précision pour trancher sur leur origine.

Mais pour la fin de l’histoire, il faudra encore patienter. Les données à haute énergie révèleront si ces positrons viennent de l’annihilation de particules de matière sombre, ou simplement de vulgaires pulsars. Combien de temps faudra-t-il encore attendre ? Le Prof. Ting n’a pas voulu le préciser. Peu importe, la communauté scientifique patientera en attendant que la collaboration AMS-02 ait suffisamment de données pour obtenir la précision nécessaire.

Si AMS-02 peut prouver que ces positrons viennent de la matière sombre, les conséquences seraient alors aussi époustouflantes que la découverte d’un nouveau continent. A l’heure actuelle, tout ce que l’on sait, c’est que cette matière  sombre correspond à 26.8% du contenu total de l’Univers. On ne la perçoit qu’à travers ses effets gravitationnels. Si AMS-02 réussi à prouver que la matière  sombre peut s’annihiler et produire des paires de positrons et d’électrons, ce serait tout simplement une révolution.

Pauline Gagnon

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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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Fermilab planning a busy 2012

Tuesday, January 3rd, 2012

This column by Fermilab Director Pier Oddone first appeared in Fermilab Today Jan. 3 .

We have a mountain of exciting work coming our way!

In accelerator operations, we need to give enough neutrinos to MINERvA to complete their low-energy run, enough anti-neutrinos to MiniBooNE to complete their run and enough neutrinos to MINOS to enable their independent neutrino velocity measurement that will follow up on last year’s OPERA results. We need to provide test beams to several technology development projects and overcome setbacks due to an aging infrastructure to deliver beam to the SeaQuest nuclear physics experiment. And we need to do all of this in the first few months of the year before a year-long shutdown starts. During the shutdown, we will modify the accelerator complex for the NOvA era and begin the campaign to double the number of protons from the Booster to deliver simultaneous beams to various experiments.

In parallel with accelerator modifications, we will push forward on many new experiments. The NOvA detector is in full construction mode, and we face challenges in the very large number of detector elements and large mechanical systems. Any project of this scale requires a huge effort to achieve the full promise of its design. We have the resources in our FY2012 budget to make a lot of progress toward MicroBooNE, Mu2e and LBNE. We will continue to work with DOE to advance Muon g-2. All these experiments are at an important stage in their development and need to be firmly established this year.

At the Cosmic Frontier, we will commission and start operation of the Dark Energy Survey at the Blanco Telescope in Chile, where the camera has arrived and is being tested. In the dark matter arena we will commission and operate the 60 kg COUPP detector at Canada’s SNOLAB and continue the run of the CDMS 15 kg detector in the Soudan Mine while carrying out R&D on future projects. We continue to have a major role in the operation of the Pierre Auger cosmic-ray observatory. In addition we should complete the first phase of the Fermilab Holometer, which will study the properties of space-time at the Planck scale.

At the Energy Frontier, we play a major role in the LHC detector operations and analysis. It should be a fabulously exciting year at the LHC as we push on the hints that we already see in the data.

Beyond construction and operation of facilities we continue our R&D efforts on the superconducting RF technology necessary for Project X and other future accelerators. We will be building the Illinois Accelerator Research Center and moving forward to connect our advanced accelerator program with industry and universities. Our rich program on theory, computation and detector technology will continue to support our laboratory and the particle physics community.

If we accomplish all that is ahead of us for 2012, it will be a year to remember and celebrate when we hit New Year’s Day 2013!


Update: I accidentally miscalculated the decay rate of K40 in a banana. There are 12 decays, per second, per banana, not 18.

Wimps, they are everywhere! They pervade the Universe to its furthest reaches; they help make this little galaxy of ours spin right round like a record (we think); and they can even be found with all the fruit in your local grocery store.

Figure 1: ( L) Two colliding galaxies galaxy clusters (Image: NASA’s Chandra X-Ray Observatory). (R) Bananas, what else? (Image: Google)

WIMPs: Weakly-Interactive Massive Particles, is an all-encompassing term used to describe any particle that has (1) mass, and (2) is unlikely to interactive with other particles. This term is amazing; it describes particles we know exist and is a generic, blanket-term that adequately describes many hypothetical particles.

Neutrinos: The Prototypical WIMP

Back in 1930, there was a bit of a crisis in the freshly established field of particle physics. The primary mechanism that mediates most nuclear reactions, known as β-decay (beta-decay), violated (at the time) one of the great pillars of experimental physics: The Law of Conservation of Energy. This law says that energy can NEVER be created or destroyed, ever. Period. Sure, energy can be converted from one type, like vibrational energy, to another type, like heat, but it can never just magically (dis)appear.

Figure 2: In β-decay, before 1930, neutrons were (erroneously) believed to decay into a high speed electron (β) and a proton (p+).

Before 1930, physicists thought that when an atom’s nucleus decayed via β-decay a very energetic electron (at the time called a β particle) would be emitted from the nucleus. From the Conservation of Energy, the energy of an electron is exactly predicted. The experimental result was pretty much as far off from the prediction as possible and implied the terrifying notion that perhaps energy was not conserved for Quantum Mechanics. Then, in 1930, the Nobel Prize-Winning physicist Wolfgang Pauli noticed that the experimental measurements of β-decay looked a bit like what one would expect if instead of one particle being emitted by a radioactive nucleus, two particles were emitted.

Prof. Pauli thought the idea of a radioactive nucleus emitting two particles, one visible (the electron) and one invisible, was horrible, silly, and unprofessional. Consequentially, he decided to pen a letter to the physics community suggesting there existed such a particle. 🙂 Using this idea and what could only be described as a level of intuition beyond that of genius, Nobel Laureate Enrico Fermi suggested that perhaps nuclear decay was actually the manifestation of a new, weak force and aptly named it the Weak Nuclear Force (note the capitalization).

To recap: 1 hypothetical particle mediated by 1 hypothetical force.

Figure 3: Prof. Pauli proposed that β-decay actually included an electrically neutral particle with little mass (χ0), in addition to the final-state electron (β) & proton (p+). This once-hypothetical particle is now known as the anti-neutrino (ν).

30 years later, in 1962, Prof. Pauli’s invisible particles (by then called neutrinos) were discovered; 20 years after that, the Weak Force was definitively confirmed; and after another 20 years, neutrinos were found to have mass.

Since 1930, hundreds of theories have invoked the existence of new particles that (1) have mass, and (2) interact weakly (note lack of capitalization) with other particles, which may/may not involve the Weak Nuclear Force (note capitalization, again). At some point in the 1980s, it was finally decided to coin a generic term that described these particles from other large classes of particles that are, say, massless or readily interact with other particles, e.g., with photons or gluons.

Dark Matter: The Elephant in the Galaxy

Kepler’s Laws of Motion & General Relativity are phenomenal at predicting the orbits of planets and solar systems around immense sources of gravity, like stars & black holes. However, there are two known astronomical observations where our predictions do not readily match the experimental results.

The first has to do with how our galaxy spins like a top. Theoretically, the more distant you are from a galaxy’s center, the slower you orbit around the center; vice versa, the closer you are to the center of the galaxy’s center, the faster you orbit around it. Experimentally, astronomers have found that after a certain distance from the galaxy’s center an object’s speed becomes roughly constant. In other words, if Earth were half as close to the galactic center as it is now, its speed will not have appreciably changed. See figure 4 (below) for nice little graph that compares what is observed (solid line) and what is predicted (dotted line). Furthermore, this is not just our galaxy; this is common to all galaxies. Weird, right?

Figure 4: (A) The theoretical prediction of how fast an object travels (velocity) around the galactic center, as a function of (radial) distance from the center. (B) The experimental observation. (Image: Penn State)

The second disagreement between theory and experiment comes from watching galaxies collide with one another. Yes, I literally mean watching galaxies collide into one another (and you thought the LHC was wicked). This is how it looks:

Figure 5: Chandra X-Ray Image of two galaxies galaxy clusters colliding. The pink regions represent the visible portions of the galaxies; the blue regions represent the invisible (dark matter) portions, as calculated from gravitational lensing. (Image: NASA)

Astronomers & astrophysicists can usually determine how massive galaxies & stars are by how bright they are; however, the mass can also be determined by a phenomenon called gravitational lensing (a triumph of General Relativity). When NASA’s Chandra X-Ray telescope took this little snapshot of two galaxies (pink) passing right through each other it was discovered, rather surprisingly, that the mass deduced from the brightness of the galaxies was only a fraction of the mass deduced from gravitational lensings (blue). You can think of this as physically feeling more matter than what can visibly be seen.

What is fascinating is that these problems (of cosmic proportion) wonderfully disappear if there exists in the universe a very stable (read: does not decay), massive, weakly-interacting particle. Sounds familiar? It better because this type of WIMP is commonly known as Dark Matter! Normally, if a theory does not work, then it is just thrown out. What makes General Relativity different is that we know it works; it has made a whole slew of correct predictions that are pretty unique. Predicting the precession of the perihelion of the planet Mercury is not as easy as it sounds. I am probably a bit biased but personally I think it is a very simple solution to two “non-trivial” problems.

Bananas: A Daily Source of K-40

Since I bought a bunch of bananas this morning, I thought I would add a WIMP-related fact about bananas. Like I mentioned earlier, β-decay occurs when a proton neutron decays into a neutron proton by emitting an electron and an anti-neutrino. From a particle physics perspective, this occurs when a down-type quark emits a W boson (via the Weak Force) and becomes an up-type quark. The W boson, which by our definition is a WIMP itself, then decays into an electron (e) and an anti-neutrino (ν – a WIMP). This is how a neutron, which has two down-type & one up-type quark, becomes a proton, which has one-down type & two up-type quarks.

Figure 6: The fully understood mechanism of β-decay in which a neutron (n0) can decay into a proton (p+) when a d-type quark (d) in a neutron emits a W boson (W) and becomes an u-type quark (u). The W boson consequentially decays into an electron (e) and an anti-neutrino (νe).

This type of nuclear transmutation often occurs when a light atom, like potassium (K), has too many neutrons. Potassium-40, which has 19 protons & 21 neutrons, makes up about 0.01% of all naturally forming potassium. Bananas are an exceptionally great source of this vital element, about 450 mg worth, and consequentially have about 45 μg (or ~6.8·1017 atoms) of the radioactive K-40 isotope. This translates to roughly 18 12 nuclear decays (or 18 12 neutrinos), per second, per banana. Considering humans and bananas have coexisted for quite a while in peaceful harmony, minus the whole humans-eat-banans thing, it is my professional opinion that bananas are perfectly safe. 🙂

Dark Matter Detection: CRESST

Okay, I have to be honest: I have a secret agenda in writing about WIMPs. The Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) Experiment Collaboration will be announcing some, uh… interesting results at a press conference tomorrow, as a part of the Topics in Astroparticle & Underground Physics Conference (TAUP 2011). I have no idea what will be said or shown aside from this press release that states the “latest results from the CRESST Experiment provide an indication of dark matter.”


With that, I bid you adieu & Happy Colliding.

– richard (@bravelittlemuon)


As an undergraduate physics major, I was introduced to the Heisenberg uncertainty principle, which states that it is impossible to measure the exact momentum and position of an object at the same time.  This is not caused by inadequacies in our experiments.  Instead, it implies a fundamental limit to our ability to predict the future of a system because we cannot precisely determine its present state.  Such a conclusion is shocking to any physicist. Even Einstein himself refused to accept it.  

Visible matter accounts for only 5 percent of the universe. CDMS hopes to identify the dark matter contained in the remainder of the universe. Courtesy: SLAC/Nicolle Rager

Shocking as the principle is, my university education at least prepared me for the uncertainty of the subatomic world.  What I wasn’t taught was how much uncertainty is embedded in the day-to-day life of a physicist.   A little over a week ago, the mine where my experiment is housed experienced a fire.  The name of my experiment is the Cryogenic Dark Matter Search, or CDMS.  

Before I tell you about the fire, let me explain the purpose of CDMS. Scientists have gathered a large body of evidence that tells us most of the matter in the universe is not in a form that we can see.   Matter that we can see takes on the form of stars, planets, moons, comets, interstellar dust etc..  Dark matter is instead composed of a form of matter that we have never observed on Earth.  My experiment is attempting to probe this dark matter component of the universe and will help us understand what  dark matter is really made of.  CDMS is located approximately 1 km, or a little more than half a mile, underground inside the Soudan Underground Laboratory – up near the Boundary Waters of northern Minnesota.  This unusual location allows us to use the earth as a barrier to cosmic rays.  These can produce signals that  will confuse our attempts to observe dark matter.

The CDMS with sheilding surrounding the silver cryostat where the detectors are housed. Credit: Fermilab

So while housing the experiment deep underground is necessary for its function, it can make for some unexpected challenges. The day of the fire, I and my colleagues waited anxiously, hour-by-hour for the latest news on the attempts to extinguish it.  Luckily, the fire was not in the lab, but was instead in the mine shaft.  Since this shaft serves as the entry and exit to the laboratory, it was still quite a serious situation.  In the end, the fire was put out after heroic efforts on the part of the Minnesota Department of Natural Resources, which operates the laboratory, and the various emergency responders.  Thankfully no one was injured and the damage to the mine shaft and infrastructure were minimal compared to our initial fears.  Since last week, the laboratory staff has been busy restoring power to the underground lab and assessing damage to the mine infrastructure.   As of this Monday, a few scientists have finally been allowed restricted access to the lab.  They are  beginning to assess the status of CDMS.

Before the fire broke out, we were in the midst of an engineering run.  The purpose of this run was to commission a new design for our detectors.   We were very excited about the results of this run because they would demonstrate the power of the new detector design.  This is a necessary step towards convincing our funding agencies that we are ready for the next step of building a much bigger experiment.   Now everything has come to a screeching halt as we continue to wait to find out when we will be able to resume our work. 

 Even without the drama of the mine fire, these past few weeks are a very tense time for a postdoc, such as myself, who is in the process of applying for faculty positions.   I was one of the lucky few this year who was able to land several interviews at top universities.   These interviews are grueling sessions where one must meet and talk to many people over the course of a few days.  During a packed series of 30-45 minute interviews, where one often doesn’t even get a few minutes break in between sessions, you must simultaneously explain your research and try to find out as much about the university as possible. 

The interview rounds are largely finished for this year.  Now it is the time when the schools begin making offers to their first-choice candidates.  Some of these decisions will make or break the dreams of young physicists.  On the part of the universities, its a very large investment, especially because the recent downturn in the economy has prohibited many schools from making hires in the past few years.

 Anxiety runs high on all sides as I continue to wait for news of my future and that of CDMS…

–Lauren Hsu