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Posts Tagged ‘CRESST’

La matière sombre et l’énergie sombre sont bien en évidence à la conférence de physique des particules de la Société de Physique Européenne à Vienne. Bien que les physiciens et physiciennes comprennent maintenant assez bien les constituants de base de la matière, tout ce que l’on voit sur la Terre, dans les étoiles et les galaxies, cette énorme quantité de matière ne représente que 5 % du contenu total de l’Univers. Pas étonnant alors qu’autant d’efforts soient déployés pour élucider le mystère de la matière sombre (27 % de l’Univers) et de l’énergie sombre (68 %).

Depuis le Big Bang, non seulement l’Univers s’étend mais cette expansion va en accélérant. Quelle énergie alimente cette accélération ? Nous l’appelons énergie sombre. Cela demeure absolument inconnu mais l’équipe du Dark Energy Survey cherche à obtenir des éléments de réponse. Ces scientifiques vont examiner un quart du ciel de l’hémisphère sud, cataloguant l’emplacement, la forme et la distribution d’objets astronomiques tels que des amas galactiques (regroupements de galaxies) et de supernovæ (étoiles en explosion). Leur but est de recueillir de l’information sur 300 millions de galaxies et 2500 supernovæ.

Les galaxies se sont formées grâce à l’effet attractif de la gravité, qui a permis à la matière de se regrouper, malgré l’effet dispersif de l’énergie sombre, qui disperse la matière avec l’expansion de l’Univers. Les scientifiques de DES étudient essentiellement comment les grandes structures telles que les amas galactiques se sont développées dans le temps en observant des objets situés à différentes distances et dont la lumière provient de différentes époques dans le temps. Avec plus de données, ces scientifiques espèrent mieux comprendre la dynamique de l’expansion.

La matière sombre est tout aussi inconnue. Jusqu’ici, elle ne s’est manifestée qu’à travers ses effets gravitationnels. Nous pouvons “sentir” sa présence mais pas la voir, puisqu’elle n’émet aucune lumière, contrairement à la matière ordinaire contenue dans les étoiles et supernovæ. Comme si l’Univers entier était rempli de fantômes.

Une douzaine de détecteurs, utilisant des techniques différentes, essaient d’attraper ces particules fantômes. Pas facile de les traquer quand on ne sait ni comment, ni même si ces particules interagissent avec la matière. Elles doivent cependant interagir très rarement car autrement, elles auraient déjà été décelées. On utilise donc des détecteurs massifs dans l’espoir qu’une de ces particules de matière sombre frappe un noyau d’un des atomes du détecteur, induisant une petite vibration décelable. Les différentes équipes de scientifiques tentent de sonder toute la gamme de possibilités. Celles-ci dépendent de la masse possible des particules de matière sombre et leur affinité à interagir avec la matière.

Le graphe ci-dessous illustre la possibilité qu’une particule de matière sombre interagisse avec un noyau (axe vertical) en fonction de leur masse (axe horizontal). Cela couvre une vaste région de possibilités qu’il faut tester. Chaque courbe sur le graphe représente les résultats d’une expérience différente. Les régions au-dessus de ces courbes représentent les possibilités qui sont exclues. La partie gauche du graphe est la plus difficile à explorer car plus les particules de matière noire sont légères, plus la vibration induite est petite.

CRESST-limit

La Collaboration CRESST utilise de petits cristaux opérant à très basse température. Ils peuvent déceler la hausse de température minime que provoquerait une particule de matière sombre en frappant un noyau atomique. Cela leur a permis de réussir là où des dizaines d’expériences précédentes avaient échoué : la recherche de particules très légères. C’est ce que l’on peut voir sur le graphe. Toutes les possibilités au-dessus du trait continu rouge dans le coin supérieur gauche sont désormais exclues. Jusqu’ici, cette zone n’était accessible qu’aux expériences du Grand Collisionneur de Hadron (LHC) du CERN (non incluses dans ce graphe), mais au prix de plusieurs suppositions. CRESST vient d’ouvrir tout un monde de possibilités. Les particules de matière sombre légères n’ont qu’à bien se tenir.

Pauline Gagnon

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Dark matter and dark energy feature prominently at the European Physics Society conference on particle physics in Vienna. Although physicists now understand pretty well the basic constituents of matter, all what one sees on Earth, in stars and galaxies, this huge amount of matter only accounts for 5% of the whole content of the Universe. Not surprising then that much efforts are deployed to elucidate the nature of dark matter (27% of the Universe), and dark energy (68%).

Since the Big Bang, the Universe is not only expanding, but this expansion is also accelerating. So which energy fuels this acceleration? We call it dark energy. This is still something absolutely unknown but the Dark Energy Survey (DES) team is determined to get some answers. To do so, they are searching a quarter of the southern sky, mapping the location, shape and distribution of various astronomical objects such as galactic clusters (large groups of galaxies) and supernovae (exploding stars). Their goal is to record information on 300 million galaxies and 2500 supernovae.

Galaxies formed thanks to gravity that allowed matter to cluster. But this happened against the dispersive effect of dark energy, since the expansion of the Universe scattered matter away. The DES scientists essentially study how large structures such as galactic clusters evolved in time by looking at objects at various distances, and whose light comes from different times in the past. With more data, they hope to better understand the dynamic of expansion.

Dark matter is just as unknown. So far, it has only manifested itself through gravitational effects. We can “feel” its presence but we cannot see it, since it emits no light, unlike regular matter found in stars and supernovae. As if the whole Universe was full of ghosts. A dozen detectors, using different techniques, are trying to find dark matter particles.

Not easy to catch such elusive particles when no one knows how and if these particles interact with matter. Moreover, these particles must interact very rarely with regular matter (otherwise, they would already have been found), the name of the game is to use massive detectors, in the hope one nucleus from one of the detector atoms will recoil when hit by a dark matter particle, inducing a small but detectable vibration in the detector. The experiments search for a range of possibilities, depending on the mass of the dark matter particles and how often they can interact with matter.

The plot below shows how often dark matter particles could interact with a nucleus (vertical axis) as a function of their mass (horizontal axis). This spans a wide region of possibilities one must test. The various curves indicate what has been achieved so far by different experiments. All possibilites above the curves are excluded. The left part of the plot is harder to probe since the lighter the dark matter particles is, the smaller the vibration induced.

CRESST-limitThe CRESST Collaboration uses small crystals operating at extremely low temperature. They are sensitive to the temperature rise that would occur if a dark matter particle deposited the smallest amount of energy. This allowed them to succeed where tens of previous experiments had failed: looking for very light particles. This is shown on the plot by the solid red curve in the upper left corner. All possibilities above are now excluded. So far, this area was only accessible to the Large Hadron Collider (LHC) experiments (results not shown here) but only when making various theoretical hypotheses. CRESST has just opened a new world of possibilities and they will sweep nearly the entire area in the coming years. Light dark matter particles better watch out.

Pauline Gagnon

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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)

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