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LAB WATCH 13 WINTER 2015 N° 36 Could astronomers finally have detected the first particles of dark matter, the invisible substance that makes up more than 80% of the mass of the Universe? The detection a year ago of a mysterious signal in the form of X-rays from distant galaxy clusters suggests it may indeed be the case. Having successively eliminated other possible causes, several teams of researchers reached the same conclusion: the signal can only be explained by the presence of dark matter in the clusters. A mysterious X-ray spike The astronomers pointed the XMMNewton and Chandra space observatories at the Perseus Cluster of galaxies, located 240 million light years away and thought to contain the largest density of dark matter in the Universe. To their surprise, they discovered an X-ray spike at an energy of around 3.5 kilo-electron volts (3.5 keV), which does not correspond to any known chemical element. “This makes dark mat ter the most convincing explanation to date,” says Yann Mambrini from the LPT,1 who co-authored the article supporting this bold hypothesis.2 So what do these particles look like? At the moment, two scenarios are put forward. In the model preferred by the teams that made the discovery, 3 the observed signal corresponds to the decay of a so-called sterile neutrino, a hypothetical particle whose existence was first postulated by physicists in the early 2000s. Unlike the three flavors of neutrinos already known,4 this fourth potential neutrino does not interact with any other particle except through gravitation. Such sterile neutrinos may decay into an ordinary neutrino and a photon with an energy of 3.5 keV. Lightweight particles However, Mambrini and his colleagues believe that the signal is more likely to result from the annihilation of dark matter particles of a different nature. “When they collide, they could produce a sort of Higgs boson, but much lighter than the one discovered at the Large Hadron Collider (LHC),” the physicist explains. “This boson, already predicted by some models, would in turn decay into two photons of 3.5 keV each.” These dark matter candidate particles are a hundred times lighter than an electron. This property is in sharp contrast with that of “weakly interacting massive particles,” or WIMPs, the favorite candidates until now. Unlike WIMPs, these extremely lightweight particles could help explain why there are so few satellite galaxies orbiting the Milky Way. But researchers remain cautious. “Even if the observed signal turns out to be statistically significant, a more mundane astrophysical explanation cannot be ruled out,” says Gianfranco Bertone, an associate professor at the University of Amsterdam (Netherlands) on leave from the IAP.5 New measurements of our own galaxy should settle the question. In particular, the launch next year of Japan’s X-ray space telescope Astro-H, should provide new insights. ii Astrophysics. An unexplained signal from distant galaxy clusters may turn out to be the first detection of dark matter, a mysterious component of the Universe. BY JULIEN BOURDET © CHANDRA : NASA/CXC/SAO/E. BULBUL ET AL. An X-ray image of the Perseus Cluster taken by the Chandra space telescope. 1. Laboratoire de physique théorique d’Orsay (CNRS / Université Paris-Sud). 2. Y. Mambrini et al., “Generating x-ray lines from annihilating dark matter,” Physical Review D, 2014. 90(3): 1-10. 3. E. Bulbul et al., “Detection of an Unidentified Emission Line in the Stacked X-Ray Spectrum of Galaxy Clusters,” Astrophysical Journal, 2014. 789(1): 13-36 ; A. Boyarsky et al., “unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster” (working paper, 2014). http://arxiv.org/abs/1402.4119. 4. Electron, Muon, and Tau neutrinos. 5. Institut d’astrophysique de Paris (CNRS / UPMC). yann.mambrini@th.u-psud.fr Dark Matter Detected at Last? BY JULIEN BOURDET


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