The Mystery of the Galactic Center Could Be Solved in the LHC at CERN

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A new calculation opened the door to controversies that the excess of gamma rays (observed with the Femmi telescope) is due to a dark matter particle with the right mass which can be created in the particle accelerator at CERN.

This is one of the most controversial observations of the recent times, but there may be an explanation for a mysterious excess of high-energy photons detected in the center of the Milky Way. A recent analysis suggests that the signal could be due to dark matter particles of a certain type; if this interpretation was correct, such particles would have the right mass to be created and detected in the Large Hadron Collider (LHC) at CERN.

After a break of two years, the LHC is scheduled to restart the proton collisions this summer. As confirmed by the journal Nature, finding the particle will be one of the priorities in this second phase of tests.

The tests won’t just detect what causes the galactic gamma-ray emission, but will reveal what dark matter – the mysterious substance believed to account for 85% of the mass of the universe – is made of. In addition, as an indication of the existence of supersymmetric particles, a generic framework proposed for decades will extend the standard model of particle physics.

“We could face the most promising explanation for the galactic center proposed to date,” says Dan Hooper, from the particle physics laboratory Fermilab in Chicago. He adds that “there are others that are not far back”.

In 2009, Lisa and Hooper Goodenough, two graduate students at the University of New York, were the firsts to decipher the enigmatic data signal provided by the gamma-ray space telescope Fermi at NASA. They proposed the idea that excess in photons could be due to the dark matter. Colliding with each other, it is possible that two dark matter particles disintegrate, as it occurs when an ordinary matter particle collides with its antimatter counterpart. Such interaction generates a cascade of unstable particles, and ultimately they produce the gamma-rays.

However, the theory behind the dark matter particle, which some have dubbed hooperon or gooperon in honor of its proponents, was soon in trouble because it was not following the physical supersymmetric models supported by physicists. Although the simplest supersymmetric extension of the standard model (MSSM – the acronym of “minimal supersymmetric standard model”) compares the dark matter particles with the estimated mass of the hooperons – among 25 and 30 gigaelectronvolts (GeV) – other experiments have provided compelling evidence to rule out such a light particle.

For many physicists, this theory sounds obsolete. “A new theory would be necessary”, says Sascha Caron, a particle physicist at the Radboud University of Nijmegen and leader of the team that has made the latest estimates. Skeptics, meanwhile, suggest that the excess of gamma rays have much more mundane explanations, such as emissions from neutron stars or supernova remnants.

However, in late 2014, the estimates of the mass of the hypothetical particle have started to be questioned. New estimates of the amount of X-rays from known sources suggest that dark matter particles compatible with the signal seen by Fermi could be much more heavy. “The excess can be explained with particles up to 200 GeV”, says Simona Murgia, a physicist at the University of California at Irvine.

From the galactic center to the big bang

Considering this possibility, Caron and colleagues recalculated the predictions of the MSSM and found another possible explanation for the excess: a particle called neutralino – explaining the dark matter in a supersymmetric context. The neutralino is said to be massive enough so as not to have been excluded so far by other experiments, but light enough to be produced during the second phase of the LHC operations.

Also, the Caron’s model predicts a consistent amount of dark matter created during the big bang, which corresponds to the recent observations of the cosmic microwave background, made by the Planck satellite at ESA. For the researcher, it is hard to think of a coincidence.

Caron’s group members are not the only who have re-analyzed data from Fermi in the light of the new estimates for the mass of the hypothetical particle. Last November, Patrick Fox from the Fermilab laboratory, and other collaborators conducted similar but less detailed calculations and also pointed to a neutralino. Katherine Freese, the director of the Nordic Institute for Theoretical Physics – NORDITA, based in Stockholm, said that she and her group had found indications that the excess could be explained with a kind of dark matter predicted by some less attractive models than the MSSM supersymmetric model.

The solution might be just around the corner. “Besides being produced in the LHC, a neutralino should also be available to the next generation of underground experiments that attempt to detect dark matter passing through the Earth”, says Albert De Roeck, a physicist at LHC. If the excess of gamma rays is due to the neutralino, “the first signs of dark matter should appear very soon”, concludes the researcher.

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