Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Dresden, Germany have analyzed data from the HADES particle detector and concluded that the so-called "dark photons" are not the constituents of dark matter. Dark photons, or U bosons, are hypothetical particles that had thus far been the main candidate for that role, and this new result could make the search for the dark matter particle even more challenging than before.
According to the latest observations, about 70 percent of the mass-energy in our universe is made out of dark energy, about 25 percent is made out of dark matter, and only the remaining roughly 5 percent is made out of the ordinary, baryonic matter that we are most familiar with.
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We know that dark matter clumps together under the force of gravity, affecting the way light bends around visible matter. However, it does not emit or reflect enough electromagnetic radiation to be observed directly, which makes it very hard to study. Physicists have long been looking for the particle that makes up dark matter, but so far they've been disappointed. Researchers have now come to believe that dark matter is comprised by unknown particles which do not fit into the Standard Model of particle physics.
One of the most promising candidates was a hypothetical particle named a "U boson," or "dark photon." The particle owes its nickname to the fact that it would have characteristics similar to photons, but with a mass, meaning it would have a weak interaction with standard, baryonic matter (photons are considered to be massless; if they weren't, they wouldn't be able to move at the speed of light in the vacuum).
But now, researchers at HZDR have concluded that these dark photons cannot be the particle that physicists were looking for. To come to that conclusion, the scientists analyzed data from the High-Acceptance Di-Electron Spectrometer (HADES) detector, collided over ten billion protons and atomic nuclei, collecting information on the resulting debris. The results exclude the possibility that these are the particle that they had been looking for.
In light of the new information, the search for the dark matter particle becomes even more challenging. For the time being, scientists have only found negative results excluding possible candidates, but no positive hint to help them better direct their search.
The research was published on the journal Physical Letters B.