5th Force of Nature: "The Portal Between the Visible and Hidden Universe"

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Physicists in Italy are about to start up a new experiment designed to hunt for hypothetical particles such as the “dark photon” and carriers of a possible fifth force of nature. "Dark Photons would provide a 'portal between the visible and hidden sectors' and might also help solve other problems, such as the muon’s anomalous magnetic moment," says Mauro Raggi of the University of Rome “La Sapienza”.



The Positron Annihilation into Dark Matter Experiment (PADME), located at the National Institute of Nuclear Physics (INFN) laboratories in Frascati outside Rome, will blast a thin diamond target with energetic positrons and record the mass of any exotic new particles produced in the collisions continues a report in today's Physics World.

The group at Frascati will mainly target the dark photon, which is a heavy version of the ordinary photon. Predicted by various extensions of the Standard Model, it would interact with both dark matter and ordinary matter. Dark photons are not themselves usually considered to be dark matter, since they would carry relatively little mass and would tend to have decayed earlier in the history of the universe.

Dark photons are also being pursued at experiments in other laboratories, such as CERN in Geneva and the Jefferson Lab in Virginia, US. But according to Raggi, PADME will have the edge in being able to search for the “missing mass” of dark photons – allowing the particles to be detected even when they leave no visible decay products.

The experiment will involve recording collisions that happen when positrons from the Frascati lab’s linear accelerator collide with the electrons in a 100 μm-thick film of diamond. 

The resulting annihilations would normally yield two ordinary photons but, if the dark photon exists, just a single visible photon would be created. The mass of the missing particle could then be calculated by subtracting the measured 4D space–time-momentum of the single visible photon in each case from that of the incoming positron, plotting a spectrum of this missing mass and then reading off the mass value of the spectral peak.


Image credit: Astronomers have found clear indications that clumps of dark matter are the nursing grounds for new born galaxies about twelve billion light years away. A single nest of dark matter can nurture several young galaxies. A scientifically accurate artistic image of galaxies twelve billion light years away is at the top of the page. The blue nebulosity is dark matter. Denser regions are white. The blue-white regions correspond to the dark matter clumps or dark matter halos where young galaxies are forming. (Image created by Naomi Ishikawa and Takaaki Takeda, National Astronomical Observatory of Japan)

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