Dark matter is a mysterious substance that seems to permeate the universe, but detecting it is tricky on account of it not interacting with regular matter very often. Scientists have a list of particles that they're working through as possible suspects, and now that list is a little shorter. Experiments at CERN have ruled out some types of dark photons, inching us closer to finding the elusive dark matter.
Decades of astronomical observations have shown that there is far more mass out there than just the stuff we can see. Dubbed dark matter, this extra mass is effectively invisible to us, only observed through its gravitational interactions with normal matter and light. For every hypothesized particle that could be responsible, there are many experiments dedicated to trying to detect them, but so far all have come up empty.
One of the main candidate particles is the dark photon. Regular photons are elementary particles that act as carriers of the electromagnetic force, constituting light and radio waves. It's thought that dark matter's influence might be the result of a new force, carried by a hypothetical particle named the dark photon.
While experiments like HADES in Germany have reported no success hunting this particle in the past, that didn't stop CERN scientists having another look with an experiment called NA64. Using the Super Proton Synchrotron (SPS) accelerator, this experiment involved blasting a 100-GeV electron beam at a target and analyzing what was produced.
Of course, if any dark photons were created in the blasts they would be invisible, but the scientists can detect them in other ways. A produced dark photon would ignore the walls of the experiment and float off into the ether – carrying with it a little bit of energy. Since the team knows how much energy should be present in the aftermath, if there's any less than that they can tell that dark photons were created.
The NA64 team analyzed data collected between 2016 and 2018, which totaled about a hundred billion electron collisions. And amongst all that, there were no signs of dark photons.
This doesn't necessarily rule out the existence of dark photons as a whole – instead, it simply means they don't exist within the energy range that this experiment was testing for. From these results, the researchers conclude that the interaction between photons and dark photons may be much weaker than previously thought.
That means that dark photons with a mass of 1 MeV interact with an electron with a force that's at least 100,000 times weaker than the electromagnetic force a photon carries. For a dark photon with a mass of 200 MeV, that difference is at least 1,000 times weaker.
These new, more stringent boundaries help scientists hone the search. Even tighter limits – and ideally, a detection – could come when the SPS comes online again after its upgrade, which is due in 2021.
The research was published online at ArXiv.
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