One of the biggest puzzles in physics is that 85% of the matter in our universe is “dark”: it does not interact with the photons of the conventional electromagnetic force and is therefore invisible to our eyes and telescopes. Although the composition and origin of dark matter are a mystery, we know it exists because astronomers observe its gravitational pull on ordinary visible matter such as stars and galaxies.
The NA64 experiment – which started operations earlier this year – uses a unique set-up to hunt down a specific type of dark matter particle called the dark photon.
Some theories suggest that dark matter consists of a family of new particles and forces, just like our visible world. In addition to gravity, dark matter particles could interact with visible matter through a new force, which has so far escaped detection. Just as the electromagnetic force is carried by the photon, this dark force is thought to be transmitted by a particle called the dark photon. It is predicted to have a subtle interaction (a “mixing”, in particle physics jargon) with the regular photon and therefore act as a mediator between visible and dark matter.
“To use a metaphor, an otherwise impossible dialogue between two people not speaking the same language (visible and dark matter) can be enabled by a mediator (the dark photon), who understands one language and speaks the other one,” explains Sergei Gninenko, spokesperson for the NA64 collaboration.
“Theories predict that dark photons could also explain the longstanding discrepancy observed in measurements with muons (known as the “g-2 anomaly”). Our experiment will be able to test this, and that’s why we are so excited,” continues Gninenko.
CERN’s NA64 experiment looks for signatures of this visible-dark interaction using a simple but powerful physics concept: the conservation of energy. A beam of electrons coming from the Super Proton Synchrotron accelerator, whose initial energy is known very precisely (100 GeV), is aimed at a detector and the energy that it deposits is measured further downstream. Interactions between incoming electrons and atomic nuclei in the detector produce visible photons. If theories of dark forces are correct, however, these ordinary photons could occasionally transform into dark photons, which simply escape the detector and carry away a large fraction of the initial electron energy.
Therefore, the signature of the dark photon is an event registered in the detector with a large amount of “missing energy” that cannot be attributed to a process involving only ordinary particles, thus providing a strong hint of the dark photon’s existence.
NA64 began operations in July for a period of two weeks, and the collaboration completed a second four-week run on 9 November. Although no signs of dark photons have been found so far, the results have already set new limits on the strength of the visible-dark-matter interaction. Significantly more data accumulated in the coming years will allow the team to narrow the search further.
If confirmed, the existence of the dark photon would represent a breakthrough in our understanding the longstanding dark matter mystery.