Without dark matter, most galaxies in the universe would not hold together. Scientists are pretty sure about this. However, they have not been able to observe dark matter and the particles that comprise it directly. They have only been able to infer its presence through the gravitational pull it exerts on visible matter.
One hypothesis is that dark matter consists of particles that interact with each other and with visible matter through a new force carried by a particle called the dark photon. In a recent study, the collaboration behind the NA64 experiment at CERN describes how it has tried to hunt down such dark photons.
NA64 is a fixed-target experiment. A beam of particles is fired onto a fixed target to look for particles and phenomena produced by collisions between the beam particles and atomic nuclei in the target. Specifically, the experiment uses an electron beam of 100 GeV energy from the Super Proton Synchrotron accelerator. In the new study, the NA64 team looked for dark photons using the missing-energy technique: although dark photons would escape through the NA64 detector unnoticed, they would carry away energy that can be identified by analysing the energy budget of the collisions.
The team analysed data collected in 2016, 2017 and 2018, which together corresponded to a whopping hundred billion electrons hitting the target. They found no evidence of dark photons in the data but their analysis resulted in the most stringent bounds yet on the strength of the interaction between a photon and a dark photon for dark-photon masses between 1 MeV and 0.2 GeV.
These bounds imply that a 1-MeV dark photon would interact with an electron with a force that is at least one hundred thousand times weaker than the electromagnetic force carried by a photon, whereas a 0.2-GeV dark photon would interact with an electron with a force that is at least one thousand times weaker. The collaboration anticipates obtaining even stronger limits with the upgraded detector, which is expected to be completed in 2021.
Read more about how the experiment is searching for dark matter in this EP newsletter article.