With the results of their latest search published on 4 January on arXiv, the NA64 collaboration constrains a large fraction of the available parameter space of two models that could explain the long-standing muon g-2 anomaly and thermal dark matter via a dark boson Z’. During their first data taking run in the M2 beamline in CERN’s North Area, they demonstrate the powerful role of muon beams in dark-matter searches.
As a possible new-physics contribution, muons scattering off the nuclei in the target could produce a hypothetical dark boson Z’, followed by its invisible decay into either a pair of neutrinos or a pair of other dark-matter candidates, depending on the underlying model.
The signature would be a loss of energy in that is not spotted in any of the NA64 detectors. For this, a 160 GeV tertiary muon beam derived from the primary SPS proton beam is fired onto an electromagnetic calorimeter acting as an active target. The experimentalists searched for events in which one final-state muon has a momentum of less than 80 GeV together with no detectable energy in any downstream calorimeter within the collected data of 2 x 1010 muons on target.
As no event matching this condition was observed in the signal region, the researchers concluded for one model that it would be only possible for a dark boson Z’ within a mass window of about 6 MeV up to 40 MeV to explain both the anomaly and dark matter, while the second case would be possible for a hypothetical dark boson decaying into dark-matter candidates cannot be heavier than 40 MeV.
The NA64 measurement are among the first ones searching for dark sectors weakly coupling to muons. The experimentalists are confident that they will cover the available parameter space of in the future by using higher beam intensities. “Using a muon beam opens a new window to explore other well-motivated new physics scenarios, such as benchmark dark-photon models, scalar portals, millicharged particles or lepton-flavour violating processes,” says deputy spokesperson Paolo Crivelli.