The best constraints on multi-GeV dark matter interactions are from underground searches for nuclei recoiling off non-relativistic dark matter particles in the Galactic halo. However, these searches are essentially blind to few-GeV or lighter dark matter, whose nuclear scattering transfers invisibly small kinetic energy to a recoiling nucleus. The Beam Dump eXperiment (BDX) at Jefferson Lab proposes an approach to search for dark matter in this lower mass range by producing it in an electron beam-dump and then detecting its scattering in a small ( ̴m3) downstream detector.
A dark photon produced by electron bremsstrahlung in a beam dump would decay into a pair of light dark matter particles (χ). This so called “invisible decay” represents a possible complementary decay channel to the “visible” decay into e+-e- pairs searched by HPS. The kinematics of the decay process is strongly forward peaked with the dark matter particles produced with almost the same electron beam energy. Due to the very low interaction probability with ordinary matter, the dark matter beam will cross undisturbed the beam-dump and the dirt before reaching the detector. On the contrary, most of the produced Standard Model particles will be stopped in the dump itself and in the surrounding shielding.
A fraction of these relativistic dark matter particles then would scatter off nucleons or electrons in the detector volume via a dark photon exchange. The χ-proton scattering will produce a slow recoil proton with ̴MeV energy whereas the χ-electron scattering will give rise to an electromagnetic shower generated by a ̴GeV recoil electron. BDX aims to measure both signals by using an electromagnetic calorimeter made by scintillating crystals. The calorimeter will be completely surrounded by layers of active veto detectors and passive shielding to reduce as much as possible the beam-unrelated cosmogenic background that, interacting with the detector nuclei, could produce signals that mimicking the signals expected from the χ-proton scattering. To quantify the background rejection capability of BDX and finalize the experimental setup design, a campaign of cosmogenic background measurements will be performed in 2015-2016 at the Laboratory Nazionali del Sud in Catania. For this purpose the different components of a complete BDX prototype are presently under construction in Catania and Genova.
A future BDX experiment performed downstream of the beam-dump at one of the high intensity JLab experimental Halls, receiving up to 1022 electrons-on-target at 12 GeV of incident energy in a one-year period, will be sensitive to large regions of dark matter parameter space, exceeding the discovery potential of existing and planned experiments by two orders of magnitude in the MeV-GeV dark matter mass range. Moreover, it will be the first experiment to search for accelerated light dark matter by looking at both the χ-proton and χ-electron scattering processes at the same time. Finally, combined with existing other experimental limits from visible and invisible decay, it will give a crucial contribution for eventually closing the door for dark photons as the explanation for the Muon g-2 anomaly.
M. Bondì, M. De Napoli, E. Leonora, N. Randazzo.
Thomas Jefferson National Accelerator Facility, Virginia, USA.
Stanford Linear Accelerator Center (SLAC), California, USA.
Institut de Physique Nucléaire (IPN), Orsay, France.
Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada.