Longer-Lived Mediators from Charged Mesons and Photons at Neutrino Experiments

Abstract: Since many of the dark-sector particles interact with Standard Model (SM) particles in multiple ways, they can appear in experimental facilities where SM particles appear in abundance. In this study, we explore a particular class of longer-lived mediators that are produced from photons, charged mesons, neutral mesons, and $e^\pm$ that arise in proton-beam fixed-target-type neutrino experiments. This class of mediators encompasses light scalars that appear in theories like extended Higgs sectors, muon(electro)philic scalars, etc. We evaluate the sensitivities of these mediators at beam-based neutrino experiments such as the finished ArgoNeuT, ongoing MicroBooNE, SBND, ICARUS, and the upcoming DUNE experiment. We realize that scalars are more enhanced while produced from three-body decay of charged mesons, especially if they are muonphilic in nature. For scenarios that contain muonphilic scalars, these experiments can probe unexplored regions of parameter space that can explain the current discrepancy in the anomalous magnetic moment of muons. The sensitivity of electrophilic scalars at the DUNE Near Detector can explore new regions. We also show that Bethe-Heitler scattering processes can be used to probe flavor-specific lepton final states even for the mediator masses below twice the lepton mass.