Muon g-2 and searches for a new leptophobic sub-GeV dark boson in a missing-energy experiment at CERN


Abstract in English

The 3.6 sigma discrepancy between the predicted and measured values of the anomalous magnetic moment of positive muons can be explained by the existence of a new dark boson Z_mu with a mass in the sub-GeV range, which is coupled predominantly to the second and third lepton generations through the L_mu - L_tau current . After a discussion of the present phenomenological bounds on the Z_mu coupling, we show that if the Z_mu exists, it could be observed in the reaction mu+Z to mu+Z+Z_mu of a muon scattering off nuclei by looking for an excess of events with large missing muon beam energy in a detector due to the prompt bremsstrahlung Z_mu decay Z_mu to u u into a couple of neutrinos. We describe the experimental technique and the preliminary study of the feasibility for the proposed search. We show that this specific signal allows for a the search for the Z_mu with a sensitivity in the coupling constant alpha_mu > 10^{-11}, which is 3 orders of magnitude higher than the value required to explain the discrepancy. We point out that the availability of high-energy and -intensity muon beams at CERN SPS provides a unique opportunity to either discover or rule out the Z_mu in the proposed search in the near future. The experiment is based on the missing-energy approach developed for the searches for invisible decays of dark photons and (pseudo)scalar mesons at CERN and is complementary to these experiments.

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