Search for muon-philic new light gauge boson at belle II

Motivated by the long-lasting 3.5σ discrepancy in the anomalous magnetic moment of muon, we consider a new muon-specific force mediated by a light gauge boson, X, with mass mX < 2mμ and the coupling constant gX ∼ (10−4, 10−3) . We show that the Belle II experiment has a robust chance to probe such a light boson in e+e− → μ+μ− + X channel and cover the most interesting parameter space explaining the discrepancy with the planned target luminosity, $$ \int dt\ \mathcal{L}=50\ {\mathrm{ab}}^{-1} $$. The clean signal of muon-pair plus missing energy at Belle II can be a smoking gun for the new gauge boson. We expect that the (invisibly decaying) muon-philic light (mX ≲ 2mμ) gauge boson can be probed down to gX ≲ 1.5 × 10−4(4.6 × 10−4,  2.3 × 10−4) for 50 (1, 10) ab−1 search.