摘要
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 m <sub>X</sub> < 2m <sub>μ</sub> and the coupling constant gX ∼ (10 <sup>−4</sup> , 10 <sup>−3</sup> ). We show that the Belle II experiment has a robust chance to probe such a light boson in e <sup>+</sup> e <sup>−</sup> → μ <sup>+</sup> μ <sup>−</sup> + X channel and cover the most interesting parameter space explaining the discrepancy with the planned target luminosity, ∫dtL=50ab−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 (m <sub>X</sub> ≲ 2m <sub>μ</sub> ) gauge boson can be probed down to g <sub>X</sub> ≲ 1.5 × 10 <sup>−4</sup> (4.6 × 10 <sup>−4</sup> , 2.3 × 10 <sup>−4</sup> ) for 50 (1, 10) ab <sup>−1</sup> search.