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

Abstract

Motivated by the long-lasting $3.5\sigma$ discrepancy in the anomalous magnetic moment of muon, we consider a new muon-specific force mediated by a light gauge boson, $X$, with mass $m_X<2m_{\mu }$ and the coupling constant $g_X\sim (10^{-4},10^{-3})$. We show that the Belle~II experiment has a robust chance to probe such a light boson in $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}+X$ channel and cover the most interesting parameter space explaining the discrepancy with the planned target luminosity, $\int dt\mathcal{L}=50\mathrm{a}{\mathrm{b}}^{-1}$. 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 ($m_X\le 2m_{\mu }$) gauge boson can be probed down to $g_X\ge 1.5\times 10^{-4}(4.6\times 10^{-4},2.3\times 10^{-4})$ for 50 (1, 10) ab${}^{-1}$ search.

21 pages, 12 figures, references added, version to appear in JHEP