Abstract
We propose new perspective in searching for axion-like particles (ALPs) from quark and lepton flavor physics: measurements of the time-dependent CP asymmetry in $B^0 \to K_S^0 \pi^0 \gamma$ and the branching ratio of $B_s \to e^\pm \mu^\mp$ decay possess, along with the anomalous magnetic moment of muon. In the mass range of sub-GeV, accessible by the flavorful ALPs search, the experimental sensitivity for these flavor observables reaches the maximum at around the pion mass scale (called the {\it sweetest} spots), where a couple of loopholes (unexplored regions) for the ALP parameter space have heretofore been present, because of an unavoidable contamination with pion background events. The proposed complementary probes can precisely determine the ALP coupling to photon at these {\it sweetest} spots/loopholes, and will significantly help cover whole parameter spaces in the ALP search including the present loopholes in the future.
Highlights
The axion is one of the mostly anticipated hypothetical particles predicted beyond the standard model (SM) of particle physics
The predicted deviation from the SM can be larger than Oð10Þ%, and turns out to be fairly irrespective to the presence of new physics in the anomalous magnetic moment of muon, or Υ decays, or μ → eγ, which will in the future fully cover the current sweetest spots/loopholes
Complementary measurements for the CP asymmetry in B0 → K0Sπ0γ and Bs → eÆμ∓, in conjunction with the anomalous magnetic moment of muon, can precisely determine the axionlike particles (ALPs) coupling to photon at around 140 MeV, and will give a significant help to fully cover the ALP parameter space including the regions which cannot be explored by existing and prospected direct search experiments
Summary
The axion is one of the mostly anticipated hypothetical particles predicted beyond the standard model (SM) of particle physics. The limit on such 140 MeV ALPs can be placed by monophoton and triphoton searches accessible at LEP, Tevatron and LHC, through ZðγÃÞ → a þ γ (a 1⁄4 ALP) [15,16,17,18,19] This channel contaminates with the neutral pion events such as Z → π0 þ γ, which is a rare decay channel and has not observed or yet uncovered even for the SM prediction. On the other side of the same coin, that the existence of the loopholes may provide us with “sweet spots” in the ALP search: As seen in Fig. 1 such an ALP is to be short-lived enough to decay to diphoton inside detectors, as is the case of the neutral pion. Significant help to explore all the parameter space of the ALP
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