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Abstract
We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), X, at the B-factory experiment Belle II. We focus on production of the ALP in the tau decays τ → Xl with l = e, μ, followed by its decay via X → l−l+. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating τ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime. The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.
Highlights
The SM particles, are both inversely proportional to fa, the QCD axion should be very light (10−6 eV < ma < 10−2 eV) and feebly interacting, with a very long lifetime [5]
We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), X, at the B-factory experiment Belle II
We focus on production of the ALP in the tau decays τ → Xl with l = e, μ, followed by its decay via X → l−l+
Summary
We focus on two representative benchmark scenarios. In Scenario 1, gτe and gee are switched on, and the other couplings are set to zero. We focus on the di-lepton final state and do not consider the di-photon decay mode as a signature of interest in this work. We summarize these two scenarios in table 1. The X branching fraction plots show that BR(X → γγ) exceeds BR(X → e−e+) for mX 0.6 GeV in Scenario 1, despite the loop suppression in (X → γγ) This observation can be understood from the fact that Γ(X → lα−lα+) ∝ mX m2α, while Γ(X → γγ) ∝ m3X , 1For a study on this coupling and LFV at Belle II, we refer the reader to ref.
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