Abstract
Lepton flavor violating interactions are absent in the standard model but are expected in various beyond standard models. In this work, the potential of the future circular electron-positron collider to probe the four fermion lepton flavor couplings via the $e^{+}e^{-}\rightarrow e^{\pm}\tau^{\mp}$ process is revisited by means of an effective field theory approach. We provide constraints at $95\%$ CL on the dimension-six Wilson coefficients including major sources of background processes and considering realistic detector effects at four expected operation energies $\sqrt{s}=157.5$, $162.5$, $240$ and $365$ GeV according to their corresponding integrated luminosities. We demonstrate that statistical combination of the results from four center-of-mass energies improves the sensitivity to the LFV couplings significantly. We compare the results with the prospects from Belle II with $50~ ab^{-1}$ and other studies at electron-positron colliders.
Highlights
In the standard model (SM) with massless neutrinos, processes with lepton flavor violation (LFV) interactions are forbidden [1]
Any observation of LFV in the charged lepton sector would be an obvious hint to the presence of physics beyond the SM, and can be an indirect way to search for beyond-the-SM scenarios
Lepton flavor violation processes are absent in the SM but appear in many extensions of the SM
Summary
In the standard model (SM) with massless neutrinos, processes with lepton flavor violation (LFV) interactions are forbidden [1]. Experimental observations of neutrino oscillations show that neutrinos are massive and mix with each other which leads to the violation of lepton flavor conservation [2]. LFV enters into the charged lepton sector from the neutrino sector via radiative corrections which are extremely suppressed because of the smallness of the ratio of neutrino mass to the W boson mass [3,4]. The predicted branching fraction, for example, for τ− → lþl−l0− decays are approximately ≲10−54, where l 1⁄4 e, μ [4]. An increase of several orders of magnitude is predicted in some extensions of the SM, such as the supersymmetric SM [5,6,7], resulting in branching fractions observable in experiments. Any observation of LFV in the charged lepton sector would be an obvious hint to the presence of physics beyond the SM, and can be an indirect way to search for beyond-the-SM scenarios
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