Indirect constraints on lepton-flavor-violating quarkonium decays

Within an effective-field-theory framework, we present a model-independent analysis of the potential of discovering new physics by searching for lepton flavor violation in heavy quarkonium decays and, more in general, we study the phenomenology of lepton-flavor-violating (LFV) 2 quark-2 lepton (2q2l) operators with two charm or bottom fields. We compute the constraints from LFV muon and tau decays on the new-physics operators that can induce LFV processes involving c (c) over bar and b (c) over bar systems, thus providing a comprehensive list of indirect upper limits on processes such as j/psi -> ll ', gamma(ns) -> gamma(ns) -> ll 'gamma etc., which can be sought at BESIII, Belle II, and the proposed super tau-charm factory. We show that such indirect constraints are so stringent that they prevent the detection of quarkonium decays into e mu. In the case of decays of quarkonia into lt (l = e, mu), we find that an improvement by 2-3 orders of magnitude on the current sensitivities is in general required in order to discover or further constrain new physics. However, we show that cancellations among different contributions to the LFV tau decay rates are possible, such that gamma(nS) -> l tau. lt can saturate the present experimental bounds. We also find that, interestingly, searches for LFV Z decays, Z -> l tau, at future e(+) e(+) colliders are complementary probes of 2q2l operators with third generation quarks.

Automated summary

Within the effective-field-theory framework, this study presents a model-independent analysis of the potential of discovering new physics through lepton flavor violation in heavy quarkonium decays. It computes the constraints from LFV muon and tau decays on new-physics operators and provides a comprehensive list of indirect upper limits on LFV processes. The results show that the constraints are stringent and make the detection of quarkonium decays into e mu difficult, while improvements in sensitivities are required for the detection of decays into lt. Additionally, searches for LFV Z decays at future e(+) e(+) colliders are found to be complementary probes of 2q2l operators with third generation quarks.