New physics in the angular distribution of Bc- → J/ψ(→μ+μ-)τ-(→π-ντ)ντ decay

In Bc- -> J/psi(-> mu (+)mu (-))tau (-)nu tau decay, the three-momentum p tau- cannot be determined accurately due to the decay products of tau (-) inevitably include an undetected nu (tau). As a consequence, the angular distribution of this decay cannot be measured. In this work, we construct a measurable angular distribution by considering the subsequent decay tau (-) -> pi (-)nu (tau). The full cascade decay is Bc- -> J/psi(-> mu (+)mu (-))tau (-)(-> pi (-)nu (tau))nu tau, in which the three-momenta p mu+,p mu-, and p pi- can be measured. The five-fold differential angular distribution containing all Lorentz structures of the new physics (NP) effective operators can be written in terms of twelve angular observables I-i(q(2), E-pi). Integrating over the energy of pion E-pi, we construct twelve normalized angular observables Ii(q(2)) and two lepton-flavor-universality ratios RPL,TJ/psi(q(2)). Based on the B-c -> J/psi form factors calculated by the latest lattice QCD and sum rule, we predict the q(2) distribution of all Ii and RPL,TJ/psi both within the Standard Model and in eight NP benchmark points. We find that the benchmark BP2 (corresponding to the hypothesis of tensor operator) has the greatest effect on all I-i and RPL,TJ/psi, except I5. The ratios RPL,TJ/psi are more sensitive to the NP with pseudo-scalar operators than the I-i. Finally, we discuss the symmetries in the angular observables and present a model-independent method to determine the existence of tensor operators.

Automated summary

In the study of Bc- -> J/psi(-> mu (+)mu (-))tau (-)nu tau decay, a measurable angular distribution is constructed by considering the subsequent decay tau (-) -> pi (-)nu (tau). The research predicts the q(2) distribution of angular observables and lepton-flavor-universality ratios within the Standard Model and at various NP benchmark points, highlighting the sensitivity of the ratios to the presence of pseudo-scalar operators. The study also discusses symmetries in angular observables and proposes a model-independent method to determine the existence of tensor operators.