Model-Independent Bounds on $R(J/\psi)$

TL;DR

This paper derives a model-independent bound on the ratio of branching fractions for Bc+ decays to J/psi with tau and mu neutrinos, using theoretical constraints and lattice QCD data, aligning with experimental results.

Contribution

It provides a novel, model-independent bound on R(J/psi) by combining dispersive relations, heavy-quark symmetry, and lattice QCD, improving the understanding of Bc+ decay ratios.

Findings

Bound on R(J/psi): 0.20 to 0.39 at 95% CL

Agreement with LHCb measurement within 1.3 sigma

Rules out some model form factors

Abstract

We present a model-independent bound on $R(J/\psi) \! \equiv \! \mathcal{BR} (B_c^+ \rightarrow J/\psi \, \tau^+\nu_\tau)/ \mathcal{BR} (B_c^+ \rightarrow J/\psi \, \mu^+\nu_\mu)$. This bound is constructed by constraining the form factors through a combination of dispersive relations, heavy-quark relations at zero-recoil, and the limited existing determinations from lattice QCD. The resulting 95\% confidence-level bound, $0.20\leq R(J/\psi)\leq0.39$, agrees with the recent LHCb result at $1.3 \, \sigma$, and rules out some previously suggested model form factors.