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

We present a model-independent bound on $R(J/\psi) ! \equiv ! $\mathcal{B}\mathcal{R}$ (B_c^+ \rightarrow J/\psi , \tau^+\nu_\tau)/ $\mathcal{B}\mathcal{R}$ (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, $0.20\le R(J\mathrm{/}\psi )\le 0.39$, agrees with the recent LHCb result at $1.3\sigma$, and rules out some previously suggested model form factors.