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A number of recent experimental measurements suggest the possibility of abreakdown of lepton ($\ell$) universality in exclusive $b \to c \ell \nu_\ell$semileptonic meson decays. We analyze the full differential decay rates forseveral such processes, and show how to extract combinations of the underlyinghelicity amplitudes that are completely independent of $m_\ell$. Ratios ofthese combinations for different $\ell$ (as well as some combinations for asingle value of $\ell$) therefore equal unity in the Standard Model and providestringent tests of lepton universality. Furthermore, the extractions assume theform of weighted integrals over the differential decay rates and therefore areuseful even in situations where data in some regions of allowed phase space maybe sparse.

Tests of the Standard Model in $B \to D\ell \nu_\ell$, $B \to D^* \ell \nu_\ell$ and $B_c \to J/\psi \, \ell \nu_\ell$