V_cb determination from inclusive b → c decays: an alternative method

The determination of V_cb relies on the Heavy-Quark Expansion and the extraction of the non-perturbative matrix elements from inclusive b → c decays. The proliferation of these matrix elements complicates their extraction at 1/m_b⁴ and higher, thereby limiting the V_cb extraction. Reparametrization invariance links different operators in the Heavy-Quark expansion thus reducing the number of independent operators at 1/m_b⁴ to eight for the total rate. We show that this reduction also holds for spectral moments as long as they are defined by reparametrization invariant weight-functions. This is valid in particular for the leptonic invariant mass spectrum (q²), i.e. the differential rate and its moments. Currently, V_cb is determined by fitting the energy and hadronic mass moments, which do not manifest this parameter reduction and depend on the full set of 13 matrix elements up to 1/m_b⁴. In light of this, we propose an experimental analysis of the q² moments to open the possibility of a model-independent V_cb extraction from semileptonic decays including the 1/m_b⁴ terms in a fully data-driven way.