Dipartimento di Fisica

The Standard Model of fundamental interactions accurately describes experimental results over an enormous range of energy scales, but we have mounting evidence that physics beyond the SM should exist. As this New Physics (NP) has so far eluded direct searches at the high-energy Large Hadron Collider (LHC) and can be out of its reach, precision measurements play an increasingly important role. In the flavour sector, a few anomalies have been observed in such measurements at the LHCb, Belle and Babar experiments: they involve semileptonic decays of the B meson and imply a violation of the Lepton Flavour Universality (LFU) which is at the heart of the SM.

Charged current semileptonic B decays are also the main avenue to measure the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix elements Vcb and Vub, and since many years the inclusive and exclusive determinations of |Vcb| are in sharp tension. As this is unlikely to be a sign of NP, the problem has been described as the Vcb puzzle. In fact, the present experimental and theoretical situation is quite confusing and we need new experimental analyses and theoretical calculations to clarify it.

We propose a coordinated programme of theoretical calculations, experimental measurements, and phenomenological analyses to improve our understanding of b→c l nu transitions. Our team brings together internationally known experts in semileptonic B decays active in italian institutions with complementary expertise in lattice QCD, phenomenology and experiment. We will focus on three lines of research, which build on the expertise of the team and contain several elements of innovation. In particular, we plan

i. to develop and validate a new method [Gambino2020] to compute inclusive semileptonic heavy quark decays in lattice QCD, which should verify and improve the inclusive determination of Vcb;

ii. to perform two ground-breaking measurements with LHCb data: the four-dimensional analysis of B→D*l nu with l=e,mu
to test LFU and search for NP, and the first measurement of inclusive moments in Bs semileptonic decays, to improve the estimates of the semileptonic Bs width and our knowledge of the hadronic Xcs states in view of subsequent LFU analyses at LHCb;

iii. a global model independent analysis of all the available inclusive and exclusive semileptonic b→c l nu data (l=e,mu), including the results of point ii, and of all form factor determinations to constrain NP and LFU violation and to extract Vcb in a reliable way.