Constraining new physics in b → cℓν transitions

Abstract

B decays proceeding via b → c ν transitions with = e or µ are tree-level processes in the Standard Model. They are used to measure the CKM element V cb , as such forming an important ingredient in the determination of e.g. the unitarity triangle; hence the question to which extent they can be affected by new physics contributions is important, specifically given the long-standing tension between V cb determinations from inclusive and exclusive decays and the significant hints for lepton flavour universality violation in b → cτ ν and b → s decays. We perform a comprehensive model-independent analysis of new physics in b → c ν, considering all combinations of scalar, vector and tensor interactions occuring in single-mediator scenarios. We include for the first time differential distributions of B → D * ν angular observables for this purpose. We show that these are valuable in constraining non-standard interactions. Specifically, the zero-recoil endpoint of the B → D ν spectrum is extremely sensitive to scalar currents, while the maximum-recoil endpoint of the B → D * ν spectrum with transversely polarized D * is extremely sensitive to tensor currents. We also quantify the room for e-µ universality violation in b → c ν transitions, predicted by some models suggested to solve the b → cτ ν anomalies, from a global fit to B → D ν and B → D * ν for the first time. Specific new physics models, corresponding to all possible tree-level mediators, are also discussed. As a side effect, we present V cb determinations from exclusive B decays, both with frequentist and Bayesian statistics, leading to compatible results. The entire numerical analysis is based on open source code, allowing it to be easily adapted once new data or new form factors become available. arXiv:1801.01112v3 [hep-ph] 19 Dec 2018In the context of the Standard Model (SM), the element V cb of the Cabibbo-Kobayashi-Maskawa (CKM) matrix can be determined in various ways:• from a global fit including e.g. meson-antimeson mixing observables [1,2],[4] for a recent review).When considering the SM as the low-energy limit of a more fundamental theory of "new physics" (NP), these determinations are not applicable model-independently in general. Specifically, flavour-changing neutral current processes like meson-antimeson mixing could easily be affected by NP, invalidating the global fit. In this case, a potential disagreement between the global fit and the tree-level determinations from semi-leptonic B decays can signal the presence of NP. However, even the tree-level processes could in principle be significantly affected by NP. This possibility has been considered in the past in particular in view of the long-standing tensions between V cb from different decay channels. Clearly, these tensions could be due to statistical fluctuations or underestimated theoretical uncertainties. While the inclusive decay can be computed to high precision in an expansion in α s and 1/m c,b , see e.g.[3] for a recent overview, the exclusive decays require the knowledge of hadronic form...

Global fit to b → cτν transitions

“…However, we note that the observables considered here are mostly ratios, reducing the overall form-factor sensitivity. We consider a heavy quark effective theory (HQET) [91,92] parametrization, including corrections of order α s , Λ QCD /m b,c and partly Λ 2 QCD /m 2 c , mostly following [16,57]. In the heavy-quark limit all form factors either vanish or reduce to a common functional form, the Isgur-Wise function ξ(q 2 ) [93].…”

“…The detailed parametrization of the different form factors can be found in Ref. [16,57]. The main difference to the latter article is the introduction of the z 3 term in the leading Isgur-Wise function, that renders the fit compatible with the extrapolation of the recent lattice data [83,95] to large recoil.…”

Revisiting the new-physics interpretation of the b → cτν data

“…A comprehensive analysis of bounds on NP affecting b → c ν transitions can be found in ref. [65]. A summary of the recent data (averages) is shown in Table I, which is compared to the SM predictions which are obtained as specified in Sec.…”

The V puzzle: An update

“…The differential B → D ( * ) ν data also provide bounds on new physics. Indeed, the known differential distributions already place stringent bounds on all possible single-mediator models of new physics [25], and none of these models can contribute to solve the V cb puzzle in a significant way [25,26], see also [27]. Even though it is unlikely to signal new physics, the |V cb | puzzle is still very important because i) it may be a signal that something is not yet understood in either the inclusive or exclusive analysis, with possible implications on the interpretation of R(D ( * ) ) as well, and ii) it limits the accuracy of |V cb | which affects FCNC studies in an important way.…”