A 7.9 Sigma Tension In Measuring V(cb) Of The CKM Matrix

In February 2020, a post at this blog reviewed the values of nine CKM matrix elements (which in turn can be derived from not more than four parameters, which are experimentally determined in the Standard Model).

The square of the absolute value of each element is equal to the probability of the transition described in the element to the second type of quark in the subscript taking place, given that a W+ boson has been emitted by a quark of the type of the first type of quark shown in the subscript.

One of those nine elements is V(cb) which has a global best fit value of:

(41.47 ± 0.70) *10^-3

Unitarity constraints (since the sum of the probabilities of all possible transitions from a single quark should equal 100%) suggest that V(cb) may be a bit high.

A new study, limited to fits to decays of neutral B mesons to a negatively charged vector D meson (D∗− (2010), which has as valence quarks an anti-charm quark and a d quark) together with a positively charged lepton (i.e. a positron, anti-muon, or anti-tau lepton) and a neutrino corresponding to it, using new methods, comes up with a lower value for element V(cb) at quite high precision. The result from this study is:

 (35.10 ± 0.41) * 10^-3.

This is a 7.9 sigma tension between this measurement and the global average which is a serious tension.

There could be a methodological error or an error in estimating the margin of error in the new measurement, but given the stark difference, this tension deserves further study.

[Submitted on 21 Aug 2020]

Revisiting fits to B0→D∗−ℓ+νℓ to measure |Vcb| with novel methods and preliminary LQCD data at non-zero recoil

Daniel Ferlewicz, Phillip Urquijo, Eiasha Waheed

We present a study of fits to exclusive B0→D∗−ℓ+νℓ measurements for the determination of the Cabbibo-Kobayashi-Maskawa matrix element magnitude |Vcb|, based on the most recent Belle untagged measurement. Results are obtained with the Caprini-Lellouch-Neubert (CLN) and Boyd-Grinstein-Lebed (BGL) form factor parameterisations, with and without the inclusion of preliminary Lattice QCD measurements of form factors at non-zero hadronic recoil from the JLQCD collaboration. The CLN and BGL fits are also studied in different scenarios with reduced theoretical assumptions, and at higher order expansions respectively. To avoid bias from high systematic uncertainty correlations we use a toy MC approach with a Cholesky decomposition of the covariance matrix. We find that (1)ηEW|Vcb|=(35.2±0.2±0.8)×10−3 for CLN and (34.9±0.3±1.0)×10−3 for BGL(1,0,2) without input from Lattice QCD. The errors quoted correspond to statistical and systematic uncertainties, respectively. We find no evidence to support lepton flavour dependence on the measurement of |Vcb| but find some tension in the results associated with the ratio of form factors R1. We show how input from JLQCD allows for well defined fit results with reduced model dependence in CLN and BGL. The results obtained using preliminary values are consistent between different orders of parameterisations, ultimately providing a method for a model-independent exclusive measurement of |Vcb|. Using preliminary inputs from the JLQCD collaboration, (1)ηEW|Vcb| is found to be approximately (35.1±0.07±0.4)×10−3 in BGL(2,2,2).