On the superiority of the |V_{cb}|-gamma plots over the unitarity triangle plots in the 2020s

Abstract

The unitarity triangle (UT) plots played already for three decades an important role in the tests of the Standard Model (SM) and the determination of the CKM parameters. As of 2022, among the four CKM parameters, |V_{us}| and beta are already measured with respectable precision, while this is not the case of |V_{cb}| and gamma . In the case of |V_{cb}| the main obstacle are the significant tensions between its inclusive and exclusive determinations from tree-level decays and it could still take some years before a unique value of this parameter will be known. The present uncertainty in gamma of 4^circ from tree-level decays will be reduced to 1^circ by the LHCb and Belle II collaborations in the coming years. Unfortunately in the common UT plots |V_{cb}| is not seen and the experimental improvements in the determination of gamma from tree-level decays at the level of a few degrees are difficult to appreciate. In view of these deficiencies of the UT plots with respect to |V_{cb}| and gamma and the central role these two CKM parameters will play in this decade, the recently proposed plots of |V_{cb}| versus gamma extracted from various processes appear to be superior to the UT plots in the flavour phenomenology of the 2020s. We illustrate this idea with Delta F=2 observables Delta M_s, Delta M_d, varepsilon _K and with rare decays B_srightarrow mu ^+mu ^-, B_drightarrow mu ^+mu ^-, K^+rightarrow pi ^+nu bar{nu } and K_Lrightarrow pi ^0nu bar{nu }. In particular the power of varepsilon _K, mathcal {B}(K^+rightarrow pi ^+nu bar{nu }) and mathcal {B}(K_Lrightarrow pi ^0nu bar{nu }) in the determination of |V_{cb}|, due to their strong dependence on |V_{cb}|, is transparently exhibited in this manner. Combined with future reduced errors on gamma and |V_{cb}| from tree-level decays such plots can better exhibit possible inconsistencies between various determinations of these two parameters, caused by new physics, than it is possible with the UT plots. This can already be illustrated on the example of the recently found 2.7sigma anomaly in B_srightarrow mu ^+mu ^-.