Light quark flavour breaking effects in B meson decay constants

Light quark flavour breaking effects in B meson decay constants

In the standard model of particle physics, there are six types of quarks whimsically called up u, down d, strange s, charm c, bottom b and top t in order of increasing mass. These bind together to form two classes of hadrons, mesons consisting of a quark – antiquark pair and baryons with three quarks. An example of a baryon is the proton (uud) while examples of mesons are the pion (ud), kaon (us), B (ub) and Bs (sb). These mesons decay to a lepton (usually the electron) and a neutrino. The decays are parametrised by ‘meson decay constants’, from which, together with experimentally determined leptonic decay rates elements of the CKM matrix can be found. These are fundamental parameters of the Standard Model and their determination can help to isolate possible new physics. Presently recent B physics experiments have left us with a number of heavy flavour physics puzzles which will be more intensively investigated by Belle II.

Figure 1: Left panel: A ‘fan’ plot for the light quark ud and us decay constants (normalised with the average ud, us decay constant) for one lattice spacing against the pion mass; Right panel: The heavy b quark ub, sb decay constants, again normalised, for several lattice spacings also plotted against the pion mass.

In our Lattice QCD programme we have developed a method by expanding about a point in the light quark mass parameter space where the u, d, s quark masses are initially set equal, and then extrapolating to the physical quark masses while keeping the average quark mass constant. The original expansions were for hadron masses, and lead to a ‘fan’ plot where the different hadron masses radiate from their common point. They were extended to decay constants for the pion and kaon mesons, [1], as shown in the left panel of the figure. We have now further extended this programme to the heavier quark masses and in particular the b, [2]. In the right panel of the figure we give the equivalent plot for the B and Bs mesons, together with the present (FLAG16) values.

[1] V.G. Bornyakov et al., Phys. Lett. B 767 (2017) 366.

[2] S. Hollitt et al., PoS(LATTICE2018)268, arXiv:1811.06677 [hep-lat].