Bmeson semileptonic form factors from unquenched lattice QCD

Abstract

The semileptonic process, B −→ π lν, is studied via full QCD lattice simulations. We use unquenched gauge configurations generated by the MILC collaboration. These include the effect of vacuum polarization from three quark flavors: the s quark and two very light flavors (u/d) of variable mass allowing extrapolations to the physical chiral limit. We employ Nonrelativistic QCD to simulate the b quark and a highly improved staggered quark action for the light sea and valence quarks. We calculate the form factors f+(q 2 ) and f0(q 2 ) in the chiral limit for the range 16 GeV 2 ≤ q 2 < q 2 max and obtainCombining this with a preliminary average by the Heavy Flavor Averaging Group (HFAG'05) of recent branching fraction data for exclusive B semileptonic decays from the BaBar, Belle and CLEO collaborations, leads to |V ub | = 4.22(30)(51) × 10 −3 . PLEASE NOTE APPENDIX B with an ERRATUM, to appear in Physical Review D, to the published version of this e-print (Phys.Rev.D 73, 074502 (2006)). Results for the form factor f+(q 2 ) in the chiral limit have changed significantly. The last two sentences in this abstract should now read; "We calculate the form factors f+(q 2 ) and f0(q 2 ) in the chiral limit for the range 16 GeV 2 ≤ q 2 < q 2 max and obtainCombining this with a preliminary average by the Heavy Flavor Averaging Group (HFAG'05) of recent branching fraction data for exclusive B semileptonic decays from the BaBar, Belle and CLEO collaborations, leads to |V ub | = 3.55(25)(50) × 10 −3 ."

Lattice QCD calculation of form factors describing the rare decaysB→K*ℓ+ℓ−

“…We chose two ensembles (c007 and c02) with a common, coarse lattice spacing on which to test quark mass dependence and one ensemble (f0062) with a fine lattice spacing which has approximately the same Goldstone pion mass as on the c007 ensemble. A calculation of B → πlν form factors on a similar subset of MILC lattices [72] found very mild quark mass dependence and no statistically significant dependence on the lattice spacing. Since the signal-to-noise ratio is much worse for correlation functions involving vector mesons in place of pseudoscalar mesons, we chose to invest our computational effort in obtaining a large statistical sample on these three ensembles rather than including more ensembles.…”

“…The specific form of the action is the same Oðv 4 Þ action as was used in earlier work by the HPQCD collaboration (e.g. [72]). Because we make use of an effective field theory to treat the b quark, the net energy of a B meson is obtained by adding a contribution associated with the b quark mass to the energy of the B meson in the Monte Carlo calculation E sim .…”

“…However past experience in evaluating q à using the BLM procedure suggests this scale is higher than TABLE III. Heavy quark and heavy-light current renormalization constants (for the parameters as in Table II) [72,76,80]. For the tensor current matching, the matching scale is taken to be m b .…”

The semileptonic decays of B/B s meson in the perturbative QCD approach: a short review

“…We also made careful phenomenological analysis for these pQCD predictions and found, in general, the following points: (a) For all the considered B/B s semileptonic decays, the next-to-leading order (NLO) pQCD predictions for their decay rates agree well with the data and those from other different theoretical methods; (b) For R(D) and R(D * ), the pQCD predictions agree very well with the data, the BaBar's anomaly of R(D ( * ) ) are therefore explained successfully in the standard model by employing the pQCD approach; and (c) We defined several new ratios R l,τ D and R l,τ Ds , they may be more sensitive to the QCD dynamics which controls the B/B s → (D ( * ) , D ( * ) s ) transitions than the old ratios, we therefore strongly suggest LHCb and the forthcoming Super-B experiments to measure these new ratios.Key Words B/B s meson semileptonic decays; The pQCD factorization approach; Form factors; Branching ratios; LHCb experiments ( * ) (s) ) transitions. The traditional methods or approaches to calculate the relevant transition form factors are the light cone QCD sum rules (LCSR) [39][40][41][42][43][44][45][46][47][48][49][50][51], the heavy quark effective theory (HQET) [1,15,[52][53][54][55] and the lattice QCD (LQCD) techniques [56][57][58]. In the pQCD approach, however, one can make direct perturbative calculation for the form factors for B → (π, K, etc) transitions [59][60][61][62][63][64].…”

“…Key Words B/B s meson semileptonic decays; The pQCD factorization approach; Form factors; Branching ratios; LHCb experiments ( * ) (s) ) transitions. The traditional methods or approaches to calculate the relevant transition form factors are the light cone QCD sum rules (LCSR) [39][40][41][42][43][44][45][46][47][48][49][50][51], the heavy quark effective theory (HQET) [1,15,[52][53][54][55] and the lattice QCD (LQCD) techniques [56][57][58]. In the pQCD approach, however, one can make direct perturbative calculation for the form factors for B → (π, K, etc) transitions [59][60][61][62][63][64].…”