G. Peter Lepage

Exclusive processes in quantum chromodynamics: Evolution equations for hadronic wavefunctions and the form factors of mesons☆

Abstract The predictions of quantum chromodynamics for meson form factors at large momentum transfer are given. Evolution equations are derived which determine the structure of hadronic wavefunctions at short distances from their form at large distances. The eigenvalues of the evolution equations appear as exponents in anomalous logarithm corrections to the nominal power law of form factors determined by dimensional counting. The results lead to detailed tests of the spin and scaling structure of QCD at short distances. The predictions for the charged pion, kaon and rho form factors and the γ → π 0 transition form factor of the photon are absolutely normalized at asymptotic momentum transfer.

Neutral B meson mixing in unquenched lattice QCD

We study B{sub d} and B{sub s} mixing in unquenched lattice QCD employing the MILC Collaboration gauge configurations that include u, d, and s sea quarks based on the improved staggered quark (AsqTad) action and a highly improved gluon action. We implement the valence light quarks also with the AsqTad action and use the nonrelativistic NRQCD action for the valence b quark. We calculate hadronic matrix elements necessary for extracting Cabibbo-Kobayashi-Maskawa matrix elements from experimental measurements of mass differences {delta}M{sub d} and {delta}M{sub s}. We find {xi}{identical_to}f{sub B{sub s}}{radical}(B-circumflex{sub B{sub s}})/f{sub B{sub d}}{radical}(B-circumflex{sub B{sub d}})=1.258(33), f{sub B{sub d}}{radical}(B-circumflex{sub B{sub d}})=216(15) MeV, and f{sub B{sub s}}{radical}(B-circumflex{sub B{sub s}})=266(18) MeV. We also update previous results for decay constants and obtain f{sub B{sub d}}=190(13) MeV, f{sub B{sub s}}=231(15) MeV, and f{sub B{sub s}}/f{sub B{sub d}}=1.226(26). The new lattice results lead to updated values for the ratio of Cabibbo-Kobayashi-Maskawa matrix elements |V{sub td}|/|V{sub ts}| and for the standard model prediction for Br(B{sub s}{yields}{mu}{sup +}{mu}{sup -}) with reduced errors. We determine |V{sub td}|/|V{sub ts}|=0.214(1)(5) and Br(B{sub s}{yields}{mu}{sup +}{mu}{sup -})=3.19(19)x10{sup -9}.

B-meson decay constant from unquenched lattice QCD.

We present determinations of the -meson decay constant f(B) and f(B)(s)/f(B) using the MILC Collaboration unquenched gauge configurations, which include three flavors of light sea quarks. The mass of one of the sea quarks is kept around the strange quark mass, and we explore a range in masses for the two lighter sea quarks down to m(s)/8. The heavy quark is simulated using nonrelativistic QCD, and both the valence and sea light quarks are represented by the highly improved (AsqTad) staggered quark action. The good chiral properties of the latter action allow for a more accurate chiral extrapolation to physical up and down quarks than has been possible in the past. We find f(B)=216(9)(19)(4)(6) MeV and f(B)(s)/f(B)=1.20(3)(1).

Flavor-Symmetry Restoration and Symanzik Improvement for Staggered Quarks

We resolve contradictions in the literature concerning the origins and size of unphysical flavor-changing strong interactions generated by the staggered-quark discretization of QCD. We show that the leading contributions are tree-level in

Perturbative quantum chromodynamics

\order(a^2)

The

and that they can be removed by adding three correction terms to the link operator in the standard action. These corrections are part of the systematic Symanzik improvement of the staggered-quark action. We present a new improved action for staggered quarks that is accurate up to errors of

B

\order(a^4,a^2\alpha_s)

and

--- more accurate than most, if not all, other discretizations of light-quark dynamics.

B_s

From the perspective of the hadronic physics of a decade ago it seems incredible that there now exists a viable, fundamental theory of the strong interactions. In fact, quantum chromodynamics is radically different from the picture of hadronic phenomena which was envisioned in the 1960’s. In contrast to the hadronic bootstrap, the quark and gluon quanta of QCD represent fundamental hadronic constituents, the elementary carriers of the electromagnetic and weak currents. In contrast to a strong-coupling model, the quark and gluon interactions of QCD approach scale-invariance at short distances and can be computed as a perturbative expansion in an asymptotically small coupling constant.1,2 As a consequence, the large momentum-transfer strong, electromagnetic, and weak interactions of hadrons are patterned after elementary short-distance gluon and quark subprocesses.

Meson Decay Constants from Lattice QCD

We present a new determination of the B and Bs meson decay constants using nonrelativistic quantum chromodynamics (NRQCD) b-quarks, highly improved staggered quark (HISQ) light and strange valence quarks and the MILC collaboration Nf=2+1 lattices. The new calculations improve on HPQCD’s earlier work with NRQCD b-quarks by replacing AsqTad with HISQ valence quarks, by including a more chiral MILC fine ensemble in the analysis, and by employing better tuned quark masses and overall scale. We find fB=0.191(9)  GeV, fBs=0.228(10)  GeV and fBs/fB=1.188(18). Combining the new value for fBs/fB with a recent very precise determination of the Bs meson decay constant based on HISQ b-quarks, fBs=0.225(4)  GeV, leads to fB=0.189(4)  GeV. With errors of just 2.1% this represents the most precise fB available today.