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 \order(a^2) 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 \order(a^4,a^2\alpha_s) --- more accurate than most, if not all, other discretizations of light-quark dynamics.Comment: 7 page

Pion Form Factor in the kTk_T Factorization Formalism

Based on the light-cone (LC) framework and the $k_T$ factorization formalism, the transverse momentum effects and the different helicity components' contributions to the pion form factor $F_{\pi}(Q^2)$ are recalculated. In particular, the contribution to the pion form factor from the higher helicity components ($\lambda_1+\lambda_2=\pm 1$), which come from the spin-space Wigner rotation, are analyzed in the soft and hard energy regions respectively. Our results show that the right power behavior of the hard contribution from the higher helicity components can only be obtained by fully keeping the $k_T$ dependence in the hard amplitude, and that the $k_T$ dependence in LC wave function affects the hard and soft contributions substantially. As an example, we employ a model LC wave function to calculate the pion form factor and then compare the numerical predictions with the experimental data. It is shown that the soft contribution is less important at the intermediate energy region.Comment: 21 pages, 4 figure

The size of the pion from full lattice QCD with physical u, d, s and c quarks

We present the first calculation of the electromagnetic form factor of the π meson at physical light quark masses. We use configurations generated by the MILC collaboration including the effect of u, d, s and c sea quarks with the Highly Improved Staggered Quark formalism. We work at three values of the lattice spacing on large volumes and with u/d quark masses going down to the physical value. We study scalar and vector form factors for a range in space-like q2 from 0.0 to -0.13 GeV2 and from their shape we extract mean square radii. Our vector form factor agrees well with experiment and we find hr2iV = 0:403(18)(6) fm2. For the scalar form factor we include quark-line disconnected contributions which have a significant impact on the radius. We give the first results for SU(3) flavour-singlet and octet scalar mean square radii, obtaining: hr2isinglet S = 0:506(38)(53)fm2 and hr2ioctet S = 0:431(38)(46)fm2. We discuss the comparison with expectations from chiral perturbation theory

Deriving the existence of BBˉ∗B\bar{B}^* bound states from the X(3872) and Heavy Quark Symmetry

We discuss the possibility and the description of bound states between $B$ and $\bar{B}^*$ mesons. We argue that the existence of such a bound state can be deduced from (i) the weakly bound X(3872) state, (ii) certain assumptions about the short range dynamics of the $D\bar{D}^*$ system and (iii) heavy quark symmetry. From these assumptions the binding energy of the possible $B\bar{B}^*$ bound states is determined, first in a theory containing only contact interactions which serves as a straightforward illustration of the method, and then the effects of including the one pion exchange potential are discussed. In this latter case three isoscalar states are predicted: a positive and negative C-parity $^3S_1-{}^3D_1$ state with a binding energy of $20\,{\rm MeV}$ and $6\,{\rm MeV}$ below threshold respectively, and a positive C-parity $^3P_0$ shallow state located almost at the $B\bar{B}^*$ threshold. However, large uncertainties are generated as a consequence of the $1/m_Q$ corrections from heavy quark symmetry. Finally, the newly discovered isovector $Z_b(10610)$ state can be easily accommodated within the present framework by a minor modification of the short range dynamics.Comment: 21 pages, 3 figures; a sign error in the potential has been corrected and new predictions have been compute

V_cs from D_s to {\phi}l{\nu} semileptonic decay and full lattice QCD

We determine the complete set of axial and vector form factors for the Ds to {\phi}l{\nu} decay from full lattice QCD for the first time. The valence quarks are implemented using the Highly Improved Staggered Quark action and we normalise the appropriate axial and vector currents fully nonperturbatively. The q^2 and angular distributions we obtain for the differential rate agree well with those from the BaBar experiment and, from the total branching fraction, we obtain Vcs = 1.017(63), in good agreement with that from D to Kl{\nu} semileptonic decay. We also find the mass and decay constant of the {\phi} meson in good agreement with experiment, showing that its decay to K{\bar{K}} (which we do not include here) has at most a small effect. We include an Appendix on nonperturbative renormalisation of the complete set of staggered vector and axial vector bilinears needed for this calculation.Comment: 19 pages, 13 figure

New results on heavy hadron spectroscopy with NRQCD

We present results for the spectrum of b-bbar bound states in the quenched approximation for three different values of the lattice spacing. Results for spin-independent splittings are shown to have good scaling behaviour; spin-dependent splittings are more sensitive to discretisation effects. We discuss what needs to be done to match the experimental spectrum.Comment: 3 pages, contribution to Lattice'9