arXiv subjects

Randy Lewis

Publications and source records attributed to Randy Lewis.

101 records · Page 6Linked to original sources

arXiv1998

Isospin violation and the proton's neutral weak magnetic form factor

The effects of isospin violation on the neutral weak magnetic form factor of the proton are studied using two-flavour chiral perturbation theory. The first nonzero contributions appear at O(p^4) in the small-momentum expansion, and the O(p^5) corrections are also calculated. The leading contributions from an explicit Delta(1232) isomultiplet are included as well. At such a high order in the chiral expansion, one might have expected a large number of unknown parameters to contribute. However, it is found that no unknown parameters can appear within loop diagrams, and a single tree-level counterterm at O(p^4) is sufficient to absorb all divergences. The momentum dependence of the neutral weak magnetic form factor is not affected by this counterterm.

hep-ph↗

arXiv1998

S-wave charmed mesons in lattice NRQCD

Heavy-light mesons can be studied using the 1/M expansion of NRQCD, provided the heavy quark mass is sufficiently large. Calculations of the S-wave charmed meson masses from a classically and tadpole-improved action are presented. A comparison of O(1/M), O(1/M^2) and O(1/M^3) results allows convergence of the expansion to be discussed. It is shown that the form of discretized heavy quark propagation must be chosen carefully.

hep-lat↗

arXiv1998

O(1/M^3) effects for heavy-light mesons in lattice NRQCD

The masses of spin-singlet and spin-triplet S-wave mesons containing a single heavy quark are computed in the quenched approximation. The light quark action and gauge field action are both classically-improved and tadpole-improved, and the couplings to the heavy quark are organized by the 1/M expansion of tadpole-improved NRQCD. At each of two lattice spacings, near 0.22fm and 0.26fm, meson masses are obtained for heavy quarks spanning the region between charmed and bottom mesons. Results up to O(1/M), O(1/M^2) and O(1/M^3) are displayed separately, so that the convergence of the heavy quark expansion can be discussed. Also, the effect of each term in the O(1/M^3) contribution is computed individually. For bottom mesons the 1/M-expansion appears to be satisfactory, but the situation for charmed mesons is less clear.

hep-lat↗

arXiv1997

Radiative and non radiative muon capture on the proton in heavy baryon chiral perturbation theory

We have evaluated the amplitude for muon capture by a proton, mu + p --> n + nu, to O(p^3) within the context of heavy baryon chiral perturbation theory (HBChPT) using the new O(p^3) Lagrangian of Ecker and Mojzis (E&M). We obtain expressions for the standard muon capture form factors and determine three of the coefficients of the E&M Lagrangian, namely, b_7, b_{19}, and b_{23}. We describe progress on the next step, a calculation of the radiative muon capture process, mu + p --> n + nu + gamma.

hep-ph↗

arXiv1997

Muon capture by a proton in heavy baryon chiral perturbation theory

The matrix element for muon capture by a proton is calculated to O(p^3) within heavy baryon chiral perturbation theory using the new O(p^3) Lagrangian of Ecker and Mojzis. External nucleon fields are renormalized using the appropriate definition of the wave function renormalization factor Z_N. Our expression for Z_N differs somewhat from that found in existing literature, but is the one which is consistent with the Lagrangian we use and the one which ensures, within our approach, the nonrenormalization of the vector coupling as required by the conserved vector current. Expressions for the standard muon capture form factors are derived and compared to experimental data and we determine three of the coefficients of the Ecker - Mojzis Lagrangian, namely, b_7, b_{19}, and b_{23}.

hep-ph↗

arXiv1996

Light vector meson decay constants and the renormalization factor from a tadpole-improved action

The rho, K* and phi decay constants and the vector current renormalization factor are studied by using an O(a^2) classically-improved, tadpole-improved action. Tree-level calculations are used to show how the classical improvement of the action, involving next-nearest-neighbour timesteps, is transferred to the matrix elements. Simulations are performed on coarse lattices and compared to Wilson results from both coarse and fine lattices. The improved action data are found to resemble Wilson data obtained at 1/3 of the lattice spacing, which is the same degree of improvement that is seen by comparing the mass spectra.

hep-lat↗

arXiv1996

The rho meson decay constant using a tadpole-improved action

The rho meson decay constant and the associated renormalization factor are computed in the quenched approximation on coarse lattices using a tadpole-improved action which is corrected at the classical level to O(a^2). The improvement is displayed by comparing to Wilson action calculations.

hep-lat↗

arXiv1995

Momentum dependent quark mass in two-point correlators

A momentum dependent quark mass may be incorporated into a quark model in a manner consistent with dynamically broken chiral symmetry. We use this to study the high $Q^2$ behavior of the vector, axialvector, scalar and pseudoscalar two-point correlation functions. Expanding the results to order $1/Q^6$, we show the correspondence between the dynamical quark mass and the vacuum condensates which appear in the operator product expansion of QCD. We recover the correct leading logarithmic $Q^2$ dependence of the various terms in the OPE, but we also find substantial subleading corrections which are numerically huge in a specific case. We conclude by using the vector minus axialvector correlator to estimate the $π^+ - π^0$ electromagnetic mass difference.

hep-ph↗

arXiv1994

What can a relativistic quark model tell us about charmed mesons?

A relativistic quark model is extended to incorporate chiral and gauge symmetries. We obtain the DD*pi and DD*gamma couplings and find that the ratio Gamma(D*0->D0pi0)/Gamma(D*0->D0gamma) constrains the charm-quark mass close to 1.45 GeV. Large 1/mc corrections appear in the heavy-quark contribution to the DD*gamma coupling. The model is extended further to describe seven excited D meson states. We find that semileptonic B decays into the ground and excited D meson states do not account for the total semileptonic decay width of the B0. The nonresonant contributions to the processes B0->D(*)pi+ell+nu appear to be large enough to account for the discrepancy.

hep-ph↗

arXiv1994

Hadronic contribution to the photon vacuum polarization: a theoretical estimate

A simple model for the hadronic contribution to the photon vacuum polarization function $Π_{had}(q^2)$, for spacelike momenta, is presented. For small momenta, the two loop contribution from the pseudoscalar meson octet is computed from the chiral Lagrangian. The light quark contribution (which at low momentum gives the ${\cal O}(q^6)$ counterterm in the chiral Lagrangian) is calculated within a relativistic constituent quark model incorporating the momentum dependence of the quark mass. The perturbative gluons of QCD are included in a standard fashion. The total result is close to an estimate of $Π_{had}(q^2)$ that is obtained directly from $e^+e^-\to hadrons$ data. We further use our results for $Π_{had}(q^2)$ to calculate the ${\cal O}(e^4)$ hadronic contribution to lepton magnetic moments and to calculate $α_{QED}(M_Z^2)$. A simpler model of constituent quarks with momentum independent masses gives less favourable results.

hep-ph↗

arXiv1994

Effects of technicolor on standard model running couplings

We discuss the running couplings in the standard model, SU(3$)_C \times $SU(2$)_L \times $U(1$)_Y$, when the Higgs sector is replaced by SU($N_{TC})$ technicolor. Particular attention is given to the running of the couplings at momentum scales where technicolor is nonperturbative, and in this region we apply a relativistic constituent technifermion model. This model has been tested against the known running of the QED coupling due to nonperturbative QCD. An understanding of this low momentum running allows the calculation of the couplings at a higher scale, $Λ_{pert}$, where technicolor becomes perturbative. We provide numerical values for the changes in the three standard model couplings between $m_Z$ and $Λ_{pert}$ due to technicolor, assuming separately ``one doublet'' and ``one family'' technicolor models. The distinction between a running and walking technicolor coupling is also considered.

hep-ph↗