arXiv subjects

S. Gardner

Publications and source records attributed to S. Gardner.

86 records · Page 5Linked to original sources

arXiv1999

Additional Isospin-Breaking Effects in ε^\prime/ε

In the analysis of ε^\prime/εit has been traditional to consider the isospin-breaking effects arising from electroweak-penguin contributions and from π^0-ηand π^0-η^\prime mixing, yet additional isospin-violating effects exist. In particular, we study the isospin violation which arises from the u-d quark mass difference in the hadronization of the gluonic penguin operator, engendering contributions of an effective ΔI=3/2 character. Using chiral perturbation theory and the factorization approximation for the hadronic matrix elements, we find within a specific model for the low-energy constants that we can readily accommodate an increase in ε^\prime/εby a factor of two.

hep-ph↗

arXiv1999

Isospin Violation in ${\bf B\to ππ}$ Decays

An isospin analysis of $B\to ππ$ decays yields $\sin 2α$, where $α$ is the usual CKM angle $α\equiv {\rm arg} [-V_{td} V_{tb}^\ast/(V_{ud}V_{ub}^\ast)]$ without hadronic uncertainty if isospin is a perfect symmetry. Yet isospin symmetry is broken not only by electroweak effects but also by the $u$ and $d$ quark mass difference --- the latter drives $π^0-η,η'$ mixing and converts the isospin-perfect triangle relation between the $B\to ππ$ amplitudes to a quadrilateral. The error incurred in $\sin 2α$ through the neglect of the resulting isospin-violating effects can be significant, particularly if $\sin 2α$ is small.

hep-ph↗

arXiv1999

Extracting Br(omega->pi^+ pi^-) from the Time-like Pion Form-factor

We extract the G-parity-violating branching ratio Br(omega->pi^+ pi^-) from the effective rho-omega mixing matrix element Pi_{rho omega}(s), determined from e^+e^- -> pi^+ pi^- data. The omega->pi^+ pi^- partial width can be determined either from the time-like pion form factor or through the constraint that the mixed physical propagator D_{rho omega}^{mu nu}(s) possesses no poles. The two procedures are inequivalent in practice, and we show why the first is preferred, to find finally Br(omega->pi^+ pi^-) = 1.9 +/- 0.3%.

hep-ph↗

arXiv1998

Isospin Violation in Hadronic B-Decays

I present two disparate examples of isospin violation in hadronic B-decays. In the first, the presence of ρ^0-ωmixing in the decay B^\mp\toρ^\mpρ^0(ω)\toρ^\mpπ^+ π^- permits the extraction of sgn(\sin α), where αis the usual angle of the unitarity triangle, with minimal hadronic uncertainty. In the second, the presence of π^0-η,η' mixing can obscure the extraction of \sin 2αfrom an isospin analysis in B\toππdecays.

hep-ph↗

arXiv1998

How Isospin Violation Mocks ``New'' Physics: $π^0-η,η'$ Mixing in $B\to ππ$ Decays

An isospin analysis of $B\toππ$ decays yields $\sin 2α$, where $α$ is the usual CKM angle $α\equiv arg [-V_{td}V_{tb}^\ast /(V_{ud}V_{ub}^\ast)]$, without hadronic uncertainty if isospin is a perfect symmetry. Isospin, however, is broken not only by electroweak effects but also by the u and d quark mass difference. The latter generates $π^0-η, η'$ mixing and converts the isospin-perfect triangle relation between the $B\toππ$ amplitudes to a quadrilateral. The combined isospin-violating effects impact the extracted value of $\sin 2α$ in a significant manner, particularly if the latter is small.

hep-ph↗

arXiv1998

Rho-Omega Mixing and the Pion Form Factor in the Time-like Region

We determine the magnitude, phase, and $s$-dependence of $ρ$-$ω$ ``mixing'' in the pion form factor in the time-like region through fits to $e^+e^- \ra π^+ π^-$ data. The associated systematic errors in these quantities, arising from the functional form used to fit the $ρ$ resonance, are small. The systematic errors in the $ρ$ mass and width, however, are larger than previously estimated.

hep-ph↗

arXiv1998

Rho-Omega Mixing and Direct CP Violation in Hadronic B-Decays

The extraction of CKM-matrix-element information from hadronic B-decays generally suffers from discrete ambiguities, hampering the diagnosis of physics beyond the Standard Model. We show that a measurement of the rate asymmetry, which is CP-violating, in $B^{\pm}\toρ^{\pm}ρ^0(ω)\toρ^{\pm}π^+π^-$, where the invariant mass of the $π^+π^-$ pair is in the vicinity of the $ω$ resonance, can remove the mod($π$) uncertainty in $α\equiv arg [-V_{td} V_{tb}^\ast/(V_{ud}V_{ub}^\ast)]$ present in standard analyses.

hep-ph↗

arXiv1997

Direct CP Violation in $B^\pm\toρ^\pmπ^+π^-$ in the $ρ^0$-$ω$ Interference Region

We study direct CP violation in $B^\pm \to ρ^\pmρ^0(ω) \to ρ^\pmπ^+π^-$ and focus specifically on the rate asymmetry in the $ρ^0$-$ω$ interference region. Here the strong phase is dominated by isospin violation, so that it can be essentially determined by $e^+e^- \to ρ^0 (ω) \to π^+π^-$ data. We find the CP-violating asymmetry to be of the order of 20% at the $ω$ invariant mass. Moreover, it is robust with respect to the estimable strong-phase uncertainties, permitting the extraction of $\sinα$ from this channel.

hep-ph↗

arXiv1995

Nuclear Production of Charmonium at ELFE Energies

I discuss the diffractive production of charmonium from nuclei and the anomalous nuclear attenuation --- termed ``color transparency'' --- attributed to such processes, focusing on the few tens of GeV energy regime germane to the ELFE project. A critical review of color transparency calculations is presented, and the major sources of uncertainty are emphasized.

nucl-th↗

arXiv1995

Isospin-Violating Meson-Nucleon Vertices as an Alternate Mechanism of Charge-Symmetry Breaking

We compute isospin-violating meson-nucleon coupling constants and their consequent charge-symmetry-breaking nucleon-nucleon potentials. The couplings result from evaluating matrix elements of quark currents between nucleon states in a nonrelativistic constituent quark model; the isospin violations arise from the difference in the up and down constituent quark masses. We find, in particular, that isospin violation in the omega-meson--nucleon vertex dominates the class IV CSB potential obtained from these considerations. We evaluate the resulting spin-singlet--triplet mixing angles, the quantities germane to the difference of neutron and proton analyzing powers measured in elastic $\vec{n}-\vec{p}$ scattering, and find them commensurate to those computed originally using the on-shell value of the $ρ$-$ω$ mixing amplitude. The use of the on-shell $ρ$-$ω$ mixing amplitude at $q^2=0$ has been called into question; rather, the amplitude is zero in a wide class of models. Our model possesses no contribution from $ρ$-$ω$ mixing at $q^2=0$, and we find that omega-meson exchange suffices to explain the measured $n-p$ analyzing power difference~at~183 MeV.

nucl-th↗

arXiv1995

Charge-Symmetry-Breaking Potentials from Isospin-Violating Meson-Baryon Coupling Constants

We consider charge-symmetry violations in the nucleon-nucleon force which result from isospin-violating meson-baryon coupling constants. The vector mesons are assumed to couple to the nucleon's electromagnetic current, which we decompose into isoscalar and isovector quark components. We compute these currents in the context of a constituent quark model. The isospin violations in the meson-baryon couplings arise from the difference in the up and down constituent quark masses. We show that class IV charge-symmetry-breaking potentials arise in the resulting $ω$ and $ρ$ exchange contributions to the $NN$ force. The magnitude of these contributions is consistent with that phenomenologically required by the measured difference of $n$ and $p$ analyzing powers in elastic $\vec{n}-\vec{p}$ scattering at 183 MeV.

nucl-th↗

arXiv1994

Pion interferometry with pion-source-medium interactions

An extended pion source, which can be temporarily created by a high energy nuclear collision, will also absorb and distort the outgoing pions. We discuss how this effect alters the interferometric pattern of the two-pion momentum correlation function. In particular, we show that the two-pion correlation function decreases rapidly when the opening angle between the pions increases. The opening-angle dependence should serve as a new means of obtaining information about the pion source in the analysis of experimental data.

nucl-th↗

arXiv1994

Relativistically generated asymmetry in the missing momentum distribution from the $(e,e'p)$ reaction

We calculate the asymmetry in the missing-momentum distribution from the $(e,e'p)$ reaction in a relativistic formalism. Longitudinal and transverse response functions are evaluated in parallel kinematics as a function of the three-momentum transfer to the nucleus. Analytic expressions for the responses are obtained in a relativistic plane-wave impulse approximation. These expressions reveal a large asymmetry in the momentum distribution, at the plane-wave level, induced by the lower components of the bound-state wave functions. These relativistic effects contaminate any attempt to infer color transparency from a measurement of the asymmetry in the $(e,e'p)$ reaction.

nucl-th↗

arXiv1993

Dynamical Color Correlations in a $SU(2)_c$ Quark Exchange Model of Nuclear Matter

The quark exchange model is a simple realization of an adiabatic approximation to the strong-coupling limit of Quantum Chromodynamics (QCD): the quarks always coalesce into the lowest energy set of flux tubes. Nuclear matter is thus modeled in terms of its quarks. We wish to study the correlations imposed by total wavefunction antisymmetry when color degrees of freedom are included. To begin with, we have considered one-dimensional matter with a $SU(2)$ color internal degree of freedom only. We proceed by constructing a totally antisymmetric, color singlet {\it Ansatz} characterized by a variational parameter $λ$ (which describes the length scale over which two quarks in the system are clustered into hadrons) and by performing a variational Monte Carlo calculation of the energy to optimize $λ$ for a fixed density. We calculate the $q-q$ correlation function as well, and discuss the qualitative differences between the system at low and high density.

nucl-th↗