Chung-Wen Kao

Generalized Drell-Hearn-Gerasimov sum rule at order ) in chiral perturbation theory

Abstract We calculate the forward spin-dependent photon-nucleon Compton amplitudes S1 and S2 as functions of photon energy ν and mass Q2 at the next-to-leading ( O (p 4 ) ) order in chiral perturbation theory, from which we extract the contribution to a generalized Drell-Hearn-Gerasimov sum rule at low Q2. The result indicates a much rapid Q2 variation of the sum rule than a simple dimensional analysis would yield.

Burkhardt-Cottingham sum rule and forward spin polarizabilities in heavy baryon chiral perturbation theory

We study spin-dependent sum rules for forward virtual Compton scattering (VVCS) off the nucleon in heavy baryon chiral perturbation theory (HBChPT) at order O(p 4 ). We show how these sum rules can be evaluated from low energy expansions (in the virtual photon energy) of the forward VVCS amplitude. We study in particular the Burkhardt-Cottingham sum rule in HBChPT and the higher terms in the low energy expansion, which can be related to generalized forward spin polarizabilities of the nucleon. The dependence of these observables on the photon virtuality Q 2 can be accessed, at small and intermediate Q 2 values, from existing and forthcoming data at

Leading chiral corrections to the nucleon generalized parton distributions

Using heavy baryon chiral perturbation theory we study the leading chiral corrections to the complete set of nucleon generalized parton distributions (GPDs). We compute the leading quark mass and momentum transfer dependence of the moments of nucleon GPDs through the nucleon off-forward twist-2 matrix elements. These results are then applied to get insight on the GPDs and their impact parameter space distributions.

Nucleon spin polarizability at order O(p{sup 4}) in chiral perturbation theory

We calculate the forward spin-dependent photon-nucleon Compton amplitude as a function of photon energy at next-to-leading (O(p{sup 4})) order in chiral perturbation theory, from which we extract the contribution to the nucleon spin polarizability. The result shows a large correction to the leading order contribution. (c) 2000 The American Physical Society.

Many body methods and effective field theory

In the framework of pionless nucleon–nucleon effective field theory we study different approximation schemes for the nuclear many body problem. We consider, in particular, ladder diagrams constructed from particle–particle, hole–hole, and particle–hole pairs. We focus on the problem of finding a suitable starting point for perturbative calculations near the unitary limit (kFa)→∞ and (kFr)→0, where kF is the Fermi momentum, a is the scattering length and r is the effective range. We try to clarify the relationship between different classes of diagrams and the large g and large D approximations, where g is the fermion degeneracy and D is the number of space–time dimensions. In the large D limit we find that the energy per particle in the strongly interacting system is 1/2 the result for free fermions.

U-boson and the HyperCP exotic events

Abstract We show that the very light spin-1 gauge U-boson of the extra U ( 1 ) ′ gauge model in the framework of the supersymmetric Standard Model extension can be a good candidate of the new light particle suggested by the HyperCP experiment. We demonstrate that the flavor changing neutral currents (FCNCs) for the HyperCP events in the decay of Σ + → p μ + μ − can be generated at both tree and loop levels. In particular, we find that the loop induced s → d U transition due to the tensor-type interaction with the dimension-5 electric dipole operator plays a very important role on the FCNCs. Our explanation of the HyperCP data with the spin-1 U-boson is different from that based on a light pseudoscalar Higgs boson or sgoldstino in the literature. In particular, the U-boson involves a rich phenomenology in particle physics as well as cosmology.

Closed-time path integral formalism and medium effects of nonequilibrium QCD matter

We apply the closed-time path integral formalism to study the medium effects of nonequilibrium gluon matter. We derive the medium modified resummed gluon propagator to the one loop level in nonequilibrium in the covariant gauge. The gluon propagator we derive can be used to remove the infrared divergences in the secondary parton collisions to study the thermalization of minijet parton plasma at BNL RHIC and CERN LHC.

Two-Photon-Exchange and {gamma}Z-Exchange Corrections to Parity-Violating Elastic Electron-Proton Scattering

Leading electroweak corrections play an important role in precision measurements of the strange form factors. We calculate the two-photon-exchange (TPE) and {gamma}Z-exchange corrections to the parity-violating asymmetry of the elastic electron-proton scattering in a simple hadronic model including the finite size of the proton. We find both can reach a few percent and are comparable in size with the current experimental measurements of strange-quark effects in the proton neutral weak current. The effect of {gamma}Z exchange is in general larger than that of TPE, especially at low momentum transfer Q{sup 2}{<=}1 GeV{sup 2}. Their combined effects on the values of G{sub E}{sup s}+G{sub M}{sup s} extracted in recent experiments can be as large as -40% in certain kinematics.

New predictions for generalized spin polarizabilities from heavy baryon chiral perturbation theory

We extract the next-to-next-to-leading order results for spin-flip generalized polarizabilities (GPs) of the nucleon from the spin-dependent amplitudes for virtual Compton scattering at

Infrared behavior of the gluon propagator in nonequilibrium situations

\mathcal{O}({p}^{4})