Hidenaga Yamagishi

Dilepton and photon emission rates from a hadronic gas

Abstract We analyze the dilepton and photon emission rates from a hadronic gas using chiral reduction formulas and a virial expansion. The emission rates are reduced to pertinent vacuum correlation functions, most of which can be assessed from experiment. Our results indicate that in the low mass region, the dilepton and photon rates are enhanced compared to most of the calculations using chiral Lagrangians. The enhancement is further increased through a finite pion chemical potential. An estimate of the emission rates is also made using Haags expansion for the electromagnetic current. The relevance of these results to dilepton and photon emission rates in heavy-ion collisions is discussed.

A Master Formula for Chiral Symmetry Breaking

We derive a master formula for chiral {ital SU}(2){times}{ital SU}(2) breaking, based on the Veltman-Bell equations and the Peierls-Dyson relation. Our approach does not rely on the use of the soft pion limit or an expansion around the chiral limit, and yields exact results for on-shell pions. Threshold theorems for {pi}{ital N}{r_arrow}{pi}{ital N},{gamma}{ital N}{r_arrow}{pi}{ital N}, {pi}{ital N}{r_arrow}{pi}{pi}{ital N},{gamma}{ital N}{r_arrow}{gamma}{pi}{ital N},{gamma}{ital N}{r_arrow}{pi}{pi}{ital N} and {pi}{ital N}{r_arrow}{pi}{gamma}{ital N} are recovered, and corrections to them are given. The reactions {pi}{r_arrow}{ital e}{nu}{gamma}, {pi}{r_arrow}{ital e}{nu}{ital e}{sup +}{ital e}{sup {minus}},{gamma}{pi}{r_arrow}{gamma}{pi} and {gamma}{gamma}{r_arrow}{pi}{pi} are also discussed. A general formula for {pi}{pi} scattering and a new one loop effective action are obtained. The new effective action reproduces the KSFR relation, and yields specific estimates for the pion polarizabilities. A detailed comparison with baryon-free chiral perturbation theory to one loop is made. An extension of our effective action to two loops is outlined. Copyright {copyright} 1996 Academic Press, Inc.

Master formula approach to chiral symmetry breaking: ππ-scattering

Abstract The master formula approach to chiral sysmmetry breaking is used to analyze the phase shifts for ππ scattering in the elastic region. The results are in excellent agreement with the data for the phase shifts up to the K K threshold. In the resonance region the bulk of the vector current contribution is found to be fixed by the electroproduction data in a model independent way. Our analysis shows that the ππ data near threshold in the scalar channel favor a large quark condensate in the vacuum, i.e. 〈 q q〉 ∼ −(240 MeV ) 3 .

Scattering in soliton models and boson-exchange descriptions.

We analyze the relationship between the conventional one-boson-exchange model (OBE) for nuclear forces with the description of the nucleon as a soliton within the context of the two-dimensional sine-Gordon model. We find that the soliton-soliton {ital S} matrix contains poles and residues compatible with the ones used in the OBE. Implications of this result for the four-dimensional case are briefly discussed.

CHIRAL SOLITONS VERSUS SOFT PION THEOREMS IN LARGE Nc QCD

It is shown that large-Nc (number of colors) QCD does not obey conventional soft pion theorems, if the nucleon is a soliton.

Is quantization of QCD unique at the non-perturbative level?

We find that QCD in covariant gauge yields zero for the topological susceptibility, even at the nonperturbative level. The result is derived in two ways, one using translational invariance, and the other using the BRST Hamiltonian. Comparison with the canonical formalism suggests that QCD is not uniquely defined at the nonperturbative level. Supporting evidence is also provided in 1+1 dimensions. Our results imply that the strong CP problem admits a trivial resolution in covariant gauge, but obstacles remain for the U(1) problem.

On-shell approach to pion-nucleon physics

We discuss an on-shell approach to pion-nucleon physics that is consistent order by order in a 1/f{sub {pi}} expansion with the chiral reduction formula, crossing, and relativistic unitarity. A number of constraints between the on-shell low-energy parameters are derived at tree level in the presence of the pion-nucleon sigma term, and found to be in fair agreement with experiment. We analyze the nucleon form factors, and the {pi}N{r_arrow}{pi}N scattering amplitude to one-loop, as well as {pi}N{r_arrow}{pi}{pi}N to tree level. We use the latter to derive a new constraint for the pion-nucleon sigma term at threshold. We compare our results to both relativistic and nonrelativistic chiral perturbation theory, and discuss the convergence character of the expansion in light of experiment. {copyright} {ital 1997} {ital The American Physical Society}

Electromagnetic emission rates and spectral sum rules

Abstract The electromagnetic emission rates at SPS energies satisfy spectral constraints in leading order in the pion and nucleon densities. These constraints follow from the strictures of broken chiral symmetry. We saturate these constraints using available data, leading to model independent emission rates from a hadronic gas. With a simple fire-ball scenario, only large nucleon densities may account for the present CERES data.

ABNORMAL PROPAGATION IN THE SKYRME MODEL

It is shown that the Skyrme Lagrangian exhibits abnormal propagation effects similar to those analyzed by Velo and Zwanziger for higher spin fields. The difficulty does not arise for the vector stabilized versions. Implications for the chiral soliton program are briefly discussed.

Instability of flat spacetime in semiclassical gravity

Abstract The semiclassical theory of gravity is considered in which an asymptotically flat background metric is coupled to quantized matter. We show that, in general, there are modes with spacelike wave vectors for small metric fluctuations around flat spacetime. Besides the usual axioms of quantum field theory in flat spacetime, the proof rests on the existence of a hard trace anomaly in the energy-momentum tensor due to matter self-couplings. Two possible interpretations of the result are discussed.