119144340
Infrared renormalization of two-loop integrals and the chiral expansion of the nucleon mass
Abstract We describe details of the renormalization of two-loop integrals relevant to the calculation of the nucleon mass in the framework of manifestly Lorentz-invariant chiral perturbation theory using infrared renormalization. It is shown that the renormalization can be performed while preserving all relevant symmetries, in particular chiral symmetry, and that renormalized diagrams respect the standard power counting rules. As an application we calculate the chiral expansion of the nucleon mass to order O ( q 6 ) .
2811077
Generalized polarizabilities and the chiral structure of the nucleon
Abstract We discuss the virtual Compton scattering reaction e − p → e − pγ at low energies. We present results for the generalized polarizabilities of the nucleon obtained in heavy baryon chiral perturbation theory at O ( p 3 ).
119066839
Power counting in baryon chiral perturbation theory including vector mesons
It is demonstrated that using a suitable renormalization condition one obtains a consistent power counting in manifestly Lorentz-invariant baryon chiral perturbation theory including vector mesons as explicit degrees of freedom.
8088855
Virtual Compton scattering off the nucleon in chiral perturbation theory
We investigate the spin-independent part of the virtual Compton scattering (VCS) amplitude off the nucleon within the framework of chiral perturbation theory. We perform a consistent calculation to third order in external momenta according to Weinberg`s power counting. With this calculation we can determine the second- and fourth-order structure-dependent coefficients of the general low-energy expansion of the spin-averaged VCS amplitude based on gauge invariance, crossing symmetry, and the discrete symmetries. We discuss the kinematical regime to which our calculation can be applied and compare our expansion with the multipole expansion by Guichon, Liu, and Thomas. We establish the connection of our calculation with the generalized polarizabilities of the nucleon where it is possible. {copyright} {ital 1997} {ital The American Physical Society}
117594500
Spontaneous Symmetry Breaking and the Goldstone Theorem
So far we have concentrated on the chiral symmetry of the QCD Hamiltonian and the explicit symmetry breaking by the quark masses. We have discussed the importance of chiral symmetry for the properties of Green functions with particular emphasis on the relations among different Green functions as expressed through the chiral Ward identities. Now it is time to address a second aspect which, for the low-energy structure of QCD, is equally important, namely, the concept of spontaneous symmetry breaking. A (continuous) symmetry is said to be spontaneously broken or hidden if the ground state of the system is no longer invariant under the full symmetry group of the Hamiltonian. In this chapter we will first illustrate this by means of a discrete symmetry and then turn to the case of a spontaneously broken continuous global symmetry.
1603412
Chiral perturbation theory Success and challenge
Chiral perturbation theory is the effective field theory of the strong interactions at low energies. We will give a short introduction to chiral perturbation theory for mesons and will discuss, as an example, the electromagnetic polarizabilities of the pion. These have recently been extracted from an experiment on radiative π photoproduction from the proton (γp → γπn) at the Mainz Microtron MAMI. Next we will turn to the one-baryon sector of chiral perturbation theory and will address the issue of a consistent power counting scheme. As examples of the heavy-baryon framework we will comment on the extraction of the axial radius from pion electroproduction and will discuss the generalized polarizabilities of the proton. Finally, we will discuss two recently proposed manifestly Lorentz-invariant renormalization schemes and illustrate their application in a calculation of the nucleon electromagnetic form factors. PACS. 11.10.Gh Renormalization – 11.30.Rd Chiral symmetries – 13.40.-f Electromagnetic processes and properties – 13.40.Gp Electromagnetic form factors – 13.60.Fz Elastic and Compton scattering – 13.60.Le Meson production
119364125
Exotic hadrons from dynamical clustering of quarks in ultrarelativistic heavy ion collisions
Results from a model study on the formation of exotic quark clusters at the hadronization stage of a heavy ion collision are presented. The dynamical quark molecular dynamics (qMD) model which is used is sketched, and results for exotica made of up to six (anti-)quarks are shown. The second part focuses on pentaquarks. The rapidity distribution are shown, and the distribution of strangeness is found to yield an indicator of thermalization and homogenization of the deconfined quark system. Relative Θ+ yields are found to be lower than thermal model estimates.
119408128
The Effect of Dynamical Parton Recombination on Event-by-Event Observables
Abstract Within a dynamical quark recombination model, we explore various proposed event-by-event observables sensitive to the microscopic structure of the QCD-matter created at RHIC energies. Charge ratio fluctuations, charge transfer fluctuations and baryon-strangeness correlations are computed from a sample of central Au + Au events at the highest RHIC energy available ( s N N = 200 GeV ). We find that for all explored observables, the calculations yield the values predicted for a quark–gluon plasma only at early times of the evolution, whereas the final state approaches the values expected for a hadronic gas. We argue that the recombination-like hadronization process itself is responsible for the disappearance of the predicted deconfinement signals. This might explain why no fluctuation signatures for the transition between quark and hadronic matter was ever observed in the experimental data up to now.
125800824
Critical review of quark gluon plasma signatures
We discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions, using insight gained from non‐equilibrium models (three‐fluid hydrodynamics and the hadronic transport model UrQMD). It is demonstrated that these two models ‐ although they do treat the most interesting early phase of the collisions quite differently (thermalizing QGP vs. coherent color fields with virtual particles) ‐ both yield a reasonable agreement with a large variety of the available heavy ion data. Hadron/hyperon yields, including J/Ψ meson production/suppression, strange matter formation, dileptons, and directed flow (bounce‐off and squeeze‐out) are investigated. Observations of interesting phenomena in dense matter are reported. However, we emphasize the need for systematic future measurements to search for simultaneous irregularities in the excitation functions of several observables in order to come close to pinning the properties of hot, dense QCD matter from data.
118819388
Hadron ratios: Chiral symmetry restauration vs. nonequilibrium quark dynamics
Hadron production in recent nucleus-nucleus collisions experiments at the SPS and at RHIC have been analyzed in various thermal model fits. Here it is shown that the fit cannot be unique: the chemical freeze-out close to the phase transition to a QGP should be accompanied by shifting baryon masses in medium, which can yield considerably smaller break-up temperatures. Also a simple microscopic quark molecular dynamics model yields hadron ratios in nice agreement with those data. Hence, additional information, e.g., from HBT, is needed to determine the freeze-out parameters from data.
18764006
Nonequilibrium dynamics of a hadronizing quark-gluon plasma
We investigate the hadronic cooling of a quark droplet within a microscopic model. The color flux tube approach is used to describe the hadronization of the quark phase. The model reproduces experimental particle ratios equally well compared to a static thermal hadronic source. Furthermore, the dynamics of the decomposition of a quark-gluon plasma is investigated and time dependent particle ratios are found.