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}
119340330
Infrared and extended on-mass-shell renormalization of two-loop diagrams
Abstract Using a toy model Lagrangian we demonstrate the application of both infrared and extended on-mass-shell renormalization schemes to multiloop diagrams by considering as an example a two-loop self-energy diagram. We show that in both cases the renormalized diagrams satisfy a straightforward power counting.
119365626
Introduction to Chiral Perturbation Theory
This article provides a pedagogical introduction to the basic concepts of chiral perturbation theory and is designed as a text for a two-semester course on that topic. Section 1 serves as a general introduction to the empirical and theoretical foundations which led to the development of chiral perturbation theory. Section 2 deals with QCD and its global symmetries in the chiral limit; the concept of Green functions and Ward identities reflecting the underlying chiral symmetry is elaborated. In Section 3 the idea of a spontaneous breakdown of a global symmetry is discussed and its consequences in terms of the Goldstone theorem are demonstrated. Section 4 deals with mesonic chiral perturbation theory and the principles entering the construction of the chiral Lagrangian are outlined. Various examples with increasing chiral orders and complexity are given. Finally, in Sect. 5 the methods are extended to include the interaction between Goldstone bosons and baryons in the single-baryon sector, with the main emphasis put on the heavy-baryon formulation. At the end, the method of infrared regularization in the relativistic framework is discussed.
15270133
BARYON CHIRAL PERTURBATION THEORY IN MANIFESTLY LORENTZ-INVARIANT FORM
A successful effective field theory program requires besides the most general effective Lagrangian a perturbative expansion scheme for observables in terms of a consistent power counting method. We discuss a renormalization scheme for manifestly Lorentz-invariant baryon chiral perturbation theory generating a simple and consistent power counting for renormalized diagrams. The approach may be used in an iterative procedure to renormalize higher-order loop diagrams and also allows for implementing a consistent power counting when vector mesons are explicitly included.
16778234
Virtual Compton Scattering and Generalized Polarizabilities of the Nucleon in Heavy Baryon Chiral Perturbation Theory
The spin-independent part of the virtual Compton scattering (VCS) amplitude from the nucleon is calculated within the framework of heavy baryon chiral perturbation theory (HBChPT). The calculation is performed to third order in external momenta according to chiral power counting. The relation of the tree-level amplitudes to what is expected from the low-energy theorem is discussed. We relate the one-loop results to the structure coefficients of a low-energy expansion for the model-dependent part of the VCS amplitude recently defined by Fearing and Scherer. Finally we discuss the connection of our results with the generalized polarizabilities of the nucleon defined by Guichon, Liu and Thomas.
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 ).
92219017
Charged particle ratio fluctuations and charge transfer fluctuations from a recombination approach
126346677
Collective Flow Signals the Quark-Gluon Plasma
A critical discussion of the present status of the CERN experiments on hadron collective flow is given. We emphasize the importance of the flow excitation function from 1 to 50 A·GeV: here the hydrodynamic model has predicted the collapse of the ]]> v_1$ flow and of the v2 flow at ∼10 A·GeV; at 40 A·GeV it has been recently observed by the NA49 Collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy we interpret this observation as potential evidence for a first order phase transition at high baryon density ρB. A detailed discussion of the collective flow as a barometer for the equation of state (EoS) of hot dense matter at RHIC follows. Here, hadronic rescattering models can explain 2 GeV/c. This is interpreted as evidence for the production of superdense matter at RHIC with initial pressure far above hadronic pressure, p > 1 GeV/fm3. We suggest that the fluctuations in the flow, v1 and v2, should be measured in future since ideal hydrodynamics predicts that they are larger than 50% due to initial state fluctuations. Furthermore, the QGP coefficient of viscosity may be determined experimentally from the fluctuations observed.
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.
201855044
Parton Recombination And Fluctuations Of Conserved Charges
We study various fluctuation and correlation signals of the d econfined state using a dynamical recombination approach (quark Molecular Dynamics, qMD). We a nalyse charge ratio fluctuations, charge transfer fluctuations and baryon-strangeness corre lations as a function of the center of mass energy with a set of central Pb+Pb/Au+Au events from AGS energies on ( Elab = 4 AGeV) up to the highest RHIC energy available ( √ sNN = 200 GeV) and as a function of time with a set of central Au+Au qMD events at √ sNN = 200 GeV with and without applying our hadronization procedure. For all studied quantities, the results start fr om values compatible with a weakly coupled QGP in the early stage and end with values compatible wit h the hadronic result in the final state. We show that the loss of the signal occurs at the same ti me as hadronization and trace it back to the dynamical recombination process implemented in our model.
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.