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S. Scherer

Publications and source records attributed to S. Scherer.

At least 91 records · Page 5Linked to original sources
arXiv1999

Physics opportunities at RHIC and LHC

Nonequilibrium models (three-fluid hydrodynamics, UrQMD, and quark molecular dynamics) are used to discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions from the SPS via RHIC to LHC. It is demonstrated that these models -- although they do treat the most interesting early phase of the collisions quite differently (thermalizing QGP vs. coherent color fields with virtual particles) -- all 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. The role of future experiments with the STAR and ALICE detectors is pointed out.

hep-ph↗
arXiv1999

Real and Virtual Compton Scattering at Low Energies

These lectures give a pedagogical introduction to real and virtual Compton scattering at low energies. We will first discuss real Compton scattering off a point particle as well as a composite system in the framework of nonrelativistic quantum mechanics. The concept of electromagnetic polarizabilities is introduced. We then address a description of the Compton-scattering tensor within quantum field theory with particular emphasis on the derivation of low-energy theorems. The importance of a consistent treatment of hadron structure in the use of electromagnetic vertices is stressed. Finally, the reader is introduced to the notion of generalized polarizabilities in the rapidly expanding field of virtual Compton scattering.

nucl-th↗
arXiv1998

Virtual Compton Scattering--Generalized Polarizabilities of Nucleons and Pions

Virtual Compton scattering off nucleons and pions at low energies is discussed. Predictions for the generalized polarizabilities of the nucleon are presented within the framework of heavy-baryon chiral perturbation theory and the linear sigma model. First results for the generalized polarizabilities of the charged pion in chiral perturbation theory at ${\cal O}(p^4)$ are shown.

hep-ph↗
arXiv1998

Low-energy and low-momentum representation of the virtual Compton scattering amplitude

We perform an expansion of the virtual Compton scattering amplitude for low energies and low momenta and show that this expansion covers the transition from the regime to be investigated in the scheduled photon electroproduction experiments to the real Compton scattering regime. We discuss the relation of the generalized polarizabilities of virtual Compton scattering to the polarizabilities of real Compton scattering.

nucl-th↗
arXiv1998

Structure analysis of the virtual Compton scattering amplitude at low energies

We analyze virtual Compton scattering off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, once the irregular nucleon pole terms have been subtracted in a gauge-invariant fashion, is free of poles and kinematical zeros. The covariant treatment naturally allows one to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the $ep\to e'p'γ$ reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon.

nucl-th↗
arXiv1997

Polarization Phenomena in Small-Angle Photoproduction of e+e- Pairs and the Gerasimov-Drell-Hearn Sum Rule

Photoproduction of $e^+e^-$ pairs at small angles is investigated as a tool to determine the functions $f_1$ and $f_2$ entering the real-photon forward Compton scattering amplitude. The method is based on an interference of the Bethe-Heitler and the virtual Compton scattering mechanisms, generating an azimuthal asymmetry in the $e^+$ versus $e^-$ yield. The general case of a circularly polarized beam and a longitudinally polarized target allows one to determine both the real and imaginary parts of $f_1$ as well as $f_2$. The imaginary part of $f_2$ requires target polarization only. We calculate cross sections and asymmetries of the reaction $p(γ,e^+e^-)p$, estimate corrections and backgrounds, and propose suitable kinematical regions to perform the experiment. Our investigation shows that photoproduction of $e^+e^-$-pairs off the proton and light nuclei may serve as a rather sensitive test of the validity of the Gerasimov-Drell-Hearn sum rule.

nucl-th↗
arXiv1996

Chiral dynamics of rare eta decays and virtual Compton scattering off the nucleon

Two different topics relevant for DA$Φ$NE and CEBAF, respectively, are covered. First we analyze the rare radiative eta decay mode $η\toππγγ$ within the framework of chiral perturbation theory at ${\cal O}(p^4)$. We then discuss virtual Compton scattering off the nucleon at low energies. Predictions for the two spin-independent generalized polarizabilities are shown.

hep-ph↗
arXiv1996

Generalized polarizabilities and the spin-averaged amplitude in virtual Compton scattering off the nucleon

We discuss the low-energy behavior of the spin-averaged amplitude of virtual Compton scattering (VCS) off a nucleon. Based on gauge invariance, Lorentz invariance and the discrete symmetries, it is shown that to first order in the frequency of the final real photon only two generalized polarizabilities appear. Different low-energy expansion schemes are discussed and put into perspective.

nucl-th↗
arXiv1996

Virtual Compton scattering off spin-zero particles at low energies

We discuss the low-energy behavior of the virtual Compton scattering amplitude off a spin-zero target. We first compare various methods of obtaining a low-energy expression based either on the soft-photon approximation or the use of Ward-Takahashi identities. We point out that structure-dependent terms are defined with respect to a low-energy approximation of the pole terms which commonly is separated from the full amplitude. We derive a general expression for the structure-dependent terms in an expansion in terms of the momenta $k_1$ and $k_2$ of the initial and final virtual photon, respectively, up to and including terms of order ${\cal O}(k^4)$. At order ${\cal O}(k^2)$ two terms appear which are related to the usual electric and magnetic polarizabilities of real Compton scattering. At order ${\cal O}(k^4)$ we find nine new structures of which five can only be determined using virtual photons.

nucl-th↗
arXiv1996

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.

nucl-th↗
arXiv1996

Virtual Compton scattering off the nucleon at low energies

We investigate the low-energy behavior of the four-point Green's function $Γ^{μν}$ describing virtual Compton scattering off the nucleon. Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive the leading terms of an expansion of the operator in the four-momenta $q$ and $q'$ of the initial and final photon, respectively. The model-independent result is expressed in terms of the electromagnetic form factors of the free nucleon, i.e., on-shell information which one obtains from electron-nucleon scattering experiments. Model-dependent terms appear in the operator at $O(q_αq'_β)$, whereas the orders $O(q_αq_β)$ and $O(q'_αq'_β)$ are contained in the low-energy theorem for $Γ^{μν}$, i.e., no new parameters appear. We discuss the leading terms of the matrix element and comment on the use of on-shell equivalent electromagnetic vertices in the calculation of ``Born terms'' for virtual Compton scattering.

nucl-th↗
arXiv1996

The rare decay $η\toππγγ$ in chiral perturbation theory

We investigate the rare radiative eta decay modes $η\toπ^+π^-γγ$ and $η\toπ^0π^0γγ$ within the framework of chiral perturbation theory at $O(p^4)$. We present photon spectra and partial decay rates for both processes as well as a Dalitz contour plot for the charged decay.

hep-ph↗
arXiv1995

The Anomalous Decay eta -> pi pi gamma gamma

We investigate the rare radiative eta decay modes eta -> pi+ pi- gamma gamma and eta -> pi0 pi0 gamma gamma within the framework of chiral lagrangians at o(p^4) and present photon spectra for both processes.

hep-ph↗
arXiv1995

Compton scattering from a pion and off-shell effects

We discuss the most general form of the electromagnetic vertex of an off-shell pion. The framework of chiral perturbation theory is used to illustrate the representation dependence of the electromagnetic three-point Green's function. For that purpose we discuss the concept of field transformations which, in comparison with the standard Gasser and Leutwyler Lagrangian, generate equivalent Lagrangians containing additional terms at order p^4 proportional to the lowest-order equation of motion. We consider Compton scattering from a pion to show that calculations involving different off-shell effects in the s- and u-channel pole diagrams may nevertheless lead to the same on-shell Compton scattering amplitude. This is a result of the equivalence theorem which states that two equivalent Lagrangians predict the same S-matrix elements even though they may generate a different off-shell behaviour of Green's functions. We conclude that off-shell effects are not only model dependent but also representation dependent.

nucl-th↗
arXiv1995

$γγ\toπ^0π^0$ and $η\toπ^0γγ$ at $O(p^6)$ in the NJL model

We discuss the processes $γγ\toπ^0π^0$ and $η\toπ^0 γγ$ at $O(p^6)$ in the momentum expansion. The calculation involves tree--level, one--loop and two--loop diagrams of a chiral effective lagrangian which is obtained by a bosonization of the NJL model. The importance of integrating out meson resonances (reduction) is pointed out. Our final results for the total cross section of $γγ\toπ^0π^0$ are in good agreement with the experimental data of the Crystal Ball Collaboration. For the width of the $η\toπ^0γγ$ decay we obtain the value $0.11$ eV which has to be compared with the experimental value of $(0.84 \pm 0.18)$ eV. Alternatively, taking empirical parameters from a vector--meson--dominance model the prediction for the decay width is $0.35$ eV. We present a prediction for the differential decay probability as a function of $m_{γγ}^2/m_η^2$.

hep-ph↗