In-medium Effects Research Articles

Influence of the nuclear medium on inclusive electron and neutrino scattering off nuclei

We present a model for inclusive electron and neutrino scattering off nuclei paying special attention to the influence of in-medium effects on the quasielastic scattering and pion-production mechanisms. Our results for electron scattering off {sup 16}O are compared to experimental data at beam energies ranging from 0.7 to 1.5 GeV. The good description of electron scattering serves as a benchmark for neutrino scattering.

Low-energy thermal photons from meson–meson bremsstrahlung

Within an effective hadronic model including electromagnetic interactions via a U em ( 1 ) gauge, we re-investigate photon bremsstrahlung from a hot hadronic gas as expected to be formed in relativistic heavy-ion collisions at SPS energies. We calculate photon emission from the reactions π π → π π γ and π K → π K γ by an explicit (numerical) evaluation of the multi-dimensional phase space integral. This, in particular, allows to avoid the commonly employed soft photon approximation (SPA), as well as to incorporate final-state thermal enhancement factors. Both improvements are shown to result in an appreciable increase of the photon production rate over previous hadronic calculations. Upon convolution over a thermal fireball we find an improvement in the description of recent low transverse-momentum WA98 data at SPS. The influence of both Landau–Pomeranchuk–Migdal and in-medium effects on “ σ” and ρ-meson exchanges are briefly discussed.

Heavy quarkonia production in p+p collisions from the PHENIX experiment

Quarkonia provide a sensitive probe of the properties of the hot dense medium created in high-energy heavy-ion collisions. Hard scattering processes result in the production of heavy quark pairs that interact with the collision medium during hadronization. These in-medium interactions convey information about the fundamental properties of the medium itself and can be used to examine the modification of the QCD confining potential in the collision environment. Baseline measurements from p+p and d+Au collision systems are used to distinguish cold nuclear matter effects while measurements from heavy-ion collision systems are used to quantify in-medium effects. The PHENIX experiment has the capability of detecting heavy quarkonia at 1.2 < |η| < 2.2 via the μ+μ− decay channel and at |η| < 0.35 via the e+e− decay channel. Recent runs have resulted in the collection of a high statistics p+p data set that provides an essential baseline reference for heavy-ion measurements and allow for further critical evaluation of heavy quarkonia production mechanisms. The latest PHENIX results for the production of the J/ψ mesons in p+p collisions are presented.

In-medium properties of light vector mesons

The photoproduction of vector mesons on various nuclei has been studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory. All three vector mesons ρ, ω and ϕ are observed via their decay to e+e−. The possible in-medium effects on the properties of the ρ meson are of particular interest. The ρ spectral function is extracted from the data on carbon, iron and titanium, and compared to the spectrum from liquid deuterium, which is relatively free of nuclear effects. We observe no effects on the mass of the ρ meson, some widening in titanium and iron is observed consistent with the collisional broadening.

In-medium effects on particle production in heavy ion collisions

The effect of possible in-medium modifications of nucleon–nucleon ( NN) cross sections on particle production is investigated in heavy ion collisions (HIC) at intermediate energies. In particular, using a fully covariant relativistic transport approach, we see that the density dependence of the inelastic cross sections appreciably affects the pion and kaon yields and their rapidity distributions. However, the ( π − / π + ) - and ( K 0 / K + ) -ratios depend only moderately on the in-medium behavior of the inelastic cross sections. This is particularly true for kaon yield ratios, since kaons are more uniformly produced in high density regions. Kaon potentials are also suitably evaluated in two schemes, a chiral perturbative approach and an effective meson–quark coupling method, with consistent results showing a similar repulsive contribution for K + and K 0 . As a consequence we expect rather reduced effects on the yield ratios. We conclude that particle ratios appear to be robust observables for probing the nuclear equation of state (EoS) at high baryon density and, particularly, its isovector sector.

MESON2006 SUMMARY: THEORY

This is a summary of theoretical plenary contributions to the biennial hadron physics conference Meson2006, which was the ninth in this series. The topics covered in the meeting include low energy pion-pion and pion-nucleon interactions, photoproduction and hadronic production of light mesons and baryons, in-medium effects, recent developments in charmed mesons, charmonia and B mesons, the status of exotica, and some related topics such as final state interactions. In this contribution we review and summarize the plenary talks presented by theorists at the meeting, and emphasize some of the main points of their presentations. Where appropriate we will add brief comments on some aspects of QCD spectroscopy. Finally, following tradition, we conclude with a new Feynman story.

SEARCH FOR IN-MEDIUM MODIFICATIONS OF THE ρ MESON

The photoproduction of vector mesons on various nuclei has been studied using the Cebaf Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory, and here we present preliminary results. All three vector mesons, ρ, ω, and ϕ, are observed via their decay to e+e-, in order to reduce the effects of final state interactions in the nucleus. Of particular interest is possible in-medium effects on the properties of the ρ meson. The ρ spectral function is extracted from the data on various nuclei, carbon, iron, and titanium, and compared to the spectrum from liquid deuterium, which is relatively free of nuclear effects. We observe no effects on the mass of the ρ meson, however, there is some widening of the resonance in titanium and iron, which is consistent with expected collisional broadening.

FLOW OF STRANGE PARTICLES FROM FOPI AT SIS

Study of strange particles produced at (sub)threshold energies in nucleus-nucleus collisions can deliver insights into fundamental questions about the in-medium properties of hadrons in dense baryonic matter. Many theoretical calculations for the production and propagation of strangeness at SIS energies lead to various predictions concerning the existence and the magnitude of in-medium effects for strange particles in nuclear matter. FOPI measured the flow of strange particles in Ni + Ni collisions at 1.93A GeV and the results are compared with transport models which are favorable to in-medium effect.

Quarkonium Production in PHENIX

Quarkonia provide a sensitive probe of the properties of the hot dense medium created in high energy heavy ion collisions. Hard scattering processes result in the production of heavy quark pairs that interact with the collision medium during hadronization. These in medium interactions convey information about the fundamental properties of the medium itself and can be used to examine the modification of the QCD confining potential in the collision environment. Baseline measurements from the d+Au and p+p collision systems can be used to distinguish cold nuclear matter effects while measurements from heavy ion collision systems, Au+Au and Cu+Cu, can be used to quantify in-medium effects. PHENIX results for the production of the J / ψ for a diverse set of collision systems and energies and for the ϒ in p+p collisions are presented.

Electroweak boson detection in the ALICE muon spectrometer

The ALICE capabilities for W and Z detection at LHC are discussed. The contributions to single muon transverse momentum distribution are evaluated. The expected performance of the muon spectrometer for detecting W and Z bosons via their muonic decay is presented. Possible application for the study of in-medium effects in ultrarelativistic heavy ion collisions is discussed.

Charged current neutrino-nucleus interactions at intermediate energies

We have extended our model for charged current neutrino-nucleus interactions developed in Phys. Rev. C 73, 065502 (2006) to neutral current reactions. For the elementary neutrino-nucleon interaction, we take into account quasielastic scattering, {delta} excitation, and the excitation of the resonances in the second resonance region. Our model for the neutrino-nucleus collisions includes in-medium effects such as Fermi motion, Pauli blocking, nuclear binding, and final-state interactions. They are implemented by means of the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) coupled-channel transport model. This allows us to study exclusive channels, namely pion production and nucleon knockout. We find that final-state interactions modify considerably the distributions through rescattering, charge-exchange, and absorption. Side-feeding induced by charge-exchange scattering is important in both cases. In the case of pions, there is a strong absorption associated with the in-medium pionless decay modes of the {delta}, while nucleon knockout exhibits a considerable enhancement of low-energy nucleons because of rescattering. At neutrino energies above 1 GeV, we also obtain that the contribution to nucleon knockout from {delta} excitation is comparable to that from quasielastic scattering.

Dilepton Spectra from HADES

In-medium effects on K and ω mesons, which have been reported in the past, confirm the importance of experiments which study hadron properties in excited and normal nuclear matter with penetrating probes. The HADES experiment at GSI uses proton and nuclear beams to record experimental data on elementary and nuclear collisions. This second generation high acceptance detector system allows to detect and separate electron-positron pairs in a high multiplicity environment with a hadron suppression factor of 10−9. Preliminary results on dilepton production in C+C collisions at 2 GeV per nucleon are presented. The comparison with UrQMD calculation shows agreement at the 20% level except at invariant masses below 100 MeV where deviations of up to 50% have been traced to shortcomings in the analysis.

Electromagnetic radiation and in-medium effects

The theory of thermal photon and dilepton emission from a hot and dense hadronic gas, as well as from the Quark-Gluon Plasma, is reviewed in the context of extracting in-medium properties of the matter constituents. In phenomenological applications to ultrarelativistic heavy-ion collisions we focus on recent photon and dilepton spectra as measured by WA98 and CERES/NA45, respectively, at CERN-SPS energies.

Dilepton production at SIS energies

In this short review the results of detailed studies for dilepton production from p+A andA+A reactions at SIS energies are presented. The calculations are based on a semi-classical BUU transport model that includes the off-shell propagation of vector mesons and evaluates the width of the vector mesons dynamically. Different scenarios of in-medium modifications of vector mesons, such as collisional broadening and dropping vector meson masses, are investigated and the possibilities for an experimental observation of in-medium effects in p+A reactions at 1–4 GeV are discussed for a variety of nuclear targets.

D-mesons: In-medium effects at FAIR

The D-meson spectral density at finite temperature is obtained within a self-consistent coupled-channel approach. For the bare meson–baryon interaction, a separable potential is taken, whose parameters are fixed by the position and width of the Λc(2593) resonance. The quasiparticle peak stays close to the free D-meson mass, indicating a small change in the effective mass for finite density and temperature. Furthermore, the spectral density develops a considerable width due to the coupled-channel structure. Our results indicate that the medium modifications for the D-mesons in nucleus-nucleus collisions at FAIR (GSI) will be dominantly on the width and not, as previously expected, on the mass.

In-Medium Effects on Phase Space Distributions of Antikaons Measured in Proton-Nucleus Collisions

Differential production cross sections of K+/- mesons have been measured in p + C and p + Au collisions at 1.6, 2.5, and 3.5 GeV proton beam energy. At beam energies close to the production threshold, the K- multiplicity is strongly enhanced with respect to proton-proton collisions. According to microscopic transport calculations, this enhancement is caused by two effects: the strangeness exchange reaction NY --> K- NN and an attractive in-medium K- N potential at saturation density.

Modelling nuclear effects in neutrino scattering

We have developed a model to describe the interactions of neutrinos with nucleons and nuclei via charged and neutral currents, focusing on the region of the quasielastic and Δ ( 1232 ) peaks. For ν N collisions a fully relativistic formalism is used. The extension to finite nuclei has been done in the framework of a coupled-channel BUU transport model where we have studied exclusive channels taking into account in-medium effects and final state interactions.

Attenuation of ϕ mesons in γA reactions

We present a theoretical analysis of inclusive photoproduction of ϕ mesons in nuclei. In particular the dependence of the total ϕ meson yield on the target mass number is investigated. The calculations are done using the semi-classical BUU transport approach that combines the initial state interaction of the incoming photon with the coupled-channel dynamics of the final state particles. The conditions of the calculations are chosen such as to match the set-up of a recent experiment performed at SPring8/Osaka. Whereas the observables prove to be rather sensitive to the ϕ self energy in the medium, the attribution of deviations from the standard scenario to a particular in-medium effect seems to be impossible.

In-medium effects on electromagnetic probes

We discuss some of the aspects of the physics of relativistic nuclear collisions, in particular those having to do with the observation of electromagnetic radiation. We concentrate on what such measurements tell us about the local, in-medium properties of the environment from which they emerge. The contribution from different sources are considered: that from the partonic sector of QCD, and that from the confined hadronic phase. Specifically, we discuss the observation of real photons and of lepton pairs at the SPS and at RHIC, and make predictions for the LHC. The role of jets is discussed.

The properties of the D meson in dense matter

We study the D meson spectral density in dense matter in the framework of a coupled-channel self-consistent calculation taking, as bare meson–baryon interaction, a separable potential. Our coupled-channel model generates dynamically the Λc(2593) resonance. The medium modifications of the D meson properties due to Pauli blocking and the dressing of D mesons, nucleons and pions are also discussed. We found that the coupled-channel effects in the self-consistent process reduce the in-medium effects on the D meson compared to previous works.