Search

It is demonstrated that the presence of a phase transition in heavy-ion collisions, at beam energies that probe dense quantum chromodynamics (QCD) matter, leads to a significant enhancement of the dilepton yield at low invariant mass and requires the embedding of low beam energies per produced pion due to the extended emission time. In addition, the temperature of low-mass dileptons shows a modest decrease due to the mixed phase. The emission of dileptons in the SIS18-SIS100 beam energy range is studied by augmenting the ultra-relativistic quantum molecular dynamics (UrQMD) transport model with a realistic density-dependent equation of state, as well as two different phase transitions. This is achieved by extending the molecular dynamics interaction part of the UrQMD model to a density-dependent interaction potential with a high-density minimum, leading to a phase transition and metastable coexisting high-density states. Together with a high-precision measurement, these simulations will be able to constrain the existence of a phase transition in QCD up to densities of several times nuclear saturation density.

Accounting for the effects of local parity nonconservation in a strongly interacting medium is implemented within the framework of the Monte Carlo model. Predictions are obtained for the distributions of the invariant masses of di-muons and di-electrons from the decays of light vector mesons in Pb–Pb collisions at the LHC energy, taking into account the resolution of the detecting systems. The influence of fluctuations of the axial chemical potential is estimated. [ABSTRACT FROM AUTHOR]

Results of searching for narrow heavy resonances on the basis of their decays into a pair of leptons in the CMS experiment at the LHC are reported. The 2016–2018 LHC RUN2 data with the integrated luminosity of fb obtained in proton–proton collisions at TeV are used. Limits are imposed on the cross sections for production of new neutral gauge bosons. [ABSTRACT FROM AUTHOR]

We compute the dilepton production rate from deconfined phase of quark-gluon plasma using the quasi-particle model (QPM). The computation is based on an extension of our previous work. In this model, the finite quark mass is replaced by the effective quark mass which is considered as the sum of square of current mass, coupling of thermal and current mass and square of thermal mass. This leads to the modest enhancement in the dilepton production rate. The results indicate that the emission rate is also an increasing function of chemical potential. The study implies that the model results are significantly enhanced with the effective quark mass as compared to thermal quark mass in the low mass region and not altered in the intermediate mass region. So the model using the effective quark mass fits well in the production of dilepton rates. [ABSTRACT FROM AUTHOR]

It is established that a strong magnetic field is generated along with quark–gluon plasma in heavy-ion collision. This unique scenario offers an opportunity to study and analyze the impact of the magnetic field on the evolution of the plasma. We calculate the dilepton yield from quark–gluon plasma in a magnetic environment by considering a suitably modified magnetized effective quark mass (MEQM). Further, we study the dilepton yield for different values of magnetic field and different values of chemical potential with MEQM. The results obtained are very encouraging and we compare it with recently reported theoretical results. [ABSTRACT FROM AUTHOR]

A search for narrow resonances in dielectron and dimuon invariant mass spectra has been performed using data obtained from proton–proton collisions at s=13 TeV collected with the CMS detector. The integrated luminosity for the dielectron sample is 2.7 fb−1 and for the dimuon sample 2.9 fb−1. The sensitivity of the search is increased by combining these data with a previously analyzed set of data obtained at s=8 TeV and corresponding to a luminosity of 20 fb−1. No evidence for non-standard-model physics is found, either in the 13 TeV data set alone, or in the combined data set. Upper limits on the product of production cross section and branching fraction have also been calculated in a model-independent manner to enable interpretation in models predicting a narrow dielectron or dimuon resonance structure. Limits are set on the masses of hypothetical particles that could appear in new-physics scenarios. For the ZSSM′ particle, which arises in the sequential standard model, and for the superstring inspired Zψ′ particle, 95% confidence level lower mass limits for the combined data sets and combined channels are found to be 3.37 and 2.82 TeV, respectively. The corresponding limits for the lightest Kaluza–Klein graviton arising in the Randall–Sundrum model of extra dimensions with coupling parameters 0.01 and 0.10 are 1.46 and 3.11 TeV, respectively. These results significantly exceed the limits based on the 8 TeV LHC data.

We investigate the contribution of partonic interactions between incident nucleons and produced mesons to the dilepton continuum in heavy-ion reactions. A schematic model is employed in order to calculate the resulting Drell–Yan yield as a function of collision energy and for different scenarios concerning the formation of hadrons. The secondary Drell–Yan process is found to be significant for invariant masses m ≈ 1. 5 GeV in central A + A collisions around s NN ≈ 1 5 − 2 0 GeV, but less so at much lower or much higher energies. Secondary Drell–Yan dileptons should be observable in experiments due to their distinct rapidity dependence as a characteristic deviation from the well-known (primary) Drell–Yan contribution due to N + N interactions. The predicted strong dependence of the secondary Drell–Yan contribution on the nuclear collision energy can be used to experimentally probe the effective hadronic formation time of secondaries. [ABSTRACT FROM AUTHOR]

Abstract Dilepton spectra are a classic probe to study ultra-relativistic heavy ion collisions. At RHIC energies, the dimuon continuum is dominated by correlated pairs from semi-leptonic decays of charm and bottom hadrons and the Drell-Yan process. The dimuon spectra contain information on heavy flavor angular correlations, which can constrain the relative contributions from different heavy flavor production mechanisms. Studying heavy flavor correlations in p +Au collisions may provide further insight on cold nuclear matter effects. Measurements of the Drell-Yan cross-section can provide constraints to PDFs, as well as further our understanding in initial state interactions in p +Au collisions. In this talk, we report measurements of μμ pairs from charm, bottom, and Drell-Yan in p + p and p +Au collisions at s N N = 200 GeV. A further shape analysis is applied to the heavy flavor pair correlations to extract the relative contributions to heavy flavor production mechanisms. [ABSTRACT FROM AUTHOR]

Dilepton and direct photon measurements in heavy ion collisions provide access to the early stages of the collision. Dilepton continuum measurements at SPS and RHIC show an excess consistent with broadening of the rho meson spectral function as a result of hadronic interactions. Direct photon measurements at RHIC and the LHC show an excess of low p T photons over the expected yield. This excess has a large azimuthal anisotropy with respect to the collision plane. Understanding the direct photon excess and its anisotropy has become an outstanding question in the field of heavy ion physics. This proceeding consists of a summary of new dilepton and direct photon experimental results shown at the 2017 International Conference on Ultrarelativistic Nucleus-Nucleus Collisions and presents them in the context of our current understanding of these electromagentic probes. [ABSTRACT FROM AUTHOR]

The project NICA (Nuclotron-based Ion Collider fAcility) is aimed to study hot and baryon rich QCD matter in heavy ion collisions in the energy range up to s NN = 11 GeV . The heavy ion program includes a study of collective phenomena, dilepton, hyperon and hypernuclei production under extreme conditions of highest baryonic density. This program will be performed at a fixed target experiment BM@N and with MPD detector at the NICA collider. [ABSTRACT FROM AUTHOR]

Recent theory developments in electromagnetic radiation from relativistic heavy-ion collisions are reviewed. Electromagnetic observables can serve as a thermometer, a viscometer, and tomographic probes to the collision system. The current status of the “direct photon flow puzzle” is highlighted. [ABSTRACT FROM AUTHOR]

The NICA (Nuclotron-based Ion Collider fAcility) project is under realization at the Joint Institute for Nuclear Research (JINR, Dubna). The main goal of the project is a study of hot and dense strongly interacting matter in heavy ion collisions (up to Au) in the energy range up to s N N = 11 GeV . Two modes of operation are foreseen, collider and extracted beam operations, with two detectors: MPD and BM@N. In the Au + Au collider mode the expected average luminosity is L = 10 27 cm − 2 s − 1 . The proposed experimental program allows one to search for possible manifestations of the phase transitions and critical phenomena. [ABSTRACT FROM AUTHOR]

In this review we address the dynamics of relativistic heavy-ion reactions and in particular the information obtained from electromagnetic probes that stem from the partonic and hadronic phases. The out-of-equilibrium description of strongly interacting relativistic fields is based on the theory of Kadanoff and Baym. For the modeling of the partonic phase we introduce an effective dynamical quasiparticle model (DQPM) for QCD in equilibrium . In the DQPM, the widths and masses of the dynamical quasiparticles are controlled by transport coefficients that can be compared to the corresponding quantities from lattice QCD. The resulting off-shell transport approach is denoted by Parton–Hadron–String Dynamics (PHSD) and includes covariant dynamical transition rates for hadronization and keeps track of the hadronic interactions in the final phase. It is shown that the PHSD captures the bulk dynamics of heavy-ion collisions from lower SPS to LHC energies and thus provides a solid basis for the evaluation of the electromagnetic emissivity, which is calculated on the basis of the same dynamical parton propagators that are employed for the dynamical evolution of the partonic system. The production of direct photons in elementary processes and heavy-ion reactions is discussed and the present status of the photon v 2 “puzzle”–a large elliptic flow v 2 of the direct photons experimentally observed in heavy-ion collisions–is addressed for nucleus–nucleus reactions at RHIC and LHC energies. The role of hadronic and partonic sources for the photon spectra and the flow coefficients v 2 and v 3 is considered as well as the possibility to subtract the QGP signal from the experimental observables. Furthermore, the production of e + e − or μ + μ − pairs in elementary processes and A + A reactions is addressed. The calculations within the PHSD from SIS to LHC energies show an increase of the low mass dilepton yield essentially due to the in-medium modification of the ρ -meson and at the lowest energy also due to a multiple regeneration of Δ -resonances. Furthermore, pronounced traces of the partonic degrees-of-freedom are found in the intermediate dilepton mass regime ( 1.2 GeV < M < 3 GeV ) at relativistic energies, which will also shed light on the nature of the very early degrees-of-freedom in nucleus–nucleus collisions. [ABSTRACT FROM AUTHOR]

The spectrum of dileptons produced by the quark-gluon plasma in an ultrarelativistic nucleus-nucleus collision depends only, to a good approximation, on the transverse mass M t of the dilepton. This scaling is exact as long as transverse flow is negligible, and the system is in local thermal equilibrium. We implement a state-of-the-art modelization of kinetic and chemical equilibration in the early stages of the evolution to study the modifications of the spectrum. Violations of M t scaling resulting from these effects are evaluated as a function of the shear viscosity to entropy ratio (η / s) that controls the equilibration time. We determine the dependence of the spectrum on system size, centrality, rapidity, and collision energy. We show that the quark-gluon plasma produces more dileptons than the Drell-Yan process up to invariant masses of order M ∼ 4 GeV at LHC energies. Due to different kinematics, for a given M t , the dependence of the dilepton yield on M is opposite for the two processes, so that experiment alone can in principle determine which process dominates. [ABSTRACT FROM AUTHOR]

We review the current state of research on electromagnetic probes in the context of heavy-ion collisions. The focus is on thermal photons and dileptons which provide unique insights into the properties of the created hot and dense matter. This review is intended to provide an introductory overview of the topic as well as a discussion of recent theoretical and experimental results. In particular, we discuss the role of vector-meson spectral functions in the calculation of photon and dilepton rates and present recent results obtained from different frameworks. Furthermore, we will highlight the special role of photons and dileptons to provide information on observables such as the temperature, the lifetime, the polarization and the electrical conductivity of the produced medium as well as their use to learn about chiral symmetry restoration and phase transitions. [ABSTRACT FROM AUTHOR]

The nuclear matter under extreme conditions of temperatures ( T) and baryonic densities ( n) undergoes a phase transition to quark gluon plasma (QGP). It is expected that such extreme conditions can be achieved by colliding nuclei at ultrarelativistic energies. In the present review, the suitability of photons and dileptons as diagnostic tools of QGP has been discussed. The photon and dilepton spectra originating from heavy-ion collisions at LHC energies have been explicitly displayed in this article. Results from SPS and RHIC have been discussed adequately with appropriate references. The role of single electron spectra originating from the decays of heavy flavoured mesons on QGP detection has also been discussed briefly. [ABSTRACT FROM AUTHOR]

The production of low-mass dileptons, including those from ρ , ω and ϕ light vector meson decays, provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions. In particular, strangeness production can be studied through ϕ meson measurements, while the detailed description of the full dilepton mass spectra down to the kinematic threshold can be used to reveal in-medium modifications of hadron properties and the thermal emission arising from the medium. Measurements in pp and p–A, respectively, provide a reference for the observations in heavy-ion collisions and give insight on soft particle production in cold nuclear matter. Dilepton production is studied with the ALICE apparatus at the LHC both at central ( | y | < 0.9 ) and forward ( 2.5 < y < 4 ) rapidity, respectively in the dielectron and dimuon channel. In this contribution, we present recent results on low-mass dimuon production at forward rapidity in Pb–Pb and p–Pb collisions. [ABSTRACT FROM AUTHOR]

The Compressed Baryonic Matter (CBM) experiment at the upcoming Facility for Antiproton and Ion Research (FAIR) will explore the phase diagram of nuclear matter at very high net-baryon densities and moderate temperatures in nucleus–nucleus collisions at beam energies up to 45 A GeV . One of the key diagnostic probes is electromagnetic radiation. In order to minimize systematic errors of this challenging measurement, CBM aims at an investigation of dielectron and dimuon pairs in the full mass range from the photon point up to charmonium. The results of performance studies for dilepton measurements and the status of the detector developments are presented. [ABSTRACT FROM AUTHOR]

High-energy collisions of various nuclei, so called 'Little Bangs' are observed in various experiments of heavy ion colliders. The time evolution of the strongly interacting quark-gluon plasma created in heavy ion collisions can be described by hydrodynamical models. After expansion and cooling, the hadrons are created in a freeze-out. Their distribution describes the final state of this medium. To investigate the time evolution one needs to analyze penetrating probes, such as direct photon or dilepton observables, as these particles are created throughout the evolution of the medium. In this paper we analyze an 1+3 dimensional analytic solution of relativistic hydrodynamics, and we calculate dilepton transverse momentum and invariant mass distributions. We investigate the dependence of dilepton production on time evolution parameters, such as emission duration and equation of state. Using parameters from earlier fits of this model to photon and hadron spectra, we compare our calculations to measurements as well. The most important feature of this work is that dilepton observables are calculated from an exact, analytic, 1+3D solution of relativistic hydrodynamics that is also compatible with hadronic and direct photon observables. [ABSTRACT FROM AUTHOR]

Abstract: We review the current state of photon and dilepton measurements at RHIC, emphasizing that of the theoretical work seeking to interpret them. We highlight the progress made recently in the modelling of relativistic nuclear collisions, and explore the effect on electromagnetic observables. Some outstanding puzzles are presented. [Copyright &y& Elsevier]

Abstract: A search for narrow, high-mass resonances decaying to electron or muon pairs has been performed using pp collision data collected at by the CMS experiment in 2011. The data sample corresponds to an integrated luminosity of approximately 5 fb−1. The event yields observed in the signal regions are consistent with predictions of the standard model backgrounds, and upper limits on the cross section times branching fraction for a resonance decaying to dileptons are extracted from a shape analysis of the dilepton invariant mass distribution. The resulting mass limits at 95% confidence level are 2330 GeV for the in the Sequential Standard Model, 2000 GeV for the superstring-inspired resonance, 890 (540) GeV for the Stueckelberg extension with the mass parameter (0.04), and 2140 (1810) GeV for Kaluza–Klein gravitons with the coupling parameter of 0.10 (0.05). These limits are the most stringent to date. [Copyright &y& Elsevier]

Abstract: We review the basic concepts and tools to study hadronic properties in the medium. Beginning with a discussion of spontaneous breaking of chiral symmetry of QCD in vacuum and its possible restoration at high temperature/density, we introduce current correlation functions and outline methods of constraining them through sum rules. This is followed by a discussion on the evaluation of the vector meson self-energy using effective Lagrangians in the framework of thermal field theory. [Copyright &y& Elsevier]

Abstract: It has been shown that the evolution of collectivity in ultra-relativistic heavy ion collision is manifested in the variation of various HBT radii with invariant mass (M) extracted from the correlation functions of two lepton pairs. The value of the radial velocity () can be estimated from the ratio of the distributions of single photons to lepton pairs for various M windows. It has been argued that the variation of radial flow with appropriate kinematic variables can be used as an indicator of a phase transition from initially produced partons to hadrons. We also consider the elliptic flow () of the matter as probed by the single electron spectra originating from the semileptonic decays of heavy mesons. The measured values of and the nuclear suppression factor () at RHIC energy have been reproduced simultaneously by including both the collisional and radiative processes within the scope of perturbative quantum chromodynamics. The and have been predicted for LHC energy. [Copyright &y& Elsevier]

Abstract: These proceedings summarize the di-electron spectrum measurements by PHENIX in and collisions at , along with a comparison to the expectations from hadronic sources and various model predictions. An overview of the latest PHENIX results on open heavy flavor and quarkonia production, measured through the electron and muon channels at mid-rapidity and forward/backward rapidity for , and collisions is also presented. [Copyright &y& Elsevier]

We report on ω mesons' production via radiative decay mode ( ω → π γ, π → 2 γ) in $\sqrt s $ = 200 GeV Au+Au collisions at RHIC-PHENIX. The main difficulty of this analysis is the huge combinatorial background inevitable for reconstructing particles from the three- body decay mode. The emphasis is on the feasibility study using both simulation and real data to search the best parameters that improve $S/\sqrt B $ . For this purpose, we examine several sets of energy and momentum cuts for selecting π and γ. To evaluate the remaining background, we consider three background sources; correlated/uncorrelated background and K contribution, then we subtract them. The amount of these contributions is determined by a fit on a mass spectrum. The result suggests that ω meson production is suppressed in central Au+Au collisions at high pT region. [ABSTRACT FROM AUTHOR]

Abstract: We consider the radiation of photons from quarks scattering on color-magnetic monopoles in the Quark-Gluon Plasma. The calculation is performed in the classical, non-relativistic approximation and results are compared to photon emission from Coulomb scattering of quarks, known to provide a significant contribution to the photon emission rates from the QGP. The present study is a first step towards understanding whether this scattering process can give a sizable contribution to dilepton production in heavy-ion collisions. [Copyright &y& Elsevier]

Abstract: The NA60 experiment has studied low-mass muon pair production in proton-nucleus collisions with a system of Be, Cu, In, W, Pb and U targets using a 400 GeV/c proton beam at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons η, ρ, ω, and ϕ new measurement of the electromagnetic transition form factors of the η and ω was performed. The values found are and . They agree with the previous available measurements, improving their uncertainty thanks to the higher statistics, and confirm the discrepancy with the prediction of the Vector Meson Dominance model in the case of the electromagnetic form factor of the ω meson. The comparison of the data to an improved calculation including also a direct point-like term is discussed. The nuclear dependence of the production cross sections of the ω, ρ and ϕ mesons was also studied. An increase of the ϕ production with respect to the ω is observed from p-Be to p-U collisions. Parameterizing the nuclear dependence of the production cross sections with the power law , one finds and . A possible bias induced by the use of the light Be target is found. If it is excluded, the results are and , showing an increase of and a decrease of . [Copyright &y& Elsevier]

Abstract: We report measurements of electron pair production in elementary and reactions at with the HADES spectrometer. For the first time, the electron pairs were reconstructed for reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass is about an order of magnitude larger in reactions as compared to . A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in reactions are compatible with a superposition of elementary and collisions, leaving little room for additional electron pair sources in such light collision systems. [Copyright &y& Elsevier]

Abstract: We derive the forward photoproduction amplitude for the diffractive reaction in the momentum space, within the formalism of -factorization. Predictions for the reaction are given using unintegrated gluon distribution from the literature. We calculate the total cross section as a function of photon–proton center of mass energy and the invariant mass distribution of the lepton pair. We also discuss whether the production of timelike virtual photons can be approximated by continuing to the spacelike domain . The present calculation provides an input for future predictions for exclusive hadroproduction in the reaction. [Copyright &y& Elsevier]

Abstract: Recent results on direct photons and dileptons from the PHENIX experiment opened up a possibility of landscaping electro-magnetic radiation over various kinetic energies in heavy ion collisions. Results on direct photon measurement in Au+Au collisions at are discussed from the point of view of structure function and isospin effect. It is supported by the first measurement of direct photons at in the same collisional system. [Copyright &y& Elsevier]

This paper addresses the theoretical analysis of dilepton spectra in C+C collisions at GSI-SIS energies. Theoretical predictions for the recent data of the HADES Collaboration at SIS energies are made with the help of a hadron-string transport model, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. A mass shift of the ρ meson due to kinematical effects is discussed. [ABSTRACT FROM AUTHOR]

Direct photon production in Au+Au collisions at √ s NN =200 GeV has been measured in the p Trange of 1< p T<5 GeV/ c. The yield as a function of centrality is in agreement with published data of the RHIC 2002 run, and with results from a new method that explores very low mass dileptons. The result is compared to several theoretical calculations, and it is found that the measurement is not inconsistent with calculations including thermal photon contributions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The exclusive photoproduction of pairs from nucleons close to the vector meson production threshold () results from two main processes: the emission of Bethe–Heitler pairs and the photoproduction of - and ω-mesons decaying into pairs. The Bethe–Heitler amplitudes are purely electromagnetic and reflect mostly the nucleon magnetic structure. The amplitudes arising from vector meson production and decay are derived from and amplitudes supplemented by the vector meson dominance assumption. The vector meson photoproduction amplitudes are calculated using a relativistic and unitary coupled-channel approach to meson–nucleon scattering. They depend sensitively on the coupling of vector fields to baryon resonances. The differential cross sections display interference patterns. The interference of Bethe–Heitler pair production with vector meson decay is quite small in the domain of validity of our model for all angles of the emitted pair. The interference of - and ω-mesons in the channel can be large. It is constructive for the reaction and destructive for the reaction. We discuss the shape and magnitude of the pair spectra produced in the and reactions as functions of the pair emission angle and of the total center of mass energy . In the particular kinematics under consideration, our results suggest that the vector meson contribution can be determined quite accurately from experimental spectra by subtracting the Bethe–Heitler term and neglecting the small interference of Bethe–Heitler pairs with vector meson decays. [Copyright &y& Elsevier]

The identification of the light mesons in the HADES spectrometer is based on an invariant mass reconstruction of their decay into e+e- pairs. In the dilepton (e+e-) signal reconstruction, particularly important is the reduction of a huge combinatorial background arising from wrong combinations of e+ and e- to unlike sign pairs. Methods of the dilepton signal and combinatorial background reconstruction in the HADES spectrometer will be presented. [ABSTRACT FROM AUTHOR]

Abstract: We consider a hard jet production tagged by a muon pair in ultrarelativistic heavy ion collisions. The process cross section is calculated by the CompHEP Monte Carlo generator taking into account full interference pattern at LHC energies. We have found that reasonable statistics, ∼1000 events per 1 month of LHC run with lead beams, can be expected for realistic geometrical acceptance and kinematic cuts. The transverse momentum imbalance due to interactions of jet partons in the medium is evaluated for pair + jet correlation, as well as for the correlation between pair and a leading particle in a jet. Theoretical and experimental uncertainties of these observables are discussed. [Copyright &y& Elsevier]

We first briefly review the theoretical basis for calculations of changes of hadronic properties in dense nuclear matter. These changes have usually been investigated by means of relativistic heavy-ion reactions. Here we discuss that observable consequences of such changes can also be seen in more elementary reactions on nuclei. Particular emphasis is put on a discussion of actual observables in photonuclear reactions; we discuss in detail η- and vector-meson production. We show that photoproduction of η’s can yield essential information on in-medium properties of the S11(1535) resonance while the φ meson properties will probably not be accessible through the K+K- decay channel. However, for ω mesons the π0γ decay channel, due to its reduced final state interaction, looks more promising in this respect. Completely free of final state interactions is dilepton production in the few GeV range. We show that the sensitivity of this decay channel to changes of hadronic properties in medium in photonuclear reactions on nuclei is as large as in ultrarelativistic heavy-ion collisions. Finally we discuss the fact that hadron production in nuclei at 10–20 GeV photon energies can give important information on the hadronization process. [Copyright &y& Elsevier]

The π− p→ e + e − n and π+ n→ e + e − p reaction cross sections are calculated below and in the vicinity of the vector-meson (ϱ0,ω) production threshold. These processes are largely responsible for the emission of e +e− pairs in pion-nucleus reactions and contribute to the dilepton spectra observed in relativistic heavy ion collisions. They are dominated by the decay of low-lying baryon resonances into vector-meson-nucleon channels. The vector mesons materialize subsequently into e + e− pairs. Using π N→ϱ0 N and π N→ω N, amplitudes calculated in the center of mass energy interval 1.4 < √ s<1.8 GeV, we compute the π− p→ e + e − n and π+ n→ e + e − p reaction cross sections in these kinematics. Below the vector-meson production threshold, the π0−ω interference in the e + e− channel appears largely destructive for the π− p→ e + e − n cross section and constructive for the π+ n→ e + e − p cross section. The pion beam and the HADES detector at GSI offer a unique possibility to measure these effects. Such data would provide strong constraints on the coupling of vector-meson-nucleon channels to low-lying baryon resonances. [ABSTRACT FROM AUTHOR]

The study of the πN→ρ0N and πN→ωN amplitudes below and close to the vector meson production threshold (1.4<√ of s<1.8 GeV) reveals a rich structure arising from the presence of baryon resonances in this energy range. These resonances are reflected in the interference pattern of the e+e− decays of the ρ0- and ω-mesons produced in π−p and π+n reactions. We discuss the shape and magnitude of the ρ0–ω interference in the π−p→e+e−n and π+n→e+e−p reaction cross sections as functions of the total center of mass energy s<1.8 GeV) reveals a rich structure arising from the presence of baryon resonances in this energy range. These resonances are reflected in the interference pattern of the e+e− decays of the ρ0- and ω-mesons produced in π−p and π+n reactions. We discuss the shape and magnitude of the ρ0–ω interference in the π−p→e+e−n and π+n→e+e−p reaction cross sections as functions of the total center of mass energy √ of s. We find contrasted results: the interference is largely destructive for the π−p→e+e−n cross section but constructive for the π+n→e+e−p cross section. An experimental study of these reactions would provide significant constraints on the coupling of vector meson–nucleon channels to low-lying baryon resonances. [Copyright &y& Elsevier]