Dielectron Cross Section Measurements in Nucleus-Nucleus Reactions at1.0AGeV

Abstract

We present measured dielectron production cross sections for Ca+Ca, C+C, He+Ca, and d+Ca reactions at 1.0 A·GeV. Statistical uncertainties and systematic effects are smaller than in previous DLS nucleus-nucleus data. For pair mass M ≤ 0.35 GeV/c 2 : 1) the Ca+Ca cross section is larger than the previous DLS measurement and current model results, 2) the mass spectra suggest large contributions from π 0 and η Dalitz decays, and 3) dσ/dM ∝ A P ·A T . For M > 0.5 GeV/c 2 the Ca+Ca to C+C cross section ratio is significantly larger than the ratio of A P ·A T values. 1Dielectrons produced in heavy-ion collisions are attractive probes for studying dynamical properties of nucleus-nucleus interactions. The e + e − pairs do not undergo significant rescattering in the reaction, thus the kinematics of the pairs retains information about their production. This is of particular interest if the e + e − pairs are produced by processes, such as pion annihilation, that must occur during in the hot, dense phase of the collisions. Use of this probe has produced interesting results at both Bevalac [1] and SPS [2] energies. We present in this letter the latest measurements of dielectron production from the Dilepton Spectrometer (DLS) Collaboration in nucleus-nucleus reactions at a beam kinetic energy of 1.0 A·GeV.The DLS collaboration has previously reported on dielectron production in several colliding systems [1,3,4]. The first generation DLS data from p+Be, Ca+Ca, and Nb+Nb [1,3] reactions provided the first observations of dielectrons produced at Bevalac energies. Early calculations suggested that such data could be dominated by contributions from π + π − annihilation [5,6]. Subsequent models of AA collisions in this energy regime [7][8][9] calculated that e + e − pairs of invariant mass below about 0.4 GeV/c 2 are produced primarily from conventional hadronic sources, such as pn bremsstrahlung and Dalitz decay processes (π 0 , ∆, and η), but that contributions from π + π − annihilation were needed to explain the Ca+Ca data at higher pair masses. Models that focus on density induced changes in the ρ-meson mass provide alternative descriptions of the pair yield at the higher masses [10,11]. Within the limited statistics of the first generation DLS data, it was not possible to distinguish among the models that provided results for specific DLS measurements.After improvements to the DLS apparatus [13][14][15], a second generation of measurements was obtained: first from p+p and p+d reactions at a number of energies [4], and then from the Ca+Ca, C+C, He+Ca, and d+Ca reactions presented in this letter. Each of these data sets contain significantly more pairs than earlier DLS data. To increase our sensitivity to the effects of multiple hadronic interactions (e.g. π + π − annihilation and multi-step resonance excitation), the nucleus-nucleus reactions were chosen to have different numbers of participant nucleons, but identical isospin and similar internal nuclear motion. 2A description of the DLS apparatus has been published [13...

Dielectron production in Ar + KCl collisions at 1.76AGeV

“…Dilepton spectra measured by the CERES [4] and NA60 [5] experiments at CERN-SPS energies (40A-158A GeV) pointed to a significant in-medium modification of the ρ meson spectral function, as signaled by a large additional yield (excess) of lepton pairs in the invariant mass region below the ρ meson pole mass. At the much lower beam energies of 1A-2A GeV the DLS [6] collaboration at the Bevalac observed a similar dielectron excess over the "hadronic cocktail," that is, the cocktail resulting from meson decays in the late (freeze-out) phase of the collision. However, in contrast to the situation at higher energies, for a long time this excess could not be satisfactorily explained by theoretical models and became the so-called "DLS puzzle."…”

“…To do so we have compiled in Fig. 6 the excess yields integrated over the mass region M ee = 0.15-0.5 GeV/c 2 from all available reaction systems [6,8,9]. For comparison, inclusive π 0 and η multiplicities measured in photon calorimetry with the TAPS detector [10,11] are plotted as well.…”

“…4 The somewhat smaller pion multiplicity per A part of Ca + Ca can be attributed to meson 4 We consider here the systems Ar + KCl and Ca + Ca as being equivalent in size and isospin. The excess yield, defined in the mass range M ee = 0.15-0.5 GeV/c 2 and extrapolated to 4π , is shown for HADES Ar + KCl and C + C data (solid triangles) [8,9] as well as for DLS data (open triangles) [6]. The π 0 and η results are from TAPS measurements in C + C (squares, solid curves) and Ca + Ca (circles, long-dashed curves) collisions [10,12].…”

Dielectron Mass Spectra fromAu+AuCollisions atsNN=200 <

“…Dilepton measurements have been a subject of experimental investigations since the early days of heavy ion collisions [7][8][9][10][11][12]. Of particular interest, measurements from CERES and NA60 ( ffiffiffiffiffiffiffi ffi s NN p ¼ 8.75-17.2 GeV) showed a clear enhancement in the mass region below ∼0.7 GeV=c 2 when compared to known hadronic sources.…”

Finite pion width effects on the rho-meson

“…Experimentally this question is studied in measurements of the dilepton production rates in heavy ion collisions. Recent experiments by the CERES and DLS collaborations [1][2][3] show that the low lepton pair mass spectrum is significantly enhanced in the range between 300 MeV and 600 MeV compared to the yield that one expects from the corresponding rates in pp-collisions.…”