R. Welsh

Dielectron cross-section measurements in nucleus-nucleus reactions at 1-A/GeV

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 dilepton spectrometer (DLS) nucleus-nucleus data. For pair mass M{le}0.35 GeV/c{sup 2} we obtain (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 {pi}{sup 0} and {eta} Dalitz decays, and (3) d{sigma}/dM{proportional_to}A{sub P}A{sub T}. For M{gt}0.5 GeV/c{sup 2} the Ca+Ca to C+C cross section ratio is significantly larger than the ratio of A{sub P}A{sub T} values. {copyright} {ital 1997} {ital The American Physical Society}

Dielectron production in Ca+Ca collisions at 1.0 and 2.0 A GeV

Abstract Dileptons should be a good probe of the early stage of nucleus-nucleus collisions and may carry information on pion dynamics in nuclear matter. We have observed a dielectron signal well above estimated contributions from Dalitz decays of particles or resonances. At 1.0 A GeV, the dielectron mass spectrum is significantly different for Ca+Ca and p+ Be reactions. The dielectron yield scales as the first power of the product of the projectile and target masses.

The dilepton spectrometer

Abstract The dilepton spectrometer (DLS) at Lawrence Berkeley Laboratorys Bevalac has been designed and constructed to investigate the production of electron-positron pairs with low mass and low transverse momentum in proton-nucleus and nucleus-nucleus collisions for incident-beam kinetic energies of 5 A GeV and less. This article briefly recalls the physics objectives of the program, discusses the methodology of the measurement, presents details of the design of the spectrometer and the detector elements, and reports on their performance. Selected experimental results are given to illustrate the capability of the DLS and to demonstrate the level to which it is possible to realize the physics objectives with the spectrometer.

Relative dielectron yields in p + p and p + d interactions from Ebeam = 1.0–4.9 GeV

Abstract The beam energy and invariant mass dependence of the dielectron yield in p + d interactions relative to the yield in p + p interactions is presented for incident kinetic energies from 1.0–4.9 GeV. The ratio of the yield in p + d interactions to that in p + p interactions decreases from 10.5±1.6 at 1.0 GeV to 1.96±0.08 at 4.9 GeV for electron pairs with invariant masses ⩾ 0.15 GeV/ c 2 . The large ratio at 1.0 GeV suggests that dielectron production in the p + d system is dominated by a p + n process. The beam energy dependence of the ratio indicates that this p + n contribution decreases with respect to the other dielectron sources as the incident energy is increased.

Antiproton production at 0° in collisions of 11A GeV/c Au beams on Au, Cu and Al targets

Abstract We have studied the production of antiprotons in Au+nucleus collisions at 11A GeV/c in experiment 878 at the BNL-AGS facility. We present rapidity distributions at p t =0 for antiprotons. We discuss how these distributions change as a function of target and centrality, and compare our measured cross sections to the predictions of the cascade models ARC and RQMD.

Dielectron yields in p+d and p+p collisions at 4.9 GeV

Abstract The dielectron yield in p + d and p + p collisions at a beam kinetic energy of 4.9 GeV has been measured using the Dilepton Spectrometer (DLS) at the Bevalac. The measured ratio of the yield in p + d to that in p + p collisions, 1.92±0.06, is in disagreement with the assumptions of model calculations applied to our ealier p +Be data, where it was found that p + n bremsstrahlung dominated other sources. While the measured ratio is consistent with a hadron-like origin of the dielectrons, the contributions of known hadronic decays are smaller than the measured yield from p + p collissions.

Low-mass dileptons in pA and AA collisions

Abstract The low-mass ( M + e − continuum has not yet been understood even though the large yield has been documented in a number of high-energy hadron-nucleon and hadron-nucleus experiments. In heavy-ion collisions, effects resulting from the presence of the excited nuclear medium are expected. An understanding of dilepton production at excitation energies in the few-GeV range will aid in the interpretation of dilepton production at ultra-relativistic energies, where it may be a probe of the quark-gluon plasma. The objective of the Dilepton Spectrometer (DLS) collaboration at the BEVALAC is to study the production of e + e − pairs in nucleon-nucleon, proton-nucleus and nucleus-nucleus collisions at bombarding kinetic energies between 1 and 5 A · GeV. The transverse momentum distributions, the mass dependence, and the excitation function of dilepton production give information about the production mechanisms.

Dielectron production in p+Be and Ca+Ca collisions at the Bevalac

Abstract Dielectron production has been measured in p+Be collisions at 1.0, 2.1 and 4.9 GeV and in Ca+Ca at 1.0 and 2.0 GeV A . The observation of a structure in the mass cross section at about two times the pion mass suggests that pion annihilation is the dominant production mechanism in p+Be above 2 GeV. Comparison of preliminary results from p+Be and Ca+Ca at 1 GeV A is presented.

Dielectron production in A-A reactions at 1.0 A.GeV.

Abstract Measured dielectron cross sections from Ca+Ca, C+C, He+Ca, and d+Ca reactions at 1.0 A·GeV are presented. The invariant mass spectrum from Ca+Ca collisions is discussed and compared with results from model calculations. The four A-A measurements are used to explore the physics content of the e + e − signal that is not explained by the models.

Dilepton production from p-p to CaCa at the Bevalac

Abstract The DLS collaboration has recently completed a high statistics study of dilepton production at the Bevalac. In particular, we have measured dielectrons (e + e − ) from p - p and p - d collisions to understand the basic dilepton production mechanisms in the energy range from 1.05 – 4.9 GeV. These data can be used to determine the basic processes which contribute to nucleon-nucleon dilepton production such as hadronic bremsstrahlung, vector meson processes, and hadronic Dalitz decay. The data show that a simple elastic bremsstrahlung calculation is insufficient to explain the data. Theoretical models are compared with the data. A new high statistics study of Ca  Ca at 1.05 A·GeV has been made to study the collectivity of A - A collisions.