J. Carroll

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}

p-/sup 4/He elastic scattering at 1. 05 GeV. [Differential cross sections, interference]

The elastic differential cross sections of 7.0-GeV/c /sup 4/He scattered from hydrogen (equivalent T/sub p/ = 1.05 GeV) has been measured over a range 0.17 < or = - t < or = 0.85(GeV/c)/sup 2/. The experiment has very low background (< or = 2%), adequate t resolution (< or = 5%), a good absolute normalization (15%), and a statistical accuracy in the interference region of about 3%. The data show a shallow first minimum and a shape very similar to the recent data from Centres dEtudes Nucleaires de Saclay.

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.

Evidence for the noneikonal effects in the tensor and vector analyzing powers in p-d elastic scattering at 800 MeV☆

Abstract Deuteron tensor and vector analyzing powers have been measured for the first time in p- d elastic scattering at 800 MeV proton laboratory energy for a - t range between 0.032 and 1.038 (GeV/ c ) 2 . The data provide a very sensitive test of the multiple scattering theory at intermediate energies. The inclusion of the noneikonal correction in the Glauber diffraction theory is essential in the description of the data, especially for -t ⪆0.2 (GeV/ c 2 ), where discrepancies of up to 150% are observed with the theory based on the eikonal approximation.

Elastic scattering of 2 GeV polarized deuterons from hydrogen

Abstract Large deuteron vector and tensor asymmetries have been measured for the first time at imtermediate energies. We have used a 2 GeV polarized deuteron beam whose tensor and vector components were 0.75 and 0.25, respectively. The tensor and vector quantities P yy and P yy were extracted from data obtained with the beam polarization normal to the scattering plane, and P xx was obtained when the polarization was precessed into the scattering plane. Analysis of the data using multiple scattering theory demonstrates the considerable sensitivity of the data to the quadrupole form factor of the deuteron and to the double spin flip components of the NN amplitudes.

Latest results on “subthreshold” production of kaons and antiprotons

Abstract We present our recent results on “subthreshold” production of K − , K + , and antiprotons in relativistic nuclear collisions. At a NN center-of-mass (cm) angle of 0° we have measured: the inclusive cross sections for production of K − from Si+Si collisions at incident energies of 1.0–2.1 A GeV, and from C+C, and Ca+Ca at 2.1 A GeV; as well as the yield of K + from Si+Si at 1.26 A GeV. At a cm angle of 90° we have measured the inclusive cross section for production of K − in Si+Si collisions at 2.1 A GeV. The systematics of the excitation function, the mass dependence, and the angular dependence are compared with similar data on sub-threshold pion yields. The present status of the five antiproton candidate events is discussed.

Pion double charge exchange on 3He and 4He

Abstract We have measured the differential cross section d 2 σ/ d Ω d T for the double charge exchange reactions 4He(π+, π−)4p and 3He(π−, π+)3n at 140, 200 and 295 MeV. By fitting this data with a simple phase-space model we have estimated the integrated cross section d σ/ d Ω and the total cross section. The 4He data agree qualitatively with the predictions of a meson exchange calculation and with one version of the successive single charge exchange model. The 3He cross sections are considerably larger than those of 4He and in some cases the missing mass plot shows a strong enhancement, which resembles a broad three-nucleon resonance.

Neutral-pion production in central collisions of relativistic 12C and 40Ar on 208Pb☆

Abstract The production of π0 mesons in central collisions of 12C and 40Ar on 208Pb has been examined at a number of energies from 1. to 2.1 GeV/N, as part of the first systematic study as a function of beam energy and charged-particle multiplicity of the rate of neutral-pion production in relativistic heavy-ion collisions. For comparison, corresponding π± data acquired with the same detector system have also been presented. The results indicate that, for a given target-projectile combination, both neutral- and charged-pion production increase smoothly by a factor of approximately 2.9 in this energy range. The measurement of charged-particle multiplicity is discussed in order to indicate how the characteristic distributions observed in this study were used to parameterize the experimental results. The average multiplicity of both pions and nucleons projected from these data is found to disagree with statistical models of particle production. The significance of the Nπ±/Ncharge and Nπ±/Nπ0 ratios observed as a function of beam energy and charged-particle multiplicity is discussed in the context of understanding the nature of the collision process.

Particle-gamma coincidence measurements in 12C + 12C and 12C + Pb collisions at 2.1 GeV/nucleon incident energy

Abstract A particle-gamma coincidence experiment has been performed with a 2.1 GeV per nucleon 12 C beam from the Bevalac. Data were taken with C and Pb targets. The γ-ray spectra are almost independent of the energy or the kind of charged particles detected in coincidence, mainly protons and deuterons. These γ-ray spectra are interpreted as resulting from π 0 decay, and are consistent with known π 0 production rates. A search for a possible decay of singly-charged anomalons into a gamma and a deuteron (or unbound proton-neutron system) has been done by studying the γp and γd invariant mass distributions. The upper limits for such a process are found to be 2 to 20% of the deuteron production rate, for anomalon masses for 200 to 400 MeV above the deuteron mass, with an anomalon mean lifetime of up to 10 −9 s, depending on which kind of decay process is considered.