Search

During the 1999 lead run, CERES has measured hadron and electron-pair production at 40 A GeV/c beam momentum with the spectrometer upgraded by the addition of a radial TPC. Here the analysis of lambda and antilambda will be presented.

We have for the first time identified e + e − -pairs produced by strongly varying electromagnetic fields in distant SPt collisions at 200 GeV/u projectile energy at the CERN-SPS. The differential cross section for the mass range 10 MeV/ c 2 ≤ m ee ≤ 100 MeV/ c 2 and the polar angle range 141 mrad ≤ θ e ≤ 260 mrad agrees well with a lowest-order perturbation QED-calculation.

Elliptic flow and two-particle azimuthal correlations of charged hadrons and high-p(T) pions (p(T)1 GeV/c) have been measured close to midrapidity in 158A GeV/c Pb+Au collisions by the CERES experiment. Elliptic flow (v(2)) rises linearly with p(T) to a value of about 10% at 2 GeV/c. Beyond p(T) approximately 1.5 GeV/c, the slope decreases considerably, possibly indicating a saturation of v(2) at high p(T). Two-pion azimuthal anisotropies for p(T)1.2 GeV/c exceed the elliptic flow values by about 60% in midcentral collisions. These nonflow contributions are attributed to nearside and back-to-back jetlike correlations, the latter exhibiting centrality dependent broadening.

Energy and angular correlations of protons and α particles were measured in coincidence with fission fragments in the reactions 200 MeV, 254 MeV 37Cl + 124Sn and 318 MeV 28Si + 141Pr. The light-particle yield is dominated by statistical emission from composite nucleus and from fission fragments, but α spectra also display the characteristics of evaporation during scission. We give Coulomb-field induced light-particle emission as a novel and hitherto only consistent explanation of this emission process.

The mechanism of the reaction $^{27}\mathrm{Al}(^{16}\mathrm{O}, ^{12}\mathrm{C}\ensuremath{\alpha})^{27}\mathrm{Al}$ at 65.6-MeV incident energy has been examined with use of coincidence techniques. It is shown that the broad bell-shaped peak observed in $^{12}\mathrm{C}$ singles spectra corresponds to an $\ensuremath{\alpha}$-transfer-like process leading to fixed excitation energy in the $\ensuremath{\alpha}$+$^{27}\mathrm{Al}$ system. The $\ensuremath{\alpha}$-$^{12}\mathrm{C}$ angular correlation, however, is forward-peaked along the momentum-transfer direction, suggesting that the decay time of the system is less than required for equilibration into $^{31}\mathrm{P}$.

Energy and angular correlations between alpha particles and fission-like fragments in the reaction of35Cl+Ag at 10 MeV/n have been measured. The coincident yield is dominated by two mechanisms: sequential emission from both fragments (M α =0.137±0.014) and alpha emission from the composite system prior to scission (M α =0.124±0.012). The observation of nuclear shadowing for sequential emission from the fragments allowed us to estimate the “lifetime” of the 218 MeV excited143Gd composite system as (1–3)×10−20s. It is argued that this rather long time reflects the slow evolution in shape of the composite system and might be termed “sticking time,” since statistical-model lifetimes are typically an order of magnitude shorter. Observed also are small contributions of nonequilibrium emission in the forward direction (M α =0.005±0.001) and a component displaying the characteristics of emission from the neck (M α =0.010±0.002).

One of the basic assumptions of the statistical model of nuclear reactions is that the decay amplitudes to different channels are uncorrelated. This assumption is expected to be no longer valid in the presence of direct reactions. First experimental evidence for channel-channel correlations (CCC) in compound nuclear (CN) scattering has been found from a study of the inelastic scattering of protons at isobaric analogue resonances (IAR)1–4).