Target fragments in collisions of 1.8 GeV/nucleon Fe nuclei with photoemulsion nuclei, and the cascade-evaporation model

Nuclear photographic emulsion is used to study the dependence of the characteristics of target-nucleus fragments on the masses and impact parameters of interacting nuclei. The data obtained are compared in all details with the calculation results made in terms of the Dubna version of the cascade-evaporation model (DCM).     

Transverse energy distributions in Si-nucleus collisions at 10 GeV/nucleon

Transverse energy distributions have been measured for collisions of 10 GeV/nucleon Si with targets of Al, Cu and Pb using a combination of a NaI wall and a uranium based sampling calorimeter. The measured cross sectionsdσ/dE _T anddE _T /dη are consistent with full stopping and an increase, with increasing values ofE _T , of energy flow into large angles.     

The transverse-energy distributions of 32S-nucleus collisions at 200 GeV per nucleon

Transverse-energy distributions have been measured for the collisions of the ³²S nucleus with Al, Ag, W, Pt, Pb, and U target nuclei, at an incident energy of 200 GeV per nucleon. The shapes of these distribution reflect the geometry of the collisions, including the deformation effects. For central collisions, the transverse-energy production in the region −0.1<η_lab<2.9 increases approximately as A^0.5, where A is the atomic mass number of the target. This increase is accompanied by a relative depletion in the forward region η_lab > 2.9. These results are compared with those obtained under similar conditions with incident ¹⁶O nuclei. A comparison is also made with the predictions of a Monte Carlo generator based on the dual parton model. Finally, we give estimates of the energy density reached and its dependence on the atomic mass number of the projectile.     

Measurement of multiplicity distributions in oxygen-tungsten collisions at 200 GeV per nucleon

Multiplicity distributions of charged particles have been measured in the pseudorapidity region 0.9 <η _lab < 4.9 for oxygen-tungsten collisions at an incident energy of 200 GeV per nucleon. The multiplicity cross-sectiondσ/dN as measured in the backward hemisphere (0.9 <η _lab < 2.9) is found to be very similar in shape to the transverse energy distributiondσ/dE _T published previously by this group, reflecting the particular geometry of nucleus-nucleus collisions. Pseudorapidity distributionsdN/dη are presented and their dependence onE _T is discussed. The average transverse energy per charged particle is studied versusE _T and is found to be constant within our systematic error of approximately 10%.     

Evidence for a negative-parity spin-doublet of nucleon resonances at 1.88 GeV

Evidence is reported for two nucleon resonances with spin-parity J^P=1/2⁻$J^P=1/2^{-}$ and J^P=3/2⁻$J^P=3/2^{-}$ at a mass just below 1.9 GeV. The evidence is derived from a coupled-channel analysis of a large number of pion and photo-produced reactions. The two resonances are nearly degenerate in mass with two resonances of the same spin but positive parity. Such parity doublets are predicted in models claiming restoration of chiral symmetry in high-mass excitations of the nucleon. Further examples of spin parity doublets are found in addition. Alternatively, the spin doublet can be interpreted as member of the 56-plet expected in the third excitation band of the nucleon. Implications for the problem of the missing resonances are discussed.     

J/psi production in Indium-Indium collisions at 158 GeV/nucleon

The NA60 experiment studies muon pair production at the CERN Super Proton Synchrotron. In this Letter we report on a precision measurement of J/psi in In-In collisions. We have studied the J/psi centrality distribution, and we have compared it with the one expected if absorption in cold nuclear matter were the only active suppression mechanism. For collisions involving more than approximately 80 participant nucleons, we find that an extra suppression is present. This result is in qualitative agreement with previous Pb-Pb measurements by the NA50 experiment, but no theoretical explanation is presently able to coherently describe both results.     

On the transverse energy distributions in the central rapidity region of 16O+Pb collisions at 200 GeV per nucleon

We point out that the recent data obtained by the NA35 collaboration at the CERN SPS on E_T distributions in ¹⁶O + Pb collisions can be understood within the wounded nucleon model with a simple geometrical calculation of the number of interacting nucleons in ¹⁶O. Although the large E_T trigger increases the average E_T per final state hadron, we are unable to explain in this way the observed rather large value of this quantity.     

Fission and fragmentation of 208Pb nuclei in collisions with gold nuclei at an energy of 158 GeV per nucleon

The fission and fragmentation of ultrarelativistic ²⁰⁸Pb nuclei in collisionswith gold nuclei were studied by using a beam from the SPS accelerator at CERN at an energy of 158 GeV per nucleon. The detectors of the target area of the NA45/CERES spectrometer were used in respective measurements. The value obtained for the fission cross section is 301 ± 44 mb, where about 77% of events stem from the electromagnetic interaction of colliding nuclei, while the remaining part is the contribution of peripheral nuclear interactions. The spallation of lead nuclei that involves the formation of heavy fragments occurs only in collisions where the impact parameter satisfies the condition b > 10 fm. A complete disintegration of lead nuclei to intermediate-mass fragments and light particles is observed in some peripheral collisions.     

Hadronic expansion dynamics in central Pb+Pb collisions at 158 GeV per nucleon

Two-particle correlation functions of negative hadrons over wide phase space, and transverse mass spectra of negative hadrons and deuterons near mid-rapidity have been measured in central Pb+Pb collisions at 158 GeV per nucleon by the NA49 experiment at the CERN SPS. A novel Coulomb correction procedure for the negative two-particle correlations is employed making use of the measured oppositely charged particle correlation. Within an expanding source scenario these results are used to ext ract the dynamic characteristics of the hadronic source, resolving the ambiguities between the temperature and transverse expansion velocity of the source, that are unavoidable when single and two particle spectra are analysed separately. The source shape, the total duration of the source expansion, the duration of particle emission, the freeze-out temperature and the longitudinal and transverse expansion velocities are deduced. Received: 25 November 1997 / Published online: 24 March 1998     

Formation of six-nucleon systems and nuclei in 16Op collisions at a momentum of 3.25 GeV/c per nucleon

The results obtained by studying the production of six-nucleon systems and nuclei that is accompanied by the yield of charged pions in ¹⁶Op collisions at a momentum of 3.25 GeV/c per nucleon are presented. It is shown that the correlation of the yield of mirror ³H (³He) nuclei with the production of π ⁺ (π ^?) mesons is due to the charge-exchange process involving an oxygen-nucleus proton (neutron) knocked out predominantly from an alpha-particle cluster.     

Interactions of 10.6 GeV/nucleon gold nuclei in nuclear emulsion

Stacks of nuclear emulsion were exposed at the Brookhaven AGS to a 10.6 GeV/n beam of gold nuclei in the BNL 868 experiment. A preliminary analysis of the main features of the interactions of these nuclei is reported here and compared with those observed previously at lower energies,E<1 GeV/n. The multiplicity and pseudo-rapidity distributions of the singly charged particles emitted both in central and in minimum bias samples of interactions have been analyzed in order to look for any non random effects indicative of the nuclear state during the interactions. The fragmentation characteristics of the gold projectiles appear to be different at 10.6 GeV/n from those at ≦1 GeV/n incident energy. Fission of the gold projectiles, which was relatively frequent at low energy, is almost entirely suppressed. Some of alpha particles from the projectile are emitted with very large transverse momenta. In gold-emulsion interactions the residual target nuclei are significantly smaller than in oxygen and sulfur interactions in emulsion.

     

Production of baryon resonances in collisions of 4He nuclei with carbon nuclei at a momentum of 4.2 GeV/c per nucleon

Under conditions of 4π geometry, Δ⁰-and Δ⁺⁺-resonance production in collisions of ⁴He nuclei with carbon nuclei at a momentum of 4.2 GeV/c per nucleon was investigated for the first time. The resonance masses and widths are estimated by analyzing the experimental and background invariant-mass distributions of (pπ ^±) pairs. Contributions to pion production from Δ-resonance decays, as well as from direct pion production, are determined. The results are compared with corresponding data obtained for CC collisions at the same primary momentum of 4.2 GeV/c per nucleon. By and large, the results obtained here comply with the existence of a collective mechanism of delta-isobar excitation in nuclei.     

Azimuthal correlations of projectile and target fragments in collisions between gold nuclei of energy 10.6 GeV per nucleon and emulsion nuclei

Intra-and intergroup azimuthal correlations of projectile and target fragments are found in collisions between gold and emulsion nuclei. The statistical significance of these correlations is high. The methodological distortions associated with the measurement errors are investigated in detail and are taken into account.     

Laboratory angular distributions for the production of charged secondaries in inelastic proton-proton collisions at 102 GeV/c

Lab angular distributions for the production of charged secondary particles in inelastic proton-proton collisions are presented. Data are separately displayed for protons and for positive and negative mesons (pions and kaons combined).     

Search for direct photons from S-Au collisions at 200 GeV/nucleon

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1<y<2.65 and 0.4 GeV/c<p??<2.0 GeV/c the yield andp??-dependence of the observed photons are well reproduced by hadron decays. Furthermore, their production rate is found to be proportional to the charged particle density. The systematic errors comparing the measured and expected photon yield result in an upper limit of 14% for the emission of direct photons in central S-Au collisions. For a photon source with a yield depending quadratically on the charged particle density the limit can be reduced to 7%.     

Search for direct photons from S-Au collisions at 200 GeV/nucleon

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1<y<2.65 and 0.4 GeV/c<p⊥<2.0 GeV/c the yield andp⊥-dependence of the observed photons are well reproduced by hadron decays. Furthermore, their production rate is found to be proportional to the charged particle density. The systematic errors comparing the measured and expected photon yield result in an upper limit of 14% for the emission of direct photons in central S-Au collisions. For a photon source with a yield depending quadratically on the charged particle density the limit can be reduced to 7%.     

Flow effects in 84Kr induced collisions in emulsion at 0.95 GeV per nucleon

Using emulsion detector the collective flow signals in inelastic interactions of ⁸⁴Kr nuclei with Ag(Br) at 950 MeV/nucleon are studied. A transverse momentum analysis is performed to determine the reaction plane. The bounce-off of spectator fragments is observed. In azimuthal distributions relative to the reaction plane squeeze-out and side-splash of participants are seen. Received: 27 September 1999