Search Results (26 total)

We present measurements from BNL AGS Experiment 864 of the _Λ³H yield and of an upper limit on the _Λ⁴H yield in central 11.5A  GeV∕c  Au+Pt collisions. The measurements span a rapidity range from center of mass, y_c.m., to y_c.m.+1 and a transverse momentum range of 0<p_t≤1.5  GeV∕c. We compare these results with E864 measurements of stable light nuclei and particle unstable nuclei yields of the same baryon number. The implications of these results for the coalescence of strange clusters are discussed.

We report measurements from experiment E864 at the BNL-AGS of the yields of particle unstable light nuclei in central collisions of ¹⁹⁷Au with beam momentum of 11.5A GeV/c on ¹⁹⁷Pt. Yields are reported as a function of rapidity for the nuclei ⁴H, ⁴Li, ⁵He, and ⁵Li in the rapidity range from y_c.m. to y_c.m.+0.8 and in the transverse momentum range of approximately 0.1<~p_T/A<~0.4 GeV/c. The yields are compared to previously reported yields and trends for the production of stable light nuclei. The nonobservation of two excited states ⁵He_{16.75 MeV}^* and ⁵Li_{16.66 MeV}^* is used to set an upper limit on the yields of these states.

We present the final results from Experiment 864 of a search for charged and neutral strange quark matter produced in interactions of 11.5 GeV/c per nucleon Au beams with Pt or Pb targets. Searches were made for strange quark matter with A>~5. Approximately 3×10¹⁰ 10% most central collisions were sampled and no strangelet states with A<~100 were observed. We find 90% confidence level upper limits of approximately 10^-8 per central collision for both charged and neutral strangelets. These limits are for strangelets with proper lifetimes greater than 50 ns. Also limits for H⁰-d and pineut production are given. The above limits are compared with the predictions of various models. The yields of light nuclei from coalescence are measured and a penalty factor for the addition of one nucleon to the coalescing nucleus is determined. This is useful in gauging the significance of our upper limits and also in planning future searches for strange quark matter.

We present Experiment 864's measurement of invariant antideuteron yields in 11.5A GeV/c Au+Pt collisions. The analysis includes 250×10⁶ triggers representing 14×10⁹ 10% central interactions sampled for events with high mass candidates. We find (1/2πp_t)d²N/dydp_t  =  3.5±1.5(stat)_-0.5^+0.9(syst)×10^-8 GeV^-2 c² for 1.8<y<2.2, 〈 p_t〉  =  0.35 GeV/c ( y_c.m.  =  1.6) and 3.7±2.7(stat)_-1.5^+1.4(syst)×10^-8 GeV^-2 c² for 1.4<y<1.8, 〈 p_t〉  =  0.26 GeV/c, and a coalescence parameter B₂̅ of 4.1±2.9(stat)_-2.4^+2.3(syst)×10^-3 GeV² c^-3. Implications for coalescence and antimatter annihilation are discussed.

We report on measurements by the E864 experiment at the BNL-AGS of the yields of light nuclei in collisions of ¹⁹⁷Au with beam momentum of 11.5A GeV/c on targets of ²⁰⁸Pb and ¹⁹⁷Pt. The yields are reported for nuclei with baryon number A=1 up to A=7, and typically cover a rapidity range from y_c.m. to y_c.m.+1 and a transverse momentum range of approximately 0.1<~p_T/A<~0.5 GeV/c. We calculate coalescence scale factors B_A from which we extract model-dependent source dimensions and collective flow velocities. We also examine the dependences of the yields on baryon number, spin, and isospin of the produced nuclei.

Light nuclei can be produced in the central reaction zone via coalescence in relativistic heavy-ion collisions. Experiment 864 at BNL has measured the production of ten light nuclei with nuclear number A  =  1 to A  =  7 at rapidity y≃1.9 and p_T/A≤300 MeV/c. Data were taken with a Au beam of momentum of 11.5A GeV/c on a Pb or Pt target with different experimental settings. The invariant yields show a striking exponential dependence on nuclear number with a penalty factor of about 50 per additional nucleon. Detailed analysis reveals that the production may depend on the spin factor of the nucleus and the nuclear binding energy as well.

We present measurements from Brookhaven AGS Experiment 864 of neutron invariant multiplicity in 11.5A GeV/c Au+Pb collisions. The measurements span a rapidity range from center of mass to beam rapidity (y_beam=3.2) and are presented as a function of event centrality. The results are compared with E864 measurements of proton invariant multiplicity and an average n/p ratio at hadronic freeze-out of 1.19±.08 is determined for the rapidity range y=1.6 to y=2.4. We discuss briefly the implications of this ratio within a simple equilibrium model of the collision system.

We present results from Experiment 864 for antiproton production and antideuteron limits in Au + Pb collisions at 11.5 GeV/c per nucleon. We have measured invariant multiplicities for antiprotons for rapidities 1.4<y<2.4 at low transverse momentum as a function of collision geometry. When compared with the results from Experiment 878 our measurements suggest a significant contribution to the measured antiproton yield from the decay of strange antibaryons. We have also searched for antideuterons and see no statistically significant signal. Thus, we set upper limits on the production at approximately 3×10^-7 per 10% highest multiplicity Au+Pb interaction.

We present results of a search for neutral strange quark matter (strangelets) in 11.6A GeV/c Au+Pb reactions from the 1995 run of experiment E864 at the Brookhaven Alternating Gradient Synchrotron. We have sampled approximately 1.3 billion 10% most central Au+Pb interactions and have observed no statistically significant signal for neutral strangelet states with baryon number in the range 6<A<100. We set upper limits on the production of these exotic states at the level of 8×10^-8 per central collision for mass >20 GeV/c². These limits are the first limits reported on the production of heavy neutral strangelets. They complement searches for positively and negatively charged strangelets also conducted by our collaboration. We discuss the implications of these results on strangelet production mechanisms and the stability of strange quark matter.

We present results of a search for strange quark matter (strangelets) in 11.5A  GeV/c  Au+Pb collisions from the 1994 and 1995 runs of experiment E864 at Brookhaven's Alternating Gradient Synchrotron. We observe no strangelet candidates and set a 90% confidence level upper limit of approximately 3×10^-8 per 10% central interaction for the production of |Z|  =  1 and |Z|  =  2 strangelets over a large mass range and with metastable lifetimes of about 50 ns or more. These results place constraints primarily on quark-gluon plasma based production models for strangelets.

We present the first results from the E864 Collaboration on the production of antiprotons in 10% central 11.5A  GeV/c  Au+Pb nucleus collisions at the Brookhaven Alternating Gradient Synchrotron. We report invariant multiplicities for antiproton production in the kinematic region 1.4<y<2.2 and 50<p_T<300  MeV/c, and compare our data with a first collision scaling model and previously published results from the E878 Collaboration. The differences between the E864 and E878 antiproton measurements and the implications for antihyperon production are discussed.

We have conducted a search for bound states of a negative pion and a number of neutrons (pineuts) using the E814 spectrometer. A beam of ²⁸Si at a momentum of 14.6A GeV/c was used to bombard targets of Al, Cu, Sn, and Pb. We describe our experimental technique, present measured upper limits for pineut production, and discuss the significance of our results.

We present the transverse energy distributions, dσ/dE_T and dE_T/dη, and the charged particle multiplicity distributions, dσ/dN_c and dN_c/dη, produced in p+Al and p+Pb collisions at 14.6 GeV/c. The data exhibit a weak correlation between these global variables. While a significant increase of the mean multiplicity with the mass of the target is observed, the transverse energy distributions show little target dependence. The dE_T/dη distribution shifts backward as the mass of the target increases, indicating the presence of rescattering. The data are compared to the predictions of the relativistic quantum molecular dynamic model (RQMD) and Fritiof event generators. RQMD reproduces well the main features of the data while Fritiof predicts too forward peaked transverse energy and particle multiplicity pseudorapidity distributions.

An investigation of the production of neutron-rich isotopes from the fragmentation of ²⁸Si projectiles at p_lab=14.6 GeV/c per nucleon was performed using the BNL-AGS-E814 spectrometer. We have measured the inclusive production cross sections of neutron-rich fragments (⁶He, ⁸He, ⁸Li, ⁹Li, ¹⁰Be, ¹¹Be, and ¹³B). We have also measured the transverse momentum distributions for ⁶He and ⁹Li, and the forward and transverse energy distributions associated with ⁶He production. The momentum distributions were analyzed in the context of the Goldhaber model. The question of whether the fragments are produced in the decay of the projectile following its electromagnetic excitation was also investigated.

We have studied in detail the electromagnetic dissociation of ²⁸Si projectiles at 14.6 GeV/(c nucleon), interacting with Pb, Sn, Cu, and Al targets. Exclusive cross sections were measured for several decay channels, including final states involving the emission of protons, neutrons, and α particles. Excitation energy distributions for the 1n+²⁷Si and 2p+²⁶Mg decay channels were reconstructed with a resolution of 2 MeV, using a constrained kinematic fit. The energy distributions obtained for 1n+²⁷Si are in good agreement with the σ(γ,n) photoneutron cross sections multiplied by the virtual photon spectrum obtained in the Weizsäcker-Williams approximation. A search for the double photon excitation process, based on the dependence of the cross sections on the target atomic number, was performed.

Inclusive double differential multiplicities d²N/dy dp_t and related quantities have been measured for protons and deuterons produced in 14.6A GeV/c Si+Al and Si+Pb collisions using the E814 forward spectrometer at the AGS at BNL. Collision ‘‘centrality’’ is determined by measuring N_c, the total charged particle multiplicity in the pseudorapidity range 0.85<η<3.8. For both systems Si + Al and Si + Pb, an increase in the proton rapidity distribution dN/dy at midrapidity and a corresponding decrease at higher rapidities are observed with increasing N_c. For Si+Pb, Boltzmann slope parameters T_B increase significantly in the most central collisions. The measured distributions exhibit a centrality dependence even when σ/σ_geo≲10%, where full overlap between the Si and Pb nuclei occurs in a simple geometric picture. The proton rapidity distribution dN/dy is presented for the symmetric system Si+Al over the entire rapididty interval. The total number of protons, which is the integral of this quantity over rapidity, varies with N_c. Results are compared with various model calculations, mostly using the hadronic cascade codes ARC and RQMD. No significant nuclear transparency is observed, indicating that large baryon and energy densities are produced in these collisions.

We have measured cross sections for the synthesis of nuclei of mass A≤4 in collisions of 14.6A GeV/c ²⁸Si nuclei with targets of Pb, Cu, and Al. The data are measured at close to center-of-mass rapidities, and are unique in their exploration of the centrality dependence of nucleosynthesis. Simple coalescence models that were used to study nucleosynthesis at lower energies are inadequate for the description of our measurements. Our data and improved models are used to extract parameters related to the size of the interaction volume at freeze-out.

We present the results of an analysis of two charged particle and transverse energy correlations in Si+Pb collisions at BNL AGS at 14.6 GeV/c per nucleon. The measured semi-inclusive normalized two particle pseudorapidity corrleation function exhibits short range correlations similar to the correlations observed in hadron-hadron and hadron-nucleus collisions at higher energies, although the observed correlations are smaller than the values scaled from hp and hA data. Estimates, provided by the observed correlations, of the intermittency indices as well as of the parameters of the cluster model are presented. Predictions using the fritiof event generator, which at this level of statistical accuracy show no pseudorapidity correlations, are not in agreement with our data. Azimuthal angle two particle correlations show nonzero back-to-back correlations in the central region (consistent with fritiof predictions) and are almost flat in the projectile fragmentation region. We also present results on the transverse energy azimuthal correlation function, which are similar to those from the two particle correlation function.

We present a systematic study of transverse energy (E_T) production in collisions of 11.4A GeV/c Au and 14.6A GeV/c Si ions with targets of Al, Au, and Pb. Comparison of data for Au+Au and Si+Al indicates that, for the heavier system, there is an increase in the amount of stopping which is accompanied by a decrease in the width of the dE_T/dη distribution. The ratio of the maximum E_T observed for the two systems is significantly greater than the ratio of the total energy available in the center of mass frame.

A model of H dibaryon production is developed for high-energy p+A collisions. The model is based on the coalescence of a neutron onto a Ξ⁰ baryon, and rates for the processes involved are estimated from data collected at Fermilab. A form for the H's differential cross section is developed, and its total cross section is estimated to be approximately 1 μb.

We have measured antiproton production cross sections as functions of centrality in collisions of 14.6 GeV/c per nucleon ²⁸Si ions with targets of Al, Cu, and Pb. For all targets, the antiproton yields increase linearly with the number of projectile nucleons that have interacted, and show little target dependence. We discuss the implications of this result on the production and absorption of antiprotons within the nuclear medium.

Collisions of ²⁸Si+Al, Cu, and Pb at E_lab=14.6 GeV/nucleon were studied at the Brookhaven National Laboratory Alternating Gradient Synchrotron. Charged particle multiplicity was measured over the pseudorapidity interval 0.875≤η≤3.86 with a silicon pad detector. A strong correlation is seen between the multiplicity and the transverse energy measured in the interval -0.5≤η≤0.8. Correlation with the energy going forward after the collision and comparison with calculations indicate that rescattering is required to explain the data. The data are compared under the assumption of Koba-Nielson-Olesen scaling. The measured multiplicity scales approximately with the total number of participant nucleons and less well with the available center-of-mass kinetic energy.

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of ²⁸Si into p+ ²⁷Al at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p-²⁷Al invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in ²⁸Si and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszäcker-Williams approximation is combined with experimental data on the photonuclear reaction ²⁸Si(γ,p) ²⁷Al, and slight differences are observed only at low final-state energy.

We present the rapidity and transverse momentum distributions of protons and neutrons from collisions between 14.6 GeV/nucleon beams of ²⁸Si and targets of Al, Cu, and Pb. The data were measured in the forward spectrometer/target calorimeter detectors of the E814 apparatus. The results indicate the existence of two distinct domains, one of beam rapidity projectilelike nucleons, and the second of participant nucleons. From the former, the in-medium inelastic nucleon-nucleon cross section is deduced. It is found to agree, within 10%, with the ‘‘free’’ value of 30 mb although under present conditions one of the two colliding nucleons has been struck before with a high probability. We compare with the present data the predictions of a fragmentation model as well as of models dealing explicitly with the heavy-ion collision and particle creation and emission.

Electromagnetic dissociation cross sections for one, two, and three nucleon removal were measured for an E_lab/A=14.6 GeV ²⁸Si beam impinging on ²⁷Al, ¹²⁰Sn, and ²⁰⁸Pb targets. The measured cross sections exhibit an approximate Z_T² dependence and are well reproduced by calculations convoluting the virtual photon spectrum with the experimental σ(γ,N) photonuclear cross sections. Comparison with other experimental results and energy dependence of the cross sections are discussed.