Search Results (14 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.

The reactions ¹³⁹La + ²⁷Al and ¹³⁹La + ⁶⁵Cu at E / A=45 MeV have been modeled by combining a Boltzmann-Nordheim-Vlasov dynamical model calculation with a statistical sequential binary decay model code. For the reaction ¹³⁹La + ²⁷Al, the major features of the experimental data are adequately described by the model calculations. These features include the inclusive fragment cross sections and the total charge and source velocity distributions of multiple fragment events. Other finer features, such as charge-Dalitz plots and the branching ratios between events of different multiplicity, are not reproduced by the calculation. The failure of the calculations is even greater for the reaction ¹³⁹La + ⁶⁵Cu, in which only the source velocity distributions of multiple fragment events are reproduced. Because the source velocity can be a measure of how much of the target is incorporated into the projectile in inverse kinematics reactions, this suggests that the earliest stages of the reaction, described by the dynamical calculation, are adequately characterized by the model. It is the later stages of the reaction, when fragments are emitted, where the model calculation appears to fail. However, there are some indications that statistical decay in the reaction ¹³⁹La + ⁶⁵Cu has occurred, whether by sequential binary decays or some type of prompt multifragmentation.

Projectilelike fragments were detected and characterized in terms of A, Z, and energy for the reactions ³⁷Cl on ⁴⁰Ca and ²⁰⁹Bi at E=7.3 MeV/nucleon, and ³⁵Cl on ²⁰⁹Bi at E=15 MeV/nucleon, at angles close to the grazing angle. Mass and charge distributions were generated in the N-Z plane as a function of energy loss, and parametrized in terms of their centroids, variances, and coefficients of correlation. The results are compared to the predictions of two current models based on a stochastic nucleon exchange mechanism. The drifts of the charge and mass centroids for the system ³⁷Cl on ⁴⁰Ca are consistent with a process of mass and charge equilibration mediated by nucleon exchange between the two partners, followed by evaporation. The asymmetric systems show a strong drift toward larger asymmetry, with the production of neutron-rich nuclei. It is concluded that this is the result of a net transfer of protons from the light to the heavy partner, and a net flow of neutrons in the opposite direction. Model predictions fail to reproduce in detail the evolution of the centroids for asymmetric systems. The variances for all systems increase with energy loss, as would be expected from a nucleon exchange mechanism. However, the variances for the reaction ³⁷Cl on ⁴⁰Ca are higher than those expected from that mechanism, and the variances for the reaction ³⁵Cl on ²⁰⁹Bi start decreasing after about 100 MeV of energy loss. The coefficients of correlation indicate that the transfer of nucleons between projectile and target is correlated, as expected from Q-value constraints to the valley of β stability.

Multifragment events from 35, 40, 45, and 55 MeV/nucleon ¹³⁹La+¹²C, ²⁷Al, ⁴⁰Ca, ⁵¹V, ^natCu, and ¹³⁹La reactions can be assigned to sources characterized by their velocity. At each bombarding energy, the probabilities of threefold, fourfold and fivefold events increase substantially with decreasing source velocity, but are independent of the target mass. To remove the bombarding-energy dependence, a simple transformation has been applied which gives the excitation energy of the fused system in the simple incomplete-fusion picture. These ‘‘excitation functions’’ appear to independent of both the system and bombarding energy.