Search Results (45 total)

The analyzing power for proton-carbon elastic scattering in the Coulomb-nuclear interference region of momentum transfer, 9.0×10^-3<-t<4.1×10^-2 (GeV/c)², was measured with a 21.7  GeV/c polarized proton beam at the Alternating Gradient Synchrotron of Brookhaven National Laboratory. The ratio of hadronic spin-flip to nonflip amplitude, r₅, was obtained from the analyzing power to be Rer₅=0.088±0.058 and Imr₅=-0.161±0.226.

The ripple phases of a range of mixed-acid phosphatidylcholine derivatives with one saturated C₁₈ chain and the other chain of variable length or unsaturation were studied using differential scanning calorimetry, x-ray diffraction, and freeze-fracture electron microscopy. The temperature dependences of their ripple wavelength (λ), stacking repeat distance (d_s), and the monoclinic angle θ_m defining their unit cell were measured and found to be consistent with an inherently asymmetric ripple phase with an amplitude that slowly increases with temperature. The temperature spans of the ripple phases of the saturated mixed-chain derivatives and the derivatives containing unsaturated chains were found to be larger and smaller, respectively, than those of homoacyl derivatives of the same equivalent chain length. This was shown to be consistent with the sliding-chain model proposed by Cevc (Biochemistry 30, 7186–7197, 1991). The tendency of phosphatidylcholine derivatives to form asymmetric ripple phases and the possible molecular organization of such phases are discussed in terms of different models. X-ray diffraction evidence was found for the existence of a secondary ripple phase with a wavelength about 1.8 times greater than the conventional pretransition ripple phase. This phase is formed in samples freshly cooled from the liquid-crystal phase and tends to persist longer than the conventional ripple phase on cooling to the gel phase. Freeze-fracture electron microscopy suggests that it is symmetric in cross section possibly reflecting the interaction of two opposing pretransition ripples.

A high-sensitivity search for the strangeness S  =  -2 H dibaryon (uuddss) was conducted at the Brookhaven Alternating Gradient Synchrotron (AGS) using the reaction ³He(K^-,K⁺)Hn at P_K^-  =  1.8 GeV/c. The sensitivity was independent of H lifetime and decay modes. No evidence for H production was observed. In a mass range extending from about 50 to 380 MeV/c² below the ΛΛ threshold of 2.231 GeV/c², the resulting upper limits on the H-production cross section are in the range of 0.058 to 0.021 μb/sr, approximately 1 order of magnitude below a theoretical calculation.

A strangelet search in Si+Pt and Au+Pt collisions at alternating-gradient synchrotron (AGS) energies, using a focusing spectrometer, sensitive to mass per charge of 3-14 GeV/c² was conducted during the 1992 and 1993 heavy ion runs at the AGS. The null results thereof are presented as upper limits on the invariant production cross section, in the range of 10^-5-10^-4 mb c³/GeV², and model dependent sensitivity limits in the range of 10^-7-10^-5 per collision. Measurements of the production cross sections of several nonstrange nuclear systems, from p to ⁷Be and ⁸Li, the background of the strangelet search, are also presented.

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.

Recent estimates suggest that the production of strange exotic objects, such as the hypothetical H dibaryon (H), is surprisingly large in Au collisions at AGS energies. Two possible bound states of the H, namely H-³He and H-d, would be well separated in mass from known nuclear isotopes and therefore could be uniquely identified by experiment. We report the null result of a first search for these objects in Au+Pt collisions at the AGS, and limit their production cross sections for stable systems to ∼10^-5 mb/(GeV)², near y=0.6 and p_t/Z=0.18 GeV/c.

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.

Recently, highly relativistic Au beams have become available at the Brookhaven National Laboratory, Alternating Gradient Synchrotron. Inclusive production cross sections for composite particles, d, t, ³He, and ⁴He, in 11.5A GeV/c Au+Pt collisions have been measured using a beam line spectrometer. For comparison, composite particle production was also measured in Si+Pt and p+Pt collisions at similar beam momenta per nucleon (14.6A GeV/c and 12.9 GeV/c, respectively). The projectile dependence of the production cross section for each composite particle has been fitted to A_proj^α. The parameter α can be described by a single function of the mass number and the momentum per nucleon of the produced particle. Additionally, the data are well described by momentum-space coalescence. Comparisons with similar analysis of Bevalac A+A data are made. The coalescence radii extracted from momentum-space coalescence fits are used to determine reaction volumes (‘‘source size’’) within the context of the Sato-Yazaki model.

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.

During the recent commissioning of Au beams at the Brookhaven Alternating Gradient Synchrotron facility, experiment 886 measured production cross sections for π^±, K^±, p, and p¯ in minimum bias Au+Pt collisions at 11.5A GeV/c. Invariant differential cross sections, Ed³σ/dp³, were measured at several rigidities (p/Z≤1.8 GeV/c) using a 5.7° (fixed-angle) focusing spectrometer. For comparison, particle production was measured in minimum bias Si+Pt collisions at 14.6A GeV/c using the same apparatus and in p+Pt collisions at 12.9 GeV/c using a similar spectrometer at KEK. When normalized to projectile mass, A_proj, the measured π^± and K^± cross sections are nearly equal for the p+Pt and Si+Pt reactions. In contrast to this behavior, the π^- cross section measured in Au+Pt shows a significant excess beyond A_proj scaling of the p+Pt measurement. This enhancement suggests collective phenomena contribute significantly to π^- production in the larger Au+Pt colliding system.

For the Au+Pt reaction, the π⁺ and K⁺ yields also exceed A_proj scaling of p+Pt collisions. However, little significance can be attributed to these excesses due to larger experimental uncertainties for the positive rigidity Au beam measurements. For antiprotons, the Si+Pt and Au+Pt cross sections fall well below A_proj scaling of the p+Pt yields indicating a substantial fraction of the nuclear projectile is ineffective for p¯ production. Comparing with p+Pt multiplicities, the Si+Pt and Au+Pt antiproton yields agree with that expected solely from ‘‘first’’ nucleon-nucleon collisions (i.e., collisions between previously unstruck nucleons). In light of expected p¯ annihilation in the colliding system, such projectile independence is unexpected without additional (projectile dependent) sources of p¯ production. In this case, the data indicate an approximate balance exists between absorption and additional sources of antiprotons. This balance is remarkable given the wide range of projectile mass spanned by these measurements.

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.

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.

The branching ratio for the Λ weak radiative decay Λ→nγ has been measured. Three statistically independent results from the same experiment (Brookhaven E811) are reported here. They are combined with a previously published measurement, also from Brookhaven E811, to yield a result of (Λ→nγ) / (Λ→anything)=(1.75±0.15)×10^-3, based on 1800 events after background subtraction. This represents a factor of 75 increase in statistics over the previous world total. A comparison with recent theoretical papers shows that no existing model provides a completely satisfactory description of all data on weak radiative decays. A search is also reported for the radiative capture process K^-p→Σ(1385)γ at rest. No signal was observed and an upper limit on the branching ratio of [K^-p→Σ(1385)γ] / [K^-p→anything]<4×10^-4 (90% C.L.) was determined.

The branching ratio for the Λ weak radiative decay has been measured to be B(Λ→n+γ)/(Λ→anything) =[1.78±0.24(stat)±_0.16^0.14(syst)]×10^-3. A low-energy kaon beam was used to produce the Λ hyperons via the reaction K^-+p→Λ+π⁰ at rest. Photons from the signal channel and π⁰ decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

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.

A dynamic thermally induced cooperative first-order phase transition between ripple phases is described for fully hydrated mixtures of dipalmitoylphosphatidylcholine and 5-mole % cholesterol bilayers. The ripple-ripple phase transition has not been observed for pure phosphatidylcholine systems. This transition has not been observed for pure phosphatidylcholine systems. This transition is more likely driven by changes in headgroup-water-headgroup interactions than by a change in acyl chain tilt. Implications for mechanical processes within a lipid bilayer are discussed.

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.

The energy spectrum of γ rays produced in proton-antiproton annihilation at rest in hydrogen and deuterium has been measured with good energy resolution and high statistics. The data were taken using the PS183 magnetic pair spectrometer at CERN's Low Energy Antiproton Ring. A detailed search of the γ spectrum yielded no evidence of γ transitions to narrow states with widths consistent with the spectrometer resolution. In hydrogen, upper limits (95% C.L.) are obtained for the yield of narrow states of (2-5)×10^-4 for states below 1700 MeV/c², and (5-10)×10^-4 for states between 1700 and 1800 MeV/c². In deuterium, the corresponding upper limits are (1-7)×10^-3 for masses between 700 and 1730 MeV/c², and (1-2)×10^-3 for masses between 1750 and 1835 MeV/c². These results contradict reports of the observation of narrow lines made by previous experiments.

Total cross sections, angular, and mass distributions for the reaction π^-p→π⁰π⁰n have been measured for p_π^-(lab)=7–140 MeV/c above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, ξ, of -0.98±0.52. The ππ scattering lengths a_I for isospin I=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are a₀=(0.207±0.028)m_π^-1 and a₂=(-0.022±0.011)m_π^-1. These values are consistent with chiral symmetry broken by the Weinberg ππ interaction and the effects of the f₀(975) scalar meson.

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.

Collisions of ²⁸Si+Al, Cu, Pb at E_lab/A=14.6 GeV were studied in a calorimetry-based experiment at the BNL Alternating Gradient Synchrotron. Transverse-energy production was measured for pseudorapidities -0.5<η<0.8. Correlations with the spectra and multiplicity of neutrons and protons emitted into a forward 0.8° cone demonstrate quantitatively the large amount of nuclear stopping observed in these reactions. Calculations in hadronic-fireball or nucleon-nucleon based models underpredict the measured transverse-energy production for Si+Pb and indicate the need to include rescattering of secondaries and /or contributions from target fragmentation.