Search Results (43 total)

We present a new phenomenology for burn propagation inside a thermal explosion based on dynamic radiography. Radiographic images were obtained of an aluminum cased solid cylindrical sample of a plastic bonded formulation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. The phenomenology observed is ignition followed by cracking in the solid accompanied by the propagation of a radially symmetric front of increasing proton transmission. This is followed by a further increase in transmission through the sample, ending after approximately 100  μs. We show that these processes are consistent with the propagation of a convective burn front followed by consumption of the remaining solid by conductive particle burning.

We present particle spectra for charged hadrons π^±, K^±, p, and p̅ from pp collisions at sqrt[s]=200  GeV measured for the first time at forward rapidities (2.95 and 3.3). The kinematics of these measurements are skewed in a way that probes the small momentum fraction in one of the protons and large fractions in the other. Large proton to pion ratios are observed at values of transverse momentum that extend up to 4  GeV/c, where protons have momenta up to 35 GeV. Next-to-leading order perturbative QCD calculations describe the production of pions and kaons well at these rapidities, but fail to account for the large proton yields and small p̅ /p ratios.

The first observation of an azimuthal cross section asymmetry with respect to the charge of the incoming lepton beam is reported from a study of hard exclusive electroproduction of real photons. The data have been accumulated by the HERMES experiment at DESY, in which the HERA 27.6 GeV electron or positron beam scattered off an unpolarized hydrogen gas target. The observed asymmetry is attributed to the interference between the Bethe-Heitler process and the deeply virtual Compton scattering (DVCS) process. The interference term is sensitive to DVCS amplitudes, which provide the most direct access to generalized parton distributions.

Precise measurements of the spin structure functions of the proton g₁^p(x,Q²) and deuteron g₁^d(x,Q²) are presented over the kinematic range 0.0041≤x≤0.9 and 0.18  GeV²≤Q²≤20  GeV². The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function g₁ⁿ is extracted by combining proton and deuteron data. The integrals of g₁^p,d at Q²=5  GeV² are evaluated over the measured x range. Neglecting any possible contribution to the g₁^d integral from the region x≤0.021, a value of 0.330±0.011(theo)±0.025(exp)±0.028(evol) is obtained for the flavor-singlet axial charge a₀ in a leading-twist next-to-next-to-leading-order analysis.

The sizes, temperatures, and free neutron-to-proton ratios of the initial interaction zones produced in the collisions of 40 MeV/nucleon ⁴⁰Ar+¹¹²Sn and 55 MeV/nucleon ²⁷Al+¹²⁴Sn are derived using total detected neutron plus charged particle multiplicity as a measure of the impact parameter range and number of participant nucleons. The size of the initial interaction zone, determined from a coalescence model analysis, increases significantly with decreasing impact parameter. The temperatures and free neutron-to-proton ratios in the interaction zones are relatively similar for different impact parameter ranges and evolve in a similar fashion.

The transfer of polarization from a high-energy positron to a Λ0 hyperon produced in semiinclusive deep-inelastic scattering has been measured. The data have been obtained by the HERMES experiment at DESY using the 27.6 GeV longitudinally polarized positron beam of the HERA collider and unpolarized gas targets internal to the positron (electron) storage ring. The longitudinal spin-transfer coefficient is found to be DLL′Λ=0.11±0.10(stat)±0.03(syst) at an average fractional energy carried by the Λ0 hyperon ⟨z⟩=0.45. The dependence of DLL′Λ on both the fractional energy z and the fractional longitudinal momentum xF is presented.

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.

The kinetic-energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A, and 47A MeV ⁶⁴Zn with ⁵⁸Ni, ⁹²Mo, and ¹⁹⁷Au. For each system investigated, the double-isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10–25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable with those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when antisymmetrized molecular dynamics transport model calculations predict entry into the final evaporative or fragmentation stage of deexcitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed.

Particle production of identified charged hadrons, π^±,K^±,p, and p̅ in Au+Au collisions at sqrt[s_NN]= 200 GeV, has been studied as a function of transverse momentum and collision centrality at y=0 and y~1 by the BRAHMS experiment at Brookhaven National Laboratory's Relativistic Heavy-Ion Collider. Significant collective transverse flow at kinetic freeze-out has been observed in the collisions. The magnitude of the flow rises with the collision centrality. Proton and kaon yields relative to the pion production increase strongly as the transverse momentum increases and also increase with centrality. Particle yields per participant nucleon show a weak dependence on the centrality for all particle species. Hadron production remains relatively constant within one unit around midrapidity in Au+Au collisions at sqrt[s_NN]= 200 GeV.

Calorimetric and coalescence techniques have been employed to probe equilibration for hot nuclei produced in heavy-ion collisions of 35 to 55 MeV/nucleon projectiles with medium mass targets. Entrance channel mass asymmetries and energies were selected so that very hot composite nuclei of similar mass and excitation would remain after early stage preequilibrium particle emission. Intercomparison of the properties and deexcitation patterns for these different systems provides evidence for the production of hot nuclei with decay patterns relatively independent of the specific entrance channel.

An extensive experimental survey of the features of the disassembly of a small quasiprojectile system with A~36, produced in the reactions of 47 MeV/nucleon ⁴⁰Ar  +  ²⁷Al, ⁴⁸Ti, and ⁵⁸Ni, has been carried out. Nuclei in the excitation energy range of 1–9 MeV/nucleon have been investigated employing a new method to reconstruct the quasiprojectile source. At an excitation energy ∼5.6 MeV/nucleon many observables indicate the presence of maximal fluctuations in the deexcitation processes. These include the normalized second moments of the Campi plot and normalized variances of the distributions of order parameters such as the atomic number of the heaviest fragment Z_max and the total kinetic energy. The evolution of the correlation of the atomic number of the heaviest fragment with that of the second heaviest fragment and a bimodality test are also consistent with a transition in the same excitation energy region. The related phase separation parameter, S_p, shows a significant change of slope at the same excitation energy. In the same region a Δ-scaling analysis for of the heaviest fragments exhibits a transition to Δ = 1 scaling, which is predicted to characterize a disordered phase. The fragment topological structure shows that the rank-sorted fragments obey Zipf's law at the point of largest fluctuations, providing another indication of a liquid gas phase transition. The Fisher droplet model critical exponent τ ∼ 2.3 obtained from the charge distribution at the same excitation energy is close to the critical exponent of the liquid gas phase transition universality class. The caloric curve for this system shows a monotonic increase of temperature with excitation energy and no apparent plateau. The temperature at the point of maximal fluctuations is 8.3±0.5  MeV. Taking this temperature as the critical temperature and employing the caloric curve information we have extracted the critical exponents β,γ, and σ from the data. Their values are also consistent with the values of the universality class of the liquid gas phase transition. Taken together, this body of evidence strongly suggests a phase change in an equilibrated mesoscopic system at, or extremely close to, the critical point.

We have measured rapidity densities dN/dy of π^± and K^± over a broad rapidity range (-0.1<y<3.5) for central Au+Au collisions at sqrt[s_NN]=200  GeV. These data have significant implications for the chemistry and dynamics of the dense system that is initially created in the collisions. The full phase-space yields are 1660±15±133 (π⁺), 1683±16±135 (π^-), 286±5±23 (K⁺), and 242±4±19 (K^-). The systematics of the strange to nonstrange meson ratios are found to track the variation of the baryochemical potential with rapidity and energy. Landau-Carruthers hydrodynamics is found to describe the bulk transport of the pions in the longitudinal direction.

A search for an exotic baryon resonance with S=-2, Q=-2 has been performed in quasireal photoproduction on a deuterium target through the decay channel Ξ^-π^-→Λπ^-π^-→pπ^-π^-π^-. No evidence for a previously reported Ξ^--(1860) resonance is found in the Ξ^-π^- invariant mass spectrum. An upper limit for the photoproduction cross section of 2.1 nb is found at the 90% confidence level. The photoproduction cross section for the Ξ⁰(1530) is found to be between 9 and 24 nb.

Charged-particle pseudorapidity densities are presented for the d+Au reaction at sqrt[s_NN]=200   GeV with -4.2≤η≤4.2. The results, from the BRAHMS experiment at BNL Relativistic Heavy-Ion Collider, are shown for minimum-bias events and 0%–30%, 30%–60%, and 60%–80% centrality classes. Models incorporating both soft physics and hard, perturbative QCD-based scattering physics agree well with the experimental results. The data do not support predictions based on strong-coupling, semiclassical QCD. In the deuteron-fragmentation region the central 200 GeV data show behavior similar to full-overlap d+Au results at sqrt[s_NN]=19.4   GeV.

We report on a study of the transverse momentum dependence of nuclear modification factors R_dAu for charged hadrons produced in deuteron  +  gold collisions at sqrt[s_NN]=200  GeV, as a function of collision centrality and of the pseudorapidity (η=0, 1, 2.2, 3.2) of the produced hadrons. We find a significant and systematic decrease of R_dAu with increasing rapidity. The midrapidity enhancement and the forward rapidity suppression are more pronounced in central collisions relative to peripheral collisions. These results are relevant to the study of the possible onset of gluon saturation at energies reached at BNL RHIC.

Transverse momentum spectra and rapidity densities, dN/dy, of protons, antiprotons, and net protons (p-p̅ ) from central (0%–5%) Au+Au collisions at sqrt[s_NN]=200  GeV were measured with the BRAHMS experiment within the rapidity range 0≤y≤3. The proton and antiproton dN/dy decrease from midrapidity to y=3. The net-proton yield is roughly constant for y<1 at dN/dy∼7, and increases to dN/dy∼12 at y∼3. The data show that collisions at this energy exhibit a high degree of transparency and that the linear scaling of rapidity loss with rapidity observed at lower energies is broken. The energy loss per participant nucleon is estimated to be 73±6  GeV.

The reaction systems, ⁶⁴Zn+⁵⁸Ni, ⁶⁴Zn+⁹²Mo, ⁶⁴Zn+¹⁹⁷Au, at 26, 35, and 47  A  MeV, have been studied both in experiments with a 4π detector array, NIMROD, and with antisymmetrized molecular dynamics model calculations employing effective interactions corresponding to soft and stiff equation of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained for both EOS’s. No conclusive preference for either EOS has been observed. Neither of the above direct observables nor the strength of the elliptic flow are also sensitive to changes in the in-medium nucleon-nucleon cross sections. A detailed analysis of the central collision events revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with ¹⁹⁷Au at 47A  MeV a significant radial expansion takes place. For reactions with ⁵⁸Ni and ⁹²Mo at 47A  MeV semitransparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain.

Fusion-evaporation reactions induced by 110  MeV ¹¹B and radioactive ¹¹C on ⁸⁷Rb targets have been studied by measuring evaporation residue–light particle coincidences. The proton to α particle ratio in each reaction has been derived and compared with predictions from statistical model calculations. These calculations account rather well for the experimental data, when a small empirical adjustment of the emission barrier is performed, in agreement with earlier results. No evidence is found for predicted temperature and isospin modification of the binding energies. The possibility of a further study of isospin and temperature dependent effects in fusion-evaporation reactions with radioactive beams is discussed.

A wide variety of observables indicate that maximal fluctuations in the disassembly of hot nuclei with A∼36 occur at an excitation energy of 5.6±0.5  MeV∕nucleon and temperature of 8.3±0.5  MeV. Associated with this point of maximal fluctuations are a number of quantitative indicators of apparent critical behavior. The associated caloric curve does not appear to show a flattening such as that seen for heavier systems. This suggests that, in contrast to similar signals seen for liquid-gas transitions in heavier nuclei, the observed behavior in these very light nuclei is associated with a transition much closer to the critical point.

Intermediate mass fragment (Z>2) emission in ¹²⁴Sn,¹²⁴Xe+¹²⁴Sn,¹¹²Sn reactions at 28  MeV∕nucleon were studied using neutron ion multidetector for reaction oriented dynamics, a 4π charged particle detection system. A number of observables, such as isotopic yield distributions, energy spectra of light charged particles and intermediate mass fragments, isotopic and isobaric yield ratios, and average neutron to proton ratios are investigated. These observables show significant dependence on the isospin N∕Z of the reacting system. It is observed that the formation of neutron-rich clusters are correlated with the excess neutrons in the composite system and depends on the temperature of the emitting source. The origin of light particles and fragments was studied through observations of rapidity distribution as a function of collision violence. With increasing centrality, the heavier ⁶He isotope is found to be emitted closer to the midrapidity region than the lighter ³He isotope. The emission of heavy fragments from the midrapidity region becomes increasingly favorable for fragments with higher charge Z. The results suggest that the midrapidity region is not only neutron rich but also a rich source of heavy fragment (cluster) formation.

We present spectra of charged hadrons from Au+Au and d+Au collisions at sqrt[s_NN]=200   GeV measured with the BRAHMS experiment at RHIC. The spectra for different collision centralities are compared to spectra from p+p̅ collisions at the same energy scaled by the number of binary collisions. The resulting ratios (nuclear modification factors) for central Au+Au collisions at η=0 and η=2.2 evidence a strong suppression in the high p_T region (>2  GeV/c). In contrast, the d+Au nuclear modification factor (at η=0) exhibits an enhancement of the high p_T yields. These measurements indicate a high energy loss of the high p_T particles in the medium created in the central Au+Au collisions. The lack of suppression in d+Au collisions makes it unlikely that initial state effects can explain the suppression in the central Au+Au collisions.

We present ratios of the numbers of charged antihadrons to hadrons (pions, kaons, and protons) in Au+Au collisions at sqrt[s_NN]=200   GeV as a function of rapidity in the range y=0–3. While the ratios at midrapidity are approaching unity, the K^-/K⁺ and p̅ /p ratios decrease significantly at forward rapidities. An interpretation of the results within the statistical model indicates a reduction of the baryon chemical potential from μ_B≈130   MeV at y=3 to μ_B≈25   MeV at y=0.

From experimental observations of limiting temperatures in heavy ion collisions we derive the critical temperature of infinite nuclear matter T_c=16.6±0.86. Theoretical model correlations between T_c, the compressibility modulus K, the effective mass m^*, and the saturation density ρ_s are then exploited to derive the quantity (K/m^*)^1/2ρ_s^-1/3. This quantity together with calculations employing Skyrme and Gogny interactions indicates a value of K in moderately excited nuclei that is in excellent agreement with the value determined from giant monopole resonance data.

The NA44 experiment has measured single-particle inclusive spectra for charged pions, kaons, and protons as a function of transverse mass near midrapidity in 158A GeV/c Pb+Pb collisions. From the particle mass dependence of the observed m_T distributions, we are able to deduce a value of about 120 MeV for the temperature at thermal freeze-out. From the observed ratios of the rapidity densities, we find values of the chemical potentials for light and strange quarks and a chemical freeze-out temperature of approximately 140 MeV.

Nuclear caloric curves have been analyzed using an expanding Fermi gas hypothesis to extract average nuclear densities. In this approach the observed flattening of the caloric curves reflects progressively increasing expansion with increasing excitation energy. This expansion results in a corresponding decrease in the density and Fermi energy of the excited system. For nuclei of medium to heavy mass apparent densities ∼0.4ρ₀ are reached at the higher excitation energies.