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Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation., Comment: 25 pages, 22 figures, accepted for publication in Physical Review C

Spectator fragments resulting from relativistic heavy ion collisions, consisting of single protons and neutrons along with groups of stable nuclear fragments up to nitrogen $(Z=7)$, are measured in PHOBOS. These fragments are observed in Au+Au $(\sqrt{{s}_{{}_{NN}}}=19.6\phantom{\rule{0.16em}{0ex}}\mathrm{GeV})$ and Cu+Cu (22.4 GeV) collisions at high pseudorapidity $(\ensuremath{\eta})$. The dominant multiply-charged fragment is the tightly bound helium $(\ensuremath{\alpha})$, with lithium, beryllium, and boron all clearly seen as a function of collision centrality and pseudorapidity. We observe that in Cu+Cu collisions, it becomes much more favorable for the $\ensuremath{\alpha}$ fragments to be released than lithium. The yields of fragments approximately scale with the number of spectator nucleons, independent of the colliding ion. The shapes of the pseudorapidity distributions of fragments indicate that the average deflection of the fragments away from the beam direction increases for more central collisions. A detailed comparison of the shapes for $\ensuremath{\alpha}$ and lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.

Pseudorapidity distributions of charged particles emitted in Au+Au, Cu+Cu, d+Au, and p+p collisions over a wide energy range have been measured using the PHOBOS detector at the BNL Relativistic Heavy-Ion Collider (RHIC). The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with |{eta}

New angular distributions for the $^{7}\mathrm{Li}$($^{18}\mathrm{O}$,$^{17}\mathrm{N}$)$^{8}\mathrm{Be}$ reaction at an energy of ${E}_{\text{lab}}$($^{18}\mathrm{O}$)$=$114 MeV for the ground states of $^{8}\mathrm{Be}$ and $^{17}\mathrm{N}$ and the excited states of $^{17}\mathrm{N}$ were measured. These data and $^{18}\mathrm{O}$($d,{}^{3}$He)$^{17}\mathrm{N}$ reaction data taken at ${E}_{d}=52$ MeV were analyzed within the coupled-reaction-channels method using $^{7}\mathrm{Li}+^{18}\mathrm{O}$ and $^{18}\mathrm{O}$ + $d$ optical potentials deduced from previous elastic and inelastic scattering results. Shell-model spectroscopic amplitudes were used in the analysis. Both reactions are dominated by single proton transfer. Calculations show that heavy-ion reactions of the type studied in this work can be used to identify final-state spins when measurements are carried to small angles.

The cross section for the reaction p+{sup 6}Li{yields}{eta}+{sup 7}Be was measured at an excess energy of 11.28 MeV above threshold by detecting the recoiling {sup 7}Be nuclei. A dedicated set of focal plane detectors was built for the magnetic spectrograph Big Karl and was used for identification and four-momentum measurement of {sup 7}Be. A differential cross section of nb/(d{sigma}/d{Omega})=[0.69{+-}0.20(stat.){+-}0.20(syst.)] sr for the ground state plus 1/2{sup -} was measured. The result is compared to model calculations.

The energy and angular dependence of double differential cross sections d{sup 2{sigma}}/d{Omega}dE were measured for p,d,t, {sup 3,4,6}He, {sup 6,7,8}Li, {sup 7,9,10}Be, {sup 10,11}B, and C produced in collisions of 1.2, 1.9, and 2.5 GeV protons with a Ni target. The shape of the spectra and angular distributions almost does not change whereas the absolute value of the cross sections increases by a factor {approx}1.7 for all ejectiles in this beam energy range. It was found that energy and angular dependencies of the cross sections cannot be reproduced by microscopic models of intranuclear cascade including coalescence of nucleons coupled to statistical model for evaporation of particles from excited, equilibrated residual nuclei. The inclusion of nonequilibrium processes, described by a phenomenological model of the emission from fast and hot moving sources, resulting from break up of the target nucleus, leads to very good reproduction of data. Cross sections of these processes are quite large, exhausting approximately half of the total production cross sections. Due to good reproduction of energy and angular dependencies of d{sup 2{sigma}}/d{Omega}dE it was possible to determine total production cross sections for all studied ejectiles. Results obtained in this work point to the analogous reaction mechanism for protonmore » induced reactions on Ni target as that observed previously for Au target in the same beam energy range.« less

Inclusive measurements of pion production in proton-proton collisions in the forward direction were undertaken at 400 and 600 MeV at the cooler synchrotron (COSY) at Forschungszentrum Julich using the Big Karl spectrograph. The high resolution in the pi(+) momentum ensured that there was an unambiguous separation of the pp ->pi(+)d/pi(+)pn channels. Using these and earlier data, the ratio of the production cross sections could be followed through the Delta region and compared with the predictions of final state interaction theory. Deviations are strongly influenced by long-range terms in the production operator and the tensor force in the final pn system. These have been investigated in a realistic pp ->pi(+)d/pi(+)pn calculation that includes S reversible arrow D channel coupling between the final nucleons. A semiquantitative understanding of the observed effects is achieved. .

New angular distributions for the {sup 7}Li({sup 10}B,{sup 9}Be){sup 8}Be reaction at an energy of E{sub lab}({sup 10}B)=51 MeV for the ground and excited states of {sup 8}Be were analyzed within the coupled-reaction-channels method. The reactions are dominated by single proton transfer at forward angles except for that to the {sup 8}Be 11.35-MeV state, which requires multistep routes for its population. The {sup 8}Be+{sup 9}Be potential parameters for ground and excited states of {sup 8}Be were deduced by fitting {sup 7}Li({sup 10}B,{sup 9}Be){sup 8}Be and {sup 10}B({sup 7}Li,{sup 9}Be){sup 8}Be reaction data and are compared with a previously determined {sup 7}Be+{sup 9}Be potential. The {sup 8}Be+{sup 9}Be real potential is very similar to that for {sup 7}Be+{sup 9}Be, whereas the imaginary one for {sup 8}Be+{sup 9}Be has a much greater spatial extent than that for {sup 7}Be+{sup 9}Be.

Total cross sections for the quasifree pn{yields}pn{eta} reaction in the range from the kinematical threshold up to 20 MeV excess energy have been determined. At threshold they exceed corresponding cross sections for the pp{yields}pp{eta} reaction by a factor of about 3 in contrast to the factor of 6 established for higher excess energies. To large extent, the observed decrease of the ratio {sigma}(pn{yields}pn{eta})/{sigma}(pp{yields}pp{eta}) toward threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final-state interactions. The experiment was conducted using a proton beam of the cooler synchrotron COSY and a cluster jet deuteron target. The proton-neutron reactions were tagged by the spectator proton whose momentum was measured for each event. Protons and neutron outgoing from the pn{yields}pn{eta} reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.

Inclusive spectra of different isotopes of He, Li, Be, B, C, and N nuclei emerging from the collisions of {sup 12}C ions of energy 344.5 MeV with carbon, aluminum, and nickel target nuclei have been measured in the angular range of 4{degree}--36{degree} (laboratory). Experimental yields of reaction products are almost the same for the different targets. The continuous parts of momentum spectra have been decomposed into three components, corresponding to direct and damped fragmentation and to other processes, such as deep-inelastic and/or compound-nucleus reactions. At small angles (smaller than 20{degree} in the laboratory system) the fragmentation processes dominate whereas at larger angles they become negligible in comparison to the other components. The direct fragmentation contribution is very small with respect to the damped one. The obtained reduced momentum widths of the direct fragmentation bumps are significantly smaller than those predicted by the Goldhaber model but agree with results obtained from other experiments at similar energies.

The energy and angular dependence of the double differential cross sections ${d}^{2}\ensuremath{\sigma}/d\ensuremath{\Omega}\mathit{dE}$ were measured for $p,d,t,$$^{3,4,6}\mathrm{He}$, $^{6,7,8,9}\mathrm{Li}$, $^{7,9,10}\mathrm{Be}$, and $^{10,11,12}\mathrm{B}$ isotopes produced in reactions of 1.2- and 1.9-GeV protons on a Au target. The beam energy dependence of the data, supplemented by the cross sections from a previous experiment at 2.5 GeV, is very smooth. The shape of the spectra and angular distributions do not change significantly in the beam energy range from 1.2 to 2.5 GeV. However, the absolute value of the cross sections increases for all ejectiles. The intermediate mass fragment spectra and their angular distributions are very well reproduced by a phenomenological model of two emitting, moving sources, with parameters smoothly varying with energy. The double differential cross sections for light charged particles were analyzed in the framework of the microscopic model calculations of intranuclear cascade, including the coalescence of nucleons and a statistical model for evaporation of particles from excited residual nuclei. However, the energy and angular dependencies of the data agree with neither predictions of the microscopic intranuclear cascade calculations for protons nor the coalescence calculations for other light charged particles. A very good description of the data is achieved by the phenomenological inclusion of the emission of light charged particles from a ``fireball'' (i.e., a fast and hot moving source). It was found that the nonequilibrium processes are very important for the production of light charged particles. They exhaust 40%--80% of the total cross section---depending on the emitted particles. Coalescence and fireball emission yield comparable contributions to the cross sections with the exception of the $^{3}\mathrm{He}$ data where coalescence clearly dominates. For all light charged particles, the ratio of nonequilibrium processes to processes proceeding through a phase of statistical equilibrium does not change significantly between the beam energies of 1.2 and 2.5 GeV.

An experimental study of nuclear reactions between Si-28 nuclei at 200 and 300 MeV/nucleon and hydrogen or deuterium target nuclei was performed at the CELSIUS storage ring in Uppsala, Sweden, to collect information about the reactions responsible for single-event effects in microelectronics. Inclusive data on Si-28 fragmentation, as well as data on correlations between recoils and spectator protons or alpha particles are compared to predictions from the Dubna cascade model and the Japan Atomic Energy Research Institute version of the quantum molecular dynamics model. The comparison shows satisfactory agreement for inclusive data except for He fragments where low-energy sub-barrier fragments and recoiling fragments with very large momenta are produced much more frequently than predicted. The yield of exclusive data are also severely underestimated by the models whereas the charge distributions of recoils in these correlations compare well. The observed enhancement in He emission, which may well be important for the description of single-event effects, is most likely to be attributed to alpha clustering in Si-28 nuclei.

The energy and angular dependence of double differential cross sections d 2 σ/ ddE was measured for reactions induced by 2.5 GeV protons on Au target with isotopic identification of light products (H, He, Li, Be, and B) and with elemental identification of heavier intermediate mass fragments (C, N, O, F, Ne, Na, Mg, and Al). It was found that two different reaction mechanisms give comparable contributions to the cross sections. The intranuclear cascade of nucleon-nucleon collisions followed by evaporation from an equilibrated residuum describes the low energy part of the energy distributions whereas another reaction mechanism is responsible for the high energy part of the spectra of composite particles. A phenomenological model description of the differential cross sections by isotropic emission from two moving sources led to a very good description of all measured data. Values of the extracted parameters of the emitting sources are compatible with the hypothesis claiming that high energy particles emerge from preequilibrium processes consisting in a breakup of the target into three groups of nucleons; small, fast, and hot fireball of ∼8 nucleons, and two larger, excited prefragments, which emit light charged particles and intermediate mass fragments. The smaller of them contains ∼20 nucleons and moves with a velocity larger than the CM velocity of the proton projectile and the target. The heavier prefragment behaves similarly as the heavy residuum of the intranuclear cascade of nucleon-nucleon collisions.

Measurements of {eta} meson production in proton-deuteron collisions have been performed using the COSY-11 facility at COSY (Juelich). Results on total and differential cross sections for the pd{yields}{sup 3}He{eta} reaction are presented at five excess energies between Q=5.0 and Q=40.6 MeV. The angular distributions show a transition from an almost isotropic emission close to threshold to a highly anisotropic distribution at higher excess energies. The total cross sections reveal a strong {eta}-{sup 3}He final state interaction, corresponding to a scattering length of vertical bar real(a) vertical bar=(4.2{+-}0.5) fm and (a)=(0.4{+-}1.9) fm.

The PHOBOS experiment at the BNL Relativistic Heavy Ion Collider has measured the total multiplicity of primary charged particles as a function of collision centrality in Au+Au collisions at {radical}(s{sub NN})= 19.6, 130, and 200 GeV. An approximate independence of / on the number of participating nucleons is observed, reminiscent of 'wounded nucleon' scaling (N{sub ch}{proportional_to}N{sub part}) observed in proton-nucleus collisions. Unlike p+A, the constant of proportionality does not seem to be set by the pp/pp data at the same energy. Rather, there seems to be a surprising correspondence with the total multiplicity measured in e{sup +}e{sup -} annihilations, as well as the rapidity shape measured over a large range. The energy dependence of the integrated multiplicity per participant pair shows that e{sup +}e{sup -} and A+A data agree over a large range of center-of-mass energies ({radical}(s)>20 GeV), and pp/pp data can be brought to agree approximately with the e{sup +}e{sup -} data by correcting for the typical energy taken away by leading particles. This is suggestive of a mechanism for soft particle production that depends mainly on the amount of available energy. It is conjectured that the dominant distinction between A+A and p+p collisions is the multiplemore » collisions per participant, which appears to be sufficient to substantially reduce the energy taken away by leading particles.« less

Forward-backward correlations of charged-particle multiplicities in symmetric bins in pseudorapidity (eta) are studied in order to gain insight into the underlying correlation structure of particle production in Au+Au collisions. The PHOBOS detector is used to measure integrated multiplicities in bins defined within eta

Two-particle correlations of identical charged pion pairs from Au+Au collisions at {radical}(s{sub NN})=62.4 and 200 GeV were measured by the PHOBOS experiment at BNL Relativistic Heavy Ion Collider (RHIC). Data for the 15% most central events were analyzed with Bertsch-Pratt and Yano-Koonin-Podgoretskii parametrizations using pairs with rapidities of 0.4

This Rapid Communication describes the measurement of elliptic flow for charged particles in Au+Au collisions at {radical}(s{sub NN})=200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider. The measured azimuthal anisotropy is presented over a wide range of pseudorapidity for three broad collision centrality classes for the first time at this energy. Two distinct methods of extracting the flow signal were used to reduce systematic uncertainties. The elliptic flow falls sharply with increasing |{eta}| at 200 GeV for all the centralities studied, as observed for minimum-bias collisions at {radical}(s{sub NN})=130 GeV.

Angular distributions of $^{7}\mathrm{Li}+^{11}\mathrm{B}$ elastic and inelastic scattering were measured at ${E}_{\mathrm{lab}}$($^{11}\mathrm{B}$) $=44\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ for the transitions to the ground and excited states of $^{7}\mathrm{Li}$ and $^{11}\mathrm{B}$. The present data and data taken from the literature at ${E}_{\mathrm{lab}}$($^{7}\mathrm{Li}$) $=34\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ were analyzed within the optical model and coupled-reaction-channels methods. Elastic and inelastic scattering, reorientations of $^{7}\mathrm{Li}$ and $^{11}\mathrm{B}$, and the most important one- and two-step transfers were included in the channels-coupling scheme. The dependence of the $^{7}\mathrm{Li}+^{11}\mathrm{B}$ optical potential on kinetic energy was taken into account. The deformation parameters of $^{7}\mathrm{Li}$ and $^{11}\mathrm{B}$ and optical model parameters for interaction of these nuclei in ground and excited states were deduced. The contributions of one- and two-step transfers to the elastic and inelastic scattering were estimated.

We present results on charged particle production at very low transverse momenta in the 15% most central Au+Au collisions at {radical}(s{sub NN})=200 GeV obtained with the PHOBOS detector at the Relativistic Heavy Ion Collider. The invariant yields were measured at midrapidity in the transverse momentum ranges from 30 to 50 MeV/c for charged pions, 90 to 130 MeV/c for charged kaons and 140 to 210 MeV/c for protons and antiprotons. No significant enhancement in low transverse momentum particle production is observed as compared to extrapolations of identified particle spectra measured at an intermediate p{sub T} range. The spectra tend to flatten at low p{sub T}, consistent with the expectations of transverse expansion of the system.

The paper is devoted to the experimental determination of the space-time characteristics for the target multifragmentation in p(8.1GeV) + Au collisions. The experimental data on the fragment charge distribution and kinetic energy spectra are analyzed within the framework of the statistical multifragmentation model. It is found that the partition of hot nuclei is specified after expansion of the target spectator to a volume equal to Vt = (2.9 +- 0.2) Vo, with Vo as the volume at normal density. However, the freeze-out volume is found to be Vf = (11 +- 3) Vo. At freeze-out, all the fragments are well separated and only the Coulomb force should be taken into account. The results are in accordance with a scenario of spinodal disintegration of hot nuclei., 11 pages, 4 figures, submitted to Phys.Rev.C

The centrality dependence of the midrapidity charged particle multiplicity in $\mathrm{Au}+\mathrm{Au}$ heavy-ion collisions at $\sqrt{{s}_{NN}}=19.6$ and $200\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ is presented. Within a simple model, the fraction of hard (scaling with number of binary collisions) to soft (scaling with number of participant pairs) interactions is consistent with a value of $x=0.13\ifmmode\pm\else\textpm\fi{}0.01(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{syst})$ at both energies. The experimental results at both energies, scaled by inelastic $p(\overline{p})+p$ collision data, agree within systematic errors. The ratio of the data was found not to depend on centrality over the studied range and yields a simple linear scale factor of ${R}_{200∕19.6}=2.03\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{syst})$.

The ratios of charged antiparticles to particles have been obtained for pions, kaons and protons near midrapidity in central Au+Au collisions at sqrt(s_NN) = 200 GeV. Ratios of / = 1.025 +/- 0.006 (stat.) +/- 0.018 (syst.), / = 0.95 +/- 0.03 (stat.) +/- 0.03 (syst.) and / = 0.73 +/- 0.02 (stat.) +/- 0.03 (syst.) have been observed. The / and / ratios are consistent with a baryochemical potential mu_B of 27 MeV, roughly a factor of 2 smaller than in sqrt(s_NN) = 130 GeV collisions. The data are compared to results from lower energies and model calculations. Our accurate measurements of the particle ratios impose stringent constraints on current and future models dealing with baryon production and transport.

The PHOBOS experiment has measured the charged particle multiplicity at mid-rapidity in Au+Au collisions at sqrt(s_NN) = 200 GeV as a function of the collision centrality. Results on dN/deta(eta , are presented as a function of . As was found from similar data at sqrt(s_NN) = 130 GeV, the data can be equally well described by parton saturation models and two-component fits which include contributions that scale as Npart and the number of binary collisions, Ncoll. We compare the data at the two energies by means of the ratio R(200/130) of the charged particle multiplicity for the two different energies as a function of . For events with >100$, we find that this ratio is consistent with a constant value of 1.14+-0.01(stat.)+-0.05(syst.).

We present a measurement of the pseudorapidity density of primary charged particles near midrapidity in Au+Au collisions at $\sqrt{{s}_{\mathrm{NN}}}=130 \mathrm{GeV}$ as a function of the number of participating nucleons. The pseudorapidity density, ${\mathrm{dN}}_{\mathrm{ch}}/d\ensuremath{\eta}{|}_{|\ensuremath{\eta}|l1}/(\frac{1}{2}〈{N}_{\mathrm{part}}〉),$ rises from $2.87\ifmmode\pm\else\textpm\fi{}0.21$ in peripheral events $(〈{N}_{\mathrm{part}}〉\ensuremath{\sim}83)$ to $3.45\ifmmode\pm\else\textpm\fi{}0.18$ in central events $(〈{N}_{\mathrm{part}}〉\ensuremath{\sim}353),$ which is $53\ifmmode\pm\else\textpm\fi{}8%$ higher than $p\overline{p}$ collisions at a similar center-of-mass energy. This is consistent with an additional contribution to charged particle production that scales with the number of binary nucleon-nucleon collisions ${(N}_{\mathrm{coll}}).$

The present data support a large anisotropy in accordance with phase shift predictions and in contrast to another recent experiment.