Search Results (24 total)

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 effects of hadronic rescattering in high energy relativistic Au+Au collisions are studied using two very different models to describe the early stages of the collision. One model is based on a hadronic thermal picture and the other on a superposition of parton-parton collisions. Operationally, the output hadrons from each of these models are used as input to a hadronic rescattering calculation. The results of the rescattering calculations from each model are then compared with rapidity and transverse momentum distributions from the BNL Relativistic Heavy Ion Collider BRAHMS experiment. In spite of the different points of view of the two models of the initial stage, after rescattering, the observed differences between the models are mostly “washed out” and both models give observables that agree roughly with each other and with experimental data.

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.

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.

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.

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.

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.

We present charged-particle multiplicities as a function of pseudorapidity and collision centrality for the ¹⁹⁷Au+¹⁹⁷Au reaction at sqrt[s_NN]  =  200 GeV. For the 5% most central events we obtain dN_ch/dη|_{η  =  0}  =  625±55 and N_ch|_{-4.7≤η≤4.7}  =  4630±370, i.e., 14% and 21% increases, respectively, relative to sqrt[s_NN]  =  130 GeV collisions. Charged-particle production per pair of participant nucleons is found to increase from peripheral to central collisions around midrapidity. These results constrain current models of particle production at the highest RHIC energy.

NA44 uses a 512-channel Si pad array covering 1.5<η<3.3 to study charged hadron production in 158A GeV Pb+Pb collisions at the CERN SPS. We apply a multiresolution analysis, based on a discrete wavelet transformation, to probe the texture of particle distributions event by event, allowing a simultaneous localization of features in space and scale. Scanning a broad range of multiplicities, we search for signals of clustering and of critical behavior in the power spectra of local density fluctuations. The data are compared with detailed simulations of detector response, using heavy-ion event generators, and with a reference sample created via event mixing. An upper limit is set on the probability and magnitude of dynamical fluctuations.

Measurements, with the BRAHMS detector, of the antiproton-to-proton ratio at midrapidities and forward rapidities, are presented for Au+Au reactions at sqrt[s_NN]  =  130 GeV, and for three different collision centralities. For collisions in the 0%–40% centrality range, we find N(p̅ )/N(p)  =  0.64±0.04_(stat)±0.06_(syst) at y≈0, 0.66±0.03±0.06 at y≈0.7, and 0.41±0.04±0.06 at y≈2. The ratios are found to be nearly independent of collision centrality and transverse momentum. The antiproton and proton rapidity densities vary differently with rapidity, and indicate a significant degree of collision transparency, although a net-baryon free midrapidity plateau (Bjorken limit) is not yet reached.

Two-particle interferometry of positive kaons is studied in Pb+Pb collisions at mean transverse momenta 〈p_T〉≈0.25 and 0.91 GeV/c. A three-dimensional analysis was applied to the lower p_T data, while a two-dimensional analysis was used for the higher p_T data. We find that the source-size parameters are consistent with the m_T scaling curve observed in pion-correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.

The invariant cross section as a function of transverse momentum for antideuterons produced in 158A GeV/c per nucleon Pb+Pb central collisions has been measured by the NA44 experiment at CERN. This measurement, together with a measurement of antiprotons, allows for the determination of the antideuteron coalescence parameter. The extracted coalescence radius is found to agree with the deuteron coalescence radius and radii determined from two particle correlations.

The NA44 Collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons, are larger than those of pions. The difference in the inverse slopes of pions, kaons, and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam, suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density dN/dy of both K⁺ and K^- increases more rapidly with system size than for π⁺ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K^-/K⁺ ratio falls with increasing system size but more slowly than the p̅ /p ratio. The π^-/π⁺ ratio is close to unity for all systems. From pBe to SPb the K⁺/p ratio decreases while K^-/p̅ increases and sqrt[(K⁺⋅K^-)/(p⋅p̅ )] stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion and baryon density and an increasing fraction of strange quarks.

Two-pion correlations from Pb+Pb collisions at 158 GeV/c per nucleon are measured by the NA44 experiment at CERN. Multidimensional fits characterize the emission volume, which is found to be larger than in S-induced collisions. Comparison to the RQMD model is used to relate the fit parameters to the actual emission volume.

Experiment NA44 has measured proton and antiproton distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon respectively. The inverse slopes of transverse mass distributions increase with system size for both protons and antiprotons but are slightly lower for antiprotons. This could happen if antiprotons are annihilated in the nuclear medium. The antiproton yield increases with system size and centrality and is largest at midrapidity. The proton yield also increases with system size and centrality, but decreases from backward rapidity to midrapidity. The stopping of protons at these energies lies between the full stopping and nuclear transparency scenarios. The data are in reasonable agreement with RQMD predictions except for the antiproton yields from sulphur-nucleus collisions.

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200 A GeV S+S and 158 A GeV Pb+Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions.

First results of the m_T dependence of π⁺π⁺ and K⁺K⁺ correlations from S+Pb collisions at 200 GeV /c per nucleon measured by the focusing spectrometer of the NA44 experiment at CERN are presented. Multidimensional fits characterize the pion and kaon emission volume. The pion radius parameter decreases with increasing p_T. Furthermore, the pion and kaon radii show a common 1/sqrt[m_T] dependence. This behavior can be interpreted as a result of a strong momentum-position correlation arising from collective flow.

Low-transverse-momentum photon production at the CERN ISR is compared in high- and low-multiplicity events. High-transverse-energy pp collisions and low-multiplicity αα and pp collisions are studied. Relative photon-to-charged-track production is found to be the same in pp minimum bias, αα minimum bias, and pp high-transverse-energy collisions, to within 15%. No low-P_T excess is seen; the limits are dominated by systematic uncertainties.

Relative yields of φ and charged K mesons are measured as functions of charged multiplicity in the central rapidity region for pp, dd, pα, and αα collisions at sqrt[s_NN]=3.15 and 44 GeV. No anomalous strange-particle production is observed in αα reactions relative to pp even at large multiplicities corresponding to events comprising less than 3.6×10^-4 of the inclusive cross section.

Rapidity and charge distributions have been measured for central charged tracks in events at center-of-mass energy 63 GeV, triggered on a π⁰ near 11° with transverse momentum greater than 2.25 GeV/c. The observed distributions are consistent with parton-parton scattering models with a spin-one gluon, and inconsistent with the exchange of a spin-zero gluon.

Measurements of the central multiplicity associated with high-p_t particles up to 2 GeV/c in p-p, p-α, and α-α collisions, and measurements of dσ / dE_t are presented. The difference in associated multiplicity, n (p-α)-n(p-p), is independent of p_t while n (α-α)-n(p-p) rises with p_t, providing new information on the "anomalous" p_t dependence of nuclear inclusive cross sections.