Scaling and fragmentation distributions of beam remnants in high Et proton-proton collisions at √s =27.4 GeV

Charged particle trajectories have been reconstructed in pp collisions triggered by transverse energies (E_t) ranging from 1 GeV to 20 GeV. The forward fragmentation function of negative particles in the beam jet is found to scale with E_t in variable x=2p_L/√s−E_t, and the slope is near to that found for small jet triggers in an earlier experiment. Above about E_t=8 GeV, positive particles scale and for 0.5<x<1.0 the ratio of positives to negatives is 1.73±0.2. The forward fragmentation functions at high E_t do not show the behavior expected from the fragmentation of diquarks and are more like that from quarks.Multiplicity and energy flow in various forward polar angle regions are also presented; they are compared to the beam jet predictions of ISAJET and found to be greater at wide angles and less at small.     

Strangelet search and particle production studies in Pb+Pb collisions at 158 AGeV/c at CERN SPS

We searched for long-lived strange quark matter particles, so-calledstrangelets, and studied particle and antiparticle production in Pb+Pb collisions at 158 GeV/c per nucleon at zero degree production angle. We give upper limits for the production of strangelets covering a mass to charge ratio up to 120 GeV/c ² and lifetimest _lab>1.2 μs and plot invariant differential production cross sections as a function of rapidity for a variety of particles.     

The transverse energy distribution in hadron-lead collisions

The transverse energy cross-sectiondσ/dE _T has been measured in the pseudorapidity region 0.6<η<2.4 for hadron-lead collisions at 200 GeV/c incident hadron momentum. TheE _T distribution extends to 40 GeV, which is twice the kinematic limit forp-p collisions at the same incident beam momentum. The distribution ofE _T is found to shift towards low pseudorapidities with increasing total transverse energy.     

Jet production from nuclei at 400 GeV/c

Results are presented from a calorimeter study on the production of jets in large-E_T proton-nucleus collisions at 400 GeV/c. Jet-like large-p_T events from eight nuclear targets are seen with a special jet trigger. As previously observed, the cross section for such events increases slightly faster than the atomic number, much like in the production of large-p_T single hadrons. Data on energy flows and multiplicities suggest that a heavy nucleus does not significantly widen the angular distribution of the beam remmants. In the central region the particle flow in hard nuclear collisions is similar to that in soft nuclear collisions.     

Baseline cold matter effects on J/ω production in AA collisions at RHIC

We present baseline calculations of initial-state shadowing and finalstate absorption effects on J/Ψ production in nucleus-nucleus collisions at the Relativistic Heavy Ion Collider. We show predictions for Au+Au and Cu+Cu collisions at √S _NN =200 GeV and Cu+Cu collisions at √S _NN =62 GeV as a function of the rapidity, y, and the number of binary nucleon-nucleon collisions, N _coll.     

Hadron Pt spectra from 0.03 to 6 GeV/c from PHOBOS

We review some of the latest results on identified and non-identified particle spectra for d+Au and Au+Au collisions at √s _NN =200 GeV from the PHOBOS Experiment at RHIC. No enhancement of hadron yields at very low PT is observed. Analysis of the high-PT spectra in d+Au and Au+Au collisions reveal the strong final state effects for central Au+Au collisions. The first measurements of identified particle spectra and particle ratios from the PHOBOS Time of Flight (ToF) detector are presented for d+Au collisions. Detailed results on centrality and pseudorapidity dependence of transverse momentum spectra, which may have important physics implications. are also shown.     

The energy dependence of nuclear reaction cross sections

The reaction cross section σ_R^pA(E) for proton-nucleus collisions varies with energy as the total cross section σ_t^pN(E) does for proton-nucleon scattering in the energy regime 100 MeV ? E ?400 GeV. We propose several empirical relations, and test them by a χ² analysis of various data. Within the framework of Glauber theory, the parameters in the relations can be linked to the cross section for a single inelastic collision.     

Excitation of theΔ (1232)-resonance in proton-nucleus collisions

The excitation of theΔ resonance is observed in proton collisions on C, Nb and Pb targets at 0.8 and 1.6 GeV incident energies. The mass E₀ and widthΓ of the resonance are determined from the invariant mass spectra of correlated (p, π^±)-pairs in the final state of the collision: The mass E₀ is smaller than that of the free resonance, however by comparing to intra-nuclear cascade calculations, this reduction is traced back to the effects of Fermi motion, NN scattering and pion reabsorption in nuclear matter.     

Significance of temperature measurements in relativistic nuclear collisions

We discuss the importance of temperature measurements for probing the properties of dense, hot hadron matter in relativistic nuclear collisions. The effects of the collective matter flow are considered. It is pointed out that information about the existence of a limiting temperatureT ^max?m _σ can only be obtained from future experimental facilities with beam energiesE _LAS>5 GeV/n. We also discuss the possibility of observing abnormal nuclear matter via a secondary, high temperature component in the particle spectra and via a shoulder in the pion multiplicity distributions.     

Cosmic-ray induced vacuum decay in the standard model

In the standard model, if the Higgs boson mass, m_H, and the top quark mass, m_t, satisfy the relationship m_t≳95GeV + 0.60 m_H, then the vacuum is unstable. However, if the top quark mass is less than 190 GeV, then the lifetime is greater than the age of the universe. There is thus a large region of parameter space in which the vacuum is unstable, but sufficiently long-lived. We examine the possibility that high energy cosmic ray collisions could induce the decay of the vacuum, and show that this region of parameter space can be excluded.     

Measurement of the azimuthal anisotropy coefficient <Emphasis Type="Italic">v</Emphasis><Subscript>2</Subscript> for the emission of α particles in nucleus-nucleus collisions at energies 1.88–10.6 GeV/nucleon

The azimuthal anisotropy of the emission of α particles in collisions of the ²²Ne, ²⁴Mg, ⁵⁶Fe, and ¹⁹⁷Au nuclei with photoemulsion nuclei has been measured at projectile energies E_pr = 1.88–10.6 GeV/nucleon. The results are compared with similar measurements for protons. It has been found that the ratio of the azimuthal anisotropy coefficients v₂ for α particles and protons is equal to 6 ± 2 at low energies E_pr ≈ 2 GeV/nucleon, whereas these coefficients coincide with each other for energies E_pr ≥ 4 GeV/nucleon. This difference may indicate that, at low projectile energies, α particles are formed predominantly at the early stage of a collective flow. Formation of α particles for E_pr ≥ 4 GeV/nucleon likely occurs at the stage of nuclear matter scattering.     

Forward a production and nuclear stopping power in d+Au collisions at RHIC

Using the forward time projection chambers of STAR we measure the centrality dependent A and ā yields in d+Au collisions at √s _NN =200 GeV at forward and backward rapidities. The contributions of different processes to particle production and baryon transport are probed exploiting the inherent asymmetry of the d+Au system. While the d side appears to be dominated by multiple independent nucleon-nucleon collisions, nuclear effects contribute significantly on the Au side. Using the constraint of baryon number conservation, the rapidity loss of baryons in the incoming deuteron can be estimated as a function of centrality. This is compared to a model and to similar measurements in Au+Au, which gives insights into the nuclear stopping power at relativistic energies.     

Partial-transparency effects in heavy-ion collisions at energies of the order of 1 GeV/nucleon

A model of heavy-ion collisions at energies E_lab ～ 1 GeV/nucleon taking into account the effects of incomplete statistical equilibrium in highly-excited nuclear matter is presented. The collision process is considered as the interaction of two flows of nucleons decelerating each other. This process is described within the framework of a relativistic kinetic approach employing the Fokker-Planck approximation. Assuming two-flow nonequilibrium the momentum distribution function is represented as a sum of two maxwellian distributions displaced by the average relative velocity of flows. Equations for time evolution of space-averaged velocities and internal energies of flows are derived. These equations contain a single model parameter, i.e. the effective deceleration length λ_d. Using the firestreak model geometry inclusive cross sections of protons and composite particles (d, t) in the reactions Ne + NaF, Ar + KCl and Ar + Pb are calculated at various values of E_lab. In contrast to the firestreak model, a complete stopping of colliding tubes in the centre-of-mass frame was not assumed in the present calculation. Composite particle spectra are calculated on the basis of the coalescence model. The theory is thoroughly compared with the experimental data. The approach suggested allows us to reproduce a two-humped structure in the rapidity distributions of the secondary particles. The experimental data analysis leads to the value λ_d = 8 fm for E_lab = 0.8 GeV/nucleon, which is in good agreement with the estimation based on experimental NN cross sections.     

Large transverse momentum particle production in αα and pp collisions at the CERN ISR

The production of charged hadrons with high p_T in αα collisions at √s=126 GeV and pp collisions at √s=31 and 63 GeV is compared, and the structure of the events associated with the high-pT particles is studied. The probability of finding associated particles close to the trigger particle increases strongly between √s=31 and 63 GeV for pp collisions. For p_T>2.5GeV/c the αα/pp cross section ratio at the same energy per nucleon is measured to be 18.7 ± 2.0, to be compared with A² = 16, and a higher associated multiplicity is observed for αα.     

Results from the first physics run of PHENIX at RHIC

First results from measurements of Au?Au collisions at $\sqrt {s_{NN} } = 130$ GeV, performed with the PHENIX experiment during the initial physics run of Brookhaven National Laboratory’s Relativistic Heavy Ion Collider in summer 2000, are summarized. Global characterization of the collisions was obtained from charged-particle multiplicity and transverse energy densities. At mid-rapidity, dN _ch /dν and dE _T /dν rise steadily with the event centrality, while the ratio <E _T >/<N _ch > remains constant. Transverse momentum spectra of charged particles up to 5 GeV/c yield a suppression of high-momentum particles in the most central events when compared to a simple scaling of binary nucleon-nucleon collisions. The scaling matches with peripheral collisions. Transverse momentum spectra of identified neutral pions from 1 to 4 GeV/c in peripheral collisions show consistency with point-like scaling with the average number of binary collisions. Yields in central collisions are significantly suppressed. Transverse momentum spectra of identified charged hadrons up to 3.5 GeV/c were measured in minimum bias events. The mass and centrality dependencies of inverse slope parameters and mean transverse momenta were determined. Inclusive spectra of electrons were measured for momenta from a few hundred MeV/c to 4 GeV/c. Inclusive photon spectra from a few hundred MeV/c to 3 GeV/c were obtained from conversion pair measurements.     

Measurement of the elliptic flux of Z ⩾ 2 charged particles in collisions of relativistic nuclei with photoemulsion nuclei

The azimuthal asymmetry is measured for the emission of Z ≥ 2 particles from the interaction of ²²Ne, ²⁴Mg, ³²S, ⁵⁶Fe, ¹⁹⁷Au, and ²⁰⁷Pb nuclei with photoemulsion nuclei that is induced by semicentral collisions characterized by projectile energies in the range E _pr = 1.88–200 GeV per nucleon and by impact-parameter (b) values in the range 0.12 ≤ b/b _max ≤ 0.70. The results of these measurements are compared with the results of similar measurements for protons. It is found that, at a low energy of E _pr ≈ 2 GeV per nucleon, the ratio of the azimuthal-anisotropy coefficients v ₂ for Z ≥ 2 particles and protons is 6 ± 2, but that, for energies in the region E _pr ≥ 4 GeV per nucleon, the coefficients in question agree with each other. This may suggest that, at low energies, Z ≥ 2 particles are predominantly formed at an early stage of the development of a collective flow. For E _pr ≥ 4 GeV per nucleon, these particles are presumably formed at the stage of nuclear-matter expansion. Other possible explanations for the results of the observation of an elliptic flux of Z ≥ 2 particles are discussed.     

Multiplicity dependence of the pion source in S+A collisions at the CERN SPS

The emission of pions from relativistic heavy-ion collisions of S+S, S+Ag and S+Pb at 200 GeV/nucleon is characterized using two-particle interferometry. The multiplicity dependence of the pion source parameters near midrapidity is studied. The transversal (R _t) and longitudinal (R _l) pion source parameters are independent of the initial nuclei in the interaction and increase with increasing multiplicity. This suggests that the freeze-out process is governed mainly by the particle multiplicity. The multiplicity dependence is weaker than that expected from a simple model of a freeze-out at a constant density.     

A semi-statistical model of high-energy collisions

The emission of particles directly created in high-energy collisions is considered to be only partly governed by statistical laws as in Fermi's model. The motion of the created particles is to some degree related to the former motion of the incoming colliding particles. In first approximation this intermediate state is built up of two extremes: two Lorentz systems are introduced from which the particles are emitted isotropically and according to statistical laws. In the centre-of-momentum system (CMS) of the collision these two systems move in the directions of the incoming particles after collision. This approximation corresponds to the two-fireball model which we regard as a representative of a mathematical method of dealing with correlations. In further approximation phase space distributions are replaced by the thermodynamic Fermi or Bose distributions for relativistic particles. Both experimental data such as the nearly constant mean transverse momentum¯p _t and phase space calculations with constant interaction volume show that the temperaturekT reaches an upper limit asymptotically at high energies; the asymptotic region begins at about 30 GeV. The comparison with experimental particle spectra from accelerators in the 10–30GeV region shows good agreement if one uses a smooth dependence ofkT on the collision energyE ₀ and, for each independent set of measurements, an individual choice of \(\bar \gamma _f \) , the mean Lorentz factor of the “fireballs” in the CMS, and of ¯n, the mean number of created particles. The p_t-distribution of pions and the dependence of¯p _t on the particle mass can also be successfully described. At very high energies the model gives production ratios of the various kinds of particles which lie within the range of the experimental determinations. The dependence of \(\bar \gamma _f \) onE ₀ is concluded to be of the form \(\bar \gamma _f \propto E_0^{1/2} /\bar n(E_0 ) \approx 0.5E_0^{1/4} \) , i.e. CMS anisotropy is related to multiplicity.     

K + production in proton-nucleus collision

We present a dynamical study ofK ⁺ production in proton-nucleus collisions from 1.2 to 2.5 GeV bombarding energy. The evolution of the proton-nucleus collision is described by a transport equation of the Boltzmann-Uehling-Uhlenbeck type. We incorporate all known sources forK ⁺ production and study their momentum and angular distributions, and the excitation function. We show that at lower energies (E _b<1.5 GeV) theNΔ andNN* channels dominate the kaon yield for light systems. At higher bombarding energies the directNN channel accounts for almost the whole cross section.     

Characteristics of charged particle production in relativistic heavy-ion collisions

Inclusive spectra of charged particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV were measured with the STAR detector at RHIC. The measured mean transverse momentum 〈p _T 〉 shows a characteristic dependence on charged particle multiplicity and beam energy in Au+Au collisions that is distinctly different from pp, $p\bar p$ and e⁺e^? collisions. A 32%±3%(syst) increase in 〈p _T 〉 from pp to Au+Au collisions was observed at 200 GeV. While the charged multiplicity was found to increase by 19%±5%(syst) from $\sqrt {s_{NN} } = 130$ GeV to 200 GeV, no significant difference in 〈p _T 〉 was found between the two energies. A comparison with model predictions is discussed.