Search

We present the first measurement of pseudorapidity densities of primary charged particles near midrapidity in Au+Au collisions at sqrt[s(NN)] = 56 and 130 GeV. For the most central collisions, we find the charged-particle pseudorapidity density to be dN/deta|(|eta

PHOBOS is a compact silicon detector designed to study the physics of Au - Au collisions at a center-of-mass energy of 200 A GeV at the Relativistic Heavy Ion Collider. The main goal of the PHOBOS experiment is a comprehensive study of various hadronic observables that should reveal the onset of new phenomena expected to occur in central collisions of gold nuclei. In this paper the detector and the physics program of the PHOBOS experiment is presented.

The production of hadrons in charged-current (anti)neutrino interactions is studied with the bubble chamber BEBC exposed to the CERN (anti)neutrino wide-band beam. Fast-hadron production in a neon target is found to be attenuated as compared to that in a hydrogen target. This feature is discussed within theoretical models based on the idea of a hadron formation length. The experimental results favour the ‘constituent’ over the ‘yo-yo’ length concept, and suggest a quark cross section in the order of 3 mb.

Most of the measured events on hydrogen, deuterium and neon targets from BEBC at CERN and the 15-foot Bubble Chamber at Fermilab are combined to measure spin-density matrix elements for ρ0 production in the current-fragmentation region in neutrino and antineutrino charged-current interactions. The spin-alignment parameter η is found to be 0.08±0.11 for neutrino and 0.41±0.11 for antineutrino interactions. The parity-odd density matrix element Im(ρ10+ρ−10) is consistent with zero in both reactions. In the combined neutrino and antineutrino data an indication of parity violation is observed: Im ρ1−1=0.38±0.16 atxB>0.2,Q2>8 GeV2 andz>0.6.

The coherent production of 3π systems by charged current antineutrino interactions on neon nuclei is studied in the bubble chamber BEBC exposed to the CERN SPS wide band beam. The coherent signal is attributed mainly to (ρπ) production, where the (ρπ) system is either non-resonant or comes from the decay of a1 mesons. The kinematical variables and the total cross section are compared with the predictions of the meson dominance model. © 1991 Springer-Verlag.

Evidence of the production of the Ds∗-(2111) meson in v μ Ne collisions is presented. The Ds∗- production rates the Ds−→φπ− branching fraction is estimated to be (2.3±0.7) × 10−3 of all charged currents having a hadronic mass W above 3 GeV.

The PHOBOS experiment at RHIC has measured the charged-particle density dN/dη at mid-rapidity for central Au+Au collisions at center of mass energies of sNN=56, and 130 GeV. We deduce that dN/dη=408±12(stat)±30(syst) and 555±12(stat)±35(syst) for collision energies of 56 GeV and 130 GeV, respectively. These numbers suggest energy densities that are some 70% higher than have been achieved in any heavy-ion collisions previously studied, and also 25–40% higher than nucleon-nucleon collisions at comparable center of mass energies.

Four data sets of charged current neutrino and antineutrino interactions with neon, deuterium and hydrogen collected from BEBC and the 15 Foot Bubble Chamber are used to study the Bose-Einstein correlations between like-sign charged pions. Two forms of the parametrization for the effect are used. No substantial differences are found between the data sets obtained with neon, deuterium and hydrogen targets. The Lorentz invariant parametrization of Goldhaber gives for the radius of the pion emission region the valuer G =0.80 ±0.04±0.16 fm and for the chaoticity parameter the value ψ=0.61±0.04±0.15. Using the Kopylov-Podgoretskii parametrization yieldsr K =1.27±0.06±0.12 fm, ψ=0.58±0.03±0.12 and for the pion source lifetimec τ=0.52±0.05±0.12 fm. The Goldhaber parametrization was used to study the effect in further detail. The same emission radius and the same strength of the effect were found for particles produced in interactions on neutrons and protons. The data are compatible with a spherical shape of the pion emission region. No multiplicity or forward-backward dependences are found. No dependence of the effect on the event kinematical variables is seen.

The diffractive production of charmed strangeD s * and possiblyD s mesons by neutrinos and antineutrinos on nucleons in hydrogen, deuterium and neon targets is observed. The slope parameter of thet distribution is 3.3±0.8 (GeV)−2. The production rate per charged current neutrino interaction with an isoscalar target times the D s + →φτ+ branching fraction is (1.03±0.27)×10−4.

The production of the meson resonances ϱ(770) (all three charge states), η(550), ω(783) andf 2(1270) in $$\bar v$$ Ne and ν Ne charged current interactions is investigated in a bubble chamber experiment with BEBC at CERN. Except for thef 2, the main features of resonance production are reasonably well described by the Lund model, although the average resonance multiplicities are overestimated by the model by (67±30)%. The average multiplicities of all resonances, including thef 2, are well reproduced by a semiempirical model, whose parameters were determined from hadron interaction data.

The longitudinal momentum spectra of mesons produced in the projectile fragmentation region ofK − p interactions at 110 GeV/c, measured in a bubble chamber experiment, are compared to two fragmentation models related to hadron production by incident leptons. The models give a qualitative description of the data. However, it is found that the mesons having a valence quark in common with the projectile tend to have higher momenta than predicted.

Distributions of the Feynman x variable have been determined for positive and negative pions in charged current neutrino-proton and antineutrino-proton reactions with hadronic energy W > 3 GeV and Bjorken x B > 0.1. The distributions have been corrected for experimental effects such as measurement errors, uncertainties in estimating the neutrino energy and particle misidentification. In the framework of the quark-parton model, the distributions yield information about the fragmentation of forward going u and d quarks and backward going uu and ud diquarks. Approximate Feynman scaling is observed for the invariant Feynman x F distributions. They can be fitted by a power law of the form (1 − | x F |) n as suggested by the dimensional counting rules. Simple isospin relations predicted by the quark-parton model are fulfilled. The fragmentation of diquarks is compared with that of protons into π ± .

By means of kinematic cuts, we select 1 421 events of the reaction K−p→K−pπ+π− at 16 GeV/c which involve the diffractive excitation of the proton. Decay of the diffractive cluster is most simply described in the Gottfried-Jackson reference frame, where a highly anisotropic decay is observed. The cluster, containing spin components J ⩾ 52, has a dominant decay mode δ++ (1236) π−.

Interactions of 200-GeV/c protons and antiprotons on hydrogen, argon, and xenon targets were studied with a streamer-chamber vertex spectrometer at the CERN SPS. Results on multiplicities, rapidity distributions, and correlations are presented and compared with predictions of current models.

Charge properties of the hadronic systems from νp and ν p scattering in BEBC are studied in the framework of the quark-parto model (QPM). The average charges 〈Q jet 〈 ν and 〈Q jet 〈 ν of a quark jet and a d-qua jet, respectively, are determined according to two different methods. The difference 〈Q jet 〈 ν − 〈Q jet 〈 ν is in agreement with e QPM value of 1. Scaling of charge and energy flow in the angular variable λ is demonstrated. The ratios ΔQ ν /ΔQ ν of charge flows in νp a ν p scattering are in accord with the QPM in both hemispheres.

Interactions of 150 GeV pions and 300 GeV pions and protons on hydrogen which result in final states with large transverse energy in the central rapidity region −0.88 < y < 0.67 were studied with a calorimeter trigger. The dependence of the cross section on the azimuthal acceptance of the trigger, on the beam energy and on the incident particle type and the jet properties of the events were investigated. No dominant two-jet structure was found. Nevertheless, the parton spectrum in the colliding hadrons appears to be relevant for large transverse energy reactions.

Large transverse energy cross sections of 300 GeV/ c pions and protons on hydrogen have been measured with a segmented calorimeter covering the central rapidity region −0.88 y π in azimuth. The selected events show large multiplicities and no jet-like event structure. Processes more complicated than the scattering of two constituents appear to dominate these inelastic collisions.

Distributions of four-momentum transfer from target proton to nucleon, d σ /d t , for 16 GeV/ c π − p reactions giving in the final state 2 to 8 pions, are analysed in terms of the F ( t ) function, which gives the average t -dependence of the matrix element squared for the reaction in question. It is shown that this function has a shape similar to the squared pion propagator for the reactions containing 3 to 8 pions in the final state. The data for 8 GeV/ c π + p reactions give results qualitatively similar to those for 16 GeV/ c π − p.

Ten prong π + p interactions at 8 GeV/ c incident momentum are analyzed. In contrast to single-particle distributions which are found to be consistent with phase space, deviations from phase space are observed in the case of some two- and more particle distributions.

The coherent production of π+ mesons by charged current neutrino interactions on neon nuclei in BEBC is compared to the predictions of the PCAC hypothesis and the meson dominance model. Good agreement is observed. © 1989 Springer-Verlag.

Total and differential cross sections ofK*−(890),K*−(890),\(\bar K^{ * 0} \)(890),K*0(890),\(\bar K^{ * 0} \)(1430) andϱ0(770) produced inK−p interactions at 110 GeV/c are presented. The cross sections of the neutral resonances show a smooth increase with energy from 10 to 110 GeV/c incident momentum. For theK*+(890) and theK*0(890), i.e. the resonances with strangenessS=+1, this rise is quite significant: their cross sections practically double between 32 GeV/c and 110 GeV/c incidentK− momentum. About 50% of the neutral kaons and 30% of charged pions produced inK−p interactions at our energy are found to be decay products of the resonances considered.

Comparing the kinematical distributions of events obtained on neon and deuterium targets in similar experimental conditions reveals a reduction of the neutrino and antineutrino charged current cross section per nucleon in neon at low Q 2 . The effect, interpreted as due to geometric shadowing of the weak propagator in interactions of neutrinos and antineutrinos with nuclei, agrees well with predictions derived from PCAC.

The cross section for coherent production of a single π- meson in charged current antineutrino interactions on neon nuclei has been measured in BEBC to be (175±25) 10-40 cm2/neon nucleus, averaged over the energy spectrum of the antineutrino wide band beam at the CERN SPS; this corresponds to (0.9±0.1) % of the total charged current {Mathematical expression} cross section. The distributions of kinematical variables are in agreement with theoretical predictions based on the PCAC hypothesis and the meson dominance model; in particular, the Q2 dependence is well described by a propagator containing a mass m=(1.35±0.18) GeV. The absolute value of the cross section is also in agreement with the model. This analysis thus provides a test of the PCAC hypothesis in the antineutrino energy range 5-150 GeV. © 1986 Springer-Verlag., 0, 42 auteurs, SCOPUS: ar.j, info:eu-repo/semantics/published

Multiplicity distributions and correlations between charged particles in the forward and back-ward c.m. hemispheres are studied inK −p interactions at 110 GeV/c and compared with other data on mesonnucleon scattering. The interpretation in terms of a simple quark-parton picture assuming that the forward multiplicity is dominated by quark fragmentation and the backward multiplicity by diquark fragmentation is supported by the experimental fact that the forward and the backward mean multiplicities are approximately equal to half of thee + e − andpp multiplicities, respectively. The 110 GeV/cK −p data show significant correlations between the numbers of slow forward and slow backward particles, whereas the multiplicities of fast forward and fast backward particles are independent.

Preliminary large transverse energy cross sections of 150, 300 GeV pions and protons on hydrogen measured with a large acceptance, segmented calorimeter are presented. Processes other than the scattering of two constituents appear to dominate this deep inelastic hadron scattering process.

Coherent production of ρ- mesons in charged current antineutrino interactions on neon nuclei is studied in the BEBC bubble chamber exposed to the CERN SPS wide band beam. The cross section is measured to be (95±25)·10-40 cm2 per neon nucleus, averaged over the beam energy spectrum. The distributions of kinematical variables and the absolute value of the cross section are in agreement with theoretical predictions based on the CVC hypothesis and the vector meson dominance model. © 1987 Springer-Verlag.