Search

In a bubble chamber experiment with BEBC the spin alignment parameter η= 1 2 (2ϱ 00 − ϱ 11 − ϱ −1−1 ) is measured for ϱ0 mesons produced in deep inelastic charged-current antineutrino and neutrono interactions on neon. In the current fragmentation region η is found to be η v =0.48±0.27 ( stat. )±0.15 ( syst. ) for v Ne and ηv=0.12±0.20 (stat.)±0.10 (syst.) for vNe interactions

This manuscript contains a detailed description of the PHOBOS experiment as it is configured for the Year 2001 running period. It is capable of detecting charged particles over the full solid angle using a multiplicity detector and measuring identified charged particles near mid-rapidity in two spectrometer arms with opposite magnetic fields. Both of these components utilize silicon pad detectors for charged particle detection. The minimization of material between the collision vertex and the first layers of silicon detectors allows for the detection of charged particles with very low transverse momenta, which is a unique feature of the PHOBOS experiment. Additional detectors include a time-of-flight wall which extends the particle identification range for one spectrometer arm, as well as sets of scintillator paddle and Cherenkov detector arrays for event triggering and centrality selection.

The early summer of 2000 saw the start of the RHIC experimental programme. The PHOBOS collaboration used data from the beginning of the running period to extract the first physics information about this exciting new regime. This first measurement consisted of pseudorapidity densities of primary charged particles near mid-rapidity in central Au + Au collisions at beam energies which gave centre-of-mass energies per pair of colliding nucleons of √snn = 56 and 130 GeV. The observed densities are higher than those observed previously in any collisions in the laboratory. In addition, the rate of increase in density between the two energies is significantly larger than that for nucleon-nucleon collisions at comparable beam energies. This paper will briefly describe the PHOBOS experiment (including the subset of the full detector installed for the early period of the RHIC beam), discuss the results of the first physics measurement and conclude with the prospects for the future, especially in the area of strangeness production.

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.

Results are presented on the reduced scaled factorial moments of order 2,3,and 4 in ν μ Ne and ν μ D 2 charged current interaction. Comparisons of the data with a Monte Carlo simulation are presented. The simulation is based on the Lund programs LEPTO 4.3 and JETSET 6.3 and including nuclear reinteractions and smearing due to measurement errors. Nuclear effects are shown to bias the estimate of the intermittency strenghts.

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.

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|1) = 408+/-12(stat)+/-30(syst) at 56 GeV and 555+/-12(stat)+/-35(syst) at 130 GeV, values that are higher than any previously observed in nuclear collisions. Compared to proton-antiproton collisions, our data show an increase in the pseudorapidity density per participant by more than 40% at the higher energy.

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.

Backward proton and pion production is studied in ν and {Mathematical expression} charged current interactions in neon. The results are compared with other experiments and theory. The complete backward proton data is compatible with protons produced by reinteractions in the nucleus. However in events with only one proton, muon variables appear correlated to those for the backward proton, as expected by the two-nucleon correlation model. © 1989 Springer-Verlag.

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 ± .

The charge structure of the final states in 16 GeV/c π±p and 8 GeV/c π+p inclusive interactions is studied. For each event the total charge of particles with rapidities larger than y is measured and the distribution of this charge is analyzed for various values of y. The results are shown to agree with predictions of the neutral isotropic cluster model.

Three- and four-body final states with strange particles are studied in π + p and π − p interactions at 16 GeV/ c . We present cross sections and investigate their energy dependence. Production mechanism, resonance production and quantum number transfer are discussed. Strong Y ∗ (1385) production is found in the reaction π + p → Λ K + π + , while the corresponding π − p reaction is dominated by production of K ∗ (890). In the NK K π channels, the K and K are produced mainly at the same vertex, i.e. non-strangeness exchange ΔS = 0 is dominant (about 75% of the cases), whereas in the Λ K ππ channels, the Λ and K are more frequently produced at opposite vertices, i.e. | ΔS | = 1 exchange is important (about 60% of the cases). Results on the polarization of the lambdas produced in the π + p reactions are given.

The charge transfer distribution, its average, dispersion and skewness are studied in K−p interactions at 110 GeV/c and lower energies. The ratio of dispersion squared to rapidity plateau height is found to be energy independent as suggested by the neutral cluster models. The short-range charge correlations in rapidity are observed at all energies; at 110 GeV/c there is also evidence for a long-range component.

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−.

The longitudinal correlations between the proton and the most forwardly produced beam-like pion ( π f ) are studied in the reactions π ± + p → π f ± + p + anything at 8, 16 and 23 GeV/c. Maxima are observed in the two-particle inclusive cross sections which can be interpreted as resulting mainly from (a) diffractive processes and (b) mechanisms of central particle production. The average multiplicity of final state particles is lower and increases more slowly with increasing energy for diffractive processes than for the central production.

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.

First observation is reported of semi-inclusive coherent diffractive charged current interactions of antineutrinos on neon nuclei. A sharp peaking towards zero is observed in the, 0, SCOPUS: ar.j, info:eu-repo/semantics/published

Results are presented for p → (ΛK+) dissociation in the reactions K−p → ΛK+K− and and π±p → ΛK+π±at 10 and 16 GeV/c. The cross sections for the low-mass ΛK+ enhancement are compatible with the energy dependence σ ∝ plab−0.3. The t′ spectra or the (ΛK+) threshold enhancement are exponential in shape. Its decay angular distribution reveals neither s-channel nor t-channel helicity conservation. The relative probabilities of the processes p → p, p → ( N π) I= 1 2 and p → (Λ K + ) dissociation are in the ratios 100 : 10 : 0.2, independent of the nature of the incident particle.

The differential cross section d σ d t′ for the charge-exchange process π+p → π0 (π+p) at 8, 16 and 23 GeV/c is presented for several regions of the π+p effective mass. It is found that the dip at t′ ≈ 0.6 (GeV/c)2 which is observed in the Δ(1236) mass band becomes a less pronounced structure in the higher mass regions. However, while the slope of the d σ d t′ distributions in the near-forward direction decreases strongly with increasing π+p mass, there is no evidence that the observed structure moves to higher values of t′ as the π+p mass increases. These results are consistent with a Regge-exchange picture where the position of the dip is determined by the exchanged trajectory, but are inconsistent with a simple geometrical picture.

A study of the reaction π + p → p π + π o at 16 GeV/ c incident momentum has been made using the prism plot analysis to reject background events arising from elastic and multineutral contaminations and to separate different reaction channels ( ϱ + p, g + p, Δ + π + , Δ ++ π o , π + (p π o ) DD ). Cross sections, invariant mass distributions and production and decay angular distributions are presented. For the channel corresponding to proton diffraction dissociation strong violation of both s - and t -channel helicity conservation is found for low values of the (p π o ) mass. We demonstrate that the prism plot method provides a better separation of background events than conventional methods using kinematic cuts.

The polarization of theΛ0 hyperon produced inK−p interactions at 110 GeV/c shows a strong dependence on the transverse momentumpt of theΛ0 and no significant dependence onxF. The polarization for largept is negative in the proton hemisphere and positive in the kaon hemisphere. In theproton hemisphere it is compared with the results of otherK−p experiments at lower energies as well as with reactions induced byp,\(\bar p\), π+, π+ andK+. The comparison is performed separately forK−p→Λ0KK+pions andK−p→Λ0+pions showing an analogous behaviour in the former channel. In thekaon hemisphere the comparison with otherK−p experiments at lower energy shows apt-dependence of theΛ0 polarization which changes with energy. The experimental results are discussed in terms of current quark models and of the Mueller-Regge phenomenology. The model predictions reproduce the trend of the data qualitatively well but fail in describing some details.

Results on the inclusive reaction π+ p→Δ++ X0 at 8, 16 and 23GeV/c are used for quantitative tests of the triple Regge formula. As predicted, the slopes ofthe missing mass (M) spectra at fixed t are independent of s. Furthermore the trajectory α(t) obtained from the s-iependence at fixed M appears consistent with that obtained from the M-dependence at fixed s. However, the pomeron intercept α (0) is found to depend on t, suggesting that no single triple Regge diagram dominates at the M and s values studied in this reaction. Provided such a t-dependence is allowed, a single term triple Regge formula gives a very satisfactory parametrisation of our data.

Angular correlations between pairs of secondary particles emitted in 4- and 6-prong events of π + p interactions at 16 GeV/ c are studied as functions of the effective mass M of the two particles involved. It is argued that this is superior to the usual technique of studying correlations as functions of Δy (the difference in the rapidities of two particles), because effects of different origins such as resonance production and Bose-Einstein interference, which all occur predominantly at Δy ≈ 0, are well separated when plotted in terms of the mass. It is shown that the angular correlations are strongly affected by the production of two-particle resonances such as ϱ 0 , f, and g mesons for ( π + π − ) pairs, and Δ ++ (1232) and Δ ++ (1890) isobars for the (p π + ) pairs. A variable D ( M ) is proposed, related to the difference in angular correlations of pairs of like and unlike charges, which is shown to be effective in enhancing resonance signals and could be useful also at higher energies.

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.

Charged particle multiplicity cross sections for 100 GeV/ c π − p inelastic interactions are presented and compared with results at other energies. The results for multiplicities n ⪖ 4 show a consistent trend in their energy dependence. The two-prong cross section curve differs as it tends to flatten off at the higher energies. The zero-prong cross section values decrease steeply as p lab −1.1 . The results for n ⩾ 2 for π − p and pp reactions appear to lie on a “universal” curve if nσ n / σ inel is plotted against 〈 n 〉/ n . This “scaling” rule is equivalent to KNO scaling, but the plot suggested here is more useful in studying low multiplicities. An interpretation in terms of a two-component model and a possible extrapolation to higher energies are proposed.

Positive correlations are observed for pion pairs of equal charge and similar momentum vectors, produced in the reactions π p→p+5 π and π p→p + 7 π at 4 to 25 GeV/ c . The results are consistent with being due to the second-order interference effects expected for pairs of identical bosons and lead to the determination of the average dimension of the “fire-ball’ from which the pions are emitted (〈 R 〉 = 1.0 ± −0.2 +0.4 fm). Attempts are made to study the fire-ball shape by selecting pions pair emitted in various polar and azimuthal directions.

Multiplicity distributions of charged particles produced in proton-proton, proton-argon and proton-xenon inelastic collisions at 200 GeV/c in various rapidity intervals are presented. Nuclear target data are analysed separately for the forward and the backward hemispheres. Results of a similar analysis for only negative particles are shown. The data are well parametrized in terms of negative binomial distributions. However, such a parametrization fails in describing thep-nucleus multiplicity distributions of charged particles for large rapidity intervals comprising both hemispheres. Parameters characteristic for the clan model of Giovannini and Van Hove are also presented.

The reactions π±p→(π±K+K−)p are studied, at 16 GeV/c, and in particular the low-mass π±K+K− system is compared with the diffractively produced 3π system in the reactions π±p→(π±π+π−)p at the same energy. Similarities are found in (a) the weak energy dependence of their cross sections, (b) the shape of the three-meson mass spectra, (c) the differential cross sections which have the same slope for the same value of the three-meson invariant mass, (d) the general characteristics of the partial-wave composition of the low-mass three-meson systems (as for the 3π case, the πKK system is produced in unnatural spin-parity states with JP=1+ dominant at low πKK mass, the state JP=0−, however, may be relatively more abundant in the πKK than in the 3π case) and (e) the exchange which is almost entirely of natural parity. We conclude that the reaction π±p→(π±K+K−p, for low values of the πKK mass, represents the diffractive dissociation process π→πKK, which is less probable than π→πππ by one order of magnitude.

The average multiplicities of charged hadrons and of π+, π− and π0 mesons, produced in\(\bar v\)Ne and νNe charged current interactions in the forward and backward hemispheres of theW±-nucleon center of mass system, are studied with data from BEBC. The dependence of the multiplicities on the hadronic mass (W) and on the laboratory rapidity (yLab) and the energy fraction (z) of the pion is also investigated. Special care is taken to determine the π0 multiplicity accurately. The ratio of average π multiplicities\(\frac{{2\left\langle {n_{\pi ^O } } \right\rangle }}{{[\left\langle {n_{\pi ^ + } } \right\rangle + \left\langle {n_{\pi ^ - } } \right\rangle ]}}\) is consistent with 1. In the backward hemisphere\(\left\langle {n_{\pi ^O } } \right\rangle \) is positively correlated with the charged multiplicity. This correlation, as well as differences in multiplicities between\(\mathop v\limits^{( - )} \) and\(\mathop v\limits^{( - )} \),\(\mathop v\limits^{( - )} \) scattering, is attributed to reinteractions inside the neon nucleus of the hadrons produced in the initial\(\mathop v\limits^{( - )} \) interaction.

A study is presented of μe events from an exposure of BEBC, filed with an heavyH 2/Ne mixture, to a wide band antineutrino beam at the CERN SPS. The promptμ + e − rate, based on the observation of 45 events withp μ>4 GeV/c andp e>0.8 GeV/c, is found to be 0.31±0.05±0.02%, in agreement with previous determinations made on much smaller statistics. The differential kinematical variables characterizing the events and the content of strange particles are as expected for the production of charmed hadrons which subsequently decay semi-leptonically. An upper limit of 2.0±10−4 at 90% C.L. is given for the production rate of promptμ + e + events. No evidence is found for a significant production of Beauty hadrons.

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.

A new set of parameters is introduced to describe charge dependence of three- and four-pion correlations. An effect similar to the GGLP effect observed for pion pairs, but stronger, is found for three- and four-pion systems. This effect, however, can be interpreted as a reflection of the GGLP effect.

Thirty-six plates of the 1-stack, which was exposed in the Po Valley expedition, were scanned for photon-electron cascades to find interactions of the highest energy. The plates were also area scanned for jets of lower energy. Measurements were made of the angular distribution of relativistic tracks. The energy of the colliding nucleons was determined for every jet. The resuits are analyzed. (M.H.R.)

Partial cross sections are presented for the various topologies and the various reaction channels in 16 GeV/ c π − p interactions. Their energy dependence is discussed. The distributions of events as a function of the squared four-momentum transfer are presented, as well as the transverse and c.m. longitudinal momenta distributions. The predictions of the Chan, Łoskiewicz and Allison model describe some of the gross features of these experimental results, but important disagreements are found, mostly in the low multiplicity channels.

Measurements of angular distributions of secondary particles in interactions of high energy primaries of cosmic radiation with the nuclei of photographic emulsion have been performed. A systematic increase in anisotropy of angular distribution in C.M. with increasing primary energy has been stated. It was found that in many high energy jets the number of secondaries in the vicinity ofπ/2 C.M. angle in smaller than that predicted by the theories of Heisenberg or Landau. For explaining these experimental results a model has been introduced in which the secondaries are emitted isotropically from two centres moving in opposite direction in the C.M. system. Some experimental arguments have been given for this isotropy. The relation between the velocity of two centres, the inelasticity coefficient, multiplicity and transversal momenta of secondaries has been discussed.

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.

Invariant single-particle cross sections for pion and proton production in π ± p interactions at 8 and 16 GeV/ c are presented in terms of integrated distributions as functions of x , reduced rapidity ζ and p ⊥ 2 , and also in terms of double differential cross sections E d 2 σ /(d x d p ⊥ 2 ) and d ζ d p ⊥ 2 ). A comparison of π ± and π − induced reactions is made and the energy dependence is discussed. It is shown that the single-particle structure function cannot be factorized in its dependece on transverse and longitudinal momentum. For the beam-unlike pion, there is an indication for factorizability in terms of rapidity and transverse momentum in a small central region.

The mechanism of pion production in eight-prong π + p reactions is discussed in terms of two models: 1. (i) the “fireball” model 2. (ii) multiperipheral CŁA model . Both models give adequate description of the general features of the experimental data, however, they fall in the prediction of fine details of the processes.

An analysis of correlations in rapidity of charged particles produced in proton and antiproton interactions on hydrogen, argon and xenon at 200 GeV is presented. Positive long-range correlations are observed in interactions on heavy targets. The dependence of rapidity correlations between the forward and backward hemisphere on the number of projectile collisions is discussed.