Inelastic cross sections at 60 and 200 GeV/nucleon are determined in a streamer chamber for 16 O on several nuclear targets. Charged particle multiplicity distributions for inelastic and central collisions are studied and compared with theoretical predictions. The inelastic cross section exhibit a geometrical dependence on nuclear radii. The multiplicity data are governed by the collision geometry. They are consistent with a picture of superposition of independent nucleon-nucleus interactions.
A search for a heavy charged gauge boson, W ′, using the decay channels W ′ → eν and W′ → τν → eνν ν is reported. The data used in the analysis were collected by the DØ experiment at the Fermilab Tevatron during the 1992-93 p p collider run from an integrated luminosity of 13.9 ± 0.8 pb −1 at s =1.8 TeV . Assuming that the neutrino from W ′ decay is stable and has a mass significantly less than m W ′ , an upper limit at the 95% confidence level is set on the cross section times branching ratio for p p → W′ → eν . A W ′ with the same couplings to quarks and leptons as the standard model W boson is excluded for m W ′ < 610 GeV/c 2 .
We report on χc1 and χc2 production in the Feynman-x range 0.1<xF<0.8 in 515GeV/c π−Be collisions. The χc states are observed via their radiative decays into J/ψ's. The resulting photons are detected either as showers in the electromagnetic calorimeter or after conversion in the target as e+e− pairs in the tracking system. The fraction of J/ψ production due to χc1 and χc2 decays is 0.443±0.041±0.035. The ratio of the χc1 to χc2 cross section is 0.57±0.18±0.06. Our results on J/ψ, ψ(2S), and χc production indicate that 0.454±0.044±0.042 of J/ψ's are produced directly.
We report the first observation of diffractively produced open charm in 800−GeV/c pp collisions of the type pp→pD*X. We measure cross sections of σdiff(D*+)=(0.185±0.044±0.054)μb and σdiff(D*−)=(0.174±0.034±0.029)μb. Our measurements are based on 4.3×109 events recorded by FNAL E690 in the fixed-target run of 1991. We compare our results with previous fixed-target charm experiments.
The dielectron yield in p + d and p + p collisions at a beam kinetic energy of 4.9 GeV has been measured using the Dilepton Spectrometer (DLS) at the Bevalac. The measured ratio of the yield in p + d to that in p + p collisions, 1.92±0.06, is in disagreement with the assumptions of model calculations applied to our ealier p +Be data, where it was found that p + n bremsstrahlung dominated other sources. While the measured ratio is consistent with a hadron-like origin of the dielectrons, the contributions of known hadronic decays are smaller than the measured yield from p + p collissions.
Pseudorapidity-interval dependence of multiplicity distributions of shower particles produced in high energy interactions of protons at 800 GeV, 4 He at ≈ 11 A GeV, and 28 Si at 14.5 A GeV in nuclear emulsions have been investigated. The multiplicity distributions and correlated moments are parametrised successfully in terms of a negative binomial distribution (NBD). The heavy-ion data for NBD agree well with the predictions of the multistring Monte Carlo code VENUS.
Measurements of charged pion and kaon production in central Pb+Pb collisions at 40, 80 and 158 AGeV are presented. These are compared with data at lower and higher energies as well as with results from p+p interactions. The mean pion multiplicity per wounded nucleon increases approximately linearly with s_NN^1/4 with a change of slope starting in the region 15-40 AGeV. The change from pion suppression with respect to p+p interactions, as observed at low collision energies, to pion enhancement at high energies occurs at about 40 AGeV. A non-monotonic energy dependence of the ratio of K^+ to pi^+ yields is observed, with a maximum close to 40 AGeV and an indication of a nearly constant value at higher energies.The measured dependences may be related to an increase of the entropy production and a decrease of the strangeness to entropy ratio in central Pb+Pb collisions in the low SPS energy range, which is consistent with the hypothesis that a transient state of deconfined matter is created above these energies. Other interpretations of the data are also discussed.
Multiplicity distributions of negatively charged particles have been studied in restricted phase space intervals for central S+S, O+Au and S+Au collisions at 200 GeV/nucleon. It is shown that multiplicity distributions are well described by a negative binomial form irrespectively of the size and dimensionality of phase space domain. A clan structure analysis reveals interesting similarities between complex nuclear collisions and a simple partonic shower. The lognormal distribution agrees reasonably well with the multiplicity data in large domains, but fails in the case of small intervals. No universal scaling function was found to describe the shape of multiplicity distributions in phase space intervals of varying size.
The inclusive production of D$^{*\pm}$ mesons in two-photon collisions is measured with the ALEPH detector at $\epem$ centre-of-mass energies from 183$\unit{GeV}$ to 209$\unit{GeV}$. A total of $360 \pm 27$ D$^{*\pm}$ meson events were observed from an integrated luminosity of 699\unit{pb^{-1}}$. Contributions from direct and single-resolved rocesses are separated using the ratio of the transverse momentum $p_{\rm t}^{\rm D^{*\pm}}$ of the D$^{*\pm}$ to the visible invariant mass $W_{\mathrm{vis}}$ of the event. Differential cross sections of D$^{*\pm}$ production as functions of $p_{\rm t}^{\rm D^{*\pm}}$ and the pseudorapidity $|\eta^{\rm D^{*\pm}}| $ are measured in the range $ 2\unit{GeV}/c < p_{\rm t}^{\rm D^{*\pm}} < 12\unit{GeV}/c $ and $ |\eta^{\rm D^{*\pm}}| < 1.5 $. They are compared to next-to-leading order (NLO)perturbative QCD calculations. The extrapolation of the integrated visible D$^{*\pm}$ cross section to the total charm cross section, based on the Pythia Monte Carlo program, yields $ \sigma (\epem \to \epem \ccbar)_ {=197\unit{GeV}} = 731 \pm 74_{\mathrm{stat}} \pm 47_{\mathrm{syst}} \pm 157_{\mathrm{extr}} \unit{pb} $.
In June 1996, the LEP centre-of-mass energy was raised to 161 GeV. Pair production of W bosons in e + e − collisions was observed for the first time by the LEP experiments. An integrated luminosity of 11 pb −1 was recorded in the ALEPH detector, in which WW candidate events were observed. In 6 events both Ws decay leptonically. In 16 events, one W decays leptonically, the other into hadrons. In the channel where both Ws decay into hadrons, a signal was separated from the large background by means of several multi-variate analyses. The W pair cross-section is measured to be σ WW = 4.23 ± 0.73 (stat.) ± 0.19 (syst.) pb. From this cross-section, the W mass is derived within the framework of the Standard Model: m W = 80.14 ± 0.34 (stat.) ± 0.09 (syst.) ± 0.03 (LEP energy) GeV/ c 2
Forum