Dijet production in diffractive deep inelastic scattering at HERA

The production of dijets in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 61 pb^-1. The dijet cross section has been measured for virtualities of the exchanged virtual photon, 5 < Q² < 100 GeV², and γ^*p centre-of-mass energies, 100 < W < 250 GeV. The jets, identified using the inclusive k_T algorithm in the γ^*p frame, were required to have a transverse energy E^* _T,jet > 4 GeV and the jet with the highest transverse energy was required to have E^* _T,jet > 5 GeV. All jets were required to be in the pseudorapidity range -3.5<η^* _jet<0. The differential cross sections are compared to leading-order predictions and next-to-leading-order QCD calculations based on recent diffractive parton densities extracted from inclusive diffractive deep inelastic scattering data.     

Inclusive processes and shadow effects in deuteron

Using the data on inclusive spectra we analyze the contribution of inelastic intermediate states to the shadow correction Δ for high-energy (E > 10 GeV) hadron-deuteron scattering. We note that the intermediate states with large masses M² ～ ER^?1 may contribute appreciably to the shadow correction, but the estimate of this contribution in the framework of the Regge-pole model for inclusive processes shows that it is small. The main contribution to the Δ is due to the small masses M < 2 GeV, which are produced by diffraction dissociation. We calculate the value Δ_inel for pd, πd and Kd scattering and estimate the energy dependence of Δ. The results are in agreement with the existing experimental data.     

Two-jet mass distributions at the CERN proton-antiproton collider

Two-jet mass distributions have been measured as a function of centre-of-mass scattering angle for high-mass jet pairs produced in proton-antiproton collisions at the CERN collider operating at a centre-of-mass energy of 630 GeV. The agreement between QCD expectations and the experimental measurements has been used to place limits on the production cross section of an object X decaying into two jets. In particular we consider the existence of a massive colour octet of vector gauge bosons (axigluons). We exclude axigluons with a width Λ_A < 0.4m_A and a mass m_A in the range 150 < m_A < 310 GeV/c² (95% CL).     

Two-photon exchange in deep inelastic scattering

We have measured the contribution of the two-photon exchange amplitude in deep inelastic muon-nucleon scattering. We find, with 95% confidence, that the real part of this amplitude is less than 0.017 of the one photon-exchange amplitude. The range of the 4-momentum transfer covered is 0.5 < Q² < 2.1 (GeV/c)².     

A study of deep inelastic hadron-hadron collisions with a large acceptance calorimeter trigger

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 < 0.67 and 2π 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.     

Transverse-momentum dependence of J/Ψ production in nuclear collisions

The suppression of J/Ψ production at transverse moment p_⊥ < 2 GeV/c in central ¹⁶O+²³⁸U→Ψ+X at 200 A GeV has been interpreted as a possible signature of quark-gluon plasma formation. We show, however, that the observed p_⊥ dependence is consistent with extrapolations from p+A→Ψ+X data, and that quasielastic initial-state parton scattering together with final-state inelastic hadronic reactions may explain the preliminary data.     

The spectrum of protons produced in pp collisions at 31 GeV total energy

Data are reported on the distributions in longitudinal and transverse momentum of protons produced in the range 0.5 < x < 1.0 (x = Feynman variable) and 0.2 < p_T² < 1.8 (GeV/c)² in proton-proton collisions at 31 GeV c.m. energy at the CERN ISR. The invariant inelastic cross section shows a peak at high longitudinal momenta. The shape of this peak suggests substantial production of states with masses up to at least 7 GeV.     

Evidence for a change of slope in large-t elastic proton-proton scattering at s=53 GeV

New experimental results are presented on proton-proton elastic scattering in the range of momentum transfer 4 GeV² < ?t < 10 GeV² at the centre-of-mass energy of √s = 53 GeV. The data have been obtained using the Split-Field Magnet detector at the CERN Intersecting Storage Rings. We observe another change of slope of the differential cross section near ?t=6.5 GeV².     

Jet-like properties of multiparticle systems produced in K+p interactions at 70 GeV/c

We present results on the jet-like properties of multiparticle systems produced in K⁺p interactions at 70 GeV/c. The data analysed in terms of several variables commonly used to study the jet structure of an event. An extensive comparison is made with jets found in electron-positron annihilations and in deep inelastic νN interactions at comparable energy. Many similarities are found between low-p_T jets in this experiment and jets observed in leptonic interactions. Our data are very well reproduced by the Field-Feynman quark fragmentation parametrisation but equally well by a longitudinal phase space model, suggesting that these similarities do not prove or disprove the universal character of the jet fragmentation.     

Comparison of scaling deviations in neutrino,electron and muon inelastic scattering

The energy dependence of the x = q²/2Mv distribution in vp and vN inelastic scattering ～2 GeV to ～60 GeV is found to be consistent with the deviations from Bjorken scaling observed in inelastic μN/eN scattering. A simple parametrization of the deviations is given.     

Study of the effective transverse momentum of partons in the proton using prompt photons in photoproduction at HERA

The photoproduction of prompt photons, together with an accompanying jet, has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb⁻¹. A study of the effective transverse momentum, 〈k_T〉, of partons in the proton, as modelled within the framework of the PYTHIA Monte Carlo, gives a value of 〈k_T〉=1.69±0.18^+0.18_−0.20 GeV for the γp centre-of-mass energy range 134<W<251 GeV. This result is in agreement with the previously observed trend in hadron–hadron scattering for 〈k_T〉 to rise with interaction energy.     

Photoproduction of K 0 and Λ at HERA and a comparison with deep inelastic scattering

Inclusive K ⁰ and Λ photoproduction has been investigated at HERA with the H1 detector at an average photon-proton center of mass energy of 200 GeV in the transverse momentum range 0.5 < p _t < 5 GeV. The production rates as a function of p _t and center of mass rapidity are compared to those obtained in deep inelastic scattering at 〈Q ²〉 = 23 GeV². A similar comparison is made of the rapidity spectra of charged particles. The rate of strangeness photoproduction is compared with $p\bar p$ measurements. The observations are also compared with next-to-leading order QCD calculations and the predictions of a Monte Carlo model.     

Preliminary results on SKAT neutrino experiment at 2–30 GeV

The data on inclusive deep inelastic neutrino scattering at 2–30 GeV were obtained with the bubble chamber SKAT. Distributions in q², x, y and mean q² dependence on neutrino energy are in agreement with quark parton model.     

Models for high-energy inclusive processes and screening in deuterium

The screening correction to the high energy (p_lab > 10 GeV/c) elastic hadron-deuteron scattering amplitude is discussed. It is found that the correction cannot completely be calculated using as input only the data for cross sections of inclusive reactions on free nucleons. Nevertheless measurements of high energy screening corrections provide sensitive tests for models of inclusive processes. In particular elastic screening does not dominate over the inelastic screening if, as indicated by recent fits, the triple pomeron coupling does not vanish for three mass-zero pomerons. If this is the case, the theory of high energy elastic scattering on heavier nuclei would have to be reconsidered. The discussion is illustrated by a numerical calculation for elastic πd scattering in the 10GeV–2000 GeV energy range.     

A measurement of the inclusive π0 and η production cross sections at high pT in p̄p and pp collisions at s = 24.3 GeV

Inclusive π⁰ and η production at large transverse momentum were studied in both p̄p and pp interactions at √s = 24.3 GeV. The experiment was performed using an internal molecular hydrogen gas jet target in the CERN SPS collider. No significant differences between production in p̄p and pp were observed in the transverse momentum range 2.5 < P_T < 5.1 GeV/c.     

Experimental cross section for dimuon production and the Drell-Yan model

We have measured the absolute cross section for dimuon production in hadron collisions at 200 GeV/c in the continuum region M = 4–8.5 GeV. In all the channels studied (pN, $\text{p}$N, π^±N and π^?H₂) the experimental cross section is significantly larger by a factor of 2.3 ± 0.5 than expected from the Drell-Yan model. Furthermore, our proton-nucleon data allow a determination of the nucleon valence structure function which agrees with the deep inelastic lepton scattering data.     

K−p elastic scattering at 14.3 GeV/c

Results are presented of a bubble chamber experiment on K^?p elastic scattering at 14.3 GeV/c, in four-momentum transfer range 0.04 < |t| < 2.74 GeV² using an initial set of 40 000 events. The total elastic cross section is (2.96 ± 0.10) mb. The results are compared with K⁺p elastic scattering data at 13.8 GeV/c, and the effective Regge trajectory is calculated using K^?p data from 5 to 100 GeV/c.     

Hadron production in 200 GeV μ-copper and μ-carbon deep inelastic interactions

The measurements of the z and p_T² distribution of hadrons produced in the interactions of 200 GeV muons with copper and carbon nuclei are shown in different x_Bj and virtual photon energy intervals. Effects of the jet scattering are seen at the lowest virtual photon energies while for energies above 70 GeV there is no evidence of these effects. Comparison with a theoretical model indicates that at high jet energies the parton fragmentation distance is greater than the nuclear radius and that the parton absorption cross section is less than 10 mb.     

Comment on the Chou-Yang model for elastic scattering

The Chou-Yang model forpp elastic scattering, modified to include the effect of coupled inelastic channels, is used to fit recent data in the region of the dip, i.e., 1.0≦|t|≦2.0 (GeV/c)² for incident momenta ranging from 35 to 250 GeV/c. Good quantitative agreement is found for an inelastic coupling which is predominantly real, as compared to the mostly imaginary elastic coupling. Implications for dips at highert, which are predicted by the model but as yet not observed experimentally, are discussed.