Measurement of Proton-dissociative Diffractive Photoproduction of Vector Mesons at Large Momentum Transfer at HERA: The ZEUS Collaboration

Diffractive photoproduction of vector mesons, γp → VY, where Y is a proton-dissociative system, has been measured in e+p interactions with the ZEUS detector at HERA using an integrated luminosity of 25 pb-1. The differential cross section, dσ/dt, is presented for 1.2 \u3c -t \u3c 12 GeV2, where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, dσ/dt ∝ (-t)-n. The slope of the effective Pomeron trajectory, measured from the W dependence of the ρ0 and φ cross sections in bins of t, is consistent with zero. The ratios dσγp→φY/dt to dσγp→ρ0Y/dt and dσγp→J/ψY/dt to dσγp→ρ0Y/dt increase with increasing -t. Decay-angle analyses for ρ0, φ and J/ψ mesons have been carried out. For the ρ0 and φ mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models

Leading Neutron Production in e\u3csup\u3e+\u3c/sup\u3e p Collisions at HERA

The production of neutrons carrying at least 20% of the proton beam energy (xL \u3e 0.2) in e+ p collisions has been studied with the ZEUS detector at HERA for a wide range of Q2, the photon virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section, ep → e′ Xn, is measured relative to the inclusive cross section, ep → e′ X, thereby reducing the systematic uncertainties. For xL \u3e 0.3, the rate of neutrons in photoproduction is about half of that measured in hadroproduction, which constitutes a clear breaking of factorisation. There is about a 20% rise in the neutron rate between photoproduction and deep inelastic scattering, which may be attributed to absorptive rescattering in the γ p system. For 0.64 \u3c xL \u3c 0.82, the rate of neutrons is almost independent of the Bjorken scaling variable x and Q2. However, at lower and higher xL values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function, F2LN(3) (x, Q2, xL), rises at low values of x in a way similar to that of the inclusive F2(x, Q2) of the proton. The total γπ cross section and the structure function of the pion, F2π (xπ, Q2) where xπ = x/(1 - xL), have been determined using a one-pion-exchange model, up to uncertainties in the normalisation due to the poorly understood pion flux. At fixed Q2, F2π has approximately the same x dependence as F2 of the proton. © 2002 Published by Elsevier Science B.V

Jet production in charged current deep inelastic e⁺p scatteringat HERA

The production rates and substructure of jets have been studied in charged current deep inelastic e⁺p scattering for Q² > 200 GeV² with the ZEUS detector at HERA using an integrated luminosity of 110.5 pb⁻¹. Inclusive jet cross sections are presented for jets with transverse energies E_{T}^{jet} > 5 GeV. Measurements of the mean subjet multiplicity, 〈n_{sbj}〉, of the inclusive jet sample are presented. Predictions based on parton-shower Monte Carlo models and next-to-leading-order QCD calculations are compared to the measurements. The value of α_{s} (M_{z}), determined from 〈n_{sbj}〉 at y_{cut} = 10⁻² for jets with 25 < E_{T}^{jet} < 119 GeV, is α_{s} (M_{z}) = 0.1202 ± 0.0052 (stat.)_{-0.0019}^{+0.0060} (syst.)_{-0.0053}^{+0.0065} (th.). The mean subjet multiplicity as a function of Q² is found to be consistent with that measured in NC DIS

Measurement of Subjet Multiplicities in Neutral Current Deep Inelastic Scattering at HERA and Determination of α\u3csub\u3es\u3c/sub\u3e

The subjet multiplicity has been measured in neutral current e+p interactions at Q2 \u3e 125 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb-1. Jets were identified in the laboratory frame using the longitudinally invariant kT cluster algorithm. The number of jet-like substructures within jets, known as the subjet multiplicity, is defined as the number of clusters resolved in a jet by reapplying the jet algorithm at a smaller resolution scale ycut. Measurements of the mean subjet multiplicity, (nsbj), for jets with transverse energies ET,jet \u3e 15 GeV are presented. Next-to-leading-order perturbative QCD calculations describe the measurements well. The value of αs(MZ), determined from (nsbj) at ycut = 10-2 for jets with 25 \u3c ET,jet \u3c 71 GeV, is αs(MZ) = 0.1187 ± 0.0017(stat.)-0.0009+0.0024(syst.) -0.0076+0.0093(th.). © 2003 Published by Elsevier Science B.V

Inclusive Jet Cross Sections in the Breit Frame in Neutral Current Deep Inelastic Scattering at HERA and Determination of α \u3csub\u3es\u3c/sub\u3e

Inclusive jet differential cross sections have been measured in neutral current deep inelastic e +p scattering for boson virtualities Q 2 \u3e 125 GeV 2. The data were taken using the ZEUS detector at HERA and correspond to an integrated luminosity of 38.6 pb -1. Jets were identified in the Breit frame using the longitudinally invariant k T cluster algorithm. Measurements of differential inclusive jet cross sections are presented as functions of jet transverse energy (E BT.jet), jet pseudorapidity and Q 2, for jets with E BT.jet \u3e 8 GeV. Next-to-leading-order QCD calculations agree well with the measurements both at high Q 2 and high E BT.jet. The value of α s(M z), determined from an analysis of dσ/dQ 2 for Q 2 \u3e 500 GeV 2, is α s(M Z=0.1212±0.0017 (stat.) +0.0023-0.0031 (syst.) +0.0028-0.0027(th.). © 2002 Elsevier Science B.V. All rights reserved

Multijet production in neutral current deep inelastic scattering at HERA and determination of α_{s}

Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities 10 5 GeV and –1 < η_{LAB}^{jet} < 2.5. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant α_{s} (M_{z}), determined from the ratio of the trijet to dijet cross sections, is α_{s} (M_{z}) = 0.1179 ± 0.0013 (stat.)_{-0.0046}^{+0.0028}(exp.)_{-0.0046}^{+0.0028}(th.)

Measurement of the Open-charm Contribution to the Diffractive Proton Structure Function

Production of D*±(2010) mesons in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb-1. Diffractive events were identified by the presence of a large rapidity gap in the final state. Differential cross sections have been measured in the kinematic region 1.

Measurement of High-Q\u3csup\u3e2\u3c/sup\u3e e\u3csup\u3e-\u3c/sup\u3e p Neutral Current Cross Sections at HERA and the Extraction of xF\u3csub\u3e3\u3c/sub\u3e: The ZEUS Collaboration

Cross sections for e- p neutral current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV using an integrated luminosity of 15.9 pb-1 collected with the ZEUS detector at HERA. Results on the double-differential cross-section d2σ/dx dQ2 in the range 185 \u3c Q2 \u3c 50 000 GeV2 and 0.0037 \u3c x \u3c 0.75, as well as the single-differential cross-sections dσ/dQ2, dσ/dx and dσ/dy for Q2 \u3e 200GeV2, are presented. To study the effect of Z-boson exchange, dσ/dx has also been measured for Q2 \u3e 10 000 GeV2. The structure function xF3 has been extracted by combining the e-p results presented here with the recent ZEUS measurements of e+ p neutral current deep inelastic scattering. All results agree well with the predictions of the Standard Model

Measurement of Deeply Virtual Compton Scattering at HERA

The cross-section for deeply virtual Compton scattering in the reaction ep → eγp has been measured with the ZEUS detector at HERA using integrated luminosities of 95.0 pb-1 of e+p and 16.7 pb-1 of e-p collisions. Differential cross-sections are presented as a function of the exchanged-photon virtuality, Q2, and the centre-of-mass energy, W, of the γ*p system in the region 5 \u3c Q2 \u3c 100 GeV2 and 40 \u3c W \u3c 140 GeV. The measured cross-sections rise steeply with increasing W. The measurements are compared to QCD-based calculations. © 2003 Elsevier B.V. All rights reserved

Measurements of Inelastic J/ψ and ψ′ Photoproduction at HERA

The cross sections for inelastic photoproduction of J/ψ and ψ′ mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 38.0 pb-1. The events were required to have 0.1 \u3c z \u3c 0.9 and 50 \u3c W \u3c 180 GeV, where z is the fraction of the incident photon energy carried by the J/ψ in the proton rest frame and W is the photon-proton centre-of-mass energy. The ψ′ to J/ψ cross-section ratio was measured in the range 0.55 \u3c z \u3c 0.9. The J/ψ data, for various ranges of transverse momentum, are compared to theoretical models incorporating colour-singlet and colour-octet matrix elements. Predictions of a next-to-leading-order colour-singlet model give a good description of the data, although there is a large normalisation uncertainty. The J/ψ helicity distribution for z \u3e 0.4 is compared to leading-order QCD predictions