126 research outputs found

    Exclusive photoproduction of J / psi mesons at HERA

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    The exclusive photoproduction of J/psi mesons, gammap → J/psip, has been studied in ep collisions with the ZEUS detector at HERA, in the kinematic range 20 lt W lt 290 GeV, where W is the photon-proton centre-of-mass energy. The J/psi mesons were reconstructed in the union and the electron decay channels using integrated luminosities of 38 pb(-1) and 55 pb(- 1), respectively. The helicity structure of J/psi production shows that the hypothesis of s-channel helicity conservation is satisfied within two standard deviations. The total cross section and the differential cross-section dsigma/dt, where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for \t\ lt 1.8 GeV2. The t distribution exhibits an exponential shape with a slope parameter increasing logarithmically with W with a value b = 4.15 +/- 0.05(stat.)(-0.18)(+0.30)(syst.) GeV-2 at W = 90 GeV. The effective parameters of the Pomeron trajectory are alpha(P)(0) = 1.200 +/- 0.009(stat.)(-0.010)(+0.004) (syst.) and alpha(P)' = 0.115 +/- 0.018(stat.)(-0.015)(+0.008) (syst.) GeV-2

    Measurements of inelastic J / psi and psi-prime photoproduction at HERA

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    The cross sections for inelastic photoproduction of J/psi and psi' 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 lt z lt 0.9 and 50 lt W lt 180 GeV, where z is the fraction of the incident photon energy carried by the J/psi in the proton rest frame and W is the photon-proton centre-of-mass energy. The psi' to J/psi cross- section ratio was measured in the range 0.55 lt z lt 0.9. The J/psi 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/psi helicity distribution for z gt 0.4 is compared to leading-order QCD predictions

    Exclusive electroproduction of J/psi mesons at HERA

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    The exclusive electroproduction of J/psi mesons, ep → epJ/psi, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15 lt Q(2) lt 0.8 GeV2 and 2 lt Q(2) lt 100 GeV2 using integrated luminosities of 69 and 83 pb(-1), respectively. The photonproton centre-of-mass energy was in the range 30 lt W lt 220 GeV and the squared four-momentum transfer at the proton vertex \t\ lt 1 GeV2. The cross sections and decay angular distributions are presented as functions of Q(2), W and t. The effective parameters of the pomeron trajectory are in agreement with those found in J/psi photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, sigma(L)/sigma(T), grows with Q(2), whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton

    Measurement of inelastic J/psi production in deep inelastic scattering at HERA

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    The inelastic production of J/psi mesons in ep collisions has been studied with the ZEUS detector at HERA using an integrated luminosity of 109 pb(-1). The J/psi mesons were identified using the decay channel J/psi -> mu(+)mu(-). The measurements were performed in the kinematic range 2 < Q(2) < 80 GeV(2), 50 < W < 250 GeV, 0.2 < z < 0.9 and -1.6 < Y(lab) < 1.3, where Q(2) is the virtuality of the exchanged photon, W is the photon-proton centre-of-mass energy, z is the fraction of the photon energy carried by the J/psi meson in the proton rest frame and Ylab is the rapidity of the J/psi in the laboratory frame. The measured cross sections are compared to theoretical predictions within the non-relativistic QCD framework including colour-singlet and colour-octet contributions, as well as to predictions based on the k(T)-factorisation approach. Calculations of the colour-singlet process generally agree with the data, whereas inclusion of colour-octet terms spoils this agreement

    Measurement of proton dissociative diffractive photoproduction of vector mesons at large momentum transfer at HERA

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    Diffractive photoproduction of vector mesons, gammap → 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, dsigma/dt, is presented for 1.2 lt -t lt 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, dsigma/dt proportional to (-t)(-n). The slope of the effective Pomeron trajectory, measured from the W dependence of the rho(0) and phi cross sections in bins of t, is consistent with zero. The ratios dsigma(gammap→phiY)/dt to dsigma(gammap→rho0Y)/dt and dsigma(gammap→J/psiY)/dt to dsigma(gammap→rho0Y)/dt increase with increasing -t. Decay- angle analyses for rho(0), phi and J/psi) mesons have been carried out. For the rho(0) and phi mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models

    Measurements of inelastic J / psi and psi-prime photoproduction at HERA

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    The cross sections for inelastic photoproduction of J/psi and psi' 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 0.4 is compared to leading-order QCD predictions

    Three- and four-jet final states in photoproduction at HERA

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    Three- and four-jet final states have been measured in photoproduction at HERA using the ZEUS detector with an integrated luminosity of 121   pb -1. The results are presented for jets with transverse energy E Tjet > 6   GeV and pseudorapidity | η jet | < 2.4, in the kinematic region given by the virtuality of the photon Q 2 < 1   GeV 2 and the inelasticity 0.2 ≤ y ≤ 0.85 and in two mass regions defined as 25 ≤ M n j < 50   GeV and M n j ≥ 50   GeV, where M n j is the invariant mass of the n-jet system. The four-jet photoproduction cross section has been measured for the first time and represents the highest-order process studied at HERA. Both the three- and four-jet cross sections have been compared with leading-logarithmic parton-shower Monte Carlo models, with and without multi-parton interactions. The three-jet cross sections have been compared to an O (α α s2) perturbative QCD calculation. © 2007 Elsevier B.V. All rights reserved

    Measurement of the Q**2 and energy dependence of diffractive interactions at HERA

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    Diffractive dissociation of virtual photons, gamma*p → Xp, has beer, studied ill ep interactions with the ZEUS detector at HERA. The data cover photon virtualities 0.17 lt Q(2) lt 0.70 GeV and 3 lt Q(2) lt 80 GeV2 with 3 lt M-X lt 38 GeV, where M-X is the mass of the hadronic final state. Diffractive events were selected by two methods: the first required the detection of the scattered proton in the ZEUS leading proton spectrometer (LPS); the second was based on the distribution of M-X. The integrated luminosities of the low- and high-Q(2) samples used in the LPS-based analysis are similar or equal to 0.9 pb(-1) and similar or equal to 3.3 pb(-1), respectively. The sample used for the M-X-based analysis corresponds to an integrated luminosity of similar or equal to 6.2 pb(-1). The dependence of the diffractive cross section on W, the virtual photon-proton centre-of-mass energy. and on Q(2) is studied. In the low-Q(2) range, the energy dependence is compatible with Regge theory and is used to determine the intercept of the Pomeron trajectory. The W dependence of the diffractive cross section exhibits no significant change from the low-Q(2) to the high- Q(2) region. In the low-Q range, little Q(2) dependence is found, a significantly different behaviour from the rapidly falling cross section measured for Q(2) gt 3 GeV2. The ratio of the diffractive to the virtual photon-proton total cross sect ion is studied as a function of W and Q(2). Comparisons are made with a model based oil perturbative QCD

    Dijet photoproduction at HERA and the structure of the photon

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    The dijet cross section in photoproduction ha's been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb(-1). The events were required to have a virtuality of the incoming photon, Q(2), of less than 1 GeV2 and a photon-proton centre-of-mass energy in the range 131 14 GeV and E-T(jet2) > 11 GeV and pseudorapidity requirements of -1 < eta(jet1,2) < 2.4. The measurements arc compared to next-to-loading-order QCD predictions. The data show particular sensitivity to the density of partons in the photon, allowing the validity of the current parameterisations to be tested

    Measurement of high Q**2 e- p neutral current cross-sections at HERA and the extraction of xF(3)

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    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 d(2)sigma/dx dQ(2) in the range 185 lt Q(2) lt 50 000 GeV2 and 0.0037 lt x lt 0.75, as well as the single-differential cross-sections dsigma/dQ(2), dsigma/dx and dsigma/dy for Q(2) gt 200 GeV2, are presented. To study the effect of Z-boson exchange, dsigma/dx has also been measured for Q(2) gt 10 000 GeV2. The structure function xF(3) 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
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