Differential cross sections for jet photoproduction in the reaction ep --> e jet X have been measured with the ZEUS detector at HERA using 82.2 pb^-1 of integrated luminosity. Inclusive jet cross sections are presented as a function of the jet transverse energy, E_T^jet, for jets with E_T^jet > 17 GeV and pseudorapidity -1 < eta^jet < 2.5, in the gamma-p centre-of-mass-energy range 142 < W_gamma-p < 293 GeV. Scaled jet invariant cross sections are presented as a function of the dimensionless variable x_T = 2 E_T^jet/W_gamma-p for <W_gamma-p> = 180 and 255 GeV. Next-to-leading-order QCD calculations give a good description of the measured differential cross sections in both magnitude and shape. The ratio of scaled jet invariant cross sections at the two <W_gamma-p> values shows clear non-scaling behaviour. A value for the strong coupling constant of alpha_s(M_Z) = 0.1224 +- 0.0001 (stat.) + 0.0022 - 0.0019 (exp.) + 0.0054 - 0.0042 (th.) has been extracted from a QCD analysis of the measured dsigma/dE_T^jet. The variation of alpha_s with E_T^jet is in good agreement with the running of alpha_s as predicted by QCD.
Measured inclusive jet cross section.
Measured scaled jet invariant cross section in two W intervals.
Ratio of the measured scaled jet invariant cross section in the two W intervals.
Inclusive jet differential cross sections have been measured in neutral current deep inelastic e+p scattering for boson virtualities Q**2>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_T,jet), jet pseudorapidity and Q**2, for jets with E_T,jet>8 GeV. Next-to-leading-order QCD calculations agree well with the measurements both at high Q**2 and high E_T,jet. The value of alpha_s(M_Z), determined from an analysis of dsigma/dQ**2 for Q**2>500 GeV**2, is alpha_s(M_Z) = 0.1212 +/- 0.0017 (stat.) +0.0023 / -0.0031 (syst.) +0.0028 / -0.0027 (th.).
Inclusive jet cross section DSIG/DQ**2 for jets of hadrons in the Breit frame.
Inclusive jet cross section DSIG/DET for jets of hadrons in the Breit frame.
Inclusive jet cross section DSIG/DETARAP for jets of hadrons in the Breit frame.
Differential cross sections for dijet photoproduction in association with a leading neutron using the reaction e^+ + p --> e^+ + n + jet + jet + X_r have been measured with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb^{-1}. The fraction of dijet events with a leading neutron in the final state was studied as a function of the jet kinematic variables. The cross sections were measured for jet transverse energies E^{jet}_T > 6 GeV, neutron energy E_n > 400 GeV, and neutron production angle theta_n < 0.8 mrad. The data are broadly consistent with factorization of the lepton and hadron vertices and with a simple one-pion-exchange model.
The differential dijet cross section as a function of ET for the inclusive data set. The second DSYS error is due to the uncertainty in the calorimeter energy scale.
The differential dijet cross section as a function of ET for the neutron-tagged data set. The second DSYS error is due to the uncertainty in the calorimeter energy scale.
The differential dijet cross section as a function of ETARAP for the inclusive data set. The second DSYS error is due to the uncertainty in the calorimeterenergy scale.
Dijet production has been studied in neutral current deep inelastic e+p scattering for 470 < Q**2 < 20000 GeV**2 with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb**{-1}. Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q**2 allows both a precise determination of alpha_s(M_Z) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the parton distribution functions of the proton. The value of alpha_s(M_Z), as determined from the QCD fit, is alpha_s(M_Z) = 0.1166 +- 0.0019 (stat.) {+ 0.0024}_{-0.0033} (exp.)} {+ 0.0057}_{- 0.0044} (th.).
The differential dijet cross section dsig/dZP1.
The differential dijet cross section dsig/dlog10(x).
The differential dijet cross section dsig/dlog10(xi).
The cross section for dijet photoproduction at high transverse energies is presented as a function of the transverse energies and the pseudorapidities of the jets. The measurement is performed using a sample of ep-interactions corresponding to an integrated luminosity of 6.3 pb^(-1), recorded by the ZEUS detector.Jets are defined by applying a k_T-clustering algorithm to the hadrons observed in the final state. The measured cross sections are compared to next-to-leading order QCD calculations. In a kinematic regime where theoretical uncertainties are expected to be small, the measured cross sections are higher than these calculations.
The dijet cross section for the full x(gamma) range as a function of the ET of the leading jet.
The dijet cross section for the full x(gamma) range as a function of the ET of the leading jet.
The dijet cross section for the full x(gamma) range as a function of the ET of the leading jet.
Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (x_gamma^OBS) and pomeron (beta^OBS) momentum participating in the production of the dijet system. The observed x_gamma^OBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section d(sigma)/d(beta^OBS) increases as beta^OBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.
Differential cross section as a function of rapidity of the two highest Et jets in event.
Differential cross section as a function of transverse energy Et of the tw o highest Et jets in event.
Differential cross section as a function of invariant mass of the GAMMA P system.
Photoproduction of beauty quarks in events with two jets and an electron associated with one of the jets has been studied with the ZEUS detector at HERA using an integrated luminosity of 120pb^-1. The fractions of events containing b quarks, and also of events containing c quarks, were extracted from a likelihood fit using variables sensitive to electron identification as well as to semileptonic decays. Total and differential cross sections for beauty and charm production were measured and compared with next-to-leading-order QCD calculations and Monte Carlo models.
Total cross sections for electrons from beauty and charm quarks.
Differential electron cross sections as a function of PT and ETARAP from beauty and charm quarks.
Differential electron cross sections as a function of PT and ETARAP from beauty and charm quarks.
Forward jet cross sections have been measured in neutral current deep inelastic scattering at low Bjorken-x with the ZEUS detector at HERA using an integrated luminosity of ${81.8 \rm pb}^{-1}$. Measurements are presented for inclusive forward jets as well as for forward jets accompanied by a dijet system. The explored phase space, with jet pseudorapidity up to 4.3 is expected to be particularly sensitive to the dynamics of QCD parton evolution at low x. The measurements are compared to fixed-order QCD calculations and to leading-order parton-shower Monte Carlo models.
Differential cross section DSIG/DQ**2 in bins of Q**2 .
Differential cross section DSIG/DX in bins of X .
Differential cross section DSIG/DET(P=4) in bins of ET(P=4) .
The production of dijets in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of $61 \pbi$. The dijet cross section has been measured for virtualities of the exchanged virtual photon, $5 < Q^2 < 100 \gev^2$, and $\gamma^{*} p$ centre-of-mass energies, 100 < W < 250 GeV. The jets, identified using the inclusive k_{T} algorithm in the $\gamma^* p$ frame, were required to have a transverse energy $E^*_{T, \rm jet} > 4 \gev$ and the jet with the highest transverse energy was required to have $E^*_{T,\rm jet} > 5 \gev$. All jets were required to be in the pseudorapidity range $-3.5 < \eta^*_{\rm jet} < 0$. 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.
Total di-jet cross section SIG as a function of Q**2 .
Distribution of D(SIG)/DQ**2 as a function of Q**2 .
Distribution of D(SIG)/DW as a function of W .
The cross section for high-E_T dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8 pb-1. The events were required to have a virtuality of the incoming photon, Q^2, of less than 1 GeV^2 and a photon-proton centre-of-mass energy in the range 142 < W < 293 GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of E_T(jet1) > 20 GeV and E_T(jet2) > 15 GeV and pseudorapidity requirements of -1 < eta(jet1,2) < 3, with at least one of the jets satisfying -1 < eta(jet) < 2.5. The measurements show sensitivity to the parton distributions in the photon and proton and effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.
Cross section D(SIG)/(ET(P=4)+ET(P=5))/2 as a function of (ET(P=4)+ET(P=5))/2 for X(C=GAMMA,OBS) > 0.75 .
Cross section D(SIG)/(ET(P=4)+ET(P=5))/2 as a function of (ET(P=4)+ET(P=5))/2 for X(C=GAMMA,OBS) <= 0.75 .
Cross section D(SIG)/ET(P=4) as a function of ET(P=4) for X(C=GAMMA,OBS) > 0.75 .
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