The small angle rear tracking detector of ZEUS

Abstract The design, construction, installation, and performance of the small angle rear tracking detector of the ZEUS experiment are described. The results on electron position measurement, electron energy correction, and background reduction at the first-level trigger are presented. The impact on the measurement of the proton structure function is discussed

Measurement of inclusive D*+- and associated dijet cross sections in photoproduction at HERA

Inclusive photoproduction of D*+- mesons has been measured for photon-proton centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of 37 pb^-1. Total and differential cross sections as functions of the D* transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the "massive charm" and "massless charm" schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant.Comment: 32 pages including 6 figure

Measurement of Jet Shapes in Photoproduction at HERA

The shape of jets produced in quasi-real photon-proton collisions at centre-of-mass energies in the range $134-277$ GeV has been measured using the hadronic energy flow. The measurement was done with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the $\eta - \phi$ plane with a cone radius of one unit. Measured jet shapes both in inclusive jet and dijet production with transverse energies $E^{jet}_T>14$ GeV are presented. The jet shape broadens as the jet pseudorapidity ($\eta^{jet}$) increases and narrows as $E^{jet}_T$ increases. In dijet photoproduction, the jet shapes have been measured separately for samples dominated by resolved and by direct processes. Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct processes describe well the measured jet shapes except for the inclusive production of jets with high $\eta^{jet}$ and low $E^{jet}_T$. The observed broadening of the jet shape as $\eta^{jet}$ increases is consistent with the predicted increase in the fraction of final state gluon jets.Comment: 29 pages including 9 figure

Angular and Current-Target Correlations in Deep Inelastic Scattering at HERA

Correlations between charged particles in deep inelastic ep scattering have been studied in the Breit frame with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb-1. Short-range correlations are analysed in terms of the angular separation between current-region particles within a cone centred around the virtual photon axis. Long-range correlations between the current and target regions have also been measured. The data support predictions for the scaling behaviour of the angular correlations at high Q2 and for anti-correlations between the current and target regions over a large range in Q2 and in the Bjorken scaling variable x. Analytic QCD calculations and Monte Carlo models correctly describe the trends of the data at high Q2, but show quantitative discrepancies. The data show differences between the correlations in deep inelastic scattering and e+e- annihilation.Comment: 26 pages including 10 figures (submitted to Eur. J. Phys. C

Dijet Angular Distributions in Direct and Resolved Photoproduction at HERA

Jet photoproduction, where the two highest transverse energy (ETjet) jets have ETjet above 6 GeV and a jet-jet invariant mass above 23 GeV, has been studied with the ZEUS detector at the HERA ep collider. Resolved and direct photoproduction samples have been separated. The cross section as a function of the angle between the jet-jet axis and the beam direction in the dijet rest frame has been measured for the two samples. The measured angular distributions differ markedly from each other. They agree with the predictions of QCD calculations, where the different angular distributions reflect the different spins of the quark and gluon exchanged in the hard subprocess

Study of Charged-Current ep Interactions at Q\u3csup\u3e2\u3c/sup\u3e ≫ 200 Gev\u3csup\u3e2\u3c/sup\u3e with the ZEUS Detector at HERA

The cross sections for the deep inelastic charged-current interactions e+p → ν̄X and e-p → νX have been measured in the kinematic region x \u3e 0.006 and Q2 \u3e 200GeV2, using the e+p and e-p data collected with the ZEUS detector during 1993 and 1994. For the integrated cross sections, σe+→ν̄X(Q2\u3e200 GeV2) = 30.3- 4.2+ 5.5 [stat] -2.6+1.6 [syst] pb σe-p→νX(Q2\u3e200 GeV2) = 54.7-9.8+15.9 [stat] -3.4+2.8 [syst] pb are obtained, in good agreement with the Standard Model expectations of 32.3pb and 65.8pb. The differential cross sections have been measured as functions of Q2, x and y. The differences between e+p and e-p scattering, which are expected from the V-A helicity structure of the weak interaction and from the quark content of the proton, are clearly observed. No significant deviation from the Standard Model predictions is found. The measured Q2 dependence of the differential cross sections for e+ and c- induced CC DIS reactions is used to determine the W mass. The measured value of MW is 79 -7 -4+8 +4 GeV, in good agreement with direct measurements performed at pp̄ colliders. A jet analysis has been performed and CC events with multiple jets have been observed. The distributions of the jet transverse momentum and rapidity as well as transverse energy flow jet profiles have been measured and are compared to the script O sign(αs) predictions of the ARIADNE and MEPS Monte Carlo models. Good agreement is observed in both the jet profiles and the jet rates. A search for charged-current events with a large rapidity gap between the observed hadronic system and the outgoing proton system yielded one candidate event in the e+p sample, corresponding to a cross section of σe+p→ν̄X(Q2 \u3e 200 GeV2; ηmax \u3c 2.5) = 0.8 -0.7+1.8 [stat] ±0.1 [syst] pb. © Springer-Verlag 1996

Measurement of the Diffractive Cross Section in Deep Inelastic Scattering

© Springer-Verlag 1996. Diffractive scattering of γ∗p → X + N, where N is either a proton or a nucleonic system with MN \u3c 4 GeV has been measured in deep inelastic scattering (DIS) at HERA. The cross section was determined by a novel method as a function of the γ∗p c.m. energy W between 60 and 245 GeV and of the mass MX of the system X up to 15 GeV at average Q2 values of 14 and 31 GeV2. The diffractive cross section dσdi f f /dMX is, within errors, found to rise linearly with W. Parameterizing the W dependence by the form dσdi f f /dMX ∞ (W2)(2αIP - 2) the DIS data yield for the pomeron trajectory αIP = 1.23±0.02(stat)±0.04(syst) averaged over t in the measured kinematic range assuming the longitudinal photon contribution to be zero. This value for the pomeron trajectory is substantially larger than αIP extracted from soft interactions. The value of αIP measured in this analysis suggests that a substantial part of the diffractive DIS cross section originates from processes which can be described by perturbative QCD. From the measured diffractive cross sections the diffractive structure function of the proton F2D(3) (β,Q2, xIP) has been determined, where β is the momentum fraction of the struck quark in the pomeron. The form F2D(3) = constant·(1/xIP)a gives a good fit to the data in all β and Q2 intervals with a = 1.46 ± 0.04(stat) ± 0.08(syst)

Measurement of the F\u3csub\u3e2\u3c/sub\u3e Structure Function in Deep Inelastic e\u3csup\u3e+\u3c/sup\u3ep Scattering Using 1994 Data from the ZEUS Detector at HERA

We present measurements of the structure function F2 in e+p scattering at HERA in the range 3.5 GeV2 \u3c Q2 \u3c 5000 GeV2. A new reconstruction method has allowed a significant improvement in the resolution of the kinematic variables and an extension of the kinematic region covered by the experiment. At Q2 \u3c 35 GeV2 the range in x now spans 6.3 · 10-5 \u3c x \u3c 0.08 providing overlap with measurements from fixed target experiments. At values of Q2 above 1000 GeV2 the x range extends to 0.5. Systematic errors below 5% have been achieved for most of the kinematic region. The structure function rises as x decreases; the rise becomes more pronounced as Q2 increases. The behaviour of the structure function data is well described by next-to-leading order perturbative QCD as implemented in the DGLAP evolution equations. © Springer-Verlag 1996

Measurement of Elastic Φ Photoproduction at HERA

The production of φ mesons in the reaction e+p → e+φp (φ → K+K-) at a median Q2 of 10-4 GeV2 has been studied with the ZEUS detector at HERA. The differential φ photoproduction cross section dσ/dt has an exponential shape and has been determined in the kinematic range 0.1 \u3c |t| \u3c 0.5 GeV2 and 60 \u3c W \u3c 80 GeV. An integrated cross section of σγp→φp = 0.96 ± 0.19+0.21-.18 μb has been obtained by extrapolating to t = 0. When compared to lower energy data, the results show a weak energy dependence of both σγp→φp and the slope of the t distribution. The φ decay angular distributions are consistent with s-channel helicity conservation. From lower energies to HERA energies, the features of φ photoproduction are compatible with those of a soft diffractive process