D* Production in Deep Inelastic Scattering at HERA

This paper presents measurements of D^{*\pm} production in deep inelastic scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The data have been taken with the ZEUS detector at HERA. The decay channel $D^{*+}\to (D^0 \to K^- \pi^+) \pi^+$ (+ c.c.) has been used in the study. The $e^+p$ cross section for inclusive D^{*\pm} production with $5<Q^2<100 GeV^2$ and $y<0.7$ is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region {$1.3<p_T(D^{*\pm})<9.0$ GeV and $| \eta(D^{*\pm}) |<1.5$}. Differential cross sections as functions of p_T(D^{*\pm}), $\eta(D^{*\pm}), W$ and $Q^2$ are compared with next-to-leading order QCD calculations based on the photon-gluon fusion production mechanism. After an extrapolation of the cross section to the full kinematic region in p_T(D^{*\pm}) and $\eta$(D^{*\pm}), the charm contribution $F_2^{c\bar{c}}(x,Q^2)$ to the proton structure function is determined for Bjorken $x$ between 2 $\cdot$ 10$^{-4}$ and 5 $\cdot$ 10$^{-3}$.Comment: 17 pages including 4 figure

Observation of Scaling Violations in Scaled Momentum Distributions at HERA

Charged particle production has been measured in deep inelastic scattering (DIS) events over a large range of $x$ and $Q^2$ using the ZEUS detector. The evolution of the scaled momentum, $x_p$, with $Q^2,$ in the range 10 to 1280 $GeV^2$, has been investigated in the current fragmentation region of the Breit frame. The results show clear evidence, in a single experiment, for scaling violations in scaled momenta as a function of $Q^2$.Comment: 21 pages including 4 figures, to be published in Physics Letters B. Two references adde

Observation of hard scattering in photoproduction events with a large rapidity gap at HERA

Events with a large rapidity gap and total transverse energy greater than 5 GeV have been observed in quasi-real photoproduction at HERA with the ZEUS detector. The distribution of these events as a function of the $\gamma p$ centre of mass energy is consistent with diffractive scattering. For total transverse energies above 12 GeV, the hadronic final states show predominantly a two-jet structure with each jet having a transverse energy greater than 4 GeV. For the two-jet events, little energy flow is found outside the jets. This observation is consistent with the hard scattering of a quasi-real photon with a colourless object in the proton.Comment: 19 pages, latex, 4 figures appended as uuencoded fil

Observation of Events with an Energetic Forward Neutron in Deep Inelastic Scattering at HERA

In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken x and Q2 in the range 3 · 10-4 \u3c xBJ \u3c 6 · 10-3 and 10 \u3c Q2 \u3c 100 GeV2

Diffractive dissociation in ep deep inelastic scattering

The deep inelastic diffractive cross section, d#sigma#_#gamma#_&quot;*_p_#-&gt;#_X_N&quot;D/dM_X, has been measured in the mass range M_X#&lt;=#15 GeV for #gamma#&quot;*p center of mass energies 60&lt;W&lt;200 GeV and photon virtualities Q&quot;2=7 to 140 GeV&quot;2. For fixed Q&quot;2 and M_X, the diffractive cross section is found to rise rapidly with W. Parameterizing the W dependence by the form d#sigma#_#gamma#_&quot;*_#-&gt;#_X_N&quot;D/dM_X#propor to#(W&quot;2)&quot;(&quot;2&quot;#alpha#&quot;_P&quot;-&quot;2&quot;) the data yielded for the t-averaged Pomeron trajectory a value of anti #alpha#_P=1.135#+-#0.008(stat)_-_0_._0_3_0&quot;+&quot;0&quot;.&quot;0&quot;1&quot;1(syst). Th is value for the Pomeron trajectory is larger than anti #alpha#_P extracted from soft interactions. The W dependence of the diffractive cross section measured in this analysis is found to be the same as that of the total cross section for scattering of virtual photons on protons. From the measured diffractive cross section the diffractive structure function of the proton F_2&quot;D&quot;(&quot;3&quot;) (#beta#,x_P,Q&quot;2) has been determined. We find the data to be consistent with the assumption that the diffractive structure function F_2&quot;D&quot;(&quot;3&quot;) factorizes according to x_PF_2&quot;D&quot;(&quot;3&quot;) (x_P,#beta#,Q&quot;2)=(x_0/x_P)&quot;nF_2&quot;P(#beta#,Q&quot;2). They are also consistent with QCD based models which incorporate factorization breaking. The rise of x_PF_2&quot;D&quot;(&quot;3&quot;) with decreasing x_P and the weak dependence of F_2&quot;P on Q&quot;2 suggest a substantial contribution from partonic interactions. (orig.)Available from TIB Hannover: RR 8919(1998-036) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of Prompt Photons with Associated Jets in Photoproduction at HERA.

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential gamma+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and x_gamma^obs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the kT-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the kT-factorisation calculations are in good agreement with the data.Comment: 22 pages, 7 eps figues, 2 table