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

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 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

Studying characteristics of diffractive deep inelastic scattering with the ZEUS Leading Proton Spectrometer

The observation of diffraction in deep inelastic scattering (DIS) at the HERA electron-proton collider generated considerable excitement. This reaction provides as opportunity to study at a parton level the mechanism responsible for diffractive and elastic reactions in hadronic collisions. The ZEUS experiment, equipped with a leading proton spectrometer (LPS), is ideally suited to observe this reaction (ep#-&gt;#epX) by detecting the proton p in addition to the electron e and photon fragment X. The reaction is approximately exponential in the momentum transfer squared t with an exponential slope b=6.6 #+-# 0.6_-_0_._6&quot;+&quot;1&quot;.&quot;2 GeV&quot;2. The inclusive cross section, expressed as F_2&quot;D&quot;(&quot;4&quot;), has been measured in the kinematic region 5 &lt; Q&quot;2 &lt;20 GeV&quot;2 and x_P&lt;0.03. An analysis of this structure function indicates that the soft Pomeron trajectory is most appropriate, although shrinkage has not yet been confirmed. The x_P dependence is observed to follow a power behavior F_2&quot;D&quot;(&quot;4&quot;) #propor to# x_P&quot;a at t=-0.16 GeV&quot;2, with a fitted to 1.08 #+-#.06_-_._1_9&quot;+&quot;.&quot;0&quot;8. The inclusive event shape variables demonstrate a final state similar to the hadronic system produced in e&quot;+e&quot;- collisions at similar energy scales. These conclusions are discussed in the context of other analyses of the same reaction at HERA, with further discussion of the problems of background and event selection faced by measurements not based on the observation of the final proton. (orig.)Available from TIB Hannover: RR 8919(1998-010) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman