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

Electron lenses for head-on beam-beam compensation in RHIC

International audienceTwo electron lenses (e-lenses) have been in operation during the 2015 RHIC physics run as part of a head-on beam-beam compensation scheme. While the RHIC lattice was chosen to reduce the beam-beam-induced resonance-driving terms, the electron lenses reduced the beam-beam-induced tune spread. This has been demonstrated for the first time. The beam-beam compensation scheme allows for higher beam-beam parameters and therefore higher intensities and luminosity. In this paper, we detail the design considerations and verification of the electron beam parameters of the RHIC e-lenses. Longitudinal and transverse alignments with ion beams and the transverse beam transfer function measurement with head-on electron-proton beam are presented

Comparison of energy flows in deep inelastic scattering events with and without a large rapidity gap

Energy flows in deep inelastic electron-proton scattering are investigated at a center-of-mass energy of 296 GeV for the range Q2 greater-than-or-equal-to 10 GeV2 using the ZEUS detector. A comparison is made between events with and without a large rapidity gap between the hadronic system and the proton direction. The energy flows, corrected for detector acceptance and resolution, are shown for these two classes of events in both the HERA laboratory frame and the Breit frame. From the differences in the shapes of these energy flows we conclude that QCD radiation is suppressed in the large-rapidity-gap events compared to the events without a large rapidity gap

Inclusive jet differential cross-sections in photoproduction at HERA

Inclusive jet differential cross sections for the reaction ep --> jet + X at Q(2) below 4 GeV2 have been measured with the ZEUS detector at HERA using an integrated luminosity of 0.55 pb(-1). These cross sections are given in the kinematic region 0.2 < y < 0.85, for jet pseudorapidities in the ep-laboratory range -1 < eta(jet) < 2 and refer to jets at the hadron level with a cone radius of one unit in the eta - phi plane. These results correspond to quasi-real photoproduction at centre-of-mass energies in the range 130-270 GeV and, approximately, for jet pseudorapidities in the interval -3 < eta(jet)(gamma p CMS) < 0. These measurements cover a new kinematic regime of the partonic structure of the photon, at typical scales up to similar to 300 GeV2 and photon fractional momenta down to x(y) similar to 10(-2). Leading logarithm parton shower Monte Carlo calculations, which include both resolved and direct processes and use the predictions of currently available parametrisations of the photon parton distributions, describe in general the shape and magnitude of the measured eta(jet) and E(T)(jet) distributions

Observation of events with a large rapidity gap in deep inelastic scattering at HERA

In deep inelastic, neutral current scattering of electrons and protons at &#8730; s = 296 GeV, we observe in the ZEUS detector events with a large rapidity gap in the hadronic final state. They occur in the region of small Bjorken x and are observed up to Q&lt;sup&gt;2&lt;/sup&gt; of 100 GeV&lt;sup&gt;2&lt;/sup&gt;. They account for about 5% of the events with Q&lt;sup&gt;2&lt;/sup&gt; &#8805; 10 GeV&lt;sup&gt;2&lt;/sup&gt;. Their general properties are inconsistent with the dominant mechanism of deep inelastic scattering, where color is transferred between the scattered quark and the proton remnant, and suggest that the underlying production mechanism is the diffractive dissociation of the virtual photon