Production of Z⁰ bosons in elastic and quasi-elastic <i>ep</i> collisions at HERA

The production of Z&lt;sup&gt;0&lt;/sup&gt; bosons in the reaction ep→eZ&lt;sup&gt;0&lt;/sup&gt;p&lt;sup&gt;(⁎)&lt;/sup&gt;, where p&lt;sup&gt;(⁎)&lt;/sup&gt; stands for a proton or a low-mass nucleon resonance, has been studied in &lt;i&gt;ep&lt;/i&gt; collisions at HERA using the ZEUS detector. The analysis is based on a data sample collected between 1996 and 2007, amounting to 496 pb&lt;sup&gt;−1&lt;/sup&gt; of integrated luminosity. The Z&lt;sup&gt;0&lt;/sup&gt; was measured in the hadronic decay mode. The elasticity of the events was ensured by a cut on ηmax&lt;3.0, where η&lt;sub&gt;max&lt;/sub&gt; is the maximum pseudorapidity of energy deposits in the calorimeter defined with respect to the proton beam direction. A signal was observed at the Z&lt;sup&gt;0&lt;/sup&gt; mass. The cross section of the reaction ep→ eZ&lt;sup&gt;0&lt;/sup&gt;p&lt;sup&gt;(⁎)&lt;/sup&gt; was measured to be σ(ep→ eZ&lt;sup&gt;0&lt;/sup&gt;p&lt;sup&gt;(⁎)&lt;/sup&gt;)=0.13±0.06(stat.)±0.01(syst.) pb, in agreement with the Standard Model prediction of 0.16 pb. This is the first measurement of Z&lt;sup&gt;0&lt;/sup&gt; production in &lt;i&gt;ep&lt;/i&gt; collisions

Measurement of the t dependence in exclusive photoproduction of Υ\Upsilon(1S) mesons at HERA

The exclusive photoproduction reaction gamma p -> Upsilon(1S) p has been studied with the ZEUS detector in ep collisions at HERA using an integrated luminosity of 468 pb^-1. The measurement covers the kinematic range 60<W<220 GeV and Q^2<1 GeV^2, where W is the photon-proton centre-of-mass energy and Q^2 is the photon virtuality. The exponential slope, b, of the t dependence of the cross section, where t is the squared four-momentum transfer at the proton vertex, has been measured, yielding b = 4.3 +2.0 -1.3 (stat.) +0.5 -0.6 (syst.) GeV^-2. This constitutes the first measurement of the t dependence of the gamma p -> Upsilon(1S) p cross section.Comment: 15 pages, 4 figures; http://dx.doi.org/10.1016/j.physletb.2012.01.00

Search for single-top production in ep collisions at HERA

A search for single-top production, ep→etX, has been performed with the ZEUS detector at HERA using data corresponding to an integrated luminosity of 0.37 fb -1. No evidence for top production was found, consistent with the expectation from the Standard Model. Limits were computed for single-top production via flavour changing neutral current transitions involving a neutral electroweak vector boson, γ or Z. The result was combined with a previous ZEUS result yielding a total luminosity of 0.50 fb -1. A 95% credibility level upper limit of 0.13 pb was obtained for the cross section at the centre-of-mass energy of √s=315 GeV. © 2012 Elsevier B.V