Measurement of (anti)deuteron and (anti)proton production in DIS at HERA

The first observation of (anti)deuterons in deep inelastic scattering at HERA has been made with the ZEUS detector at a centre-of-mass energy of 300--318 GeV using an integrated luminosity of 120 pb-1. The measurement was performed in the central rapidity region for transverse momentum per unit of mass in the range 0.3<p_T/M<0.7. The particle rates have been extracted and interpreted in terms of the coalescence model. The (anti)deuteron production yield is smaller than the (anti)proton yield by approximately three orders of magnitude, consistent with the world measurements.Comment: 26 pages, 9 figures, 5 tables, submitted to Nucl. Phys.

Forward jet production in deep inelastic ep scattering and low-x parton dynamics at HERA

Differential inclusive jet cross sections in neutral current deep inelastic ep scattering have been measured with the ZEUS detector. Three phase-space regions have been selected in order to study parton dynamics where the effects of BFKL evolution might be present. The measurements have been compared to the predictions of leading-logarithm parton shower Monte Carlo models and fixed-order perturbative QCD calculations. In the forward region, QCD calculations at order alpha_s^1 underestimate the data up to an order of magnitude at low x. An improved description of the data in this region is obtained by including QCD corrections at order alpha_s^2, which account for the lowest-order t-channel gluon-exchange diagrams, highlighting the importance of such terms in parton dynamics at low x.Comment: 25 pages, 4 figure

Measurement of dijet photoproduction for events with a leading neutron at HERA

Differential cross sections for dijet photoproduction and this process in association with a leading neutron, e+ + p -> e+ + jet + jet + X (+ n), have been measured with the ZEUS detector at HERA using an integrated luminosity of 40 pb-1. The fraction of dijet events with a leading neutron was studied as a function of different jet and event variables. Single- and double-differential cross sections are presented as a function of the longitudinal fraction of the proton momentum carried by the leading neutron, xL, and of its transverse momentum squared, pT^2. The dijet data are compared to inclusive DIS and photoproduction results; they are all consistent with a simple pion-exchange model. The neutron yield as a function of xL was found to depend only on the fraction of the proton beam energy going into the forward region, independent of the hard process. No firm conclusion can be drawn on the presence of rescattering effects.Comment: 40 pages, 18 figure

Deep inelastic inclusive and diffractive scattering at Q2Q^2 values from 25 to 320 GeV2^2 with the ZEUS forward plug calorimeter

Deep inelastic scattering and its diffractive component, $ep \to e^{\prime}\gamma^* p \to e^{\prime}XN$, have been studied at HERA with the ZEUS detector using an integrated luminosity of 52.4 pb$^{-1}$. The $M_X$ method has been used to extract the diffractive contribution. A wide range in the centre-of-mass energy $W$ (37 -- 245 GeV), photon virtuality $Q^2$ (20 -- 450 GeV$^2$) and mass $M_X$ (0.28 -- 35 GeV) is covered. The diffractive cross section for $2 < M_X < 15$ GeV rises strongly with $W$, the rise becoming steeper as $Q^2$ increases. The data are also presented in terms of the diffractive structure function, $F^{\rm D(3)}_2$, of the proton. For fixed $Q^2$ and fixed $M_X$, \xpom F^{\rm D(3)}_2 shows a strong rise as \xpom \to 0, where \xpom is the fraction of the proton momentum carried by the Pomeron. For Bjorken-$x < 1 \cdot 10^{-3}$, \xpom F^{\rm D(3)}_2 shows positive $\log Q^2$ scaling violations, while for $x \ge 5 \cdot 10^{-3}$ negative scaling violations are observed. The diffractive structure function is compatible with being leading twist. The data show that Regge factorisation is broken.Comment: 89 pages, 27 figure

A vacuum tolerant high voltage system with a low noise and low power Cockcroft–Walton photomultiplier base

We developed a high voltage system for the electromagnetic calorimeter of the KOTO detector. The system is designed around a low noise, low power Cockcroft–Walton (CW) photomultiplier tube base with a high gain preamplifier. The low power makes it suitable for operations in vacuum. The low noise and high gain allow detecting signals in the 1 MeV range. We achieved a final noise level below 180 μV[rms] for a preamplifier gain of more than 40. A vacuum tolerant control system for the CW bases power distribution was also designed. This system is able to control and monitor the high voltage of each individual base

Scaled momentum spectra in deep inelastic scattering at HERA

Charged particle production has been studied in neutral current deep inelastic ep scattering with the ZEUS detector at HERA using an integrated luminosity of 0.44 fb(-1). Distributions of scaled momenta in the Breit frame are presented for particles in the current fragmentation region. The evolution of these spectra with the photon virtuality, Q(2), is described in the kinematic region 10 < Q(2) < 41000 Ge V-2. Next-to-leading-order and modified leading-log-approximation QCD calculations as well as predictions from Monte Carlo models are compared to the data. The results are also compared to e(+)e(-) annihilation data. The dependences of the pseudorapidity distribution of the particles on Q(2) and on the energy in the gamma p system, W, are presented and interpreted in the context of the hypothesis of limiting fragmentation

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