28 research outputs found
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 values from 25 to 320 GeV with the ZEUS forward plug calorimeter
Deep inelastic scattering and its diffractive component, , have been studied at HERA with the ZEUS
detector using an integrated luminosity of 52.4 pb. The method has
been used to extract the diffractive contribution. A wide range in the
centre-of-mass energy (37 -- 245 GeV), photon virtuality (20 -- 450
GeV) and mass (0.28 -- 35 GeV) is covered. The diffractive cross
section for GeV rises strongly with , the rise becoming
steeper as increases. The data are also presented in terms of the
diffractive structure function, , of the proton. For fixed
and fixed , \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-, \xpom F^{\rm D(3)}_2 shows
positive scaling violations, while for
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
Production of Z<sup>0</sup> bosons in elastic and quasi-elastic <i>ep</i> collisions at HERA
The production of Z<sup>0</sup> bosons in the reaction ep→eZ<sup>0</sup>p<sup>(⁎)</sup>, where p<sup>(⁎)</sup> stands for a proton or a low-mass nucleon resonance, has been studied in <i>ep</i> collisions at HERA using the ZEUS detector. The analysis is based on a data sample collected between 1996 and 2007, amounting to 496 pb<sup>−1</sup> of integrated luminosity. The Z<sup>0</sup> was measured in the hadronic decay mode. The elasticity of the events was ensured by a cut on ηmax<3.0, where η<sub>max</sub> 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<sup>0</sup> mass. The cross section of the reaction ep→ eZ<sup>0</sup>p<sup>(⁎)</sup> was measured to be σ(ep→ eZ<sup>0</sup>p<sup>(⁎)</sup>)=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<sup>0</sup> production in <i>ep</i> collisions
Multi-lepton production at high transverse momentum at HERA
A search for events containing two or more high-transverse-momentum isolated
leptons has been performed in ep collisions with the ZEUS detector at HERA
using the full collected data sample, corresponding to an integrated luminosity
of 480 pb^{-1}. The number of observed events has been compared with the
prediction from the Standard Model, searching for possible deviations,
especially for multi-lepton events with invariant mass larger than 100 GeV.
Good agreement with the Standard Model has been observed. Total and
differential cross sections for di-lepton production have been measured in a
restricted phase space dominated by photon-photon collisions.Comment: 18 pages, 5 figures. v2: revised version for journal publicatio
Measurement of the t dependence in exclusive photoproduction of (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
Measurement of Isolated Photons Accompanied by Jets in Deep Inelasic Scattering
The production of isolated high-energy photons accompanied by jets has been
measured in deep inelastic ep scattering with the ZEUS detector at HERA, using
an integrated luminosity of 326 pb^{-1}. Measurements were made for exchanged
photon virtualities, Q^2, in the range 10 to 350 GeV^2. The photons were
measured in the transverse-energy and pseudorapidity ranges 4 < ET^gamma < 15
GeV and -0.7 < eta^gamma < 0.9, and the jets were measured in the
transverse-energy and pseudorapidity ranges 2.5 < ET^jet <35 GeV and -1.5 <
eta^jet < 1.8. Differential cross sections are presented as functions of these
quantities. Perturbative QCD predictions give a reasonable description of the
shape of the measured cross sections over most of the kinematic range, but the
absolute normalisation is typically in disagreement by 20-30%.Comment: 17 pages, 4 figures. Minor changes in published versio
Inclusive-Jet Cross Sections in NC DIS at HERA and a Comparison of the {\boldmath }, {\boldmath anti-} and SIScone Jet Algorithms
For the first time, differential inclusive-jet cross sections have been
measured in neutral current deep inelastic ep scattering using the anti-kT and
SIScone algorithms. The measurements were made for boson virtualities Q^2 > 125
GeV^2 with the ZEUS detector at HERA using an integrated luminosity of 82 pb^-1
and the jets were identified in the Breit frame. The performance and
suitability of the jet algorithms for their use in hadron-like reactions were
investigated by comparing the measurements to those performed with the kT
algorithm. Next-to-leading-order QCD calculations give a good description of
the measurements. Measurements of the ratios of cross sections using different
jet algorithms are also presented; the measured ratios are well described by
calculations including up to O(alphas^3) terms. Values of alphas(Mz) were
extracted from the data; the results are compatible with and have similar
precision to the value extracted from the kT analysis.Comment: 27 pages, 6 figures, 4 table
Exclusive Photoproduction of Mesons at HERA
he exclusive photoproduction reaction γ p → Υ{hooked} p has been studied with the ZEUS experiment 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. These results, which represent the analysis of the full ZEUS data sample for this channel, are compared to predictions based on perturbative QC