34 research outputs found
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
(+ c.c.) has been used in the study. The
cross section for inclusive D^{*\pm} production with
and is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region
{ GeV and }. Differential cross
sections as functions of p_T(D^{*\pm}), and 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 (D^{*\pm}), the charm
contribution to the proton structure function is
determined for Bjorken between 2 10 and 5 10.Comment: 17 pages including 4 figure
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 and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, 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 .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
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
Measurement of inclusive D+-(s) photoproduction at HERA
The first measurement of inclusive Ds+- photoproduction at HERA has been
performed with the ZEUS detector for photon-proton centre-of-mass energies 130
< W < 280 GeV. The measured cross section for 3 < pt(Ds) < 12 GeV and
|eta(Ds)| Ds X) = 3.79 +- 0.59 (stat.) +0.26-0.46 (syst.)
+- 0.94 (br.) nb, where the last error arises from the uncertainty in the Ds
decay branching ratio. The measurements are compared with inclusive D*+-
photoproduction cross sections in the same kinematic region and with QCD
calculations. The Ds cross sections lie above a fixed-order next-to-leading
order calculation and agree better with a tree-level O(alpha,alpha_s^3)
calculation that was tuned to describe the ZEUS D* cross sections. The ratio of
Ds+- to D*+- cross sections is 0.41 +- 0.07 (stat.) +0.03-0.05 (syst.) +- 0.10
(br.). From this ratio, the strangeness-suppression factor in charm
photoproduction, within the LUND string fragmentation model, has been
calculated to be gamma_s = 0.27 +- 0.05 +- 0.07 (br.). The cross-section ratio
and gamma_s are in good agreement with those obtained in charm production in
e+e- annihilation.Comment: 20 pages, 3 figures, submitted to Phys. Lett. B; 1 footnote adde
Measurement of the E**2(T,jet) / Q**2 dependence of forward jet production at HERA
The forward-jet cross section in deep inelastic ep scattering has been
measured using the ZEUS detector at HERA with an integrated luminosity of 6.36
pb^-1. The jet cross section is presented as a function of jet transverse
energy squared, E(T,jet)^2, and Q^2 in the kinematic ranges
10^-2<E(T,jet)^2/Q^2<10^2 and 2.5 10^-4<x<8.0 10^-2. Since the perturbative QCD
predictions for this cross section are sensitive to the treatment of the
log(E_T/Q)^2 terms, this measurement provides an important test. The measured
cross section is compared to the predictions of a next-to-leading order pQCD
calculation as well as to various leading-order Monte Carlo models. Whereas the
predictions of all models agree with the measured cross section in the region
of small E(T,Jet)^2/Q^2, only one model, which includes a resolved photon
component, describes the data over the whole kinematic range.Comment: 8 pages including 3 figure
Measurement of azimuthal asymmetries in deep inelastic scattering
The distribution of the azimuthal angle for the charged hadrons has been
studied in the hadronic centre-of-mass system for neutral current deep
inelastic positron-proton scattering with the ZEUS detector at HERA.
Measurements of the dependence of the moments of this distribution on the
transverse momenta of the charged hadrons are presented. Asymmetries that can
be unambiguously attributed to perturbative QCD processes have been observed
for the first time.Comment: 19 pages - 4 figures. Included referee's comment
Measurement of dijet production in neutral current deep inelastic scattering at high Q(2) and determination of alpha(s)
Dijet production has been studied in neutral current deep inelastic e(+)p scattering for 470 lt Q(2) lt 20000 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb(-1). Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q(2) allows both a precise determination of alpha (s)(M-z) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the parton distribution functions of the proton. The value of cu,(Mz), as determined from the QCD fit, is alpha (s)(M-z) = 0.1166 +/- 0.0019(stat.)(- 0.0033)(+0.0024)(exp.)(-0.0044)(+0.0057)(th.)