34 research outputs found
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
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 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 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 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 proton structure function F2 at very low Q2 at HERA
A measurement of the proton structure function F_2(x,Q^2) is presented in the
kinematic range 0.045 GeV^2 < Q^2 < 0.65 GeV^2 and 6*10^{-7} < x < 1*10^{-3}.
The results were obtained using a data sample corresponding to an integrated
luminosity of 3.9pb^-1 in e^+p reactions recorded with the ZEUS detector at
HERA. Information from a silicon-strip tracking detector, installed in front of
the small electromagnetic calorimeter used to measure the energy of the
final-state positron at small scattering angles, together with an enhanced
simulation of the hadronic final state, has permitted the extension of the
kinematic range beyond that of previous measurements. The uncertainties in F_2
are typically less than 4%. At the low Q^2 values of the present measurement,
the rise of F_2 at low x is slower than observed in HERA data at higher Q^2 and
can be described by Regge theory with a constant logarithmic slope. The
dependence of F_2 on Q^2 is stronger than at higher Q^2 values, approaching, at
the lowest Q^2 values of this measurement, a region where F_2 becomes nearly
proportional to Q^2.Comment: 23 pages including 4 figures. Revised version (v2) has a corrected
author list and implements comments from the referee. Accepted by Phys. Lett.
Measurement of exclusive omega electroproduction at HERA
The exclusive electroproduction of omega mesons, ep -> e omega p, has been
studied in the kinematic range 3<Q^2<20 GeV^2, 40<W<120 GeV and |t|<0.6 GeV^2
with the ZEUS detector at HERA using an integrated luminosity of 37.7 pb^{-1}.
The omega mesons were identified via the decay omega -> pi^+pi^-pi^0. The
exclusive (ep -> e omega p) cross section in the above kinematic region is
0.108 +- 0.014(stat.) +- 0.026(syst.) nb. The reaction ep -> e phi p, phi ->
pi^+pi^-pi^0, has also been measured. The cross sections, as well as the
cross-section ratios omega/rho and omega/phi, are presented as a function of W
and Q^2. Thus, for the first time, the properties of omega electroproduction
can be compared to those of rho^0, phi and J/psi electroproduction at high W.Comment: 15 pages including 4 figure
Measurement of the diffractive cross section in deep inelastic scattering using ZEUS 1994 data
The DIS diffractive cross section, dσdiffγ*p→XN/dMx, has been measured in the mass range Mx < 15 GeV for γ*p c.m. energies 60 < W < 200 GeV and photon virtualities Q2 = 7 to 140 GeV2. For fixed Q2 and Mx, the diffractive cross section rises rapidly with W, dσdiffγ*p→X N (Mx, W, Q2)/dMx ∝ Wadiff with adiff = 0.507 ± 0.034 (stat) +0.155-0.046 (syst) corresponding to a t-averaged pomeron trajectory of ̄αℙ = 1.127 ± 0.009 (stat) +0.039-0.012 (syst) which is larger than ̄αℙ observed in hadron-hadron scattering. The W dependence of the diffractive cross section is found to be the same as that of the total cross section for scattering of virtual photons on protons. The data are consistent with the assumption that the diffractive structure function FD(3)2 factorizes according to cursive greek chiℙFD(3)2(cursive greek chiℙ, β, Q2) = (cursive greek chi0//cursive greek chiℙ)nFD(2)2(β, Q2). They are also consistent with QCD based models which incorporate factorization breaking. The rise of cursive greek chiℙFD(3)2 with decreasing cursive greek chiℙ and the weak dependence of FD(2)2 on Q2 suggest a substantial contribution from partonic interactions