13 research outputs found
Sociologies de l'enfance
Differential cross sections for jet photoproduction in the reaction ep → e jet X have been measured with the ZEUS detector at HERA using 82.2 pb-1 of integrated luminosity. Inclusive jet cross sections are presented as a function of the jet transverse energy, ET jet, for jets with ET jet > 17 GeV and pseudorapidity -1 <ηjet <2.5, in the γp centre-of-mass-energy range 142 < Wγp < 293 GeV. Scaled jet invariant cross sections are presented as a function of the dimensionless variable xT ≡ 2ET jet/Wγp for (Wγp) = 180 and 255 GeV. Next-to-leading-order QCD calculations give a good description of the measured differential cross sections in both magnitude and shape. The ratio of scaled jet invariant cross sections at the two (Wγp) values shows clear non-scaling behaviour. A value for the strong coupling constant of αs(MZ) = 0.1224 ± 0.0001 (stat.)-0.0019 +0.0022(exp.)-0.0042 +0.0054(th.) has been extracted from a QCD analysis of the measured dσ/dET jet. The variation of αs with ET jet is in good agreement with the running of αs as predicted by QCD
Measurements of inelastic J / psi and psi-prime photoproduction at HERA
The cross sections for inelastic photoproduction of J/psi and psi' mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 38.0 pb-1. The events were required to have 0.1 0.4 is compared to leading-order QCD predictions
Leading neutron production in e+ p collisions at HERA
The production of neutrons carrying at least 20% of the proton beam energy
(\xl > 0.2) in collisions has been studied with the ZEUS detector at
HERA for a wide range of , the photon virtuality, from photoproduction to
deep inelastic scattering. The neutron-tagged cross section, ,
is measured relative to the inclusive cross section, , thereby
reducing the systematic uncertainties. For \xl > 0.3, the rate of neutrons in
photoproduction is about half of that measured in hadroproduction, which
constitutes a clear breaking of factorisation. There is about a 20% rise in the
neutron rate between photoproduction and deep inelastic scattering, which may
be attributed to absorptive rescattering in the system. For 0.64 <
\xl < 0.82, the rate of neutrons is almost independent of the Bjorken scaling
variable and . However, at lower and higher \xl values, there is a
clear but weak dependence on these variables, thus demonstrating the breaking
of limiting fragmentation. The neutron-tagged structure function,
{{F}^{\rm\tiny LN(3)}_2}(x,Q^2,\xl), rises at low values of in a way
similar to that of the inclusive \ff of the proton. The total
cross section and the structure function of the pion,
where x_\pi = x/(1-\xl), have been determined using a one-pion-exchange
model, up to uncertainties in the normalisation due to the poorly understood
pion flux. At fixed , has approximately the same
dependence as of the proton.Comment: 61 pages, 19 figure
Measurement of dijet cross-sections for events with a leading neutron in photoproduction at HERA
Differential cross sections for dijet photoproduction in association with a
leading neutron using the reaction e^+ + p --> e^+ + n + jet + jet + X_r have
been measured with the ZEUS detector at HERA using an integrated luminosity of
6.4 pb^{-1}. The fraction of dijet events with a leading neutron in the final
state was studied as a function of the jet kinematic variables. The cross
sections were measured for jet transverse energies E^{jet}_T > 6 GeV, neutron
energy E_n > 400 GeV, and neutron production angle theta_n < 0.8 mrad. The data
are broadly consistent with factorization of the lepton and hadron vertices and
with a simple one-pion-exchange model.Comment: 28 pages, 9 figures, accepted by Nuclear Physics B, correction to
neutron yiel
Search for resonance decays to an -neutrino plus jet in scattering at DESY HERA
A study of the antineutrino-jet mass spectrum in e^+p -> antineutrino+X events at center-of-mass energy 300 GeV has been performed with the ZEUS detector at HERA using an integrated luminosity of 47.7 pb^-1. The mass spectrum is in good agreement with that expected from Standard Model processes over the antineutrino -jet mass range studied. No significant excess attributable to the decay of a narrow resonance is observed. By using both e^+p -> e^+ X and e^+p -> antineutrino X data, mass-dependent limits are set on the s-channel production of scalar and vector resonant states. Couplings to first-generation quarks are considered and limits are presented as a function of the e^+q and antineutrino-q branching ratios. These limits are used to constrain the production of leptoquarks and R-parity violating squarks
Three jet production in diffractive deep inelastic scattering at HERA
Three-jet production in the reaction ep --> eXp has been studied with the
ZEUS detector at HERA using an integrated luminosity of 42.74 pb^-1. The data
were measured in the kinematic region 5 < Q^2 < 100 GeV^2, 200 < W < 250 GeV
and 23 < M_X < 40 GeV. The diffractive signal was selected by requiring a large
rapidity gap in the outgoing proton direction. Jets were reconstructed in the
centre-of-mass system of X using the exclusive k_T-algorithm. A sample of
three-jet events in diffraction has been identified. Differential cross
sections were measured as a function of the jet pseudorapidity and jet
transverse momentum with respect to the virtual photon-Pomeron axis. The jets
going in the Pomeron direction are broader than those going in the
virtual-photon direction. This is consistent with models predicting that gluons
are predominantly produced in the Pomeron direction and quarks in the
virtual-photon direction.Comment: 14 pages, 5 figures and 7 tables. Submitted to Physics Letters
High-mass dijet cross sections in photoproduction at HERA
Dijet differential cross sections for the reaction e/sup +/p to e/sup +/+jet+jet+X in the photoproduction regime have been measured with the ZEUS detector at HERA using an integrated luminosity of 42.7 pb/sup -1/. The cross sections are given for photon-proton centre-of-mass energies in the range 134 lt W lt 277 GeV. The differential cross sections as a function of the dijet mass, M/sup jj/, and of the dijet angular variables have been measured for 47 lt M/sup jj/ lt 160 GeV and compared to next-to-leading-order QCD calculations. The dijet events in the region 75 lt M/sup jj/ lt 100 GeV have been used to derive a 95% C.L. upper limit on the cross section for Z/sup 0/ photoproduction of sigma /sub e(+)p to e(+)Z(0)/X lt 5.9 ph. Upper limits on the photoproduction of new heavy resonances decaying into two jets are also presented for masses in the range between 60 GeV and 155 GeV
Measurement of three jet distributions in photoproduction at HERA
The cross section for the photoproduction of events containing three jets
with a three-jet invariant mass of M_3J > 50 GeV has been measured with the
ZEUS detector at HERA. The three-jet angular distributions are inconsistent
with a uniform population of the available phase space but are well described
by parton shower models and O(alpha alpha_s^2) pQCD calculations. Comparisons
with the parton shower model indicate a strong contribution from initial state
radiation as well as a sensitivity to the effects of colour coherence.Comment: 13 pages including 5 figure
Search for contact interactions in deep inelastic e(+)p -> e(+)X scattering at HERA
In a search for signatures of physics processes beyond the Standard Model, various eeqq vector contact-interaction hypotheses have been tested using the high-Q(2) deep inelastic neutral-current e(+)p scattering data collected with the ZEUS detector at HERA. The data correspond to an integrated luminosity of 47.7pb(-1) of e(+)p interactions at 300 GeV center-of-mass energy. No significant evidence of a contact-interaction signal has been found. Limits at the 95% confidence level are set on the contact-interaction amplitudes. The effective mass scales Lambda corresponding to these limits range from 1.7 TeV to 5 TeV for the contact-interaction scenarios considered