17 research outputs found
Observation of hard scattering in photoproduction events with a large rapidity gap at HERA
Events with a large rapidity gap and total transverse energy greater than 5
GeV have been observed in quasi-real photoproduction at HERA with the ZEUS
detector. The distribution of these events as a function of the
centre of mass energy is consistent with diffractive scattering. For total
transverse energies above 12 GeV, the hadronic final states show predominantly
a two-jet structure with each jet having a transverse energy greater than 4
GeV. For the two-jet events, little energy flow is found outside the jets. This
observation is consistent with the hard scattering of a quasi-real photon with
a colourless object in the proton.Comment: 19 pages, latex, 4 figures appended as uuencoded fil
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
Differential cross-sections of D*+- photoproduction in e p collisions at HERA
Inclusive photoproduction of D*+- in ep collisions at HERA has been measured
with the ZEUS detector for photon-proton centre of mass energies in the range
115 < W < 280 GeV and photon virtuality Q**2 < 4 GeV**2. The cross section
sigma(ep --> D*X) integrated over the kinematic region pt(D*) > 3 GeV and -1.5
< eta(D*) < 1.0 is (10.6+-1.7(stat.)+1.6(syst.)-1.3(syst.)) nb. Differential
cross sections as functions of pt(D*), eta(D*) and W are given. The data are
compared with two next-to-leading order perturbative QCD predictions. For a
calculation using a massive charm scheme the predicted cross sections are
smaller than the measured ones. A recent calculation using a massless charm
scheme is in agreement with the data.Comment: 18 pages including 4 figure
Comparison of energy flows in deep inelastic scattering events with and without a large rapidity gap
Energy flows in deep inelastic electron-proton scattering are investigated at a center-of-mass energy of 296 GeV for the range Q2 greater-than-or-equal-to 10 GeV2 using the ZEUS detector. A comparison is made between events with and without a large rapidity gap between the hadronic system and the proton direction. The energy flows, corrected for detector acceptance and resolution, are shown for these two classes of events in both the HERA laboratory frame and the Breit frame. From the differences in the shapes of these energy flows we conclude that QCD radiation is suppressed in the large-rapidity-gap events compared to the events without a large rapidity gap
Measurement of the reaction gamma* p --> phi p in deep inelastic e+ p scattering at HERA
The production of mesons in the reaction (), for GeV and for
virtual photon-proton centre of mass energies () in the range 42-134 GeV,
has been studied with the ZEUS detector at HERA. When compared to lower energy
data at similar , the results show that the \gamma^*p \rightarrow \phid
p cross section rises strongly with . This behaviour is similar to that
previously found for the cross section. This
strong dependence cannot be explained by production through soft pomeron
exchange. It is, however, consistent with perturbative QCD expectations, where
it reflects the rise of the gluon momentum density in the proton at small .
The ratio of , which has previously been
determined by ZEUS to be 0.065 0.013 (stat.) in photoproduction at a mean
of 70 GeV, is measured to be 0.18 0.05 (stat.) 0.03 (syst.) at
a mean of 12.3 GeV and mean of 100 GeV and is thus
approaching at large the value of 2/9 predicted from the quark charges of
the vector mesons and a flavour independent production mechanism.Comment: 22 pages, including 4 figures. Includes some minor editorial change
Study of D*(2010)+- Production in ep Collisions at HERA
We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D*(+) --> (D-0 --> K(-)pi(+)) pi(+) (+ c.c.). Clear signals in the mass difference Delta M=M(D*)-M(D-0) as well as in the M(K pi) distribution at the D-0 mass are found. The ep cross section for inclusive D*(+/-) production with Q(2) c ) over bar cX)=(0.45+/-0.11(-0.22)(+0.37)) mu b at root s=296 GeV and [W]=198 GeV. The average gamma p charm cross section sigma(gamma p --> c $($) over bar cX) is found to be (6.3+/-2.2(-3.0)(+6.3)) mu b at [W]=163 GeV and (16.9+/-5.2(-8.5)(+13.9)) mu b at [W]=243 GeV. The increase of the total charm photoproduction cross section by one order of magnitude with respect to low energy data experiments is well described by QCD NLO calculations using singular gluon distributions in the proton
Exclusive rho0 production in deep inelastic electron - proton scattering at HERA
The exclusive production of mesons in deep inelastic electron-proton
scattering has been studied using the ZEUS detector. Cross sections have been
measured in the range GeV for centre of mass
(c.m.) energies from 40 to 130 GeV. The cross section
exhibits a dependence and both
longitudinally and transversely polarised 's are observed. The
cross section rises strongly with increasing c.m.
energy, when compared with NMC data at lower energy, which cannot be explained
by production through soft pomeron exchange. The data are compared with
perturbative QCD calculations where the rise in the cross section reflects the
increase in the gluon density at low . the gluon density at low .Comment: 22 pages, latex, 4 figures appende
Dijet cross-sections in photoproduction at HERA
Dijet production by almost real photons has been studied at HERA with the
ZEUS detector. Jets have been identified using the cone algorithm. A cut on xg,
the fraction of the photon energy participating in the production of the two
jets of highest transverse energy, is used to define cross sections sensitive
to the parton distributions in the proton and in the photon. The dependence of
the dijet cross sections on pseudorapidity has been measured for xg
and xg . The former is sensitive to the gluon momentum density in the
proton. The latter is sensitive to the gluon in the photon. The cross sections
are corrected for detector acceptance and compared to leading order QCD
calculations.Comment: 21 pages, latex, 4 figures appended as uuencoded fil
Diffractive hard photoproduction at HERA and evidence for the gluon content of the pomeron
Inclusive jet cross sections for events with a large rapidity gap with
respect to the proton direction from the reaction with quasi-real photons have been measured with the ZEUS detector. The cross
sections refer to jets with transverse energies ~GeV. The data
show the characteristics of a diffractive process mediated by pomeron exchange.
Assuming that the events are due to the exchange of a pomeron with partonic
structure, the quark and gluon content of the pomeron is probed at a scale
. A comparison of the measurements with model predictions
based on QCD plus Regge phenomenology requires a contribution of partons with a
hard momentum density in the pomeron. A combined analysis of the jet cross
sections and recent ZEUS measurements of the diffractive structure function in
deep inelastic scattering gives the first experimental evidence for the gluon
content of the pomeron in diffractive hard scattering processes. The data
indicate that between 30\% and 80\% of the momentum of the pomeron carried by
partons is due to hard gluons.Comment: 25 pages, latex, 4 figures appended as uuencoded file. No change
w.r.t. previous version (just avoiding warning latex messages while
processing