160 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.
Photoproduction of mesons associated with a leading neutron
The photoproduction of mesons associated with a leading
neutron has been observed with the ZEUS detector in collisions at HERA
using an integrated luminosity of 80 pb. The neutron carries a large
fraction, {}, of the incoming proton beam energy and is detected at
very small production angles, { mrad}, an indication of
peripheral scattering. The meson is centrally produced with
pseudorapidity {
GeV}, which is large compared to the average transverse momentum of the neutron
of 0.22 GeV. The ratio of neutron-tagged to inclusive production is
in the photon-proton
center-of-mass energy range { GeV}. The data suggest that the
presence of a hard scale enhances the fraction of events with a leading neutron
in the final state.Comment: 28 pages, 4 figures, 2 table
Measurement of the open-charm contribution to the diffractive proton structure function
Production of D*+/-(2010) mesons in diffractive deep inelastic scattering has
been measured with the ZEUS detector at HERA using an integrated luminosity of
82 pb^{-1}. Diffractive events were identified by the presence of a large
rapidity gap in the final state. Differential cross sections have been measured
in the kinematic region 1.5 < Q^2 < 200 GeV^2, 0.02 < y < 0.7, x_{IP} < 0.035,
beta 1.5 GeV and |\eta(D*+/-)| < 1.5. The measured cross
sections are compared to theoretical predictions. The results are presented in
terms of the open-charm contribution to the diffractive proton structure
function. The data demonstrate a strong sensitivity to the diffractive parton
densities.Comment: 35 pages, 11 figures, 6 table
Measurement of beauty production in deep inelastic scattering at HERA
The beauty production cross section for deep inelastic scattering events with
at least one hard jet in the Breit frame together with a muon has been
measured, for photon virtualities Q^2 > 2 GeV^2, with the ZEUS detector at HERA
using integrated luminosity of 72 pb^-1. The total visible cross section is
sigma_b-bbar (ep -> e jet mu X) = 40.9 +- 5.7 (stat.) +6.0 -4.4 (syst.) pb. The
next-to-leading order QCD prediction lies about 2.5 standard deviations below
the data. The differential cross sections are in general consistent with the
NLO QCD predictions; however at low values of Q^2, Bjorken x, and muon
transverse momentum, and high values of jet transverse energy and muon
pseudorapidity, the prediction is about two standard deviations below the data.Comment: 18 pages, 4 figure
Observation of isolated high-E_T photons in deep inelastic scattering
First measurements of cross sections for isolated prompt photon production in
deep inelastic ep scattering have been made using the ZEUS detector at the HERA
electron-proton collider using an integrated luminosity of 121 pb^-1. A signal
for isolated photons in the transverse energy and rapidity ranges 5 < E_T^gamma
< 10 GeV and -0.7 < eta^gamma < 0.9 was observed for virtualities of the
exchanged photon of Q^2 > 35 GeV^2. Cross sections are presented for inclusive
prompt photons and for those accompanied by a single jet in the range E_T^jet
\geq 6 GeV and -1.5 \leq eta^jet < 1.8. Calculations at order alpha^3alpha_s
describe the data reasonably well.Comment: 16 pages, 5 figure
Isolated tau leptons in events with large missing transverse momentum at HERA
A search for events containing isolated tau leptons and large missing
transverse momentum, not originating from the tau decay, has been performed
with the ZEUS detector at the electron-proton collider HERA, using 130 pb^-1 of
integrated luminosity. A search was made for isolated tracks coming from
hadronic tau decays. Observables based on the internal jet structure were
exploited to discriminate between tau decays and quark- or gluon-induced jets.
Three tau candidates were found, while 0.40 +0.12 -0.13 were expected from
Standard Model processes, such as charged current deep inelastic scattering and
single W-boson production. To search for heavy-particle decays, a more
restrictive selection was applied to isolate tau leptons produced together with
a hadronic final state with high transverse momentum. Two candidate events
survive, while 0.20 +-0.05 events are expected from Standard Model processes.Comment: 28 pages, 4 figures, 3 tables, accepted by Phys. Lett. B. Updated
with minor changes to the text requested by the journal refere
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
Decay and Fission Hindrance of Two- and Four-Quasiparticle K Isomers in Rf 254
Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73)μs have been discovered in the heavy Rf254 nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the Kπ=8-, ν2(7/2+[624],9/2-[734]) two-quasineutron and the Kπ=16+, 8-ν2(7/2+[624],9/2-[734])⊗ - 8-π2(7/2-[514],9/2+[624]) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N=150 isotones. The four-quasiparticle isomer is longer lived than the Rf254 ground state that decays exclusively by spontaneous fission with a half-life of 23.2(1.1)μs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state
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
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