220 research outputs found
Multijet production in neutral current deep inelastic scattering at HERA and determination of α_{s}
Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities 10 5 GeV and –1 < η_{LAB}^{jet} < 2.5. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant α_{s} (M_{z}), determined from the ratio of the trijet to dijet cross sections, is α_{s} (M_{z}) = 0.1179 ± 0.0013 (stat.)_{-0.0046}^{+0.0028}(exp.)_{-0.0046}^{+0.0028}(th.)
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 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
Jet production in charged current deep inelastic e⁺p scatteringat HERA
The production rates and substructure of jets have been studied in charged current deep inelastic e⁺p scattering for Q² > 200 GeV² with the ZEUS detector at HERA using an integrated luminosity of 110.5 pb⁻¹. Inclusive jet cross sections are presented for jets with transverse energies E_{T}^{jet} > 5 GeV. Measurements of the mean subjet multiplicity, 〈n_{sbj}〉, of the inclusive jet sample are presented. Predictions based on parton-shower Monte Carlo models and next-to-leading-order QCD calculations are compared to the measurements. The value of α_{s} (M_{z}), determined from 〈n_{sbj}〉 at y_{cut} = 10⁻² for jets with 25 < E_{T}^{jet} < 119 GeV, is α_{s} (M_{z}) = 0.1202 ± 0.0052 (stat.)_{-0.0019}^{+0.0060} (syst.)_{-0.0053}^{+0.0065} (th.). The mean subjet multiplicity as a function of Q² is found to be consistent with that measured in NC DIS
Multijet production in neutral current deep inelastic scattering at HERA and determination of alpha_s
Multijet production rates in neutral current deep inelastic scattering have
been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2.
The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s)
= 318 GeV using the ZEUS detector and correspond to an integrated luminosity of
82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster
algorithm in the longitudinally invariant inclusive mode. Measurements of
differential dijet and trijet cross sections are presented as functions of jet
transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with
E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD
calculations describe the data well. The value of the strong coupling constant
alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections,
is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.)
{+0.0064}_{-0.0046}(th.)Comment: 22 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
Measurement of event shapes in deep inelastic scattering at HERA
Inclusive event-shape variables have been measured in the current region of
the Breit frame for neutral current deep inelastic ep scattering using an
integrated luminosity of 45.0 pb^-1 collected with the ZEUS detector at HERA.
The variables studied included thrust, jet broadening and invariant jet mass.
The kinematic range covered was 10 < Q^2 < 20,480 GeV^2 and 6.10^-4 < x < 0.6,
where Q^2 is the virtuality of the exchanged boson and x is the Bjorken
variable. The Q dependence of the shape variables has been used in conjunction
with NLO perturbative calculations and the Dokshitzer-Webber non-perturbative
corrections (`power corrections') to investigate the validity of this approach.Comment: 7+25 pages, 6 figure
Beauty photoproduction measured using decays into muons in dijet events in ep collisions at =318 GeV
The photoproduction of beauty quarks in events with two jets and a muon has
been measured with the ZEUS detector at HERA using an integrated luminosity of
110 pb. The fraction of jets containing b quarks was extracted from the
transverse momentum distribution of the muon relative to the closest jet.
Differential cross sections for beauty production as a function of the
transverse momentum and pseudorapidity of the muon, of the associated jet and
of , the fraction of the photon's momentum participating in
the hard process, are compared with MC models and QCD predictions made at
next-to-leading order. The latter give a good description of the data.Comment: 32 pages, 6 tables, 7 figures Table 6 and Figure 7 revised September
200
Search for a narrow charmed baryonic state decaying to D^*+/- p^-/+ in ep collisions at HERA
A resonance search has been made in the D^*+/- p^-/+ invariant-mass spectrum
with the ZEUS detector at HERA using an integrated luminosity of 126 pb^-1. The
decay channels D^*+ -> D^0 pi^+_s -> (K^- pi^+) pi^+_s and D^*+ -> D^0 pi^+_s
-> (K^- pi^+ pi^+ pi^-) pi^+_s (and the corresponding antiparticle decays) were
used to identify D^*+/- mesons. No resonance structure was observed in the
D^*+/- p^-/+ mass spectrum from more than 60000 reconstructed D^*+/- mesons.
The results are not compatible with a report of the H1 Collaboration of a
charmed pentaquark, Theta^0_c.Comment: 22 pages, 7 figures, 1 table; minor text revisions; 2 references
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