The DIS diffractive cross section, , has been measured in the mass range GeV for c.m. energies GeV and photon virtualities to 140 GeV. For fixed and , the diffractive cross section rises rapidly with ..., with corresponding to a t-averaged pomeron trajectory of which is larger than 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 factorizes according to . They are also consistent with QCD based models which incorporate factorization breaking. The rise of with decreasing and the weak dependence of on suggest a substantial contribution from partonic interactions.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The photon–proton total cross section has been measured in the process
e
+
p→
e
+
γp→
e
+
X with the ZEUS detector at HERA. Events were collected with photon virtuality
Q
2<0.02 GeV
2 and average
γp ...center-of-mass energy
W
γp
=209 GeV in a dedicated run, designed to control systematic effects, with an integrated luminosity of 49 nb
−1. The measured total cross section is
σ
tot
γp
=174±1 (stat.)±13 (syst.) μb. The energy dependence of the cross section is compatible with parameterizations of high-energy
pp and
p
p
̄
data.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Elastic and proton–dissociative photoproduction of \(\rho^0\), \(\phi\) and \(J/\psi\) vector mesons (\(\gamma p\rightarrow Vp\), \(\gamma p\rightarrow VN\), respectively) have been measured in ...\(e^+p\) interactions at HERA up to \(-t=3\) GeV\(^2\), where t is the four-momentum transfer squared at the photon–vector–meson vertex. The analysis is based on a data sample in which photoproduction reactions were tagged by detection of the scattered positron in a special-purpose calorimeter. This limits the photon virtuality, \(Q^2\), to values less than 0.01 GeV\(^2\), and selects a \(\gamma p\) average center-of-mass energy of \(\langle W\rangle\) = 94 GeV. Results for the differential cross sections, \(\mbox{d}\sigma/\mbox{d}t\), for \(\rho^0\), \(\phi\) and \(J/\psi\) mesons are presented and compared to the results of recent QCD calculations. Results are also presented for the t-dependence of the pion-pair invariant-mass distribution in the \(\rho^0\) mass region and of the spin-density matrix elements determined from the decay-angle distributions. The Pomeron trajectory has been derived from measurements of the W dependence of the elastic differential cross sections \(\mbox{d}\sigma/\mbox{d}t\) for both \(\rho^0\) and \(\phi\) mesons.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process \(e^+p \rightarrow e^+ + \mbox{dijet} + e^- + X\) with the ZEUS detector at HERA using an ...integrated luminosity of 38.5 pb\(^{-1}}\). Events with photon-proton centre-of-mass energies, \(W_{\gamma p}\), between 134 and 269 GeV and a photon virtuality, \(Q^2\), less than 1 GeV\(^2\) were selected requiring at least two jets of transverse energy \(E_T^{\rm jet1(2)} >7(6)\) GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, \(p_T^{\rm rel}\). The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum \(p_T^{b} > p_T^{\rm min} =\) 5 GeV and pseudorapidity \(|\eta^{b}| <\) 2. The extrapolated cross section is \(1.6 \pm 0.4 (stat.)^{+0.3}_{-0.5} (syst.) ^{+0.2}_{-0.4} (ext.) \mbox{nb}\). The result is compared to a perturbative QCD calculation performed to next-to-leading order.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The subjet multiplicity has been measured in neutral current e+p interactions at Q2>125 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb−1. Jets were identified in the ...laboratory frame using the longitudinally invariant kT cluster algorithm. The number of jet-like substructures within jets, known as the subjet multiplicity, is defined as the number of clusters resolved in a jet by reapplying the jet algorithm at a smaller resolution scale ycut. Measurements of the mean subjet multiplicity, 〈nsbj〉, for jets with transverse energies ET,jet>15 GeV are presented. Next-to-leading-order perturbative QCD calculations describe the measurements well. The value of αs(MZ), determined from 〈nsbj〉 at ycut=10−2 for jets with 25<ET,jet<71 GeV, is αs(MZ)=0.1187±0.0017(stat.)+0.0024−0.0009(syst.)+0.0093−0.0076(th.).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Inclusive photoproduction of mesons has been measured for photon-proton centre-of-mass energies in the range GeV and photon virtuality 1 GeV. The data sample used corresponds to an integrated ...luminosity of 37 pb. Total and differential cross sections as functions of the transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the 'massive charm' and 'massless charm' schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The exclusive electroproduction of
ω mesons,
ep→
eωp, 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
ω mesons were identified via the decay
ω→
π
+
π
−
π
0. The exclusive cross section in the above kinematic region is
σ
ep→
eωp
=0.108±0.014(stat.)±0.026(syst.) nb. The reaction
ep→
eφp,
φ→
π
+
π
−
π
0, has also been measured. The cross sections, as well as the ratios
σ
γ
∗p→ωp
/σ
γ
∗p→ρ
0p
and
σ
γ
∗p→ωp
/σ
γ
∗p→φp
, are presented as a function of
W and
Q
2. Thus, for the first time, the properties of
ω electroproduction can be compared to those of
ρ
0,
φ and
J/
ψ electroproduction at high
W.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
First inclusive measurements of isolated prompt photons in photoproduction at the HERA
ep collider have been made with the ZEUS detector, using an integrated luminosity of 38.4 pb
−1. Cross sections ...are given as a function of the pseudorapidity and the transverse energy (
η
γ
,
E
T
γ
) of the photon, for
E
T
γ
>5
GeV in the
γp centre-of-mass energy range 134–285 GeV. Comparisons are made with predictions from Monte Carlo models having leading-logarithm parton showers, and with next-to-leading-order QCD calculations, using currently available parameterisations of the photon structure. For forward
η
γ
(proton direction) good agreement is found, but in the rear direction all predictions fall below the data.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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