Measurements of open charm production cross sections in deep-inelastic
ep
scattering at HERA from the H1 and ZEUS Collaborations are combined. Reduced cross sections
for charm production are obtained ...in the kinematic range of photon virtuality 2.5≤
Q
2
≤2000 GeV
2
and Bjorken scaling variable 3⋅10
−5
≤
x
≤5⋅10
−2
. The combination method accounts for the correlations of the systematic uncertainties among the different data sets. The combined charm data together with the combined inclusive deep-inelastic scattering cross sections from HERA are used as input for a detailed NLO QCD analysis to study the influence of different heavy flavour schemes on the parton distribution functions. The optimal values of the charm mass as a parameter in these different schemes are obtained. The implications on the NLO predictions for
W
±
and
Z
production cross sections at the LHC are investigated. Using the fixed flavour number scheme, the running mass of the charm quark is determined.
The cross-section for deeply virtual Compton scattering in the reaction ep→eγp has been measured with the ZEUS detector at HERA using integrated luminosities of 95.0 pb−1 of e+p and 16.7 pb−1 of e−p ...collisions. Differential cross-sections are presented as a function of the exchanged-photon virtuality, Q2, and the centre-of-mass energy, W, of the γ∗p system in the region 5<Q2<100 GeV2 and 40<W<140 GeV. The measured cross-sections rise steeply with increasing W. The measurements are compared to QCD-based calculations.
The photoproduction of isolated photons, both inclusive and together with a jet, has been measured with the ZEUS detector at HERA using an integrated luminosity of 374 pb−1. Differential cross ...sections are presented in the isolated-photon transverse-energy and pseudorapidity ranges 6<ETγ<15 GeV and −0.7<ηγ<0.9, and for jet transverse-energy and pseudorapidity ranges 4<ETjet<35 GeV and −1.5<ηjet<1.8, for exchanged-photon virtualities Q2<1 GeV2. Differential cross sections are also presented for inclusive isolated-photon production as functions of the transverse energy and pseudorapidity of the photon. Higher-order theoretical calculations are compared to the results.
A search for single-top production, ep→etX, has been performed with the ZEUS detector at HERA using data corresponding to an integrated luminosity of 0.37 fb−1. No evidence for top production was ...found, consistent with the expectation from the Standard Model. Limits were computed for single-top production via flavour changing neutral current transitions involving a neutral electroweak vector boson, γ or Z. The result was combined with a previous ZEUS result yielding a total luminosity of 0.50 fb−1. A 95% credibility level upper limit of 0.13 pb was obtained for the cross section at the centre-of-mass energy of s=315 GeV.
The production of neutrons carrying at least 20% of the proton beam energy (
x
L
>
0.2
) in
e
+
p collisions has been studied with the ZEUS detector at HERA for a wide range of
Q
2, the photon ...virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section,
ep→
e′
Xn, is measured relative to the inclusive cross section,
ep→
e′
X, thereby reducing the systematic uncertainties. For
x
L> 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
γp system. For 0.64<
x
L<0.82, the rate of neutrons is almost independent of the Bjorken scaling variable
x and
Q
2. However, at lower and higher
x
L values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function,
F
LN(3)
2(
x,
Q
2,
x
L), rises at low values of
x in a way similar to that of the inclusive
F
2(
x,
Q
2) of the proton. The total
γπ cross section and the structure function of the pion,
F
π
2(
x
π
,
Q
2) where
x
π
=
x/(1−
x
L), have been determined using a one-pion-exchange model, up to uncertainties in the normalisation due to the poorly understood pion flux. At fixed
Q
2,
F
π
2 has approximately the same
x dependence as
F
2 of the proton.
The production of isolated high-energy photons accompanied by jets has been measured in deep inelastic ep scattering with the ZEUS detector at HERA, using an integrated luminosity of 326 pb−1. ...Measurements were made for exchanged photon virtualities, Q2, in the range 10 to 350 GeV2. The photons were measured in the transverse-energy and pseudorapidity ranges 4<ETγ<15 GeV and −0.7<ηγ<0.9, and the jets were measured in the transverse-energy and pseudorapidity ranges 2.5<ETjet<35 GeV and −1.5<ηjet<1.8. Differential cross sections are presented as functions of these quantities. Perturbative QCD predictions give a reasonable description of the shape of the measured cross sections over most of the kinematic range, but the absolute normalisation is typically in disagreement by 20–30%.
A
bstract
In this extended analysis using the ZEUS detector at HERA, the photoproduction of isolated photons together with a jet is measured for different ranges of the fractional photon energy,
x
γ
...meas
, contributing to the photon-jet final state. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges 6
< E
T
γ
<
15 GeV and −0
.
7
< η
γ
<
0
.
9, and for jet transverse-energy and pseudorapidity ranges 4
< E
T
jet
<
35 GeV and −1
.
5
< η
jet
<
1
.
8, for an integrated luminosity of 374 pb
−1
. The kinematic observables studied comprise the transverse energy and pseudorapidity of the photon and the jet, the azimuthal difference between them, the fraction of proton energy taking part in the interaction, and the difference between the pseudorapidities of the photon and the jet. Higher-order theoretical calculations are compared to the results.
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
P
<0.035
,
β<0.8,
p
T(D
∗±)>1.5
GeV and
|η(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.
A
bstract
The cross sections for inelastic photoproduction of
J/ψ
and
ψ
′ mesons have been measured in
ep
collisions with the ZEUS detector at HERA, using an integrated luminosity of 468 pb
−1
...collected in the period 1996–2007. The
ψ
′ to
J/
ψ cross section ratio was measured in the range 0
.
55
< z <
0
.
9 and 60
< W <
190 GeV as a function of
W
,
z
and
p
T
. Here
W
denotes the photon-proton centre-of-mass energy,
z
is the fraction of the incident photon energy carried by the meson and
p
T
is the transverse momentum of the meson with respect to the beam axis. The
J/ψ
cross sections were measured for 0
.
1
< z <
0
.
9, 60
< W <
240 GeV and
p
T
>
1 GeV. Theoretical predictions within the non-relativistic QCD framework including NLO colour-singlet and colour-octet contributions were compared to the data, as were predictions based on the
k
T
-factorisation approach.
The production of Z 0 bosons in the reaction e p a e Z 0 p (a) , where p (a) stands for a proton or a low-mass nucleon resonance, has been studied in ep collisions at HERA using the ZEUS detector. ...The analysis is based on a data sample collected between 1996 and 2007, amounting to 496 pb - 1 of integrated luminosity. The Z 0 was measured in the hadronic decay mode. The elasticity of the events was ensured by a cut on IDT max < 3.0 , where IDT max is the maximum pseudorapidity of energy deposits in the calorimeter defined with respect to the proton beam direction. A signal was observed at the Z 0 mass. The cross section of the reaction e p a e Z 0 p (a) was measured to be I (e p a e Z 0 p (a)) = 0.13 +/- 0.06 (stat.) +/- 0.01 (syst.) pb , in agreement with the Standard Model prediction of 0.16 pb. This is the first measurement of Z 0 production in ep collisions.
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