A
bstract
Two-particle azimuthal correlations have been measured in neutral current deep inelastic
ep
scattering with virtuality
Q
2
>
5 GeV
2
at a centre-of-mass energy
s
= 318 GeV recorded with the ...ZEUS detector at HERA. The correlations of charged particles have been measured in the range of laboratory pseudorapidity
−
1
.
5
< η <
2
.
0 and transverse momentum 0
.
1
< p
T
<
5
.
0 GeV and event multiplicities
N
ch
up to six times larger than the average 〈
N
ch
〉 ≈ 5. The two-particle correlations have been measured in terms of the angular observables
c
n
{2}
= 〈〈
cosn
Δ
φ
〉〉, where
n
is between 1 and 4 and ∆
φ
is the relative azimuthal angle between the two particles. Comparisons with available models of deep inelastic scattering, which are tuned to reproduce inclusive particle production, suggest that the measured two-particle correlations are dominated by contributions from multijet production. The correlations observed here do not indicate the kind of collective behaviour recently observed at the highest RHIC and LHC energies in high-multiplicity hadronic collisions.
A
bstract
The exclusive photoproduction reactions
γp
→
J/ψ
(1
S
)
p
and
γp
→
ψ
(2
S
)
p
have been measured at an
ep
centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated ...luminosity of 373 pb
−
1
. The measurement was made in the kinematic range 30
< W <
180 GeV,
Q
2
<
1 GeV
2
and |
t
|
<
1 GeV
2
, where
W
is the photon-proton centre-of-mass energy,
Q
2
is the photon virtuality and
t
is the squared four-momentum transfer at the proton vertex. The decay channels used were
J/ψ
(1
S
)
→ μ
+
μ
−
,
ψ
(2
S
)
→ μ
+
μ
−
and
ψ
(2
S
)
→ J/ψ
(1
S
)
π
+
π
−
with subsequent decay
J/ψ
(1
S
)
→ μ
+
μ
−
. The ratio of the production cross sections,
R
=
σ
ψ
(2
S
)
/σ
J/ψ
(1
S
)
, has been measured as a function of
W
and |
t
| and compared to previous data in photoproduction and deep inelastic scattering and with predictions of QCD-inspired models of exclusive vector-meson production, which are in reasonable agreement with the data.
A new measurement of inclusive-jet cross sections in the Breit frame in neutral current deep inelastic scattering using the ZEUS detector at the HERA collider is presented. The data were taken in the ...years 2004–2007 at a centre-of-mass energy of
318
GeV
and correspond to an integrated luminosity of
347
pb
-
1
. The jets were reconstructed using the
k
t
-algorithm in the Breit reference frame. They have been measured as a function of the squared momentum transfer,
Q
2
, and the transverse momentum of the jets in the Breit frame,
p
⊥
,
Breit
. The measured jet cross sections are compared to previous measurements and to perturbative QCD predictions. The measurement has been used in a next-to-next-to-leading-order QCD analysis to perform a simultaneous determination of parton distribution functions of the proton and the strong coupling, resulting in a value of
α
s
(
M
Z
2
)
=
0.1142
±
0.0017
(experimental/fit)
-
0.0007
+
0.0006
(model/parameterisation)
-
0.0004
+
0.0006
(scale)
, whose accuracy is improved compared to similar measurements. In addition, the running of the strong coupling is demonstrated using data obtained at different scales.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract The exclusive photoproduction reactions γp → J/ψ(1S)p and γp → ψ(2S)p have been measured at an ep centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated ...luminosity of 373 pb −1. The measurement was made in the kinematic range 30 < W < 180 GeV, Q 2 < 1 GeV2 and |t| < 1 GeV2, where W is the photon-proton centre-of-mass energy, Q 2 is the photon virtuality and t is the squared four-momentum transfer at the proton vertex. The decay channels used were J/ψ(1S) → μ + μ − , ψ(2S) → μ + μ − and ψ(2S) → J/ψ(1S)π + π − with subsequent decay J/ψ(1S) → μ + μ − . The ratio of the production cross sections, R = σ ψ(2S) /σ J/ψ(1S), has been measured as a function of W and |t| and compared to previous data in photoproduction and deep inelastic scattering and with predictions of QCD-inspired models of exclusive vector-meson production, which are in reasonable agreement with the data.
A
bstract
Collective behaviour of final-state hadrons, and multiparton interactions are studied in high-multiplicity
ep
scattering at a centre-of-mass energy
s
= 318 GeV with the ZEUS detector at ...HERA. Two- and four-particle azimuthal correlations, as well as multiplicity, transverse momentum, and pseudorapidity distributions for charged-particle multiplicities
N
ch
≥ 20 are measured. The dependence of two-particle correlations on the virtuality of the exchanged photon shows a clear transition from photoproduction to neutral current deep inelastic scattering. For the multiplicities studied, neither the measurements in photoproduction processes nor those in neutral current deep inelastic scattering indicate significant collective behaviour of the kind observed in high-multiplicity hadronic collisions at RHIC and the LHC. Comparisons of PYTHIA predictions with the measurements in photoproduction strongly indicate the presence of multiparton interactions from hadronic fluctuations of the exchanged photon.
A
bstract
Charm production in charged current deep inelastic scattering has been measured for the first time in
e
±
p
collisions, using data collected with the ZEUS detector at HERA, corresponding to ...an integrated luminosity of 358 pb
−1
. Results are presented separately for
e
+
p
and
e
−
p
scattering at a centre-of-mass energy of
s
= 318 GeV within a kinematic phase-space region of 200 GeV
2
<
Q
2
< 60000 GeV
2
and
y
< 0.9, where
Q
2
is the squared four-momentum transfer and
y
is the inelasticity. The measured cross sections of electroweak charm production are consistent with expectations from the Standard Model within the large statistical uncertainties.
Production of exclusive dijets in diffractive deep inelastic
e
±
p
scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 372 pb
-
1
. The measurement was ...performed for
γ
∗
–
p
centre-of-mass energies in the range
90
<
W
<
250
GeV
and for photon virtualities
Q
2
>
25
GeV
2
. Energy flows around the jet axis are presented. The cross section is presented as a function of
β
and
ϕ
, where
β
=
x
/
x
I
P
,
x
is the Bjorken variable and
x
I
P
is the proton fractional longitudinal momentum loss. The angle
ϕ
is defined by the
γ
∗
–dijet plane and the
γ
∗
–
e
±
plane in the rest frame of the diffractive final state. The
ϕ
cross section is measured in bins of
β
. The results are compared to predictions from models based on different assumptions about the nature of the diffractive exchange.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
Isolated photons with high transverse energy have been studied in deep inelastic
ep
scattering with the ZEUS detector at HERA, using an integrated luminosity of 326 pb
−1
in the range of ...exchanged-photon virtuality 10-350 GeV
2
. Outgoing isolated photons with transverse energy 4 <
E
T
γ
< 15 GeV and pseudorapidity − 0.7 <
η
γ
< 0.9 were measured with accompanying jets having transverse energy and pseudorapidity 2.5 <
E
T
jet
< 35 GeV and −1
.
5
< η
jet
<
1
.
8, respectively. Differential cross sections are presented for the following variables: the fraction of the incoming photon energy and momentum that is transferred to the outgoing photon and the leading jet; the fraction of the incoming proton energy transferred to the photon and leading jet; the differences in azimuthal angle and pseudorapidity between the outgoing photon and the leading jet and between the outgoing photon and the scattered electron. Comparisons are made with theoretical predictions: a leading-logarithm Monte Carlo simulation, a next-to-leading-order QCD prediction, and a prediction using the
k
T
-factorisation approach.
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.
(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image).A combination is presented of all inclusive deep inelastic cross sections previously published by the H1 and ZEUS ...collaborations at HERA for neutral and charged current ... scattering for zero beam polarisation. The data were taken at proton beam energies of 920, 820, 575 and 460 GeV and an electron beam energy of 27.5 GeV. The data correspond to an integrated luminosity of about 1 fb... and span six orders of magnitude in negative four-momentum-transfer squared, ..., and Bjorken x. The correlations of the systematic uncertainties were evaluated and taken into account for the combination. The combined cross sections were input to QCD analyses at leading order, next-to-leading order and at next-to-next-to-leading order, providing a new set of parton distribution functions, called HERAPDF2.0. In addition to the experimental uncertainties, model and parameterisation uncertainties were assessed for these parton distribution functions. Variants of HERAPDF2.0 with an alternative gluon parameterisation, HERAPDF2.0AG, and using fixed-flavour-number schemes, HERAPDF2.0FF, are presented. The analysis was extended by including HERA data on charm and jet production, resulting in the variant HERAPDF2.0Jets. The inclusion of jet-production cross sections made a simultaneous determination of these parton distributions and the strong coupling constant possible, resulting in ... An extraction of ... and results on electroweak unification and scaling violations are also presented.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
PDF
