Measurement of the luminosity in the ZEUS experiment at HERA II Adamczyk, L.; Andruszkow, J.; Bold, T. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2014, Letnik:
744
Journal Article
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The luminosity in the ZEUS detector was measured using photons from electron bremsstrahlung off protons. In 2001 the HERA collider was upgraded for operation at higher luminosity. At the same time ...the luminosity-measuring system of the ZEUS experiment was modified to tackle the expected higher photon rate and synchrotron radiation. The existing lead-scintillator calorimeter was equipped with radiation hard scintillator tiles and shielded against synchrotron radiation. In addition, a magnetic spectrometer was installed to measure the luminosity independently using photons converted in the beam-pipe exit window. The redundancy provided a reliable and robust luminosity determination with a systematic uncertainty of 1.7%. The experimental setup, the techniques used for luminosity determination and the estimate of the systematic uncertainty are reported.
The very forward region of a detector at a linear e/sup +/e/sup -/ collider is a particularly challenging area for instrumentation. In the TESLA detector, two calorimeters, BeamCal (Beam Calorimeter) ...and LumiCal (Luminosity Calorimeter) are planned. The BeamCal is positioned just adjacent to the beampipe. It will be hit by beamstrahlung remnants giving a deposition of several tens of TeV per bunch crossing. The distribution of this energy will be measured to assist in tuning the beams. Single high-energy electrons will be identified and measured. High-energy electron identification is particularly important to veto backgrounds to new particle searches. Several technological options for BeamCal are discussed. Monte Carlo simulations are presented for a diamond/tungsten sandwich structure and compared to results obtained for a heavy element crystal calorimeter. First, tests of sensors are described. The LumiCal will measure larger polar angles than the BeamCal. It will provide a high-precision (O(10/sup -4/)) luminosity measurement from Bhabha scattering. Monte Carlo simulations to optimize the shape and the structure of the calorimeter are presented.
ZEUS inclusive diffractive-cross-section measurements have been used in a DGLAP next-to-leading-order QCD analysis to extract the diffractive parton distribution functions. Data on diffractive dijet ...production in deep inelastic scattering have also been included to constrain the gluon density. Predictions based on the extracted parton densities are compared to diffractive charm and dijet photoproduction data.
The dissociation of virtual photons,
γ
⋆
p
→
X
p
, in events with a large rapidity gap between
X and the outgoing proton, as well as in events in which the leading proton was directly measured, has ...been studied with the ZEUS detector at HERA. The data cover photon virtualities
Q
2
>
2
GeV
2
and
γ
⋆
p
centre-of-mass energies
40
<
W
<
240
GeV
, with
M
X
>
2
GeV
, where
M
X
is the mass of the hadronic final state,
X. Leading protons were detected in the ZEUS leading proton spectrometer. The cross section is presented as a function of
t, the squared four-momentum transfer at the proton vertex and
Φ, the azimuthal angle between the positron scattering plane and the proton scattering plane. It is also shown as a function of
Q
2
and
x
P
, the fraction of the proton's momentum carried by the diffractive exchange, as well as
β, the Bjorken variable defined with respect to the diffractive exchange.
The reduced cross sections for ep deep inelastic scattering have been measured with the ZEUS detector at HERA at three different centre-of-mass energies, 318, 251 and 225 GeV. From the cross ...sections, measured double differentially in Bjorken x and the virtuality, Q2, the proton structure functions FL and F2 have been extracted in the region 5×10−4<x<0.007 and 20<Q2<130 GeV2.
The exclusive electroproduction of
J/
ψ mesons,
ep→
epJ/
ψ, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges
0.15<Q
2<0.8
GeV
2
and
2<Q
2<100
GeV
...2
using integrated luminosities of 69 and
83
pb
−1
, respectively. The photon–proton centre-of-mass energy was in the range
30<W<220
GeV
and the squared four-momentum transfer at the proton vertex
|t|<1
GeV
2
. The cross sections and decay angular distributions are presented as functions of
Q
2,
W and
t. The effective parameters of the pomeron trajectory are in agreement with those found in
J/
ψ photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of
s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections,
σ
L
/
σ
T
, grows with
Q
2, whilst no dependence on
W or
t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.
Inclusive-jet cross sections have been measured in the reaction ep→e+jet+X for photon virtuality Q2<1 GeV2 and γp centre-of-mass energies in the region 142<Wγp<293 GeV with the ZEUS detector at HERA ...using an integrated luminosity of 300 pb−1. Jets were identified using the kT, anti-kT or SIScone jet algorithms in the laboratory frame. Single-differential cross sections are presented as functions of the jet transverse energy, ETjet, and pseudorapidity, ηjet, for jets with ETjet>17 GeV and −1<ηjet<2.5. In addition, measurements of double-differential inclusive-jet cross sections are presented as functions of ETjet in different regions of ηjet. Next-to-leading-order QCD calculations give a good description of the measurements, except for jets with low ETjet and high ηjet. The influence of non-perturbative effects not related to hadronisation was studied. Measurements of the ratios of cross sections using different jet algorithms are also presented; the measured ratios are well described by calculations including up to O(αs2) terms. Values of αs(MZ) were extracted from the measurements and the energy-scale dependence of the coupling was determined. The value of αs(MZ) extracted from the measurements based on the kT jet algorithm is αs(MZ)=0.1206−0.0022+0.0023(exp.)−0.0035+0.0042(th.); the results from the anti-kT and SIScone algorithms are compatible with this value and have a similar precision.
The ZEUS inclusive differential cross-section data from HERA, for charged and neutral current processes taken with e + and e- beams, together with differential cross-section data on inclusive jet ...production in e + p scattering and dijet production in \(\gamma p\) scattering, have been used in a new NLO QCD analysis to extract the parton distribution functions of the proton. The input of jet-production data constrains the gluon and allows an accurate extraction of \(\alpha_s(M_Z)\) at NLO; \(\alpha _{s} {\left( {M_{Z} } \right)} = 0.1183 \pm 0.0028{\left( {\exp .} \right)} \pm 0.0008{\left( {{\text{model}}} \right)}.\) An additional uncertainty from the choice of scales is estimated as \(\pm 0.005\). This is the first extraction of \(\alpha_s(M_Z)\) from HERA data alone.
Exclusive electroproduction of
ϕ mesons has been studied in
e
±
p
collisions at
s
=
318
GeV
with the ZEUS detector at HERA using an integrated luminosity of
65.1
pb
−1
. The
γ
∗
p
cross section ...is presented in the kinematic range
2
GeV
2
<
Q
2
<
70
GeV
2
,
35
GeV
<
W
<
145
GeV
and
|
t
|
<
0.6
GeV
2
. The cross sections as functions of
Q
2
,
W,
t and helicity angle
θ
h
are compared to cross sections for other vector mesons. The ratios
R of the cross sections for longitudinally and transversely polarized virtual photons are presented as functions of
Q
2
and
W. The data are also compared to predictions from theoretical models.
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