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.
The high-precision HERA data allows searches up to TeV scales for beyond the Standard Model contributions to electron–quark scattering. Combined measurements of the inclusive deep inelastic cross ...sections in neutral and charged current ep scattering corresponding to a luminosity of around 1 fb−1 have been used in this analysis. A new approach to the beyond the Standard Model analysis of the inclusive ep data is presented; simultaneous fits of parton distribution functions together with contributions of “new physics” processes were performed. Results are presented considering a finite radius of quarks within the quark form-factor model. The resulting 95% C.L. upper limit on the effective quark radius is 0.43⋅10−16 cm.
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
The production of beauty and charm quarks in
ep
interactions has been studied with the ZEUS detector at HERA for exchanged four-momentum squared 5
< Q
2
<
1000 GeV
2
using an integrated ...luminosity of 354 pb
−1
. The beauty and charm content in events with at least one jet have been extracted using the invariant mass of charged tracks associated with secondary vertices and the decay-length significance of these vertices. Differential cross sections as a function of
Q
2
, Bjorken
x
, jet trans- verse energy and pseudorapidity were measured and compared with next-to-leading-order QCD calculations. The beauty and charm contributions to the proton structure functions were extracted from the double-differential cross section as a function of
x
and
Q
2
. The running beauty-quark mass,
m
b
at the scale
m
b
, was determined from a QCD fit at next-to-leading order to HERA data for the first time and found to be
m
b
(
m
b
) = 4.07 ± 0.14 (fit)
− 0.07
+ 0.01
(mod.)
− 0.00
+ 0.05
(param.)
− 0.05
+ 0.08
(theo.) GeV.
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.
A simultaneous fit of parton distribution functions (PDFs) and electroweak parameters to HERA data on deep inelastic scattering is presented. The input data are the neutral current and charged ...current inclusive cross sections which were previously used in the QCD analysis leading to the HERAPDF2.0 PDFs. In addition, the polarization of the electron beam was taken into account for the ZEUS data recorded between 2004 and 2007. Results on the vector and axial-vector couplings of the Z boson to u- and d-type quarks, on the value of the electroweak mixing angle and the mass of the W boson are presented. The values obtained for the electroweak parameters are in agreement with Standard Model predictions.
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.
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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