Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel pK0S→pπ+π−. A peak is observed in the pK0S invariant mass spectrum at ...1528±2.6(stat)±2.1(syst) MeV. Depending on the background model, the naive statistical significance of the peak is 4–6 standard deviations and its width may be somewhat larger than the experimental resolution of σ=4.3–6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Θ+(uudds̄) pentaquark baryon. No signal for an hypothetical Θ++ baryon was observed in the pK+ invariant mass distribution. The absence of such a signal indicates that an isotensor Θ is excluded and an isovector Θ is unlikely.
Inclusive-jet and dijet differential cross sections have been measured in neutral current deep inelastic
ep scattering for exchanged boson virtualities
Q
2
>
125
GeV
2
with the ZEUS detector at ...HERA using an integrated luminosity of 82 pb
−1. Jets were identified in the Breit frame using the
k
T
cluster algorithm. Jet cross sections are presented as functions of several kinematic and jet variables. The results are also presented in different regions of
Q
2
. Next-to-leading-order QCD calculations describe the measurements well. Regions of phase space where the theoretical uncertainties are small have been identified. Measurements in these regions have the potential to constrain the gluon density in the proton when used as inputs to global fits of the proton parton distribution functions.
Single-spin asymmetries have been measured for semi-inclusive electroproduction of π+, π−, π0 and K+ mesons in deep-inelastic scattering off a longitudinally polarised deuterium target. The ...asymmetries appear in the distribution of the hadrons in the azimuthal angle φ around the virtual photon direction, relative to the lepton scattering plane. The corresponding analysing powers in the sinφ moment of the cross section are 0.012±0.002(stat.)±0.002(syst.) for π+, 0.006±0.003(stat.)±0.002(syst.) for π−, 0.021±0.005(stat.)±0.003(syst.) for π0 and 0.013±0.006(stat.)±0.003(syst.) for K+. The sin2φ moments are compatible with zero for all particles.
Double-spin asymmetries of semiinclusive cross sections for the production of identified pions and kaons have been measured in deep inelastic scattering of polarized positrons on a polarized ...deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with reanalyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.
Differential inclusive-jet cross sections have been measured for different jet radii in neutral current deep inelastic ep scattering for boson virtualities Q2>125 GeV2 with the ZEUS detector at HERA ...using an integrated luminosity of 81.7 pb−1. Jets were identified in the Breit frame using the kT cluster algorithm in the longitudinally inclusive mode for different values of the jet radius R. Differential cross sections are presented as functions of Q2 and the jet transverse energy, ET,Bjet. The dependence on R of the inclusive-jet cross section has been measured for Q2>125 and 500 GeV2 and found to be linear with R in the range studied. Next-to-leading-order QCD calculations give a good description of the measurements for 0.5⩽R⩽1. A value of αs(MZ) has been extracted from the measurements of the inclusive-jet cross section dσ/dQ2 with R=1 for Q2>500 GeV2: αs(MZ)=0.1207±0.0014(stat.)−0.0033+0.0035(exp.)−0.0023+0.0022(th.). The variation of αs with ET,Bjet is in a good agreement with the running of αs as predicted by QCD.
Mean values and differential distributions of event-shape variables have been studied in neutral current deep inelastic scattering using an integrated luminosity of 82.2 pb
−1 collected with the ZEUS ...detector at HERA. The kinematic range is
80
<
Q
2
<
20
480
GeV
2
and
0.0024
<
x
<
0.6
, where
Q
2
is the virtuality of the exchanged boson and
x is the Bjorken variable. The data are compared with a model based on a combination of next-to-leading-order QCD calculations with next-to-leading-logarithm corrections and the Dokshitzer–Webber non-perturbative power corrections. The power-correction method provides a reasonable description of the data for all event-shape variables studied. Nevertheless, the lack of consistency of the determination of
α
s
and of the non-perturbative parameter of the model,
α
0
¯
, suggests the importance of higher-order processes that are not yet included in the model.
The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. ...The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy \(\nu\) and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio \(R_M^{h}\) at low \(\nu\) and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the \(\nu\)-dependence of \(R_M^{h}\) is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the \(\nu\)- and z-dependence of \(R_M^{h}\).
A new measurement of the proton structure function
F2 (
x,
Q
2) is reported for momentum transfers squared
Q
2 between .5 GeV
2 and 5000 GeV
2 and for Bjorken
x between 3 · 10
−5 and 0.32 using data ...collected by the HERA experiment H1 in 1994. The data represent an increase in statistics by a factor of ten with respect to the analysis of the 1993 data. Substantial extension of the kinematic range towards low
Q
2 and
x has been achieved using dedicated data samples and events with initial state photon radiation. The structure function is found to increase significantly with decreasing x, even in the lowest accessible
Q
2 region. The data are well described by a Next to Leading Order QCD fit and the gluon density is extracted.
Jet substructure and differential cross sections for jets produced in the photoproduction and deep inelastic
ep scattering regimes have been measured with the ZEUS detector at HERA using an ...integrated luminosity of
82.2
pb
−
1
. The substructure of jets has been studied in terms of the jet shape and subjet multiplicity for jets with transverse energies
E
T
jet
>
17
GeV
. The data are well described by the QCD calculations. The jet shape and subjet multiplicity are used to tag gluon- and quark-initiated jets. Jet cross sections as functions of
E
T
jet
, jet pseudorapidity, the jet–jet scattering angle, dijet invariant mass and the fraction of the photon energy carried by the dijet system are presented for gluon- and quark-tagged jets. The data exhibit the behaviour expected from the underlying parton dynamics. A value of
α
s
(
M
Z
)
of
α
s
(
M
Z
)
=
0.1176
±
0.0009
(
stat.
)
−
0.0026
+
0.0009
(
exp.
)
−
0.0072
+
0.0091
(
th.
)
was extracted from the measurements of jet shapes in deep inelastic scattering.
PDF
