1,758 research outputs found
An NLO QCD analysis of inclusive cross-section and jet-production data from the ZEUS experiment
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 data constrains the gluon and allows an accurate extraction of
\alpha_s(M_Z) at NLO;
\alpha_s(M_Z) = 0.1183 \pm 0.0028(exp.) \pm 0.0008(model)
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.Comment: 37 pages, 14 figures, to be submitted to EPJC. PDFs available at
http://durpdg.dur.ac.uk/hepdata in LHAPDFv
Measurement of charm fragmentation ratios and fractions in photoproduction at HERA
The production of D^*+, D^0, D^+, D_s^+ and Lambda_c^+ charm hadrons and
their antiparticles in ep scattering at HERA was measured with the ZEUS
detector using an integrated luminosity of 79 pb^-1. The measurement has been
performed in the photoproduction regime with the exchanged-photon virtuality
Q^2 < 1 GeV^2 and for photon-proton centre-of-mass energies in the range 130 <
W < 300 GeV. The charm hadrons were reconstructed in the range of transverse
momentum p_T(D, Lambda_c) > 3.8 GeV and pseudorapidity |eta(D, Lambda_c)| <
1.6. The production cross sections were used to determine the ratio of neutral
and charged D-meson production rates, R_u/d, the strangeness-suppression
factor, gamma_s, and the fraction of charged D mesons produced in a vector
state, P_v^d. The measured R_u/d and gamma_s values agree with those obtained
in deep inelastic scattering and in e^+e^- annihilations. The measured P_v^d
value is smaller than, but consistent with, the previous measurements. The
fractions of c quarks hadronising as a particular charm hadron, f(c -> D,
Lambda_c), were derived in the given kinematic range. The measured open-charm
fragmentation fractions are consistent with previous results, although the
measured f(c -> D^*+) is smaller and f(c -> Lambda_c^+) is larger than those
obtained in e^+e^- annihilations. These results generally support the
hypothesis that fragmentation proceeds independently of the hard sub-process.Comment: 29 pages, 5 figures, 6 tables; minor text revision
Inclusive jet cross sections and dijet correlations in photoproduction at HERA
Inclusive jet cross sections in photoproduction for events containing a
meson have been measured with the ZEUS detector at HERA using an integrated
luminosity of . The events were required to have a
virtuality of the incoming photon, , of less than 1 GeV, and a
photon-proton centre-of-mass energy in the range . The measurements are compared with next-to-leading-order (NLO) QCD
calculations. Good agreement is found with the NLO calculations over most of
the measured kinematic region. Requiring a second jet in the event allowed a
more detailed comparison with QCD calculations. The measured dijet cross
sections are also compared to Monte Carlo (MC) models which incorporate
leading-order matrix elements followed by parton showers and hadronisation. The
NLO QCD predictions are in general agreement with the data although differences
have been isolated to regions where contributions from higher orders are
expected to be significant. The MC models give a better description than the
NLO predictions of the shape of the measured cross sections.Comment: 43 pages, 12 figures, charm jets ZEU
Search for a Standard Model Higgs Boson in the H â ZZ â â^+â^-vv Decay Channel with the ATLAS Detector
A search for a heavy standard model Higgs boson decaying via HâZZââ^+â^-ΜΜÌ
, where â=e, ÎŒ, is presented. It is based on proton-proton collision data at âs=7ââTeV, collected by the ATLAS experiment at the LHC in the first half of 2011 and corresponding to an integrated luminosity of 1.04ââfb^(-1). The data are compared to the expected standard model backgrounds. The data and the background expectations are found to be in agreement and upper limits are placed on the Higgs boson production cross section over the entire mass window considered; in particular, the production of a standard model Higgs boson is excluded in the region 340<m_H<450ââGeV at the 95% confidence level
Search for new physics in the dijet mass distribution using 1 fb^(â1) of pp collision data at âs = 7 TeV collected by the ATLAS detector
Invariant mass distributions of jet pairs (dijets) produced in LHC protonâproton collisions at a centre-of-mass energy âs=7TeV have been studied using a data set corresponding to an integrated luminosity of 1.0 fb^(-1) recorded in 2011 by ATLAS. Dijet masses up to âŒ4 TeV are observed in the data, and no evidence of resonance production over background is found. Limits are set at 95% C.L. for several new physics hypotheses: excited quarks are excluded for masses below 299 TeV, axigluons are excluded for masses below 3.32 TeV, and colour octet scalar resonances are excluded for masses below 1.92 TeV
Search for Pair Production of a New b' Quark that Decays into a Z Boson and a Bottom Quark with the ATLAS Detector
A search is reported for the pair production of a new quark bâČ with at least one bâČ decaying to a Z boson and a bottom quark. The data, corresponding to 2.0ââfb^(-1) of integrated luminosity, were collected from pp collisions at âs=7ââTeV with the ATLAS detector at the CERN Large Hadron Collider. Using events with a b-tagged jet and a Z boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum bâČ candidates is tested for an enhancement. No evidence for a bâČ signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of bâČ quarks with masses m_bâČ<400ââGeV that decay entirely via bâČâZ+b. In the case of a vectorlike singlet bâČ mixing solely with the third standard model generation, masses m_bâČ<358ââGeV are excluded
Measurement of the pseudorapidity and transverse momentum dependence of the elliptic flow of charged particles in leadâlead collisions at âsNN = 2.76 TeV with the ATLAS detector
This Letter describes the measurement of â elliptic flow of charged particles in leadâlead collisions at
âsNN = 2.76 TeV using the ATLAS detector at the Large Hadron Collider (LHC). The results are based on
an integrated luminosity of approximately 7 ÎŒb^(â1). Elliptic flow is measured over a wide region in pseudorapidity,
|η| < 2.5, and over a broad range in transverse momentum, 0.5 < p_T < 20 GeV. The elliptic flow
parameter v_2 is obtained by correlating individual tracks with the event plane measured using energy
deposited in the forward calorimeters. As a function of transverse momentum, v_2(p_T) reaches a maximum
at p_T of about 3 GeV, then decreases and becomes weakly dependent on p_T above 7â8 GeV. Over
the measured pseudorapidity region, v_2 is found to be only weakly dependent on η, with less variation
than observed at lower beam energies. The results are discussed in the context of previous measurements
at lower collision energies, as well as recent results from the LHC
Search for first generation scalar leptoquarks in pp collisions at âs = 7 TeV with the ATLAS detector
We report a search for first generation scalar leptoquarks using 1.03 fb^(â1) of protonâproton collisions data produced by the Large Hadron Collider at âs = 7 TeV and recorded by the ATLAS experiment. Leptoquarks are sought via their decay into an electron or neutrino and a quark, producing events with two oppositely charged electrons and at least two jets, or events with an electron, missing transverse momentum and at least two jets. Control data samples are used to validate background predictions from Monte Carlo simulation. In the signal region, the observed event yields are consistent with the background expectations. We exclude at 95% confidence level the production of first generation scalar leptoquark with masses m_(LQ) <660 (607) GeV when assuming the branching fraction of a leptoquark to a charged lepton is equal to 1.0 (0.5)
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