3,665 research outputs found
The ATLAS liquid argon hadronic end-cap calorimeter: construction and selected beam test results
ATLAS has chosen for its Hadronic End-Cap Calorimeter (HEC) the copper-liquid
argon sampling technique with flat plate geometry and GaAs pre-amplifiers in
the argon. The contruction of the calorimeter is now approaching completion.
Results of production quality checks are reported and their anticipated impact
on calorimeter performance discussed. Selected results, such as linearity,
electron and pion energy resolution, uniformity of energy response, obtained in
beam tests both of the Hadronic End-Cap Calorimeter by itself, and in the ATLAS
configuration where the HEC is in combination with the Electromagnetic End-Cap
Calorimeter (EMEC) are described.Comment: 4 pages, 2 figures,IPRD04 conferenc
Measurement of parton shower observables with OPAL
A study of QCD coherence is presented based on a sample of about 397,000 e+e- hadronic annihilation events collected at âs = 91 GeV with the OPAL detector at LEP. The study is based on four recently proposed observables that are sensitive to coherence effects in the perturbative regime. The measurement of these observables is presented, along with a comparison with the predictions of different parton shower models. The models include both conventional parton shower models and dipole antenna models. Different ordering variables are used to investigate their influence on the predictions
Diquark condensation at strong coupling
The possibility of diquark condensation at sufficiently large baryon chemical
potential and zero temperature is analyzed in QCD at strong coupling. In
agreement with other strong coupling analysis, it is found that a first order
phase transition separates a low density phase with chiral symmetry
spontaneously broken from a high density phase where chiral symmetry is
restored. In none of the phases diquark condensation takes place as an
equilibrium state, but, for any value of the chemical potential, there is a
metastable state characterized by a non-vanishing diquark condensate. The
energy difference between this metastable state and the equilibrium state
decreases with the chemical potential and is minimum in the high density phase.
The results indicate that there is attraction in the quark-quark sector also at
strong coupling, and that the attraction is more effective at high baryon
density, but for infinite coupling it is not enough to produce diquark
condensation. It is argued that the absence of diquark condensation is not a
peculiarity of the strong coupling limit, but persists at sufficiently large
finite couplings.Comment: 10 pages, 2 figures. An important discussion concerning the extension
of the results to finite couplings adde
Inclusive production of charged hadrons in photonâphoton collisions
The inclusive production of charged hadrons in the collisions of quasi-real photons (e+eââe+eâ+Xe+eââe+eâ+X) has been measured using the OPAL detector at LEP. The data were taken at e+eâe+eâ centre-of-mass energies from 183 to 209 GeV209 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant αsαs. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed
Measurement of the Cross-Section for the Process Gamma-Gamma to Proton-Antiproton at sqrt(s_ee) = 183 - 189 GeV with the OPAL Detector at LEP
The exclusive production of proton-antiproton pairs in the collisions of two
quasi-real photons has been studied using data taken at sqrt(s_ee) = 183 GeV
and 189 GeV with the OPAL detector at LEP. Results are presented for
proton-antiproton invariant masses, W, in the range 2.15 < W < 3.95 GeV. The
cross-section measurements are compared with previous data and with recent
analytic calculations based on the quark-diquark model.Comment: 6 pages, 8 figures, Photon 2003 conference proceeding
Local Hadronic Calibration
The scheme of the hadronic calibration is discussed. Based on the cluster reconstruction an effective noise suppression is achieved. In a first step clusters are classified as electromagnetic or hadronic clusters. The weighting scheme to correct for the different e/pion response in the ATLAS calorimeter is presented. Dead material corrections and out of cluster corrections yield finally a signal which is rather close to the energy deposited by the final state particles in the ATLAS calorimeter. The constants and algorithms are derived from single pion MC studies and tested with jets. The validation of the scheme using testbeam data is presented as well
Colour reconnection in e+e- -> W+W- at sqrt(s) = 189 - 209 GeV
The effects of the final state interaction phenomenon known as colour
reconnection are investigated at centre-of-mass energies in the range sqrt(s) ~
189-209 GeV using the OPAL detector at LEP. Colour reconnection is expected to
affect observables based on charged particles in hadronic decays of W+W-.
Measurements of inclusive charged particle multiplicities, and of their angular
distribution with respect to the four jet axes of the events, are used to test
models of colour reconnection. The data are found to exclude extreme scenarios
of the Sjostrand-Khoze Type I (SK-I) model and are compatible with other
models, both with and without colour reconnection effects. In the context of
the SK-I model, the best agreement with data is obtained for a reconnection
probability of 37%. Assuming no colour reconnection, the charged particle
multiplicity in hadronically decaying W bosons is measured to be (nqqch) =
19.38+-0.05(stat.)+-0.08 (syst.).Comment: 30 pages, 9 figures, Submitted to Euro. Phys. J.
Determination of alpha_s using Jet Rates at LEP with the OPAL detector
Hadronic events produced in e+e- collisions by the LEP collider and recorded
by the OPAL detector were used to form distributions based on the number of
reconstructed jets. The data were collected between 1995 and 2000 and
correspond to energies of 91 GeV, 130-136 GeV and 161-209 GeV. The jet rates
were determined using four different jet-finding algorithms (Cone, JADE, Durham
and Cambridge). The differential two-jet rate and the average jet rate with the
Durham and Cambridge algorithms were used to measure alpha(s) in the LEP energy
range by fitting an expression in which order alpah_2s calculations were
matched to a NLLA prediction and fitted to the data. Combining the measurements
at different centre-of-mass energies, the value of alpha_s (Mz) was determined
to be
alpha(s)(Mz)=0.1177+-0.0006(stat.)+-0.0012$(expt.)+-0.0010(had.)+-0.0032(theo.)
\.Comment: 40 pages, 17 figures, Submitted to Euro. Phys. J.
Scaling violations of quark and gluon jet fragmentation functions in e+e- annihilations at sqrt(s) = 91.2 and 183-209 GeV
Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and
flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are
measured in e+e- annihilations from data collected at centre-of-mass energies
of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are
defined by hemispheres of inclusive hadronic events, while the biased jet
measurements are based on three-jet events selected with jet algorithms.
Several methods are employed to extract the fragmentation functions over a wide
range of scales. Possible biases are studied in the results are obtained. The
fragmentation functions are compared to results from lower energy e+e-
experiments and with earlier LEP measurements and are found to be consistent.
Scaling violations are observed and are found to be stronger for the
fragmentation functions of gluon jets than for those of quarks. The measured
fragmentation functions are compared to three recent theoretical
next-to-leading order calculations and to the predictions of three Monte Carlo
event generators. While the Monte Carlo models are in good agreement with the
data, the theoretical predictions fail to describe the full set of results, in
particular the b and gluon jet measurements.Comment: 46 pages, 17 figures, Submitted to Eur. Phys J.
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