424 research outputs found
Determination of QCD condensates from tau-decay data
We have used the latest data from the ALEPH Collaboration to extract values
for QCD condensates up to dimension d=12 in the V-A channel and up to dimension
d=8 in the V, A and V+A channels. Performing 2- and 3-parameter fits, we obtain
new results for the correlations of condensates. The results are consistent
among themselves and agree with most of the previous results found in the
literature.Comment: 22 page
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Search for Charged Higgs Bosons at LEP
A search for pair-produced charged Higgs bosons is performed with the L3
detector at LEP using data collected at centre-of-mass energies between 189 and
209GeV, corresponding to an integrated luminosity of 629.4/pb. Decays into a
charm and a strange quark or into a tau lepton and its neutrino are considered.
No significant excess is observed and lower limits on the mass of the charged
Higgs boson are derived at the 95% confidence level. They vary from 76.5 to
82.7GeV, as a function of the H->tv branching ratio
Evidence for an Excess of Soft Photons in Hadronic Decays of Z^0
Soft photons inside hadronic jets converted in front of the DELPHI main
tracker (TPC) in events of qqbar disintegrations of the Z^0 were studied in the
kinematic range 0.2 < E_gamma < 1 GeV and transverse momentum with respect to
the closest jet direction p_T < 80 MeV/c. A clear excess of photons in the
experimental data as compared to the Monte Carlo predictions is observed. This
excess (uncorrected for the photon detection efficiency) is (1.17 +/- 0.06 +/-
0.27) x 10^{-3} gamma/jet in the specified kinematic region, while the expected
level of the inner hadronic bremsstrahlung (which is not included in the Monte
Carlo) is (0.340 +/- 0.001 +/- 0.038) x 10^{-3} gamma/jet. The ratio of the
excess to the predicted bremsstrahlung rate is then (3.4 +/- 0.2 +/- 0.8),
which is similar in strength to the anomalous soft photon signal observed in
fixed target experiments with hadronic beams.Comment: 37 pages, 9 figures, Accepted by Eur. Phys. J.
Study of Inclusive J/psi Production in Two-Photon Collisions at LEP II with the DELPHI Detector
Inclusive J/psi production in photon-photon collisions has been observed at
LEP II beam energies. A clear signal from the reaction gamma gamma -> J/psi+X
is seen. The number of observed N(J/psi -> mu+mu-) events is 36 +/- 7 for an
integrated luminosity of 617 pb^{-1}, yielding a cross-section of
sigma(J/psi+X) = 45 +/- 9 (stat) +/- 17 (syst) pb. Based on a study of the
event shapes of different types of gamma gamma processes in the PYTHIA program,
we conclude that (74 +/- 22)% of the observed J/psi events are due to
`resolved' photons, the dominant contribution of which is most probably due to
the gluon content of the photon.Comment: 13 pages, 8 figures, Accepted by Phys. Lett.
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
Study of Tau-pair Production in Photon-Photon Collisions at LEP and Limits on the Anomalous Electromagnetic Moments of the Tau Lepton
Tau-pair production in the process e+e- -> e+e-tau+tau- was studied using
data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000.
The corresponding integrated luminosity is 650 pb^{-1}. The values of the
cross-section obtained are found to be in agreement with QED predictions.
Limits on the anomalous magnetic and electric dipole moments of the tau lepton
are deduced.Comment: 20 pages, 9 figures, Accepted by Eur. Phys. J.
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Search for Doubly-Charged Higgs Bosons at LEP
Doubly-charged Higgs bosons are searched for in e^+e^- collision data
collected with the L3 detector at LEP at centre-of-mass energies up to 209 GeV.
Final states with four leptons are analysed to tag the pair-production of
doubly charged Higgs bosons. No significant excess is found and lower limits at
95% confidence level on the doubly-charged Higgs boson mass are derived. They
vary from 95.5 GeV to 100.2 GeV, depending on the decay mode. Doubly-charged
Higgs bosons which couple to electrons would modify the cross section and
forward-backward asymmetry of the e^+e^- -> e^+e^- process. The measurements of
these quantities do not deviate from the Standard Model expectations and
doubly-charged Higgs bosons with masses up to the order of a TeV are excluded
Flavour Independent Search for Neutral Higgs Bosons at LEP
A flavour independent search for the CP-even and CP-odd neutral Higgs bosons
h and A is performed in 624/pb of data collected with the L3 detector at LEP at
centre-of-mass energies between 189 and 209GeV. Higgs boson production through
the e^+e^- -> Z h and the e^+e^- ->h A processes is considered and decays of
the Higgs bosons into hadrons are studied. No significant signal is observed
and 95% confidence level limits on the hZZ and hAZ couplings are derived as a
function of the Higgs boson masses. Assuming the Standard Model cross section
for the Higgs-strahlung process and a 100% branching fraction into hadrons, a
95% confidence level lower limit on the mass of the Higgs boson is set at
110.3GeV
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