Search results for "perturbative QCD"
showing 10 items of 174 documents
Study of jets produced in association with aWboson inppcollisions ats=7 TeVwith the ATLAS detector
2012
We report a study of final states containing a W boson and hadronic jets, produced in proton-proton collisions at a center-of-mass energy of 7 TeV. The data were collected with the ATLAS detector at the CERN LHC and comprise the full 2010 data sample of 36 pb^-1. Cross sections are determined using both the electron and muon decay modes of the W boson and are presented as a function of inclusive jet multiplicity, N_jet, for up to five jets. At each multiplicity, cross sections are presented as a function of jet transverse momentum, the scalar sum of the transverse momenta of the charged lepton, missing transverse momentum, and all jets, the invariant mass spectra of jets, and the rapidity d…
Bremsstrahlung and the underlying event structure
1990
Abstract In this talk we address the problem of the structure of the underlying event and its dependence upon a specific trigger. Generally speaking, events selected without any special trigger, like the minimum bias events, need not have the same characteristics as those selected by the jet trigger or through the W and Z-boson selection criteria. We use perturbative QCD in order to unfold part of the structure. In particular we concentrate on the contribution to the underlying event structure which comes from initial state bremsstrahlung. In the present paper we discuss the relationship between transverse momentum and transverse energy for W and Z-boson production both in the low as well a…
LHC data challenges the contemporary parton-to-hadron fragmentation functions
2014
We discuss the inclusive high-pT charged-particle production in proton-proton collisions at the LHC. The experimental data are compared to the NLO perturbative QCD calculations employing various sets of parton-to-hadron fragmentation functions. Most of the theoretical predictions are found to disastrously overpredict the measured cross sections, even if the scale variations and PDF errors are accounted for. The problem appears to arise from the presently too hard gluon-to-hadron fragmentation functions.
Pre-thermalization dynamics: initial conditions for QGP at the LHC and RHIC from perturbative QCD
1997
I discuss how the initial conditions for QGP-production in ultrarelativistic heavy ion collisions at the LHC and RHIC can be computed from perturbative QCD.
Production of Muons from Heavy Flavor Decays at Forward Rapidity inppand Pb-Pb Collisions atsNN=2.76 TeV
2012
The ALICE Collaboration has measured the inclusive production of muons from heavy-flavor decays at forward rapidity, 2.5 < y < 4, in pp and Pb-Pb collisions at root s(NN) = 2.76 TeV. The p(t)-differential inclusive cross section of muons from heavy-flavor decays in pp collisions is compared to perturbative QCD calculations. The nuclear modification factor is studied as a function of p(t) and collision centrality. A weak suppression is measured in peripheral collisions. In the most central collisions, a suppression of a factor of about 3-4 is observed in 6 < p(t) < 10 GeV/c. The suppression shows no significant p(t) dependence.
Rapidity dependence of particle production in ultrarelativistic nuclear collisions
2002
We compute the rapidity dependence of particle and transverse energy production in ultrarelativistic heavy ion collisions at various beam energies and atomic numbers using the perturbative QCD + saturation model. The distribution is a broad gaussian near $y=0$ but the rapid increase of particle production with the beam energy will via energy conservation strongly constrain the rapidity distribution at large $y$.
BLM scale for the pion transition form factor
2001
The NLO Brodsky-Lepage-Mackenzie (BLM) scale for the pion transition form factor has been determined. To achieve that, a consistent calculation up to nf-proportional NNLO contributions to both the hard-scattering amplitude and the perturbatively calculable part of the pion distribution amplitude has been performed. By combining and matching the results obtained for these two amplitudes, a proper cancellation of collinear singularities has been established and the gamma5 ambiguity problem (related to the use of the dimensional regularization method) has been resolved by using the naive-gamma5 as well as the 't Hooft-Veltman (HV) schemes. It has been demonstrated that the prediction for the p…
Hydrodynamics of nuclear collisions with initial conditions from perturbative QCD
1997
We compute the longitudinal hydrodynamic flow in ultrarelativistic heavy ion collisions at $\sqrt{s}$ = 5500 GeV by using boost non-invariant initial conditions following from perturbative QCD. The transfer of entropy and energy from the central region to larger rapidities caused by boost non-invariance is determined and the associated decrease in the lifetime of the system is estimated.
Perturbative QCD study of Bs decays to a pseudoscalar meson and a tensor meson
2014
We study two-body hadronic B-s -> PT decays, with P(T) being a light pseudoscalar (tensor) meson, in the perturbative QCD approach. The CP-averaged branching ratios and the direct CP asymmetries of the Delta S = 0 modes are predicted, where Delta S is the difference between the strange numbers of final and initial states. We also define and calculate experimental observables for the Delta S = 1 modes under the B-s(0)-(B) over bar (0)(s) mixing, including CP averaged branching ratios, time-integrated CP asymmetries, and the CP observables C-f, D-f and S-f. Results are compared to the B-s -> PV ones in the literature, and to the B -> PT ones, which indicate considerable U-spin symmetry breaki…
Azimuthal Anisotropy ofπ0Production inAu+AuCollisions atsNN=200 GeV: Path-Length Dependence of Jet Quenching and the Role of Initial Geometry
2010
We have measured the azimuthal anisotropy of pi(0) production for 1 < p(T) < 18 GeV/c for Au + Au collisions at root s(NN) = 200 GeV. The observed anisotropy shows a gradual decrease for 3 less than or similar to p(T) less than or similar to 7-10 GeV/c, but remains positive beyond 10 GeV/c. The magnitude of this anisotropy is underpredicted, up to at least similar to 10 GeV/c, by current perturbative QCD (PQCD) energy-loss model calculations. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and fluctuations is insufficient to account for this discrepancy. Calculations that implement a path-length dependence steeper than wh…