13868349
Dilepton production as a measure of QGP thermalization time
We discuss dilepton production from a quark–gluon plasma which has a time-dependent anisotropy in momentum space. A phenomenological model for the hard momentum scale, phard(τ) and the plasma anisotropy parameter, ξ(τ), is presented. The model interpolates between early-time 1 + 1 free streaming behavior (τ τiso) and late-time ideal 1 + 1 hydrodynamical behavior (τ τiso). Using this model, we find that the dilepton rate arising from electromagnetic annihilations of quarks in the kinematic range 3 <pT< 8 GeV is sensitive to the assumed isotropization time of the system, τiso. Therefore high-energy dilepton production can be used to probe the degree of momentum-space isotropy of a quark–gluon plasma produced in relativistic heavy-ion collisions and the time of onset of hydrodynamic expansion of the QGP.
117317073
Anisotropic hydrodynamics - basic concepts
Due to the rapid longitudinal expansion of the quark-gluon plasma created in relativistic heavy ion collisions, potentially large local rest frame momentum-space anisotropies are generated. The magnitude of these momentum-space anisotropies can be so large as to violate the central assumption of canonical viscous hydrodynamical treatments which linearize around an isotropic background. In order to better describe the early-time dynamics of the quark gluon plasma, one can consider instead expanding around a locally anisotropic background which results in a dynamical framework called anisotropic hydrodynamics. In this proceedings contribution we review the basic concepts of the anisotropic hydrodynamics framework presenting viewpoints from both the phenomenological and microscopic points of view.
118399487
Generalization of Gunion-Bertsch formula for soft gluon emission
We generalize the most extensively used Gunion-Bertsch formula for the soft gluon emission in a perturbative QCD. We show that the corrections arising due to this generalization could be very important for the phenomenology of the hot and dense matter produced in the heavy-ion collisions.
119256392
Constraining the onset of viscous hydrodynamics
Abstract We derive two general criteria that can be used to constrain the initial time of the onset of 2nd-order conformal viscous hydrodynamics in relativistic heavy-ion collisions. We show this explicitly for 0+1 dimensional viscous hydrodynamics and discuss how to extend the constraint to higher dimensions.
118662616
Constraining relativistic viscous hydrodynamical evolution
We show that by requiring positivity of the longitudinal pressure it is possible to constrain the initial conditions one can use in second-order viscous hydrodynamical simulations of ultrarelativistic heavy-ion collisions. We demonstrate this explicitly for (0+1)-dimensional viscous hydrodynamics and discuss how the constraint extends to higher dimensions. Additionally, we present an analytic approximation to the solution of (0+1)-dimensional second-order viscous hydrodynamical evolution equations appropriate to describe the evolution of matter in an ultrarelativistic heavy-ion collision.
115544761
High-energy dileptons from an anisotropic quark-gluon plasma
We calculate leading-order dilepton yields from a quark-gluon plasma which has a time-dependent anisotropy in momentum space. Such anisotropies can arise during the earliest stages of quark-gluon plasma evolution due to the rapid longitudinal expansion of the created matter. A phenomenological model for the proper time dependence of the parton hard momentum scale, p_hard, and the plasma anisotropy parameter, xi, is proposed. The model describes the transition of the plasma from a 0+1 dimensional collisionally-broadened expansion at early times to a 0+1 dimensional ideal hydrodynamic expansion at late times. We find that high-energy dilepton production is enhanced by pre-equilibrium emission up to 50% at LHC energies, if one assumes an isotropization/thermalization time of 2 fm/c. Given sufficiently precise experimental data this enhancement could be used to determine the plasma isotropization time experimentally.
123410491
Z plus jets production at the tevatron
We present preliminary results on inclusive Z(→ee) boson plus jets production in pp collisions at s√ = 1.96 TeV, based on 1.7 fb-1 of data collected by the CDF experiment. Inclusive jet measurements are performed as function of pTjet and jet multiplicity for jets in the region of pTjet > 30 GeV/c and |yjet | < 2.1. Results are compared to NLO pQCD predictions and are found to be in good agreement.
17360856
Jet Production at CDF
The Run 2 at Tevatron will define a new level of precision for QCD studies in hadron collisions. Both collider experiments, CDF and D0, expect to collect up to 15 fb{sup -1} of data in this new run period. The increase in instantaneous luminosity, center-of-mass energy (from 1.8 TeV to 2 TeV) and the improved acceptance of the detectors will allow stringent tests of the Standard Model (SM) predictions in extended regions of jet transverse energy, E{sub T}{sup jet}, and jet pseudorapidity, {eta}{sup jet}. In the following, a review of some of the most important QCD results from Run 1 is presented, together with first preliminary Run 2 measurements (based on the very first data collected by the experiment) and future prospects as the integrated luminosity increases.
121540832
A very forward luminosity monitor for the ALEPH detector at LEP
Abstract The design of a very small angle luminosity monitor for the ALEPH detector at LEP is described. The detector consists of four calorimeter modules made of alternating layers of tungsten and scintillators plus one layer of silicon strips for shower localization. Tests of the performance of the calorimeter and initial results obtained in LEP are also described.
119515666
Study of jet properties at the Tevatron
The Run II at the Tevatron will define a new level of precision for QCD studies in hadron collisions. Both collider experiments, CDF and D0, expect to collect up to 8 fb{sup -1} of data in this new run period. The increase in instantaneous luminosity, center-of-mass energy (from 1.8 TeV to 2 TeV) and the improved acceptance of the detectors will allow stringent tests of the Standard Model (SM) predictions in extended regions of jet transverse momentum, P{sub T}{sup jet}, and jet rapidity, Y{sup jet}. The hadronic final states in hadron-hadron collisions are characterized by the presence of soft contributions (the so-called underlying event) from initial-state gluon radiation and multiple parton interactions between remnants, in addition to the jets of hadrons originated by the hard interaction. A proper comparison with pQCD predictions at the parton level requires an adequate modeling of these soft contributions which become important at low P{sub T}{sup jet}. In this letter, a review of some of the most important QCD results from Run II is presented.
123275219
Measurement of the Inclusive Z+Jet(s) Production Cross Section at CDF
We present preliminary results on inclusive Z(→ ee) boson plus jets production in p p ¯ collisions at s = 1.96 TeV , based on 1 fb −1 of data collected by the CDF experiment between February 2002 and March 2006. Inclusive jet measurements are performed as a function of p T jet and jet multiplicity for jets in the region p T jet > 30 GeV/c and | y jet | 2.1 . Results are compared to NLO pQCD predictions.