Jet Physics at the LHC

Abstract

The production and propagation of high pT probes can explore the mechanisms of parton energy loss, deconfinement in the medium, and shed light on the relevant phys- ical mechanisms and the microscopic properties of the medium formed in the heavy-ion collisions. In particular, partonic energy loss results in significant modification of spectra. The LHC opened a new era in heavy-ion physics bringing, among several, the hard probes to unreached region of the phase-space. Some of the physics results are discussed. High energy partons interact with the medium and loose energy, primarily through induced gluon radiation and, to a smaller extent, elastic scattering (1). The theory of parton energy loss in hot matter has come a long way from the jet quenching predictions by Bjorken and others (2) describing radiative energy loss by a fast parton. Experimentally, several methods are used to address such questions, generally through comparison of the relative production of single particles suppression at RHIC to fully reconstructed jets at the LHC in nuclear collisions to expectations from a superposition of independent nucleon-nucleon collisions. The thermal scale, set by the medium temperature, is very important for the parton energy loss. More specifically, the coupling between partons and medium at large scales, of the order of the initial parton energy, may be weak, governed by perturbative dynamics, and factorize from the medium. At lower scales of the order of 1 GeV, the coupling between the and the medium may be strong. Jet reconstruction and measurements of the properties aim to quantify the in-medium energy loss and capture the dynamics of quenching. At low particle momenta, the underlying event, even if considered as a background contribution to the hard probes because uncorrelated to the hard parton scattering, is an important element of the hadronic environment consisting of complex contributions, spanning over nonperturbative and perturbative QCD and including sensitivities to multiscale and low x physics. Jet reconstruction in heavy-ion collisions proceed against this large background, resulting very challenging. Dedicated algorithms and background subtraction techniques have been developed and optimized to reconstruct the collimated spray of particles associated with the original parton and contained in the cone radius fixed by the specific algorithm (4, 5). At the LHC, fully reconstructed jets are available over a wide pT range at √ sNN = 2.76 TeV confirming and extending the suppression pattern already observed for charged particles both at these energies and at lower collision energies (i.e. RHIC). However, although the original parton energy is better reconstructed in a than by tagging only a fast hadron, the single inclusive suppression is similar to that of single hadrons. This can be understood if parton energy loss is predominantly through radiation outside the cone radius used in the reconstruction algorithm.