Jet Physics in CMS Heavy Ion Programme

In this note, we report on the study of jet+ jet, γ + jet and Z(→ μμ) + jet production in heavy ion collisions. In particular, we review the capability of the CMS detector to observe the energy losses of quarkand gluon-initiated jets in a dense QCD matter. 1 Jet quenching: jet + jet, γ + jet and Z + jet production Hard jet production is considered to be an efficient probe for formation of super-dense matter – quark-gluon plasma (QGP) [1] in future experiments on heavy ion collisions at LHC [2, 3]. High pT parton pair (dijet) from a single hard scattering is produced at the initial stage of the collision process (typically, at <∼ 0.01 fm/c). It then propagates through the QGP formed due to mini-jet production at larger time scales (∼ 0.1 fm/c), and interacts strongly with the comoving constituents in the medium. The actual problem is to study the energy losses of a hard partonic jet evolving through the dense matter. We know two possible mechanisms of energy losses: (1) radiative losses due to gluon ”bremsstrahlung” induced by multiple scattering [4, 5, 6, 7] and (2) collisional losses due to the final state interactions (elastic rescatterings) of high pT partons off the medium constituents [8, 9, 10]. It is known that the jet rescattering intensity strongly increases with temperature. Thus formation of a super-dense and hot partonic matter in heavy ion collisions (with initial temperature up to T ∼ 1 GeV) at LHC energies [11] should result in significantly larger jet energy losses as compared with the case of hadronic gas at T <∼ 0.2 GeV. Although the radiative energy losses of a high energy parton have been shown to dominate over the collisional losses by up to an order of magnitude [5], a direct experimental verification of this phenomenon remains an open problem. Indeed, with increasing of hard parton energy the maximum of the angular distribution of bremsstrahlung gluons shifts towards the parent parton direction. This means that measuring the jet energy as a sum of the energies of final hadrons moving inside an angular cone with a given finite size θ0 will allow the bulk of the gluon radiation to belong to the jet and thus the major fraction of the initial parton energy to be reconstructed. Therefore, the medium-induced radiation will, in the first place, soften particle energy distributions inside the jet, increase the multiplicity of secondary particles, but will not affect the total jet energy. It was recently shown [6] that the radiation of energetic gluons in a QCD medium is essentially different from the Bethe-Heitler independent radiation pattern. Such gluons have formation times exceeding the mean free path for QCD parton scattering in the medium. In these circumstances the coherent effects play a crucial role leading to a strong suppression of the medium-induced gluon radiation. This coherent suppression is a QCD analogue of the Landau-PomeranchukMigdal effect in QED. It is important to notice that the coherent LPM radiation induces a strong dependence of the jet energy on the jet cone size θ0 [12]. On the other hand, the collisional energy losses represent an incoherent sum over all rescatterings. It is almost independent of the initial parton energy. At the same time, the angular distribution of the collisional energy loss is essentially different from that of the radiative one. The bulk of ”thermal” particles knocked out of the dense matter by elastic scatterings fly away in almost transverse direction relative to the hard jet axis. As a result, the collisional energy loss turns out to be practically independent on θ0 and emerges outside the narrow jet cone. Thus the relative contribution of collisional losses would likely become significant for jets with finite cone size propagating through the hot plasma under LHC conditions [12]. In the case of boost-invariant expanding quark-gluon fluid, created in central ultrarelativistic AA collision, partons are produced on the hypersurface of equal proper time τ = √ t2 − z2 [13]. The total energy losses ∆Etot (the sum of the radiative and the collisional losses) experienced by a hard parton due to multiple scattering in the dense matter are the result of averaging over dijet production vertices (R, φ), transfer momenta squared Q in a single rescattering, and space-time evolution of the medium: ∆Etot = 1 sin θ 2π ∫