Abstract
In a series of previous papers, we have presented a new approach, based on perturbative QCD, for the evolution of a jet in a dense quark-gluon plasma. In the original formulation, the plasma was assumed to be homogeneous and static. In this work, we extend our description and its Monte Carlo implementation to a plasma obeying Bjorken longitudinal expansion. Our key observation is that the factorisation between vacuum-like and medium-induced emissions, derived in the static case, still holds for an expanding medium, albeit with modified rates for medium-induced emissions and transverse momentum broadening, and with a modified phase-space for vacuum-like emissions. We highlight a scaling relation valid for the energy spectrum of medium-induced emissions, through which the case of an expanding medium is mapped onto an effective static medium. We find that scaling violations due to vacuum-like emissions and transverse momentum broadening are numerically small. Our new predictions for the nuclear modification factor for jets RAA, the in-medium fragmentation functions, and substructure distributions are very similar to our previous estimates for a static medium, maintaining the overall good qualitative agreement with existing LHC measurements. In the case of RAA, we find that the agreement with the data is significantly improved at large transverse momenta pT ≳ 500 GeV after including the effects of the nuclear parton distribution functions.
Highlights
These data, the experimental efforts must be accompanied by theoretical progress, aiming at understanding the jet-medium interactions from first principles
The physical picture underlying our approach is anchored in a remarkable property emerging from perturbative QCD: the parton cascades are factorised between vacuum-like emissions (VLEs), which are triggered by the virtuality of the initial parton and occur at early times, and mediuminduced emissions (MIEs), which are triggered by collisions with the plasma constituents and can occur anywhere inside the medium
We have demonstrated that the factorisation in time between vacuum-like emissions and medium-induced emissions remains valid in the expanding plasma and established the new phase space for in-medium VLEs to leading logarithmic accuracy
Summary
We shall describe the generalisation of our physical picture in refs. [1,2,3] to the case of a plasma which obeys longitudinal expansion. [1,2,3] to the case of a plasma which obeys longitudinal expansion. We shall describe the generalisation of our physical picture in refs. The consequences of this expansion for the transverse momentum broadening and for the medium-induced radiation have been explored at length in the literature In what follows we shall build upon such previous studies to incorporate the medium expansion in our unified description for the in-medium parton showers, including both vacuum-like and medium-induced emissions
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