Evolution of mechanism of parton energy loss with transverse momentum at the RHIC and LHC in relativistic collision of heavy nuclei

Abstract

We analyse the suppression of particle production at large transverse momenta in (0–5% most) central collisions of gold nuclei at = 200 GeV and lead nuclei at = 2.76 TeV. Full next-to-leading-order radiative corrections at , and nuclear effects such as shadowing and parton energy loss are included. The parton energy loss is implemented in a simple multiple scattering model, where the partons lose an energy ϵ = λ × dE/dx per collision, where λ is their mean-free path. We take ϵ = κE for a treatment which is suggestive of the Bethe–Heitler (BH) mechanism of incoherent scatterings, for the LPM mechanism, and ϵ = constant for a mechanism which suggests that the rate of energy loss (dE/dx) of the partons is proportional to the total path length (L) of the parton in the plasma, as the formation time of the radiated gluon becomes much larger than L. We find that while the BH mechanism describes the nuclear modification factor RAA for pT ⩽ 5 GeV/c (especially at RHIC energy), the LPM and more so the constant dE/dx mechanism provides a good description at larger pT. This confirms the earlier expectation that the energy loss mechanism for partons changes from BH to LPM for pT ⩾ λ⟨k2T⟩, where λ ≈ 1 fm and ⟨k2T⟩ ≈ 1 GeV2 is the average transverse kick squared received by the parton per collision. The energy loss per collision at = 2.76 TeV is found to be about twice of that at 0.2 TeV.