Revisiting scaling properties of medium-induced gluon radiation

TL;DR

The paper shows that medium-induced radiation in hard processes cannot always be equated with parton energy loss due to interference between initial- and final-state radiation. By using a QED toy model and applying it to large-$x_{F}$ quarkonium production, it demonstrates that the induced radiated energy can scale with the incoming energy and behave like Bethe–Heitler radiation of an asymptotic massive color charge, rather than the conventional parton energy loss. This framework predicts strong, energy-scaling nuclear suppression in large-$x_{F}$ quarkonium and potentially open heavy flavour, while DY remains largely insensitive to parton energy. The results suggest a unified mechanism for various nuclear suppression phenomena and highlight observable-dependent transitions across rapidity regions.

Abstract

Discussing the general case of a hard partonic production process, we show that the notion of parton energy loss is not always sufficient to fully address medium-induced gluon radiation. The broader notion of gluon radiation associated to a hard process has to be used, in particular when initial and final state radiation amplitudes interfere, making the medium-induced radiated energy different from the energy loss of any well-identified parton. Our arguments are first presented in an abelian QED model, and then applied to large-xF quarkonium hadroproduction. In this case, we show that the medium-induced radiated energy is qualitatively similar (but not identical) to the radiative energy loss of an "asymptotic massive parton" undergoing transverse momentum broadening when travelling through the nucleus. In particular, it scales as the incoming parton energy, which suggests to reconsider gluon radiation as a possible explanation of large-xF quarkonium suppression in p-A collisions. We expect a similar effect in open heavy-flavour and possibly light-hadron hadroproduction at large xF, depending on the precise definition of the nuclear suppression factor in the latter case.

Figures (11)

• Figure 1: Atomic mass number dependence ($\sigma(p$--$A)\propto A^\alpha$) of $J/\psi$ (circles) and Drell-Yan (squares) production ($4<M<8.4$ GeV) measured in $p$--A collisions at $\sqrt{s_{_{pA}}}=38.8$ GeV as a function of $x_{_F}$. E866/NuSea data from Leitch:1999eaBadier:1981ci.
• Figure 2: Large angle: (a) Particle produced at mid-rapidity in A--A collisions. Small angle: (b) particle produced in $p$--A collisions, (c) particle produced at large rapidity in A--A collisions.
• Figure 3: Model for large angle scattering in QED in "$p$--$p$" collisions. The blob represents the hard process, the photon is radiated by the incoming (left) and outgoing (right) electron.
• Figure 4: Photon emission diagrams in "A--A" collisions, where the outgoing electron rescatters in the medium.
• Figure 5: An electron produced in the vacuum at an initial time $t=0$ by a hard process radiates a photon.