Jet modification in the next decade: A pedestrian outlook

Abstract

Abstract. In this review, intended for non-specialists and beginners, we recount the current status of the theory of jet modification in dense matter. We commence with an outline of the "traditional" observables which may be calculated without recourse to event generators. These include single and double hadron suppression, nuclear modification factor versus reaction plane etc. All of these measurements are used to justify both the required underlying physical picture of jet modification as well as the final obtained values of jet transport coefficients. This is followed by a review of the more modern observables which have arisen with the ability to reconstruct full jets, and the challenges faced therein. This is followed by a preview of upcoming theoretical developments in the field and an outlook on how the interface between these developments, phenomenological improvements, and upcoming data will allow us to quantitatively determine properties of the medium which effect the modification of hard jets.

The medium modification of high-virtuality partons

“…Almost 20 years past the discovery of jet quenching in the suppression of high transverse momentum (high-p T ) hadrons at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) [1,2], and a decade past the observation of modification of reconstructed jets at the Large Hadron Collider (LHC) at CERN [3,4], there is a measure of consensus regarding the overarching mechanism among the practitioners of the field. It is now widely held that jets are multi-scale objects [5][6][7][8], that start as a single parton with an off-shellness or virtuality µ far above the ambient medium scale of temperature T , or Debye mass m D gT ∼ T , in a heavy-ion collision, and continue to lose this off-shellness in a cascade of partonic splits, that take place over an extended lifetime.…”

“…Within this cascade, it is also generally accepted that there are at least two distinct stages [7,[9][10][11]: A higher virtuality stage (HVS), where the off-shellness of a given parton is greater than that generated by multiple scattering (MS) in the medium, i.e. µ 2 µ 2 M S ∼ qτ , where τ is the lifetime of the parton, and q is the transverse momentum transport coefficient defined for a massless parton as,…”

“…2) gained by a single parton traversing a length L, transverse to its original direction (designated as the negative z-axis). This higher virtuality stage is often referred to as the radiation stage [7] or Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) stage [12][13][14][15]. In the higher virtuality stage, the frequency of hard emissions outweighs that of hard scatterings at a similar scale, and the effect of these emissions can be calculated either by using the DGLAP equation or on the basis of event generators based on this equation.…”