MEDIUM-modified fragmentation functions

Abstract

Abstract. We extend the Sudakov formalism from vacuum to opaque media by supplementing the splitting functions with an adittional term given by the medium-induced gluon radiation spectrum. We then solve the DGLAP evolution equations to obtain the medium-modified fragmentation functions. In this way, both the additional energy loss and the modification of the QCD evolution by the medium are included in a consistent manner. As a phenomenological application, we compute the suppression of the high-pt yields in heavy-ion collisions by convoluting the obtained fragmentation functions with the perturbative spectrum. Modification of the vacuum splitting functionsA hard parton produced in a heavy ion collision will travel trough the opaque medium losing virtuality till its eventual hadronization, which takes place in vacuum for large enough transverse momentum. The medium accelerates the QCD evolution of the parton by inducing successive soft gluon radiations. Under the BDMPS approximation[1], the spectrum of gluons emitted by the parent parton can be written as a function of two parameters which completely characterize the medium: the medium length L and the transport coefficientq [2]. The properties of the medium are encoded in the transport coefficient,q, which can be related to the average squared transverse momentum transferred to the parton per mean free path.To compute medium modified fragmentation functions (MMFF), we extend the fact that the formalism [1,2] provides the vacuum splitting functions in the collinear limit,The total splitting function is the sum of both (see also [4] and [5] ),See that eq. 1 is only valid at z → 1 due to the use of the medium induced gluon spectra, which relies on high energy approximations. So, at this level, the only difference between our total g → gg and q → qg splittings will be still a Casimir color factor. To extend it to all z, we take the full LO splitting functions P V AC . Small z corrections are also introduced in P M ED , whose effect is found to be small (see fig. 1 below) at the level of the Sudakovs. Sudakov FactorsThe Sudakov form factor is defined in the standard waywhich can be interpreted as the probability for a parton a not to radiate resolvable partons when evolving between the scales t 0 and t. In fig. 1 we show the effect of supplementing the Sudakov form factor with a medium term in the splitting function as given by eq. (1). This is done for two energies E =10 GeV, of interest for RHIC, and E = 100 GeV, of interest for the LHC. Increasing the value of the transport coefficient leads to an enhancement on the radiation and, hence, to more suppressed Sudakov factors. The Sudakov for gluons is approximately enhanced by the color factor 9/4 with respect to that for quarks. Figure 1. Left: Sudakov form factors for a parent parton of energy E jet = 10 GeV. Upper and lower curves correspond to quarks and gluons respectively. We use different colors for different medium densities. Solid/dashed lines account for colinear and large x corrected splitting functions...

A Monte Carlo study of jet fragmentation functions in PbPb and pp collisions at sqrt{s}=2.76 TeV

“…[17]). In the past few years, MC codes for in-medium shower simulations developed for hadronic collisions have also become available [18][19][20][21][22]. They have been based on the success of MC shower simulations in the vacuum such as PYTHIA and HERWIG [23].…”

Inclusive transverse momentum distribution of hadrons in jets produced in PbPb and pp collisions at the LHC: Data versus jet-quenching Monte Carlos

“…Since the start of the heavy-ion program at the LHC, with its plethora of new hard observables available, new theoretical approaches have been developed to study the interaction of energetic partons with the hot and dense medium [8]. In particular, realistic Monte Carlo (MC) codes for the simulation of in-medium parton shower evolution have been constructed from "QCD vacuum" event generators [9][10][11][12][13][14] such as PYTHIA [15,16] and HERWIG [17]. JEWEL (Jet Evolution With Energy Loss) [9] implements elastic and inelastic medium interactions which lead to distinctive modifications of the jet fragmentation pattern, including Landau-Pomeranchuk-Migdal destructive interference effects in a probabilistic framework [18].…”

“…JEWEL (Jet Evolution With Energy Loss) [9] implements elastic and inelastic medium interactions which lead to distinctive modifications of the jet fragmentation pattern, including Landau-Pomeranchuk-Migdal destructive interference effects in a probabilistic framework [18]. In Q-PYTHIA [13,14], medium effects are introduced via an extra term in the QCD splitting functions arising from the multiple-soft scattering approximation. In PYQUEN [10], gluon radiation is associated with each parton scattering in the hot and dense medium and interference effects are included through the modified radiation spectrum as a function of the medium temperature (i.e.…”

Medium modified fragmentation function: equivalence of the ACSX and QW formalisms in the high- Q 2 region

“…The interaction of partons with the medium affects high transverse momentum hadron production, thus it can be reflected by the parton fragmentation function. In this paper, we study the equivalence of the medium-evolved fragmentation function (ACSX) [10,11] and the quenching weight fragmentation function (QW) [12], which can provide useful information on the properties of the QGP.…”