Abstract
We present the medium-modified energy collimation in the leading-logarithmic approximation (LLA) and next-to-leading-logarithmic approximation (NLLA) of QCD. As a consequence of more accurate kinematic considerations in the argument of the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) fragmentation functions (FFs) we find a new NLLA correction $\mathcal{O}({\ensuremath{\alpha}}_{s})$ which accounts for the scaling violation of DGLAP FFs at small $x$. The jet shape is derived from the energy collimation within the same approximations and we also compare our calculations for the energy collimation with the event generators pythia6 and YaJEM for the first time in this paper. The modification of jets by the medium in both cases is implemented by altering the infrared sector using the Borghini-Wiedemann model. The energy collimation and jet shapes qualitatively describe a clear broadening of showers in the medium, which is further supported by YaJEM in the final comparison of the jet shape with CMS PbPb data at center-of-mass energy 2.76 TeV. The comparison of the biased versus unbiased YaJEM jet shape with the CMS data shows a more accurate agreement for biased showers and illustrates the importance of an accurate simulation of the experimental jet-finding strategy.
Highlights
The phenomenon of jet quenching was first established experimentally through the observed suppression of highpT hadrons in nucleus-nucleus (A-A) collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) [1,2,3,4]
A high-energy jet of half opening angle Θ0, energy E and virtuality Q 1⁄4 EΘ0 produced in a nucleus-nucleus collision was considered, followed by the production of one concentric sub-jet of opening angle Θ and transverse momentum k⊥ 1⁄4 xEΘ where the bulk xE ∼ E of the jet energy is contained [32,33]
fragmentation functions (FFs) DBAðx; EΘ0; xEΘÞ determine the probability that a parton A produced at large pT ∼ E in a high-energy collision fragments into a hard sub-jet B of transverse momentum xEΘ, which we write in the third argument of the FF
Summary
The phenomenon of jet quenching was first established experimentally through the observed suppression of highpT hadrons in nucleus-nucleus (A-A) collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) [1,2,3,4]. The computation of the energy collimation was first performed analytically in the vacuum [32] and subsequently modeled in the medium [33] by means of the inclusive spectrum of partons provided by the medium-modified solution of DGLAP FFs at large x ∼ 1 For this purpose, a high-energy jet of half opening angle Θ0, energy E and virtuality Q 1⁄4 EΘ0 produced in a nucleus-nucleus collision was considered, followed by the production of one concentric sub-jet of opening angle Θ and transverse momentum k⊥ 1⁄4 xEΘ where the bulk xE ∼ E of the jet energy is contained [32,33]. FFs DBAðx; EΘ0; xEΘÞ determine the probability that a parton A produced at large pT ∼ E in a high-energy collision fragments into a hard sub-jet B of transverse momentum xEΘ, which we write in the third argument of the FF
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