Jet quenching via jet collimation

The strong modifications of dijet properties in heavy ion collisions measured by ATLAS and CMS provide important constraints on the dynamical mechanisms underlying jet quenching. In this work, we show that the transport of soft gluons away from the jet cone — jet collimation — can account for the observed dijet asymmetry with values of q̂ L that lie in the expected order of magnitude. Further, we show that the energy loss attained through this mechanism results in a very mild distortion of the azimuthal angle dijet distribution. The kinematical reach of the LHC allows for systematic studies involving fully reconstructed calorimetric jets to be carried out, and for the ensuing analyses to be significant beyond the experimental uncertainties inherent to the identification of jets in a fluctuating high multiplicity environment. The measurement of the dijet asymmetry AJ = (ET1 −ET2)/(ET1 +ET2) in Pb-Pb collisions at √ s = 2.76 TeV by both ATLAS [1, 2] and CMS [3, 4] provides a clear example of the extent to which such full jet measurements can contribute to furthering the understanding of the dynamical processes responsible for jet quenching. In the analyzed dijet event samples, the transverse energies of the jets are identified within cones of radius R (0.4 for ATLAS, 0.5 for CMS). For the leading jet, it is ET1 > 100 GeV (ATLAS), 120 GeV (CMS), while for the recoiling jet — found at an azimuthal separation ∆Φ = |Φ1 − Φ2| > π/2 (ATLAS) and 2π/3 (CMS) — it is ET2 > 25 GeV (ATLAS), 50 GeV (CMS). The measured event asymmetry distribution shows qualitative features consistent with a substantial medium induced energy loss of the recoiling jet. In a nutshell, the fraction of energy lost from the recoiling jet cone is larger in heavy ion collisions than in the proton-proton case, and grows with increasing centrality of those collisions. Crucially, this effect is accompanied by a distortion of the dijet azimuthal distribution which is, at most, very mild. As such, this measurement carries the hallmark of an underlying dynamical mechanism capable of carrying a substantial amount of energy away from the jet cone without any significant deflection of the jet direction. ar X iv :1 10 7. 19 64 v1 [ he pph ] 1 1 Ju l 2 01 1 Jet quenching via jet collimation 2 In the 10% most central events (for which the observed effect is stronger), the average amount of energy lost from the recoiling jet cone in addition to the protonproton case falls within the bounds [5] 10 GeV < ∆E < 21 GeV, (for the ATLAS data in [1]) , (1) 8 GeV < ∆E < 18 GeV, (for the CMS data in [3]) . (2) Here, the lower bounds were obtained by assuming that all recoiling jets traverse the medium. The upper bounds resulted from taking only a fraction (∼ 0.5, the ratio of symmetric AJ = 0 dijet events in Pb-Pb and proton-proton) as interacting with the medium. A mechanism — jet collimation — that leads to both large energy degradation and no sizeable azimuthal displacement was proposed in [5]. Soft gluons radiated at small angles result in a very small azimuthal displacement of the jet. These gluons, and in fact all partons in the parton shower, undergo Brownian motion during their passage through the medium and thus accumulate an average squared transverse momentum q̂ per unit path length, 〈k T 〉 ∼ q̂L ∼ q̂τ . Thus, in the presence of a medium, the average formation time τ ∼ ω/k T for partons of energy ω is