Jet (de)coherence in Pb-Pb collisions at the LHC

We study the modifications of jets created in heavy-ion collisions at LHC energies. The inherent hierarchy of scales governing the jet evolution allows to distinguish a leading jet structure, which interacts coherently with the medium as a single color charge, from softer sub-structures that will be sensitive to effects of color decoherence. We argue how this separation comes about and show that this picture is consistent with experimental data on reconstructed jets at the LHC, providing a quantitative description simultaneously of the jet nuclear modification factor, the missing energy in di-jet events and the modification of the fragmentation functions. In particular, we demonstrate that effects due to color decoherence are manifest in the excess of soft particles measured in fragmentation functions in Pb-Pb compared to proton-proton collisions.

• well defined objects in perturbation theory * • ideal hard probes for extracting properties of the medium! by now a plenitude of direct evidence for the central pre-diction that the coupling strength of gluons decreases with increasing energy and momentum. 8 Note that several of the individual points in the figure summarize hundreds of independent measurements, all of which must be-and are-fitted with only one adjustable parameter (the quark-gluon coupling measured at the Z-boson mass).
The actual history was different. The need for asymptotic freedom in describing the strong interaction was deduced from much more indirect clues, and QCD was originally proposed as the theory of the strong interaction because it is essentially the unique quantum field theory To avoid confusion, I should state that, when I discuss high-temperature QCD in this article, I'm assuming that the net baryon density (quarks minus antiquarks) is very small. Conversely, when I discuss high-density QCD, I mean a high net density of quarks at low temperature, but well above the ordinary quark density of cold nuclear matter. Temperature and net baryon density are generally taken as the two independent variables of the phase diagram for hadronic matter.
Asymptotic freedom implies that QCD physics gets simpler at very high temperature. That would seem implausible if you tried to build up the high-temperature phase by accounting for the production and interaction of all the different mesons and baryon resonances that are energetically accessible at high temperature. Hoping to bypass this forbidding e + e + e + e + ee -ee -q q g q q FIGURE 3. IN HIGH-ENERGY e + eannihilations into strongly interacting particles, the many-particle final state is observed (left) to consist of two or occasionally three (or, very rarely, four or more) "jets" of particles leaving the collision in roughly the same directions. QCD predicts their production rates and angular and energy distributions by assuming that (right) a single primary quark or gluon underlies each jet. The jets are explained by asymptotic freedom, which tells us that the probability is small for emitting a quark or gluon that drastically alters the flow of energy and momentum.

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"2 jet" "3 jet" * free from problems related to hadronic fragmentation functions… • probabilistic picture, factorization • jet scales -perturbative evolution • angular ordering -essential for small x • MLLA + Local-Parton-Hadron-Duality • In medium scales? (before doing the math) to ⇤ QCD by invoking the Local Parton-Hadron Duality hypothesis. The resulting parton spectrum can then be directly compared to hadron spectra by introducing an energy independent scaling factor.
The collimation property of vacuum jets can be inferred directly from the fact that D vac only depends on the jet energy and cone angle in terms of Q, which is the largest scale of the process. The separation of intrinsic jet and medium scales allow to find the modified fragmentation function directly via the jet calculus rule, where D med q (x, p ⇥ , L) is the distribution of primary quarks [21]. Here we point out two crucial points concerning Eq. (6). First and foremost, the subscript of the resulting distribution refers to the coherent jet (color) structure that survives the medium interactions at this level of approximation. In other words, vacuum and written as the sum of two components, D jet med (x; Q, L) = D coh med (x; Q, L) + D decoh med (x; Q, L) , (7) where D coh med is the coherent modified jet spectrum found from Eq. (6) and the decoherence of in-cone vacuum radi-was f 1.5 G depic heren at all the e o⇤ th the su ever, an en lision Eq. (7 sured thin-s mediu dip a the h is val small shoul distri are the general features encoded in the hard emission currents in eqs. (2.22) and d generalizes the picture of medium-induced decoherence described in Section 5 . [7,8]. us wrap up the discussion by considering hard gluon emissions. While interferences dy strongly suppressed for ⌅ > m D /⇥ qq in the "dipole regime" due to longitudinal nce e⇥ects (the LPM e⇥ect), the same is not true for the "saturation" regime where √x D med (x, )