((HOT)) Liliana Model Set 143

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By using \(Z +\) jet events, we can select quark and gluon-initiated jets, with the information of their initial transverse momentum scale (provided that the \(Z-\) boson transverse momentum is a good proxy for the back-to-back parton). We parallel the results obtained from reconstructed jets in vacuum and in the presence of an expanding medium as provided by the Monte Carlo model. We further compare the Monte Carlo results to the analytical expectations from Eq. (1). Our results are always presented in terms of a quark to gluon ratio. In vacuum, for the observables discussed and at lowest order, this ratio obeys the well known scaling \(C_F/C_A\) (Casimir scaling). We study how this scaling gets modified in the presence of in-medium radiation.

where the lower indices vac indicate the vacuum reference of each Monte Carlo model and \(vac + med\) the full Monte Carlo simulation that naturally includes both vacuum-like and medium-induced radiation. The upper index specifies the sample flavour. The \(\langle \ \rangle \) denotes the fact that we will use the average value of the observable in some bin (typically a \(p_T\) bin). This allow us to isolate pure medium-induced emissions in the definition of \(R({\mathcal {O}})_{med}\).

Figure 5 summarises the hadronization results while Fig. 6 summarises the results when only ISR is included. In this case, we also show, for direct comparison, the results obtained from the Hybrid approach (recall that ISR and hadronization are activated separately in each sample of either model).

From Fig. 6, it is clear that both generators show stable ratios for high \(p_T^Z\) at fairly similar values. This observation is rather remarkable since the colour structure of the showers in the two models is very different. In JEWEL the colour structure of the shower is modified by interaction with medium scattering centres. In particular, by each gluon exchanged with the coloured medium, the incoming parton has its colour field rotated. The Hybrid model preserves the colour structure of the PYTHIA generated shower. The only modification of colour structure is induced by the partons that are stopped in the medium due to energy loss effects and thus will not contribute to the reconstructed jet. Taking into account the holographic scaling from (5), we would naively expect quark- and gluon-initiated jets to be more alike in the Strong/Weak Hybrid model than in JEWEL. We see in Fig. 6 that this is indeed the case, but the difference is not significant. One of the contributing factors might be the presence of additional radiation sources (particles) that naturally occur during JEWEL parton shower evolution, with respect to Hybrid (where energy losses are applied to a fully developed vacuum-like shower). From our studies in Sect. 3.1, the convolution of pQCD medium-induced energy loss, together with some decoherence effects during the parton shower evolution, induce similar energy losses for quark- and gluon-initiated jets. Overall, the JEWEL Casimir ratio increases and approaches the one obtained from the Hybrid model.

At sufficiently high \(p_T\) the R ordering of the Hybrid model curves is the opposite to what JEWEL predicts. For both JEWEL and the Hybrid model, at low \(p_T\), the behaviour is dominated by gluon-like (which radiate more), thus we observe a larger value of \(R(\varDelta p_T)_{med}\). At larger \(p_T\), quark-like jets dominate and thus the value of \(R(\varDelta p_T)_{med}\) decreases. For a fixed \(p_T\) bin (sufficiently high), increasing R leads to recovery of radiation for both quark- and gluon-like jets. However, this is more relevant for gluon-like jets, and therefore the scaling decreases with R in JEWEL. However, in the Hybrid model, what happens is that in fact, a part of the jet population (for a certain \(p_T\) bin) comes from jets which lost a lot of energy, and thus migrated from higher \(p_T\) bins. Therefore, as one changes the radius, there is a competition between a population of jets which did not lose much energy vs a population of higher energy jets which radiated a lot and ended in a lower \(p_T\) bin. From the behaviour seen as R varies, we see that this second group significantly modifies the dependence on R, when comparing to JEWEL. 2b1af7f3a8