Sorting Out Quenched Jets

Abstract

We introduce a new "quantile" analysis strategy to study the modification of jets as they traverse through a droplet of quark-gluon plasma. To date, most jet modification studies have been based on comparing the jet properties measured in heavy-ion collisions to a proton-proton baseline at the same reconstructed jet transverse momentum (pT ). It is well known, however, that the quenching of jets from their interaction with the medium leads to a migration of jets from higher to lower pT , making it challenging to directly infer the degree and mechanism of jet energy loss. Our proposed quantile matching procedure is inspired by (but not reliant on) the approximate monotonicity of energy loss in the jet pT . In this strategy, jets in heavy-ion collisions ordered by pT are viewed as modified versions of the same number of highest-energy jets in proton-proton collisions, and the fractional energy loss as a function of jet pT is a natural observable (QAA). Furthermore, despite non-monotonic fluctuations in the energy loss, we use an event generator to validate the strong correlation between the pT of the parton that initiates a heavy-ion jet and the pT of the vacuum jet which corresponds to it via the quantile procedure (p quant T ). We demonstrate that this strategy both provides a complementary way to study jet modification and mitigates the effect of pT migration in heavy-ion collisions.

Nuclear modification factors for jet fragmentation

“…proposed in [43] in the analysis of the nuclear effects on single jets. It would be interesting to check whether such methods could give us a more direct, experimental, access to the genuine modifications in the jet fragmentation function near x = 1.…”

Jets as a probe of the quark-gluon plasma

“…It has also been pointed out in [33] that the apparent lack of dependence of the jet R AA on the collision energy, despite the larger energy loss anticipated in the hotter plasma produced at higher energies, could be due to simultaneous effects of different spectra and different energy loss at the two energies. This highlights the importance of measuring energy loss in a way that is less sensitive to the vacuum spectra than the R AA , for example [34].…”

“…The average energy loss of jets as a function of their p T can be measured from the (p Tdependent) shift of the jet-production spectrum to lower p T in heavy-ion collisions compared to proton-proton collisions [34]. It was proposed in [34] that this can be used to mitigate the effects of selection biases in jet modification observables by adjusting the p T range of a heavy-ion jet measurement relative to the p T range of the proton-proton measurement to compensate for the average energy loss of jets at that p T . Since this procedure only corrects for the average energy loss, the samples of heavy-ion and proton-proton jets that are compared will still not be identical.…”

Quenching effects in the cumulative jet spectrum

“…Recently, there has been efforts toward mitigating such bias effects by investigating novel observables or by using machine learning techniques [9]. One alternative, that we will investigate in detail in the current work, is to introduce a quantile procedure [10] to reconstruct a p T that is closer to the initial jet p T before quenching sets in. In contrast to R AA , the quantile procedure uses the tail-cumulative of the jet spectrum and momentum ratio to reduce the bias coming from the steepness of the spectrum.…”