Jet substructure measurements in pp and Pb-Pb collisions at s NN − − − √ =5.02 TeV with ALICE
JJet substructure measurements in pp and Pb–Pbcollisions at √ s NN = . TeV with ALICE
James Mulligan for the ALICE Collaboration a , b a Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA b Physics Department, University of California, Berkeley, CA 94720, USA
E-mail: [email protected]
We report jet substructure measurements in pp and Pb–Pb collisions at √ s NN = .
02 TeV with theALICE detector. Charged-particle jets were reconstructed at midrapidity with the ALICE trackingdetectors using the anti- k T algorithm with resolution parameters R = . R = .
4. In ppcollisions, the groomed jet momentum fraction, z g , and the groomed jet radius, θ g ≡ R g / R , aremeasured for the first time using the Dynamical Grooming method. Additionally, new systematicmeasurements of the infrared and collinear (IRC) safe ungroomed jet angularities are presented.In heavy-ion collisions, we measure z g and θ g with the Soft Drop grooming algorithm. The largeunderlying event in heavy-ion collisions poses a challenge for the reconstruction of groomed jetobservables, since fluctuations in the background can cause groomed splittings to be misidentified.By using strong grooming conditions to reduce this background, we report these observables fullycorrected for detector effects and background fluctuations for the first time, and compare them toseveral theoretical models. HardProbes20201-6 June 2020Austin, Texas © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] S e p et substructure measurements in pp and Pb–Pb collisions at √ s NN = . TeV with ALICE
1. Introduction
The substructure of jets can be used to study fundamental aspects of QCD in both pp and Pb–Pbcollisions [1]. Jet grooming techniques, such as Soft Drop [2] and Dynamical grooming [3], reducenon-perturbative effects in pp collisions by selectively removing soft large-angle radiation, whichallows for well-controlled comparisons of measurements to pQCD calculations [4–7]. Groomingtechniques have also been applied to heavy-ion collisions, in order to explore whether jet quenchingin the quark-gluon plasma modifies the hard substructure of jets [8–16]. Several measurementsof groomed jet observables have been made in pp and heavy-ion collisions at the LHC and RHIC[17–22]. Ungroomed observables, such as jet angularities [23], provide a complementary way tostudy QCD in both pp and Pb–Pb [17, 24, 25] collisions, and offer the ability to systematicallyvary the observable definition in a way that is theoretically calculable, and give sensitivity to thepredicted scaling of non-perturbative shape functions [26, 27].In what follows, we reconstruct charged-particle jets at midrapidity with the ALICE [28]tracking detectors using the anti- k T algorithm [29, 30] with resolution parameters R = . R = .
4. All presented results are corrected for detector effects (in both pp and Pb–Pb collisions)and background fluctuations (in Pb–Pb collisions) using an iterative unfolding algorithm [31].
2. Jet substructure in proton–proton collisions
The Dynamical grooming algorithm [3] identifies a single “splitting” by re-clustering theconstituents of a jet with the Cambridge-Aachen algorithm [32], and traversing the primary Lundplane [33] to identify the splitting that maximizes: z i ( − z i ) p T , i (cid:16) ∆ R i R (cid:17) a , where z i is the longitudinalmomentum fraction of the i th splitting, ∆ R i is the rapidity-azimuth ( y , ϕ ) separation of the daughters,and a is a continuous free parameter. Since the grooming condition defines a maximum rather thanan explicit cut (as in the case of Soft Drop), every jet will always return a tagged splitting. We focuson the two kinematic observables that characterize the splitting: the groomed jet radius, θ g ≡ R g / R ≡ (cid:112) ∆ y + ∆ ϕ / R , and the groomed momentum fraction, z g ≡ p T , subleading /( p T , leading + p T , subleading ) .Figure 1 shows the z g and θ g distributions in pp collisions for several values of the groomingparameter a . For small values of a , the grooming condition favors splittings with symmetriclongitudinal momentum, which is reflected in the distributions skewing towards large- z g and small- θ g . As a increases, the grooming condition favors splittings with large angular separation, whichis reflected in the distributions skewing towards small- z g and large- θ g . The results are compared toPYTHIA [34], which describes the data well. The class of IRC-safe jet angularities [23] are defined as λ κβ = (cid:213) i ∈ jet (cid:18) p T , i p T, jet (cid:19) κ (cid:18) ∆ R i R (cid:19) β (1)for κ = β >
0. 2 et substructure measurements in pp and Pb–Pb collisions at √ s NN = . TeV with ALICE g z g z d σ d j e t, i n c σ ALICE Preliminary = 5.02 TeV s pp T k Charged jets anti-| < 0.5 jet η = 0.4 | R c < 80 GeV/
T, ch jet p
60 < = 0.1 a Dynamical Grooming: = 1.0 a Dynamical Grooming: = 2.0 a Dynamical Grooming: Sys. uncertaintyPYTHIA8 Monash 2013 g z PY T H I A D a t a ALI-PREL-352113 g θ g θ d σ d j e t, i n c σ ALICE Preliminary = 5.02 TeV s pp T k Charged jets anti-| < 0.5 jet η = 0.4 | R c < 80 GeV/
T, ch jet p
60 < = 0.1 a Dynamical Grooming: = 1.0 a Dynamical Grooming: = 2.0 a Dynamical Grooming: Sys. uncertaintyPYTHIA8 Monash 2013 g R g θ PY T H I A D a t a ALI-PREL-352108
Figure 1:
Measurements of z g (left) and θ g (right) in pp collisions with Dynamical grooming [3] for threevalues of the grooming parameter a , along with comparison to PYTHIA Monash 2013 [34]. Figure 2 shows the λ κ = β distributions in pp collisions for R = . R = . β . As β increases, the distributions skew towards small λ κ = β , since ∆ R i / R is smaller than unity. For larger R , the distributions are narrower than for smaller R , as expecteddue to the collinear nature of jet fragmentation. The results are compared to PYTHIA [34], whichdescribes the data reasonably well but with some deviations to be further explored. =1 κβ λ = κ β λ d σ d j e t σ ALICE Preliminary = 5.02 TeV s pp T k Charged jets anti-| < 0.7 jet η = 0.2, | R c < 80 GeV/ chT,jet p
60 < = 1 β = 1.5 β = 2 β = 3 β Sys. uncertaintyPYTHIA8 Monash2013 =1 κβ λ PY T H I A D a t a ALI-PREL-352078 =1 κβ λ = κ β λ d σ d j e t σ ALICE Preliminary = 5.02 TeV s pp T k Charged jets anti-| < 0.5 jet η = 0.4, | R c < 80 GeV/ chT,jet p
60 < = 1 β = 1.5 β = 2 β = 3 β Sys. uncertaintyPYTHIA8 Monash2013 =1 κβ λ PY T H I A D a t a ALI-PREL-352093
Figure 2:
Measurements of jet angularities λ κ = β in pp collisions with for R = . R = . β , along with comparison to PYTHIA Monash 2013 [34]. et substructure measurements in pp and Pb–Pb collisions at √ s NN = . TeV with ALICE
3. Jet substructure in heavy-ion collisions
In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction ofgroomed jet observables, since fluctuations in the background can cause groomed splittings to bemisidentified [35]. We present measurements [36] of z g and θ g with the Soft Drop grooming algo-rithm that are fully corrected for detector effects and background fluctuations, leveraging strongergrooming conditions than in previous measurements. Figures 3 and 4 show these measurements inPb–Pb collisions together with their comparison to those from pp collisions, for central (0–10%)and semi-central (30–50%) Pb–Pb collisions, respectively.We find that the z g distributions in Pb–Pb collisions are consistent with those in pp collisions,whereas a significant narrowing of the θ g distributions in Pb–Pb collisions relative to pp collisionsis observed. These measurements are compared to a variety of jet quenching models: JETSCAPE[37–39], Caucal et al. [13, 40], Chien et al. [8], Qin et al. [10], Pablos et al. [15, 41, 42], and Yuanet al. [14, 43]. All models considered are consistent with the z g measurements. Many of the modelscapture the narrowing effect observed in the θ g distributions, although with quantitative differences.This behavior is consistent with models implementing an incoherent interaction of the jet showerconstituents with the medium, but also consistent with medium-modified “quark/gluon” fractionswith fully coherent energy loss. By isolating the theoretically well-controlled hard substructure ofjets, these measurements provide direct connection to specific jet quenching physics mechanisms,and offer the opportunity for future measurements to definitively disentangle them. g z g z d σ d j e t, i n c σ = 0.88 AAtagged f = 0.89, pptagged f ALICE Preliminary = 5.02 TeV NN s T k Charged jets anti-| < 0.7 jet η = 0.2 | R c < 80 GeV/
T, ch jet p
60 < =0 β =0.2, cut z Soft Drop pp 10% − Pb 0 − PbSys. uncertainty g z pp P b − P b JETSCAPE, MATTER+LBT (Prel.) c < 100 GeV/ T,jet p Caucal et al., 75 < c < 130 GeV/ T,jet p Chien et al., 100 < c < 120 GeV/ T,jet p Qin et al., 90 < = 0 res L Pablos et al., T π = 2/ res L Pablos et al., ∞ = res L Pablos et al.,
ALI–PREL–352940 g θ g θ d σ d j e t, i n c σ g R = 0.88 AAtagged f = 0.89, pptagged f ALICE Preliminary = 5.02 TeV NN s T k Charged jets anti-| < 0.7 jet η = 0.2 | R c < 80 GeV/
T, ch jet p
60 < =0 β =0.2, cut z Soft Drop pp 10% − Pb 0 − PbSys. uncertainty g θ pp P b − P b JETSCAPE, MATTER+LBT (Prel.) c < 100 GeV/ T,jet p Caucal et al., 75 < = 0 res L Pablos et al., T π = 2/ res L Pablos et al., ∞ = res L Pablos et al.,
Yuan et al., med q/gYuan et al., quark L = 5 GeVqYuan et al.,
ALI–PREL–352930
Figure 3:
Measurements of z g (left) and θ g (right) in 0–10% central Pb–Pb collisions compared to ppcollisions for R = .
2, along with comparison to several theoretical models [36].
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