Introduction JEWEL Role of uctuations Medium's response to jets Outlook

Deciphering jet quenching with JEWEL

Korinna Zapp

LIP (Lisbon) & CERN

CERN TH Colloquium, 11. 04. 2018

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook A heavy ion collision

I 1600 primary charged hadrons per unit rapidity ∼ I p+p at comparable √s: dNch/dη 4 5 ' − Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Heavy ion collisions: bird's eye view

I heavy ion collisions create strongly interacting system of high density

I Bjorken's energy density estimate: 1 d E⊥ 25 GeV fm3 0 2 d / ' πR τ0 η η=0 ' Bjorken, Phys. Rev. D 27 (1983) 140 ALICE, Phys. Rev. C 94 (2016) 034903

I there has to be re-scattering in the nal state

I scattering drives a system towards thermal equilibrium

I How and to what extent does the nal state in heavy ion collisions equilibrate?

I proof that there is re-scattering: jet quenching

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jets in heavy ion collisions

I proton-proton collisions: 2 jets with balancing transverse momentum

I in heavy ion collisions: signicant softening of jets thermalisation of a far-from-equilibrium system → jet quenching informs us about equilibration in QCD → Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What is needed to ge beyond qualitative statements

1 a αβ QCD = Ψ(¯ iD/ m)Ψ F F + gauge-xing + ghost L − − 4 αβ a L L

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What is needed to ge beyond qualitative statements

1 a αβ QCD = Ψ(¯ iD/ m)Ψ F F + gauge-xing + ghost L − − 4 αβ a L L

physics: solve QCD exactly

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What is needed to ge beyond qualitative statements

1 a αβ QCD = Ψ(¯ iD/ m)Ψ F F + gauge-xing + ghost L − − 4 αβ a L L

physics: solve QCD exactly impossible → /

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What is needed to ge beyond qualitative statements

1 a αβ QCD = Ψ(¯ iD/ m)Ψ F F + gauge-xing + ghost L − − 4 αβ a L L physics: dynamical model tool: e.g.: JEWEL Monte Carlo event generator NB: NOT rst-principles QCD

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Outline

Introduction: jets and jet quenching

Jet quenching in JEWEL

The di-jet asymmetry and the role of uctuations

Jet sub-structure and the medium's response to jets

Outlook

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Outline

Introduction: jets and jet quenching

Jet quenching in JEWEL

The di-jet asymmetry and the role of uctuations

Jet sub-structure and the medium's response to jets

Outlook

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jet quenching warm-up

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jet quenching warm-up

I jets produced in earliest phase of heavy ion collision I calibrated probe: well understood in p+p I jet production in heavy ion collisions unmodied (short distance process) except for nuclear eects in pdf's

I jet quenching allows to observe process of equilibration soft observables see result of equilibration

I jets give access to scale dependence of medium properties

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What is a jet: operationally

A jet is

I a collimated spray of hadrons. I dened by the jet nding algorithm. I result of fragmentation of an energetic quark or gluon. Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What is a jet: theoretically

I energetic quarks and gluons are produced in hard scattering processes

I factorisation of the cross section: 1 Z X 2 2 2 σ(P1, P2)= dx1 dx2 fi (x1, Q )fj (x2, Q )σˆij (x1P1, x2P2, αs, Q ) i,j 0

I σˆij : partonic cross section

I has perturbative expansion in αs known: NNLO for di-jets, NLO for up to ∼ 5 jets

I short distance physics: insensitive to nature of incoming hadrons i.e. no nuclear modications 2 I fi (x, Q ): parton distribution function nuclear pdf ts available

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jet sub-structure

I jets have characteristic sub-structure dictated by QCD radiation

I in collinear limit QCD cross sections factorise: 2 dQ dφ αs dσn+1 dσn dz (z) ≈ Q2 2π 2π P

I naive radiation probability

2 Qmax 1 Z d 2 Z  2  σn+1 Q d αs αs 2 Qmax Π1 = 2 z (z) ln 2 ≡ σn Q 2π P ≈ 2π Q0 2 min Q0 z

I Π1 > 1 for suciently hard processes Π1 is not a probability → I have to resum can be done analytically and explicitly in Monte Carlo event generators

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jet sub-structure

I no-emission probability: Sudakov form factor

 Q2 1  max 2 2 2 Z dQ Z αs ∆(Qmax, Q ) = exp  dz (z) − Q2 2π P  Q2 zmin

I basis for Monte Carlo implementation I parton showers resum these logs to leading log accuracy with some sub-leading terms, NLO parton showers nearly complete

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jets: summary

Jets in p+p

I jets in proton-proton collisions fairly well understood

I jet production I jet sub-structure I jet reconstruction using dedicated algorithms

most commonly used at LHC: anti-k⊥

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jets: summary

Jets in p+p

I jets in proton-proton collisions fairly well understood

I jet production I jet sub-structure I jet reconstruction using dedicated algorithms

most commonly used at LHC: anti-k⊥

Jets in A+A

I jet production in heavy ion collisions understood only nuclear modication in pdf's

I cross-check: vector boson production

I observed: strong modications of jets

I due to re-scattering in nal state

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Suppression of single-inclusive jets

1.5 AA

R ATLAS Preliminary |y| < 2.8

anti-k t R = 0.4 jets, sNN = 5.02 TeV

1

0.5 0 - 10 % 2015 Pb+Pb data, 0.49 nb-1 30 - 40 % 2015 pp data, 25 pb-1 60 - 70 % 0 1000 200 300 400 500 600 900 p [GeV] T ATLAS-CONF-2017-009

I suppression of jets by factor 2 relative to expectation from p+p need to scale p+p reference by number of hard N+N collisions

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Di-jet momentum asymmetry

CMS, Phys. Lett. B 712 (2012) 176

I enhancement of asymmetric congurations

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Intra-jet energy distribution: Jet prole

CMS, Phys. Lett. B 730 (2014) 243

I suppression of activity at intermediate r

I increase near the edge of the jet

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Particle distribution outside jets

40 pp reference CMS Particle yield vs. ∆r pp 27.4 pb-1 (5.02 TeV) PbPb 404 µb-1 (5.02 TeV) 30 r anti-k R=0.4 jets, p > 120 GeV, |η |<1.6 ∆ T

dN T jet d jet 1

N 20 0.7 < ptrk< 1 GeV 3 < ptrk< 4 GeV 12 < ptrk< 16 GeV T T T Y= 1 < ptrk< 2 GeV 4 < ptrk< 8 GeV 16 < ptrk< 20 GeV 10 T T T 2 < ptrk< 3 GeV 8 < ptrk< 12 GeV 0.7 < ptrk< 20 GeV T T T 0 0 0.2 0.4 0.6 0.8 1 40 PbPb 40 PbPb 40 PbPb 40 PbPb 50-100% 30-50% 10-30% 0-10%

30 30 30 30 r r r r ∆ ∆ ∆ ∆ dN dN dN dN d d d d jet jet jet jet 1 1 1 1

N 20 N 20 N 20 N 20 Y= Y= Y= Y= 10 10 10 10

0 0 0 0 0 PbPb0.2 - pp0.4 0.6 0.8 01 PbPb0.2 - pp0.4 0.6 0.8 01 PbPb0.2 - pp0.4 0.6 0.8 01 PbPb0.2 - pp0.4 0.6 0.8 1 15 50-100% 15 30-50% 15 10-30% 15 0-10% pp 10 10 10 10 - Y

PbPb 5 5 5 5 Y

0 0 0 0

0 0.2 0.4 0.6 0.8 010 0.2 0.4 0.6 0.8 010 0.2 0.4 0.6 0.8 010 0.2 0.4 0.6 0.8 1 ∆r ∆r ∆r ∆r CMS, CMS-HIN-16-020, arXiv:1803.00042 I enhancement of soft particles far away from jet

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Groomed shared momentum fraction

CMS-HIN-16-006

min(p⊥,1, p⊥,2) zg = p⊥ sharing between two hardest prongs p⊥,1 + p⊥,2 →

I suppression of symmetric congurations I and/or enhancement of very asymmetric ones → distributions separately normalised to unity Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Summary of experimental results

I jet production suppressed by factor 2 up to p⊥'s of 1 TeV ∼ I hard structures inside jets survive largely unmodied I enhancement of soft activity at edges of jet I and far away from jet

jet substructure: energetic components close to jet axis reduction of jet energy energetic core stays intact

intermediate part reduced

low energy components enhanced & move away from jet axis

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What happens to jets in medium?

Scenario I: hard partons don't resolve quasi-particles

I interactions between jet & medium at large coupling

I AdS/CFT techniques

Scenario II: hard partons do resolve quasi-particles

I jet  medium interactions at weak(ish) coupling

I perturbative techniques

I thermalisation through elastic re-scattering (slow)

I parton energy loss through QCD bremsstrahlung

I destructive interference in multiple scattering LPM eect relevant scale: momentum transfer q between hard parton and medium

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Jets and thermalisation

In both scenarios jet quenching related to thermalisation:

I early stages of HIC and jets are far-from-equilibrium systems

I jet quenching can be seen as jet thermalisation

I eective kinetic theory describing thermalisation Arnold, Moore, Yae, JHEP 0301 (2003) 030 Kurkela, Zhu, Phys. Rev. Lett. 115 (2015) no.18, 182301

dfp = 1↔2[fp] + 2↔2[fp] + exp[fp] − dτ C C C

I 1↔2: splitting/merging rate in presence of multiple scattering including LPMC eect

I 2↔2: elastic scattering rate C I both processes also responsible for parton energy loss

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What happens to jets in medium?

2 I time it takes to radiate a gluon: tform ω/k ≈ ⊥ I time needed for jet evolution medium size ∼ I jet evolution & re-scattering happen simultaneously I multi-scale problem I cannot be solved exactly analytic results for special cases

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook What is known analytically

I single gluon emission o a hard parton in eikonal limit multiple soft or single hard scattering h

Gyulassy, Wang, Nucl. Phys. B 420 (1994) 583 Baier, Dokshitzer, Mueller, Peigne, Schi, Nucl. Phys. B 484 (1997) 265 Zakharov, JETP Lett. 65 (1997) 615 Wiedemann, Nucl. Phys. B 588 (2000) 303 Gyulassy, Levai, Vitev, Nucl. Phys. B 594 (2001) 371 Wang, Guo, Nucl. Phys. A 696 (2001) 788

I non-Abelian LPM eect plays important role

I 2-gluon emission Arnold, Chang, Iqbal, JHEP 1610 (2016) 124 Casalderrey-Solana, Pablos, Tywoniuk, JHEP 1611 (2016) 174

I radiation o colour dipole Mehtar-Tani, Salgado, Tywoniuk, JHEP 1204 (2012) 064 & JHEP 1210 (2012) 197

I non-eikonal kinematics Apolinário, Armesto, Milhano, Salgado, JHEP 1502 (2015) 119

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Outline

Introduction: jets and jet quenching

Jet quenching in JEWEL

The di-jet asymmetry and the role of uctuations

Jet sub-structure and the medium's response to jets

Outlook

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook JEWEL: Basic idea and assumptions

Basic idea

I complexity of problem asks for Monte Carlo event generator

I consistent dynamical model of jet evolution in medium

I anchored in analytical understanding of pQCD Assumptions 1. medium as seen by jet: collection of quasi-free partons 2. use infra-red continued perturbation theory to describe all jet-medium interactions 3. formation times govern the interplay of dierent sources of radiation 4. use results from eikonal limit to include LPM-eect

Zapp, Krauss & Wiedemann, JHEP 1303 (2013) 080

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook JEWEL in a nutshell

I jet production in initial N+N collisions: ME+PS

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook JEWEL in a nutshell

I jet production in initial N+N collisions: ME+PS I re-scattering: ME+PS

I generates elastic & inelastic processes

I with leading log correct relative rates I general kinematics

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook JEWEL in a nutshell

I jet production in initial N+N collisions: ME+PS I re-scattering: ME+PS

I generates elastic & inelastic processes

I with leading log correct relative rates I general kinematics I emission with shortest formation time is realised

I all emissions (vacuum & medium induced) treated equally I hard structures remain unperturbed

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook JEWEL in a nutshell

I jet production in initial N+N collisions: ME+PS I re-scattering: ME+PS I generates elastic & inelastic processes I with leading log correct relative rates I general kinematics I emission with shortest formation time is realised I all emissions (vacuum & medium induced) treated equally I hard structures remain unperturbed I LPM interference Zapp, Stachel, Wiedemann, JHEP 1107 (2011) 118 I also governed by formation times I without kinematic restrictions Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Probabilistic formulation of the LPM-eect

I naive MC purely incoherent I consider gluon radiation with two momentum transfers Wiedemann, Nucl. Phys. B 588(2000),303

I analytical calculation interpolates between incoherent production coherent production

τ1 L τ1 L   k k GB GB

q2 q1

q2 q1

L L 2 ω gluon formation time I τ1 2 ≡ (k + q1) → momentum transfers within formation time act coherently

Deciphering→ jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Coherent emission

Kinematics

I coherent scattering centres act as one one momentum transfer:

3 (1) d I Z 2 ω dq A(q) R(k, q) dωdk ∝ | | two momentum transfers:

3 (2) d I Z 2 2 ω dq1 dq2 A(q1) A(q2) R(k, q1 + q2) dωdk ∝ | | | |

I consistent determination of scattering centres and formation time Emission probability

I suppression compared to incoherent emission by factor 1/Ncoh Ncoh: number of coherent momentum transfers Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Probabilistic formulation of the LPM-eect

10 ω−3 1 analytical results: ω−3/2 0.1 dI −3/2 for dω ω ω < ωc ω

d ∝

/ 0.01 I d 3 d I − d ω for ω > ωc 0.001 ω ∝ 0.0001 deviation in infra-red

0.00001 due to regularisation 0.01 0.11 ωc/E ω/E

200 L 2 ∆E L for L < Lc 150 ∝ ] 30 for c GeV ∆E L L > L [ 100 25 L2 E ∝ 20 ∆ 15 50 10 5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0 2 4 6 8 10 L/Lc Zapp, Stachel, Wiedemann, Phys. Rev. Lett. 103 (2009) 152302 Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Probabilistic formulation of the LPM-eect

10 ω−3 1 analytical results: ω−3/2 0.1 dI −3/2 for dω ω ω < ωc ω

d ∝

/ 0.01 I d 3 d I − d ω for ω > ωc 0.001 ω ∝ 0.0001 deviation in infra-red

0.00001 due to regularisation 0.01 0.11 ωc/E ω/E

200 L 2 ∆E L for L < Lc 150 ∝ ] 30 for c GeV ∆E L L > L [ 100 25 L2 E ∝ 20 ∆ 15 50 10 5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0 2 4 6 8 10 L/Lc Zapp, Stachel, Wiedemann, Phys. Rev. Lett. 103 (2009) 152302 Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Outline

Introduction: jets and jet quenching

Jet quenching in JEWEL

The di-jet asymmetry and the role of uctuations

Jet sub-structure and the medium's response to jets

Outlook

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Di-jet asymmetry  a reminder

Milhano, Zapp, Eur. Phys. J. C 76 (2016) no.5, 288

di-jet asymmetry in PbPb 0-10%, p ,1 > 120 GeV ⊥ 0.012 b CMS PbPb data JEWEL+PYTHIA PbPb p⊥,1 p⊥,2 0.01 JEWEL+PYTHIA pp AJ = − event fraction p⊥,1 + p⊥,2 0.008 b

b b b b b 0.006 b

b 0.004 b naive interpretation: b 0.002 b

0 b b b 1.4 1.2 1

MC/Data 0.8 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 AJ

CMS, Phys. Lett. B 712 (2012) 176

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook The role of geometry

di-jet production points 8 0.016 di-jet asymmetry in PbPb 6 0.014 J 2.5 ] A 4 0.012 -2 JEWEL+PYTHIA full geometry /d

2 0.01 N

d 2 0 0.008 N/(dx dy) [fm y [fm] 2 di-jet -2 0.006 d N 1.5 di-jet -4 0.004 1/ 1/N

-6 0.002 1 -8 0 -8 -6 -4 -2 0 2 4 6 8 x [fm] 0.5

0 1.4 1.2 1 Ratio 0.8 0.6 0 0.2 0.4 0.6 0.8 1 AJ

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook The role of geometry

di-jet production points 8 0.016 di-jet asymmetry in PbPb 6 0.014 J 2.5 ] A 4 0.012 -2 JEWEL+PYTHIA full geometry /d

2 0.01 N

d 2 0 0.008 N/(dx dy) [fm y [fm] 2 di-jet -2 0.006 d N 1.5 di-jet -4 0.004 1/ 1/N

-6 0.002 1 -8 0 -8 -6 -4 -2 0 2 4 6 8 x [fm] 0.5 di-jet production points 8 7

6 0 6 1.4 ]

4 -2 5 1.2 2 4 1

0 Ratio N/(dx dy) [fm y [fm] 3 2 0.8 -2 d

2 di-jet 0.6 -4 1/N 0 0.2 0.4 0.6 0.8 1 -6 1 AJ -8 0 -8 -6 -4 -2 0 2 4 6 8 x [fm]

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook The role of geometry

di-jet production points 8 0.016 di-jet asymmetry in PbPb 6 0.014 J ] A

-2 2.5 4 0.012 JEWEL+PYTHIA full geometry /d

2 0.01 N JEWEL+PYTHIA central production d 0 0.008 2 N/(dx dy) [fm y [fm] 2 di-jet -2 0.006 d N di-jet -4 0.004 1/ 1.5 1/N

-6 0.002 1 -8 0 -8 -6 -4 -2 0 2 4 6 8 x [fm] 0.5 di-jet production points 8 7

6 0 6 1.4 ]

4 -2 5 1.2 2 4 1

0 Ratio N/(dx dy) [fm y [fm] 3 2 0.8 -2 d

2 di-jet 0.6 -4 1/N 0 0.2 0.4 0.6 0.8 1 -6 1 AJ -8 0 -8 -6 -4 -2 0 2 4 6 8 x [fm]

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in p+p

1. recoil from extra emissions mostly from initial state & rst nal state emission

initial and final di-jet asymmetry in pp J

A 9 JEWEL+PYTHIA reconstructed jets /d 8 N JEWEL+PYTHIA ME+PS level d 7

di-jet 6 N

1/ 5 4 3 2 1 0 1.4 1.2 1 Ratio 0.8 0.6 0 0.2 0.4 0.6 0.8 1 AJ

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in p+p

1. recoil from extra emissions mostly from initial state & rst nal state emission

2. jet p⊥ < parton p⊥ due to incomplete reconstruction radiation outside reconstructed jet & multi-jet congurations

I one parton fragments softer more radiation, lower p⊥ particles I typically, this is the sub-leading parton I more particles lost when clustering jets lower p⊥ jet → p loss of quark jets in pp γ-jet events in JEWEL+PYTHIA ⊥ ⊥ (in) p

/ 0.2 ⊥ p ∆

0.15

(in) 0.1 25 GeV < p < 50 GeV 50 GeV < p(in)⊥ < 100 GeV 100 GeV < p⊥(in) < 150 GeV (in)⊥ 0.05 150 GeV < p < 200 GeV 200 GeV < p(in)⊥ < 250 GeV 250 GeV < p(in)⊥ < 300 GeV ⊥ 0 0 0.2 0.4 0.6 0.8 1 m(in)/p(in) ⊥ Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in p+p

1. recoil from extra emissions mostly from initial state & rst nal state emission

2. jet p⊥ < parton p⊥ due to incomplete reconstruction radiation outside reconstructed jet & multi-jet congurations

I one parton fragments softer more radiation, lower p⊥ particles I typically, this is the sub-leading parton I more particles lost when clustering jets lower p⊥ jet →

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in p+p

1. recoil from extra emissions mostly from initial state & rst nal state emission

2. jet p⊥ < parton p⊥ due to incomplete reconstruction radiation outside reconstructed jet & multi-jet congurations

I one parton fragments softer more radiation, lower p⊥ particles I typically, this is the sub-leading parton I more particles lost when clustering jets lower p⊥ jet →

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in p+p

1. recoil from extra emissions mostly from initial state & rst nal state emission

2. jet p⊥ < parton p⊥ due to incomplete reconstruction radiation outside reconstructed jet & multi-jet congurations

I one parton fragments softer more radiation, lower p⊥ particles I typically, this is the sub-leading parton I more particles lost when clustering jets lower p⊥ jet →

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in p+p

1. recoil from extra emissions mostly from initial state & rst nal state emission

2. jet p⊥ < parton p⊥ due to incomplete reconstruction radiation outside reconstructed jet & multi-jet congurations

I one parton fragments softer more radiation, lower p⊥ particles I typically, this is the sub-leading parton I more particles lost when clustering jets lower p⊥ jet →

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in p+p

1. recoil from extra emissions mostly from initial state & rst nal state emission

2. jet p⊥ < parton p⊥ due to incomplete reconstruction radiation outside reconstructed jet & multi-jet congurations

I one parton fragments softer more radiation, lower p⊥ particles I typically, this is the sub-leading parton I more particles lost when clustering jets lower p⊥ jet →

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Contributions to di-jet asymmetry in A+A

1. recoil from extra emissions: same as in p+p hard structures are not perturbed by medium eects 2. hardness of fragmentation:

I jets with soft fragmentation lose more p⊥ due to rescattering amplies initial asymmetry due to vacuum-like fragmentation → 3. energy loss uctuations number of extra emissions is O(1)

p loss of quark jets in PbPb γ-jet events in JEWEL+PYTHIA ⊥ ⊥ (in)

p 0.5 / ⊥ p

∆ 0.4

0.3

0.2 25 GeV < p(in) < 50 GeV 50 GeV < p(in)⊥ < 100 GeV ⊥(in) 0.1 100 GeV < p < 150 GeV 150 GeV < p(in)⊥ < 200 GeV (in)⊥ 0 200 GeV < p < 250 GeV 250 GeV < p(in)⊥ < 300 GeV ⊥ 0 0.2 0.4 0.6 0.8 1 m(in)/p(in) ⊥ Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Outline

Introduction: jets and jet quenching

Jet quenching in JEWEL

The di-jet asymmetry and the role of uctuations

Jet sub-structure and the medium's response to jets

Outlook

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Soft Drop: measuring the splitting function?

Dasgupta, Fregoso, Marzani, Salam, JHEP 1309 (2013) 029 Larkoski, Marzani, Soyez, Thaler, JHEP 1405 (2014) 146

I Soft Drop procedure: identies hardest 2-prong structure in a jet

min(p⊥,1, p⊥,2) I groomed shared momentum fraction zg = p⊥,1 + p⊥,2 1 calculation: P(zg ) + P( zg ) I p(zg ) = 1 2 − Θ(zg zcut) R / dz P(z) + P(1 z) − zcut −

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Soft Drop: measuring the splitting function?

Dasgupta, Fregoso, Marzani, Salam, JHEP 1309 (2013) 029 Larkoski, Marzani, Soyez, Thaler, JHEP 1405 (2014) 146

I Soft Drop procedure: identies hardest 2-prong structure in a jet

min(p⊥,1, p⊥,2) I groomed shared momentum fraction zg = p⊥,1 + p⊥,2 1 calculation: P(zg ) + P( zg ) I p(zg ) = 1 2 − Θ(zg zcut) R / dz P(z) + P(1 z) − zcut − � ������� �������� �������� ������++� �� ��� ���

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σ ��� (2015) no.11, 111501 �� > ���� ���

� 91 ��� �

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�� Larkoski, Marzani, Thaler, Phys. Rev. D Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Soft Drop: measuring the splitting function?

Dasgupta, Fregoso, Marzani, Salam, JHEP 1309 (2013) 029 Larkoski, Marzani, Soyez, Thaler, JHEP 1405 (2014) 146

I Soft Drop procedure: identies hardest 2-prong structure in a jet

min(p⊥,1, p⊥,2) I groomed shared momentum fraction zg = p⊥,1 + p⊥,2 1 calculation: P(zg ) + P( zg ) I p(zg ) = 1 2 − Θ(zg zcut) R / dz P(z) + P(1 z) − zcut − � ������� �������� �������� ������++� �� ��� ���

� �� = ���� β= � �� > �� ��� > Can this be used to measure the � �σ �� ��� ��� I � �� > ��� ���

σ ��� (2015) no.11, 111501 eective splitting function in �� > ���� ��� � 91 ��� medium? �

� ��� ��� ��� ��� ��� ��� ���

�� Larkoski, Marzani, Thaler, Phys. Rev. D Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook For experts: The Soft Drop algorithm

Soft Drop/modied Mass Drop Tagger algorithm:

1. cluster jet with anti-k⊥ 2. re-cluster with Cambridge/Aachen (based on angles)

3. undo last clustering step and compute zg and ∆R12 β 4. if zg > zcut(∆R12/R) stop else reject softer prong and go back to 3

Larkoski, Marzani, Soyez, Thaler, JHEP 1405 (2014) 146

The CMS measurement:

I zcut = 0.1, β = 0

I R = 0.4

I CMS analysis requires also ∆R12 > 0.1 removes large fraction of jet sample

CMS-HIN-16-006

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook

Reminder: zg distribution in A+A

-1 µ -1 sNN = 5.02 TeV, pp 27.4 pb , PbPb 404 b η β ∆ 1.6 Centrality: 0-10%1.6 anti-k R = 0.4, | | < 1.3 1.6 Soft Drop = 0, z = 0.1, R12 > 0.1 CMS T jet cut 140 < p < 160 GeV 160 < p < 180 GeV 180 < p < 200 GeV 1.4 T,jet 1.4 T,jet 1.4 T,jet Data 1.2 1.2 1.2

1 1 1

PbPb/pp 0.8 PbPb/pp 0.8 PbPb/pp 0.8

0.6 JEWEL 0.6 0.6 Coherent antenna BDMPS 2 0.4 q = 1 GeV/fm , L = 5 fm 0.4 0.4 q = 2 GeV/fm2, L = 5 fm 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 1.6 1.6 1.6 zg zg zg 200 < p < 250 GeV 250 < p < 300 GeV 300 < p < 500 GeV 1.4 T,jet 1.4 T,jet 1.4 T,jet

1.2 1.2 1.2

1 1 1

PbPb/pp 0.8 PbPb/pp 0.8 PbPb/pp 0.8

0.6 0.6 SCET 0.6 Chien-Vitev HT q = 4 GeV/fm2 0 0.4 0.4 g = 1.8 Coherent 0.4 g = 2.2 Incoherent 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 zg zg zg

CMS-HIN-16-006 Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Medium's response to energy deposited by jets

I common assumption: immediate thermalisation I JEWEL: three options

1. ignore recoiling thermal partons 2. extract source term for hydrodynamic description of medium Flörchinger, Zapp, EPJC 74 (2014) no. 12, 3189 3. include recoiling partons

I recoiling partons becomes colour neighbour of hard parton I recoiling partons do not re-interact other limiting case I have so subtract thermal component of recoil momentum Kunnawalkam Elayavalli, Zapp, JHEP 1707 (2017) 141

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Medium response: practical considerations

jet

background

I ideal situation: at background  can be subtracted

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Medium response: practical considerations

jet

background

I ideal situation: at background  can be subtracted

I more realistic: uctuating background  can be subtracted on average, have to unfold

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Medium response: practical considerations

jet correlated jet background

background background

I ideal situation: at background  can be subtracted

I more realistic: uctuating background  can be subtracted on average, have to unfold

I adding medium response: correlated background

I part of the background is correlated with jet medium response → I activity above uncorrelated background I correlated background cannot and should not be subtracted

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Medium response: practical considerations

jet correlated jet background

background background

I ideal situation: at background  can be subtracted I more realistic: uctuating background  can be subtracted on average, have to unfold

I adding medium response: correlated background

I part of the background is correlated with jet medium response → I activity above uncorrelated background I correlated background cannot and should not be subtracted I nally: also uctuations in correlated part of background matter

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook zg distribution in A+A Milhano, Wiedemann, Zapp, Phys. Lett. B 779 (2018) 409

JEWEL+PYTHIA Pb+Pb (0 10%) √s = 5.02 TeV JEWEL+PYTHIA Pb+Pb (0 10%) √s = 5.02 TeV − NN − NN

g 9 12 z p+p p+p R /d 8 without medium response ∆ without medium response J 10 N anti-k R=0.4 jets /d anti-k R=0.4 jets J d 7 jet ⊥ jet ⊥ J p > 140 GeV N p > 140 GeV d N ⊥ ⊥ J 8 6 SoftDrop zcut = 0.1; β = 0; ∆R12 > 0.1 SoftDrop zcut = 0.1; β = 0 1/ N

5 1/ 6 4 3 4 2 2 1 0 0 1.4 1.4 1.2 1.2 1 1

PbPb/pp 0.8 PbPb/pp 0.8 0.6 0.6 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 zg ∆R12

I w/o medium response: jets get narrower

broad jets more eected by medium more likely to fail p⊥ cuts → → also seen in other observables

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook zg distribution in A+A Milhano, Wiedemann, Zapp, Phys. Lett. B 779 (2018) 409

JEWEL+PYTHIA Pb+Pb (0 10%) √s = 5.02 TeV JEWEL+PYTHIA Pb+Pb (0 10%) √s = 5.02 TeV − NN − NN

g 9 12 z p+p p+p R /d 8 without medium response ∆ without medium response J 10 N with medium response /d with medium response J d 7 J anti-k R=0.4 jets N anti-k R=0.4 jets d N jet ⊥ jet ⊥ 6 p > 140 GeV J 8 p > 140 GeV 1/ ⊥ N ⊥ SoftDrop zcut = 0.1; β = 0; ∆R12 > 0.1 SoftDrop zcut = 0.1; β = 0 5 1/ 6 4 3 4 2 2 1 0 0 1.4 1.4 1.2 1.2 1 1

PbPb/pp 0.8 PbPb/pp 0.8 0.6 0.6 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 zg ∆R12

I w/o medium response: jets get narrower

I w/ medium response: additional component with large ∆R12 & small zg

I additional p⊥ from medium response promotes very asymmetric congurations above zcut

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Quantising contribution from medium response

parton level, 4-momentum subtraction i ⊥ (jet)

p JEWEL+PYTHIA Pb+Pb, entire jet

/ 0.5 JEWEL+PYTHIA Pb+Pb, leading sub-jet ⊥ (rec) JEWEL+PYTHIA Pb+Pb, sub-leading sub-jet p

h ( ) 0.4 anti-k R=0.4; p J > 140 GeV ⊥ ⊥

0.3

0.2

0.1

0 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 zg

I this is parton level I average fraction of sub-jet p⊥ coming from medium response I much more important for softer prong I more important at low zg

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Relating this to an observable

JEWEL+PYTHIA Pb+Pb (0 10%) √s = 5.02 TeV − NN

g 25

/d p+p

NJ with medium response d 20 J without medium response N anti-k R=0.4 1/ 15 ⊥ P p ∆R g = i ⊥,i iJ (J) 10 p⊥

5

0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 g

I rst radial moment of p⊥ distribution in jet I w/o medium response g decreases  jets get narrower

I w/ medium response g increases again  contribution from recoils mostly at large ∆R

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Sub-jet girth

JEWEL+PYTHIA Pb+Pb (0 10%) √s = 5.02 TeV JEWEL+PYTHIA Pb+Pb (0 10%) √s = 5.02 TeV − NN − NN i i

g p+p g p+p 0.08 with medium response 0.1 with medium response without medium response without medium response

sub-jet 0.07 sub-jet h h 0.08 0.06

0.05 0.06 0.04 anti-k R=0.4 jets 0.04 anti-k R=0.4 jets 0.03 jet jet p >⊥140 GeV p >⊥140 GeV 0.02 SoftDrop⊥ z = 0.1; β = 0; ∆R > 0.1 SoftDrop⊥ z = 0.1; β = 0; ∆R > 0.1 cut 12 0.02 cut 12 0.01 leading prong sub-leading prong 0 0 1.4 1.4 1.2 1.2 1 1

PbPb/pp 0.8 PbPb/pp 0.8 0.6 0.6 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 zg zg

I w/o medium response sub-jet girth decreases everything gets narrower

I w/ medium response sub-jets get broader

I largest increase in softer prong at low zg I this is an observable and maybe it can be measured. . .

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Outline

Introduction: jets and jet quenching

Jet quenching in JEWEL

The di-jet asymmetry and the role of uctuations

Jet sub-structure and the medium's response to jets

Outlook

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Conclusions

I In heavy ion collisions a dense and apparently equilibrated, strongly interacting system is produced.

I Jet quenching is the partial thermalisation of a far-from-equilibrium system in this medium.

I Jets are a calibrated probe of equilibration.

I Increasingly dierential measurements require advanced tools capable of describing multi-particle nal states.

I JEWEL is a fully dynamical model for jet quenching based on pQCD.

I Examples for how of it can be used to understand jet quenching:

I di-jet asymmetry and the role of uctuations, I jet sub-structure and medium response.

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Outlook

Large systems:

I future of LHC running: more sub-structure, more boson-jet, more rare congurations

I jet sub-structure needed to understand jet modications

I and medium modications

I requires appropriate tools

Small systems:

I small systems share many similarities with large systems in soft sector

I but hard probes seem to be unmodied

I understand origins of soft signatures

I study jet quenching in small systems

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Role of JEWEL

I very little known for certain about jet quenching hard to make denitive statements based on a single model → all the more important that theoretical ideas can be tested/falsied → I JEWEL makes minimal assumptions & is linked to clear physical picture can make signicant contributions to central questions → Do jets resolve quasi-particles in the medium? or At what scale are quasi-particles resolved?

I at soft scale medium does not support quasi-particle description

I high enough momentum transfers resolve quasi-particle structure

I in this case expect power-law tail in scattering cross section

I need a dynamical model JEWEL →

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook

Deciphering jet quenching with JEWEL K. Zapp Introduction JEWEL Role of uctuations Medium's response to jets Outlook Event generation

I jet production MEs & ISR: PYTHIA Sjostrand et al., JHEP 0605 26

I nuclear PDFs: EPS09 Eskola, Paukkunen & Salgado, JHEP 0904 (2009) 065

I jet evolution in medium: JEWEL I medium: do whatever you like, e.g.

I geometry: Glauber model Eskola et al., Nucl. Phys. B 323 37 distribution of jets and temperature prole 3 4 I EOS: ideal quark-gluon gas n T &  T ⇒ ∝ ∝ I boost-invariant longitudinal expansion Bjorken, PRD 27 (1983) T (τ) τ −1/3 n(τ) τ −1 & (τ) τ −4/3 ⇒ ∝ ⇒ ∝ ∝ I initial conditions: Ti = 486 MeV at τi = 0.6 fm for √sNN = 2.76 TeV Shen and Heinz, Phys. Rev. C 85 (2012) 054902

I hadronisation: PYTHIA string fragmentation

Deciphering jet quenching with JEWEL K. Zapp