Photon-Tagged Jet Fragmentation Functions and Jet Shapes in Pp and Pbpb Collisions with the CMS Detector

Photon-tagged jet fragmentation functions and jet shapes in pp and PbPb collisions with the CMS detector

Kaya Tatar Massachusetts Institute of Technology for the CMS Collaboration

Hard Probes 2018, Aix-Les-Bains, France October 2, 2018

Kaya Tatar 1 Hard Probes 2018 Introduction Study modification of parton shower JHEP 05 (2018) 006 Gives info about the dynamics of hot QCD matter Tools : Jet fragmentation function (FF) ● Longitudinal distribution of momentum Jet shapes (JS) ● Distribution of jet energy in transverse direction FF and JS provide different, but complementary info jet

r EPJC 77 (2017) 379 p p / b P p h b z ~ T P p jet T z Kaya Tatar 2 Hard Probes 2018 Inclusive jet vs photon+jet

Photon+jet Inclusive jet Photon-tag controls initial state Compares samples with different initial states Produced partons : quark fraction enhanced Produced partons : mix of quarks and gluons -- > Probe quark jet modification -- > Insight for gluon modification when combined with inclusive jet

PRC 90 (2014) 024908

Enhancement

Depletion

(high ξ – > low-p particle) T arXiv:1809.08602 Kaya Tatar 3 Hard Probes 2018 Observables : ξjet

● Take tracks (charged particles) inside the jet cone.

● Project the track momentum to jet axis. γ ● Divide jet momentum by the projected track momentum.

● The natural log of this ratio is called ξjet.

: 3-momentum vector of the jet

: 3-momentum vector of the track

● Based on final jet energy (after quenching) ● Measured previously using inclusive jets

projected to jet axis

Kaya Tatar 4 Hard Probes 2018 γ Observables : ξT

● Take tracks (charged particles) inside the jet cone.

● Construct transverse momentum vectors for track and photon γ

● Invert the track transverse momentum

● Follow the same logic as for ξjet.

: transverse momentum vector of the photon

: transverse momentum vector of the track

● Based on photon energy ● Measured for the first time for reconstructed jets

- projected to axis

Kaya Tatar 5 Hard Probes 2018 Object Selections Photons Jets Tracks JHEP 04 (2017) 039 γ p > 60 GeV/c trk T anti-kT, R=0.3 pT > 1 GeV/c γ |η | < 1.44 jet pT > 30 GeV/c ΔR(jet, track) < 0.3 |ηjet| < 1.6 Bkg tracks subtracted via MB event mixing Δφ(photon, jet) > 7π /8 inclusive jets, bkg jets subtracted via MB event mixing

Background sources Tracks from underlying event (UE) –> Subtracted via Min Bias event mixing Mis-identified (fake) jets –> Subtracted via Min Bias event mixing photons from neutral meson decays rejected using shower shape cut, remaining bkg fraction estimated via template fit Kaya Tatar 6 Hard Probes 2018 Background subtraction for tracks

arXiv:1801.04895 isolated-photon+jet event

Raw tracks γ inside jet cone

MB event Bkg tracks inside jet cone

γ Raw – Bkg (Bkg track subtracted)

Kaya Tatar 7 Hard Probes 2018 jet γ arXiv:1801.04895 Results : ξ vs ξT

ξjet ● Based on reconstructed jet energy (energy after quenching)

Kaya Tatar 8 Hard Probes 2018 jet γ arXiv:1801.04895 Results : ξ vs ξT

jet γ ξ ξT ● Based on reconstructed jet energy (energy after quenching) ● Based on photon energy jet ● shifted to left compared to γ ξ ξT ● Out-of-cone radiation, photon+multijet

Kaya Tatar 9 Hard Probes 2018 jet γ arXiv:1801.04895 Results : ξ vs ξT

jet γ ξ ξT Transition at jet ≈ 2.5 and γ ≈ 3 –> p trk ≈ 3 GeV ξ ξT T Central PbPb collisions – > enhancement of low-p particles and a depletion of high-p particles T T γ modified stronger compared to jet ξT ξ Kaya Tatar 10 Hard Probes 2018 jet γ arXiv:1801.04895 ξ and ξT vs Theory

SCET (JHEP 11 (2016) 155) G Hybrid (JHEP 1410 (2014) 019, JHEP 1603 (2016) 053 ) ● Weak coupling : high-Q² processes using pQCD ● Framework decomposing Soft Collinear and Glabuer models ● Strong coupling : low-Q² interactions between CoLBT-hydro (Phys. Lett. B, 777 (2018) 86) parton shower and medium ● Couples LBT for jet evolution with (3+1)D hydrodynamics ● Weak and strong coupling are combined ● Combines pQCD approach with hydro simulation of medium

Turnover at jet ≈ 2.5 and γ ≈ 3 –> p trk ≈ 3 GeV ξ ξT T Large enhancement from particles after turnover -- > Models tend to underpredict this -- > Medium response important for CoLBT and Hybrid

Other ingredients considered by theory ● Medium-induced radiation ● Effects of medium on hadronization Kaya Tatar 11 Hard Probes 2018 γ-tagged jet shape arXiv:1809.08602

ρ(r) normalized to unity over r < 0.3

Results are corrected for detector resolution, particle reco.

pp results are NOT smeared

Central PbPb collisions – > a larger fraction of jet energy at large distances from the jet axis.

Kaya Tatar 12 Hard Probes 2018 inclusive vs γ-tagged jet shape JHEP 05 (2018) 006 arXiv:1809.08602

Inclusive jets 3

2 ρ(r) normalized to unity over r < 1.0 (inclusive jets) Enhancement r < 0.3 (γ+jet) 1 Depletion

SCET [JHEP 05 (2016) 023] γ+jet : G Larger enhancement at large r. Smaller depletion at intermediate r. LBT [Phys. Lett. B, 782 (2018) 707] ● Increased quark fraction (70-80%) ? Models describe data well. ● Lower jet p threshold (higher fraction of quenched jets) ? T

Kaya Tatar 13 Hard Probes 2018 Summary

● FF and jet shapes (JS) measured for jets tagged with isolated-photons.

– Constrains the initial parton kinematics and probes quark-jet modification.

● FF modification – > excess of low-pT particles and depletion of high-pT particles inside the jet cone.

– FF observable wrt photon energy – > robust measurement, larger modification

● JS modification – > a larger fraction of jet energy is carried at large distances from the jet axis.

– No large depletion at intermediate distances.

● Models seem to describe both longitudinal and transverse jet structure.

arXiv:1801.04895 arXiv:1809.08602

Acknowledgements : The MIT group's work was supported by US DOE-NP. Kaya Tatar 14 Hard Probes 2018 BACKUP

Kaya Tatar 15 Hard Probes 2018 Inclusive jet shape JHEP 05 (2018) 006

ρ(r) normalized to unity over r < 1. 3

2 Enhancement

1 Depletion

Kaya Tatar 16 Hard Probes 2018 Inclusive jet shape Phys. Lett. B 730 (2014) 243

Kaya Tatar 17 Hard Probes 2018 Bkg subtraction for jets

γ Raw jets (bkg track subtracted)

MB event

Bkg jets (bkg track subtracted)

γ Raw – Bkg (bkg track and bkg jet subtracted)

Kaya Tatar 18 Hard Probes 2018 Analysis – bkg photons

● Observables are constructed using photons, jets and tracks.

Neutral meson decay

h0 ->γγ

Phys. Lett. B 785 (2018) 14

Background source photons from neutral meson decays ● rejected with shower shape cut ● 2 photons are reconstruced as single with a wider shower shape prompt ● dominates the sideband region : 0.011 < σ < 0.017 ηη + h0 ->γγ h0 ->γγ Energy weighted width of shower : σ ηη

shower shape Kaya Tatar 19 Hard Probes 2018 Background from photons

● σηη < 0.01 selects narrow shower shape, supresses background from neutral meson decays, however there is still contamination.

● Purity = fraction of the prompt photons among candidates

– Estimated using template fit method. Fit the distribution for σηη < 0.01 with Signal (prompt photon) template from MC with isolated photon events arXiv:1801.04895 Bkg (neutral meson) template from non-isolated photons in data

Phys. Lett. B 785 (2018) 14 s e i r t n isolated E non-isolated

Σ iso = 2R (tot energy in a cone of R=0.4 around the photon) - (ave. energy from a strip of 2π x 2R) 2π Kaya Tatar 20 Hard Probes 2018 Smearing jet spectra

● Jet energy resolution and jet angular resolution JINST 12 (2017) P10003 differ between pp and PbPb due to underlying event – Estimate relative resolution between pp and PbPb using simulations – Smear jet spectra in pp using this relative resolution

● Smearing jet energy – Parametrize jet energy resolution via

RECO 2 2 pT 2 S N σ = C + + pGEN pGEN ( pGEN)2 ( T ) √ T T – Fit C, S and N parameters and apply relative resolution via (S2 −S2 ) (N 2 −N 2 ) σ = (C 2 −C2 )+ PbPb pp + PbPb pp rel PbPb pp pGEN ( pGEN )2 √ T T ● Smearing jet azimuthal angle

2 2 – Use same parametrization as in jet energy RECO GEN 2 S N σ (|φ −φ |)= C + + resolution pGEN (pGEN )2 √ T T – Apply relative resolution in the same fashion

Kaya Tatar 21 Hard Probes 2018 γ-tagged jet FF - ξjet arXiv:1801.04895

: 3-momentum vector of the jet : 3-momentum vector of the track

Kaya Tatar 22 Hard Probes 2018 γ arXiv:1801.04895 γ-tagged jet FF - ξT

: transverse mom. vector of the photon : transverse mom. vector of the track

Kaya Tatar 23 Hard Probes 2018 γ-tagged jet shape arXiv:1809.08602

ρ(r) normalized to unity over r < 0.3.

Results are corrected for detector resolution, particle reco.

pp results are NOT smeared.

Kaya Tatar 24 Hard Probes 2018 γ-tagged jet shape : pp vs MC

arXiv:1809.08602

Kaya Tatar 25 Hard Probes 2018