Jet Substructure at the LHC Wouter Waalewijn LANL - January 8, 2015 - PowerPoint PPT Presentation

Jet Substructure at the LHC Wouter Waalewijn LANL - January 8, 2015

Outline • Introduction • Jet Charge • Jet Mass • Hadronization of Jets • Quark/Gluon Discrimination • Conclusions

Introduction

What is a Jet? Energetic quarks and gluons Produce jets of hadrons radiate and hadronize q q ¯ q q g g ¯ q q q g q g g g g g 4

Jet Algorithms • Repeatedly cluster nearest “particles” p i , p j → p i + p j • Cut off by jet “radius” R distance = ( ∆ y ) 2 + ( ∆ φ ) 2 p T Azimuthal angle 2 o 15 o 90 o φ Rapidity y 5

Jet Algorithms • Repeatedly cluster nearest “particles” p i , p j → p i + p j • Cut off by jet “radius” R • Default at LHC: anti- k T (Cacciari, Salam, Soyez) p T p T Azimuthal angle Azimuthal angle 2 o 15 o 90 o φ Rapidity φ Rapidity y y (arXiv:0802.1189) 6

Jets at the LHC • Most measurements involve jets as signal or background 7

Jet Cross Sections • Bin by jet multiplicity to improve background rejection 
 × 10 0 0 Events / bin Events/bin (b) All jets, e µ → 30 H → WW ATLAS Prelim. H WW* -1 ∫ � s = 8 TeV, L d t = 20.3 fb 20 Obs stat ± DY � Exp syst ± Top Higgs 10 VV � Misid WW 0 � 0 1 2 3 4 5 6 7 (ATLAS-CONF-2013-030) Number of jets n (ATLAS-CONF-2014-060) j • Large logarithms lead to large theory uncertainties no jets above this p T ln 2 p cut σ ( H + 0 jets) ∝ 1 − 6 α s T + . . . m H π (Berger, Marcantonini, Stewart, Tackmann, WW; Banfi, Monni, Salam, Zanderighi, Becher, Neubert, Rothen; Stewart, Tackmann, Walsh, Zuberi; Liu, Petriello; Boughezal, Focke, Li, Liu; Jaiswal, Okui, …)

Jet Substructure for Boosted Objects • New heavy particles could produce boosted top, W, Higgs 
 decay products lie within one “fat” jet • Distinguish from QCD jets using jet substructure • Avoids combinatorial background Hadronic decay of top quark (ATLAS-CONF-2013-052) 9

Top Tagging in Z 0 → t ¯ t 20 Efficiency [%] ) [pb] s = 8 TeV Obs. 95% CL upper limit Obs. 95% CL upper limit ATLAS Preliminary Simulation 18 3 10 Exp. 95% CL upper limit Exp. 95% CL upper limit s =8 TeV t -1 L dt = 14.3 fb t ∫ 16 Exp. 1 Exp. 1 uncertainty uncertainty → σ σ + jets, combined µ BR(Z’ Exp. 2 Exp. 2 σ σ uncertainty uncertainty + jets, boosted 2 14 µ 10 Leptophobic Z’ (LO x 1.3) Leptophobic Z’ (LO x 1.3) e + jets, combined 12 ATLAS Preliminary e + jets, boosted × 10 Z’ 10 σ 8 1 6 -1 10 4 2 -2 10 0 0 0.5 1 1.5 2 2.5 3 3.5 0.5 1 1.5 2 2.5 3 m [TeV] Z’ mass [TeV] t t (ATLAS-CONF-2013-052) b • One leptonic and one hadronic top ν Z 0 ¯ t ` • Boosted analysis crucial for large ¯ m Z 0 u d ¯ b 10

Jet Substructure for Quark/Gluon Discrimination • New physics often more quarks than QCD backgrounds • Extensive Pythia study (Gallicchio, Schwartz) • Charged track multiplicity and jet “girth” are good 
 group of 5 Gluon Rejection Gluon Rejection best pair p i charge * girth 3 ( y i − y J ) 2 + ( φ i − φ J ) 2 X p 10 T girth = charged mult R=0.5 p J subjet mult R sub =0.1 Gluon Rejection � T girth R=0.5 i ∈ jet Gluon rejection optimal kernel 1st subjet R=0.5 avg k T of R sub =0.1 2 • More variables only give 
 10 mass/Pt R=0.3 decluster k T R sub =0.1 jet shape Ψ (0 . 1) marginal improvement | pull | R=0.3 planar flow R=0.3 10 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Quark Jet Acceptance Quark acceptance (arXiv:1106.3076) 11

Events/0.14e ATLAS Preliminary Jet Mass and Charge 500 ∫ -1 s = 8 TeV, L dt = 5.8 fb + µ Data 2012 400 - Data 2012 µ =0.3 κ + MC@NLO t t µ - µ MC@NLO t t + Motivation: Background (MC) µ 300 - Background (MC) µ 200 • Measured at the LHC Data/MC 100 • Benchmark for our ability 
 0 0 -2 0 2 Jet charge [e] (ATLAS-CONF-2013-086) to calculate substructure • Test and improve Monte Carlo: 
 GeV ∫ ATLAS 1 -1 2010 Data, L = 35 pb 0.025 anti-k R=1.0 Herwig and Pythia differ Systematic unc. t d m σ d 300 < p < 400 GeV Total unc. T 0.02 N = 1, |y| < 2 1 σ Pythia PV Herwig++ 0.015 0.01 0.005 0.2 0 0 20 40 60 80 100 120 140 160 180 200 Jet mass [GeV] (arXiv:1203.4606)

Jet Charge Krohn, Lin, Schwartz, WW (arXiv:1209.2421) 
 WW (arXiv:1209.3091)

Defining Jet Charge (1 / σ ) d σ /dQ κ [e − 1 ] ⇣ p i Pythia ⌘ κ X T Q κ = Q i κ = 0 . 5 p J T i ∈ jet (Feynman, Field) Q κ [ e ] � • If too small: sensitive to soft hadrons contamination → κ • If too large: only sensitive to most energetic hadron 
 κ need more statistics 14