Top tagging at CMS. Torben Dreyer on behalf of the CMS Collaboration. BOOST 2017, Buffalo

Transcription

1 Torben Dreyer on behalf of the CMS Collaboration BOOST 2017, Buffalo

2 Motivation LHC at s = 13 TeV More top quarks with high momentum (Standard Model and new physics) Top tagging more important than ever Understanding in data crucial Measurement of top tagging efficiencies Study performance of new algorithms Studies of top tagging in CMS [CMS-DP ] 2

3 Outline Top tagging validation in 2016 data [CMS-DP ] Top tagging with Energy Correlation Functions [CMS-PAS-EXO ] Boosted Event Shape Tagger (BEST) [CMS-DP ] XCone Studies [CMS-DP ] Studies of top tagging in CMS 3

4 Top tagging validation in 2016 data CMS-DP

5 CMS default top tagger Based on: Groomed jet mass (mmdt / Soft drop) Ungroomed N-subjettiness ratio τ3/τ2 Subjet b-tagging anti-kt jets with R = 0.8 (AK8) Pileup correction: Charged Hadron Subtraction or PUPPI Different N-subjettiness working points with and without subjet b-tag [CMS-PAS-JME ] 5

6 Validation in data ν Muon+jets tt selection W High pt muon p Large boost expected t lepton b-jet p t use non isolated muons top jet probe jet re he e p r is m ph e e is ch ni hem o ic dr ha ton p le φ = 2/3 π b-jet μ Tag and probe method Leptonic decay leg as tag B-tag in leptonic hemisphere Probe jet: AK8 (CA15) jet in hadronic hemisphere 6

7 Validation in 2016 data Soft drop groomed AK8 jets Different categories for tt simulation (matching to generator particles) Merged top Merged W Merged b+light quark Not merged Template fit soft drop mass distribution new method scale factors for different tt categories [CMS-DP ] 7

8 Kinematic distributions [CMS-DP ] 8

9 Softdrop mass pass and fail region Fit in pass and fail regions Fail: τ3/τ2 > 0.54 Pass: τ3/τ2 < 0.54 [CMS-DP ] 9

10 Efficiency measurement [CMS-DP ] Efficiency: ε = Npass/(Npass+Nfail) For each tt contribution Scale factors: sf = εdata/εsimulation Fits performed in different pt bins Full set of systematic uncertainties studied New for 2016 data JEC, JER, pileup, muon/trigger scale factors, b-tagging, PDF, and scale uncertainties 10

11 Mass cut efficiency Mass cut not included for the fit Tails important for semi-merged and not merged contributions Extra mass cut efficiency εmass = Npass, mass cut/npass For data use data distribution directly Post fit MC (all categories) for simulation Total efficiency: ε = εno mass εmass [CMS-DP ] 11

12 Efficiency and scale factors [CMS-DP ] 12

13 Top tagging with Energy Correlation Functions CMS-PAS-EXO

14 ECF BDT tagger Developed in the context of a dark matter search [CMS-PAS-EXO ] Use of energy correlation functions in CA15 jets e.g. 3 point ECF: [CMS-PAS-EXO ] with: ECF ratios proposed in [arxiv: ] similar to τ3/τ2 several dimensionless ECFs ratios show separation power [CMS-PAS-EXO ] BDT with 11 ECF ratios, frec, and τ32sd 14

15 ECF BDT validation in 2016 data Validation on top jets in lepton+jets tt decays and quark/gluon jets in Z+jets Good performance in data [CMS-PAS-EXO ] [CMS-PAS-EXO ] 15

16 Boosted Event Shape Tagger (BEST) CMS-DP

17 Boosted Event Shape Tagger Heavy particle jet classification and separation to light jets Targets top, H, W, Z Idea: go to different hypothesized reference frames Isotropic in correct rest frame N-prong structure Neural network for classification Based on [Phys. Rev. D 94, (2016)] First discussed at BOOST 2016 lab frame top rest frame 17

18 Input observables Fox-Wolfram Moments: Eigenvalues of Sphericity tensor: [CMS-DP ] p To Longitudinal Jet Asymmetry: Recluster constituents AK4 jets Hi gg s e fra m e [CMS-DP ] m fra 18

19 Neural network Neural network for classification 41 input observables Simultaneous separation between top, W, H, Z, and light flavor 5 output nodes Each node optimized for another particle classification Visualization in 2D plane [CMS-DP ] Best separation for top and Higgs 19

20 Tagging efficiency for top quarks Top tagging efficiency Based on msd, τ32, b-tag, BEST NN output Working point with background efficiency ~1% [CMS-DP ] Stable as a function of jet pt and number of pileup vertices [CMS-DP ] 20

21 XCone Studies CMS-DP

22 XCone Exclusive jet algorithm based on N-jettiness [I. W. Stewart, F. J. Tackmann et al., JHEP 1511 (2015) 072] N-jettiness: Measure how particles are distributed along N axis in the event Returns fixed number of jets Conical jets Jets stay separated in the boosted regime Smooth transition between resolved and [JHEP 1511 (2015) 072] boosted regime 22

23 Optimal number of jets Find the optimal N-jet to reconstruct the event Use N-jettiness Different methods studied Noptimal: [CMS-DP ] tt simulation μ+jets smallest N with τn τn-1 > -30 Reconstruct event with Noptimal XCone jets Can be used to count jets resolved and boosted regime 23

24 XCone jets Same kinematic cuts on XCone and AK4 jets similar jet multiplicity [CMS-DP ] tt simulation, μ+jets [CMS-DP ] [CMS-DP ] Similar behavior of jet pt and HT 24

25 Top mass from XCone jets Mass reconstructed from the 3 leading XCone, AK4 jets compared to AK8 Soft drop mass tt simulation, μ+jets [CMS-DP ] [CMS-DP ] Similar performance between AK4 and XCone for low pt Better performance by XCone in the boosted region 25

26 Summary Top tagging very important at 13 TeV Top tagging used and studied in 2016 data Jet substructure well described by simulation [CMS-DP ] Performance well understood in data and simulation New methods studied in simulation 26

27 Back up

28 13 TeV simulation studies CMS-PAS-JME

29 13 TeV simulation studies Studied variables: CMSTT v1 variables min. pairwise mass top mass HepTopTagger V2 (HTT V2) variables mass Groomed masses: Pruned, Filtered, Trimmed, Soft drop N-subjettiness ratio τ3/τ2 Qjets-volatility Shower deconstruction log(x) [CMS-PAS-JME ] 29

30 13 TeV simulation studies Different combinations studied Similar performance HTT V2 good performance for low pt Good performance for msd + N-subjettiness for high pt [CMS-PAS-JME ] 30

31 13 TeV simulation studies [CMS-PAS-JME ] msd + N-subjettiness (+ subjet b-tag) Steep turn on Stable in pt default option for high pt region Additional study of HepTopTagger V2 for low pt region 31

32 Validation in 2015 data CMS-PAS-JME

33 Validation in 2015 data AK8 jets with PUPPI applied pt > 400 GeV Soft drop mass and pt after selection [CMS-PAS-JME ] 33

34 Validation in 2015 data Loose top tag (τ3/τ2 < 0.8, 105 < msd < 210 GeV) Cut on respective variable omitted [CMS-PAS-JME ] 34

35 Hep Top Tagger V2 CA15 jets HTT V2 variables after loose top tag Cuts on respective variable omitted Good description of mass HTT V2 mass [CMS-PAS-JME ] and frec [CMS-PAS-JME ] [CMS-PAS-JME ] 35

36 2015 scale factors Efficiency: Scale factors: sf = εdata/εsimulation AK8 jets MSD + N-subjettiness [CMS-PAS-JME ] CA15 jets HTT V2 36

37 Use of top tagging in 2015 data

38 tt resonance search Top tagging used in search for tt resonances Lepton+jets and fully hadronic channel [JHEP 1707 (2017) 001] 38

39 Validation for tt resonances Well description of soft drop mass Lepton+jets and fully hadronic channel [JHEP 1707 (2017) 001] 39