Top-tagging at high jet multiplicity

Transcription

1 Top-tagging at high jet multiplicity Sebastian Schätzel Universität Heidelberg, Germany Using Jet Substructure Workshop University of Oregon, Eugene, USA April 2013 HEPTopTagger fat jet quantities mass p T multiplicity of substructure objects Thanks to Tilman Plehn and Michael Spannowsky for suggestions

2 The past clean back-to-back ttbar events 2

3 High jet multiplicity 4 tops from gluino pairs benchmark model: <pt distributions of tops> mass (GeV) g t LSP 100 3

4 Top Taggers prong-based 3 subjets in fat jet substructure kinematics compatible with t bw bqq fat jet shapes mass and pt of fat jet N-subjettiness... example: HEPTopTagger (mass windows) performance at high jet multiplicity? 4

5 The setup signal: Herwig background: QCD dijets, PYTHIA Delphes (0.1x0.1 cells) ECM = 14 TeV distributions scaled to 10 fb-1 not ATLAS simulation no pile-up 5

6 The jet environment SUSY 6

7 The jet environment QCD 7

8 Interlude: Pythia practical reason for using it: 8M events at hand for multi-jets better use multi-leg generator take efficiencies in present study with grain of salt final numbers from data 8

9 ±10% if no pile-up S.S., BOOST2012 9

10 with PU: up to 40% off, more in tails this is at low multiplicity! POWHEG slightly better 10

11 Master distributions SUSY QCD experimentally: ATLAS standard jets: anti-k T R=0.4 ntag n njet m integrated distributions 11

12 HEPTopTagger (HTT) fat jet (C/A R=1.5) Plehn et al., arxiv: HTT top candidate 4-momentum pt underlying event, pile-up I. identify hard subjets undo C/A clustering until all subjets after filtering II. 3 subjets compatible with top decay? a) Filtering: remove UE and PU min have m<50 GeV drop subjets that contribute less Δ R ij recluster with small R=min (0.3, ) 2 keep 5 hardest subjets than 20% to parent mass exclusively recluster constituents to 3 subjets b) call these subjets substructure objects p t = p apply kinematic cuts [ m ij m m 0.85 W,1.15 W m123 mt mt additional cuts (backup) any mij mw: pt,t > 200 GeV, 140 < mt < 200 GeV ] 12

13 HEPTopTagger efficiency significance improvement w.r.t. SUSY analysis without top quark identification more fake tops at high multiplicity (accidental mass window hits) akt04 jets w/pt>50 GeV 13

14 normal tight loose N filt jet mw window ±15% ±10% ±20% mcut (GeV) HEPTopTagger filtering max R filt jet 14

15 normal tight loose N filt jet mw window ±15% ±10% ±20% mcut (GeV) HEPTopTagger filtering max R filt jet tight > normal > loose 2 tags > 1 tag 15

16 Unintegrated efficiencies SUSY QCD x3 x3 x1.5 x3 16

17 Conclusions HEPTopTagger high multiplicity: larger chance to hit mass windows accidentally 2nd tag: need 3 more jets tight filtering reduces fake rate improves S/B 17

18 Fat jet pt and top pt keep keep QCD configurations passing HTT are more signal-like 18

19 Leading fat jet mass keep 19

20 Sum of fat jet masses keep 20

21 Number of fat jets QCD SUSY 21

22 Improvement in significance w.r.t. SUSY analysis without top quark identification pt > m several fat jets > 1 fat jet 22

23 Hard substructure in fat jet HTT breakdown of fat jet into substructure objects (80% mass-drop, mcut = 50 GeV) keep 23

24 Improvement in significance counting does not help 24

25 Improvement in significance for njet 7: comparable performance S / B improvement: factor 2 25

26 Uncertainties relative uncertainty on efficiency (njet 7) HEPTopTagger subjet calibration 4% fat jet mass >350 GeV 5% fat jet pt >500 GeV 3% sum fat jet mass > 800 GeV? 26

27 Summary Delphes study of high jet multiplicity events one SUSY model vs. QCD (PYTHIA) HEPTopTagger (prong-based): jet combinatorics increase fake rate (accidental hits of mass windows) tight filtering helps similar performance from cuts on m(fat jet), pt(fat jet 1) substructure counting and cut on m(fat jet 1) perform worse ATLAS study underway (Maddalena Giulini) 27

28 Backup 28

29 Additional HEPTopTagger cuts 2 i for m 0 : m123 m 12 +m 13 +m 23 kinematics described by 2 angles: cos θ = m23/m123 sphere with radius m123 mt Φ = atan m13/m12 cut away background tt QCD cos θ = W+jets Φ= 29 Plehn et al., arxiv: