Measurement of the jet mass distribution in boosted top quark decays at 8 TeV in CMS

Transcription

1 Measurement of the jet mass distribution in boosted top quark decays at 8 TeV in CMS, Johannes Haller, Roman Kogler

2 itroduction jet mass mjet: invariant mass of all stable particles in a jet boosted top quarks: decay products merge reconstruction of the full top quark in one jet mjet ~ mtop analysis aim: measurement of the differential tt cross section as a function of mjet on particle level motivation: mjet used to identify boosted top quarks e.g. used by the CMS top-tagger test the modeling of jets in simulation extraction of the top quark mass [CMS-PAS-JME ] 1

3 top quark mass measurements top quark plays an important role in the Standard Model (SM) b W top mass important for consistency checks of the SM t LHC allows precise top mass measurements b W l ν (world combination 2014) t measurements based on reconstructed decay products measurement of mass parameter in simulation ( ) theoretically not well defined connection to well defined mass uncertainty ~ 1 GeV [Phys. Rev. D93 (2016) ] 2

4 top quark mass measurements measurements of well defined top quark masses: e. g. measurement of the pole mass from the inclusive tt cross section measurement of the inclusive tt cross section in data comparison to theory predictions dependence on not large [arxiv: ] sensitivity limited 3

5 alternative method measure cross section as a function of a sensitive observable top quark jet mass calculated in: [S. Fleming, A. H. Hoang et al., Phys. Rev. D77 (2008) ]... [A. H. Hoang, A. Pathak et al., JHEP 12 (2015) 059] boosted top quarks in lepton-lepton collisions all decay products in one hemisphere ( jet ) calculation in an effective field theory study of and its relation to well defined mass schemes no LHC calculations available yet interesting to compare to data top quark mass in a well defined renormalization scheme proof of principle with 8 TeV data (low statistics) 4

6 phase space definition goal: phase space calculable theoretically and measurable experimentally theory constraints: all decay products in the jet large pt veto on additional jets [S. Fleming, A. H. Hoang et al., Phys. Rev. D77 (2008) ] experimental constraints: enough statistics pt not too large large jets small background => measurement in lepton + jets channel Cambridge/Aachen (CA) jets with R = 1.2 and pt > 400 GeV 5

7 selection on particle level measurement of the jet mass of the leading jet in tt-decays particle level! electron/muon+jets channel selection on particle level >= 1 jet with pt > 400 GeV, ƞ < 2.5 >= 2 jets with pt > 150 GeV, ƞ < 2.5 veto on additional jets with pt > 150 GeV simulated tt events 1 electron/muon pt > 45 GeV, ƞ < 2.5 mjet [GeV] 6

8 selection on particle level measurement of the jet mass of the leading jet in tt-decays particle level! electron/muon+jets channel selection on particle level >= 1 jet with pt > 400 GeV, ƞ < 2.5 >= 2 jets with pt > 150 GeV, ƞ < 2.5 veto on additional jets with pt > 150 GeV 1 electron/muon pt > 45 GeV, ƞ < 2.5 ΔR(second jet, lepton) < 1.2 mleading jet > m2nd jet+lepton simulated tt events mjet [GeV] 7

9 selection on detector level reconstruction and selection based on [Phys.Rev. D93 (2016) 1, ] small distance between lepton and b-jet non isolated leptons CA 1.2 one b-tag similar selection as on particle level 2012 CMS dataset with 19.7 fb-1 at 8 TeV leading jet mass spectrum mjet [GeV] 8

10 unfolding particle level distribution x folded with detector effects detector effects described by a response matrix A unfolding: obtain true distribution x from the detector level distribution y response matrix determined in simulation [S.Schmitt, JINST 7 (2012) T10003] use regularized unfolding method (TUnfold) 9

11 response matrix response matrix (POWHEG+PYTHIA): measurement phase space divided into two pt bins 400 GeV < pt < 500 GeV pt > 500 GeV 10-3 additional sideband regions: more information on migrations ev ev ev G G G 1 <4 0 < p T,1 <2,2 < p T p T, < < reduction of model dep. effects measurement phase space 10

12 differential cross section e/μ+jets pt > 400 GeV slight overestimation of cross section in simulation consistent with other cross section measurements in boosted events [Phys. Rev. D93 (2016) (ATLAS), arxiv: (CMS)] 11

13 top quark mass extraction no LHC calculations available yet extraction of the top quark mass from simulated templates just to estimate the uncertainty of the method on 8 TeV only shape of mjet is sensitive to mtop use normalized cross section 12

14 top quark mass extraction calculate for every template: data bins simulation bins perform a fit to the covariance matrix distribution minimum best fit value for result: mtop [GeV] large statistical and model uncertainties with 8 TeV data expect large improvement with more data on 13 TeV 13

15 conclusion measurement of the differential tt production cross section as a function of mjet at 8 TeV on particle level sensitivity to mtop calculable on particle level independent determination of the top quark pole mass large uncertainties on mtop with the 8 TeV data 8 TeV: statistical uncertainty dominant expected improvement with 13 TeV data 14

16 Back Up

17 unfolding bias tests response matrix from simulation: different MC generators different modeling of tt production difference in the mjet spectrum variation of the Q scale unfolding should be independent of such differences mjet [GeV] needs to be tested pt, leading jet [GeV] 16

18 unfolding bias tests influence of the simulation model: e/μ+jets response matrix: POWHEG pseudo data: MADGRAPH e d i s n bi more bias tests: variation of mtop variation of the Q scale variation of the shower model mjet [GeV] differences included as a model uncertainty 17

19 systematic uncertainties model uncertainties e/μ+jets systematic uncertainties e/μ+jets statistical uncertainties dominant on 8 TeV expect improvement with more statistics with the 13 TeV data 18

20 The CMS detector Compact Muon Solenoid (CMS) general purpose detector usage of the Particle Flow algorithm 19

21 unfolding true distribution x folded with detector effects detector effects described by a response matrix A regularized unfolding method: obtain true distribution x from the reconstructed distribution y by a maximum likelihood fit [S.Schmitt, JINST 7 (2012) T10003] amplification of statistical fluctuations suppress with a regularization term 20