1 measurements at 8 tev of ttbar events with additional particles in the final state boris mangano...

Measurements at 8 TeV of TTbar events

with additional particles in the final state

Boris Mangano (ETH Zürich)

on behalf of the CMS collaboration

LHCP 2014Boris Mangano page 2

Outline

TTbar plus what?

- TTbar+jets

- TTbar+BBbar

- TTbar+V:

o TTbar+gamma

o TTbar+Wo TTbar+Z

8TeV results released during the last 12 months

Were these processes seen also at 8TeV ?

new results

PAS TOP-12-041

PAS TOP-13-010

PAS TOP-13-011

8 TeV

TOP-12-018

PAS TOP-12-024

TOP-12-014

7 TeV


Motivation

TTbar plus what?

- TTbar+jets

- TTbar+BBbar

- TTbar+V:

o TTbar+gamma

o TTbar+Wo TTbar+Z

About ½ of TTbar events contain extra jets with |h| < 2.4, pT>30 GeV. Possibility to test higher order QCD calculations.

Irreducible non-resonant background for ttH (H->bb) analysis.

Potential to verify if couplings to the top quark are compatible with SM or altered by BSM effects.


tt+jets @ 8TeV (19.6 /fb)PAS TOP-12-041Event selection:

- Dilepton OS events (el and mu)- For same-flavour pairs, Z-veto

and require MET- N-jets >=2- N-bjets >=1

Unfolding:- cross-section in N-jets bins is

presented at particle level

Extra jets definition:- Kinematic reconstruction

method to identify b-jets from ttbar decay.

- Other extra jets are then analysed

Main source of uncertainty:Up to 20% uncertainty from jet-energy-scale and signal model uncertainties.


The whole is greater than the sum of its parts (Aristotle)

MC@NLO + Herwig does not generate sufficiently large jet multiplicities.

tt+jets @ 8TeV (19.6 /fb)



Leading extra jet pT (GeV)

tt+jets @ 8TeV (19.6 /fb)

Second Leading extra jet pT (GeV)

These distributions are at reconstruction level


PAS TOP-13-010Event selection:- Dilepton OS events (el and mu)- For same-flavour pairs, Z-veto and

require MET- N-jets >=4- N-bjets (tight) >=2

tt+bb@ 8TeV (19.6 /fb)

Main source of uncertainty:20-25% total uncertainty. Mostly from uncertainties in b-tagging discriminator templates.

Normalization of simulated samples:- Performed by means of a fit to the

measured b-tagging algorithm discriminator.

- Templates for different tt+X samples (including tt+bb signal) are taken from simulation.



First additional jet Second additional jet

Simultaneous template fit of two data distributions




Factor 0.38 to account for difference in selection efficiency between ttbb and ttjj (from simulation)

Cross-section ratio at particle level:


using again selection efficiencies from simulation


PAS TOP-13-011Event selection:

- one isolated central m with pT > 26 GeV - Veto on extra leptons- N-jets >=4, with pT > 55, 45, 35, 20 GeV- N-bjets >=1- One central photon ET > 25 GeV- DR (photon,muon) > 0.7

tt+gamma @ 8TeV (19.7 /fb)

Main source of uncertainty:20-25% total uncertainty from modelling of the background template.

Signal extraction:

- Template fit of charged-hadron-isolation discriminant distribution in data.

- Templates for signal and misidentified photons from simulations.



N-1 plots for some of the observables used to select the signal photons.




Fit of distribution of thecharged-hadron-isolation discriminant

Inclusive ttbar cross section (TOP-12-017)


ttW and ttZ @8TeV (19.5 /fb)

- Both ttW and ttZ processes are very rare and the measurement of their cross sections can be used to test the internal consistency of SM.

TOP-12-036

pre-print to be released in the next days

- ttZ production allows the probe of tZ coupling.


ttW in same-sign dilepton

Signal region definition:- Tight prompt-lepton selection- same-sign dilepton (el or mu)- third lepton vetoed only if it makes a

Z-candidate- N-jets >= 3- N-bjets >=1- lepton pT>40 GeV and HT > 155 GeV

Analysis type:counting experiment

Against misidentified prompt leptons and ttbar background

Against WZ and other diboson processes. Remove overlap with ttZ analysis.

Final optimization against ttbar background


ttW in same-sign dileptonDominant background:

- ttbar with misidentified leptons is estimated from data. Uncertainty is about 50%.

Other backgrounds:

- ttZ, WZ and other rarer processes are estimated from simulations.

- Z+jets with mis-measured lepton charge estimated from data

Six signal regions to exploit larger S/B in plus-plus channels.


ttZ in trilepton


Signal region definition:- Three tight leptons (el or mu)- Two same-flavour opposite-sign

making a Z-candidate- N-jets >= 4- N-bjets (loose) >=2 & N-bjet

(medium) >=1

Against misidentified leptons and ttbar background

Against WZ and other diboson processes. Remove overlap with ttW analysis.


ttZ in trileptonDominant background:

- non-top-quark background processes with b-jets from ISR. Uncertainty is about 50%.

Other backgrounds:

- TTbar with misidentified leptons is estimated from data.

- Rarer, and irreducible, SM processes estimated from simulation

Only one signal region is used for the statistical interpretation of the data


ttZ in four-lepton


Signal regions definition:- Four tight leptons (el or mu)- Two same-flavour opposite-sign

making a Z-candidate- Veto on second Z-candidate- N-bjets =1- N-bjets >1

Against WZ and lepton misidentification

Two non-overlapping signal regions

Against ZZ background


ttZ in four-leptonBackgrounds:

- ZZ(*) in four leptons is estimated from MC

- Z+X events with misidentified leptons

Limits and benefits of this channel:

- statistically limited

- Large S/B ratio


ttV (V = W or Z)

Results in agreement with SM predictions:

cross section ttW = 206 ± 23 fbcross section ttZ = 197 ± 25 fb

Numbers from MG5_aMC@NLO. They are compatible with first NLO calculations from MCFM and PowHel.


Summary

TTbar plus what?

- TTbar+jets

- TTbar+BBbar

- TTbar+V:

o TTbar+gamma

o TTbar+Wo TTbar+Z

8TeV results released during the last 12 months

PAS TOP-12-041

PAS TOP-13-010

PAS TOP-13-011

TOP-12-036

pre-print to be released in the next days

New Results

Latsis Symposium 2013 page 22Boris Mangano

Backup slides


ttV (V = W or Z)



tt+jets

1 measurements at 8 tev of ttbar events with additional particles in the final state boris mangano...

Documents

tev of ttbar events

tev slide

boris manganolhcp

outline ttbar

motivation ttbar

ttbar decay

fb slide

tev results