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Study of single top quark production with the CMS detector
Natalia Tsirova
D.V. Skobeltsyn Institute of Nuclear Physics, Moscow State University

for the CMS collaboration

QFTHEP 2013 25 June


Outline


Single top processes and motivation t-channel measurements




Cross section Charge asymmetry



Associated tW production Summary



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Single top
Single top quark production:
s-channel
N. PR PR PR K D D D idonakis: 83, 091503 (2011) 81, 054028 (2010) 82, 054018 (2010)

t-channel

tW production

LHC @ 7 TeV LHC @ 8 TeV

4.59 pb 5.6 pb

64.57 pb 87.8 pb

15.6 pb 22.4 pb

Single top features:



Cross section proportional to |Vtb|2 => allows direct measurement Wtb vertex enables tests of V­A structure Test of b-quark structure function Sensitive to new physics, e.g. anomalous couplings, 4th generation, W', H+

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t-channel cross section (7 TeV)

JHEP12(2012)035

Data: 1.17 / 1.56 fb-1 (muon / electron) Event selection Single isolated lepton (muon or electron) p >20 GeV/c ||<2.1 (muon) T p >30 GeV/c ||<2.5 (electron) T


One central b-jet from top decay Additional light-quark jet often in forward region Additional b-jet can be present (softer pT) Cut to reduce QCD multijet events: MtW >40 GeV/c2 (muon), MET>35 GeV (electron) g b b QCD
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Main backgrounds:

g top pair W+jets multivariate analyses (BDT & NN)
j'

Two approaches: template fit ( | | analysis)


t-channel cross section (7 TeV)
| | analysis: likelihood fit to | | distribution (pseudorapidity of light jet)
j' j'


JHEP12(2012)035

Specific signal region: 2 jets 1 b-tag category 130 GeV/c < TopMass < 220 GeV/c Data-driven W+jets modelling: yield and | |-template extracted from data from sideband region of TopMass
j'



Combination:

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t-channel cross section (7 TeV)
jets 0 b-tag 1 b-tag 2 b-tags 2 3 4

JHEP12(2012)035

Multivariate analyses: use multivariate methods (BDT, NN) to obtain a powerful discriminator between signal and background processes W+jets Signal Top pairs BDT
Used to check of modeling of input variables Used for cross section measurement

NN

The measured cross section:

Combination:

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t-channel cross section (8 TeV)
| | analysis: the same strategy as for 7 TeV
j'


CMS PAS TOP-12-011



Data: 5.0 fb-1 (muon channel only) Signal region: 2 jets 1 b-tag category 130 GeV/c < TopMass < 220 GeV/c Jet pT>60 GeV/c Cut to reduce QCD multijet events: MtW >50 GeV/c 2 Data-driven top pair modelling | |-template is obtained using the 3-jets 2-tags data sample Data-driven W+jets modelling: as for 7 TeV from data from sideband region of TopMass
j'



Likelihood fit to | | distribution results in
j'

Ratio cross section (8 TeV) / cross section (7 TeV):
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t-channel cross-section
7 TeV: | |, BDT and NN analyses combined with BLUE method:
j'

8 TeV: |Vtb| extraction Assuming |Vtd|, |Vts| << |Vtb| :

calculated with |Vtb|=1

7 TeV: 8 TeV: f
LV

is a left-handed vector coupling, fLV=1 in SM
LV

Constraining |Vtb | to the interval [0, 1] and setting f (7 Tev) and

= 1 yields:
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(8 TeV) @ 95% CL


Charge asymmetry (8 TeV)


CMS PAS TOP-12-038

The top quark inherits the sign of the charge from the light quark q' The cross section ratio depends on light quark PDF, sensitive to new physics (due to Wtb vertex presence) An effective handle to constrain different parton distribution function models





| | analysis: template fit to pseudorapidity of the light jet
j'


Data: 12.2 fb-1 the same event selection as for t-channel cross-section measurement signal region: 2 jets 1 b-tag category 130 GeV/c < TopMass < 220 GeV/c

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Charge asymmetry (8 TeV)
Measurements:

CMS PAS TOP-12-038

SM predictions (Kidonakis):

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Associated tW production (7 TeV)
Data: 4.9 fb
-1

Phys. Rev. Lett. 110, 022003 (2013)

Event selection 2 leptons with opposite charge p >20 GeV/c ||<2.4 (muon) T p >20 GeV/c ||<2.5 (electron) T One or two jets with p >30 GeV/c and ||<2.4, at least T one jet is b-tagged MET>30 GeV Additional cuts: 2 m >20 GeV/c ll 2 2 veto events with 81 GeV/c 60 GeV (specific cut in cut-based analysis) T Main backgrounds:
HT: scalar sum of the pT of the leptons, jets, and MET

top pairs Two complementary approaches:

Z+jets multivariate analysis (BDT) cut-based analysis as a cross-check
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Associated tW production (7 TeV)

Phys. Rev. Lett. 110, 022003 (2013)

BDT analysis: combine 4 variables in a BDT to increase ttbar/tW separation power Input variables: HT, pT of the system, pT of jet, angle between MET and the closest lepton

3 categories considered: 1 jet 1 tag, 2 jets 1 tag, 2 jets 2 tags Simultaneous fit: three categories in the three final states (ee, e and ) Binned likelihood fit to BDT output simultaneously in all channels (ee, e,) Cross-check with count-based analysis

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Associated tW production (7 TeV)
Measured cross-section: BDT 16 (+5 -4) pb Cut-based 15 (+-5) pb

Phys. Rev. Lett. 110, 022003 (2013)

Significance: BDT 4 Cut-based 3.5

|Vtb| extraction Assuming |Vtd|, |Vts| << |Vtb| :

calculated with |Vtb|=1

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Summary

The first measurements of single top production properties are published: t-channel and associated tW production cross sections, |Vtb|, charge asymmetry


The next round of analyses are in progress: differential cross sections, top mass and rare s-channel production


Searches for the deviation from SM prediction in single top are in progress


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