Top Pair Production
Cross Section Measurements
and Searches for top Squarks at 13 TeV in CMS
Luca Scodellaro
Instituto de Física de Cantabria - CSIC
X CPAN DAYS
October 29
th-31
st, 2018
Salamaca (Spain)
1Tag
Introduction
u The top quark is expected to play a special role in the standard model (SM) of elementary particles:
o Yukawa coupling close to one: special role in electroweak symmetry breaking and new physics?
o Main background for several beyond SM searches (SUSY)
u Experimentally, top pair production measurements allow to constrain theory (PDF, QCD parameters) and detector
modeling
u In this talk, an overview of CMS results on the measurement of the top pair production cross section and on the search for top supersymmetric partners is presented
o Emphasis is given on analyses with a significant contribution from Spanish institutions, namely on final states with two
oppositely charged leptons
2
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Role of Spanish Institutions
u Large involvement of Spanish institutions in the coordination of SUSY and top analyses in CMS
u SUSY Physics Analysis Group:
o MC SUSY subgroup convener: Luca Scodellaro (IFCA)
o TBT SUSY subgroup convener (till 18/09): Pablo Martinez (IFCA) o Combination tool contact: Carlos Erice (Oviedo)
u TOP Physics Analysis Group:
o Single Top subgroup convener: Enrique Palencia (Oviedo) o Conference Support contact: Enrique Palencia (Oviedo) o Trigger contact: Sergio Sanchez (Oviedo)
o MC production contact: Enrique Palencia (Oviedo) o DQM, PVT contact: Carlos Erice (Oviedo)
o Combination tool contact: Pietro Vischia (Oviedo, now UCL)
3
Top Pair Production
Cross Section Measurements
4
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Top Pair Production at LHC 5
+
o Quark annihilation (~10%)
-29 -22
o Dilepton final state with electrons and muon is the golden channel o Large experience in exploiting this
channel for measuring top pair production cross section from Spanish institutions since the beginning of the LHC era
u Production mechanisms in proton-proton collisions:
o Gluon fusion (~90%)
u Theoretical calculations available NNLO+NNLO (twiki)
o σtt (13TeV)= 832 +20 (scale) ± 35 (PDF+αS) +23 (mass) pb
u Decay mainly to bW. Final states given by W boson decays:
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tt Production at 13 TeV
u Cut&Count method
u Event selection: e±μ pair,
≥2 jets, ≥1 b-tagged jet
u Main syst. uncertainties:
o Lepton efficiencies, JES, choice of NLO generator
6
-
EPJC 77 (2017) 172
Events
0 5 10
103
×
Data t t
Non W/Z V t VV + t
tW ±
±µ
→ e γ* Z/
(13 TeV) 2.2 fb-1
CMS
(f) •* e±µ + ± ≥ 2 jets
Number of b jets
0 1 2 ≥ 3
Data/MC 0.6 1 1.4
±
Source Dstt (pb) Dstt/stt (%) Experimental
Trigger efficiencies 9.9 1.2
Lepton efficiencies 18.9 2.3
Lepton energy scale <1 0.1
Jet energy scale 17.4 2.1
Jet energy resolution 0.8 0.1
b tagging 11.0 1.3
Mistagging <1 0.1
Pileup 1.5 0.2
Modeling
µFandµR scales <1 0.1
tt NLO generator 17.3 2.1
tt hadronization 6.0 0.7
Parton shower scale 6.5 0.8
PDF 4.9 0.6
Background
Single top quark 11.8 1.5
VV <1 0.1
Drell–Yan <1 0.1
Non-W/Z leptons 2.6 0.3
ttV <1 0.1
Total systematic 37.8 4.6
(no integrated luminosity)
Integrated luminosity 18.8 2.3
Statistical 8.5 1.0
Total 43.0 5.3
σtt = 815 ± 9 (stat) ± 38 (syst) ± 19 (lumi) pb
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tt Production at 13 TeV
u Selecting events with a e+e−, μ+μ- or e±μ pair
u Template fit to multi-
differential distributions, split in categories of b-tagged jet and additional non-b- tagged jet multiplicities
u Overall systematic uncertainty constrained within 4%
u Additional fit to extract top quark mass and strong
coupling constant
7
-
±
Events/GeV
500
1000 Data
Signal Background Syst
MC Stat
[GeV]
Additional jet pT
40 60 80 100 120 140 160 180 200
pred.obs. 0.81
1.2
CMS Preliminary 35.9 fb-1 (13 TeV)
Events/GeV
10 20 30
40 Data
Signal Background Syst
MC Stat
[GeV]
Additional jet pT
50 100 150 200 250 300
pred.obs. 0.81
1.2
CMS Preliminary 35.9 fb-1 (13 TeV)
CMS-PAS-TOP-17-001
σtt = 803 ± 2 (stat) ± 25 (syst) ± 20 (lumi) pb
eμ
1 b-tag. jet 1 add. jet
eμ
2 b-tag. jet 2 add. jet
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tt Production at 13 TeV
u Measurements of top pair
production cross section in all the decay channels
u l+jets channel also reaches high precision through a likelihood fit to 44 orthogonal categories
u All-hadronic channel penalized by JES, modeling and b-tagging uncertainties
8
[pb]
t
σt
200 400 600 800 1000 1200 1400
CMS Preliminary σtt summary, s = 13 TeV Feb 2018 NNLO+NNLL PRL 110 (2013) 252004
0.001
± ) = 0.118 (MZ
αs
= 172.5 GeV, mtop
scale uncertainty
uncertainty αS
⊕ scale
total stat
(lumi) (syst) ± (stat) ±
t± σt
* Preliminary µ CMS, dilepton e
, 50 ns = 43 pb-1
PRL 116 (2016) 052002, Lint
µ
CMS, dilepton e 746 ± 58 ± 53 ± 36 pb
µ CMS, dilepton e
, 25 ns = 2.2 fb-1
EPJC 77 (2017) 172, Lint
µ
CMS, dilepton e 815 ± 9 ± 38 ± 19 pb
CMS, l+jets *
, 50 ns = 42 pb-1
CMS-PAS TOP-15-005, Lint
CMS, l+jets * 836 ± 27 ± 84 ± 100 pb
CMS, l+jets
, 25 ns = 2.2 fb-1
JHEP 09 (2017) 051, Lint
CMS, l+jets 888 ± 2 ± 26 ± 20 pb
CMS, all-jets *
= 2.53 fb-1
CMS-PAS TOP-16-013, Lint
CMS, all-jets * 834 ± 25 ± 118 ± 23 pb
ABM12 ) = 0.113 (mZ
αs
PRD 89 (2015) 054028 CT14 PRD 93 (2016) 033006 MMHT14 EPJC 75 (2015) 5 NNPDF3.0 JHEP 04 (2015) 040
-
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tt Production at 5 TeV
u First measurement at 5.02 TeV:
o 27.4/fb collected in 2015
u Multilepton final states:
o l+jets: fit in b-jet categories o Dilepton: cut&count
u Good agreement with theoretical predictions:
σtt = 69.5±6.1(stat)±5.6(syst) ±1.6(lumi) pb σtt = 68.9 +1.9(scale)±2.3(PDF) +1.4(αS) pb u Used to constrain gluon PDF at high
momentum fraction
9
-
JHEP 03 (2018) 115
µF2 = 105 GeV2
HERA DIS + CMS W± + σtt - HERA DIS + CMS W±
DIS + W± + σtt / DIS + W± -
x
Gluon PDF relative uncertainty
CMS NNLO MC Method
0.8 1 1.2
10-3 10-2 10-1
0.8 1 1.2
10-3 10-2 10-1
NNLO
-2.3 -1.0
(GeV)
miss
pT
0 20 40 60 80 100 120 140
Events / 10 GeV
1
10−
1 10 102
103
104 Data
t t
Non-W/Z WV tW
γ* Z/
lumi Stat ⊕
(5.02 TeV) 27.4 pb-1
CMS µ±µ + Z veto±
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[TeV]
s
2 4 6 8 10 12 14
cross section [pb]tInclusive t
10 102
103
CMS Preliminary Feb 2018
* Preliminary
-1) 8.8 fb Tevatron combined 1.96 TeV (L ≤
-1) CMS dilepton,l+jets* 5.02 TeV (L = 27.4 pb
-1) 7 TeV (L = 5 fb CMS eµ
-1) CMS l+jets 7 TeV (L = 2.3 fb
-1) CMS all-jets 7 TeV (L = 3.54 fb
-1) 8 TeV (L = 19.7 fb CMS eµ
-1) CMS l+jets 8 TeV (L = 19.6 fb
-1) CMS all-jets 8 TeV (L = 18.4 fb
, 50 ns) 13 TeV (L = 43 pb-1
CMS eµ
-1) 13 TeV (L = 2.2 fb CMS eµ
, 50 ns) CMS l+jets* 13 TeV (L = 42 pb-1
-1) CMS l+jets 13 TeV (L = 2.2 fb
-1) CMS all-jets* 13 TeV (L = 2.53 fb
NNLO+NNLL (pp) ) p NNLO+NNLL (p
Czakon, Fiedler, Mitov, PRL 110 (2013) 252004
)=0.113]
(MZ
αs
0.001 [*
) = 0.118 ± (MZ
αs
= 172.5 GeV, NNPDF3.0, mtop
[TeV]
13 s
600 800 1000
NNPDF3.0 MMHT14
CT14 ABM12*
tt Production (Summary)
u Good agreement with theoretical calculation over all the explored range of center-of-mass energy
10
-
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tt Production in pPb Collisions
u First observation of tt production in proton-nucleus collisions:
o 174/nb at √sNN = 8.02 TeV (2016)
u l+jets final states:
o Likelihood fit to the reconstructed mass of the hadronically decaying W for events with 0, 1 and ≥2 b-jets
u Combined result:
u σ(pPb -> tt+X) = 45 ± 8 nb
u Consistent with QCD predictions u Probe of nuclear PDF at high
Bjorken-x
11
-
PRL 119 (2017) 242001
0 20 40 60 80 100
[nb]
σ CMS
(Top++)
NNLO+NNLL
⋅ K NLO MCFM CT14+EPPS16
(Top++)
NNLO+NNLL
⋅ K NLO MCFM CT10+EPS09
=8.16 TeV) sNN
-1, ( pPb, 174 nb
e+jets +jets µ l+jets
NNLO+NNLL Top++
CT14
NNLO+NNLL Top++
=8 TeV) CT10 s
-1, ( pp, 19.6 fb
(8 TeV) NNLO+NNLL σ
(8.16 TeV) NNLO+NNLL
⋅σ Data scaled by A
JHEP 1608 (2016) 029 eµ
EPJC 77 (2017) 15 l+jets
⊕syst Exp. unc.: stat stat
scales
⊕ Th. unc.: pdf pdf
[GeV]
mjj'
50 100 150 200 250 300
Events
5 10 15 20 25 30
35 CMS
= 8.16 TeV) sNN
-1, pPb (174 nb
≥2b)
≥4j ( + µ±
/ e±
Data correct t t
wrong t t
background
1Tag
yt 2
− −1 0 1 2
tdy
σd σ1
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
(13 TeV) 35.9 fb-1
CMSPreliminaryPreliminaryPreliminaryPreliminaryPreliminaryPreliminaryPreliminaryPreliminaryPreliminaryPreliminary Dilepton: particle
Data
Powheg v2+Pythia8 Powheg v2+Herwig++
MG5_aMC@NLO+Pythia8 [FxFx]
yt
2
− −1 0 1 2
DataTheory
0.9 1 1.1
1.2 Stat. ⊕ Syst.
Stat.
t GeV pT
0 100 200 300 400 500
-1 GeVt Tdpσd σ1
4
10− 3
10− 2
10−
(13 TeV) 35.9 fb-1
CMSPreliminaryPreliminary Dilepton: parton
Data
Powheg v2+Pythia8 Powheg v2+Herwig++
MG5_aMC@NLO+Pythia8 [FxFx]
t GeV pT
0 100 200 300 400 500
DataTheory 1
1.2
Syst.
Stat. ⊕ Stat.
Differential tt Cross Sections
u Differential cross sections for several kinematic variables measured in l+jets and dilepton final states
u Unfolding to parton and particle level to correct for detector resolution effects
u Some disagreement observed
o In particular, top pT more steeply falling than predicted
12
-
) [GeV]
(tl
pT 5
10−
−4
10
−3
10
−2
10
]-1
σd1 [GeV )σ(tdpnorm lT
(13 TeV) 35.8 fb-1
parton level +jets e/µ
CMS Data
stat Sys ⊕ Stat
P8 OWHEG P
NNLO QCD+NLO EW H++
OWHEG P
MG5 P8 [FxFx]
0 100 200 300 400 500 600 700 800 ) [GeV]
(tl
pT
0.8 1 Theory Data 1.2
h)|
|y(t 0.2
0.4 0.6 0.8 σd1 σ)|d|y(tnorm h 1
(13 TeV) 35.8 fb-1
particle level +jets e/µ
CMS Data
stat Sys ⊕ Stat
P8 OWHEG P
CS HERPA S
H++
OWHEG P
MG5 P8 [FxFx]
0 0.5 1 1.5 2 2.5
h)|
|y(t 0.95
1 1.05 1.1
DataTheory
PRD 97 (2018) 112003
CMS-PAS-TOP-17-014
Searches for top Squarks
13
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Top Squark Searches 14
p
p !t1
!t1
t
! χ01
! χ01 t
p
p !t1
!t1
! χ+1
! χ−1
b W−
! χ01
! χ01 W+ b
u Searches mainly interpreted in models with the top squark decaying into a LSP neutralino via an intermediate top quark or chargino
u Strategy depends on stop and LSP masses, and on their difference Δm=mstop-mX0
u We focused on two regions:
o Degenerate top and stop o Three-body decays
1TmT2(eag
Degenerate Stop: Strategy 15
[GeV]
Arbitrary units / 5 GeV 3−10
2
10−
−1
10
CMS
13 TeV Simulation Preliminary
= 52.5 GeV
0
χ1
= 227.5 GeV, m∼ t1
m~
= 30 GeV
0
χ1
= 205 GeV, m∼ t1
m~
= 7.5 GeV
0
χ1
= 182.5 GeV, m∼ t1
m~
t t
(GeV) MT2
0 20 40 60 80 100 120
tSignal/t
0.6 0.8 1 1.2 1.4
CMS-PAS-SUS-18-003
u Stop and top pair production exhibit very similar kinematics
u Sensitivity to stop production through precise control of top pair production estimate
u Analysis performed in events with an electron-muon pair, two jets and one b-tagged jet
u Increase of sensitivity at higher stop masses and larger Δm obtained fitting the mT2(eμ) observable
1Tag Events / 5 GeV 0 5 10
103
×
= 30 GeV
0 χ1
= 205 GeV, m∼ t1
m~
Data t t tW Residual SM
CMS
(13 TeV) 35.9 fb-1
Preliminary
(GeV) MT2
0 20 40 60 80 100
Data/Pred.0.6
0.8 1 1.2 1.4
Degenerate Stop: Backgrounds
u Top pair production contributes to ~94% of SM background
o Normalized to NNLO+NNLL cross section
o Other backgrounds from tW, diboson, ttZ and ttW events
u No excess over the SM predictions is observed
16
u Modeling uncertainties for each bin of the MT2
distribution:
o Main contributions from jet energy scale and from lepton and b-tagging efficiencies o Total experimental
uncertainties: 2-4%
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Degenerate Stop Results
u Top squark excluded for masses up 210 GeV in models with ΔM≈mΧ0
u Exclusions up to 240 GeV in models with ΔM=7.5 GeV
17
180 190 200 210 220 230 240
0.5 1 1.5 2 2.5 3 3.5 4
4.5 Observed
Expected Expected 1σ Expected 2σ
= 175 GeV
0
χ1
- m∼ t1
m~
(GeV)
t1
m~
95% CL limit on signal strength
CMS Preliminary 35.9 fb-1 (13 TeV)
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Three Body Decay Search
u Targeting the mass region with mW<mstop-mX0<mtop
u Baseline selection requires two oppositely charged leptons and missing transverse energy pTmiss>140 GeV
u Two search regions defined depending on the presence of a b-tagged jet in the event:
o No-tag region is sensitive to models with Δm ≈ mW o Tag region is sensitive to models with Δm ≈ mtop
u Search regions further divided in bins of pTmiss with optimized sensitivity to different stop masses
18
arXiv:1807.07799, accepted by JHEP
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Three Body Decay Strategy
u Simultaneous fit to mT2(ll) distributions observed in the different search regions:
o mT2(ll) shape for the dominant backgrounds from top pair, tW and WW production validated in dedicated control regions
19
) [GeV]
(ll mT2
0 50 100
Events / 20 GeV
1 10 102
103
104
Data Bkg. uncert.
Pre-fit Fit b+s
t t tW WW
) l
→ 3 WZ (
Z t t
ν) l2
→ 2 ZZ (
Drell-Yan Minor bkg.
) GeV = 225
1
χ0
, m∼
GeV = 350
t~1
1 (m χ0
t∼
1→
~t
1,
~t t1
~
channel) SR3 tags+ISR (eµ
CMS 35.9 fb-1 (13 TeV)
) [GeV]
ll
T2( m
0 50 100
SM exp.Data 0.511.5
) [GeV]
ll
T2( m
0 50 100
Events / 20 GeV
1 10 102
103
104
Data Bkg. uncert.
Pre-fit Fit b+s
t t tW WW
) l
→ 3 WZ (
Z t t
ν) 2 l
→ 2 ZZ (
Drell-Yan Minor bkg.
) GeV = 225
1
χ0
, m∼
GeV = 350
t~1
1 (m χ0
t∼
1→
~t
1,
~t t1
~
channel) SR3 0tag+ISR (eµ
CMS 35.9 fb-1 (13 TeV)
) [GeV]
ll
T2( m
0 50 100
SM exp.Data 0.511.5
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Three Body Decay Results
u Observed limits extend up to 420 (360) GeV for top squark (neutralino) masses in the region mW≤Δm≤mtop
u In a model with t->bX±->bW±X0, top squark masses are excluded in the range 225-325 GeV for Δm≈2mW
20
[GeV]
t1
m~
200 400 600 800 1000 1200
[GeV] 10 χ∼m
0 100 200 300 400 500 600 700 800
3
10− 2
10− 1
10−
1 10 102
(13 TeV) 35.9 fb-1
CMS
1
χ0
t ∼
→ t1
, ~ t1
~ t1
→~ pp
NLO+NLL excl.
theory
1 σ Observed ±
experiment
1 σ Expected ±
95% CL upper limit on cross section [pb]
[GeV]
t1
m~
200 400 600 800 1000 1200
[GeV] 10 χ∼m
0 100 200 300 400 500 600 700 800
3
10− 2
10− 1
10−
1 10 102
(13 TeV) 35.9 fb-1
CMS
1
χ0
∼ b W+ 1→ χ+
b ∼
1→
~t
1,
~t t1
→~ pp
1)/2
χ0
+ m∼ t~1
= (m
1
χ±
m∼
NLO+NLL excl.
theory
1 σ Observed ±
experiment
1 σ Expected ±
95% CL upper limit on cross section [pb]
1Tag
Overview of CMS Results
u Summary of mass limits of top squark pair production via an intermediate top quark (left) or a gaugino (right) in various final states
21
200 400 600 800 1000 1200
[GeV]
~t
m
0 100 200 300 400 500 600 700 800 900
[GeV] 10χ∼m
CMS
1
χ0 +∼ W
→ b
1+
χ∼
→ b
~t ,
~t
~t
pp → July 2018
(13 TeV) 35.9 fb-1
+mb
01
∼χ
= m mt~
+mb
+mW
01
∼χ
= m mt~
+mt
01
∼χ
= m mt~
Expected Observed 1711.00752, 0-, 1-, 2-lep (stop)
T2) 1705.04650, 0-lep (M 1805.05784, soft 1-lep + 0-lep 1801.01846, soft 2-lep
)
1
χ∼0
t + m = 0.5(m~
2
χ∼0 1, χ±
m∼
200 400 600 800 1000 1200
[GeV]
~t
m
0 100 200 300 400 500 600 700 800 900
[GeV] 10χ∼m
CMS
1
χ0
∼ t(*)
t → , ~
~t
~t
pp → July 2018
(13 TeV) 35.9 fb-1
+mb
1
χ0
∼
= mt
m~
+mb
+mW
01
∼χ
= mt
m~
+mt
01
∼χ
= mt
m~
0 χ1
∼
+ mt
= mt
m~
Expected Observed 1707.03316, 0-lep (stop)
1706.04402, 1-lep (stop) 1711.00752, 2-lep (stop) 1805.05784, soft 1-lep + 0-lep 1805.05784, soft 1-lep (MVA)
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Summary
u Top quark pair production has been tested at CMS at high precision over a large range of pp collisions center-of-mass energy:
o Good agreement with theoretical predictions o Excellent test for PDF and QCD parameter
u Final states with two top quark also expected by several beyond SM scenarios
u Searches for top squark pair production with 2016 LHC data have been presented:
o No sign for new physics observed over SM backgrounds
o Latest searches focusing into phase space corners and other decay modes
o New and upgrades searches expected with full RunII LHC data (4-5 time increase of luminosity)
22
Backup Material
23
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tt Production at 7 and 8 TeV - 24
[pb]
t
σt
50 100 150 200 250 300
CMS Preliminary σtt summary, s = 7 TeV
(*) Superseded by results shown below the line
June 2016 NNLO+NNLL PRL 110 (2013) 252004
0.001
± ) = 0.118 (MZ
αs
= 172.5 GeV, mtop
scale uncertainty
uncertainty αS
PDF ⊕
scale ⊕ total stat
(lumi)
± (syst)
± (stat)
t± σt
ABM12 ) = 0.113 (MZ
αs
PRD 89 (2015) 054028 CT14 PRD 93 (2016) 033006 MMHT14 EPJ C75 (2015) 5 NNPDF3.0 JHEP 04 (2015) 040
Effect of LHC beam energy uncertainty: 3.2 pb (not included in the figure)
CMS, l+jets (*) 164 ± 3 ± 12 ± 7 pb Lint=0.8-1.1 fb-1
CMS, dilepton (*) 170 ± 4 ± 16 ± 8 pb Lint=1.1 fb-1
µ (*)
had+
CMS, τ 149 ± 24 ± 26 ± 9 pb Lint=1.1 fb-1
CMS, all jets (*) 136 ± 20 ± 40 ± 8 pb Lint=1.1 fb-1
CMS combined 166 ± 2 ± 11 ± 8 pb Lint=0.8-1.1 fb-1
CMS, l+jets 161.7 ± 6.0 ± 12.0 ± 3.6 pbLint=5.0 fb-1
µ
CMS, dilepton e 173.6 ± 2.1 −+ 4.0 4.5± 3.8 pb Lint=5.0 fb-1
had+l
CMS, τ 143 ± 14 ± 22 ± 3 pb Lint=2.2 fb-1
+jets τhad
CMS, 152 ± 12 ± 32 ± 3 pb Lint=3.9 fb-1
CMS, all jets 139 ± 10 ± 26 ± 3 pb Lint=3.5 fb-1
[pb]
t
σt
100 150 200 250 300 350 400
CMS Preliminary σtt summary, s = 8 TeV Feb 2017 NNLO+NNLL PRL 110 (2013) 252004
0.001 ) = 0.118± (MZ
αs
= 172.5 GeV, mtop
scale uncertainty
uncertainty αS
PDF ⊕ scale ⊕
ABM12
) = 0.113 (MZ
αs
PRD 89 (2015) 054028
CT14 PRD 93 (2016) 033006
MMHT14 EPJ C75 (2015) 5
NNPDF3.0 JHEP 04 (2015) 040 total stat
(lumi) (syst) ± (stat) ±
t± σt
Effect of LHC beam energy uncertainty: 4.2 pb (not included in the figure)
CMS, lepton+jets
EPJ C77 (2017) 15, Lint = 19.6 fb-1 228.5 ± 3.8 ± 13.7 ± 6.0 pb
τh
CMS, lepton+
PLB 739 (2014) 23, Lint = 19.6 fb-1 257 ± 3 ± 24 ± 7 pb
µ) µ, e CMS, dilepton (ee, µ
JHEP 02 (2014) 024, Lint = 5.3 fb-1 239.0 ± 2.1 ± 11.3 ± 6.2 pb
µ CMS, dilepton e
JHEP 08 (2016) 029, Lint = 19.7 fb-1 244.9 ± 1.4 −+ 5.5 6.3± 6.4 pb
CMS, all jets
EPJ C76 (2016) 128, Lint = 18.4 fb-1 275.6 ± 6.1 ± 37.8 ± 7.2 pb
