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Measurement of the strong coupling constant by CMS Juska Pekkanen on - - PowerPoint PPT Presentation

Jul 22, 2023 250 likes •406 views

QCD@LHC 2019 Measurement of the strong coupling constant by CMS Juska Pekkanen on behalf of the CMS Collaboration juska@cern.ch SUNY Buffalo July 16, 2019 The strong coupling constant S Not a constant but running wildly Leads to

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QCD@LHC 2019

Measurement of the strong coupling constant by CMS Juska Pekkanen on behalf of the CMS Collaboration

juska@cern.ch

SUNY Buffalo July 16, 2019

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 2/14

The strong coupling constant αS

Not a constant but running wildly

  • Leads to confinement and

asymptotic freedom of QCD

The only free parameter in QCD

  • Considering quark masses fixed

Enters every QCD theory calculation

  • Contributes e.g. to Higgs production σ calculation

⋆ Affects also BSM searches looking for deviations in Higgs σ!

  • Precision measurement important but hard
  • δ(αEM) = 0.2 parts per billion, δ(αS) = 107 ppb (∼1%)

Determined in lattice QCD or collider experiments

  • Best precision from lattice calculations, but reliability of

uncertainty estimates under discussions

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 3/14

History of αS measurements

1985 1990 1995 2000 2005 2010 2015 2020 Year 0.100 0.105 0.110 0.115 0.120 0.125 World average αS(mZ)

Strong coupling constant value vs year

First known world average value from G. Altarelli in 1989

  • Heavily extrapolated to mZ, but still within 1σ!

Lattice calculations and collider experiment results

combined in world averages

  • Uncertainty estimation and choosing results not trivial

Converging to αs(mZ) = 0.118, but uncertainty grown

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 4/14

Measurements from CMS

CMS has produced O(10) αs extractions with 7–13 TeV Here I present four interesting and/or recent results Arbitrary choice, apologies if your favourite not included Following analyses made the cut this time:

  • αs extraction from tt cross section @ 7 TeV
  • αs extraction from jet substructure in tt events
  • αs extraction from triple-differential dijet cross section
  • αs extraction from tt cross section @ 13 TeV
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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 5/14

αs from tt production cross section

Inclusive σtt from a previous CMS analysis is compared to

NNLO QCD predictions

  • 2.3fb−1 @ 7 TeV, arXiv:1208.2671

αs(mZ) determined by fixing mt (and vice versa)

  • Favoured αs from a PDF fit χ2 scan

Five different NNLO PDF sets tested First αs(mZ) result with NNLO at a hadron collider and the

first αs extraction using top-quark production

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 6/14

Results

PDG 2018

Extracted αs values mostly below the 2012 world average

  • ... but world average came down from that!

With the latest world average results ∼within 1 σ Main result from NNPDF2.3: αs(mZ) = 0.1151+0.0028 −0.0027

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 7/14

αs from triple-differential dijet cross section

Triple differential σdijet measured from

dijet events with 20fb−1 of 8 TeV data

  • σdijet(pave

T , rapidity separation y∗, boost yb)

Dijet processes sensitive to αs PDF fits done with the measeured

σdijet and DIS data from HERA

  • αs(mZ) inferred by repeating PDF fit

with it as a free parameter

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 8/14

Results

1000 200 300 500

pT, avg [GeV]

10

4

10

3

10

2

10

1

100 101 102 103 104 105 106 107 108 d 3 dpT, avgdybdy * [pb /GeV]

19.7 fb

1 (8 TeV) NLOJET++ (NLO EW NP) NNPDF 3.0 = pT, maxe0.3y* anti kt R = 0.7

CMS

yb < 1 0 y* < 1 (×102) yb < 1 1 y* < 2 (×102) yb < 1 2 y* < 3 (×101) 1 yb < 2 0 y* < 1 (×101) 1 yb < 2 1 y* < 2 (×101) 2 yb < 3 0 y* < 1 (×100)

Measured triple-differential σdijet agree well with the

NLOJET++ and NNPDF 3.0 prediction

  • Some deviation in the boosted region yb > 1

Extracted strong coupling: αs(mZ) = 0.1199+0.0034 −0.0025

  • Theory scale uncertainty dominating: +0.0031

−0.0020

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 9/14

αs from jet substructure data in tt events

Measurement of jet substructure in tt → ℓ± + jets events

  • Using the full 2016 dataset; 36 fb−1 at

s = 13 TeV

Jet substructure sensitive to the strenght of QCD

  • Here angle between subjets most sensitive

αs(mZ) extracted at LO+LL theory accuracy

  • Measured distributions compared to POWHEG + PYTHIA8

predictions, ’preferred’ αs determined in χ2 scan

t

ν l+ W + b

t

W – b q q'

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 10/14

Results

0.08 0.09 0.1 0.11 0.12 0.13 0.14

)

Z

(m

FSR S

α

100 200 300 400 500 600 700

2

χ (N) λ ε

g

z

g

R ∆

(13 TeV)

  • 1

35.9 fb

CMS

POWHEG+PYTHIA 8 bottom jets charged particles LO+LL, 2-loop CMW 0.08 0.09 0.1 0.11 0.12 0.13 0.14

)

Z

(m

FSR S

α

100 200 300 400 500 600 700

2

χ

g

R ∆ Uncertainties Experimental Model FSR scale Total (13 TeV)

  • 1

35.9 fb

CMS

POWHEG+PYTHIA 8 bottom jets charged particles

0.015 + 0.013 −

) = 0.115

Z

(m

S

α LO+LL, 2-loop CMW

Out of many jet substructure variables, the angle between

groomed subjets ∆Rg gives the best αs extraction result

αs is determined from a FSR-process Result: αs = 0.115+0.015 −0.013

  • Precision limited by FSR scale unc. in PYTHIA8, +0.014

−0.012

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 11/14

αs from tt production cross section (revisited)

Inclusive σtt is measured from dileptonic tt events

  • Full 2016 dataset; 36 fb−1 @ 13 TeV

αs(mZ) determined by fixing mt (and vice versa) and

comparing obtained σtt to NNLO prediction

  • Preferred αs value from a χ2 scan

Four different NNLO PDF sets tested

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 12/14

Results

ABMP16 MMHT14 CT14 NNPDF3.1

αS(mZ) χ2

CMS

35.9 fb-1 (13 TeV) 2.5 5 7.5 10 0.105 0.11 0.115 0.12 0.125

CMS

35.9 fb-1 (13 TeV)

σtt

_ ABMP16nnlo mt(mt) = 160.86 GeV NNPDF3.1nnlo mt(mt) = 162.56 GeV MMHT14nnlo mt(mt) = 163.47 GeV CT14nnlo mt(mt) = 163.30 GeV PDG 2018

αS(mZ)

0.105 0.11 0.115 0.12

Good agreement between all four NNLO PDF sets Extracted αs values (again) below the world average Just to pick one, NNPDF3.1 gives αs(mZ) = 0.1140+0.0039 −0.0033

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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 13/14

Summary

1990 1995 2000 2005 2010 2015 2020 Year 0.095 0.100 0.105 0.110 0.115 0.120 0.125 0.130 αS(mZ)

World average CMS Measurement with total uncertainty CMS Measurement without scale uncertainty

CMS results well in line with the world average Uncertainties much larger than in combined values

  • Theory scale unc. dominate in σdijet & jet substructure

Similar unc. with 2.3fb−1 7 TeV and 36fb−1 13 TeV in σtt

  • Syst. limited, leading uncertainties from σtt and PDF
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QCD@LHC 2019 Juska Pekkanen, juska@cern.ch July 16, 2019 14/14

References & further reading

Links to the presented CMS results:

  • 1. "Determination of the top-quark pole mass and strong coupling constant from the

tt production cross section in pp collisions at

s = 7 TeV" arXiv:1307.1907

  • 2. "Measurement of the triple-differential dijet cross section in proton-proton

collisions at

s = 8 TeV and constraints on parton distribution functions"

arXiv:1705.02628

  • 3. "Measurement of jet substructure observables in tt events from proton-proton

collisions at

s = 13TeV" arXiv:1808.07340v2

  • 4. "Measurement of the tt production cross section, the top quark mass, and the

strong coupling constant using dilepton events in pp collisions at

s = 13 TeV"

arXiv:1812.10505

Excellent review on αs measurements by G. Dissertori: "The Determination of the Strong Coupling Constant" 1506.05407

A contribution to: "The Standard Theory up to the Higgs discovery - 60 years of CERN", L. Maiani and G. Rolandi (editors)