Top quark mass

Kaven Yau Wong On behalf of:

The ATLAS Collaboration

Top quark mass

measurements using the ATLAS detector at the LHC

International Conference on High Energy Physics 2014

Valencia, Spain, July 4 th , 2014

Outline

• Top quark mass analyses

• Single lepton channel

• Dilepton channel

• Pole mass from cross-section

• tt̄ mass difference

• Conclusion

Template method

• Extensively used in ATLAS top quark mass analyses:

• Model the distribution of an observable with a function of n parameters

• Replace the n parameters by their top quark mass dependency

• Use this template to fit the data and measure the top quark mass

Single lepton channel

• Uses 3 dimensional template technique (4.7 fb -1 , 7 TeV)

• Reconstructed top quark mass: m t reco (depends on m top , JSF and bJSF)

• Reconstructed W boson mass: m W reco (depends on JSF)

• Reconstructed R lb (depends on bJSF and m top )

• The events are fully reconstructed using a kinematic likelihood fit

ATLAS-CONF-2013-046

Single lepton channel ATLAS-CONF-2013-046

At least 2 b-tagged jets

Exactly 1 b-tagged jet

Single lepton channel ATLAS-CONF-2013-046

m

= 172.31 ± 0.75 stat+JSF+bJSF ± 1.35 syst GeV

Dilepton channel

• Measure the top quark mass using the template method on m lb

• All channels are combined (4.7 fb -1 , 7 TeV)

• Chosen observable: m lb

• Defined as the average invariant masses of the lepton-b-jet systems ATLAS-CONF-2013-077

m

m

Dilepton channel

• Template:

• Signal: Gaussian+Landau

• Background: Landau (3%)

ATLAS-CONF-2013-077

Dilepton channel ATLAS-CONF-2013-077

m

= 173.09 ± 0.64 stat ± 1.50 syst GeV

Pole mass from cross section

• Cross section measurement in 7 and 8 TeV (Dilepton channel)

• Exploits the strong dependence of σ tt on the top quark pole mass

 Top quark pole mass: mass as a free particle. May differ from reconstructed top quark mass by O(1 GeV)

arXiv:1406.5375

Pole mass from cross section

Measurements using different PDFs

Pole mass extracted maximizing the likelihood:

Combining the 7 and 8 TeV measurements:

m

(pole) =172.9−2.6 +2.5 GeV

arXiv:1406.5375

Pole mass from cross section

Submitted to:

EPJC

arXiv:1406.5375

tt̄ mass difference

• Analysis in tt̄ single lepton channel (4.7 fb -1 , 7 TeV)

• Using a χ 2 kinematic fit to reconstruct the top quark pair in each event

• Used to define observable:

Physics Letters B728 (2014)

pp. 363-379

tt̄ mass difference

Signal parametrized by the sum of two Gaussians

Background parametrized by a single Gaussian

Physics Letters B728 (2014)

pp. 363-379

tt̄ mass difference

Δm = 0.67 ± 0.61 stat ± 0.41 syst GeV

Physics Letters B728 (2014) pp. 363-379

Measurement dominated by the b fragmentation model systematic uncertainty

P OWHEG +(E VT G EN versus P YTHIA )

World combination

More about the world combination in Dr. Maria Aldaya’s talk at 12:55

Conclusions

All the top quark mass measurements have greatly reduced their total uncertainty

• The tt̄ mass difference is compatible with zero

• Very competitive top quark pole mass measurement

m

(pole) =172.9−2.6 +2.5 GeV

Δm = 0.67 ± 0.61 stat ± 0.41 syst GeV

Full version of the analyses

• Single lepton

• http://cds.cern.ch/record/1547327

• Dilepton

• http://cds.cern.ch/record/1562935

• All hadronic

• http://cds.cern.ch/record/1431895

• Pole mass

• http://arxiv.org/abs/1406.5375

• tt̄ mass difference

• http://dx.doi.org/10.1016/j.physletb.2013.12.010

BACKUP

Pole mass from cross section arXiv:1406.5375

Signal MC generators

• Single lepton

• P OWHEG +P YTHIA

• Dilepton

• P OWHEG +P YTHIA

• Pole mass

• P OWHEG +P YTHIA

• tt̄ mass difference

• P YTHIA