1. Greg Heath University of Bristol
Top physics results
Greg Heath
University of Bristol

2. UK HEP forum, Cosener's, 7th September 2011
Motivation
Large cross-section
About 20× Tevatron at 7 TeV
Opportunity to study “bare” quark
Decays before hadronisation
Handles on New Physics
Higgs mass uncertainties
New phenomena coupling to top
Top as background to discovery channels
Exercise full detector and reconstruction in complex multi-parton events
Leptons, jets, MET, b-tagging, tau-id
UK HEP forum, Cosener's, 7th September 2011

3. Top production and decay
Top pair production from initial gg pair dominates
Top pair decay modes
All hadronic (6 jets) – large BR, large QCD background
Lepton+jets (4 jets) – smaller BR (per lepton), smaller background
Dileptonic (2 jets) – smallest BR, cleanest
Also consider single top production
Theory predictions
UK HEP forum, Cosener's, 7th September 2011

4. UK HEP forum, Cosener's, 7th September 2011
Top pair cross-section
UK HEP forum, Cosener's, 7th September 2011

5. UK HEP forum, Cosener's, 7th September 2011
2011 dilepton analyses
Robust cut and count analyses
B-tagging improves efficiency
Systematic errors dominate
ATLAS
CMS
UK HEP forum, Cosener's, 7th September 2011

6. Lepton+jets analyses with b-tags
b-tagging probability as input to likelihood fits to signal+background
Combined fit to jet multiplicity bins
CMS lepton+jets with b-tags
ATLAS single lepton with b-tags
m+jets
e+jets
m+jets
e+jets
UK HEP forum, Cosener's, 7th September 2011

7. Lepton+jets untagged (ATLAS)
2011 data (0.7 fb-1)
Likelihood fit to event kinematics
UK HEP forum, Cosener's, 7th September 2011

8. All-hadronic measurement (CMS)
Kinematic fit to events with b-tagging
Fit mass distribution to estimate t-tbar contribution
UK HEP forum, Cosener's, 7th September 2011

9. Dilepton measurements with ts
CMS – cut-based selection including b-tagging
ATLAS – m+jets background minimised using BDT
UK HEP forum, Cosener's, 7th September 2011

10. Cross-section summary
Individual combinations (l+jets and dilepton)
ATLAS
CMS
UK HEP forum, Cosener's, 7th September 2011

11. Other cross-section measurements
Single top
t-channel
s-channel and tW
Resonance searches
Charge asymmetry
UK HEP forum, Cosener's, 7th September 2011

12. UK HEP forum, Cosener's, 7th September 2011
Single top production
Three EW diagrams contribute to production of single top quarks at hadron colliders
t-channel production has the largest predicted cross-section
Main backgrounds t-tbar and W+jets
UK HEP forum, Cosener's, 7th September 2011

13. t-channel cross-section
t-channel (dominant)
Central b-tagged jet
Forward light jet
Leptonic decay of W
Isolated lepton
Large MT(ln)
Variety of approaches
Backgrounds are large
2 jets
3 jets
Boosted decision tree (BDT)
Cut-based
UK HEP forum, Cosener's, 7th September 2011

14. t-channel cross-section
ATLAS 2011 cut-based:
CMS 2010 combined:
(dominated by BDT)
UK HEP forum, Cosener's, 7th September 2011

15. Wt and s-channel single top searches
95% CL
1.2s
ATLAS
signal exp
16±6
total exp
285±17
observed
296
UK HEP forum, Cosener's, 7th September 2011

16. Searches for t-tbar resonance
ATLAS l+jets
Reject extra jets from radiation
ATLAS dilepton
CMS l+jets
Kinematic fit and b-tagging
UK HEP forum, Cosener's, 7th September 2011

17. UK HEP forum, Cosener's, 7th September 2011
Charge asymmetry
Small effect predicted in SM
Top quark h distribution narrower than antiquark
But dominant gg production is symmetric
Measured quantity:
Theory predicts small positive asymmetry
Lepton+jets selection
Fit to identify top and anti-top
Unfold measured asymmetry
CMS result:
Theory:
AC=0.013±0.001
Measured:
AC=-0.016±
ATLAS result:
Theory:
AC=0.006
Measured:
AC=-0.024±0.016±0.023
UK HEP forum, Cosener's, 7th September 2011

18. UK HEP forum, Cosener's, 7th September 2011
Top properties
UK HEP forum, Cosener's, 7th September 2011

19. mt in lepton+jets (ATLAS 2011)
Template method
Pick the “best” jet combination hypothesis per event
Kinematic fit with mW constraint
Simultaneous fit to mt and Jet Scale Factor
UK HEP forum, Cosener's, 7th September 2011

20. mt in lepton+jets (CMS 2010)
Ideogram method
Kinematic fit and per-event likelihood vs mt
Uses all jet combination hypotheses
JES as systematic correction
Additional CMS measurement in dilepton events:
UK HEP forum, Cosener's, 7th September 2011

21. Measurement of pole mass
ATLAS
Direct measurements yield MC mass parameter mt
Theoretical cross-section calculations depend on the pole mass
Mass uncertainty 4× smaller than cross-section uncertainty
Pole mass looks smaller than MC mass
UK HEP forum, Cosener's, 7th September 2011

22. t-tbar mass difference
Ideogram method modified
Lepton+jets
Use lepton to distinguish t and tbar
Fit mass of hadronic top only
2011 data (1.09 fb−1)
CMS
UK HEP forum, Cosener's, 7th September 2011

23. UK HEP forum, Cosener's, 7th September 2011
Spin correlation
ATLAS
Parton level
Data vs MC
Top quark decays before hadronisation
Spin correlations at production propagated to decay products
Charged lepton from W decay carries full spin information
Measure distribution of Df between leptons in dilepton events
Fit to a mixture of SM and uncorrelated templates
Results consistent with SM expectations
SM fraction FSM=1.06±0.21
UK HEP forum, Cosener's, 7th September 2011

24. W helicity measurement
ATLAS
Test of V−A structure of top quark decay
Angular distribution of lepton and b quark in W rest frame
Fit to LH, RH and longitudinally polarised W
Results consistent with SM expectations
Longitudinal fraction F0=0.75±0.08
Improved limits on anomalous couplings
UK HEP forum, Cosener's, 7th September 2011

25. UK HEP forum, Cosener's, 7th September 2011
Summary and outlook
LHC is a top factory
Initial measurements in l+jets and dilepton channels now systematics dominated
Cross-section precision (7%) approaching that of theory prediction
Single top production confirmed
Top quark properties consistent with SM (so far)
Top is an active field in searches for New Physics
Many studies just beginning
UK HEP forum, Cosener's, 7th September 2011

26. UK HEP forum, Cosener's, 7th September 2011
List of sources
UK HEP forum, Cosener's, 7th September 2011

27. Top pair cross section measurements
L+jets:
ATLAS 2010 analysis: 3, 4, >4-jet subsamples (ATLAS-CONF )
CMS 2010 analysis similar (arXiv: )
B-tagging gives best results
Untagged analysis on 2011 data (ATLAS-CONF )
Dilepton:
ATLAS fb-1 (ATLAS-CONF )
Simple cut-based, no b-tagging
CMS fb-1 (CMS PAS TOP )
Combinations
ATLAS combine the above two results (ATLAS-CONF )
CMS combine the two 2010 measurements (arXiv: )
All-hadronic and tau dilepton
CMS all-hadronic measurement on 2011 data (CMS PAS TOP )
ATLAS all-hadronic search on 2010 data (ATLAS-CONF )
ATLAS mu+tau measurement (ATLAS-CONF )
CMS mu+tau measurement (CMS PAS TOP )
UK HEP forum, Cosener's, 7th September 2011

28. Production measurements; single top
Spin correlation (ATLAS-CONF )
Charge asymmetry
(ATLAS-CONF )
(CMS PAS TOP )
Single top
t-channel measurement (ATLAS-CONF )
t-channel measurement (arXiV: )
s-channel search (ATLAS-CONF )
Search for Wt (ATLAS-CONF )
UK HEP forum, Cosener's, 7th September 2011

29. Top mass and properties, exotica
Template method (ATLAS-CONF )
Mass from cross-section (ATLAS-CONF )
Ideogram method (CMS PAS TOP )
Mass in dileptons (included in arXiV: )
Mass difference (CMS PAS TOP )
W polarisation (ATLAS-CONF )
Resonance search
Dilepton channel (ATLAS-CONF )
Lepton+jets channel (ATLAS-CONF )
CMS search (CMS PAS TOP )
Exotic searches
Search for FCNC (ATLAS-CONF )
Search for anomalous MET (ATLAS-CONF )
UK HEP forum, Cosener's, 7th September 2011