T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 1 Rencontres de Moriond QCD 2016 La Thuile, Aosta Valley, Italy Measurements @ ATLAS & CMS Sunday, March 20 th , 2016 Top Quark Mass Thomas McCarthy 1 on behalf of the ATLAS & CMS Collaborations 1 Max-Planck-Institut für Physik, München
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T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 1
Rencontres de Moriond QCD 2016 La Thuile, Aosta Valley, Italy
Measurements @ ATLAS & CMS
Sunday, March 20th, 2016
Top Quark Mass
Thomas McCarthy1on behalf of the ATLAS & CMS Collaborations
1Max-Planck-Institut für Physik, München
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 2
General Overview
1 Overview & Highlights of LHC Run 1 mtop Measurements (√s = 7 & 8 TeV)
2 LHC Combinations from Run 1
3 Summary & Outlook
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResultsLink to ATLAS Top Quark Public Results
Link to CMS Top Quark Public Resultshttps://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsTOP
A nice summary of LHC mtop
measurements @ Run 1…G. Cortiana (MPI Munich) Submitted to Reviews in Physics
“Top-quark mass measurements: review and perspectives”
http://arxiv.org/abs/1510.04483
pp
Direct measurements from kinematics & event reconstructionAlternative measurements (e.g. from σtt)-
Focus today on a select number of analyses (many more results available, see backup or below)
mtop a fundamental parameter of the Standard Model Short lifetime O(10-25) ➙ no bound hadronic states formed Direct access to top quark properties via its decay products Better knowledge of mtop ➙ better performance in analyses with top backgrounds
CMS-PAS-TOP-13-004 *Only most precise measurements in each channel summarized on this page. See links to additional analyses in overviews on later slides.
2D Template Method mtop extracted together with global JSF (compared w/ 1D)
Updated since Moriond 2015!
b-Dependent JEC Data Statistics Backgrounds In-Situ JEC
Jet energies modified by constraints from kinematic fits using known W boson mass Multi-jet background is modelled using an event-mixing technique (able to get fsig ~ 78%!) Likelihood built from S & B shapes, splitting signal into correct or wrong (from simulation)
b-Dependent JEC Semileptonic B-decay modelling Data Statistics
Left: large reduction in (mostly combinatorial) background by applying goodness-of-fit (Pgof) weighting Above: Also evaluate mt separately in different bins of kinematic variables sensitive to top quark production & decay
Final measured result (1D vs hybrid compared):
CERN-PH-EP-2015-234
Precision of ~0.3%!
Best-precision measurement in a single channel at CMS
Analysis strategy similar to 7 TeV and 7/8 TeV all-hadronic
Fitted jet energies used to reconstruct final observable
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 8
MC Mass
Pole Mass
Physics Analysis Summary (PAS)
√s = 7 TeV
√s = 8 TeVLepton(s) + J/𝜓 Events @ 8 TeV
Measured Result:
Very small BR, but sensitivity to mtop w/o using jets to build observable (avoid JES/bJES)
Variation on semileptonic / dileptonic channels, requiring at least one of the top quarks have a final leptonic state For the main leptonic top quark decay, aim to identify cases where b-quark decays via (b ➙J/𝜓 + X ➙ µ+µ- + X)
Remains statistically limited for now
Top Quark pT ME-PS Matching Threshold Renormalization Scale
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 9
MC Mass
Pole Mass
Will supersede previous result
√s = 7 TeV
√s = 8 TeVSecondary Vertices + Lepton(s)
Sensitivity to mtop primarily from leptons (e/µ) and via decay lengths of charged hadrons (from b-quark decay) Target signal events in both semileptonic and dileptonic channels:
e + jets µ + jets
ee µµ eµDileptonic tt Channel:
Semileptonic tt Channel:
--
Reconstruct charged hadrons (left: J/𝜓, but also D0, D*) resulting from b-quark decays Allows one to retain a stronger sensitivity to mtop without inclusion of jets (where related systematics would otherwise dominate) Ultimately select msvl (invariant mass of lepton / secondary vertex) as mtop-sensitive observable Inclusion of b-quark fragmentation studies (leading systematic)
More generalized version of J/𝜓 analysis
Measured Result:
Dominant systematics: top quark pT & b-quark fragmentation
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016
tW +
bW
proton
proton
b
+
q q
13
Direct mtop Measurements
Advantages:Access to mtop through EW production of top quarks Orthogonal datasets w.r.t. other channels ➙ combinations
Lower cross-sections and higher backgrounds
Disadvantages:
Single-Top Enriched Channel
Single-Top Enriched Events
ATLAS-CONF-2014-0558 TeV 1D template m(lb) & NN
tt BR ~ 25%-(1)
(1) W Branching Ratios from: K.A. Olive et al. (Particle Data Group), Chin. Phys. C, 38, 090001 (2014) and 2015 update (Value quoted is for single top quarks and includes all lepton types: e/µ/𝜏 except hadronic 𝜏 decays)
(Would be ~33% including hadronic tau decays)
e + jets µ + jets
e µ + jetst-channel with µ + jets8 TeV CMS-PAS-TOP-15-001
Planned measurements of mtop @ √s = 13 TeV Higher √s ➙ greater % of boosted top quarks As always, trade off between with stats / syst Avoiding jets ➙ avoid (b)JES systematics…but b-quark fragmentation / top pT systematics increase New methods should aim to strike optimal balance (see nice LHC talk in EW session last week) [link]
Great success by ATLAS & CMS in making precision mtop measurements during Run 1! Several novel approaches with a variety of final-state signatures (& differing backgrounds)
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 32
√s = 7 TeV
√s = 8 TeV
MC Mass
Pole Mass
Employs ideogram method Investigated precision of 1D or 2D method (2D method simultaneously extracts global JES) Likelihood variable based on S & B parameterizations:
Signal split into three permutation cases: (correct, incorrect, unmatched) from simulation Event weights lessen impact of incorrect jet-parton associations from kinematic fit (keeping total number of events unchanged)
Jet energies modified by constraints from kinematic fits Jet Energy Scale (JES)
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 39
MC Mass
Pole Mass
√s = 7 TeV
√s = 8 TeV+ Jets Channel @ 8 TeV
Comparisons between 1D, 2D and hybrid template method Hybrid: incorporates prior knowledge about JSF using a Gaussian constraint Best precision obtained in hybrid case as in all-had channel
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 46
Pole Mass √s = 8 TeV
MC Mass √s = 7 TeVEur.Phys.J. C74 (2014) 3109Pole Mass from Cross-Section at √s = 7 & 8 TeV
Measured Result:
Predominantly a high-precision cross-section calculation measurement Pole mass (mtop) can simultaneously be extracted from dependence on XS Work specifically in the eµ channel (oppositely charged leptons)
where
}Solve for&
Allows for solution of system of equations below Simultaneous measurements:
Look @ Number of Data Events with 1 or 2 b-tagged Jets (N1 & N2, respectively)
Inclusive cross-section ( ) Efficiency to reconstruct and tag a b-quark jet ( )
Luminosity Parton Distribution Functions (PDFs)
Dominant Uncertainties (Largely Theoretical):Uncertainties on 𝛼s
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 47
MC Mass
Pole MassPole Mass from Cross-Section at √s = 7 & 8 TeV
Measurements of σtt together with NNLO theoretical prediction allow for the extraction of the top quark pole mass (mt) Relies on a given choice of PDF sets and 𝛼s (quoted result uses NNPDF3.0, 𝛼s = 0.118 ± 0.001)
Consistent results using CT14 and MMHT2014 Parameterize functional dependence of σtt on mt as exponential function
Maximization of final likelihood yields measured value (after 7/8 TeV combination):
Submitted to JHEP
√s = 8 TeV
√s = 7 TeV
Precision cross-section measurements performed at √s = 7 & 8:
Great improvement previous measurement:
Above: Measured values (black markers) with the vertical bars indicating the total 1-σ uncertainty. The theoretical predictions are shown as the coloured bands.
Three points above correspond to different mt hypotheses(for which σtt fit is repeated) ∆mt/mt ~ 1%
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 48
MC Mass
√s = 8 TeVPole Mass
√s = 7 TeVPhys.Lett.B. 738 (2014) 526Pole Mass from Cross-Section at √s = 7 TeV
Measured Result:
!Measured value taken from dileptonic (eµ) channel result (most precise) Result gives a Baysian contour interval on mtop under external constraint of 𝛼s
Baysian analysis to determine a value of mtop (pole mass) based on measured and theoretical σtt
Very good precision on 𝛼s:
Sensitivity to Uncertainty on 𝛼s
But even shifting by this amount greatly affects ∆mtop:
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 49
Alternate Measurements
Advantages:Can take advantage of ‘tag and probe’ method of lepton + jets channel (relatively low backgrounds) Complimentary to direct top quark mass measurements as there no distinction is made between top/anti-top
Very sensitive to detector’s b- vs. b-jet response Differences in W- / W+ production also significant systematic uncertainty
T.G.McCarthy - Top Quark Mass @ ATLAS & CMS Rencontres de Moriond QCD Session, March 20, 2016 51
√s = 7 TeV
√s = 8 TeV
MC Mass
Pole MassTop/Anti-top Quark Mass Difference √s = 7 TeV
Performed separately for e+jets and µ+jets channels Final likelihood-based fit performed separately for each distribution (as shown above) Results show ∆mt = |mtop - manti-top| consistent with 0 as required by CPT theorem
Average mt also consistent with other measurements