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Top Quark Mass Measurements at Hadron Colliders G. WATTS (UW/SEATTLE, CPPM) For the DZERO, CDF, CMS, and ATLAS collaborations July 15, 2014.

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Presentation on theme: "Top Quark Mass Measurements at Hadron Colliders G. WATTS (UW/SEATTLE, CPPM) For the DZERO, CDF, CMS, and ATLAS collaborations July 15, 2014."— Presentation transcript:

1 Top Quark Mass Measurements at Hadron Colliders G. WATTS (UW/SEATTLE, CPPM) For the DZERO, CDF, CMS, and ATLAS collaborations July 15, 2014

2 The Top Quark G. Watts (UW/Seattle) FFP Marseille 2 Just like other Fermions Except: The next heaviest quark! The Mass gives the top quark a special role in the Standard Model Only fermion which has a significant coupling to the Higgs Plays key roll in many important physics processes Flavor physics, Electro-weak processes It plays a special roll in a number of Beyond the Standard Model theories as well

3 The Top Mass G. Watts (UW/Seattle) FFP Marseille 3 By far the most precisely measured quark mass! While it behaves like any other quark in the Standard Model, its mass gives it a unique role. Only version for which the coupling to the Higgs is important Stability of the SM Higgs potential at high scales A consistency check for the Standard Model!

4 G. Watts (UW/Seattle) FFP Marseille 4 Each measurement deserves at least a seminar I have chosen a few extra results Current World Average: GeV. Known to better than 0.5 %!! Higgs mass is known to better than 0.3% Top is easier to discover: Top is harder to reconstruct:

5 G. Watts (UW/Seattle) FFP Marseille 5 Tevatron LHC

6 Decays G. Watts (UW/Seattle) FFP Marseille 6 Dilepton events Clean, but low statistics ~4% Lepton + Jet events Good compromise Reasonable background ~30% All Hadronic events Huge multi-jet background ~44% Top mass has been measured in all decay channels. Classified by the Ws’ decay

7 The Tevatron & The LHC G. Watts (UW/Seattle) FFP Marseille 7 The Tevatron is coming out with its final results

8 G. Watts (UW/Seattle) FFP Marseille 8 Does not always give you 4-vectors (neutrinos!) Detector/Object resolutions (e.g. Jet Energy Scale) Background contamination Incorrect reconstruction (e.g. bad jet assignment) Top mass width Etc. Two common methods to address this: Matrix Element Uses all the information Computationally very expensive Template Method Flexible, subsets the information used “Fairly easy” to implement Detector gives you 4-vectors. Use Griffiths! What do we measure? The Pole mass? The MC mass?

9 The Jet Energy Scale G. Watts (UW/Seattle) FFP Marseille 9 Common curse for all methods Experiments normally measure in independent control sample. Resolution not good enough for a state- of-the-art top mass measurement. In situ Jet Energy Scale measurement Two poorly measured objects One very well measured object Global fit over the full sample Scale all jets by a constant factor to achieve constraint Lepton+Jets Flavor Jet Energy Scale

10 The Matrix Element Approach G. Watts (UW/Seattle) FFP Marseille 10 A reverse Monte Carlo MC Generates 100K events Distributions of kinematic variables for all objects “Map of kinematic phase space” Turn that around Given a single event in data, how dense a part of kinematic phase space is it in?

11 ME – Multiple Steps G. Watts (UW/Seattle) FFP Marseille 11 ALPGEN + Pythia Detector Simulation Reconstruct ion 4 vectors of reconstructed objects Normalization Sum over all possible jet assignments Which jet is the first tops? Which jets belong to the W?

12 ME – Multiple Steps G. Watts (UW/Seattle) FFP Marseille 12 ALPGEN + Pythia Detector Simulation Reconstruct ion 4 vectors of reconstructed objects 10 dimensional integral over phase space Mass of the tops, W’s Directions of the b-quarks Lepton and neutrino direction Note no mention of data 4-vectors yet!

13 ME – Multiple Steps G. Watts (UW/Seattle) FFP Marseille 13 ALPGEN + Pythia Detector Simulation Reconstruct ion 4 vectors of reconstructed objects Sum over incoming parton flavors All neutrino solutions The Leading Order Matrix Element Given all the phase space parameters Weight for the kinematics values Uses all available information At leading order

14 ME – Multiple Steps G. Watts (UW/Seattle) FFP Marseille 14 ALPGEN + Pythia Detector Simulation Reconstruct ion 4 vectors of reconstructed objects PDF’s Phase Space Factor Transverse momenta of incoming partons

15 ME – Multiple Steps G. Watts (UW/Seattle) FFP Marseille 15 ALPGEN + Pythia Detector Simulation Reconstruct ion 4 vectors of reconstructed objects

16 G. Watts (UW/Seattle) FFP Marseille 16 In used at DZERO since Run I Total error is equivalent to March world average! 3 years of work (old result): Use different top mass in the Matrix Elements Vary the Jet Energy Scale in the transfer functions

17 What Did 3 years get? G. Watts (UW/Seattle) FFP Marseille 17 Speed (CPU) to allow better MC stats X100 increase means MC stats error drops from ~0.25 GeV to ~0.05 GeV. New Jet Energy Scale Calibrations ISR modeling Constrain by studies in Drell-Yan data Gives an experimental bound to ISR mis-modeling

18 Template Method G. Watts (UW/Seattle) FFP Marseille 18 Use a likelihood to estimate template compatibility Make it for each sample Can do in two dimension Jet energy scale Top mass

19 Top Mass In Dilepton Events G. Watts (UW/Seattle) FFP Marseille 19 Very little SM background! CDF’s basic selection: Observe 520 events, expect 78% purity ATLAS’ basic selection: Observe 2913, expect 96% purity Really excellent top lab Except… There are no 4-vectors for the two!!

20 Template Method G. Watts (UW/Seattle) FFP Marseille 20 Need distributions that are strongly correlated with the top mass Template method to figure out the top mass ATLAS Two permutations (take smallest) Good separation power

21 CDF Template Variables G. Watts (UW/Seattle) FFP Marseille 21 Fully reconstruct the top mass There are not enough constraints to solve for solution! Grid in the azimuthal angles

22 Statistics Isn’t The Problem… G. Watts (UW/Seattle) FFP Marseille 22 Broad peak, but decent separation power. Leading systematic: Jet Energy Scale! This measurement is statistics limited. Can something be done?

23 Statistics Isn’t The Problem… G. Watts (UW/Seattle) FFP Marseille 23 Broad peak, but decent separation power. Leading systematic: Jet Energy Scale! This measurement is statistics limited. Can something be done? CDF creates a second template variable: And combines the two, optimizing for minimal error

24 Dilepton Top Mass Results G. Watts (UW/Seattle) FFP Marseille 24 Standard Template Method Jet Energy Scale isn’t fit: not enough constraints Statistics already making a big difference here

25 Top Mass in All Hadronic Decays (CDF & CMS) G. Watts (UW/Seattle) FFP Marseille 25 44% of all decays. Largest single decay class. Overwhelmed by SM QCD background! 6 Jets After CMS requires 6 jets Estimated signal purity is 3% Signal Efficiency is 3.5%!

26 Improving the Purity G. Watts (UW/Seattle) FFP Marseille 26 Unique Handles: (CDF) Mass of the pairs of light quark jets

27 Improving the Purity G. Watts (UW/Seattle) FFP Marseille 27 Raise CMS’s purity to 39% Additional kinematic selection Raise CMS’s purity to 54% CDF has a purity of 57%

28 Extracting the Mass G. Watts (UW/Seattle) FFP Marseille 28 The Template Method

29 Lepton + Jets From CMS G. Watts (UW/Seattle) FFP Marseille 29 Analysis is very similar to the All-Jets analysis from CMS Initial selection is > 100K events and 94% pure QCD background is negligible! A simple kinematic fit to clean up incorrect jet assignments Largest systematic error is the flavor dependent Jet Energy Scale (0.41 GeV)

30 Conclusions G. Watts (UW/Seattle) FFP Marseille 30 If you believe BICEP2!

31 Awaiting the next world Combination… G. Watts (UW/Seattle) FFP Marseille 31 Current World Combination Tevatron Combination CMS Combination

32 Systematic Errors G. Watts (UW/Seattle) FFP Marseille 32

33 G. Watts (UW/Seattle) FFP Marseille 33 ATLAS Lepton+Jets Template

34 G. Watts (UW/Seattle) FFP Marseille 34 ATLAS dilepton 7 TeV CDF dilepton

35 G. Watts (UW/Seattle) FFP Marseille 35 CDF all jets CMS all jets

36 G. Watts (UW/Seattle) FFP Marseille 36 CMS All Jets 7 TeV

37 G. Watts (UW/Seattle) FFP Marseille 37 Tevatron Combination DZERO Lepton+Jets ME


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