Top Quark Mass Measurements at Hadron Colliders
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Top Quark Mass Measurements at Hadron CollidersG. WATTS (UW/SEATTLE, CPPM)
For the DZERO, CDF, CMS, and ATLAS collaborations
July 15, 2014
G. Watts (UW
/Seattle) FFP 2014 - Marseille
2The Top QuarkJust like other Fermions
Except:ππ‘ 40Γππ
The next heaviest quark!
The Mass gives the top quark a special role in the Standard Model
β’ Only fermion which has a significant coupling to the Higgsβ’ Plays key roll in many important physics processes
β’ Flavor physics, Electro-weak processesβ’ It plays a special roll in a number of Beyond the Standard
Model theories as well
G. Watts (UW
/Seattle) FFP 2014 - Marseille
3The Top MassWe have known almost since it was discovered.
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.
β’ Only version for which the coupling to the Higgs is importantβ’ Stability of the SM Higgs
potential at high scalesA consistency check for the
Standard Model!β’ Shows up in a number of production
loopsβ’ at the LHC contains a top loopβ’ Heavy Flavor physics (e.g. )
production
G. Watts (UW
/Seattle) FFP 2014 - Marseille
4Top Mass Is A Precision
Measurement
Each measurement deserves at least a
seminar
I have chosen a few extra results
Current World Average: 173.3 GeV.Known to better than 0.5 %!!
Higgs mass is known to better than 0.3%
Top is easier to discover: at TeV at TeV
No clean easy to see peak l!All final states involve jets
Top is harder to reconstruct:
G. Watts (UW
/Seattle) FFP 2014 - Marseille
5Te
vatro
nLH
C
G. Watts (UW
/Seattle) FFP 2014 - Marseille
6Decays
Dilepton eventsClean, but low statistics~4%
Lepton + Jet eventsGood compromiseReasonable background~30%
All Hadronic eventsHuge multi-jet background~44%
Top mass has been measured in all decay
channels.
π‘ π‘βπ+ΒΏππ βπΒΏ
Classified by the Wsβ decay
G. Watts (UW
/Seattle) FFP 2014 - Marseille
7The Tevatron & The LHCThe Tevatron is coming out with its final results
β’ of data at TeVβ’ Well understood detectorβ’ Sophisticated analysis techniques
The LHC is just coming online in the worldβ’ TeV results well developedβ’ 8 TeV results just appearingβ’ Statistics are much better due to the much higher
The much larger statistics will eventually open the door to new measurement techniques.
G. Watts (UW
/Seattle) FFP 2014 - Marseille
8Extracting from Data
β’ Does not always give you 4-vectors (neutrinos!)β’ Detector/Object resolutions (e.g. Jet Energy Scale)β’ Background contaminationβ’ Incorrect reconstruction (e.g. bad jet assignment)β’ Top mass widthβ’ Etc.
Two common methods to address this:
Matrix Element Uses all the informationComputationally very expensive
Template Method Flexible, subsets the information usedβFairly easyβ to implement
Detector gives you 4-vectors. Use Griffiths!
What do we measure? The Pole mass? The MC mass?
G. Watts (UW
/Seattle) FFP 2014 - Marseille
9The Jet Energy ScaleCommon curse for all methods
β’ Experiments normally measure in independent control sample.
β’ Resolution not good enough for a state-of-the-art top mass measurement.
In situ Jet Energy Scale measurement
πβππ β²Two poorly measured
objectsOne very
well measured
object
Many techniques will constrain to be as part of the global fitting process.
Global fit over the full sampleβ’ Scale all jets by a constant
factor to achieve constraint
Lepton+Jets
Flavor Jet Energy Scale
G. Watts (UW
/Seattle) FFP 2014 - Marseille
10The Matrix Element
ApproachA reverse Monte Carlo
MC Generates
100K events
Distributions of kinematic
variables for all objects
βMap of kinematic
phase spaceβ
Turn that aroundGiven a single event in data, how dense a
part of kinematic phase space is it in?
Repeat for all major backgrounds and signal:
π
G. Watts (UW
/Seattle) FFP 2014 - Marseille
11ME β Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
π (ππ‘ππ )= 1ππππ
π‘ π‘ (ππ‘ππ )βπ=1
24
π€ π
Normalization
Sum over all possible jet assignmentsβ’ Which jet is the first tops?β’ Which jets belong to the
W?
A weight reflecting the probability of those jet assignmentsβ’ -tagging
probabilities
G. Watts (UW
/Seattle) FFP 2014 - Marseille
12ME β Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
π (ππ‘ππ )= 1ππππ
π‘ π‘ (ππ‘ππ )βπ=1
24
π€ πβ«ππ ππ12ππ1
2 ππ22ππ 2
2ππ βππ1π₯ππ1
π¦ ππ2π₯ππ2
π¦
10 dimensional integral over phase spaceβ’ Mass of the tops, Wβsβ’ Directions of the b-quarksβ’ Lepton and neutrino direction
Note no mention of data 4-vectors yet!
G. Watts (UW
/Seattle) FFP 2014 - Marseille
13ME β Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
π (ππ‘ππ )= 1ππππ
π‘ π‘ (ππ‘ππ )βπ=1
24
π€ πβ«ππ ππ12ππ1
2 ππ22ππ 2
2ππ βππ1π₯ππ1
π¦ ππ2π₯ππ2
π¦
βπ πππ‘ππ ππππ£πππ ,π
β
|ππ‘π‘|2
Sum over incoming parton flavorsAll neutrino solutions
The Leading Order Matrix Elementβ’ Given all the phase space
parametersβ’ Weight for the kinematics
valuesβ’ Uses all available
informationβ’ At leading order
G. Watts (UW
/Seattle) FFP 2014 - Marseille
14ME β Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
π (ππ‘ππ )= 1ππππ
π‘ π‘ (ππ‘ππ )βπ=1
24
π€ πβ«ππ ππ12ππ1
2 ππ22ππ 2
2ππ βππ1π₯ππ1
π¦ ππ2π₯ππ2
π¦
βπ πππ‘ππ ππππ£πππ ,π
β
|ππ‘ π‘|2 π β² (π1 ) π β² (π2 )
β (ππΌπ½π1πΌπ2π½ )2βππ12 ππ2
2Ξ¦6
PDFβs
Phase Space FactorTransverse
momenta of incoming partons
G. Watts (UW
/Seattle) FFP 2014 - Marseille
15ME β Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
π (ππ‘ππ )= 1ππππ
π‘ π‘ (ππ‘ππ )βπ=1
24
π€ πβ«ππ ππ12ππ1
2 ππ22ππ 2
2ππ βππ1π₯ππ1
π¦ ππ2π₯ππ2
π¦
βπ πππ‘ππ ππππ£πππ ,π
β
|ππ‘π‘|2 π β² (π1 ) π β² (π2 )
β (ππΌπ½π1πΌπ2π½ )2βππ 12 ππ 2
2Ξ¦6π (π₯ , π¦ ;π , π β ,β¦)
Transfer Functionsβ’ Given a generated jet with what is the probability DZERO
will reconstruct values x and y?β’ Detector and reconstruction resolution
G. Watts (UW
/Seattle) FFP 2014 - Marseille
16DZERO using the ME
MethodIn used at DZERO since Run I
GeV3.6
GeV
Total error is equivalent to March world average!
3 years of work (old result):
β’ Use different top mass in the Matrix Elements
β’ Vary the Jet Energy Scale in the transfer functions
G. Watts (UW
/Seattle) FFP 2014 - Marseille
17What Did 3 years get?
β’ Speed (CPU) to allow better MC statsβ’ X100 increase means MC stats error
drops from ~0.25 GeV to ~0.05 GeV.
β’ New Jet Energy Scale Calibrationsβ’ ISR modeling
β’ Constrain by studies in Drell-Yan data
β’ General modeling improvements
The variable is sensitive to Z boson recoil ().Gives an experimental bound to ISR mis-modeling
Systematic error on reduced from ~0.25 to 0.06 GeV
G. Watts (UW
/Seattle) FFP 2014 - Marseille
18Template Method
Using a distribution sensitive to :
Simulated sample at GeV
Simulated sample at GeV
Simulated sample at GeV
Use a likelihood to estimate template
compatibility
Make it for each sample
ππ‘
Can do in two dimensionβ’ Jet energy scaleβ’ Top mass
G. Watts (UW
/Seattle) FFP 2014 - Marseille
19Top Mass In Dilepton
Events4% of all decays, split into , and .
Very little SM background!
CDFβs basic selection: Observe 520 events, expect 78% purityATLASβ basic selection: Observe 2913, expect 96% purity
Really excellent top labExceptβ¦
For 2 !!! There are no 4-vectors for the two!!
G. Watts (UW
/Seattle) FFP 2014 - Marseille
20Template Method
Need distributions that are strongly correlated
with the top massTemplate method to
figure out the top mass
ATLASThe average in the event
Two permutations (take smallest)Avoid the missing resolution
Good separatio
n power
G. Watts (UW
/Seattle) FFP 2014 - Marseille
21CDF Template Variables
Fully reconstruct the top massProblem: detector measures missing
There are not enough constraints to solve for solution!The weighting method
π1
π2
Grid in the azimuthal anglesβ’ Fit for the top mass at
each grid location.β’ Resulting is the template
variable.β’ Weight by fit .
The fit includes terms for:β’ All the measurements (2 leptons, two jets, missing )β’ Top mass and the (constrained) W mass
G. Watts (UW
/Seattle) FFP 2014 - Marseille
22Statistics Isnβt The
Problem⦠Broad peak, but decent separation power.
Leading systematic:Jet Energy Scale!
This measurement is statistics limited.Can something be done?
G. Watts (UW
/Seattle) FFP 2014 - Marseille
23Statistics Isnβt The
Problem⦠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:
GeV β’ Depends on 4-vector of leptonsβ’ Direction of jetsβ’ No Jet Energy Scale, no Missing
And combines the two, optimizing for minimal errorπ π‘
πππ‘=π€ βππ‘πππ‘+ (1βπ€ ) βππ‘
πππ‘
G. Watts (UW
/Seattle) FFP 2014 - Marseille
24Dilepton Top Mass
ResultsStandard Template MethodJet Energy Scale isnβt fit: not enough constraints
Statistics already making a big difference here
G. Watts (UW
/Seattle) FFP 2014 - Marseille
25Top Mass in All Hadronic
Decays (CDF & CMS)44% of all decays. Largest single decay class.
Overwhelmed by SM QCD background!
6 Jets
After CMS requires 6 jets4 jets with GeV5th with GeV6th with GeV
Estimated signal purity is 3%Signal Efficiency is 3.5%!
G. Watts (UW
/Seattle) FFP 2014 - Marseille
26Improving the Purity
Unique Handles:2 -jets
2 Look for -tagged jetsPerform kinematic fit:β’ Know β’ The two are the same2
(CDF)
Mass of the pairs of light quark jets
is the well measured value of 80.4 GeV
Mass of the pairs of light quark jets
free parameters
Every jet permutation is triedMinimum is kept
G. Watts (UW
/Seattle) FFP 2014 - Marseille
27Improving the Purity
1. Requiring the fit to converge2. Very basic cuts on the
Raise CMSβs purity to 39%
Additional kinematic selection
CMS: CDF: Neural Network
Raise CMSβs purity to 54%CDF has a purity of 57%
G. Watts (UW
/Seattle) FFP 2014 - Marseille
28Extracting the Mass
ππ‘ππππ ππ‘
β
The Template MethodFit for both Jet Energy Scale and
G. Watts (UW
/Seattle) FFP 2014 - Marseille
29Lepton + Jets From CMS
Full TeV result: 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
β’ Each possible jet assignment gives β’ Each is weighted by the fit probability
Largest systematic error is the flavor dependent Jet Energy Scale (0.41 GeV)
G. Watts (UW
/Seattle) FFP 2014 - Marseille
30Conclusions
Field is still rapidly evolving World average submitted in
March CDF dilptons and all-hadronic DZERO matrix element CMS all-hadronic and
lepton+jets What is next?
Tevatron will finish putting out βfinalβ mass measurements
LHCβs statistics and purity mean it should quickly surpass the Tevatron.
LHC Run 2 projections Other measurements with the
quark mass Top and anti-top have
consistent masses measurements that can
clarify which mass we measure.
Becoming like the W massβ¦
If you believe BICEP2
!
G. Watts (UW
/Seattle) FFP 2014 - Marseille
31Awaiting the next world
Combinationβ¦
Current World Combination
Tevatron CombinationCMS Combination
Β±0.95 Β±0.76 Β±0.64
G. Watts (UW
/Seattle) FFP 2014 - Marseille
32
Systematic Errors
G. Watts (UW
/Seattle) FFP 2014 - Marseille
33ATLAS Lepton+Jets Template
G. Watts (UW
/Seattle) FFP 2014 - Marseille
34ATLAS dilepton 7 TeV CDF dilepton
G. Watts (UW
/Seattle) FFP 2014 - Marseille
35
CDF all jets CMS all jets
G. Watts (UW
/Seattle) FFP 2014 - Marseille
36CMS All Jets 7 TeV
G. Watts (UW
/Seattle) FFP 2014 - Marseille
37
Tevatron Combination
DZERO Lepton+Jets ME
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