top quark mass measurements at hadron colliders g. watts (uw/seattle, cppm) for the dzero, cdf, cms,...
TRANSCRIPT
Top Quark Mass Measurements at Hadron CollidersG. WATTS (UW/SEATTLE, CPPM)
For the DZERO, CDF, CMS, and ATLAS collaborations
July 15, 2014
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ille2The Top Quark
Just like other Fermions
Except:
𝑚𝑡 40×𝑚𝑏
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
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ille3The Top Mass
We 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.
• 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!
• Shows up in a number of production loops• at the LHC contains a top loop• Heavy Flavor physics (e.g. )
production
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ille4Top 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:
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Tevatr
on
LHC
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ille6Decays
Dilepton events
Clean, but low statistics~4%
Lepton + Jet events
Good compromiseReasonable background~30%
All Hadronic events
Huge multi-jet background~44%
Top mass has been measured in all decay
channels.
𝑡 𝑡→𝑊+¿𝑏𝑊 −𝑏¿
Classified by the Ws’ decay
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ille7The Tevatron & The LHC
The Tevatron is coming out with its final results
• of data at TeV• Well understood detector• Sophisticated analysis techniques
The LHC is just coming online in the world
• TeV results well developed• 8 TeV results just appearing• Statistics are much better due to the much higher
The much larger statistics will eventually open the door to new measurement techniques.
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ille8Extracting from Data
• 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 ElementUses all the informationComputationally very expensive
Template MethodFlexible, 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?
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ille9The Jet Energy Scale
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
𝑊→𝑞𝑞 ′
Two poorly measured
objects
One very well
measured object
Many techniques will constrain to be as part of the global fitting process.
Global fit over the full sample• Scale all jets by a constant
factor to achieve constraint
Lepton+Jets
Flavor Jet Energy Scale
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The Matrix Element ApproachA 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?
Repeat for all major backgrounds and signal:
𝑃
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ME – Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
𝑃 (𝑚𝑡𝑜𝑝 )= 1𝜎𝑜𝑏𝑠
𝑡 𝑡 (𝑚𝑡𝑜𝑝 )∑𝑖=1
24
𝑤 𝑖
Normalization
Sum over all possible jet assignments• Which jet is the first tops?• Which jets belong to the
W?
A weight reflecting the probability of those jet assignments• -tagging
probabilities
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ME – Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
𝑃 (𝑚𝑡𝑜𝑝 )= 1𝜎𝑜𝑏𝑠
𝑡 𝑡 (𝑚𝑡𝑜𝑝 )∑𝑖=1
24
𝑤 𝑖∫𝑑𝜌 𝑑𝑚12𝑑𝑀1
2 𝑑𝑚22𝑑𝑀 2
2𝑑𝜌 ℓ𝑑𝑞1𝑥𝑑𝑞1
𝑦 𝑑𝑞2𝑥𝑑𝑞2
𝑦
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!
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ME – Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
𝑃 (𝑚𝑡𝑜𝑝 )= 1𝜎𝑜𝑏𝑠
𝑡 𝑡 (𝑚𝑡𝑜𝑝 )∑𝑖=1
24
𝑤 𝑖∫𝑑𝜌 𝑑𝑚12𝑑𝑀1
2 𝑑𝑚22𝑑𝑀 2
2𝑑𝜌 ℓ𝑑𝑞1𝑥𝑑𝑞1
𝑦 𝑑𝑞2𝑥𝑑𝑞2
𝑦
∑𝑝 𝑎𝑟𝑡𝑜𝑛 𝑓𝑙𝑎𝑣𝑜𝑟𝑠 ,𝜈
❑
|𝔐𝑡 𝑡|2
Sum over incoming parton flavorsAll 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
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ME – Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
𝑃 (𝑚𝑡𝑜𝑝 )= 1𝜎𝑜𝑏𝑠
𝑡 𝑡 (𝑚𝑡𝑜𝑝 )∑𝑖=1
24
𝑤 𝑖∫𝑑𝜌 𝑑𝑚12𝑑𝑀1
2 𝑑𝑚22𝑑𝑀 2
2𝑑𝜌 ℓ𝑑𝑞1𝑥𝑑𝑞1
𝑦 𝑑𝑞2𝑥𝑑𝑞2
𝑦
∑𝑝 𝑎𝑟𝑡𝑜𝑛 𝑓𝑙𝑎𝑣𝑜𝑟𝑠 ,𝜈
❑
|𝔐𝑡 𝑡|2 𝑓 ′ (𝑞1 ) 𝑓 ′ (𝑞2 )
√ (𝜂𝛼𝛽𝑞1𝛼𝑞2
𝛽 )2−𝑚𝑞 1
2 𝑚𝑞 2
2Φ6
PDF’s
Phase Space FactorTransverse
momenta of incoming partons
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ME – Multiple Steps
ALPGEN + Pythia
Detector Simulation
Reconstruction
4 vectors of reconstructed objects
𝑃 (𝑚𝑡𝑜𝑝 )= 1𝜎𝑜𝑏𝑠
𝑡 𝑡 (𝑚𝑡𝑜𝑝 )∑𝑖=1
24
𝑤 𝑖∫𝑑𝜌 𝑑𝑚12𝑑𝑀1
2 𝑑𝑚22𝑑𝑀 2
2𝑑𝜌 ℓ𝑑𝑞1𝑥𝑑𝑞1
𝑦 𝑑𝑞2𝑥𝑑𝑞2
𝑦
∑𝑝 𝑎𝑟𝑡𝑜𝑛 𝑓𝑙𝑎𝑣𝑜𝑟𝑠 ,𝜈
❑
|𝔐𝑡 𝑡|2 𝑓 ′ (𝑞1 ) 𝑓 ′ (𝑞2 )
√ (𝜂𝛼𝛽𝑞1𝛼𝑞2
𝛽 )2−𝑚𝑞 1
2 𝑚𝑞 2
2Φ6𝑊 (𝑥 , 𝑦 ;𝜌 , 𝜌 ℓ ,…)
Transfer Functions• Given a generated jet with what is the probability DZERO
will reconstruct values x and y?• Detector and reconstruction resolution
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DZERO using the ME Method
In used at DZERO since Run I
GeV3.6
GeV
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
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What Did 3 years get?• 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
• 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
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Template MethodUsing 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
𝑚𝑡
Can do in two dimension• Jet energy scale• Top mass
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Top Mass In Dilepton Events
4% of all decays, split into , and .
Very little SM background!
CDF’s basic selection: Observe 520 events, expect 78% purityATLAS’ basic selection: Observe 2913, expect 96% purity
Really excellent top lab
Except…
For 2 !!!There are no 4-vectors for the two!!
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Template Method
Need distributions that are strongly correlated
with the top mass
Template method to figure out the top mass
ATLAS
The average in the eventTwo permutations (take smallest)Avoid the missing resolution
Good separatio
n power
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CDF Template VariablesFully reconstruct the top mass
Problem: detector measures missing
There are not enough constraints to solve for solution!
The weighting method𝜙1
𝜙2
Grid in the azimuthal angles
• Fit for the top mass at each grid location.
• Resulting is the template variable.• Weight by fit .
The fit includes terms for:• All the measurements (2 leptons, two jets, missing )• Top mass and the (constrained) W mass
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Statistics Isn’t The Problem… Broad peak, but decent
separation power.
Leading systematic:Jet Energy Scale!
This measurement is statistics limited.Can something be done?
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Statistics Isn’t 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 • Depends on 4-vector of leptons• Direction of jets• No Jet Energy Scale, no Missing
And combines the two, optimizing for minimal error
𝑀 𝑡𝑓𝑖𝑡=𝑤 ∙𝑚𝑡
𝑓𝑖𝑡+ (1−𝑤 ) ∙𝑚𝑡𝑎𝑙𝑡
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Dilepton Top Mass ResultsStandard Template MethodJet Energy Scale isn’t fit: not enough constraints
Statistics already making a big difference here
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Top 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 jets
4 jets with GeV5th with GeV6th with GeV
Estimated signal purity is 3%Signal Efficiency is 3.5%!
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Improving the Purity
Unique Handles:
2 -jets
2
Look for -tagged jets
Perform kinematic fit:• Know • 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
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Improving the Purity
1. Requiring the fit to converge2. Very basic cuts on the
Raise CMS’s purity to 39%
Additional kinematic selection
CMS: CDF: Neural Network
Raise CMS’s purity to 54%CDF has a purity of 57%
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Extracting the Mass
𝑚𝑡𝑟𝑒𝑐𝑜 𝑚𝑡
❑
The Template Method
Fit for both Jet Energy Scale and
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Lepton + Jets From CMSFull 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
• Each possible jet assignment gives • Each is weighted by the fit probability
Largest systematic error is the flavor dependent Jet Energy Scale (0.41 GeV)
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Conclusions
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 “final” mass measurements
LHC’s 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 mass…
If you believe BICEP2
!
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Awaiting the next world Combination…
Current World Combination
Tevatron Combination
CMS Combination
±0.95 ±0.76 ±0.64
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Systematic Errors
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ille33ATLAS Lepton+Jets Template
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ille34ATLAS dilepton 7 TeV CDF dilepton
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CDF all jets CMS all jets
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CMS All Jets 7 TeV
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Tevatron Combination
DZERO Lepton+Jets ME