top quark mass measurements at hadron colliders

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: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

G. Watts (UW

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.

• Only version for which the coupling to the Higgs is important• Stability of the SM Higgs

potential at high scalesA 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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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

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.

𝑡 𝑡→𝑊+¿𝑏𝑊 −𝑏¿

Classified by the Ws’ decay

G. Watts (UW

7The Tevatron & The LHCThe 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.

G. Watts (UW

8Extracting 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 Element Uses all the informationComputationally 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?

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9The Jet Energy ScaleCommon 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

objectsOne 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

G. Watts (UW

10The Matrix Element

ApproachA reverse Monte Carlo

MC Generates

100K events

Distributions of kinematic

variables for all objects

“Map of kinematic

phase space”

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:

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11ME – Multiple Steps

ALPGEN + Pythia

Detector Simulation

Reconstruction

4 vectors of reconstructed objects

𝑃 (𝑚𝑡𝑜𝑝 )= 1𝜎𝑜𝑏𝑠

𝑡 𝑡 (𝑚𝑡𝑜𝑝 )∑𝑖=1

𝑤 𝑖

Normalization

Sum over all possible jet assignments• Which jet is the first tops?• Which jets belong to the

A weight reflecting the probability of those jet assignments• -tagging

probabilities

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ALPGEN + Pythia

Detector Simulation

Reconstruction

𝑤 𝑖∫𝑑𝜌 𝑑𝑚12𝑑𝑀1

2 𝑑𝑚22𝑑𝑀 2

2𝑑𝜌 ℓ𝑑𝑞1𝑥𝑑𝑞1

𝑦 𝑑𝑞2𝑥𝑑𝑞2

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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ALPGEN + Pythia

Detector Simulation

Reconstruction

∑𝑝 𝑎𝑟𝑡𝑜𝑛 𝑓𝑙𝑎𝑣𝑜𝑟𝑠 ,𝜈

|𝔐𝑡𝑡|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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ALPGEN + Pythia

Detector Simulation

Reconstruction

|𝔐𝑡 𝑡|2 𝑓 ′ (𝑞1 ) 𝑓 ′ (𝑞2 )

√ (𝜂𝛼𝛽𝑞1𝛼𝑞2𝛽 )2−𝑚𝑞12 𝑚𝑞2

PDF’s

Phase Space FactorTransverse

momenta of incoming partons

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ALPGEN + Pythia

Detector Simulation

Reconstruction

|𝔐𝑡𝑡|2 𝑓 ′ (𝑞1 ) 𝑓 ′ (𝑞2 )

√ (𝜂𝛼𝛽𝑞1𝛼𝑞2𝛽 )2−𝑚𝑞 12 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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16DZERO using the ME

MethodIn used at DZERO since Run I

GeV3.6

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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17What 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

G. Watts (UW

18Template Method

Using a distribution sensitive to :

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

G. Watts (UW

19Top Mass In Dilepton

Events4% 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 labExcept…

For 2 !!! There are no 4-vectors for the two!!

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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

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21CDF Template Variables

Fully reconstruct the top massProblem: detector measures missing

There are not enough constraints to solve for solution!The weighting method

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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22Statistics Isn’t 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

23Statistics 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 error𝑀 𝑡

𝑓𝑖𝑡=𝑤 ∙𝑚𝑡𝑓𝑖𝑡+ (1−𝑤 ) ∙𝑚𝑡

𝑎𝑙𝑡

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24Dilepton Top Mass

ResultsStandard Template MethodJet Energy Scale isn’t fit: not enough constraints

Statistics already making a big difference here

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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%!

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26Improving the Purity

Unique Handles:2 -jets

2 Look for -tagged jetsPerform kinematic fit:• Know • The two are the same2

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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27Improving 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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28Extracting the Mass

𝑚𝑡𝑟𝑒𝑐𝑜 𝑚𝑡

The Template MethodFit for both Jet Energy Scale and

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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

• 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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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 “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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31Awaiting the next world

Combination…

Current World Combination

Tevatron CombinationCMS Combination

±0.95 ±0.76 ±0.64

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Systematic Errors

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33ATLAS Lepton+Jets Template

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34ATLAS dilepton 7 TeV CDF dilepton

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CDF all jets CMS all jets

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36CMS All Jets 7 TeV

G. Watts (UW

Tevatron Combination

DZERO Lepton+Jets ME

top quark mass measurements at hadron colliders

watts uwseattle ffp

higgs mass

measured quark mass

mc mass

marseille4top mass

quark mass measurements

pole mass

jet energy scaleg

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