# Presentation: 577761

## Top Mass Measurements at CDF

- Erik Brubaker
- University of Chicago
- for the CDF Collaboration
- Moriond EWK
- Mar 11-18, 2006

## The Top Quark

- Feels strong, electroweak, gravitational forces.
- Short-lived—doesn’t hadronize (=5×10-25 s).
- Especially interesting due to its mass
- Most massive particle at ~175 GeV/c2.
- More massive than b quark by factor of 40.
- Studied directly only at the Tevatron.

- Mass
- (GeV/c2)

- Moriond EWK

- March 13, 2006

## Top Production, Decay, Selection

- Mass analyses use t-tbar pairs produced strongly.
- ~85% quark annihilation,~15% gluon fusion.
- Top always decays to W boson and b quark.
- Events classified by decay of W to leptons or quarks
- Dilepton: 2*e|, MET, 2 jt
- L+jets: e|,MET,4 jt
- All-hadronic: 6 jt
- B tagging (displaced vertex) improves S/B ratio.

- L+Jets

- Dilepton

**15%**

**85%**

- Moriond EWK

- March 13, 2006

## Why Measure the Top Quark Mass?

- Fundamental parameter of Standard Model.
- Unexpectedly large: mb x 40
- Related through radiative corrections to other EW observables.
- Very important for precision tests of SM.
- With mW, constrains mH.
- SM Yukawa coupling ~ 1  Special role in EWSB??

- Moriond EWK

- March 13, 2006

## A Difficult Measurement

- Complicated events
- Only ~50% of l+jets events have leading 4 jets from tt decay.
- 12 ways to interpret 4 jets  4 partons.

- Jet energy resolution effect andjet energy scale systematics
- Resolution 85%/√ET  statistical uncertainty.
- Systematic 3%  systematic uncertainty.
- Background contamination
- Well understood S:B of 1:1–10:1.
- Must be treated properly to avoid bias.
- Run IIa goal for Mtop (2 fb-1)
- CDF: 3 GeV/c2 precision
- Tevatron: 2 GeV/c2 precision

- Moriond EWK

- March 13, 2006

## Top Mass at CDF

- Robust program of top mass measurements
- Good statistics  precision measurements
- Many analysis techniques with different sensitivities  high confidence
**Five new results using 680-750 pb****-1****.**

|  | Pretag | 1 b tag | 2 b tags |
| --- | --- | --- | --- |
| Dilepton | 64 | 27 | 7 |
| Lepton + Jets | 360 | 252 | 57 |
| All-hadronic |  | O(600) |  |

***NEW**!*

***NEW**!*

***NEW**!*

***NEW**!*

***NEW**!*

***NEW**!*

**Events (S+B) in 680–750 pb****-1**

- Moriond EWK

- March 13, 2006

## How to Weigh Truth

- Pick a kinematic observable (e.g. reconstructed mass).
- Create “templates” using events simulated with different Mtop values (+ background).
- Perform maximum likelihood fit to extract measured mass.

- Build likelihood directly from PDFs, matrix element(s), and transfer functions that connect quarks and jets.
- Integrate over unmeasured quantities (e.g. quark energies).
- Calibrate measured mass and error using  simulation.

- TEMPLATES

- MATRIX ELEMENT

- Both techniques rely on good Monte Carlo and detector simulation!

- Kinematicinformation

- + Dynamicalinformation

- Moriond EWK

- March 13, 2006

## Template Analysis (L+Jets) Overview

**M****top**

- mtreco

- Parameters

- Observables

******JES**

- mjj

-  Kinematic fit to reconstruct top quarkmass in each event

-  JES: deviation from nominalin units of external calib.

-  Invariant mass of possible W decay jets

- Ext. max. likelihood using4 subsamples; constrainJES = 0 ± 1 c

**This method,****318 pb****-1****: **
**PRL 96, 022004;****PRD 72, 032003**

- Moriond EWK

- March 13, 2006

## Template (L+Jets) Results—680 pb-1

**m****t****reco** in data w/ fit overlaid

**m****jj** in data w/ fit overlaid

******JES** = -0.3 ± 0.6 (stat. + Mtop) ******c**

- 40% improvement indominant JES systematic!

**2-tag**

**1-tag(T)**

**1-tag(L)**

**0-tag**

**2-tag**

**1-tag(T)**

**1-tag(L)**

**0-tag**

- Moriond EWK

- March 13, 2006

## Template (L+Jets) Results—680 pb-1

- Likelihood contours in Mtop-JES plane

| Systematic | Mtop(GeV/c2) |
| --- | --- |
| Residual JES | 0.7 |
| B-jet energy scale | 0.6 |
| Bkgd JES | 0.4 |
| Bkgd Shape | 0.5 |
| ISR | 0.5 |
| FSR | 0.2 |
| Generators | 0.3 |
| PDFs | 0.3 |
| MC stats | 0.3 |
| B-tagging | 0.1 |
| TOTAL | 1.3 |

- Moriond EWK

- March 13, 2006

## Matrix Element (L+Jets) Overview

- JES sensitivity comes from W resonance.
- Uses kinematic & dynamical features of each event.
- All jet-parton assignments are considered, weighted.
- Select events with exactly 4 jets, well described by LO ME.
- Require b tag to improve sample purity.

-  Likelihood simultaneously determines Mtop, Jet Energy Scale, andsignal fraction

-  Probabilities built from matrix element, transfer functions, andparton distribution functions

**Dynamical Likelihood Method (different ME analysis), 318 pb****-1****:**
**PRL 96, 022004; hep-ex/0512009**

- New method for 680 pb-1 dataset!

- Moriond EWK

- March 13, 2006

## Matrix Element (L+jets) Technique

**Calibrate method** against MC samples.
- Shows **unbiased** measurement.
- Errors rescaled to account for observed pull width.

- Corrects **approximations** in integration:
- Angles perfectly measured
- Lepton energy perfectly measured
- Jets are from ttbar decay

- No bias!

- Moriond EWK

- March 13, 2006

## Matrix Element (LJ) Results—680 pb-1

- JES here is constant multiplicative factor.
- Edata = EMC/JES
- JES = 1.02 ± 0.02.
- Very close to central value of template method

- Virtually identical sensitivitywith fewer events!

| Systematic | Mtop (GeV/c2) |
| --- | --- |
| Add’l JES | 0.7 |
| Signal Modeling | 1.1 |
| Other | 0.4 |
| TOTAL | 1.4 |

- Moriond EWK

- March 13, 2006

## Decay Length Technique—680 pb-1

- Difficult template analysis—measure slope of exponential.
- But systematics are dominatedby tracking effects	 Small correlation with	traditional measurements!
- So far: L+jets channel, but extended acceptance—not limited to 4-jet events.
- 375 events
- Statistics limited now
- Can make significant contribution at LHC

-    B hadron decay length  b-jet boost  Mtop

**Method: **
**PRD 71, 054029**

- Moriond EWK

- March 13, 2006

## Matrix Element (Dilepton) Technique

- Harder to reconstruct Mtop in dilepton events: two neutrinos make system underconstrained.
- More amenable to ME approach
- Likelihood is similar to L+jets.
- No W resonance  no fit for JES
- Add ME for dominant bkgds:DY+jets, WW+jets, fakes
- Approximations havesignificant effect
- MC calibration essential
- Correct fitted mass forcalibration slope of 0.85
- Correct for pull widthof 1.49 (constant in Mtop)
- Analysis performed alsoon tagged subsample.

**This method,****340 pb****-1****: **
**hep-ex/0512070**

|  | Events | S:B |
| --- | --- | --- |
| 0 b tags | 64 | ~2:1 |
| 1 b tags | 27 | ~20:1 |

- Moriond EWK

- March 13, 2006

## Matrix Element (Dil) Results—750 pb-1

- Best measurement in challenging dilepton channel.
- Could reach 2 GeV (stat)sensitivity by end of run II.

| Systematic | Mtop (GeV/c2) |
| --- | --- |
| JES | 2.6 |
| Signal Modeling | 1.1 |
| Other | 1.3 |
| TOTAL | 3.1 |

- Restrict sample to b-tagged events:Mtop = 162.7 ± 4.6 ± 3.0 GeV/c2

- Moriond EWK

- March 13, 2006

## Combination of CDF Results

- Use BLUE (Best Linear Unbiased Estimator) technique.
*NIM ****A270**** 110, ****A500**** 391*.
- Accounts for correlations in systematics.
- Stat correlations in progress.
- So far only combine measurements on independent datasets (incl run I).

- Updated CDF+D0 combined result coming...

- Moriond EWK

- March 13, 2006

## Keep an Eye on This...

- See some discrepancy between L+jets, Dilepton channel Mtop measurements.
- Statistically consistent so far:
- ME(dil) vsTempl(L+jets):******2**** = 2.9/1, p=0.09**. (Accounts for correlated systematics)
- But what if it persists?
- Could there be a missing systematic?
- Would have to affects the channels differently...
- Could our assumption of SM ttbar be incorrect?
- Will be interesting to see all-hadronic measurements.

**Stay tuned...**

- Moriond EWK

- March 13, 2006

## Conclusions

- CDF has surpassed our run IIa goal of 3 GeV/c2 precision on Mtop.
- Goal assumed 2 fb-1!
- With *in situ* JES calibration, dominant “systematic” now scales as 1/sqrt(N).
- 1% uncertainty on Mtop is in sight as we concentrate on reducing remaining systematics

- Moriond EWK

- March 13, 2006

## Backup Slides

## Measure JES Using Dijet Mass

- Build templates using invariant mass mjj of allnon-tagged jet pairs.

- Rather than assuming JES and measuring MW...
- Assume MW and measure JES
- Parameterize P(mjj;JES) same as P(mtreco;Mtop)

- Moriond EWK

- March 13, 2006

## Systematics: ISR/FSR/NLO

- Method in hand to use Drell-Yan events to understand and constrain extra jets from ISR.
- Constraint scales with luminosity.
- Easily extendible to FSR.
- MC@NLO sample shows no add’l NLO uncertainty is needed.

- (t-tbar)

- Moriond EWK

- March 13, 2006