# Presentation: 810701

## The Measurement of AFB in Electron-Positron Pairs

- Gregory Veramendi (Illinois)
- for the CDF Collaboration

- _Outline_
- A Brief History of AFB
- Signal and Background
- Acceptance
- Results

## AFB

- Interference between Z and * exchanges
- -dependent cross sections
**d********/ dcos******** = A(1+cos****2********) + Bcos******
**A****FB**** = 3B / 8A**
**Directly probing the couplings**
- Reduces systematics
- Luminosity, efficiencies and acceptance

- Z

- *

- f = e, quarks

- +

## AFB at e+e- Colliders

- CESR
- PEP
- PETRA
- TRISTAN
- SLC
- measure L,R
- 	using polarized e beams
- LEP
- Tevatron needs >10 fb-1 to compete

## AFB at Hadron Colliders

## AFB beyond LEPII: Z ́,etc...

- AFB beyond LEPII: Z ́,etc...

- Looking for symmetries beyond the SM
- Various models predict new neutral, heavy bosons: Z ́s
- New resonance could interfere with  and Z.
- Complementary to cross-section search (see Muge’s talk)

- AFB

- 500 GeV/c2 Z’:

**Rosner, J.L.: Phys. Rev. D 54, 1078 (1996)**

- Z

- Zx

- Mee(GeV/c2)

## Z/ Candidates and CDF

- Z/ Candidates and CDF

- Selection (5211 candidates)
**2 high P****T**** isolated electrons**
- ET > 20 GeV

_**CDF detector**_

- CDF II Preliminary

- CDF II Preliminary

## Backgrounds for Z/ 

- Dijet dominant

| Source | # of events |  |
| --- | --- | --- |
|  | C-C | C-P |
| Dijet | 9 | 128 |
| We+X | 1.8 | 25 |
| Z | 5.6 | 7.2 |
| WZ | 1.4 | 1.7 |
| WW | 1.5 | 1.8 |
| Top | 1.1 | 0.7 |

- Background Summary

- CDF II Preliminary

- CDF II Preliminary

## Data-MC Comparisons

- CDF II Preliminary

## Data-MC: Cos(*)

- CDF II Preliminary

## Calculating AFB

- cos* in Collin-Soper frame
- Minimize ambiguity in the incoming quark Pt
- cos*>0 Forward
- cos*<0 Backward

- Calculating AFB:

- Z0/*

- Z0/*

- PZ =0

- Z-Axis

- lab frame

## Acceptance:Event Loss

- _Generator Level:_
- Acceptance is symmetric, but distribution is not
- Grey:
**|********1**** | <1,|********1**** | <3, **
**E****T**** > 20 GeV**** **
- Tends to shift AFB in a bin

- Acceptance

- Raw Distr.

- Low
- Mass

- Pole
- Region

- High
- Mass

## Acceptance:Event Migration

- Event Migration Matrix
- Large correlations between bins near Z pole
- Unfolding Problem
**Invert Matrix by fitting for A****FB**
- Large variances
**Correction factors**
- Biases result
- High Mass O.K.

- 40**GeV/c****2**

**600****GeV/c****2**

**600 GeV/c****2**

- Sum AFB in Rows:

- CDF II Preliminary

## Fit with smoothing

- Result fitting AFB
**Large uncertainties**
**Tikhonov regularization**
- Only in bins with correlations

- CDF II Preliminary

- CDF II Preliminary

## Fitting for Z Couplings

- Fit at Raw AFB Level
**Test A****FB**** is smeared**
-  show good agreement with SM

- 2=10.66/11

## Couplings Results

|  | This Study | 2 fb-1
Uncert. | Experimental values (PDG) | SM Prediction |
| --- | --- | --- | --- | --- |
| uL |  0.410.14 | 0.028 |  0.3300.016 | 0.34590.0002 |
| ur |  0.010.12 | 0.024 | -0.1760.008 | -0.15500.0001 |
| dL | -0.320.26 | 0.057 | -0.4390.011 | -0.42910.0002 |
| dr | -0.020.34 | 0.088 | -0.0230.058 |  0.07760.0001 |

- Quark Couplings

- Electron Couplings:

|  | This study | SLD+LEP | SM prediction |
| --- | --- | --- | --- |
| eV | -0.0560.018 | -0.038160.00047 | -0.03816  0.00047 |
| eA | -0.5360.190 | -0.501110.00035 | -0.5064    0.0001 |

## Acceptance*Efficiency

- Correction Factors:
**Energy Resolution**
**Kinematic and Fiducial cuts**
**Radiation from FSR and Brems**
**Electron ID efficiency**
- Assumes SM
**Allow Z couplings to float **

- Couplings fit
- used as input AFB

- CDF II Preliminary

## SM w/ measured Z couplings

- Result with correction factors
**Not useful for non-SM physics near Z-pole**

- CDF II Preliminary

- CDF II Preliminary

## Conclusions

- Unfolding Problem near Z pole
**Fit for couplings using uncorrected A****FB**
**Good agreement with current world ave.**
**Contribute to Z-quark couplings in future?**
- High mass not affected by smearing
- Nothing new above pole, yet...
- PRD on its way...

## Backup Slides

## Understanding the detector

- MisID rate (trident):0.7%

- Radiation Effects: Charge ID
**Get M****ee**** dependence from MC**
**two effects: “tridents”, ********(p****T****)**
**Trident:**

- Energy Scale and Resolution

- CDF II Preliminary

- CDF II Preliminary

## Cross-Check: SS vs OS

- SS vs. OS
**CC : Find 36 SS events in our sample**
- Have to worry about Material  Tridents
- Subtracting peak from MC
**13 ± 6****(stat)**** ± 4****(sys) ****events**
- Checks both radiation in Simulation and background estimation

- CDF II Preliminary

- CDF II Preliminary

## Dijet background

- From Data
**M****ee**** from jet sample**
**A****FB**** from background sample**
- AFB ~ 0

- CDF II Preliminary

- CDF II Preliminary

## Acceptance

- Generator Level Studies:
_**Event Loss:**_
**A: Fiducial Cuts**
**B: Kinematic Cuts**
_**Event Migration:**_
**C: E****T**** Smearing**
**D: FSR Corrections**

- Line: backward  |  Dashed: forward

- |1 | <1
- |2 | <3

- ET > 20 GeV

- cal(E)

- FSR

## Correlations in Couplings

## Theory Band

- Number of MC’s
- VPB
**g resummation**
- ZGRad
**O(********) EW corr.**
- Pythia
**Parton shower (QED+QCD)**
- Nothing w/ NLO QCD & O() EW

## Systematics

- Energy Scale
**Central: ********0.5%**
**Plug: ********1%**
- Energy Resolution
**Central: +0.5 GeV**
**Plug: +1.5 GeV**
- Material
**Central: ****±****0.015 X****o**** Copper**
**Plug: ****±****1/6 X****o**** Iron**
- Background
**QCD: ±********fake rate**
**Montecarlo:**

- Checked(negligible):
**A****FB****(dijet)**
**A****FB****(W+X)**
**Fiducial acceptance**
**Charge mis-ID**
**Trigger efficiency**
- For SM plot:
**A****FB**** input from couplings measurement**

## Fitting for AFB

- Parameterization takes f(AFBphysics) AFBraw
- Construct -log(likelihood): binomial function
- Some bins poorly constrained...
- Add smoothing function 
-  Choose  such that ’

## Add Smoothing

- Using Tikhonov regularization
- We assume parabolic form for
**Solve for three adjacent bins**
- New likelihood:
- Choose  such that ’

## Parameterizing Acceptance

- Define 4 15x15 matrices:
**Efficiency for bin i to end up in bin j**
- Mean # of events in bin i:

- Number of raw/detector level events: