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W Mass From LEP W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 6th October, 2006 Ambreesh Gupta, University of Chicago Ambreesh Gupta, University of Chicago
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W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Page 1: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

W Mass From LEPW Mass From LEP

Fermilab Wine and Cheese SeminarFermilab Wine and Cheese Seminar

6th October, 20066th October, 2006

Ambreesh Gupta, University of ChicagoAmbreesh Gupta, University of Chicago

Page 2: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

Ambreesh Gupta Fermilab Wine & Cheese 2

Outline

1. Introduction

- W Boson in the Standard Model of Particle Physics

2. W mass Measurement

- Identifying and reconstructing W’s.

- Mass extraction techniques used by LEP experiments

3. Systematic Uncertainties on W mass measurement

5. Summary

I will show results from all the four experiments with details on OPAL analyses.

Page 3: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

Ambreesh Gupta Fermilab Wine & Cheese 3

Standard Model of Particle Physics Our picture of the fundamental constituents of nature

There are about 19 (+10) free

parameters in the theory to be

determined experimentally

Standard Model predicts

relationship between

these parameters.

Page 4: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Standard Model Relations• Standard Model predicts relation between the parameters; W boson mass(MW)

and Fermi constant(GF), fine structure constant(), Z boson mass (MZ)

: electron g-2 0.004 ppm

GF : muon life-time 9 ppm

MZ : LEP 1 lineshape 23 ppm

• Precision measurements require higher order terms in the theory and help

constraint the unknown pieces.

(running of )

weakrr Δ+Δ−≡Δ

f WHWtW W

Page 5: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Precision EW

top-quark mass “predicted” by electroweak corrections prior to direct discovery

The measured W mass precision is such that Top and Higgs loops required

for consistency in the Standard Model (SM)

This gives an indirect inference on the Higgs.

Better precision on W mass constraints the Higgs Indirect measurement of W mass

- W mass known to 20 MeV from indirect measurement (LEP1 + SLD +Tevatron).

- A direct measurement of W mass with similar precision is of great interest. Measurement of the width of W boson can also be carried out at LEP providing

further checks on consistency of the SM.

Page 6: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Large Electron Positron Collider (LEP)LEP I (1989-1993) : Z physics. 18 million Z bosons produced

LEP II (1996-2000) : W physics. 80,000 W’s produced. (Energies from 161 GeV – 209 GeV) W’s produced in pairs.

Page 7: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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The Four LEP Experiments ALEPH

L3

Page 8: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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WW Production and Decay at LEP

• W’s produced in pairs at LEP

- 700 pb-1/experiment; 40,000 WW BR ~ 44%

BR ~ 46%

BR ~ 10%WW ll

WW qql

WW qqqq

Efficiency Purity

l l 70% 90%

qql 85% 90%

qqqq 85% 80%

Backgrounds

Page 9: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Event Selection

Very good agreement between expected and observed.

Event selection primarily based on multivariate relative likelihood discriminants

OPAL

Page 10: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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W Mass at LEP

• The WW cross section at s = 2Mw

sensitive to W mass

• LEP experiments collected

10 pb-1 data at s = 161 GeV

• Combined Result :

Mw = 80.40 0.21 GeV

Most of LEP 2 data at higher energies - use direct reconstruction There are two main steps in measuring W mass and width

1. Reconstruct event-by-event mass of W’s

2. Fit mass distribution Extract MW and W.

However, jet energies poorly measured ( /E ~ 12% ), neutrinos unobserved.

Kinematic fitting plays vital role

Page 11: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Kinematic Fitting Mass Reconstruction

- Identify lepton and jets (DURHAM)

-- Energy flow techniques

- Kinematic fitting

-- Use LEP beam energy as constraint

-- Total Energy = s; Total Momem. = 0;

-- Additionally, apply equal mass constraint

mw+ - mw- = 0;

Significantly improved mass resolution

Caveat - Photon radiation will change

s s’ (photon energy)

Need good WW4f theory model (~0.5% theory error )

Page 12: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Mass Reconstruction qql channel

- 1 or 2 constraint kinematics fit

- Golden channel

qqqq channel

- Well constrained events

- But, ambiguity in assigning jets to

W’s Combinatorial Background

- 5-jet event: 10 comb., 4-jet: 3-comb.

Page 13: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Fit Methods

Re-weighting

- Weight fully simulated events to create

sample with new W mass and width

parameter

- No external bias correction needed

- Need large event sample to derive stable

weights

80.33 81.33

LEP experiments used three likelihood methods to extract W mass and width

from the reconstructed mass spectrum.

1. Re-weighting

2. Breit-Wigner

3. Convolution

The primary difference between the methods is the amount of information

they try to use for the best measurement.

Page 14: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Fit Methods (continued)

Breit-Wigner

- Fit to W mass spectrum with Breit-Wigner function

- Width adjusted to account for resolution and ISR effects.

- Bias corrected by comparing to fully simulated MC.

Fitted Function (70-88) GeV mass

Convolution

- P(m1,m2|Mw,Gw)R(m1,m2)

- Build event-by-event Likelihood

- Maximize statistical sensitivity

- Need bias correction as in BW

Page 15: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Likelihood Variables - Likelihood built using three variables -- both in qqlv, qqqq channels

- ~ 400 events per bin for stable fit

- Fit for eight energy point, four channels, then combine => lots of MC needed.

5C fit mass error 5C fit mass Hadronic 4C mass

5C fit mass error 5C fit mass4C fit mass

difference

- OPAL variables

- ALEPH also

3-d fit

Page 16: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Performance of Likelihood Fucntion

Test the central value modeling with bias plot Test the uncertainty on central value with pull distribution.

OPAL

Check bias and pulls distributions. . .below a typical example

Page 17: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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OPAL W mass Very good agreement between three methods in

channel and year Strong correlation between methods

=> Combining them had only small stat. gain CV, which has the smallest expected statistical

uncertainty is used as the main method. Use of momentum cut analysis makes

significant reduction in FSI uncertainty. Final W mass and total uncertainty from

the three methods on OPAL -

Mw ΔMw (Stat.+Syst.)

CV 80.416 0.053

RW 80.405 0.052

BW 80.390 0.058

Page 18: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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LEP W Mass The combined preliminary LEP W mass

MW = 80.376 0.025 (stat) 0.022 (syst) GeV

Systematics on W mass

Source

Hadronisation

QED(ISR/FSR)

Detector

Colour Reconnection

Bose-Einstein Correlation

LEP Beam Energy

Other

Total Systematics

Statistical

Total

qql qqqq combined

14

7

10

9

2

10

4

19

5

8

35

79

11

13

8

10

0

0

9

3

21

30

36

44

40

59

22

25

33

Channel weights qqlv : 76% qqqq : 22% xs : 2%

(MeV)

Page 19: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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LEP W Width The combined preliminary LEP W width

W = 2.196 0.063(stat) 0.055(syst) GeV

Systematics on W width

Source

Hadronisation

QED(ISR/FSR)

Detector

Colour Reconnection

Bose-Einstein Correlation

LEP Beam Energy

Other

Total Systematics

Statistical

Total

qql + qqqq

(MeV)

40

6

22

27

3

5

19

55

63

84

Page 20: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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LEP Beam Energy• LEP beam energy used in event kinematics fit ΔMW/MW ΔELEP/ELEP

• Beam energy calibrated using

- Resonant De-Polarization (41- 60 GeV.)

- Extrapolated to LEP II energies NMR probes

- Main systematic error due to extrapolation

• Extrapolation checked with

1. Flux Loop

2. Spectrometer

3. Synchrotron Oscillation• Final results on LEP beam energy ( Eur. Phys. J., C 39 (2005), 253 ) - Reduction of beam energy uncertainty used in earlier W mass combination

- oldΔEbeam= 20-25 MeV ΔMW = 17 Mev

- new : ΔEbeam = 10-20 MeV ΔMW ~ 10 Mev -- OPAL Final 9 MeV

Page 21: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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LEP Beam Energy Cross Check with Data

LEP beam energy can be estimated using radiavtive return events

- Z mass precisely known

- Measured mass in radiative events sensitive to beam energy Result consistent with zero within experimental errors

Page 22: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Detector

MC modeled to represent data;

Disagreements Systematic error

Systematics from MC Modeling

Main Sources- QED/EW radiative effects

- Detector Modeling

- Hadronisation Modeling

- Background Modeling

- Final State Interaction

Page 23: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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• KoralW’s O(3) implementation adequate,

but misses

- WSR

- interference between ISR,WSR & FSR

• KandY includes

- O() corrections

- Screened Coulomb Correction

Error ~ 7 MeV

Photon Radiation

Page 24: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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• Z0 calibration data recorded annually provides

a control sample of leptons and jets (~ 45 GeV).

• Data/Mc comparison used to estimate corrections for

- Jet/Lepton energy scale/resolution

- Jet/Lepton energy linearity

- Jet/Lepton angular resolution/biases

- Jet mass

• Error is assigned from the error on correction

qqlv qqqq Combined

10 MeV 8 MeV 10 MeV

Detector Systematics

• Raw Corrected

LEP Combined:

Jet energy scale

Jet energy resolution

Page 25: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Detector Systematics: Breakdown OPAL

Page 26: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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• MC programs (JETSET,HERWIG,ARIADNE) model production of hadrons

but difference in particles and their distributions

• The difference interplays with detector response

- particle assignment to jets

- cuts applied to low momentum particles

- low resolution for neutral particles

- assumptions made on particle masses at reco.

• JETSET used by all LEP experiment with parameters tuned with

Z peak data

systematic shift estimated from shift with other hadronization models.

qqlv qqqq Combined

13 MeV 19 MeV 14 MeV

LEP Combined:

Hadronization Modeling

Page 27: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Final State Interactions

Two known sources that could potentially bias W mass

and width measurement

1. Color Reconnection

- color flow between W’s could bias their masses

- only phenomenological models exist.

- Most sensitive variable to CR is W mass itself

2. Bose-Einstein Correlation.

- coherently produced identical pions are closer in

phase space.

- BE correlation between decay products of same

W established

- Does the effect exist between W’s?

The Basic Problem: If products of hadronically decaying W’s (~0.1 fm)

interact before hadronization (~1.0 fm) Can create a mass bias.

Page 28: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Color reconnection

Only phenomenological models exist. - SK1 model produces largest shift CR strength parameter (ki)

LEP experiments estimate effect of color reconnection Measure particle flow in the inter-jet regions of the W’s - Extreme values of CR disfavored by data but it does not rule out CR - A 68% upper limit on ki is used to set a data driven uncertainty on W mass. - Combined LEP value of ki = 2.13 For this Reco. Prob., CR error ~ 120 MeV (OPAL)

Page 29: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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A,D,L,O use varations of below

- OPAL Uses P-cut 2.5 GeV for

qqqq

- ~18% loss in statistics.

- Much reduced CR systematics

125 41 MeV (ki =2.3) OPAL

- A worthwhile tradeoff!

- ALEPH 28 MeV, L3 38 MeV

Cuts and Cones: Reducing CR effect CR affects mostly soft particles between

jets changes jet direction Re-calculate Jet direction

1. Within cone of radius R

2. Cut on particle momentum P

3. Weighted particle momentum |P|

Final CR error in qqqq 35 MeV

Page 30: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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2.5 GeV P-cut to redefine jet direction also

reduces BEC W mass bias

- OPAL (default) 46 MeV (P-cut) 24 MeV. LEP experiments have measured BEC between

W’s

- Using “mixing method”

- A,D,L,O: only a fraction of Full BEC seen in

data (0.1713)

BEC in WW events

A 68% upper limit on BEC fraction seen in data

(OPAL), used to set W mass systematics

ΔMW = ( 0.33 + 044) ΔMW (Full BEC) = 19 MeV

L3 18 MeV, ALEPH 2 MeVFinal BEC error in qqqq 7 MeV

Page 31: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

Ambreesh Gupta Fermilab Wine & Cheese 31

Results: qqqq and qqlv channels

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Page 32: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Results: LEP W mass and WidthmW (LEP) = 80.376 ± 0.033 GeV W (LEP) = 2.196 ± 0.083 GeV

Page 33: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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W’s as Calibration Sample at LHC“Yesterdays sensation is today’s calibration

and tomorrows background” - Telegdi

- W’s from top decay are foreseen to provide the

absolute jet scale.

- Fast simulation studies in the past showed feasibility

- Select samples with a four jets and lepton

(electron,muon) with two jets b-tagged.

- estimated 45K events from 10 fb-1

- Cross check with Z/ +Jet sample

- Sattistics not the issue but understanding

the physics of the events.

Page 34: W Mass From LEP Fermilab Wine and Cheese Seminar Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago.

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Summary

Final results from all the four LEP experiments Final LEP combination will use combined FSI error

Much reduced FSI error in final results A new preliminary LEP combination

Total LEP W mass uncertainty decreased to 33 MeV

It took about five years after LEP shut down to get final W mass

results from all the four experiment. Now it is up to Tevatron to

better the W mass precision before LHC turns on.