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W Mass From LEP. Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago. Outline . 1 . Introduction - W Boson in the Standard Model of Particle Physics 2 . W mass Measurement - Identifying and reconstructing W’s.

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w mass from lep

W Mass From LEP

Fermilab Wine and Cheese Seminar

6th October, 2006

Ambreesh Gupta, University of Chicago

slide2

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.

Fermilab Wine & Cheese

slide3

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.

Fermilab Wine & Cheese

slide4

W

H

W

W

W

t

(running of a)

f

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.

Fermilab Wine & Cheese

slide5

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.

Fermilab Wine & Cheese

slide6

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.

Fermilab Wine & Cheese

slide7

The Four LEP Experiments

ALEPH

L3

Fermilab Wine & Cheese

ww production and decay at lep
WW Production and Decay at LEP

Backgrounds

BR ~ 10%

WW ll

  • W’s produced in pairs at LEP
  • - 700 pb-1/experiment; 40,000 WW

BR ~44%

WW qql

Efficiency Purity

l l70%90%

qql85% 90%

qqqq85% 80%

BR ~ 46%

WW qqqq

Fermilab Wine & Cheese

event selection
Event Selection
  • Event selection primarily based on multivariate relative likelihood discriminants

OPAL

Very good agreement between expected and observed.

Fermilab Wine & Cheese

w mass at lep
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

Fermilab Wine & Cheese

kinematic fitting
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 WW4f theory model (~0.5% theory error )

Fermilab Wine & Cheese

mass reconstruction
Mass Reconstruction
  • qqqq channel
  • - Well constrained events
  • - But, ambiguity in assigning jets to
  • W’sCombinatorial Background
  • - 5-jet event: 10 comb., 4-jet: 3-comb.
  • qql channel
  • - 1 or 2 constraint kinematics fit
  • - Golden channel

Fermilab Wine & Cheese

fit methods

80.33

81.33

Fit Methods
  • 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.

  • 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

Fermilab Wine & Cheese

fit methods continued
Fit Methods (continued)

Fitted Function (70-88) GeV mass

  • 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.
  • Convolution

- P(m1,m2|Mw,Gw)R(m1,m2)

- Build event-by-event Likelihood

- Maximize statistical sensitivity

- Need bias correction as in BW

Fermilab Wine & Cheese

slide15

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.
  • OPAL variables
  • ALEPH also
  • 3-d fit

5C fit mass error

Hadronic 4C mass

5C fit mass

4C fit mass

difference

5C fit mass error

5C fit mass

Fermilab Wine & Cheese

slide16

Performance of Likelihood Fucntion

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

OPAL

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

Fermilab Wine & Cheese

opal w mass

MwMw (Stat.+Syst.)

CV 80.416  0.053

RW 80.405  0.052

BW 80.390  0.058

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 ofmomentum cut analysismakes

significant reductionin FSI uncertainty.

  • Final W mass and total uncertainty from

the three methods on OPAL -

Fermilab Wine & Cheese

slide18

qqqq

combined

qql

Source

19

5

8

35

79

11

Hadronisation

QED(ISR/FSR)

Detector

Colour Reconnection

Bose-Einstein Correlation

LEP Beam Energy

Other

14

7

10

9

2

10

4

13

8

10

0

0

9

3

44

40

59

Total Systematics

Statistical

Total

21

30

36

22

25

33

LEP W Mass

Channel weights

qqlv : 76%

qqqq : 22%

xs : 2%

  • The combined preliminary LEP W mass
  • MW = 80.376  0.025 (stat)  0.022 (syst) GeV
  • Systematics on W mass

(MeV)

Fermilab Wine & Cheese

slide19

LEP W Width

  • The combined preliminary LEP W width
  • W = 2.196  0.063(stat)  0.055(syst) GeV
  • Systematics on W width

qql + qqqq

(MeV)

Source

Hadronisation

QED(ISR/FSR)

Detector

Colour Reconnection

Bose-Einstein Correlation

LEP Beam Energy

Other

40

6

22

27

3

5

19

Total Systematics

Statistical

Total

55

63

84

Fermilab Wine & Cheese

slide20

LEP Beam Energy

  • LEP beam energy used in event kinematics fit  DMW/MW DELEP/ELEP
  • Beam energy calibrated using
  • - Resonant De-Polarization (41- 60 GeV.)
  • - Extrapolated to LEP II energies NMR probes
  • - Main systematic error due toextrapolation
  • 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
  • - old : DEbeam= 20-25 MeV  DMW = 17 Mev
  • - new : DEbeam = 10-20 MeV  DMW ~ 10 Mev
    • -- OPAL Final 9 MeV

Fermilab Wine & Cheese

slide21

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

Fermilab Wine & Cheese

slide22

Detector

Systematics from MC Modeling

  • Main Sources
  • - QED/EW radiative effects
  • - Detector Modeling
  • Hadronisation Modeling
  • - Background Modeling
  • - Final State Interaction

MC modeled to represent data;

Disagreements  Systematic error

Fermilab Wine & Cheese

slide23

Photon Radiation

  • KoralW’s O(3) implementation adequate,
  • but misses
  • - WSR
  • - interference between ISR,WSR & FSR
  • KandY includes
  • - O() corrections
  • - Screened Coulomb Correction

Error ~ 7 MeV

Fermilab Wine & Cheese

slide24

Raw  Corrected

Detector Systematics

Jet energy resolution

  • 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

Jet energy scale

LEP Combined:

qqlv qqqq Combined

10 MeV 8 MeV 10 MeV

Fermilab Wine & Cheese

slide25

Detector Systematics: Breakdown

OPAL

Fermilab Wine & Cheese

slide26

Hadronization Modeling

  • 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.

LEP Combined:

qqlv qqqq Combined

13 MeV 19 MeV 14 MeV

Fermilab Wine & Cheese

final state interactions
Final State Interactions

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

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

Two known sources that could potentially bias W mass

and width measurement

1.ColorReconnection

- 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?

Fermilab Wine & Cheese

color reconnection
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)

Fermilab Wine & Cheese

slide29

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|

  • A,D,L,O use varations of below

- OPAL Uses P-cut2.5 GeVfor

qqqq

- ~18% loss in statistics.

  • - Much reduced CR systematics
  • 125  41 MeV (ki =2.3) OPAL
  • - A worthwhile tradeoff!
  • - ALEPH28 MeV, L338 MeV

Final CR error in qqqq 35 MeV

Fermilab Wine & Cheese

slide30

BEC in WW events

  • 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 (0.1713)

  • A 68% upper limit on BEC fraction seen in data

(OPAL), used to set W mass systematics

MW = ( 0.33 + 044)  MW (Full BEC) = 19 MeV

  • L3 18 MeV, ALEPH 2 MeV

Final BEC error in qqqq 7 MeV

Fermilab Wine & Cheese

slide31

Results: qqqq and qqlv channels

Fermilab Wine & Cheese

slide32

Results: LEP W mass and Width

mW (LEP) = 80.376 ± 0.033 GeV

GW (LEP) = 2.196 ± 0.083 GeV

Fermilab Wine & Cheese

slide33

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.

Fermilab Wine & Cheese

slide34

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.

Fermilab Wine & Cheese