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W boson mass and W boson mass and width measurements width measurements

at LEP2at LEP2

Hugo Ruiz, CERN – Aleph

On behalf of the LEP Collaborations

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OutlineOutline

Introduction

Measurement of MW (and w) by direct

reconstruction

Some relevant systematics:– Bose-Einstein correlations

– Colour Reconnection

Results and conclusions

Prospects

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IntroductionIntroduction mW in perspective:

– From pp colliders (transverse mass spectra, single Ws, april 2004):

mW = 80.452 0.059 GeV (CDF run1+D0 run1+UA2)

– Prediction from EW fit:

mW = 80.373 0.033 GeV (LEP1,SLD)

mW = 80.386 0.023 GeV (+Mtop)

At LEP2, W’s produced in pairs:

1996-2000: ~40k WW evts precision measurement

mW measured from direct reconstruction of W decay

products

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The measurement

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Event selectionEvent selection

Fraction

Backgrounds ZZ, Zee, Z We, qq() qq()

Efficiency ~50% ~70% ~85%

Purity ~90% ~94-99% ~85%

Fraction

Backgrounds ZZ, Zee, Z We, qq() qq()

Efficiency ~50% ~70% ~85%

Purity ~90% ~94-99% ~85%

q

q

q

q

q q

Semileptonic (qqSemileptonic (qqll)) Hadronic (4q) Hadronic (4q) LeptonicLeptonic

10% 44% 46%

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W reconstructionW reconstruction

Hadronic and semileptonic channels:

1. Reconstruct leptons and cluster jets

2. Apply a kinematic fit:– Constraints:

• E,p conservation

• In some cases, M1=M2 / 1=2

– Effects:

• alows ‘reconstruction’ of in semileptonic channel

• resolution ~7GeV ~3GeV

• decreases detector systematics

Hadronic channel:

3. Pair jets (there is a 3-fold ambiguity):

– algorithms provide ~85% of good pairing

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Reconstructed MReconstructed MWW

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mmWW and and ww extraction extraction

Extraction by fitting distribution of reconstructed MW to

either:

– MC samples generated with different MW (and W) values

– Function convoluting BW and detector effects, and then use MC to correct for residual offsets

Mass extraction

Assume SM relation between M and

Perform 1-parameter fit

Mass extraction

Assume SM relation between M and

Perform 1-parameter fit

Width extraction

Assume no relation between M and

Perform 2-parameter fit

Width extraction

Assume no relation between M and

Perform 2-parameter fit

Rely on MC simulation:– Lots of data for tuning from LEP1

– Residual discrepancies systematics

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Systematic uncertainties for mSystematic uncertainties for mWW

Source MW (MeV)Correlati

ons

Detector simulationMostly from calo energy calibration

15 Channel, Year

LEP energythrough kinematic fit

17Channel,

Year, Experiment

Fragmentation 18Channel,

Year, Experiment

Interconnection effects 9

(90 MeV in hadr.)

Year, Experiment

Expected final statistical error for LEP2 ~ 25 MeV

Largest systematic uncertainties:

next sections of the talk

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Interconnection effects on hadronic

events

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– parton shower (large Q2, pQCD)

Fragmentation (quarks hadrons):

Interconnection effectsInterconnection effects

Hard process: e+e-4q

e+

e-

W+

W-

q

_qq

_q

Interconnection effects (not included in standard MC models):

– Bose-Einstein correlations: momenta of identical bosons tend to be correlated.

d~0.1 fm

– Colour reconnection: hadronic interaction between W decays

•d(W+,W-) < 1 fm

– hadronisation (phenomenological)

Event simulation:

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Bose-Einstein Correlations Bose-Einstein Correlations (BEC)(BEC) Pairs of 00, ++ and -- tend to be bunched.

For calculation of BEC, quantum phases and space-time structure are needed only phenomenological models available.

Effect on MW:W1

W2

– Intra-W:

• not relevant for Minv

– Inter-W:

• Cause discrepancies data-MC in jet overlaps jet clustering different data-MC bias.

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BEC in WBEC in W++WW-- events events BEC effects experimentally established in Z jets at LEP1

Inter-W BEC? Analyses performed in 4 LEP experiments to search/limit them

– Observable: distance in p-space between pairs of charged pions: Q2ij=-(pi-pj)2

Inter-W BEC correlations disfavoured

Limit on systematic: MW ~ 15 MeV

L3

0 1 Q(GeV)

LEPWW/FSI/2002-02

fraction of model seen

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Colour ReconnectionColour Reconnection

Several phenomenological models used to study the effect

on mW, amongst them:

– At parton shower:

• Ariadne2: formation of some inter-W dipoles, MW ~ 70 MeV

– At hadronisation:

• Herwig-CR: hadrons created from inter-W parton pairs, MW ~ 40

MeV

• Rathsman: reduce string tension by reconnecting, MW ~ 40 MeV

• Jetset SK1: allow formation of inter-W strings, MW up to 400 MeV,

depending on a free parameter

Dedicated analyses try to observe / limit CR effects from data on 4

LEP experiments

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The particle flow analysisThe particle flow analysis

CR models predict a modified particle flow in W+W- events:

CR:

No CR:

W-

W+

W-

W+

Observable: ratio of particle flow between the inter and intra-W regions:

(A + B) / (C + D)

A

B

C

D

• Data

- SK1 (extreme parameter)- Jetset

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Results from particle flowResults from particle flow

Try to make analyses more robust to CR effects

CR Prob

For SK1:– Extreme values discarded– Preferred value of the parameter

corresponds to MW ~ 100 MeV!!

‘Asymmetry’ from experiments combined in a 2.

Cannot discard models like:– Ariadne2– Herwig-CR– Rathsman

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Towards a less CR sensitive analysis

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PrinciplePrinciple Interconnections mainly occur between low

momentum particles in the inter-W region.

Idea: modify clustering algorithm to dismiss information from those particles. This implies:– “purer” information

– loss of statistical precision

Many variations of jet algorithms tried, mainly:• Cones: perform angular cut around jet direction

• P-cuts: remove low momentum particles

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Reduction of Reduction of MMWW

Example: SK1

MW (MeV)

Model StandardCone

R=0.5 rad

SK1, kI~2 100 40

Herwig 35 10

AR 2 50 20

Rathsman

40 15

Good reduction factors are obtained for all available models

Example: Cone (R=0.5 rad), with a statistical loss of ~ 25%:

parameter

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A by-product: measure CR?A by-product: measure CR?

The difference between MW measured with ‘robust’

and standard analyses is sensitive to CR effects:

DELPHI, Cone algorithm R=0.5

DELPHI preliminary:– Exclude extreme scenarios.

– Minimum at ~1.3, P~0.5

Cone radius (rad)

ALEPH

SK1 k=2.13

MW (

GeV

)

MW

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Results

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mmWW from direct reconstruction from direct reconstruction

Non-4q 4q

mW = 22 ± 43 MeV

Results in CERN-EP/2003-091, LEPEWWG/2003-02still with standard jet algorithms

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mmWW at LEP2 at LEP2

mW = 80.412 ± 0.042 GeV

0.029 stat , 0.031 syst

LEP2 combination: World average:

mW = 80.425 ± 0.034 GeV

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ww at LEP2 at LEP2

W = 2.150 ± 0.091GeV

0.068 stat, 0.060 syst

W = 2.150 ± 0.091GeV

0.068 stat, 0.060 syst

Combination:

Detector:Detector: 29 MeV

Frag:Frag: 30 MeV

FSI:FSI: 37 MeV

Detector:Detector: 29 MeV

Frag:Frag: 30 MeV

FSI:FSI: 37 MeV

World average:

W = 2.133 ± 0.069 GeV

W = 2.133 ± 0.069 GeV

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ConclusionsConclusions

The combination of the results of the LEP experiments gives:

mW = 80.412 ± 0.042 GeV

W = 2.150 ± 0.091 GeV

mW = 80.412 ± 0.042 GeV

W = 2.150 ± 0.091 GeV

Consistency of mW within SM:

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ProspectsProspects Publication status:

– BEC studies already published for L3 Phys.Lett.B 547 (2002) and ready for OPAL (CERN-PH-EP/2004-008, to Eur. Phys. Journal C).

– CR studies ready for publication for L3 (CERN-EP/2003-12, to Phys. Lett. B).

– All the rest will be ready by this autumn/end of this year

Expected developments related with color reconnection:– Final estimation of effect

– Analysis with improved robustness. If all experiments use them:• Total error in hadronic channel: ~90 ~60 MeV.• Total error from decrease by ~3 MeV• Weight of hadronic channel in combination:

0.1% 0.3%.

+ Learn something about colour reconnection