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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