W Mass Measurements
and Electroweak
Constraints
•Happy 22nd Birthday W’s
Chris Parkes
•UA1,UA2
•LEP RIP
•W Boson Properties
•WW xsec, W BRs, Vcs, TGCs
•W Mass  Higgs mass
•The Future
•TeVatron Run II, LHC, ILC
Higgs Maxwell Particle Physics Workshop, Ne-SC Edinburgh, February 9th 2005
W Discovery
• UA1, UA2 1983
UA2, Phys.Lett.B276:354-364,1992
MW=80.350.330.17 GeV
LEP’s Legacy – Weighing the
Bosons
• Precision measurements of the weak
interactions
• The Z
LEP 1 Phase
1989-1995
•15 million Z’s
•MZ = 91187.52.1 MeV
•2 parts in 105 !
•Z=2495.22.3 MeV
LEP 2 Phase 1996-2000
• W boson measurements
• Measuring the Higgs mass ?
•MW depends on (mtop)2
•MW depends on ln (mhiggs)
WW Production at LEP
•Three Feynman
graphs with
interference gives
Six terms
•Only Born level shown
• Near threshold tchannel dominates
• Cancellations are
consequence of SM
structure
First WW Event
• 35,000 selected
WW’s at LEP2
• Luminosity
~700pb-1 per
Experiment
• Energies
161 – 209 GeV
e  e   W W   q q'q q'
Event Selection
Divide events into final states:
l l l l ( BR  10%) q q'qq' ( BR  46%)
l l qq'( BR  44%)
•Event characteristics:
•Jets, leptons
•Backgrounds
•Z, ZZ
Selections typically:
Neural Net, Likelihood
based
WW cross-section results
Final DELPHI,
ALEPH, L3
GENTLE 0.969 0.009
• Measured cross-sections corrected for QM interference
with other processes that produce the same final state
• Theoretical error at threshold (IBA) 2%
• Theoretical error above 170 Gev (LPA/DPA) 0.70.4%
Branching Ratios, Vcs
BRW qq ' 
( Vcs )
W ( Vcs )
q
W
|Vqq’|2
q’
Vud  Vus  Vub  Vcd  Vcb  Vcs
2
2
2
2
2
2
SM 67.51%
Vcs  0.976  0.014
2
Assuming measurements of other elements
•2.8 sigma excess in tau decays
2 B(W    ) /( B(W  e e )  B(W    ))  1.073  0.026
W Mass Analysis Technique
• Select Events l l qq'( BR  44%), qq'qq' ( BR  46%)
• Reconstruct lepton and jets (also gluon jets)
• Impose Kinematic constraints
– improve resolution
•E,p conservation
•M1,M2 or M1=M2
Perform maximum likelihood fit to data
•Calibrate with simulation
•Event by Event Resoultion
LEP W Mass Error Components
Statistics
FSI
EBeam
Detector
Hadronisation
O()
0
5
10
15
20
25
30
LEP Beam Energy Determination
M W
MW

E Beam
E Beam
Correlated between all experiments
• Spin precession frequency of polarised e+ebeams (EBEAM=200keV)
– Polarisation< 60 GeV Calibrate other methods
• Measurement of magnetic field of LEP
bending magnets
• Oscillations of beam around ideal orbit
From Ebeam
(Synchotron tune)
M W  10MeV
• Spectrometer
Final State Interactions
• W+W- decay vertices separation typically 0.1fm
• Typical hadronisation scale 1fm
BEC: between final state hadrons –
identical bosons (pions) close in
phase space – 35 MeV
CR: cross-talk between coloured
objects in non-perturbative QCD
region – 65 MeV
Additional systematic on W Mass
for fully-hadronic decays
•Simulation
•Measurements
World average W Mass
[0.029(stat.)
0.031 (syst)]
•
•
•
Weight of qqqq channel in LEP fit 10%
Mass difference (no FSI) 2243 MeV
Stat (no syst.) 21 MeV
•
LEP direct determination of W Width
– 2.150  0.0068(stat.) 0.0060(syst.) GeV
Measuring the Higgs Mass
Remember LEP 1 predicted the top mass !
mHiggs  114  69
45 GeV
mHiggs < 260 GeV (95% CL)
SUSY?
• SM MH
varied
• MSSM
parameters
varied
Triple Gauge Couplings
W  e(1      ) / 2mW
qW  e(    ) / mW
•O(em):
• 1-2% xsec
•W- production
angle becomes
more fwd peaked
2
Also QGCs!(WW)
And NTGCs
C, P conserving
emag. gauge invariant
WWZ, WW
The near-ish Future: TeVatron, LHC
l
u
W
d
• LEP+TeVatron Run II
MW~30 MeV
• LHC MW~15 MeV
l
• Transverse mass
– No knowledge of longitudinal 
momentum
– Transverse  momentum from
missing momentum
M
T2
W
 (2 plT pT (1  cos  ))
CDF
•Systematics limited Statistical Error 2 MeV for 10fb-1
–Lepton energy scale, use Zl+l- i.e. measure mW/mZ
–Parton distribution functions W longitudinal plepton acceptance
The Far Future: ILC
• MW~7MeV
Measure the cross-section at threshold
measure mass
Measurement made at LEP with 10pb-1
Sensitivity ~ same at direct reconstruction
The difficulties:


 0.05%
•Luminosity
•Theory:

-1
•LEP 700pb 
•To obtain error of 1MeV
•GENTLE MW=24MeV
• ILC 107 s, 100fb-1
•Full O() calculation in threshold
•Determine Background
region,~ 104 Feynman graphs
•At threshold t-channel
•Ebeam
 diagram, eL+ eR•Spectrometer, calibrate to Mz
Polarised beams can
•Z radiative return
turn off signal !
Dear All,
Having a lovely time in the
2nd nicest town in Scotland.
•WW cross-section, ±1%
•BR, Vcs
•TGCs
•W Width 2.150 0.091 GeV
•W Mass 80.412 0.042 GeV
mHiggs < 260 GeV (95% CL)
Given the state of the British postal
service, it may be measured to
~15 MeV(LHC) , ~7 MeV (ILC)
by the time this arrives …