W Mass Measurements and Electroweak Constraints
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Transcript of W Mass Measurements and Electroweak Constraints
W Mass Measurements
and Electroweak Constraints
Chris Parkes
Higgs Maxwell Particle Physics Workshop, Ne-SC Edinburgh, February 9 th 2005
•Happy 22nd Birthday W’s
•UA1,UA2
•LEP RIP
•W Boson Properties•WW xsec, W BRs, Vcs, TGCs
•W Mass Higgs mass
•The Future
•TeVatron Run II, LHC, ILC
W Discovery • UA1, UA2 1983
UA2, Phys.Lett.B276:354-364,1992 MW=80.350.330.17 GeV
LEP’s Legacy – Weighing the Bosons
• Precision measurements of the weak interactions
• The ZLEP 1 Phase 1989-1995
•15 million Z’s
•MZ = 91187.52.1 MeV
•2 parts in 105 !
Z=2495.22.3 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
• Near threshold t-channel dominates
• Cancellations are consequence of SM structure
•Three Feynman graphs with
interference gives Six terms
•Only Born level shown
First WW Event
• 35,000 selected WW’s at LEP2
• Luminosity ~700pb-1 per Experiment
• Energies 161 – 209 GeV
'' qqqqWWee
Event SelectionDivide events into final states:
%)46('' BRqqqq
•Event characteristics:•Jets, leptons•Backgrounds •Z, ZZ
Selections typically:Neural Net, Likelihoodbased
%)44(' BRqql l%)10( BRll ll
WW cross-section results
• 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) 0.70.4%
Final DELPHI, ALEPH, L3
GENTLE 0.969 0.009
Branching Ratios, Vcs
SM 67.51%
026.0073.1))()(/()(2 WBeWBWB e
Wq
q’|Vqq’|2
222222
cscbcdubusud VVVVVV
014.0976.02 csV
Assuming measurements of other elements
)(
)('
csW
cs
qqW V
VBR
•2.8 sigma excess in tau decays
W Mass Analysis Technique• Select Events • Reconstruct lepton and jets (also gluon jets)
• Impose Kinematic constraints– improve resolution
•E,p conservation•M1,M2 or M1=M2
%)46(''%),44(' BRqqqqBRqql l
Perform maximum likelihood fit to data•Calibrate with simulation
•Event by Event Resoultion
LEP W Mass Error Components
0 5 10 15 20 25 30
Statistics
FSI
O()
EBeam
Detector
Hadronisation
LEP Beam Energy Determination
• Spin precession frequency of polarised e+e- beams (EBEAM=200keV)– Polarisation< 60 GeV Calibrate other methods
• Measurement of magnetic field of LEP bending magnets
• Oscillations of beam around ideal orbit (Synchotron tune)
• Spectrometer
Beam
Beam
W
W
EE
MM Correlated between all experiments
MeVMW 10From Ebeam
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 MeVCR: 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
• Weight of qqqq channel in LEP fit 10%
• Mass difference (no FSI) 2243 MeV
• Stat (no syst.) 21 MeV
• LEP direct determination of W Width– 2.150 0.0068(stat.) 0.0060(syst.) GeV
[0.029(stat.) 0.031 (syst)]
Measuring the Higgs Mass
mHiggs < 260 GeV (95% CL)
Remember LEP 1 predicted the top mass !
GeVmHiggs6945114
SUSY?
• SM MH varied
• MSSM parameters varied
Triple Gauge Couplings
Also QGCs!(WW)And NTGCs
•O(em):• 1-2% xsec•W- production angle becomes more fwd peaked
WW me 2/)1( 2/)( WW meq
C, P conservingemag. gauge invariant
WWZ, WW
The near-ish Future: TeVatron, LHC• LEP+TeVatron Run II MW~30 MeV• LHC MW~15 MeVu
d l
l
W
))cos1(2(2 TT
lTW ppM
•Systematics limited Statistical Error 2 MeV for 10fb-1
–Lepton energy scale, use Zl+l- i.e. measure mW/mZ
–Parton distribution functions W longitudinal plepton acceptance
• Transverse mass– No knowledge of longitudinal
momentum– Transverse momentum from
missing momentumCDF
•Theory: •To obtain error of 1MeV •GENTLE MW=24MeV•Full O() calculation in threshold region,~ 104 Feynman graphs
•Ebeam•Spectrometer, calibrate to Mz
•Z radiative return
•Luminosity•LEP 700pb-1• ILC 107 s, 100fb-1
•Determine Background•At threshold t-channel diagram, eL
+ eR-
Polarised beams can turn off signal !
The Far Future: ILC MW~7MeV
The difficulties:
Measure the cross-section at thresholdmeasure mass
Measurement made at LEP with 10pb-1
Sensitivity ~ same at direct reconstruction
%05.0
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
Given the state of the British postal service, it may be measured to~15 MeV(LHC) , ~7 MeV (ILC) by the time this arrives …
The standard model
Higgs is light
mHiggs < 260 GeV (95% CL)