Perfect agreement with QED, first order computation:
Bhabha scattering:
Tasso experiment & test of point-like electrons:
Tasso: detector at collider PETRA (DESY, Hamburg), started data taking 1979, CMS up to 35 GeV reached (Tasso famous for discovery of gluon)
Lowest order calculationprovides a very gooddescription of the observeddata
Test of point-likeness of muon (@ Tasso):
→ muon substructure
QED calculation :
Due to interference with electroweak diagrams effects of the Z boson (M
Z= 91 GeV) are already seen
at
→ determination of EW parameters
Discovery of tau lepton at Mark I (SLAC), 1975:
cut on coplanarity to reject huge background of two body decays:
explanation:
Measurement of tau mass:
production threshold in
original result by Mark I
later measurement (BES, 1994)
Cross-section and point-likeness of tau lepton:
Again good agreement for total cross-sectionas function of CMS with prediction.[Forward-backward asymmetry not measured, expect similar distribution as for ]
→ another (heavy) elementary particle (lepton)
Discovery of meson → discovery of 4th quark
BNL experiment (S. Ting et. al)(Berkley national laboratory)
SLAC: Mark I(Standford linear accelerator complex)
Discovery of Y(1S) = |bb> in 1977 → third quark family!
proton beam on fixed target at Fermilab m(Y) = 9460 MeV → m(b) ~ 5000 MeV
decay OZI suppressed, only Y(4S) (m = 10.58 GeV) decays into pairs of B mesons
ISR + FSR + EW correction
Building blocks of QED Feynman Diagrams
Propagator correction
M: upper cut ofM → ∞ for correct resultlogarithmic divergence!
Instead of redefining propagator→ redefine charge (coupling)
- sign due to internal loop
How is measured charge related to charge entering the vertex in Feynman computation
→ measured charge is not a factor contributing to all vertices but:
assume charge is measured in an experiment at
[contribution from each diagram enter with factor (-1)n, n: nb. of loops]
: bare charge
F and I are highly correlated; F includes add. fermion currents
1 + - +
not divergent !
→
(PETRA)
(LEP)
Phenomenological explanationscreeing at large distances
smaller net charge