bhabha scattering - heidelberg university

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