Bhabha scattering, the process of e+ e- ⟶ e+ e- , has been studied at center-of-mass energies
from 50 to 58 GeV with the AMY detector at the KEK e+e- storage ring TRISTAN. The
study is based on a data sample of 79.7 pb-1 integrated luminosity. The differential cross
section of Bhabha scattering has been measured. The measured cross section is found
to agree fairly well with the Standard Model of the electroweak theory. The measured
cross section is also compared with various four-fermion contact interaction models, and
confidence level lower limits on the composite scale, A, are determined. In addition, the
limits on VV model are converted to SM-break-down scales, which indicate the validity of
the SM down to the distance of order ~ 10-17 cm and the electron charge radius of ~ 10-16
cm. Attempts are made in searching for an additional boson Z'. No clear signal of the
existence of a Z' boson is found up to energy of ~160 GeV/c2. The effect of transverse
beam polarization on Bhabha scattering is also studied. The ∅ dependence of Bhabha
events are fitted to the QED prediction and found to agree with the theory. However, no
quantitative conclusion on polarization effect can be drawn based on current data sample,
which does not provide enough statistics. More data is being accumulated and further study
should be carried out.
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