Violation in Mixing

2.1 Physics at � � B Factory operating at the § �Ë resonance 49
ÞÐ�� � ­Þ�Å ¬­ Ø�Å
from which the decay vertex distance along Þ can be calculated
Defining ¡Ø �
¡Þ � Þ Ð��
� Ø�Å Ø �Å
�
ÞÐ�� � ­ ¬�Å
� Ø�Å Ø �Å
� , equation (2.1) becomes
¡Þ � ­¬�Å
� Ó× ��Å
�
¬­
� Ø�Å Ø �Å
�
(2.1)
� �
Ø�Å ¡Ø Ó× ��Å ¬­ ¡Ø (2.2)
where ¡Ø is the quantity to be measured. This expression can be simplified considering that, in the § �Ë
rest frame, the � mesons have ¬�Å � � �� so that the first addend in (2.2) is small and can be neglected:
it can be written
from which one has
¡Þ ¬­ ¡Ø
¡Ø � ¡Þ
¬­
This is a linear dependence on ¡Ø, the quantity that has to be measured, from ¡Þ which is the quantity that
can be measured.
2.1.2 PEP-II.
PEP II is an � � asymmetric machine running at a center of mass energy of ��� ��Πcorresponding to
the mass of the § �Ë resonance. The electron beam (in the High Energy Ring HER) has �� ��Îand the
positron beam (in the Low Energy Ring LER) has � ��Î. Some PEP-II parameters are shown in Tab. 2-1.
PEP-II has surpassed its design goals both in term of instantaneous and integrated daily luminosity, with
significantly fewer bunches than anticipated [39].
While most of the data are recorded at the peak of the § �Ë resonance, about are taken at a center of
mass energy � �Πlower to allow for studies of non-resonant background.
PEP-II measures radiative Bhabha scattering to provide a fast monitor of the relative luminosity for operations.
BABAR derives the absolute luminosity offline from other QED processes, maily � � and � �
pairs: the systematic uncertainty on the absolute value of the luminosity is estimated to be about �� . This
error is dominated by uncertainties in the Monte Carlo generator and the simulation of the detector.
THE BABAR EXPERIMENT