Violation in Mixing

2
The BABAR Experiment
2.1 Physics at � � B Factory operating at the § �Ë resonance
2.1.1 �È asymmetry experimental measure
The primary goal of the BABAR experiment is the study of �È -violating asymmetries in the decay of neutral
� meson. Secondary goals are precision measurement of decays of bottom and charm mesons and of �
leptons, searches for rare processes accessible because of the high luminosity of PEP II B factory.
Through equation (1.44), in the previous chapter, a �È asymmetry measurement can be related to a measurement
of ¡Ø, the time interval between the � decays: ¡Ø � Ø�È ØØ��. In case the � momenta are
known, ¡Ø can be measured by the decay point distance ¡Þ. In a symmetric � � collider operating at the
§ �Ë resonance, the two � mesons are created almost at rest and the decay point distance can be calculated
taking the small phase space left once � masses have been subtracted from the § �Ë mass
� § �Ë � ��� ��Î � �� � � § �Ë ��� � ��Î�
From the total energy of each �, the kinetic energy is
getting:
Õ �� Ñ � � Ô � Ñ�¬­ � � � ��Î
¬­ � � �� � ¬
where it has been considered that ­ is almost since the system is non-relativistic. The � decay length is
then