Violation in Mixing

76 The BABAR Experiment
The drift chamber filters select events with one tight (ÔØ � � Å�Î� ) track or two loose (ÔØ � � Å�Î� )
tracks originating from the IP: track selection is based on the Ü Ý closest approach distance (� ) to the IP
and the corresponding Þ coordinate for that closest approach point (Þ ). The IP is a fixed location close to
the average beam position over many months. Tight (loose) tracks have to satisfy a vertex condition defined
as �� � � � Ñ (�� � � �� Ñ) and �Þ ÞÁÈ � � �� Ñ (�Þ ÞÁÈ � � � Ñ).
The calorimeter filters select events with either high energy deposits (��Å � � Å�Î) or high cluster
multiplicity (at least � clusters): they also require a high effective mass (� �� ��� ) calculated from
the cluster energy sums and the energy weighted centroid positions of all clusters in the event assuming
mass-less particles. A Bhabha veto filter is also used: it selects one-prong (only a positron in the back part
of the detector) and two-prong events (with both � and � detected) and it applies stringent criteria on
�Å� energy deposits relying on the track momenta and ��Ô values.
During a typical run on the § �Ë peak with an average luminosity of �� Ñ × , the physics events
represent the of the total L3 output (with a rate of � ÀÞ), while the calibration and diagnostic samples
comprise � (with a rate �� ÀÞ): the total output rate is ÀÞ.
MARCELLA BONA