Violation in Mixing

74 The BABAR Experiment
The direction of the neutral hadron is determined from the event vertex and the centroid of the neutral
cluster: no information on the energy of the cluster can be obtained.
Information from �Å� and the cylindrical RPCs is combined with the Á�Ê cluster information: the angular
resolution of the neutral hadron cluster can be derived from a sample of Ã Ä produced in the reaction
� � � �­ � Ã Ä Ã Ë ­. The Ã Ä direction is calculated from the missing momentum computed from
the measured particles in the final state. The angular resolution of the Ã Ä is of the order of � ÑÖ��: for
Ã Ä interacting in the �Å� the resolution is about twice better. Right plot in fig. (2-17) shows the angular
difference ¡� between the missing momentum and the direction of the nearest neutral hadron cluster. the
Ã Ä detection efficiency increases almost linearly with momentum: it varies between and � in the
momentum range from ��� to ����.
2.2.7 The trigger.
The PEP-II high luminosity is also due to the � Ñ bunch spacing: the bunch time spacing is �� Ò×
corresponding to a cross frequency of � ÅÀÞ. At design luminosity, beam-induced background rates
are typically about �ÀÞ each for one or more tracks in the drift chamber with ÔØ � Å�Î� or at least
one �Å� cluster with �� Å�Î. This rate is to be contrasted with the desired logging rate of less than
ÀÞ. The trigger and data acquisition subsystems are designed to record data at no more than the latter
rate: the purpose of the trigger is to reject backgrounds while selecting a wide variety of physics processes.
The total trigger efficiency is required to exceed �� for all �� events and at least �� for continuum
events. The trigger should also contribute no more than to dead time.
The BABAR trigger has two levels: Level 1 which executes in hardware and Level 3 which executes in software
after the event assembly. The Level 1 trigger system is designed to achieve very high efficiency and good
understanding of the efficiency. During normal operation, the L1 trigger is configured to have an output rate
of typically �ÀÞ, while the L3 filter acceptance for physics is � � ÀÞ.
Event signatures are used to separate signal from background. Combinations of the following global
event properties are used in the L1 trigger as general event selection criteria: charged track multiplicity,
calorimeter cluster multiplicity and event topology. These selection criteria have associated thresholds
for the following parameters: charged-track transverse momentum (ÔØ), energy of the calorimeter clusters
(� ÐÙ×), solid angle separation (�) and track-cluster match quality. The trigger definition can contain
selection criteria that differ only by the values of thresholds. A small fraction of random beam crossings and
events that failed to trigger are also selected for diagnostic purposes.
For a given trigger level, the global selection is a logical OR of a number of specific trigger selection lines,
where each line is the result of a boolean operation on any combination of trigger objects: table 2-4 shows
some examples of trigger objects.
Table 2-5 shows some trigger lines together with their L1 trigger rates and their efficiencies for various
physics processes: the star (*) symbol next to a trigger object indicates that a minimum angular separation
was required in order to count more than one object (typically � Æ ). Back-to-back topologies among clusters
MARCELLA BONA