Violation in Mixing

52 The BABAR Experiment
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Coil Cryostat 100
Calorimeter
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Figure 2-3. The BABAR detector.
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.249 ref.
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The �È decay modes of interest generally have �Ê below � and reconstructing them requires observing
anywhere two to six charged particles: in order to get a good efficiency, the BABAR detector has to cover as
much solid angle as possible. As PEP-II is an asymmetric collider, particular care must be taken to cover
the forward region: the applied boost implies that, on average, half the produced particles are in the region
with Ó× �Ð�� � ��. The accelerator bending magnets limit the maximum acceptance to �� Æ , in both
forward and backward directions, but to allow the maximum forward coverage, machine components such
as cooling systems etc, are located in the backward region. The active parts of the silicon vertex tracker cover
the polar angle between � Æ and � � Æ in the laboratory frame. This region in the lab frame corresponds
to ��� � Ó× ��Å � ��� where ��Å is the polar angle in the center-of-mass frame.
Another important parameter is the minimum measurable momentum value for both charged and neutral particles:
taking into consideration that charged pions have minimum momentum values of about ��
and that tagging kaons have minimum momentum values of about � ���, the tracking system has a
minimum acceptance value of � �� for momenta. In case of photons, the energy spectra have shown
that the minimum measurable energy value must be � Å�Î.
MARCELLA BONA