Violation in Mixing

2.2 The BABAR detector. 53
The detector geometry is cylindrical in the inner zone and hexagonal in the outermost zone: the central part
of the structure is called barrel and it’s closed forward and backward by end caps. The BABAR coordinate
system has the Þ axis along the boost direction (or the beam direction): the Ý axis is vertical and the Ü axis
is horizontal and goes towards the external part of the ring.
2.2.1 The Silicon Vertex Tracker: ËÎÌ.
The charged particle tracking system is based on the vertex detector and the drift chamber: the main
purpose of this charged particle tracking system is the efficient detection of charged particles and the
measurement of their momentum and angles with high precision. These track measurements are important
for the extrapolation to the �ÁÊ�, the �Å� and the Á�Ê: at lower momenta, the ��À measurements
are more important while at higher momenta the ËÎÌ dominates. The vertex detector is the only tracker
within a radius of Ñ from the primary interaction region: it is placed inside the support tube of the beam
magnets and consists of five layers to provide five measurements of the positions of all charged particles
with polar angles in the region � Æ ��� � Æ . Because of the presence of a �� Ì magnetic field, the
charged particle tracks with transverse momenta lower than � �� cannot reach the drift chamber
active volume. So the ËÎÌ has to provide stand-alone tracking for particles with transverse momentum less
than Å�Î� , the minimum that can be measured reliably in the ��À alone: this feature is essential
for the identification of slow pions from � £ meson decays. Because of these, the ËÎÌ has to provide
redundant measurements.
Beyond the stand-alone tracking capability, the ËÎÌ provides the best measurement of track angles which
is required to achieve design resolution for theČerenkov angle for high momentum tracks. The ËÎÌ is very
closed to the production vertex in order to provide a very precise measure of points on the charged particles
trajectories on both longitudinal (Þ) and transverse directions. The longitudinal coordinate information is
necessary to measure the decay vertex distance, while the transverse information allows a better separation
between secondary vertices coming from decay cascades.
More precisely, the design of the ËÎÌ was carried out according to some important guidelines:
¯ The number of impact points of a single charged particle has to be greater than to make a stand-alone
tracking possible, and to provide an independent momentum measure.
¯ The first three layers are placed as close as possible to the impact point to achieve the best resolution
on the Þ position of the � meson decay vertices.
¯ The two outer layers are close to each other, but comparatively far from the inner layers, to allow a
good measurement of the track angles.
¯ The ËÎÌ must withstand MRad of ionizing radiation: the expected radiation dose is Rad/day in
the horizontal plane immediately outside the beam pipe and � Rad/day on average.
¯ Since the vertex detector is inaccessible during normal detector operations, it has to be reliable and
robust.
THE BABAR EXPERIMENT