Violation in Mixing

54 The BABAR Experiment
Figure 2-4. Cross-sectional view of the ËÎÌ in a plane perpendicular to the beam axis and representation
of a detector module from the third layer.
These guidelines have led to the choice of a ËÎÌ made of five layers of double-sided silicon strip sensors:
the spatial resolution, for perpendicular tracks must be � �Ñ in the three inner layers and about
� �Ñ in the two outer layers. The three inner layers perform the impact parameter measurement, while
the outer layers are necessary for pattern recognition and low ÔØ tracking. The silicon detectors are doublesided
(contain active strips on both sides) because this technology reduces the thickness of the materials
the particles have to cross, thus reducing the energy loss and multiple scattering probability compared to
single-sided detectors. The sensors are organized in modules (see right drawing in fig. (2-4)). The ËÎÌ five
layers contain 340 silicon strip detectors with AC-coupled silicon strips.
Each detector is �Ñ-thick but sides range from � ÑÑ to � ÑÑ and there are 6 different detector
types. Each of the three inner layers has a hexagonal transverse cross-section and it is made up of 6
detector modules, arrayed azimuthally around the beam pipe, while the outer two layers consist of 16 and
18 detector modules, respectively. The inner detector modules are barrel-style structures, while the outer
detector modules employ the novel arch structure in which the detectors are electrically connected across
an angle. This arch design was chosen to minimize the amount of silicon required to cover the solid angle
while increasing the solid angle for particles near the edges of acceptance: having incidence angles on the
detector closer to � degrees at small dip angles insures a better resolution on impact points. One of the main
features of the ËÎÌ design is the mounting of the readout electronics entirely outside the active detector
volume.
The strips on the two sides of the rectangular detectors in the barrel regions are oriented parallel (� strips) or
perpendicular (Þ strips) to the beam line: in other words, the inner sides of the detectors have strips oriented
perpendicular to the beam direction to measure the Þ coordinate (Þ-size), whereas the outer sides, with
longitudinal strips, allow the �-coordinate measurement (�-side). In the forward and backward regions of the
MARCELLA BONA