Violation in Mixing

2.2 The BABAR detector. 55
two outer layers, the angle between the strips on the two sides of the trapezoidal detectors is approximately
� Æ and the � strips are tapered.
The inner modules are tilted in � by � Æ , allowing an overlap region between adjacent modules: this provide
full azimuthal coverage and is convenient for alignment. The outer modules are not tilted, but are divided
into sub-layers and placed at slightly different radii (see left drawing in fig. (2-4)).
The total silicon area in the ËÎÌ is ��� Ñ and the number of readout channels is about � . The
geometrical acceptance of ËÎÌ is � of the solid angle in the c.m. system and typically � are used in
charged particle tracking.
The Þ-side strips are connected to the read-out electronics with flexible Upilex fanout circuits glued to the
inner faces of half-modules: as a matter of fact, each module is divided into two electrically separated
forward and backward half-modules. The fanout circuits consist of conductive traces on a thin flexible
insulator (copper traces on Kapton): the traces are wire-bonded to the end of the strips.
In the two outer layers, in each module the number of Þ strips exceeds the number of read-out channels,
so that a fraction of the strips is “ganged”, i.e., two strips are connected to the same read-out channel.
The “ganging” is performed by the fanout circuits. The length of a Þ strip is about � �Ñ (case of no
ganging) or �Ñ (case of two strip connected): the ganging introduces an ambiguity on the Þ coordinate
measurement, which must be resolved by the pattern recognition algorithms. The � strips are daisy-chained
between detectors, resulting in a total strip length of up to � Ñ. Also, for the �-side, a short fanout
extension is needed to connect the ends of the strips to the read-out electronics.
Table 2-3. Parameters of the ËÎÌ layout: these characteristics are shown for each layer.
st nd rd �th �th
layer layer layer layer layer
radius (ÑÑ) 32 40 54 91-127 114-144
modules/layer 6 6 6 16 18
wafers/module 4 4 6 7 8
read-out pitch (�Ñ)
� 50-100 55-110 55-110 100 100
Þ 100 100 100 210 210
The signals from the read-out strips are processed using a new technique, bringing in several advantages.
After amplification and shaping, the signals are compared to a preset threshold and the time they exceed
this threshold (time over threshold, or ToT) is measured. This time interval is related to the charge induced
in the strip by the charged particle crossing it. Unlike the traditional peak-amplitude measurement in the
shaper output, the ToT has the advantage of an approximately logarithmic relation of the time interval to
the charge signal. This compresses the active dynamic range of the signal, ensuring a good sensitivity in
the lower range. When a particle crosses a silicon detector a cluster of adjoining strips producing a signal
THE BABAR EXPERIMENT