Violation in Mixing

64 The BABAR Experiment
Quartz Bar Sector
~2 m
~5 m
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Standoff Cone
PMT Module
Hinged Cover (12)
Plane Mirror (12)
Quartz Bar
Sector Cover
Window Frame
Assembly Flange
Quartz Window
Figure 2-11. Mechanical elements of the �ÁÊ� and schematic view of bars assembled into a mechanical
and optical sector.
of the bars is instrumented. A mirror placed at the other end on each bar reflects forward-going photons to
the instrumented end. The Čerenkov angle at which a photon was produced is preserved in the propagation,
modulo some discrete ambiguities (the forward-backward ambiguity can be resolved by the photon arrivaltime
measurement, for example). The �ÁÊ� efficiency grows together with the particle incidence angle
because more light is produced and a larger fraction of this light is totally reflected. To maximize the
total reflection, the material must have a refractive index (fused silica index is Ò � ��� ) higher than the
surrounding environment (the �ÁÊ� is surrounded by air with index Ò � � ).
Once photons arrive at the instrumented end, most of them emerge into a water-filled expansion region,
called the Standoff Box: the purified water, whose refractive index matches reasonably well that of the bars
(ÒÀ Ç � � ��), is used to minimize the total internal reflection at the bar-water interface.
The standoff box is made of stainless steel and consists of a cone, cylinder and 12 sectors of PMTs: it
contains about � liters of purify water. Each of the 12 PMTs sectors contains ��� PMTs in a closepacked
array inside the water volume: the PMTs are linear focused �� Ñ diameter photo-multiplier tubes,
lying on an approximately toroidal surface.
The �ÁÊ� occupies only � Ñ of radial space, which allows for a relatively large radius for the drift chamber
while keeping the volume of the CsI Calorimeter reasonably low: it corresponds to about � � at normal
incidence. The angular coverage is the �� of the � azimuthal angle and the � of Ó× ��Å.
Čerenkov photons are detected in the visible and near-UV range by the PMT array. A small piece of fused
silica with a trapezoidal profile glued at the back end of each bar allows for significant reduction in the area
requiring instrumentation because it folds one half of the image onto the other half. The PMTs are operated
directly in water and are equipped with light concentrators: the photo-multiplier tubes are about � Ñ away
MARCELLA BONA