Violation in Mixing

2.2 The BABAR detector. 71
The main requirements for the Á�Ê are large solid angle coverage, good efficiency and high background
rejection for muons down to momenta below ��Î. For neutral hadrons, the most important requirements
are high efficiency and good angular resolution. The momentum range in which the Á�Ê can work, starts
from about �� �� (limit due to the barrel magnetic field: lower momentum particles cannot enter the
Á�Ê), while in the forward and backward regions the lower limit is � Å�Î� . The upper limit is of order
of some ��Î, but, since even direct muons cannot have momentum values higher than ���Î�, one can
say that the Á�Ê does not have an upper limit for muons from § �Ë decays.
H.V.
Foam
Bakelite
Gas
Bakelite
Foam
Figure 2-16. The Á�Ê detector and schematic representation of RPC components.
Aluminum
X Strips
Insulator
Graphite
2 mm
2 mm
2 mm
Graphite
Insulator
Y Strips
Spacers
Aluminum
8-2000
8564A4
The Á�Ê uses the steel flux return of the magnet as muon filter and hadron absorber: the uniform magnetic
field of �� Ì is generated by a superconducting solenoid and the large iron structure needed as magnet yoke
is segmented and instrumented with Resistive Plate Chambers (RPCs) with two-coordinate readout. The
RPCs are installed in the gaps of the finely segmented steel of the barrel and the end doors of the flux return
(see fig. (2-16)). The steel segmentation has been chosen on the basis of Monte Carlo studies of muon
penetration and charged and neutral hadron interactions: the steel is segmented into � plates increasing in
thickness from Ñ for the inner nine plates to Ñ for the outermost plates. The nominal gap between
the steel plates is �� Ñ in the inner layers of the barrel and � Ñ elsewhere. There are � RPC layers
in the barrel and � in the end-caps. Each end-cap consists of hexagonal plates, divided vertically into two
parts to allow opening of the detector and has a central hole for the beam components and the magnetic
shields. In addition, two layer of cylindrical RPCs are installed between the �� and the magnet cryostat,
in order to detect particle exiting the ��.
RPCs detect streamers from ionizing particles via capacitive readout strips. The position resolution depends
on the segmentation of the readout: a value of a few mm is achievable. A cross section of an RPC is shown
schematically in fig.(2-16): the planar RPC consists of two bakelite sheets, ÑÑ thick and separated by a
gap of ÑÑ. The gap width is kept uniform by policarbonate spaces that are glued to the bakelite, spaced
THE BABAR EXPERIMENT