Violation in Mixing

72 The BABAR Experiment
at distances of about Ñ. The external surface of the bakelite are coated with graphite surfaces that are
connected to high voltage (� � �Π) and ground and protected by an insulating Mylar film.
The RPC is essentially a gas gap at atmospheric pressure enclosed between two ÑÑ-thick bakelite (phenolic
polymer) plates: the gas mixture is based on comparable quantities of Argon and Freon and a small
amount of Isobutane. A crossing charged particle produces a quenched spark that produces signals on
external pick-up electrodes. The RPCs are operated in limited streamer mode and the signal are read out
capacitively on both sides of the gap by external electrodes made of aluminum strips on a Mylar substrate.
The Á�Ê consists of a central part (barrel) and two plugs (end-caps) which complete the solid angle coverage
down to ÑÖ�� in the forward direction and � ÑÖ�� in the backward direction. The barrel extends
radially from ��� Ñ to � Ñ and is divided into sextants: the length of each sextant is ��� Ñ and the
width varies from ��� Ñ to � Ñ.
The Á�Ê detectors cover a total active area of about Ñ : there are a total of � � RPC modules, �� in
each of the six barrel sections, � in each of the four half end-doors and in the two cylindrical layers.
The modules of each chamber are connected to the gas system in series, while the high voltage is supplied
separately to each module.
Barrel modules have strips running perpendicular to the beam axis to measure the Þ coordinate and
�� strips in the orthogonal direction extending over three modules to measure �. The readout strips are
separated from the ground aluminum plane by a � ÑÑ-thick foam. The strips are connected to the readout
electronics at one end and even and odd strips are connected to different front-end cards so that a failure of
a card does not result in a total loss of signal, since a particle crossing the gap typically generates signals in
two o more adjacent strips.
The cylindrical RPC is divided into four sections, each covering a quarter of the circumference: each of
these sections has four sets of two single gap RPCs with orthogonal readout strips, the inner with helical
Ù Ú strips that run parallel to the diagonals of of the module, and the outer with strips parallel to � and Þ.
The efficiency of the RPCs is evaluated both for normal collision data and for cosmic ray muons: every
week cosmic ray data are recorded at different voltage settings and the efficiency is measured chamber-bychamber
as a function of the applied voltage (the typical voltage is 7.6kV). To calculate the efficiency in
a given chamber, nearby hits in a given layer and hits in different layers are combined to form clusters.
Two algorithms are used: the first relies only on Á�Ê information, while the second tries to match Á�Ê
clusters with the tracks reconstructed in the ��À. They both start from one-dimensional Á�Ê clusters
defined as groups of adjacent hits in one of the two readout coordinates. The first algorithm consists of
joining one-dimensional clusters (of the same readout coordinate) in different layers, in order to form twodimensional
clusters and then these two-dimensional clusters in different coordinates are combined into
three-dimensional clusters. The second algorithm extrapolates ��À charged tracks to be combined with
the Á�Ê clusters to form two- and three-dimensional clusters.
The residual distribution from straight line fits to two-dimensional clusters typically have an rms width
of less than Ñ. An RPC is considered efficient if a signal is detected at a distance of less than Ñ
from the fitted straight line in either of the two readout planes: �� of the active RPCs modules exceed an
efficiency of � . The RPC dark current is very temperature dependent: this current increases � per
MARCELLA BONA