Violation in Mixing

58 The BABAR Experiment
z Resolution (μm)
Layer 1
Layer 3
Layer 5
angle (degrees)
Layer 2
Layer 4
(a)
angle (degrees)
φ Resolutiion (μm)
Layer 1
angle (degrees)
Layer 2
Layer 3 Layer 4
Layer 5
(b)
angle (degrees)
Figure 2-6. ËÎÌ hit resolution in the Þ and � coordinate in microns, plotted as functions of the track
incident angle in degrees. Each plot corresponds to a different layer of the ËÎÌ.
and the particle identification device at its outer radius. The flat end-plates are made of aluminum: since the
BABAR events will be boosted in the forward direction, the design of the detector is optimized to reduce the
material in the forward end. The forward end-plate is made thinner ( ÑÑ) in the acceptance region of the
detector compared to the rear end-plate ( � ÑÑ), and all the electronics is mounted on the rear end-plate.
The device is asymmetrically located with respect to the IP: the forward length of 174.9 cm is chosen so that
particles emitted at polar angles of �� Æ traverse at least half of the layers of the chamber before exiting
through the front end-plate. In the backward direction, the length of 101.5 cm means that particles with
polar angles down to � �� Æ traverse at least half of the layers.
The inner cylinder is made of ÑÑ beryllium and the outer cylinder consists of two layers of carbon fiber
on a Nomex core: the inner cylindrical wall is kept thin to facilitate the matching of ËÎÌ and ��À tracks,
to improve the track resolution for high momentum tracks and to minimize the background from photon
conversions and interactions. Material in the outer wall and in the forward direction is also minimized in
order not to degrade the performance of the �ÁÊ� and the �Å�.
The region between the two cylinders is filled up by a gas mixture consisting of Helium-isobutane (� �
): the chosen mixture has a radiation length that is five times larger than commonly used argon-based
gases. � layers of wires fill the ��À volume and form � � hexagonal cells with typical dimensions
of � ¢ �� Ñ along the radial and azimuthal directions, respectively (see right plot in fig. 2-7). The
hexagonal cell configuration has been chosen because approximate circular symmetry can be achieved over
a large portion of the cell. Each cell consist of one sense wire surrounded by six field wires: the sense wires
are �Ñ gold-plated tungsten-rhenium, the field wires are �Ñ and � �Ñ gold-plated aluminum. By
MARCELLA BONA