Violation in Mixing

2.2 The BABAR detector. 59
324 1015 1749
68
35
551 973
IP
202
17.19
236
469
1618
Sense
Guard 1-2001
8583A16
Figure 2-7. Side view of the BABAR drift chamber (the dimensions are in ÑÑ) and isochrones (i.e. contours
of equal drift time of ions) in cells of layer and � of an axial super-layer. The isochrones are spaced by
Ò×.
using the low-mass aluminum field wires and the helium-based gas mixture, the multiple scattering inside
the ��À is reduced to a minimum, representing less than � � of material. The total thickness of the
��À at normal incidence is � � � .
The drift cells are arranged in super-layers of � cylindrical layers each: the super-layers contain wires
oriented in the same direction: to measure the Þ coordinate, axial wire super-layers and super-layers with
slightly rotated wires (stereo) are alternated. In the stereo super-layers a single wire corresponds to different
� angles and the Þ coordinate is determined by comparing the � measurements from axial wires and the
measurements from rotated wires. The stereo angles vary between ¦�� ÑÖ�� and ¦�� ÑÖ��.
While the field wires are at ground potential, a positive high voltage is applied to the sense wires: an
avalanche gain of approximately � ¢ � is obtained at a typical operating voltage of �� Î and a � �
helium:isobutane gas mixture.
In each cell, the track reconstruction is obtained by the electron time of flight: the precise relation between
the measured drift time and drift distance is determined from sample of � � and � � events. For each
signal, the drift distance is estimated by computing the distance of closest approach between the track and
the wire. To avoid bias, the fit does not include the hit of the wire under consideration. The estimated drift
distances and the measured drift times are averaged over all wires in a layer.
The ��À expected position resolution is lower than �Ñ in the transverse plane, while it is about ÑÑ
in the Þ direction. The minimum reconstruction and momentum measure threshold is about Å�Î� and
it is limited by the ��À inner radius. The design resolution on the single hit is about � �Ñ while the
achieved weighted average resolution is about � �Ñ. Left plot in fig. (2-8) shows the position resolution
as a function of the drift distance, separately for the left and the right side of the sense wire. The resolution
is taken from Gaussian fits to the distributions of residuals obtained from unbiased track fits: the results are
based on multi-hadron events for data averaged over all cells in layer �.
Field
THE BABAR EXPERIMENT