Violation in Mixing

2.2 The BABAR detector. 57
SVT Efficiency
1.1
1
0.9
0.8
8 bad out of 208 total
Half Modules not included
BABAR
Phi View Z View
1 2 3 4 5 1 2 3 4 5
Layer
Figure 2-5. Efficiency of ËÎÌ hit reconstruction, as measured on data, as a function of ËÎÌ layer and
readout view.
2.2.2 The drift chamber ��À.
The drift chamber is the second part of BABAR tracking system: its principal purpose is the efficient detection
of charged particles and the measurement of their momenta and angles with high precision. The ��complements the measurements of the impact parameter and the directions of charged tracks provided by
the ËÎÌ near the impact point (IP). At lower momenta, the ��À measurements dominate the errors on the
extrapolation of charged tracks to the �ÁÊ�, �Å� and Á�Ê. The reconstruction of decay and interaction
vertices outside of the ËÎÌ volume, for instance the Ã Ë decays, relies only on the ��À. For these
reasons, the chamber should provide maximal solid angle coverage, good measurement of the transverse
momenta and positions but also of the longitudinal positions of tracks with a resolution of � ÑÑ, efficient
reconstruction of tracks at momenta as low as �� and it has to minimally degrade the performance
of the calorimeter and particle identification devices (the most external detectors). The ��À also needs
to supply information for the charged particle trigger. For low momentum particles, the ��À is required
to provide particle identification by measuring the ionization loss (����Ü). A resolution of about � will
allow ��à separation up to � �� . This particle identification (PID) measurement is complementary
to that of the �ÁÊ� in the barrel region, while in the extreme backward and forward region, the ��À is the
only device providing some discrimination of particles of different mass. The ��À should also be able to
operate in presence of large beam-generated backgrounds having expected rates of about � �ÀÞ/cell in the
innermost layers.
To meet the above requirements, the ��À is a � Ñ-long cylinder (see left plot in fig. (2-7)), with an
inner radius of �� Ñ and an outer radius of � �� Ñ: it is bounded by the support tube at its inner radius
THE BABAR EXPERIMENT