Violation in Mixing

Efficiency
2.2 The BABAR detector. 61
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a)
Efficiency
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Figure 2-9. Track reconstruction efficiency in the ��À at operating voltages of �� Î and � Î as a
function of transverse momentum (left plot) and of polar angle (right plot). The efficiency is measured in
multi-hadron events.
Charged tracks are defined by five parameters (� , � , �, Þ and Ø�Ò �) and their associated error matrix:
these parameters are measured at the point of closest approach to the Þ-axis and � and Þ are the distances
of this point from the origin of the coordinate system (in the Ü Ý plane and on the Þ axix, respectively). The
angle � is the azimuth of the track, � is the dip angle relative to the transverse plane and � is the curvature.
� and � have signs that depend on the particle charge.
The track finding and the fitting procedure make use of the Kalman filter algorithm that takes into account
the detailed description of material in the detector and the full map of the magnetic field. First of all,
tracks are reconstructed with ��À hits through a stand-alone ��À algorithm: the resulting tracks are then
extrapolated into the ËÎÌ and ËÎÌ track segments are added and a Kalman fit is performed to the full set
of ��À and ËÎÌ hits. Any remaining ËÎÌ hits are then passed to the ËÎÌ stand-alone track finding
algorithms. Finally, an attempt is made to combine tracks that are only found by one of the two tracking
systems and thus recover tracks scattered in the material of the support tube.
The efficiency for track reconstruction in the ��À has been measured as a function of transverse momentum,
polar and azimuthal angles in multi-track events. These measurement rely on specific final states and
exploit the fact that the track reconstruction can be performed independently in the ËÎÌ and the ��À. The
absolute ��À tracking efficiency is determined as the ratio of the number of reconstructed ��À tracks to
the number of tracks detected in the ËÎÌ with the requirement that they fall within the acceptance of the
��À. Left plot in fig. (2-9) shows the efficiency in the ��À as a function of transverse momentum in
multi-hadron events.
b)
THE BABAR EXPERIMENT