Violation in Mixing

62 The BABAR Experiment
At design voltage of �� Î , the efficiency averages ��¦ per track above Å�Î� : the data recorded
at � Î show a reduction in efficiency by about � for tracks almost at normal incidence, indicating that
the cells are not fully efficient at this voltage (see right plot in fig. (2-9)).
Tracks/10 MeV/c
Efficiency
8000
4000
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0
0.8
0.4
a)
b)
Data
Simulation
0.0
0.0 0.1 0.2 0.3 0.4
Transverse Momentum (GeV/c)
σ (mm)
0.4
0.3
0.2
0.1
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σ z0
σ d0
0
0 1 2
3
Transverse Momentum (GeV/c)
Figure 2-10. Left plot: Monte Carlo studies of low momentum tracks in the ËÎÌ on � £ � � � events.
a) comparison with data in �� events and b) efficiency for slow pion detection derived from simulated
events. Right plot: resolution in the parameters � and Þ for tracks in multi-hadron events as a function of
the transverse momentum.
The stand-alone ËÎÌ tracking algorithms have a high efficiency for tracks with low transverse momentum:
to estimate the tracking efficiency for these low momentum tracks, a detailed Monte Carlo study was
performed. The pion spectrum was derived from simulation of the inclusive � £ production in �� events
and Monte Carlo events were selected in the same way as the data: since the agreement with MC is very
good, the detection efficiency has been derived from MC simulation. The ËÎÌ extends the capability of the
charge particle reconstruction down to transverse momenta of � � �� (see left plot in fig. (2-10)).
The resolution in the five track parameters is monitored using � � and � � pair events: the resolution
is derived from the difference of the measured parameters for the upper and lower halves of the cosmic
ray tracks traversing the ��À and the ËÎÌ. On this sample with transverse momenta above ��Î�, the
resolution for single tracks is �Ñ in � and � �Ñ in Þ . To study the dependence of resolution from
transverse momentum, a sample of multi-hadron events is used: the resolution is determined from the width
of the distribution of the difference between the measured parameters (� and Þ ) and the coordinates of the
vertex reconstructed from the remaining tracks in the event: right plot in fig. (2-10) shows the dependence
of the resolution in � and Þ as a function of ÔØ. The measured resolutions are about � �Ñ in � and � �Ñ
in Þ for ÔØ of ��Î�: these values are in good agreement with the Monte Carlo studies and in reasonable
agreement also with the results from cosmic rays.
MARCELLA BONA