Violation in Mixing

56 The BABAR Experiment
is formed. The good signal resolution in the lower range ensures a good determination of the tails of the
cluster thus improving the resolution on the impact point measurement.
The electronic noise measured is found to vary between � and � electrons ENC (equivalent noise
charge), depending on the layer and the readout view: this can be compared to the typical energy deposition
for a minimum ionizing particle at normal incidence, which is equivalent to � � electrons.
During normal running conditions, the average occupancy of the ËÎÌ in a time window of �× is about
for the inner layers, where it is dominated by machine backgrounds, and less than for the outer layers,
where noise hits dominate.
The cluster reconstruction is based on a cluster finding algorithm: first the charge pulse height of a single
pulse is calculated form the ToT value and clusters are formed grouping adjacent strips with consistent times.
The position Ü of a cluster formed by Ò strips is evaluated with an algorithm called “head-to-tail” algorithm:
Ü � Ü ÜÒ Ô ÉÒ É
ÉÒ É
where Ü� and É� are the position and the collected charge of i-th strip and Ô is the read-out pitch. This
formula always gives a cluster position within Ô� of the geometrical center of the cluster. The cluster pulse
height is simply the sum of the strip charges, while the cluster time is the average of the signal times.
The ËÎÌ efficiency can be calculated for each half-module by comparing the number of associated hits to
the number of tracks crossing the active area of the half-module. Excluding defective readout sections (9
over 208), the combined hardware and software efficiency is �� (see fig. (2-5)).
The spatial resolution of ËÎÌ hits is calculated by measuring the distance (in the plane of the sensor)
between the track trajectory and the hit, using high-momentum tracks in two prong events: the uncertainty
due to the track trajectory is subtracted from the width of the residual distribution to obtain the hit resolution.
The track hit residuals are defined as the distance between track and hit, projected onto the wafer plane and
along either the � or Þ direction. The width of this residual distribution is then the ËÎÌ hit resolution. Fig.
(2-6) shows the ËÎÌ hit resolution for Þ and � side hits as a function of the track incident angle, for each of
the five layers: the measured resolutions are in very good agreement with the Monte Carlo expected ones.
Over the whole ËÎÌ, resolutions are raging from � �Ñ (inner layers) to � �Ñ (outer layers)
for normal tracks.
For low-momentum tracks (ÔØ � Å�Î� ), the ËÎÌ provides the only particle identification information.
The measure of the ToT value enables to obtain the pulse height and hence the ionization ����Ü: the
value of ToT are converted to pulse height using a look-up table computed from the pulse shapes. The
double-sided sensors provide up to ten measurements of ����Ü per track: with signals from at least four
sensors, a � truncated mean ����Ü is calculated. For MIPs, the resolution on the truncated mean ����Ü
is approximately � : a � separation between kaons and pions can be achieved up to momentum of
� �� and between kaons and protons beyond ���.
MARCELLA BONA