Violation in Mixing

Efficiency
2.2 The BABAR detector. 73
Æ C. During the first summer of operation, the daily temperature in the IR hall was � Æ C and the maximum
hall temperature frequently exceeded Æ C: the temperature inside the steel rose to more than � Æ C so that
the dark currents in many modules exceeded the capabilities of the HV system and some RPCs had to be
temporarily disconnected. A water cooling was installed on the barrel and end door steel.
During operation at high temperature, a large fraction of the RPCs showed very high dark currents, but
also some reduction in efficiency compared to earlier measurement: the cause of the efficiency loss remains
under investigation. After the cooling was installed and the RPCs reconnected, some of them continued to
deteriorate while others remained stable, some of them (� ) at full efficiency.
1.0
0.5
MC
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0.4
160 Data
Background
0.6
0.4
0.2
0.0
0
0.3
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Momentum (GeV/c)
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Neutral Clusters
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1-2001
8583A6
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-100 0
Δφ (Degrees)
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Figure 2-17. Left plot: muon efficiency (left scale) and pion mis-identification probability (right scale) as
a function of the laboratory track momentum. Right plot: difference between the direction of reconstructed
neutral hadron cluster and the missing transverse momentum in events with a reconstructed �� decay. The
Monte Carlo simulation is normalized to the luminosity of the data.
Muon identification relies almost entirely on the Á�Ê: charged particles are reconstructed in the ËÎÌ and
��À and muon candidates are required to meet the criteria for minimum ionizing particles in the �Å�.
Charged tracks are extrapolated to the Á�Ê taking into account the non-uniform magnetic field, multiple
scattering and the average energy loss. The projected intersection with the RPC planes are computed and all
clusters within a predefined distance from the predicted intersection are associated with the track.
The performance of the muon identification has been tested on samples of muons from ���� and ��­ final
states and pions from Ã Ë and three-prong � decays: the muon detection efficiency is about � in the
momentum range of �� �� � ��� with a fake rate for pions of about � � (see left plot in fig.
(2-17)).
Ã Ä and other neutral hadrons interact in the steel of the Á�Ê and can be identified as clusters that are not
associated with a charged track: Monte Carlo studies predict that about �� of Ã Ä of more than ��Î�
momentum, produce a cluster in the cylindrical RPC or a cluster with hits in two or more planar RPC layers.
THE BABAR EXPERIMENT