Violation in Mixing

66 The BABAR Experiment
signal with the requirement that the transit time of the photon from its creation in the bar to its detection at
the PMT be consistent with the measurement error of about �� Ò×.
An unbinned maximum likelihood formalism is used to take into account all information provided by the
�ÁÊ�: the reconstruction routine provides a likelihood value for each of the five stable particle types (�,
�, �, à and Ô) if the track passes through the active volume of the �ÁÊ�. These likelihood probabilities
are calculated in an iterative process by maximizing the likelihood value for the entire event while testing
different hypotheses for each track. If enough photons are found, a fit of �� and the number of observed
signal and background photons are calculated.
In the absence of correlated systematic errors, the resolution (� ��track ) on the track Čerenkov angle should
scale as
���track � ���­ Ô
ÆÔ�
where ���­ is the single photon angle resolution. This angular resolution (obtained from di-muon events) can
be estimated to be about � ÑÖ��, in good agreement with the expected value (see left plot in fig. 2-12).
The measured time resolution is �� Ò× close to the intrinsic �� Ò× time spread of the PMTs. In di-muon
event data, the number of photo-electrons varies between for small polar angles at the center of the barrel
and �� at large polar angles: this is variation is well reproduced by Monte Carlo and can be understood by
the fact that the number of Čerenkov photons varies with the path length of the track in the radiator (smaller
path length at perpendicular incidence at the center of the barrel). Also the fraction of photons trapped by
total internal reflection rises with larger values of � Ó× � track �: this increase in the number of photons for
forward going tracks corresponds also to an increase in momentum of the tracks and thus an improvement
of the �ÁÊ� performance.
The width of the track Čerenkov angle resolution for di-muon events is �� ÑÖ�� compared to the design
goal of � ÑÖ�� (see right plot in fig. (2-12)). From the measured single track resolution versus momentum
in d-muon events and from the difference between the expectedČerenkov angles of charged pions and kaons,
the pion-kaon separation power of the �ÁÊ� can be evaluated: the expected separation between pions and
kaons at ��� is about �� �, within � of the design goal.
Left plot in fig. (2-13) shows an example of use of PID from �ÁÊ�: the Ã� invariant mass spectra are
shown with and without the use of the �ÁÊ� for kaon identification and the peak corresponds to the �
particle. Note how the �ÁÊ� contribution can reduced the background level.
The efficiency for correct identifying a charged kaon passing through the radiator and the probability of
wrongly identifying a pion as a kaon are determined using � � Ã � decays selected kinematically
from inclusive � £ production: fig. (2-13) shows kaon identification efficiency and pion mis-identification
as functions of the track momentum. The mean kaon identification efficiency is ��� ¦ � (stat) and the
mean pion mis-identification is � ¦ � (stat).
MARCELLA BONA