Violation in Mixing

4.4 PID selection 107
In the case of the global maximum likelihood fit (see Sec. 4.6), no cuts are applied to the Fisher discriminant.
Instead, signal-background discrimination is achieved by using � in the fit itself. Comparison of � for signal
and background events is described in detail in section 4.6.1.
In case of the counting analysis, a cut on the Fisher discriminant output is applied and it is chosen in order
to optimize the statistical significance Ë � Ë � , where Ë and � are the number of expected signal and
background events, respectively. After this cut, one should check that side-band background shape is wellmodeled
by the ARGUS function fitted before applying these cuts, giving confidence that the same function
can be used (see Sec. 4.6).
4.4 PID selection
The difference between a à or a � in the final state taken here into account is apparent only in the
reconstructed ¡�, for which there is a separation of less than �. The particle identification capabilities
of the BABAR detector provide additional means to distinguish the two decays. Of primary importance is the
�ÁÊ� information since the momenta of the two daughter tracks in these decays are in a region where the
mean ��À ����Ü for kaons and pions differ by only about �. In principle, the �ÁÊ� can provide better
than � separation of pions and kaons throughout the momentum region of the daughter tracks.
The maximum likelihood fit makes direct use of the � ��Ö�Ò�ÓÚ angle, � , reconstructed by the �ÁÊ�. Each
track is assigned a likelihood to be a pion or kaon based on the value of the reconstructed �. As a cross
check, a second complementary method is pursued. It employs particle selector algorithms which provide
lists of kaon and pion tracks that are used to separately identify the different final state modes.
A cut on the number of signal photons observed in the �ÁÊ� is used to improve the � resolution and reduce
the size of non-Gaussian tails. The cut Æ×�� ­ � � is used, where Æ×�� ­ is the number of observed signal
photons for the track. Protons are explicitly removed with the cut � � Ô � ÑÖ��, where � Ô is
the expected mean value of � for a proton of a given momentum. Electrons are removed by rejecting tracks
which pass a tight selector criteria.
The performance of the PID cuts are studied in the actual data using a pure sample of kaons and pions
obtained from a control sample of � £ � � � , with � � Ã � .
4.4.1 � � £ � control sample
In order to assess and parameterize the performance of the particle ID methods, without relying on Monte
Carlo simulation, one must identify a source of pions and kaons, the selection of which does not utilize
particle ID from the �ÁÊ�. An ideal control sample consists of the daughter tracks from � � à �
decays in the reaction � £ � � � � Ã � � . The � (Ã) track is always the one with the
same (opposite) charge as the � £ and cutting on the small � £ � mass difference ensures that there
is virtually no contamination from incorrectly reconstructed � candidates (i.e. a true � � Ã �
reconstructed as a � � � Ã candidate and then combined with a random track to form a � £
STRATEGY AND TOOLS FOR CHARMLESS TWO-BODY � DECAYS ANALYSIS