2 The CDF Experiment at Fermilab Contents - Harvard University ...

90 Section 2: CDF Physics Analysis
Bs
Ds K
φ
π
K K
Other B Decay (used for tagging)
000 111
000 111
000 111
000 111
Primary Vertex
Bs
Ds
Fragmentation K (used for tagging)
K
Figure 43: Diagram of the B s ! D s K; D s ! ; ! KK decay, including fragmentation
K and opposite side B used for avor tagging.
Measuring sin with B s ! D s K
Bailey has performed a feasibility study for measuring sin using the decay mode B s !
Ds K at CDF II. Since then, he and Maksimovic have improved this study and presented
it to the collaboration. This mode is unique to hadronic colliders (the B factories do not
produce B s ) and thus provides an important complement to studies using B 0 . It also has the
benet of not involving \penguin" diagrams which make many CP violation measurements
ambiguous through their non-CP violating contributions. With a relatively large decay rate,
B s ! Ds K is one of the best decay modes for measuring sin which is necessary for the
full consistency check of the Standard Model.
The theoretically expected errors are inversely proportional to a variety of dilution factors
which eectively reduce the amplitude of the signal observed. These dilution factors arise
from measurement resolutions, backgrounds, t limitations, and avor tagging ineciencies
and mistags. Using the latest CDF II estimates for these dilutions the expected error on
sin is 0.45.
To check the dependence of this measurement upon various input parameters, Bailey
wrote a toy Monte Carlo simulation and likelihood tter including backgrounds, resolution
eects, and mistag probabilities. The measurement involves an unbinned likelihood t to
four dierent decay rates (both B s and B s to both D s , K + and D s + K , ) to extract the weak
phase , a strong phase, and the ratio of the decay rates. The input parameters to the event
generation and t were varied and the resulting error on sin was studied. It was found
to scale as expected by theory. Figure 45 (left) shows the expected error as a function of
the B s mixing parameter x s = m s =,. The current limit on m s is 10:2ps ,1 [26], but