Measurements of Branching Fraction and CP-Violation Parameters of
B0 → D+ D- Decays
We report measurements of branching fraction and time-dependent CP
violating (CPV) parameters of B0 → D+ D-
decays using a data sample of 772×106
BB-pairs
collected on the Υ(4S)-resonance with the Belle detector at
the asymmetric energy e+e--collider KEKB.
Fig.1: Tree-level and penguin diagram contributing to the decay B0 → D+ D-.
b→ ccd transitions such as in B0 → D+ D- provide an interesting probe of CP violation.
According to the tree-level diagram (Figure 1 left) the expected CPV parameters
are S = - sin(2 φ1) and A = 0. Higher order penguin diagrams (Figure 1 right)
are expected to change the parameters of S and A only by a few percent~\cite{Xing2000}.
Both BaBar and Belle studied CP violation in this decay.
Belle measured using a data sample of of 535 × 106 BB-pairs~\cite{Fratina2007}:
A = 0.91 ± 0.23 (stat) ± 0.06 (syst)
S = -1.13 ± 0.37 ± 0.09
B ( B0 → D+ D- ) = (1.97 ± 0.20 ± 0.20)× 10-4
BaBar measured using a data sample of 467 × 106 BB-pairs~\cite{Aubert2009}:
C = - A = -0.07 ± 0.23 (stat) ± 0.03 (syst)
S = -0.63 ± 0.36 ± 0.05
While both measurements agree in S, only Belle measures an unexpected
large value in A.
Objective of the current analysis is to measure the branching fraction
and CPV parameters using the full and reprocessed data set.
B0 → D+ D- decays are reconstructed
in the following modes:
B0 → D+ D-
→ ( K- π+ π+ ) ( K+ π- π-)
→ ( K- π+ π+ ) ( KS0 π- )
→ ( K- π+ π+ ) ( KS0 π- π0)
Selection criteria comprise requirements on track impact parameters, PID and mass distributions.
Main source of background are e+ e- → q q (q = u, d, s, c) continuum events.
This background is suppressed by a neural network trained on event shape variables like Super-Fox-Wolfram-Moments, CLEO cones and the angle of the B meson candidate with respect
to the beam axis.
For the time-dependent measurement additional criteria such as on quality
of vertex fits or number of SVD hits are required.
Possible background peaking in the Mbc and Δ E signal
region is estimated from D+ sidebands.
Fig.2: Data distributions of B0 → D+ D- for Kππ (left), KS0π (middle) and KS0ππ0 mode (right).
Mbc data distributions for each mode are shown in
Figure 2.
The obtained and peaking background subtracted yields are 221±19
signal events in Kππ, 48±9 in KS0π
and 54±15 in KS0ππ0 mode.
Combining these three modes the following branching fraction has
been measured:
Only the Kππ and KS0π modes are used in
the time-dependent measurement. These modes show no significant peaking
background in contrast to the KS0ππ0
mode.
Fig.3: Δ t distribution and raw asymmetry of the combined CP fit of Kππ and KS0π modes in B0 → D+ D-.
Δ t distribution and the resulting raw asymmetry of the combined
CP fit of Kππ and KS0π modes are shown
in Figure 3. The following CPV parameters have been measured:
A = 0.43 ± 0.17 (stat) ± 0.04 (syst)
S = -1.06 ± 0.21 ± 0.07
Compared to the previous Belle measurement the direct CPV parameter A
decreased and its center value moved towards the expected value which
is close to 0.
