The CLEO collaboration has published a measurement of the B -> D* pi branching fractions using a high efficiency partial reconstruction technique.

• B⁰ -> D*^- pi⁺ branching fraction: (2.81 +/- 0.11 +/- 0.21 +/- 0.05) x 10^-3

Why is B⁰ -> D*^- pi⁺ a good 1st year physics topic?

• Very high reconstruction efficiency:
  
  
  • CLEO partial reconstruction efficiency for B⁰ -> D*^- pi⁺: 29%
    Includes D*^- -> D⁰ pi^- branching fraction (68%).
    Which gives 2600 events in 3.1 1/fb.
    
  • CLEO exclusive reconstruction efficiency for B⁰ -> D*^- pi⁺: 3%
    Which gives 248 events in 3.1 1/fb.
    
  
  
• Need to understand soft pion efficiency. Problem common to many analyses.

In the long term:

• Hope to measure sin(2 beta + gamma).
• One of our only handles on gamma.

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B⁰ -> D*^- pi⁺ Kinematics

Number of parameters: 4-vectors x 5 particles = 20.

Constraints:

• 8 from E and P conservation in B⁰ and D* decay.
• 5 from the particle masses.
• 1 from beam energy (gives us |P_B0| in Upsilon frame).

Measurements:

• 6 from 3-momenta of fast and slow pions.

20 parameters - 14 constraints - 6 observed quantities = 0-C Fit.

The plan

• Use B⁰ and D^* frame pion angular distributions for signal/background discrimination.
• Use angle between pions to reject background.
• Main backgrounds: Continuum and other B decay modes which produce a fast pion.

distribution is uniform.
distribution is (distinctive).

• Variables in red are measured.
• Variables in blue are calculated from measured quantities.
• Variables in black are "constants" (some smearing from beam energy).

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Angles that determine D* direction.



Angle between B⁰ and D* decay planes.



Where is the angle between the slow pion direction and opposite the fast pion direction in the Upsilon frame.

Require to be physical to reject background.

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References:

• The CLEO Collaboration. A New Measurement of B -> D* pi Branching Fractions.
  Phys.Rev.Lett. 80 (1998) 2762-2766 .
  

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owen@electron.physics.ucsb.edu