1. Measuring the B-meson’s brief but eventful life
Tim Adye
Rutherford Appleton Laboratory
Particle Physics Masterclass
16-18th March 2005
Tim Adye
BaBar B-meson lifetime

2. The BaBar Detector at SLAC
Linear Accelerator
26th May 1999: first events recorded by BaBar
2.2km ring
BaBar

3. The BaBar Collaboration
10 Countries Institutions Physicists
Tim Adye
BaBar B-meson lifetime

4. B-meson production and decay
First B decays
Second B decays e+
Measure the decay length to calculate the B lifetime t t1 t2
e ̶
Tim Adye
BaBar B-meson lifetime

5. BaBar B-meson lifetime
lower energy
high energy
Tim Adye
BaBar B-meson lifetime

6. BaBar Detector Elements (side view)
Muon
Detector
Electro-
magnet
Tracking
Detailed
Tracking
Particle
Identification
Electron &
Photon
Detector
Tim Adye
BaBar B-meson lifetime

7. Event Display Instructions
Event number
Move between
events
B and B decay to pions, kaons, etc. Their trajectories are extrapolated from the outer detector into the centre and are shown as red and yellow “tracks”: red for one B (or B) and yellow for the other.
A track is just an extrapolation of the particle’s path, so is extended past its production point. The final part of the extrapolation is shown with a narrower line to indicate this.
Particle momentum
and type
“Beamspot” where most e+e- collisions take place. This is about 1cm long
Click-move-release
mouse to measure
Rotate about
horizontal axis
Reset size
Zoom Out
Move Down
These white lines are part of the outer detector that overlap our display. They can be ignored at this scale.
Move
Horizontally
Reset view
Zoom In
Centre Up
Side view

8. Tips for Finding and Measuring B-meson decays - 1
B-mesons are created and decay along BaBar’s central axis
Look for your decay points along, or a little to the left or right of, the “beamspot” (horizontal blue line at the centre)
The B-mesons travelled along this same axis
Some tracks may not give a good extrapolation
could be from the decay of longer-lived particles, or due to scattering
You can ignore tracks that don’t help you form a decay point
even “good” tracks can’t be measured perfectly
Try rotating the event for a better view
use the slider on the right
Tim Adye
BaBar B-meson lifetime

9. Tips for Finding and Measuring B-meson decays - 2
Make sure you use an appropriate magnification
Scale=512 to 1448 is best
you may still want to briefly zoom out to see where you are
Many B-mesons will decay too quickly for you to see any separation
Note these down as zero decay distance
These will still contribute to the average
Don’t panic!
The concepts, data, and display take some getting used to. Take your time on your first few events.
Ask a demonstrator if you have problems.
Tim Adye
BaBar B-meson lifetime

10. Finding and Measuring B-meson decays - Summary
B-mesons are created and decay along the central axis
Ignore tracks that don’t help you form a decay point
Try rotating the event for a better view
Zoom factor scale=512 to 1448 is best
If you don’t see any separation  D=0
Don’t Panic – ask for help!
Tim Adye
BaBar B-meson lifetime