1. Measuring the B-meson’s brief but eventful life
Tim Adye
Rutherford Appleton Laboratory
Particle Physics Masterclass
15-17th March 2006
“Using experimental data from the BaBar detector (introduced in morning’s lecture) to measure the B-meson’s lifetime.”

2. The BaBar Detector at SLAC
Linear Accelerator
“Recorded more than 300 million BBbar decays. You will be looking at just 10!”
Million BBbar Events, as of 7/3/06 from
(subtract off-peak)
Then use cross-section nb (taken from ratio of values returned by BbkLumi) to get BBbar (this is B0B0bar + B+B-)
336 Million BB recorded since starting in May 1999
2.2km ring
BaBar

3. The BaBar Collaboration
10 Countries Institutions Physicists
Point out yourself (and any demonstrators in the photo).
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
Animated slide.
“[This] is what happens… but all we see are the decay products. From them we infer the decay distance, and hence the lifetime.”
Be prepared to answer the obvious question: “why are you measuring a lifetime difference, and how does that relate to the lifetime?”. It’s tricky to explain, but fortunately only the very smart ones (and a few teachers) spot the subtle point. You can use radioactive decay (which they have done) as an analogy. t t1 t2
e ̶
Tim Adye
BaBar B-meson lifetime

5. lower energy high energy
Emphasise the higher energy e-, giving B mesons momentum along beampipe.
high energy

6. BaBar Detector Elements (side view)
Muon
Detector
Electro-
magnet
Tracking
Detailed
Tracking
Briefly explain tracking. Can mention outer detectors for neutral particles and PID – but not required for exercise.
This is the same as the initial view in the exercise (but coloured in).
Particle
Identification
Electron &
Photon
Detector

7. Event Display Instructions
Event number
Move between
events
Particle momentum
and type
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.
Rotate about
horizontal axis
“Beamspot” where most e+e- collisions take place. This is about 1cm long
Click-move-release
mouse to measure
Open real event display here (event 183 is a simple one to show – there is a shortcut to run it in this directory, though it’s a good idea to open at the initial view before before you start), zoom in, and make the measurement. Briefly show them how to rotate and go to next event.
They have a hardcopy of this page (with p10 summary on the back).
You don’t need to explain the expert display – leave that for the students to explore for themselves, or to be introduced by the demonstrators (not all will have time for that anyway).
These white lines are part of the outer detector that overlap our display. They can be ignored at this scale.
Reset size
Zoom Out
Move Up
Side view
Move
Horizontally
Reset view
Zoom In
Centre
Move Down

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!
Can leave this up during the exercise. They also have a hardcopy on the back of the event display instructions.
Tim Adye
BaBar B-meson lifetime

11. BaBar B-meson lifetime
More Information
Particle Physics:
and follow the links
BaBar:
Includes “live” display of new events
Only show if someone asks for links
Tim Adye
BaBar B-meson lifetime