Measuring the B-meson’s brief but eventful life Tim Adye Rutherford Appleton Laboratory Particle Physics Masterclass th March 2007

2 The BaBar Detector at SLAC Linear Accelerator 2.2km ring BaBar 433 Million BB recorded since starting in May 1999

BaBar B-meson lifetime3 The BaBar Collaboration 10 Countries 77 Institutions 522 Physicists

BaBar B-meson lifetime4 What we will measure today The BaBar experiment has published 265 papers containing an enormous variety of measurements. We will make one measurement that is fundamental to most of these: the decay rate of the B-meson. Actually measure the average decay time, or “lifetime”. Comparing the lifetimes of B-mesons with that of anti-B-mesons, allows us to measure CP-violation. Emmanuel discussed CP-violation this morning. We only have time today for the first step: we will measure the average lifetime of the B-meson and anti-B-meson.

BaBar B-meson lifetime5 B-meson production and decay t1t1 t2t2 First B decays Second B decays Measure the decay length to calculate the B lifetime e+e+ e-e-

high energy lower energy

Tracking Particle Identification Electro- magnet Detailed Tracking Electron & Photon Detector Muon Detector BaBar Detector Elements (side view)

Reset view Reset size Zoom In Zoom Out CentreMove Down Move UpSide view Move Horizontally Rotate about horizontal axis Event Display Instructions Particle momentum and type Move between events Click-move-release mouse to measure Event number “Beamspot” where most e + e - collisions take place. This is about 1cm long 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. These white lines are part of the outer detector that overlap our display. They can be ignored at this scale.

BaBar B-meson lifetime9 Tips for Finding and Measuring B-meson decays 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 2.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 3.Try rotating the event for a better view use the slider on the right

BaBar B-meson lifetime10 4.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 5.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 6.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. Tips for Finding and Measuring B-meson decays - 2

BaBar B-meson lifetime11 1.B-mesons are created and decay along the central axis 2.Ignore tracks that don’t help you form a decay point 3.Try rotating the event for a better view 4.Zoom factor scale=512 to 1448 is best 5.If you don’t see any separation  D=0 6.Don’t Panic – ask for help! Finding and Measuring B-meson decays - Summary

BaBar B-meson lifetime12 More Information Particle Physics: and follow the links BaBar: Includes “live” display of new events