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.”

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 http://bbr-onlwww.slac.stanford.edu:8080/babarrc/LumInt-Run_1-5.gif (subtract off-peak) Then use cross-section 1.1289 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

The BaBar Collaboration 10 Countries 77 Institutions 593 Physicists Point out yourself (and any demonstrators in the photo). Tim Adye BaBar B-meson lifetime

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

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

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

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

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

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

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

BaBar B-meson lifetime More Information Particle Physics: http://hepwww.rl.ac.uk/pust/ and follow the links BaBar: http://www-public.slac.stanford.edu/babar/ Includes “live” display of new events Only show if someone asks for links Tim Adye BaBar B-meson lifetime