1. Pile-up vertex finding in heavy ion collisions
Marco van Leeuwen, LBNL

2. Some numbers TPC drift time ~40 ms
Time between events without pile-up: 80 ms
Interaction rate with single pile-up 12.5 kHz
Rate at Run 4 Au+Au: 3-10 kHz
Rate at Run 5 Cu+Cu: 8-25 kHz
Expected rate Au+Au RHIC II: ~ 60 kHz

3. Pile-up vertex finding
Incidence of tracks on beam axis (z-coord)
Display (not the same event)
Duncan Prindle
Step 1: identify vertex candidates
Resolution few cm
# candidates: 1 triggered interaction + 2x pile-up

4. Vertex selection parameters
Number of BEMC hits
Mean-dip
Tracks crossing central membrane
Red lines: parametrised ‘expected’ trends
Step II: assign weights (rank) based on several properties

5. Results Confusion/inefficiency reduced to a few per cent in Cu+Cu
Before
Efficiencies
After
Anthony Timmins
Confusion/inefficiency reduced to a few per cent in Cu+Cu

6. Future expectations Method tested to 20-30 kHz rates (Cu+Cu)
Anthony Timmins
Method tested to kHz rates (Cu+Cu)
Will likely work up to expected RHIC II Au+Au rates, 60 kHz
Lighter II could be problematic
(but see p+p experience)
Caveat: track density for 60 kHz larger than 25kHz Cu+Cu
However, pile-up events are minbias, so not too worried