ONLINE and offline TOF calibration tools for physics

1. TOF Meeting, 9 December 2009, CERN Chiara Zampolli for the ALICE-TOF ONLINE and offline TOFcalibration tools for physics

2. Outline • The TOF Calibration • Sources of uncertainty • Operating status • The online procedures • The offline procedures Chiara Zampolli

3. Sources of Uncertainties and Operating Status

4. Sources of Uncertainty • TIME DELAYS: an equalization of the channels of the TDCs is necessary because of the delays introduced by the electronics (mainly cable lengths and pulse line lengths). TIME SLEWING: time slewing is caused by the finite amount of charge necessary to trigger the discriminator charge fluctuations generate a time walk. Chiara Zampolli

5. Operating Status • Apart from correcting for the time delays and the time slewing effects, during reconstruction also the operating status of each channel has to be taken into account • Front-End: off/on • Noise: noisy/not-noisy • Pulser: not-ok/ok Chiara Zampolli

6. TOF Online Calibration Procedure

7. TOF Online Calibration • TOF online calibration procedure is aimed at defining the status and at a first-order estimation of the delay in time measurement of each TOF channel. • TOF online calibration is performed on three different sources: • PHYSICS runs • PULSER runs • NOISE runs with additional information concerning the Front-End: • every run • Everything included in the ALICE online calibration framework Delay Procedures running in the DAQ Status Procedure running in the DCS Status Chiara Zampolli

8. TOF Online Calibration - Delays • Determine the relative delays for each TOF channel using the tTOF-texp spectra • tTOF = time measured by TOF (T0subtracted!!) • texp = expected time of flight, from TOF geometry, assuming β= 1 and a straight line trajectory No delay, edge @ ~ 0 • Sharp edge expected at the delay value from fastest particles (β~ 1)  Delay defined using the mean <tTOF-texp> over ~ 320 ps (≈ 4 times σTOF) around the edge <tTOF-texp> Chiara Zampolli

9. Dependenceon the Number of Entries/Channel Uncertainty (RMS) on the time edge vs Number of Entries/Channel Fit to 1/N Mean Value of the Time Edge vs Number of Entries/Channel (delay=0). Stable within 10 ps. Chiara Zampolli

10. Dependence on the Number of Events pp Runs, assuming 10 hits on TOF/MB event 4 hours fill, L2 rate = 100 Hz Chiara Zampolli

11. TOF Offline Calibration Procedure

12. Offline Calibration Strategy • An algorithm with the aim to calibrate the measured times taking into account at the same time the time slewing effect and the delays introduced by electronics (refined calibration) has been developed. • To unfold the time spread due to the momentum spectra, to the particle types, and to the different track length,the algorithm is based on the comparison between the times of the reconstructed tracks (from track length and momentum measurements) and the measured times (tEXP-tTOF). • The identity of the particle (needed to choose the proper tEXP) is determined via a combinatorial algorithm. Chiara Zampolli

13. Offline Calibration Procedure • Quality cut selection applied on the available tracks • 5thorder polynomial fitting of the (texp – tTOF) vs ToT distributions for the selected tracks  channel by channel; • Setting of the 6 parameters just obtained as the calibration parameters • CALIBRATION Chiara Zampolli

14. Results obtained with 500 tracks from 2 MC Pb-Pb ev; time delay = 2 ns. Results from MC Adding time slewing and delays 2. Simulated, with effects 1.Simulated, w/o effects σa = 92.7 ps σb = 111.4 ps 3. Simulated, calibrated σc = 93.2 ps calibration σcal =~ 10 ps Chiara Zampolli

15. Dependence on the # of Selected Tracks • The residual σcal depends on the number of selected tracks used in the calibration procedure. • Given a certain number of tracks to be collected/TOF pad, the number of required events comes. Chiara Zampolli

16. Required Statistics • The number of required events to collect sufficient statistics to calibrate all the ~ 160000 TOF channels is: • Ntr = # tracks/pad to obtain a given σcal • k = selected tracks “occupancy” ~ 250/160000 (250 = # selected tracks in an event) • To obtain σcal~ 15 ps (which is well within the TOF requirements), Ntr~ 400. • The number of necessary Pb-Pb events to be collected is O(3x105). Chiara Zampolli

17. Required Statistics – pp Runs • The number of reconstructed tracks in a MB pp event is ~ 2. • Due to the much cleaner environment in which reconstruction is carried out, the track selection criteria can be much looser, so that all the reco tracks are taken into account. • For σcal~ 35 ps, the number of necessary tracks per channel is ~ 150. • Then, the number of required MB pp events is O(107). Chiara Zampolli

18. Summary and Conclusions • TOF Online and Offline Calibration procedures defined and implemented since long time • TOF Online and Offline Calibration procedures developed for collisions  So far, calibration from cosmics used • TOF Online and Offline calibration tools used to determine channel by channel: • Operating Status of the channel: • Online Calibration always used • Time Delay & Time Slewing relative to the channel: • TOF Online and Offline Calibration not to be used together: • Online Calibration to be used first • Offline Calibration to be used second  switch between the two foreseen Chiara Zampolli