ONLINE AND OFFLINE TOFCALIBRATION TOOLS FOR PHYSICS

TOF Meeting, 9 December 2009, CERN

Chiara Zampolli for the ALICE-TOF

Outline

The TOF Calibration Sources of uncertainty Operating status The online procedures The offline procedures

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Sources of Uncertainties and Operating Status

Sources of Uncertainty

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1. 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).

2. TIME SLEWING:time slewing is caused by the finite amount of charge necessary to trigger the discriminator charge fluctuations generate a time walk.

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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

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TOF Online Calibration Procedure

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 9th December 2009Chiara Zampolli

Procedures running in the DAQ

Procedure running in the DCS

Status

Status

Delay

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TOF Online Calibration - Delays Determine the relative delays for each TOF

channel using the tTOF-texp spectra• tTOF = time measured by

TOF (T0 subtracted!!)• texp = expected time of

flight, from TOF geometry, assuming β = 1 and a straight line trajectory

Sharp edge expected at the delay value from fastest particles (β ~ 1)

No delay, edge @ ~ 0

Delay defined using the mean <tTOF-texp> over ~ 320 ps (≈ 4 times σTOF) around the edge <tTOF-texp> 8

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Dependence on the Number of Entries/Channel

Fit to 1/N

Uncertainty (RMS) on the time edge vs Number of Entries/Channel

Mean Value of the Time Edge vs Number of Entries/Channel (delay=0). Stable within 10 ps.

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Dependence on the Number of Events

pp Runs, assuming 10 hits on TOF/MB event

4 hours fill, L2 rate = 100 Hz

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TOF Offline Calibration Procedure

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. 9th December 2009Chiara Zampolli 12

Offline Calibration Procedure Quality cut selection applied on the available

tracks 5th order 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

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Results from MC

σa = 92.7 ps σb = 111.4 ps

σc = 93.2 ps

Adding time slewing and delays

calibration

σcal = ~ 10 ps

2a

2c σσ

Results obtained with 500 tracks from 2 MC

Pb-Pb ev; time delay = 2 ns.

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1.Simulated, w/o effects 2. Simulated, with effects

3. Simulated, calibrated

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.

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Required Statistics The number of required events to collect sufficient

statistics to calibrate all the ~ 160000 TOF channels is:

k1NN trev

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).

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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).

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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 foreseen9th December 2009Chiara Zampolli 18