TOF calibration: status Roberto Preghenella Museo Storico della - - PowerPoint PPT Presentation

TOF calibration: status

Roberto Preghenella

Museo Storico della Fisica e Centro Studi e Ricerche ”Enrico Fermi”, Roma INFN, Sezione di Bologna ALICE Offline Week – CERN, 16/03/2010

Roberto Preghenella

TOF data flow

2

RAW DATA

01100110011010

AliTOFDecoder AliTOFRawStream ClusterFinder

time (TDC bins) TOT (TDC bins) deltaBC (BC bins) L0L1Latency (BC bins) time (TDC bins) TOT (TDC bins) deltaBC (BC bins) L0L1Latency (BC bins)

ESD DATA OCDB AliTOFDecoder PhysicsDA PreProcessor

time (TDC bins) TOT (TDC bins) deltaBC (BC bins) L0L1Latency (BC bins) 01100110011010 fTOFsignalRaw (ps) fTOFsignal (ps) TDCLatency Calibration Event-time

FEE config

TDCLatency TDCLatency Event-time time (TDC bins) TOT (TDC bins) deltaBC (BC bins) L0L1Latency (BC bins) Calibration

Roberto Preghenella

TOF calibration scheme

3 Within the TOF calibration scheme all corrections are applied in AliTOFClusterFinder using the input from raw data and OCDB fTOFsignalRaw = TDCtim

e

fTOFsignal = fTOFsignalRaw – DeltaBCcorrection

+ L0L1latency

+ TOFCTP latency – TDClatency window – Calibration – Event-Timefill TDCtim

e

→ from decoder → from raw data DeltaBCcorrection → from decoder → from raw data L0L1latency → from decoder → from raw data TOFCTP latency → from OCDB TDClatency window → from OCDB → from FEE configuration Calibration → from OCDB → offline calibration Event-Timefill → from OCDB → online/offline measurement

Roberto Preghenella

TOF calibration scheme

4 Within this calibration scheme TOF measured time recorded in ESD data (fTOFsignal) is not dependent on the FEE configuration Moreover, TOF time stored in ESD data takes into account the average event-time which is measured online on a run-by-run basis In this way, already at pass1 reconstruction, TOF time is on average consistent with the expected Time-Of-Flight t – texp  = 0 The Time-Of-Flight resolution at pass1 reconstruction is then expected to be σ2

TOF = σ2 tim e

+ σ2

T0

where σT0 is related to the beam spread in the z-coordinate

Roberto Preghenella

Timing-calibration with cosmic rays

The baseline calibration has been performed with cosmic-ray data collected during summer 2009 (no magnetic field, TOF trigger) The calibration is based on the time-of-flight τ of muons as measured by TOF τ = t – t0 and the expected time-of-flight τ(exp) assuming ultra-relativistic muons τ(exp) = L / c using about 100k selected muon events for calibration 5

Roberto Preghenella

Timing-calibration with pp data

An improvement to the cosmic-ray calibration has been performed with pp data collected last December (pass2). The calibration is based on the time-of-flight τ of matched tracks as measured by TOF τ = t – t0 and the expected time-of-flight τ(exp) assuming pion-ID where t0 is the average event-time measured run-by-run pp calibration aims at determining the parameters which could not be measured with cosmic rays because of the limited horizontal-track events collected. 6

Roberto Preghenella

PID after Timing-calibration

7 σ2

TOF = σ2 tim e

+ σ2

T0

σTOF ~ 190 ps

Roberto Preghenella 8

Online avegare event-time

TOF Pre-Processor runs at the end of each run and builds the channel-integrated raw-time spectrum:

• nly one histogram

no limits on range and binning currently 2000 bins in the range [-24.4,24.4] ns

• read DA raw-data
• perform all needed corrections

(consistency with reconstruction)

• subtract the expected time-of-flight

(straight track from nominal vertex, ultrarelativistic hypotesis)

• fit the spectrum to measure the time-edge

Roberto Preghenella 9

Raw-time spectrum

Roberto Preghenella 10

Time-edge measurements

Landau fit of the distribution close to the expected position of the edge. Use the maximum bin to correctly steer the fit. The edge is defined at the rising point of the distribution on the basis of landau-fit parameters: tedge = tM

PV – 3σlandau

Roberto Preghenella 11

Time-edge measurements

time-edge

Roberto Preghenella 12

Time-edge resolution

Time-edge resolution follows 1/√N behaviour Nhits > 100k σedge < 50 ps saturation resolution ~10 ps

Roberto Preghenella 13

Online average event-time resolution

The online T0-fill method measures the average event-time with good resolution: ~25 ps

(20 runs with very different statistics)

Roberto Preghenella 14

Offline event-by-event event-time

A combinatorial algorithm to measure the event- time on a event-by-event basis using ony TOF data has been implemented. The event-time resolution depends on the number of tracks.

Roberto Preghenella 15

Conclusions

• TOF calibration scheme has been updated in order

to obtain useful time measurements already at pass1 reconstruction

• the baseline cosmic-ray calibration has been

refined with pp data. Though, the optimal calibration is not yet achievable ( ~10 M pp events needed)

• online event-time algorithm implemented for run-

by-run measurement of average event-time

• offline event-time algorithm (combinatorial

algorithm) is available for event-by-event event-time measurement