1. Time-zero evaluation using TOF, T0 and vertex detectors
D. Caffarri – University and INFN Padova
A. Alici – P. Antonioli INFN Bologna
F. Noferini – CNAF e INFN Bologna
CERN, 24/03/ TOF meeting– T0 Analysis D. Caffarri 1

2. Outline - Introduction and some definitions
- Reference data sample used and cuts
Use of vertex information for T0 correction
T0 resolutions
- Getting the right time-zero in different case of event.
- Conclusions and outlook
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 2

3. Introduction
Aim of this analysis: exploit combined ALICE detectors information to provide the best time-zero estimate to analyse TOF data (and in turn best PID from TOF)
- The T0 detector is a small ALICE detector close to the beam pipe able to provide precise interaction time of each collision (used by TOF as a time-zero)
T0 detector in ALICE has been built for Pb-Pb collisions. In p-p collisions, due to its low acceptance region, cannot always give the signal for the time-zero of the collisions
Note TOF provided TDC and read-out electronics to T0 exactly to have the same hardware measuring time signals
CERN, 24/03/ TOF meeting – T0 analysis D. Caffarri 3

4. T0A T0C
T0 position
T0A pseudorapidity acceptance : −5 < η < − cm from nominal interaction point
T0C pseudorapidity acceptance : 2.9 < η < cm from nominal interaction point
CERN, 24/03/ TOF meeting – T0 analysis D. Caffarri 4

5. Some definitions
T0. The detector T0, the hardware system. Made of two parts T0A, T0C one on each side of the experiment as shown.
T0det. The interaction time (time-zero) as measured by the T0 detector
T0TOF A time-zero estimate using tracks that reach the TOF.
T0fill: the average time-zero estimated in the fill (used as “last resort” when other means are not available)
T0Best.  What we want to obtain.
Trying to find the best available time-zero for each event.
This depends on: track multiplicity, T0A and T0C response, vertex measurement, number of tracks reaching the TOF.
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6. Data sample and cuts
This analysis was performed on the Run pass 4
TOTAL EVENTS
“Physics” EVENTS
(we reject beam-gas, beam-empty events events with the wrong trigger configurations)
Cuts on the tracks matched with TOF:
standard collaboration cuts to consider only “good” tracks coming from the central tracker
Vertex Z:
To have the bigger efficiency on the vertex recostruction:
If ncontributors >= 3  z coordinate of the tracks vertex
If ncontributors < 3  z coordinate of the inner pixel detector vertex (vertexer Z has higer efficiency at low multplicity)
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 6

7. Primary vtx – T0 correlations: corrections
T0A*
Offset in T0C  different mean but almost the same sigma than track vtx.
T0A and T0C individually depends on vertex position.
We found corrections
T0A*=T0A(vtx)
T0C*=T0C(vtx)
to clear this dependency. Vertex position measured by ITS.
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 7

8. Estimate of T0det resolution
Using sample of events where we have both T0A* and T0C* (corrected by vertex) it is possible to estimate T0det resolution using: (T0A*-T0C*)/2 This quantity filters jitter introduced by the beam and its width is direct estimate of T0det resolution.
T0det resolution = 63 ps
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 8

9. Estimate of the jitter due to the beam
The time-zero estimated by T0det is: (T0A+T0C)/2 The width of this distribution contains the jitter introduced by the beam
Consistent with the estimate of the vertex
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 9

10. Estimate on the resolution on the T0or
The T0or is defined when or only one T0A or T0C or both give a signal. All the distributions are corrected for the vertex.
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 10

11. Getting the right time-zero
Total events selected: (100%)
Events with at least one T0 detector (“T0or”): ( 60 %)
Events with both T0A and T0C (“T0and”): 9130 (14.4 %)
These numbers are in very good agreement with MC given from T0 group.
CERN, 24/03/ TOF meeting– T0 Analysis D. Caffarri 11

12. Getting the right time-zero for TOF
- Request of at least 1 track matched to TOF (that is… when a time-zero is needed): events selected (43% of the total)
- Within this sample:
18277 events with T0or (68%)
5355 events with T0and (20%)
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 12

13. Getting the right time-zero without T0
32% of events don’t have a time-zero from T0 detector. Before to use average time-zero fill as last resort, we can obtain a time-zero event-by-event from TOF data (bayesian fit iterating over tracks making PID hypothesis)
We tested T0TOF algorithm using events when a time-zero from T0 detector (and) is available obtaining T0det-T0TOF distributions
We didn’t use here the latest calibration from the TOF detector, but we are already working in that direction.
PRELIMINARY
Low number of events
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14. TOF PID using T0best
p (GeV/c)
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15. TOTAL NUMBER
OF EVENTS
NUMBER of EVENTS WITH T0or
NUMBER of EVENTS WITH A TRACK MATCHED WITH TOF
66354
37264
26709
100%
60%
43%
NUMBER of EVENTS WITH A TRACK MATCHED WITH TOF
NUMBER of EVENTS
WITH a TRACK at TOF and T0or
NUMBER of EVENTS WITH T0 WITH TOF
(because there is no T0)
NUMBER of EVENTS with T0 fill
26709
18277
2418
6143
100%
68%
10%
22%
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 14

16. Conclusions and outlook
Combined analysis of T0, TOF and vertex detectors data to allow a precise time zero determination for TOF (and in turn PID) is well in progress.
Improvements are expected on further analysis (no need of various ad-hoc offsets used here, improvements of TOF calibration, improved track-length estimates).
- Propagation of this approach to on-going analysis (namely identified spectra) is already under way. Statistics and resolution improvements expected!
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17. BACK UP SLIDE
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18. Standard track cuts Cuts on the tracks matched with TOF:
Minimum number of cluster of the TPC = 50
Maximum χ2 per cluster of the TPC = 3.5
Require kTPCrefit
Maximum DCA to the vertex Z = 3.0 cm
Maximum number of sigma to vertex = 4
CERN, 24/03/ TOF meeting – T0 Analysis D. Caffarri 17

19. T0det after VTX corrections
CERN, 22/03/ PWG1 – T0 Analysis D. Caffarri 18