D. Caffarri – University and INFN PadovaA. Alici – P. Antonioli INFN BolognaF. Noferini – CNAF e INFN Bologna
CERN, 24/03/10 TOF meeting– T0 Analysis. D. Caffarri 1
Time-zero evaluation using TOF, T0 and vertex detectors
Outline
CERN, 24/03/10 TOF meeting – T0 Analysis. D. Caffarri 2
- 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
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Introduction
- 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
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)
CERN, 24/03/10 TOF meeting – T0 analysis. D. Caffarri 4
T0A T0C
T0A pseudorapidity acceptance : −5 < η < −4.5 - 350 cm from nominal interaction point
T0C pseudorapidity acceptance : 2.9 < η < 3.3 - 70 cm from nominal interaction point
T0 position
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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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Data sample and cutsThis analysis was performed on the Run 104892 - pass 4
TOTAL EVENTS 109.752 “Physics” EVENTS 66.354
(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 vertexIf ncontributors < 3 z coordinate of the inner pixel detector vertex (vertexer Z has higer efficiency at low multplicity)
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Primary vtx – T0 correlations: corrections
Offset in T0C different mean but almost the same sigma than track vtx.
T0A*
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.
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Using sample of events where we have both T0A* and T0C* (corrected by vertex) it is possible to estimate T0det resolution using:
(T0A*-T0C*)/2This quantity filters jitter introduced by the beam and its width is
direct estimate of T0det resolution.
Estimate of T0det resolution
T0det resolution = 63 ps
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The time-zero estimated by T0det is: (T0A+T0C)/2The width of this distribution contains the jitter introduced by the
beam
Estimate of the jitter due to the beam
T 0and T 0and det2 Jitter
2
T 0and det 63psT 0and 161ps
Jitter 148ps
Consistent with the estimate of the vertex
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The T0or is defined when or only one T0A or T0C or both give a signal. All the distributions are corrected for the vertex.
Estimate on the resolution on the T0or
T 0or T 0or det2 Jitter
2
Jitter 148psT 0or 167ps
T 0or det 77ps
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Getting the right time-zero • Total events selected: 61840 (100%)• Events with at least one T0 detector (“T0or”): 37264 ( 60 %)• Events with both T0A and T0C (“T0and”): 9130 (14.4 %)
These numbers are in very good agreement with MC given from T0 group.
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Getting the right time-zero for TOF
- Request of at least 1 track matched to TOF (that is… when a time-zero is needed): 26709 events selected (43% of the total)
- Within this sample:18277 events with T0or (68%) 5355 events with T0and (20%)
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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.
Low number of eventsPRELIMINARY
TOF PID using T0best
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p (GeV/c)
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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%
Conclusions and outlook
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- 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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BACK UP SLIDE
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Standard track cuts
Cuts on the tracks matched with TOF:
Minimum number of cluster of the TPC = 50Maximum χ2 per cluster of the TPC = 3.5 Require kTPCrefitMaximum DCA to the vertex Z = 3.0 cmMaximum number of sigma to vertex = 4
T0det after VTX corrections
CERN, 22/03/10 PWG1 – T0 Analysis. D. Caffarri 18
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