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Html version of the presentation For virtual participants in VENUS @ http://bogrid1.bo.infn.it/˜pierella/ TOFWEB/ALI-Off/venusians.html
TOF Reconstruction, PID and Time Zero Bologna-ITEP-Salerno Group F. Pierella, Bologna University
Status of the code 6Klines written in the past 2 month for Reconstruction – Matching TPC/TOF (4K) Particle Identification (1K) Determination of the event time zero (1K) Current List of TOF classesList of TOF classes Coding conventions violations under control
Reconstruction Algorithm (summary) TPC trackable tracks are exctracted from TPC data according to some cut; They are propagated geometrically till the TOF detector volumes; Each track is eventually matched with a TOF fired pad according to a weighting algorithm (charge induced on each pad taken into account).
Results for PID (an example) (2) Extrapolated Mass VS Momentum
Determination of the event time zero in Pb-Pb events (summary) The statistical method is based on the exclusive use of quantities known from TOF detector measurements and from reconstruction. The basic idea is the following: all primary particles are generated at the same time (no matter if this time is equal to zero: it has only to be a constant for all primary particles.) (the so called time zero). This information (that in the algorithm is treated as aconstrain) is propagated to TOF as the primary track times of flight are registered by TOF detector. The combinatorial algorithm comes back to initial information (just the time zero) using a chi-square minimization method (more details @ AliTOFT0.cxx).AliTOFT0.cxx
Class Design for Time Zero AliTOFT0.h/cxx – AliTOFhitT0.h/cxx
Results for Time Zero (an example) 15 tracks with global time resolution 120 ps
Computing Times (1) Reconstruction – Matching 3 per event (Pb-Pb) PID <10 per event (Pb-Pb) Determination of the event time zero Due to combinatorial character of the algorithm, the computing time depends on the number of selected tracks (see next table)
Computing Times (2) Strong dependence (as expected) on the number of tracks per set Global Time Resolution Number of tracks per set Gaussian Fit sigma for time zero distribution Computing Time per set (e.g.) 120ps 10(41±1)ps 5 120 ps 15(32±1)ps 10
TOF Detector DB A very fruitfull collaboration with the W. Peryt group has been started Defined the topology of dictionaries for TOF - TDC electronics (18 tables which allow the control of each TOF readout channel) In progress the same definition for TOF mechanics dictionaries
Answer to questions about TOF Reconstruction (1) It is possible to have different contour cuts for TOF-PID in order to have a flexible scheme for PID? Yes, of course, the AliTOFPID class has only the pointer to the external file which contains contour cuts. You can select different contour cuts by simply selecting the filename in the AliTOFPID parametrized constructor. Different contour cuts can be generated using the TCutG ROOT class and a set of different contour cuts will be provided in the CVS Repository
Answer to questions about TOF Reconstruction (2) What's the TPC tracking efficiency you are using in reconstruction? We are selecting TPC "trackable" tracks as explained essentially in TOF TDR where you can find the overall efficiency (VS momentum) of the selected TPC tracks (see "without TPC tracks" area). We cannot actually use the TPC reconstruction because it is too much expensive in terms of CPU time (1 day/ 1 central event).In any case 2 solutions are available and possible:the first one is to introduce in the matching code a special method ReadTPCReconstructedTracks() which will be used if TPC reconstructed track data are available;the second one is the introduction of the parameterisation of the TPC tracking efficiency as a function of the momentum (or transverse momentum) and correct accordingly our selected TPC track distributions.(In any case the last step TOF-PID procedure efficiency and contaminations are available).
Answer to questions about TOF Reconstruction (3) Are You using different TPC tracking efficiency e.g. for pp data? No, but (see previous answer) we mean to use different tracking efficiency both for pp VS Pb-Pb and 0.2T VS 0.4T magnetic field values.
Answer to questions about TOF Reconstruction (4) What about the possibility of use the Kalman algorithm for TOF reconstruction? When TRD Kalman tracking will be ready we will implement this algorithm for the TOF in order to improve the PID results by using TRD tracking capabilities.