Minutes of the CALCOM meeting, January 23, 1998. ================================================ Agenda: S. Stepanyan - Rate estimates for exclusive channels from E1 run data A. Stavinsky - Tracking efficiency for close-by tracks S. Taylor - TOF efficiency using hit-based tracking V. Burkert - Charged particle identification in CLAS W. Brooks - EC timing calibration and forward TOF alignment SUMMARY: ------- S. Stepanyan reported on a study of the December run data broken down into various exclusive channels (N-pi, N-eta, Delta-pi) and for various Q2 and W bins. This provides a tool for planning the upcoming e1 runs. A. Stavinsky reported on a study of the reconstruction efficiency for close tracks. He finds that for tracks with relative angles of less than 18 degrees the reconstruction efficiency begins to decrease and becomes zero for tracks with less than 4 degree, although these tracks are still well separated on the CED display. S. Taylor showed results on TOF counter efficiencies. Using hit-based tracking, and with large enough matching cuts he finds efficiencies in excess of 95% for the forward panels, and more than 90% for the large angles. He will further refine these studies. V. Burkert reported on a study done by V.B., Latifa E., and Stepan. S. It shows that the rf structure of the beam can be used to do very accurate timing required for particle identification. Positrons, pions, kaons, and protons are well separated for momenta as they occurred in the 2.4 GeV data. W. Brooks showed his results on the inner timing of the forward calorimeter, and the relative timing of EC and TOF counters. Resolutions of about 500psec have been achieved. The calorimeter timing may now be used as a cross check of the timing calibration of the panel 1 TOF counters. Volker Burkert ---------------------------------------------------------------------- Individual reports: ================== S.Stepanyan - Rates and beam time (electron triggers) estimates for some ----------- exclusive channels. Rates are estimated using the samples of December data at two energies - 300K triggers from run #7867 @ Eo = 1.645 GeV & hard.trigg. EC*CC; 550K triggers from run #8099 @ Eo = 2.445 GeV & hard.trigg. EC*CC; Some useful numbers - Number of reconstructed electrons is 33% @ Eo = 1.645 GeV & ~30% @ Eo = 2.445 GeV. Number of electrons after fedutial cuts 25% @ Eo = 1.645 GeV & ~22% @ Eo = 2.445 GeV. Ratio TBT/HBT 97% (approximately the same for both energies). Regions of first and second resonances were studied in four Q2 intervals. At the Delta region the following reactions were analyzed - ep = eppi0 ; ep = eph and ep = ep+n. Events in that reaction were selected from 2 prong (one negative and one positive) data events by applying cuts on missing mass distributions. For h events subtraction of rough estimated background (two pion events) are done. Similar procedure (2 prong data events and cuts on missing mass distributions) were used to select the events in the reactions ep = epi+n and ep = epi+D0. For reaction ep = eD++pi- 3 prong data events were analyzed. Besides the missing mass, the invariant mass distributions of 2 final hadrons (out of 3, one missing) were studied for final states - ep = eppi-(pi+); ep = eppi+(pi-) and ep = epi+pi-(p). The following are the estimated rates and the number of electron triggers needed for 5% stat.error in each (W,Q2, cosq*,f*) bin assuming 20 bins by costh*, 12 by phi* and binning of (W,Q2) as in the tables. Postricpt file contaning all drived tables is - /home/stepanya/public_html/rates.ps. EFFICIENCY FOR DUBLE TRACK RECONSTRUCTION. K.MIKHAILOV,A.STAVINSKY ----------------------- 1.Task. There are a number of physical tasks where two(many) particles with close momenta must be identified and measured with reasonable efficiency and accuracy. a) Study of correlations of identical particles with nearby momenta which are sensitive to the space-time distances between the emission points due to interference effects and the strong and Coulomb final state interaction. Such method is widely used to study of reaction mechanism in different applications (from e+e- annihilation to low(high) energy heavy ion collision). It could be also applied to study of e(gamma)p(A) interactions at CEBAF using CLAS. b)Two particles with close momenta in the final state is an important feature of the events corresponding to production of some unstable systems like hypothetic dibarion at 2.07 GeV and some others. c)The example of new important physics could be done by study of pi+pi- correlations at very small relative momenta, which now started at CERN within DIRAC project. Such measurements would submit the understanding of chiral symmetry breaking and would provide the crucial test of QCD. Is it possible to study such kind of correlation at CLAS? The problem is that two tracks expected to be so close to each other in drift chamber that events reconstruction is not the ordinary procedure. Up to some extent this problem is important for non-identical particles at close relative velocity too - these particles could produce one cluster in the first super-layer. The goal of this study was what is the present status of reconstruction program with respect to close tracks identification. 2. Procedure. While efficiency of close track reconstruction is likely independent on special physical task and is a function of particles momenta difference, we can study it for empty target run (eAl interactions) as well as for standard ep interactions. The main advantage of this choice is relativly high proton multiplicity in eA interactions with respect to pion multiplicity in ep(eA) interactions at CEBAF energy. We compare visual information from runs 7984 and 7989 (both empty target at 2.445 GeV) and make our choice for the second one because of higher single particle efficiency. Before visual selection of events of interest, we apply pre-selection cuts on electron within time-based tracking in RECSIS framework. These cuts (E_0-E>1GeV,W_2>1.2GeV) was applied to suppress low multiplicity events. After that visual selection was made as follows: pair of positive tracks (pions or mainly protons) in the same sector with visual prolection of open angle between them (psi) less then 20 degrees. The accuracy of visual measuring of the angle between particles at small psi region was estimated as 2 degrees. A number of 'others' (other negative track or positive tracks in other sectors) did not fixed, but the presence of pre-selected electron in each events was tested. Each track in selected events was fixed with its sector number. We found 154 events according to the above listed criteria for 113.000 triggers. 'Others' track (except two positive tracks and trigger's electron - 184 tracks) was used to estimate reconstruction efficiency for isolated particle (epsilon _0) which proved to be of the order of 0.6. This value was used for normalisation to exclude inevitable uncertainty due to visual selection of events. Being tested by standard RECSIS selected events fall into three classes: fully reconstructed events (2 -> 2), partly reconstructed (2 ->1), and unreconstructed (2 -> 0). Relations between its population are in accordance with binomial distribution with single track efficiency epsilon (psi). The ratio epsilon(psi) to epsilon _0 is shown in the table. Table. Relative efficiency _______________________________________________________ I psi<4 I 4 8 degrees, while hardware provides a good resolution for tracks with psi> 2 degrees. It means that the software provides efficiency of reconstruction of the order of 5-10% in the region of the effects listed in section 1, while the hardware, in principle, permits to make that reconstruction for 70-80% of events. To make appropriate solution we plan to find selection criteria for events of interest (~1 % of all data sample) on the stage of raw-data to be able to apply special reconstruction procedure for that events. 3) Conclusion. The CLAS detector (hardware) provides the possibility to study phenomena at small relative momenta of secondaries up to open angle between them of the order of > 2 degrees while present variant of RECSIS (software) provides the efficient reconstruction in the open angle region of > 8 degrees. S. Taylor: TOF counter efficiencies. --------- Results of a study of track matching to the scintillators were presented. If an exact match between a scintillator firing and a track projected to the scintillator plane was required, the matching percentage was about 80-90% in the forward angle with a drop-off toward larger angles (especially panel 4). With the exception of sector 2, which showed a dramatic drop for panels 2 and 3, all the sectors looked more or less the same. When the cut was loosened by one (i.e., the predicted scintillator could differ from the scintillator that actually fired by one), the 'efficiency' rose to greater than 95% in the forward angles and 90% in the back angles except for a drop for panel 4's, a dip at the border between panels 1 and 2, and a drop for scintillator 1 in each sector. The dip arises from the fact that the south clamshell is several inches away from its nominal position. The drop-off for scintillator one is due to the acceptance of the first scintillator plane. Note that these results are different from my previous presentation because of the resolution of a normalization problem. Results for time-based tracking were also presented, but it was pointed out that time-based tracking requires a match to the scintillators in the first place, so the plots do not say anything about the scintillators. V. Burkert, L. Elouadrhiri, S. Stepanyan. ---------------------------------------- Charged particle identification in CLAS: At the previous CALCOM meeting one of us (L.E.) presented a calibration scheme for the TOF counters based on the known rf structure of the electron beam. Here we used this method for a precise calibration of the TOF counters in conjunction with time-based tracking, to measure the charged particle masses. As a first step the TDC calibration constant (assuming linear behavior) was recalibrated using the rf structure with its known peak-to-peak distance of 2nsec. After that all TOF paddles in one sector were calibrated. For this, events were selected with the electron and charged hadron in the same sector and both in panel 1. The reconstructed hadron mass shows clear pion as well as proton mass peaks at the correct masses. The pion mass was used for the calibration. In the next step the fine tuning of the accumulated rf-peak was done for each strip by lining them all up at zero. This was done with an accuracy of better than 50psec, After completion of this procedure the only remaining ambiguity is due to the 2nsec rf structure. This ambiguity, however, is easily resolved as any miscalibration of 2 nsec corresponds to a large shift in the pion mass. To calibrate panels 2-4, hadrons were selected in these panels, and the timing adjusted to give the correct pion mass. As a final step, the relative timing constant between sectors 1,4,5,6 were determine using electrons in one sector and hadrons in one of the other sectors. This was repeated for all sectors. DSector 5 was used as the reference sector. The particle mass resolution improved tremendously over the resolution achieved with the default calibration constants, allowing the separation of positrons, pions, kaons, and protons for the momenta ranges available from the 2.4GeV data. W. Brooks; EC Timing Calibration and Forward TOF Timing Alignment --------- In previous talks a method was described for deriving the calorimeter timing offset constants which only used data internal to the calorimeter. In this method, the average time in a given view is calculated (using sqrt(adc) weights), then the average time of the three views is formed (using geometrical weights), for the inner and outer layer separately, and the quantity T(inner)-T(outer) was used iteratively to determine the stack-to-stack timing offset constants. These constants were derived for an October 4 GeV run and a December 2.4 GeV run for all of the 1,296 phototubes. This talk presented the first results for tests of this calibration method. The tests were performed by comparing the calorimeter times with the TOF times for matched tracks with momentum > 1 GeV and negative charge. The test also used a different reconstruction method from that used in the calibration procedure, namely the one which was implemented in the calorimeter reconstruction code by Stepan Stepanyan, Zhujun Li, Serguei Boiarinov, and Kevin Beard, with minor modifications by W.B. to remove bad tdc data, adjust the effective signal velocity, and implement the sqrt(adc) weighting of the times. The results of this study demonstrate that the forward calorimeter mean time relative to all the TOF's in a given plane has an error of less than 1 ns for sectors with better-aligned TOF's. The calorimeter timing relative to individual TOF bars was seen to be at the level of 500 ps, agreeing perfectly with the prediction of the calibration procedure. The misalignment of the forward TOF bars presently in the calibration map was presented as the following (sigmas): sector 1: 1.4 ns, sector 2: 1.3 ns, sector 3: 1.4 ns, sector 4: 1.9 ns, sector 5: 0.9 ns, sector 6: 0.7 ns. The systematic trends seen in the TOF data are consistent with systematic errors from using cosmic rays to do the alignment (same pattern in sectors 1/4, 2/3, 5/6), which is how the present TOF numbers were derived. A second topic addressed was the coordinates of the matched tracks for the calorimeter and forward TOF. While some irregularities are apparent in the TOF x and y calculation, there was an excellent correlation of the calorimeter i and j coordinates with the TOF x and y coordinates (both sector coordinates) for all sectors. At the present time only the TOF z coordinate is used for geometrical matching; from the data it appears that nearly all the presently matched tracks would be retained if the x and y coordinates were also used. A small fraction of the data had very wrong x or y values. In conclusion, the calorimeter timing calibration is complete at the sub-nanosecond level. The constants which will be checked into the map will average over the present forward TOF variations, and will have to be revised when the TOF numbers are refined. The calorimeter times are one of several possible vehicles which may be used to do the bar-to-bar alignment of the forward TOF bars. Future efforts will concentrate in the areas of making the existing calibration method more cpu-efficient, and will address the problem of getting to the 200 ps timing level predicted by GEANT simulation. The latter will require time walk corrections, and may make use of the RF signal and cross-calibration with the forward TOF bars.