Minutes of the CALCOM Analysis Meeting August 8, 1997.
=====================================================

Agenda:
        K. Hicks   - Comments on drift chamber TDC analysis
        F. Klein   - Progress on drift chamber tracking
        L. Qin     - Region 2 efficiency and resolution analysis 
        L.C. Smith - GSIM simulation of em background in CLAS
        K. Joo     - e p-> pi-0 e (p) reconstruction in the Delta
                     region using EC and HBT 
        J. Price   - Single photo-electron calibration for Cherenkov
                     counter S6
======================================================================

V. Burkert - Summary & comments
-------------------------------
(1) Major progress has been made in the hit-based and time-based tracking 
area using SDA (see F.K.'s report), leading to a much improved tracking 
efficiency (HBT and TBT) for the low luminosity, low energy run# 3166. The 
remaining problems have been identified and appear to solvable. The next step 
is to implement the changes into RECSIS and to continue the fine-tuning. Then 
the high energy runs should be looked at.

(2) We also have now a detailled analysis of the region 2 layer efficiency, 
the single layer position resolution (whithout TOF analysis), and magnetic 
field effects. The latter appear to be rather modest (see L. Qin's report). 
Similar analyses are needed for the region 1 and region 3 chambers. 

(3) Background simulations are making progress as well. A concerted effort of 
several people led to a more realistic implementation of the shielding and 
support structure of the CLAS magnets. This can now be used to identify 
(and eliminate!!) sources of background events seen in the June run 
(see Cole Smith's report).

(4) The analysis of exclusive reaction is progressing as well. To identify 
the e p->e p pi0 reaction in the ECAL_high triggered 4GeV runs, which is 
not possible with HBT only using electron and proton identification and 
pi0 miss. mass., the reversed direction seems to be working for events 
triggered on high energy pi0 (see K. Joo's report). An interesting physics 
question is whether such a trigger could be used to select events with 
higher Q2 than is possible with the Cherenkov counter electron id, which 
is limited to 45deg in scattering angle. 
===================================================================

Individual reports:
================== 

Franz Klein:
-----------

The status of track reconstruction was discussed.
The analysis focussed on run# 3166 (1.625 GeV, El.-Trigger in Sec.6).
Using a modified version of the SDA analysis package 
a Hit Based tracking eff. of 70-80% was achieved compared with
good events as found by eye on CED. This was achieved by inserting
a modified linking routine in SDA allowing tracks to have less than
6 superlayers. 60 % of these tracks pass the time based tracking by 
modifications to the LR ambiguity routines. The loss of remaining
40% (through time based tracking) is due mostly to a failure to 
identify electrons to obtain a start time. The missing mass for these
events e^- p -> e^- X was shown and was consistent with electrons.
However the angluar distribution for scattered electrons showed 
significant gaps and is being investigated. The plan is to 
incorporate these improvements into RECSIS. 

Liming Qin:
----------
Study of B field effects on the R2 drift time.

We analyzed run 3606 (4GeV, half of the full field). We selected hits from 
HBT (recsis) w/ a cut on the azimuthal angle (+-5 deg from midplane). 
We found the absolute layer efficiency was (98.0+-0.5)% and the single layer
resolution was 600 microns. These were obtained from single superlayer
TBT using measured RAW drift time (Only a constant offset was removed.).

We found moderate B field effect on the drift time. The maximum drift time
in the very forward section in the R2 axial superlayer is 750ns. The 
decreasing rate is 120ns per Telsa in the backward section of the same 
superlayer. The maximum drift time in the stereo superlayer is 110ns 
longer compared to the nearby section in the axial superlayer. 

The results from run 3713 (3/4 of the full field) are consistent with
the findings from 3606, though the statistics is poor. A single 
time-doca correlation function gives comparable results for all the cases.
The "drift time" in the function is scaled by the maximum drift time. 

Cole Smith:
----------

Geometry definition routines in GSIM have been modified to reflect
the most up-to-date drawings for the mini-torus coils, forward beamline 
shielding, windows and support ribs for the Saclay target chamber.  
Still to be added is the forward beampipe.  These modifications allow 
more realistic simulations of the backgrounds generated by scattered 
beam partcles emitted at small angles.

GSIM was run with electrons thrown with 0.8<p<4.0 GeV, 0<theta<20 and
 -30<phi<30 degrees (mid-plane of Sector 1).  Reconstructed energy in 
EC and SC was studied as a function of the thrown kinematics. 

1) Between 0-3 degrees, practically no events are found which deposit 
significant energy in EC.  Since there is no fwd beampipe yet in GSIM, 
interactions occur only within the walls of the beamline shielding.  
Nevertheless the energy is contained.
   
2) Between 3-5 degrees, electrons strike the end of the Pb shielding cone
located within the forward apex of the mini-torus.  This results in a large 
transverse shower of hundreds of low-energy (0-2 MeV) photons but with 
most of the charged debris absorbed within the cone.  No more than 1-2% of 
the beam energy is deposited in EC.  A large number of SC bars are hit (~80) 
with average total deposited energy of 30-40 MeV.

3) Beginning at 6 degrees, much of the shower debris begins to emerge 
from electrons scraping the surface of shielding surrounding the mini-torus 
coil/beamline region. The irregular curvature of the coils in this region 
appears to contribute to some of this leakage.  The effect is similar to 
the action of a pre-radiator in that a beam of photons results which 
carries a large fraction of the primary electron energy into the forward 
part of EC.  These events are usually accompanied by low-momentum 
outbending positrons which leave tracks in R2 and R3.  In-bending electrons 
can create showers in structures further downstream, producing additional 
tracks in R3.  During the June run many events with this pattern were seen 
in CED.

Further studies will attempt to define the optimum placement of wedge-shaped Pb 
baffles to block these preshowers.  A Web page showing the progress of this work
is at http://apollo.phys.virginia.edu/clas/june1.html.

K. Joo:
-------

Pi0-reconstruction from 2-gamma events were studied using data obtained in EC 
using run 3713 with 4 GeV beam energy, EC electron trigger in sector 6 with 
high threshold (about 1.5 GeV) and a 75% of full magnetic field setting in 
the CLAS torus, and a 6000A current in the mini torus. Pi0's were reconstructed 
with a mass of about 144 MeV and a resolution of +/-17.4 MeV.

By looking at coincident electrons with pi0, after cuttimg on the invariant mass of 
the ep system (W) for Delta resonance, a missing mass spectrum for p(e,epi0)X was 
obtained clearly showing the proton mass at 927MeV with a resolution of +/-88Mev
(correction after the meeting). 
 
It is also apparent that many events with low energy electrons in sector 3 or 4 
(most of them are below 1 GeV) are triggered by high energy photons in sector 6. 
More systematic studies are needed for electron triggered events.  

J. Price:
--------

The Cerenkov single photoelectron (SPE) peak positions have been calibrated using 
a simple fitting scheme within PAW, and the calibrations are stored within the 
database.  The data used for this calibration consisted of runs 2891 through 2898, 
documented in Logbook 1A, pp. 106 and a few pages thereafter.  The data was taken 
by self-triggering on the Cerenkov detector with a low (30mV) threshold. The fit 
was a simple Gaussian.  Several different voltages were used, which resulted in a 
plot of the SPE peak position as a function of photomultiplier tube PMT voltage 
for each of the PMTs in the Cerenkov. These plots were fit with a function of the 
form 
     y = exp (a + bx)

which was then inverted to determine the voltage required to put the SPE peak at 
ADC channel 150 for each PMT.  The voltages were then changed to the values indicated 
by the fit.  After this was done, it became apparent that a simple Gaussian was 
insufficient to properly fit the data, because the 30mV threshold cut into the 
lower side of the SPE peak.  Therefore, a "two-sided" Gaussian was used for the 
fit, which had the form

     y =  A * exp(-(x-B)^2/(2C^2))          (x > B)
          A * exp(-(x-B)^2/(2D^2))          (x < B)

This functional form provided a much better qualitative fit to the data, and produced 
a much lower chi-squared value.  The SPE peak positions produced by this function 
are systematically lower than those produced by the simple Gaussian fit.

Studies are underway to optimize the fitting procedure.