Introduction

We have a successful run this summer!! The results are shown in rcs_15cm.eps and rcs_offend.eps. The data shown include endpoint runs without radiator (runs 1429-32) and off-endpoint runs with the radiator (runs 1441-46). For all runs, the beam energy was 3.317 GeV and the calorimeter angle was at 35.0 degree. The analysis of these data relies on the difference thig-thigx (the difference of the in-plane angle from calorimeter measurement and ep kinematic prediction, labled as theta in the postscript file) and thog-thogx (out-plane angle difference, labeled as phi in the postscript file). A 2-D plot of (thig-thigx)%(thog-thogx) is shown in theta_vs_phi.eps for the endpoint (up) and off-endpoint (down) data.

In rcs_15cm.ps and rcs_offend.ps, various cuts were applied to thog-thogx (out-plane angle, labled as phiI) for endpoint and off-endpoint data, respectively.
 
 

Data analysis

Considering first the off-endpoint case, rcs_offend.eps, the upper-left plot shows all coincidence events. The peak at center is primarily due to ep scattering and secondarily to RCS, whereas the continuum is due to pi_0 decay photons. For the middle-left plot, a cut is made requiring that the in-plane angle theta be close to that expected for two-body kinematics (-0.01<thig-thigx<0.01); this clearly enhances the central peak. The lower-left plot in addition requires a signal in the veto detector, therefore picking out the ep events. 

The right plots select on the events that do not pass the veto cut; this
should give all photons if the veto detector is 100% efficient. In reality
there are still electrons present which we must take into account later.
An anti-cut on thig-thigx (called peripheral theta) is shown in the
middle-right and preferentially picks out the pi_0 decay photons, whereas a theta central cut (lower-right, -0.01<thig-thigx<0.01) selectively enhances the RCS events, with some continuum mixed in. 

As an estimation, the events above the horizontal line in the lower-right plot represent RCS events plus the contamination of ep due to the inefficiency of the veto detector. To estimate the contamination, we look at the endpoint data, rcs_15cm.eps.  The lower-left are the events that passes the veto and central theta cut, and therefore should be all
electrons. The lower-right are the events that pass the central theta cut
but failed the veto cut. These are the electrons that veto detector failed
to detect. The number of events of veto-not-detected to veto-detected is 183/2407 = 0.076. The veto efficiency is therefore 2407/(2407+183) = 93%.

Now back to rcs_offend.eps, the lower-left plot. There are 291 events.
Due to the veto-inefficiency, there will be approximately 291*0.076=22
electrons appear on the lower-right plot. The number of events above the solid horizontal line is 49. 22 out of 49 are electrons, therefore the
remaining 27 are the RCS. From the Cornell RCS cross section data we
roughly estimate about 35 events (see the RCS proposal, page 33, table 2), which is reasonable.

 
maintained by  Franck Sabatie