Optics tune

Contrary to the case of E89-009 which tagged zero degree scattered electrons, the present experiment requires the full tracking reconstruction in the electron arm. Therefore, a sieve slit and collimator plate made of dense material (such as 3/4" tungsten) will be installed at the entrance of Enge spectrometer for the calibration of the optics. Figure 32 shows that the bremstrahlung electrons are concentrated in the Splitter mid-plane which is 5 cm below the Enge spectrometer collimator opening. The bremstrahlung electrons are totally blocked by the collimator plate and additional local lead shielding outside the vacuum connection. The scattered electrons associated with the virtual photons are also peaked at the center of the bremstrahlung but they fall off slower than bremstrahlung does. Only the portion accepted by the Enge spectrometer at the focal plane is shown in figure 32 to determine the collimator and sieve slit size. Figure 33 shows correlation between focal plane parameters for sieve slit image (the sieve slit plate has 35 holes, 5 raws and 7 columns). The first plot shows the accepted events reconstructed to the sieve slit plate. It can be seen that some holes have no contributions due to the tilted optics. Comparing the simulated correlations with the measured ones, it is possible to tune the optical parameters to obtain the correct optical reconstruction code for momentum and scattering angle. Further optimization of the electron arm optical code and precise missing mass scale calibration can be performed by measuring the known masses of $\Lambda$ and $\Sigma$ with CH$_2$ target, and $^{12}_\Lambda$B ground state with C target. This calibration will reduce the ambiguity due to absolute accuracies of the electron beam energy, kaon central momentum and scattering electron central momentum.