Log entry time 01:29:50 on December 03, 2008

Entry number 251792

1. Fig. 1 shows the vertical position and the vertical angle. We can see clearly two regions which are corresponding to negative and positive charged particles. (It is quite strange that we can see some kind of cut in the lower half of the plot). Our goal is to try to understand this one.

2. The problem clearly happen in the shower or preshower. Fig. 2 shows the projected tracking position on the shower region. It is more likely to be a problem with preshower, since we can also see some effects between the left and right parts of the plot.

3. To demonstrate more it is a shower problem. We plotted the negative charged particle (Fig.3) and positive charged particle (Fig.4) with T3, which has nothing to do with BigBite side. We did not see the cut in this case, which means it is really a trigger problem.

4. Fig. 5 shows the negative particles with trigger 1. We can see the clear cut. Now there are two possibilities. One is that we are missing data or we are seeing some kind of noise.

5. In order to figure this out, we need to select good electrons. Fig. 6 shows how we select good electrons.

6. Fig. 7 shows the results after we selected good electrons. Compared to Fig. 3, we can conclude that we did not loose data and we are seeing some "noise" around 0.

7. Now the question is what are these noise? If we make sure the momentum is larger than 1.0 GeV/c, we did not see the noise on the plot. see Fig. 8.

8. Then we can look at the pions, see Fig. 9, In this case, we can see the enhancement around same region, which does not depend much on momentum. In this case, we can see that the enhancement we saw in the electron case is the leak from pion, because the PMTs problems in the preshower.

9. We can see the same problem with positive charged particle. Fig. 10.

10. The problem become bigger in the T6, since the trigger threshold is higher. We start to cut into good electrons, but we did not cut much the "noise" part. Fig. 11

11. Fig. 12 shows the pions in T6 with negative charged particle cut. The pions are strongly suppressed, suggesting that we may cut into electrons. And we still see some enhancement, suggesting the PMTs may have some problems (Too high HV? or somehow threshold is low around there)

12. Thus we plot the position vs total energy, we can see clearly around that region, the total energy is a little bit lower. Fig. 13

13. This can also be seen for position vs beta Fig. 14.

In summary, we think this is not a problem, since we did not lose data. The enhancement that we saw is due to the lack of calibration. We confirm this by look at the first pass elastic data, we did not see a clear sign of enhancement there. Fig. 15

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