this section circulated as email note to he group, 30 Sept. '97.
In the following we discuss the effects of the minimum energy cut on the resulting oscillations parameters. The summary is that smallest minimum energy cuts seem best (smallest dm2 errors), and that there seems to be something peculiar going on with the data when the cut is around 600 MeV.
Below you will find a picture with four graphs. The horizontal axis for each is the minimum visible energy (in GeV) employed for the oscillation parameter fits. The 300 day off-site data and 7.75 year off-site MC (East) are used.
The top left plot shows the values of dm2 found from the R(dm2) test (which is to say, where R, as a function of dm2, crosses one). The upper (dots) and lower (dashes) are + and - one sigma statistical errors on dm2. The lower error limit falls away at evis min cuts above 600 MeV.
The top right plot shows the R_mu (= mu_data/mu_mc) value of the (presumably) oscillated plateau in the log(L/E) plots, that is above L/E > 10^4 km/GeV. This is a measure of 1 - 0.5sinsq(2theta) (under the assumption of nu_mu<->nu_x mixing). It should not be below 1/2 (which is maximal mixing). Note that it becomes non-physical in the region of 600 MeV in the minimum evis cut.
The lower left plot is the difference between upper and lower one sigma dm2 values, on a log_10 scale (data from top left plot). This illustrates that we get the best resolution at the smallest values of minimum visible energy. [Note this may be in conflict with the results of Larry Wai, who studied the MC to see where to make the cut to get best resolution. I do not know why we have a different result].
The lower right plot show the normalization factor, derived by dividing the number of observed electron events by MC electron events in the sample (assuming 7.75 years for the MC, and 301.4 live days for the data). One sees that the MC predictions are a bit low, by 15-18% or so below 500 MeV in the evis cut, and dropping above that to 6-10%.
JohnF and I have discussed the implications of the peculiarities around 600 MeV, but we have no definitive answer. We have speculated about earth magnetic field effects and flashers near the bottom. Any ideas out there?
One thing that seems clear from this study is that the data is telling us to go for the lowest evis. This maximizes the statistics and it gets one more points at large L/E values.