1. Improvement on vertex reconstruction Motivations Many peaks in vertex distributions Further study for the <0.1MeV events S. Moriyama 6 th March 2004 XMASS meeting Namba’s plots

2. Problem in the vertex reconstruction We are using maps which have value on a 2.5cm spacing grid.  F(ix,iy,iz,ith PMT) Between grid points, F(probability to observe photoelectron by ith PMT) is obtained by a linear interpolation.  F(x,y,z, ith PMT) Namba pointed out that there must be discontinuity of dF/dx, etc, on the grid edge. When I saw likelihood distribution, there is a valley on the grid edge due to the dF/dx discontinuity.

3. How to solve? 1.Adopt higher order interpolation for the maps.  Technically complex for me. 2.Slightly modify likelihood distribution near the current best fit point.  looks good. Best fit here One example: Z(cm) Likelihood Z(cm) Likelihood Zoom up * Actual likelihood 2 nd order polynomial fit Concentration to the grid is now understood.

4. Principle of the improvement For the events wall>2cm, vertex improvement code is called. In the code, likelihood along x, y, and z axis is calculated by 1mm spacing for +/- 1cm region. For each likelihood, 2 nd order ploynomial fit is performed based on the 1mm spacing data, and adopt minimum value of the 2 nd order polynomial as a new fitted result. No modification for energy information (/home/xmass/src/reconst_v6.0)

5. Effect on the source run analysis (Namba) Improvement succeeded. MC Data Further improvement needs maps with more fine spacing.

6. Performance check revisited… Minamino?

7. Background data and MC Note: There was a bug in MC treatment after applying reconst_v4.4 (improvement on <0.1MeV analysis), but there is almost no change by the bug fix. Almost no change observed

8. How about <0.1MeV events? Detail check of wall distribution Horizontal axis (15.5-wall)**3  uniform volume plots can be seen. ~0.6kg/bin

9. Background distributions Horizontal: cm3 (125cm3==10cmFV, 1000cm3==20cmFV) Vertical: DRU Data MC 0-0.1MeV 0.1-0.2MeV 0.2-0.3MeV0.3-0.4MeV 0.4-0.5MeV0.5-1MeV 1-2MeV 2-3MeV

10. 0-0.1MeV0.1-0.2MeV 0.2-0.3MeV0.3-0.4MeV

11. Summary Improvement on the vertex reconstruction done. Still there is small grid effect. It will be improved by making fine maps. <0.1MeV problem and Kr like signals seems to be related. Further study is needed why event concentration at the detector center occurs.