Level 2 Background Reduction

The measurement of Solar neutrinos suffers from large backgrounds - Solar neutrinos contribute between 15 and 50% to the low energy data sample (between 6.5 and 20 MeV) even after all background reduction cuts have been applied. As the energy decreases, the background contamination rises steeply. On the other hand, the solar neutrino flux at a recoil energy below 6.5 MeV is much larger than the flux above, which motivates a lower analysis threshold than 6.5 MeV. Above 7 MeV, the dominant remaining background is due to cosmic ray spallation events which motivitates improvements to the spallation cut.

To deal with the increased background levels below 6.5 MeV and spallation background above 7 MeV, Super-Kamiokande tightened and changed its background rejection cuts: the new unified analysis covers the energy range from 5 to 20 MeV and replaces the Super Low Energy Analysis (SLE, 5.5 to 6.5 MeV) and Low Energy (LE, 6.5 to 20 MeV). Beyond an improved spallation cut for events where the parent cosmic ray muon deposits less energy than a minimum ionizing muon and a tightened gamma ray cut against external events below 6.5 MeV, there are three new cuts:
The clusfit fiducial cut (CLUS), the GRid INtegrated GOodness cut (GRINGO) and a Cherenkov pattern liklihood cut (PATLIK). The improved background rejection enables Super-Kamiokande to see a solar neutrino signal down to 4.5 MeV.

Gamma-Ray Cut

The dominating background below 6.5 MeV are external events (possibly gamma rays from the rocks surrounding the Super-Kamiokande tank), most of which are removed before events are written to tape by a cut on the fiducial volume utilizing on-line event vertex fitters. Only events with a vertex which is further than 2m from the inner detector wall are written to tape. The gamma-ray cut computes the distance of the event to the inner detector wall along the event's direction.

Above 6.5 MeV, events with a distance smaller than 4.5m are removed, below 6.5 MeV, events with a distance smaller than 8m are removed.

GRINGO Cut

The reconstruction of both energy and direction of an event depends crucially on the quality of the vertex fit. The GRINGO cut (grid-integrated goodness cut) compares the vertex goodness at the best fit position with surrounding positions to investigate, if the vertex reconstruction points reliably to a single position:

to investigate, if the vertex reconstruction points reliably to a single position. Events with ambiguous fit vertices are rejected. The hexagonal grid surrounding the vertex (at 0,0) is defined on a plane perpendicular to the event direction. Good events must have a larger vertex goodness shift than the displayed surface (i.e. below it) for at least 92% of the grid points (304 out of 330 points).

GRINGO reduces the background by roughly one order of magnitude below 6 MeV.

CLUS Cut

Clusfit (Cluster Fitter) is a vertex fitter designed to fit very low energy events. To remove events that were reconstructed incorrectly outside the fiducial volume, all events are re-fitted with clusfit. Events that clusfit reconstructs outside the fiducial volume are removed.