30th International Cosmic Ray Conference in Merida, Mexico Michael Smy UC Irvine Low Energy Event Reconstruction and Selection in Super-Kamiokande-III

SSM spectrum pp 7 Be pep 8B8B P( e  e ) Vacuum osc. dominant matter osc. dominant Expected P( e  e ) Why Study Low Energy Events? precision measurement of solar neutrino oscillation parameters with reactor neutrinos 0.1% GdCl 3need 0.1% GdCl 3 to tag neutrons in delayed coincidence MSW transition for solar neutrinos oscillations both require very low energy threshold & precise energy calibration Courtesy K. Bays, UC Irvine & M. Nakahata, ICRR oscillated/unoscillated 1.0 Reconstructed Total Positron Energy (MeV) expected spectrum from one year of SK data (15 reactors) with Gd (current best-fit oscillation parameters) e Gd  ’s with  E=8MeV after simple cuts (vertex correl., fiducial volume,...)

Positron Vertex Resolution from simulation of Gd delayed coincidences from 15 reactors around SK (8 MeV  cascade) vertex resolution o.k. above ~3MeV vertex resolution much better, if both e + and n are used Courtesy K. Bays, UC Irvine mean deviation from true vertex

New SK-III Low Energy Data turned on online vertex fits and SLE trigger 100% efficient at 5 MeV 2 nd reduction of data new LINAC calib. data at very low energy preliminary energy scale artificial radon injection solar neutrino elastic scattering peak observed Michael Smy, UC Irvine

SK-III Super Low Energy Trigger Courtesy Furuse-san, ICRR SLE LE Energy (MeV) Efficiency in future: record every hit! record every hit! no hardware threshold no hardware threshold threshold set by reconstruction threshold set by reconstruction

after pre-cut (electronic noise, Michel electrons) + blast shield (FRP) radioactive noise cut + spallation cut + mis-reconstruction cut + external event cut (SK-I: + 16 N cut) after pre-cut (electronic noise, Michel electrons) + blast shield (FRP) radioactive noise cut + spallation cut + mis-reconstruction cut after pre-cut (electronic noise, Michel electrons) + blast shield (FRP) radioactive noise cut + spallation cut after pre-cut (electronic noise, Michel electrons) + blast shield (FRP) radioactive noise cut Data Rate for each Reduction Step Agreement of SK-III and SK-I looks quite good! Courtesy Takeuchi-san, ICRR Energy (MeV) Number of events/day/22.5kt/0.5MeV Solid: SK-I (PRD ) Error bar: SK-III (Very Preliminary) after pre-cut (electronic noise, Michel electrons)

Data Reduction goodness: tests timing residuals of vertex fit for coincidence (0=poor, 1=good residuals) dirKS: tests azimuthal symmetry of Cherenkov cone (0=symmetric, 1=asymmetric) pattern likelihood cut:  2 test of Cherenkov angle solution external event d  : project event backwards to the inner detector boundary, calculate distance to that point d , cut d  6.5MeV) Michael Smy, UC Irvine

New Linear Accelerator Data just took LINAC data at six positions a new lowest electron energy: 4.4 MeV! new positions: z=+16m (edge of fiducial volume) study vertex reconstruction and cuts with this data in future, plan to lower beam momentum even more, if possible Michael Smy, UC Irvine

LINAC 4.4 MeV Data Michael Smy, UC Irvine

LINAC Goodness vs. dirKS (4.4MeV) Michael Smy, UC Irvine good bad good bad good bad good bad good bad good bad

LINAC Vertex Resolution (4.4 MeV) Michael Smy, UC Irvine w/o cut gdn cut gdn cut gdn cut gdn cut gdn cut gdn cut ~90cm ~100cm ~90cm ~100cm

LINAC Angular Resolution (4.4 MeV) Michael Smy, UC Irvine w/o cut gdn cut gdn cut gdn cut gdn cut gdn cut gdn cut ~35 0 good bad

Solar Peak (After Cuts) Signal event rates look consistent SK-III has already reached to the similar signal to noise ratio as SK-I in MeV in 22.5kt 2m Fid. Vol. (22.5kt) Courtesy Takeuchi-san, ICRR

meeting14 Vertex Distribution After Cuts SK-I SLE 1216daysSK-III SLE 97days Both SK-I & SK-III rate for R>10m is artificially reduced by external event cut There are more events near SK-III barrel & bottom. SK-III has lower event rates in the central & top region. Z R Courtesy Takeuchi-san, ICRR

Solar Peak MeV all SLE data (97 days livetime) d  >11m radius<13.4m height>-12.1m Michael Smy, UC Irvine

Solar Peak in Central/Top Region Central top region SK-III BG rate is smaller than SK-I in MeV in the central top region Signal rate looks consistent. Courtesy Takeuchi-san, ICRR

Rn Injection: Study Rn and Water Flow Michael Smy, UC Irvine mean position

Radon dirKS vs. Goodness (after cut) Michael Smy, UC Irvine good bad good bad

Conclusions now ~100% efficient triggering at 5 MeV new LINAC data at lower energy at lowest energy, background rate is smaller than SK-I in the central/top region of detector will attempt to expand this region in near future will lower hardware trigger threshold to zero might study reactor ’s and solar ’s at very low energy Michael Smy, UC Irvine

LINAC Efficiencies (4.4 MeV) Michael Smy, UC Irvine

LINAC Resolution (4.4MeV) Michael Smy, UC Irvine

Rotating Vertex Cut Michael Smy, UC Irvine goodness-dirKS cut no x’y’ vertex cut goodness-dirKS cut no x’y’ vertex cut no goodness-dirKS cut x’y’ vertex cut no goodness-dirKS cut x’y’ vertex cut

Compare to Low Energy Data Sample Michael Smy, UC Irvine same as for radon events Sample contains mostly Cherenkov events!