Low Energy Event Reconstruction and Selection in Super-Kamiokande-III

1. 30th International Cosmic Ray Conference in Merida, Mexico Low Energy Event Reconstruction and Selection in Super-Kamiokande-III Michael Smy UC Irvine

2. 1.0 after simple cuts (vertex correl., fiducial volume, ...) Why Study Low Energy Events? SSM n spectrum pp 0.8 0.8 oscillated/unoscillated 7Be Vacuum osc. dominant 0.6 0.6 P(ne ne) • precision measurement of solar neutrino oscillation parameters with reactor neutrinos • need 0.1% GdCl3 to tag neutrons in delayed coincidence • MSW transition for solar neutrinos oscillations • both require very low energy threshold & precise energy calibration Expected P(ne ne) pep expected spectrum from one year of SK n data (15 reactors) with Gd (current best-fit oscillation parameters) 0.4 0.4 8B 0.2 0.2 matter osc. dominant Reconstructed Total Positron Energy (MeV) g’s with SE=8MeV Gd ne Courtesy K. Bays, UC Irvine & M. Nakahata, ICRR

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

4. New SK-III Low Energy Data • turned on online vertex fits and SLE trigger • 100% efficient at 5 MeV • 2nd 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

5. 1 Efficiency 0.8 0.6 0.4 0.2 0 1 2 3 4 6 5 7 8 0 Energy (MeV) SK-III Super Low Energy Trigger • in future: • record every hit! • no hardware threshold • threshold set by reconstruction LE SLE Courtesy Furuse-san, ICRR

6. Data Rate for each Reduction Step • Agreement of SK-III and SK-I looks quite good! after pre-cut (electronic noise, Michel electrons) after pre-cut (electronic noise, Michel electrons) + blast shield (FRP) radioactive noise cut + spallation cut + mis-reconstruction cut + external event cut (SK-I: +16N 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 after pre-cut (electronic noise, Michel electrons) + blast shield (FRP) radioactive noise cut + spallation cut 4 10 3 10 2 10 Number of events/day/22.5kt/0.5MeV 10 1 Solid: SK-I (PRD73 112001) Error bar: SK-III (Very Preliminary) -1 10 12 14 6 8 10 Energy (MeV) Courtesy Takeuchi-san, ICRR

7. 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: c2 test of Cherenkov angle solution • external event dg: project event backwards to the inner detector boundary, calculate distance to that point dg, cut dg<4.5m(<6.5MeV)/8m(>6.5MeV) Michael Smy, UC Irvine

8. 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

9. LINAC 4.4 MeV Data 5 1 2 3 6 4 Michael Smy, UC Irvine

10. 1 2 3 LINAC Goodness vs. dirKS (4.4MeV) bad bad bad good good good 4 5 6 bad bad bad good good good Michael Smy, UC Irvine

11. LINAC Vertex Resolution (4.4 MeV) 1 2 ~100cm 3 ~100cm ~90cm w/o cut w/o cut w/o cut gdn cut gdn cut gdn cut 5 6 ~100cm 4 ~90cm ~90cm w/o cut w/o cut w/o cut gdn cut gdn cut gdn cut Michael Smy, UC Irvine

12. bad LINAC Angular Resolution (4.4 MeV) 3 1 2 ~350 ~350 ~350 w/o cut w/o cut gdn cut gdn cut gdn cut w/o cut ~350 4 ~350 5 ~350 6 w/o cut w/o cut w/o cut gdn cut gdn cut gdn cut good Michael Smy, UC Irvine

13. Solar Peak (After Cuts) 2m Fid. Vol. (22.5kt) • Signal event rates look consistent • SK-III has already reached to the similar signal to noise ratio as SK-I in 5.0-20MeV in 22.5kt Courtesy Takeuchi-san, ICRR

14. Vertex Distribution After Cuts Z R 0 SK-I SLE 1216days SK-III SLE 97days 100 100 • There are more events near SK-III barrel & bottom. • SK-III has lower event rates in the central & top region. Both SK-I & SK-III rate for R>10m is artificially reduced by external event cut JUN-2007 Y.Takeuchi@SK meeting 14 Courtesy Takeuchi-san, ICRR

15. Solar Peak 5-5.5 MeV all SLE data (97 days livetime) Very Preliminary dg>11m radius<13.4m height>-12.1m Michael Smy, UC Irvine

16. Solar Peak in Central/Top Region Central top region • SK-III BG rate is smaller than SK-I in 5.0-5.5MeV in the central top region • Signal rate looks consistent. Courtesy Takeuchi-san, ICRR

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

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

19. 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 n’s and solar n’s at very low energy Michael Smy, UC Irvine

20. 1 LINAC Efficiencies (4.4 MeV) 2 3 4 5 6 Michael Smy, UC Irvine

21. 1 LINAC Resolution (4.4MeV) 2 3 6 4 5 Michael Smy, UC Irvine

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

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