Supernova Relic Neutrinos ( SRN ) are a diffuse neutrino signal from all past supernovae

1. Supernova Relic Neutrinos (SRN) are a diffuse neutrino signal from all past supernovae that has never been detected. Expected number SRN events in SK 0.8 -5.0 events/year/22.5kton (10-30MeV) 0.5 -2.5 events/year/22.5kton (16-30MeV) 0.3 -1.9 events/year/22.5kton (18-30MeV) Predicted SRN flux Motivation SRN measurement enables us to investigate the history of past Supernovae. The SRN flux is related to the supernova rate in galaxies and the cosmic star formation history Ando et al (2005) (LMA) R. A. Malaney (1997) Kaplinghat et al (2000) Hartmann, Woosley (1997) Totani et all (1996) (constant SN rate)

2. SK-I : < 1.2 /cm2 /sec OLD 90% C.L. Flux limit: SK-I DATA From 2003 published result: SK-I result: M. Malek, et al, Phys. Rev. Lett. 90, 061101 (2003) (1496 days) + 90% c.l. relic both backgrounds (solid) This study used: binnedχ2limit extraction 18 MeV lower energy threshold SK-I data only 0th order inverse beta cross section two irreducible backgrounds These things have now been improved! Invisible m-e decay (dashed) Atmospheric ne (dot dashed) Two Irreducible backgrounds: 1) Atmospheric νe cc interactions 2) Decay of sub-Cherenkov ‘invisible μ’s’ from atmospheric νμinteractions

3. 16 MeV 18 MeV Reducible Backgrounds energy resolution at: pp Nuclear Spallation from cosmic m’s Solar neutrinos Radioactive backgrounds Cosmic ray muons, decay electrons Pions from neutrino interactions Electronics effects 7Be pep 8B • many backgrounds, cuts • solar n’s and spallation: largest at low energy, set energy threshold • dominant background is spallation: spa-cut has largest inefficiency • crude solar and spallation cuts in published analysis: improvement needed for lower E threshold 18 16 hep solar e recoil energy (total) (MeV)

4. energy resolution Spallation and Solar Cuts • SPALLATION is cut using correlation to cosmic ray muons • Original cut used 2-D spatial correlation, time and charge • New method allows 3-D spatial correlation, muon categorization • Stricter cut < 18 MeV • SOLAR events are cut by correlation to solar direction • New technique estimates multiple scattering, which dominates angular resolution • New cut is optimized in 1 MeV bins using MC, better reduction half-life (s) 11Be 16N 16 MeV 15C 8Li 12C 8B 18 MeV 9Li 9C 8He 12B 14B 12Be 12N 11Li 13B 13O New threshold18 16 MeV! Lowering threshold < 16 MeV too difficult due to “wall” of spallation products with long half-lives that enter sample

5. Solar and Spallation cut inefficiency SPALLATION CUT SOLAR CUT 2003 cut new cut new cut Energy range 2003 cut 16-18 MeV N/A 23% N/A 18% 18-20 MeV 7%9% 36% 9% 20-24 MeV 7% 0% 36% 9% 24-34 MeV 7% 0% 36% 0% Total signal inefficiency: (now more data included!) SK-I(1497 days)SK-II(794d)SK-III (562d) NEW (now) 22% (16-90) 31% 23% OLD (2003) 48%(18-90) N/A N/A

6. Atmospheric n background νμ CC νe CC μ/π NC elastic 2003: two channels: νμ CC spectrum modeled by decay electrons from cosmic ray m’s νe CC spectrum from MC Now: four channels: νμ CC νe CC NC elastic required by lower E threshold; spectrum from MC μ/πprod.: reduced by cuts; helps constrain NC in signal fit E of background (MeV)

7. Signal region Isotropic region Low angle events p ne N 42o n e+ 25-45o μ, π n reconstructed angle near 90o n (invisible) MC (without nm contribution) SK-I/III combined final data sample isotropic region (NC elastic) isotropic region (NC elastic) signal region (relic n/nm/ne) signal region (relic n/nm/ne) low region (μ/π) low region (μ/π) νe CC μ/π NC elastic Cherenkov angle distribution degrees

8. 2003: binned χ2 fit to center region, two background channels SK-I/III data νμ CC νe CC NC elastic μ/π > C. thr. all background relic Now: simultaneous unbinned maximum likelihood fit, four background channels, three Cherenkov angle regions. Each channel has free floating normalization E (MeV) 20-38 degrees 38-50 degrees 78-90 degrees

9. Combined Fit SK-I/II/III combined likelihood logLikelihood combined 90% c.l. ev/yr interacting in 22.5 ktons combined 90% c.l.: < 5.1 ev / yr / 22.5 ktons interacting < 2.7 /cm2/s (>16 MeV) < 1.9 /cm2/s (scaled to >18 MeV)

10. BACKUP

11. νe+ 16O  16N + e+ The main interaction mode for SRN’s in SK is charged current quasi-elastic interaction (inverse b decay) Super-Kamiokande (SK) 10 νe+ p  e+ + n 0.1 SK is 50 kton water Cherenkov detector in the Kamioka mine, Japan (2700 m.w.e). The data is divided into segments: SK-I, II, III, and IV. 10-3 SK Event Rate [/year /MeV] νe+ 16O  16F + e- 10-5 νe+ e  νe + e- 10-7 0 10 20 30 40 50 Electron energy [MeV]

12. Spallation Cut μentry point μ track dlLongitudinal NEW! where peak of DE/DX plot occurs dlTransverse OLD likelihood Relic Candidate dE/dx Plot QPeak= sum of charge in window New Cut: 16 < E < 18 MeV: 18.2% signal inefficiency 18 < E < 24 MeV: 9.2% signal inefficiency Old cut (likelihood + 150 ms hard cut) 18 < E < 34: 36% signal inefficiency p.e.’s spallation expected here distance along muon track (50 cm bins) • 4 variable likelihood cut • The 4 variables: • dlLongitudinal • dt • dlTransverse • QPeak • Use new, better μfitters • Tuned for each muon type (i.e. single, multiple, stopping μ) • Improvements allow lowering of energy threshold to 16 MeV!

13. Effwall cut Some g ray events originating from outside of fiducial volume have possibility of being reconstructed within fiducial volume of SK. In order to remove these events, we applied effwall cut which uses travel distance from tank wall. Inner detector wall Effwall reconstructed event vertex reconstructed event direction Effwall (cm) Signal Inefficiency: Old: 7% New: 2.5% old new Energy (MeV)

14. Final Backgrounds (after all relic cuts) CC Backgrounds invisible μ decay e νe CC μ > C. threshold νμ CC νe CC μ/π NC elastic μ/π NC Backgrounds: Single π-, π+ > 200 MeV (~30%) Elastic (~39%) Single π+ < 200 MeV (~11%) Single π0’s (~11%) Multiple π production (~8%) other (<1%, neglect) These 3 can be modeled as a combination of other backgrounds, and thus aren’t considered separately E (MeV)

15. Combined Fit SK-I/II/III combined likelihood logLikelihood combined 90% c.l. SK-I (~1500 days) SK-II (~790 days) SK-III (~560 days) combined ev/yr interacting in 22.5 ktons combined 90% c.l.: = 5.1 ev / yr / 22.5 ktons interacting = 2.7 /cm2/s (>16 MeV) = 1.9 /cm2/s (scaled to >18 MeV)

16. SK-I data νμ CC νe CC NC elastic μ/π > C. thr. all background relic SK-I best fit is negative fit shown is 0 relic contribution SK-I only 90% c.l. limit: < 2.4 /cm2/s (>16 MeV) < 1.6 /cm2/s (scaled to >18 MeV) E (MeV) 20-38 degrees 38-50 degrees 78-90 degrees

17. SK-II data νμ CC νe CC NC elastic μ/π > C. thr. all background relic Best fit (shown): 3.5 ev/yr interacting SK-II only 90% c.l. limit: < 7.4 /cm2/s (>16 MeV) < 5.2 /cm2/s (scaled to >18 MeV) E (MeV) 20-38 degrees 78-90 degrees 38-50 degrees

18. SK-III data νμ CC νe CC NC elastic μ/π > C. thr. all background relic Best fit (shown) : 6.5 ev/yr interacting SK-III only 90% c.l. limit: < 8.1 /cm2/s (>16 MeV) < 5.7 /cm2/s (scaled to >18 MeV) E (MeV) 20-38 degrees 38-50 degrees 78-90 degrees

19. Systematics: Inefficiency σineff SK-I: 3.5% SK-II: 4.5% SK-III: 3.1% • Define: • r = # relic events we see in data • R = # relic events actually occurring in detector • ε = efficiency (SK-I/II/III dependent) • assume ε follows a probability distribution P(ε) • assume P(ε) is shaped like Gaussian w/ width σineff • then we alter likelihood: then the 90% c.l. limit R90 is such that

20. Cuts: efficiencies and sys errors • SK-III 96% (0.3%) 94% (0.3%) 98% (0.5%) 89% (1%) >99% • 99% (0.3%) 99% (2%) 69% 77% SK-I: effwall : 98% (0.5%) C. angle: 95% (0.4%) pion like: 98% (0.2%) spall+solar: 89% (1%) 2-peak, 2-ring: >99% Correlation cut: 99% (0.3%) 1st reduction: 99% (2%) (includes: electronic noise cuts, 50 us cut) Total: 78 % SK-II 95% (0.3%) 88% (0.3%) 97% (0.5%) 87% (1.4%) >99% 99% (0.3%) 99% (2%)