Supernova Relic Neutrinos (SRN) are a diffuse neutrino signal from all past supernovae that has never been detected. 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 Predicted SRN flux Expected number SRN events in SK events/year/22.5kton (10-30MeV) events/year/22.5kton (16-30MeV) events/year/22.5kton (18-30MeV) Ando et al (2005) (LMA) R. A. Malaney (1997) Kaplinghat et al (2000) Hartmann, Woosley (1997) Totani et all (1996) (constant SN rate)

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

solar e recoil energy (total) (MeV) energy resolution at: 16 MeV 18 MeV 7 Be B8B pep hep pp Nuclear Spallation from cosmic  ’s Solar neutrinos Radioactive backgrounds Cosmic ray muons, decay electrons Pions from neutrino interactions Electronics effects many backgrounds, cuts solar ’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 Reducible Backgrounds

Spallation and Solar Cuts 11 Be 11 Li 12 N 14 B energyresolution 8B8B 9 Li 8 Li 12 B 13 B 13 O 12 Be 12 C 8 He 9C9C 15 C 16 N 16 MeV 18 MeV New threshold 18  16 MeV! Lowering threshold < 16 MeV too difficult due to “wall” of spallation products with long half-lives that enter sample 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)

16-18 MeV N/A 23% N/A 18% MeV 7% 9% 36% 9% MeV 7% 0% 36% 9% MeV 7% 0% 36% 0% Energy range2003 cut new cut Solar and Spallation cut inefficiency SOLAR CUT SPALLATION CUT 2003 cutnew cut Total signal inefficiency: 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 (now more data included!)

Atmospheric background ν μ CC ν e CC μ/π NC elastic E of background (MeV) : two channels: ν μ CC spectrum modeled by decay electrons from cosmic ray  ’s ν e CC spectrum from MC Now Now: four channels: ν μ CC ν e CC NC elastic required by lower E threshold; spectrum from MC μ/π prod.: reduced by cuts; helps constrain NC in signal fit

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

20-38 degrees38-50 degrees78-90 degrees E (MeV) SK-I/III data ν μ CC ν e CC NC elastic μ / π > C. thr. all background relic : binned χ 2 fit to center region, two background channels Now Now: simultaneous unbinned maximum likelihood fit, four background channels, three Cherenkov angle regions. Each channel has free floating normalization

Combined Fit combined 90% c.l.: < 5.1 ev / yr / 22.5 ktons interacting 16 MeV) 18 MeV) combined 90% c.l. ev/yr interacting in 22.5 ktons logLikelihood SK-I/II/III combined likelihood Comparison to Published/cm 2 /s >18 MeV Published limit1.2 cross section update to Strumia-Vissani 1.2  1.4 Gaussian statistics  Poissonian statistics in fit 1.4  1.9 New SK-I Analysis: E THRESH 18  16 MeV ε = 52%  78 % (small statistical correlation in samples) improved fitting method takes into account NC 1.9  1.6 New SK-I/II/III combined fit1.6  1.9

BACKUP

PeriodLive time# ID PMTs / % coverageComment SK-I1497 days11146 / 40%Experiment start SK-II793 days5182 / 19 %After accident SK-III 562 days11129/ 40%After repair SK-IVrunning now11129/ 40%New electronics Super-Kamiokande (SK) 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. Electron energy [MeV] SK Event Rate [/year /MeV] ν e + 16 O  16 N + e + ν e + 16 O  16 F + e - ν e + e  ν e + e - ν e + p  e + + n The main interaction mode for SRN’s in SK is charged current quasi- elastic interaction (inverse  decay)

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

Effwall cut Energy (MeV) Effwall (cm) old new Some  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. : Signal Inefficiency: Old: 7% New: 2.5% : Signal Inefficiency: Old: 7% New: 2.5% reconstructed event vertex reconstructed event direction Effwall Inner detector wall

Final Backgrounds (after all relic cuts) E (MeV) NC Backgrounds: Single π -, π + > 200 MeV (~30%) Elastic (~39%) Single π + < 200 MeV (~11%) Single π 0 ’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 CC Backgrounds invisible μ  decay e ν e CC μ > C. threshold μ/πμ/π ν μ CC ν e CC μ/π NC elastic

Combined Fit combined 90% c.l.: = 5.1 ev / yr / 22.5 ktons interacting = 2.7 /cm 2 /s (>16 MeV) = 1.9 /cm 2 /s (scaled to >18 MeV) combined 90% c.l. ev/yr interacting in 22.5 ktons logLikelihood SK-I/II/III combined likelihood SK-I (~1500 days) SK-II (~790 days) SK-III (~560 days) combined

SK-I best fit is negative fit shown is 0 relic contribution degrees degrees degrees E (MeV) SK-I data ν μ CC ν e CC NC elastic μ/π > C. thr. all background relic SK-I only 90% c.l. limit: 16 MeV) < 1.6 /cm 2 /s (scaled to >18 MeV )

SK-II Best fit (shown): 3.5 ev/yr interacting degrees degrees degrees E (MeV) SK-II only 90% c.l. limit: 16 MeV) < 5.2 /cm 2 /s (scaled to >18 MeV ) data ν μ CC ν e CC NC elastic μ/π > C. thr. all background relic

SK-III Best fit (shown) : 6.5 ev/yr interacting degrees degrees degrees E (MeV) SK-III only 90% c.l. limit: 16 MeV) < 5.7 /cm 2 /s (scaled to >18 MeV ) data ν μ CC ν e CC NC elastic μ / π > C. thr. all background relic

Systematics: Inefficiency 19 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 σ ineff – then we alter likelihood: then the 90% c.l. limit R 90 is such that σ ineff SK-I: 3.5% SK-II: 4.5% SK-III: 3.1%

Cuts: efficiencies and sys errors 20 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%) 1 st 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%) SK-III 96% (0.3%) 94% (0.3%) 98% (0.5%) 89% (1%) >99% 99% (0.3%) 99% (2%) 69% 77%