Relic Neutrino Detection in Large Water Cherenkov Detectors

1. APS/JPS Joint Meeting Kapalua, Maui, September 2005 Relic Neutrino Detection in Large Water Cherenkov Detectors Michael Smy UC Irvine

2. Relic n in Super-K spallation products from cosmic m’s atm. nm→ stealth m±→e± relic n’s Michael Smy, UC Irvine

3. Spectrum fitting in SK-I SK-I preliminary free parameter a = 0.0 : factor of SRN b = 1.30±0.2 : factor of νe g = 0.45±0.1: factor of νμ c2 = 7.2 / 13 d.o.f Visible energy [MeV] Courtesy Iida, ICRR

4. Spectrum fittingin SK-II SK-II preliminary free parameter a = 0.0 : factor of SRN b = 0.76±0.2 : factor of νe g = 0.51±0.1: factor of νμ c2 = 10.4 / 13 d.o.f Visible energy [MeV] ※In SK-II, spallation BG is remaining in the first energy bin due to worse energy resolution. Number of remaining spallation is estimated from quality of rejected spallation events. Courtesy Iida, ICRR

5. Current SK Flux Limit preliminary SK-II limit: < 3.68 /cm2/sec SK-I limit: <1.25 /cm2/sec SK-I/II Limit: < 1.08 /cm2/sec revised in NNN05 Courtesy Iida, ICRR

6. Possible Improvements • Enlarge Exposure: Increase Fiducial Volume; add SK-III, IV • Reduce Deadtime and Efficiency of Nuclear Spallation Tagging • Lower Analysis Threshold • Detect Neutrons

7. Preliminary E-Scale correction Inner Detector wall θinc • Energy scale correction done using the `Dinc’ variable Erecon Dinc Ecorr / Etrue Dwall After Corr: Dwall Before Correction: Dinc Dinc [cm] Fitting function Ecorr / Etrue Ecorr / Etrue F(x) = P1eP2x + P3 P1 = -0.540±0.0023 P2 = -0.00486±0.00004 P3 = 0.997±0.0014 X2 = 1.1 , 19 d.o.f 100 < wallsk < 200: 0.984 mean 0.1595 s 500 < wallsk < 1000: 1.006 mean 0.1475 s Courtesy Iida, ICRR

8. Position Resolution vs. Dwall SK-II MC, 20MeV, 10000events Fid V Position resolution [cm] ・Resolution means 68% in Dr distribution. ・50-70cm resolution for all energies. ・Good position resolution out of fid V!! Red : 20MeV Green: 30MeV Blue : 50MeV Dwall [cm] Courtesy Iida, ICRR

9. Result after effwall cut Decay-e MC Event rate [event / m3/ 2.5y] Data E correction is applied. F(x) is tuned for 20MeV Dwall [m] Effwall cut efficiency is corrected. Visible Energy [MeV] • After various cut, event rate becomes flat in Dwall • Energy spectrum is consistent with expected Michel spectrum • Signal increase 15.6% (volume increase is 21%) Courtesy Iida, ICRR

10. The Best Way to Cut Spallation Events Go Deep! Vertical muon intensity (cm-2s-1sr-1) From MACRO Collaboration @ 23rd International Cosmic Ray Conference, Vol. 4, Edited by D.A. Leahy, R.B. Hickws, and D. Venkatesan. Invited, Rapporteur, and Highlight Papers. Singapore: World Scientific, 1993., p.391 standard rock h gcm-2

11. Improvement of Spallation Tagging entry position Qpeak sum of 10 bins around peak muon photons from muon interaction spallation occurs here projected position of spallation product PMT 11 new 3D tag: predict spallation point along m track PMT times & m track fit to reconstruct point of emission Q histogram of such points, peak predicts point of spallation distance along muon track in m Courtesy K. Bays, UC Irvine

12. This Really Works:A Simple Example entry point muon peak of dE/dx LLONG relic n candidate LTRAN Spallation • LLONG (cm) • LLONG (cm) • LTRAN (cm) • LTRAN (cm) Courtesy K. Bays, UC Irvine

13. Likelihood Method LLONG (spallation,black) LLONG (random,red) LTRAN (spallation,black) LTRAN (random,red) QPEAK (random,red) dt (spallation,black) QPEAK (spallation,black) dt (random,flat,red) 13 Courtesy K. Bays, UC Irvine x10 pe

14. Removal of Spallation • Deadtime 18% (Compared to 37% in Publication) • Further Tuning in Progress… (dt < 10 s) stopping muons single muons 14 LTRAN (cm) dt (seconds) black – before likelihood cut, red – after likelihood cut Courtesy K. Bays, UC Irvine

15. Current SK Flux Limit preliminary SK-II limit: < 3.68 /cm2/sec SK-I limit: <1.25 /cm2/sec SK-I/II Limit: < 1.08 /cm2/sec revised in NNN05 Courtesy Iida, ICRR

16. Future SK Flux Limit SK-II limit: ≈ 2.9 /cm2/sec SK-III limit: ≈1.6 /cm2/sec SK-I limit: ≈1 /cm2/sec SK-I-III Limit: ≈ 0.75 /cm2/sec revised in NNN05 Courtesy Iida, ICRR

17. ne Neutron Tagging (with Gd)

18. 0.2 % GdCl3 Solution 18 cm 5 cm BGO Am/Be Φ=13 cm Φ=18 cm Am/Be/GdCl3-Sol. n Correlation Source detect by BGO • Mimic νe+p→e++n with α+ 9Be→12C*+n • n+p→…→n+Gd→Gd+γ’s(Σ=8 MeV) • 12C*→12C+γ(4.4 MeV) • 0.2% GdCl3 Sol. in Source: en=28% • 0.2% GdCl3 Sol. in all SK:en=90% Courtesy Watanabe, ICRR

19. 95.1% 91.9% Number of Events Number of Events Data MC Preliminary Results • dR < 2m yields 90 % efficiency • 2-component t-fit: f(t) = p1e-t/p2+p3e-t/p4+p5 (Ther. n’s leave source, return & capture on Gd. with τ =20μs) dR [cm] dR [cm] Data MC dT [×10-3 sec] dT [μs] Courtesy Watanabe, ICRR

20. Energy Spectrum of n Capture • Mean Value ~ 4.7 MeV in Data & MC MC Data Number of Events Number of Events Red: E > 4 MeV Red: Trig. Eff. Included. Energy [MeV] Energy [MeV] Courtesy Watanabe, ICRR

21. Measure Water Transparency Michael Smy, UC Irvine

22. GdCl3 Solution Michael Smy, UC Irvine

23. Intelligent Very Low E Trigger • Super-Kamiokande: Reach down to 3MeV! • Online Vertex Fitting + Prefilter: • Need ~50 CPU’s photons from vertex 4-hit Combin. Vertex PMT Michael Smy, UC Irvine

24. 4.4MeV LINAC Data @(-3.9,-0.1,-0.1)m threshold>9 4.4MeV LINAC Data @(-3.9,-0.1,-0.1)m threshold>9 Michael Smy, UC Irvine

25. Lessons from SK for a 100kton Detector • go deep! spallation background best removed by depth • as much photocathode coverage as possible: energy resolution is crucial • need good calibration system for energy scale • design detector with neutron tagging to distinguish stealth muon decays from signal • don’t choose exact cylinder; flat surfaces are bad for low energy event reconstruction • sophisticated trigger with very low threshold needed Michael Smy, UC Irvine