The Importance of Low-Energy Solar Neutrino Experiments

1. The Importance of Low-EnergySolar Neutrino Experiments Thomas Bowles Los Alamos National Laboratory Markov Symposium Institute for Nuclear Research 5/13/05 Nuclear Physics

2. Standard Solar Model Nuclear Physics

3. Comparison of measured rates and Standard Solar Model (After 30+ years of effort) Nuclear Physics

4. Flavor Content of the Solar 8B Neutrino Flux Detecting Neutrinos in SNO CC Interaction Sensitive to electron neutrinos only NC Interaction Equally sensitive to all flavours ES Interaction Sensitive to all flavors, but most sensitive to electron neutrinos Nuclear Physics

5. What We Know • Flux of 8B n’s has a large non-ne component • Survival probability Pee for En > 5 MeV is essentially independent of En • Pee for n’s of lower energy (p-p) is larger • There is no significant (> 2s) D/N asymmetry All observations are consistent with the following hypotheses: Mass-induced flavor oscillations (with LMA as the favored solution) Nuclear Physics

6. Neutrino Oscillations If neutrinos have mass leptons can mix: Flavor eigenstates are a mixture of mass eigenstates States evolve with time or distance The ne survival probability for two flavor mixing is: Nuclear Physics

7. Photomultipliers Reactor Neutrino Experiment Terrestrial Neutrinos KamLAND is a 1 kton liquid scintillator detector that observes from a number of reactors in Japan at an average distance of 180 km (NOBS - NBG)/NEXP = 0.611 ± 0.085 (stat) ± 0.041 (syst) KamLAND observes a significant deficit of neutrinos and confirms solar neutrino LMA neutrino oscillation solution Nuclear Physics

8. Neutrino Properties • What We Know • There are 3 types of neutrinos : ne , nm , nt • Neutrinos have mass and oscillate • Oscillation parameters (Dm2 and tan2q) known to ~ 30% • Neutrino masses are small • 50 meV < mn < 2.8 eV (90% CL) • Lower limit from atmospheric neutrino results • Upper limit from tritium beta decay results • Neutrinos account for at least as much mass in the Universe • as the visible stars Nuclear Physics

9. Neutrino Properties • What We Don’t Know - Neutrino Properties • Are neutrinos their own antiparticles? (Majorana n) • What is the absolute scale for neutrino mass? • Is the mass scale normal ordered or inverted hierarchy? • Are there sterile neutrinos? • What are the elements of the MNS mixing matrix? • Is CP / CPT violated in the neutrino sector? • What We Don’t Know - Neutrino Astrophysics • Is the Standard Solar Model correct? • What is the flux of solar neutrinos below 5 MeV? • What is the flux of CNO neutrinos? • What is the radial temperature distribution of the Sun? • How do neutrino properties affect supernovae? Nuclear Physics

10. Physics Program for FutureSolar Neutrino Experiments (I) • Directly observe the 99.99% of solar neutrinos • that are below 5 MeV Direct test of solar models (p-p, 7Be, CNO) Uncertainties in the solar neutrino fluxes p-p 7Be CNO 8B Present 15% 35% 100% 6% With present 12% 8% 100% 4% generation dets Future expts 1-3% 2-5% 10-20% 2-4% • Measurement of CNO neutrinos provides an important test: • 1.5% of the Sun’s energy is from the CNO cycle • CNO burning is crucial in first 108 yr convective stage • Provides test of initial metallicity of the Sun • Determine unitarity / dimension of n mixing matrix • Goal is to measure the flavor composition • of the p-p solar n’s to 1% precision in • a model-independent manner • Requires CC and ES/NC measurement • (assuming active oscillations) • Model-indep test for sterile n’s using measured • oscillation parameters (p-p + KamLAND)  Can achieve ≈ 13% sensitivity (90% CL) Nuclear Physics

11. Physics Program for FutureSolar Neutrino Experiments (II) • Use p-p neutrinos as “standard candle” Precision test for CPT violation comparing and • Model-dependent cross-check for sterile neutrinos • with ≈ 2% sensitivity (90% CL) Measurement of the p-p rate to 1% provides knowledge of q12 to allow a search for CPT violation at a scale of 10-20 GeV Compared to the present CPT test from the upper limit on the mass difference in the kaon system of 4.4 x 10-19 GeV Various scenarios imply that the sterile component of solar neutrino fluxes may be energy dependent • Provide improved precision of mixing angle • Future p-p solar neutrino experiments offer the best prospect • for improving our knowledge of q12  Low-energy solar neutrino expts must be part of any full study of sterile neutrinos • Search for n magnetic moment with improved • sensitivity (contribution  1/Te) Qsolar required to determine mn in 0n-bb decay  Expect sensitivity of 10-11mB Nuclear Physics

12. p-p Solar Neutrino Experiments:Physics Goals fTotal= fActive + fSterile Search with sterile neutrino components with an order of magnitude improved sensitivity Future Sensitivity Present limits Nuclear Physics

13. Next-Generation Solar Neutrino Experiments What is required of future experiments: Measurement of ne fluxes: Source To match To match To match current expts: projected expts: LMA prediction: p-p 15% 12% 2% 7Be 35% 8% 5% CNO 100% 100% 100% pep 100% 100% 2% 8B 6% 4% 6% Mixing parameters: To match current limits on tan2q: 3% p-p accuracy To match projected SNO, KamLAND limits: 2% p-p accuracy Nuclear Physics

14. Future Experiments - Borexino • Looks at solar 7Be line (862 keV) • Precision measurement of q12 • Will provide test of SSM for 7Be flux • Possible future extension to p-p neutrinos Nuclear Physics

15. p-p Solar Neutrino Experiments Charged-Current Experiments: LENS, MOON Goal: Measure ne component of p-p (7Be) with 1-3% (2-5%) accuracy Elastic Scattering Experiments: CLEAN, HERON, TPC, XMASS Goal: Measure ne / nm, nt component of p-p (7Be) with 1-3% (2-5%) accuracy Nuclear Physics

16. CC p-p Experiments: LENS Spokesman: Raju Raghavan 40 tons In target in 400 tons scintillator Modular design with In cells surrounded by non-In cells (2000 tons scintillator) Fundamental problem: 115In beta decay Nuclear Physics

17. CC p-p Experiments: LENS Nuclear Physics

18. LENS Count Rates • Design Parameters (assumed) • 40 tons In •  480 tons InLS, 4 kton non-InLS • 4 years of running (5 calendar years) • Detection efficiency ~ 22% for p-p, 57% for 7Be, CNO • 300 MeV/pe scintillator, 3 m attenuation length • No backgrounds • Calibrated by 8 MCi 51Cr source Source Statistical Accuracy p-p 2.3% 7Be 2.8% CNO 5.8% pep 11.8% Issue: estimated cost ~ $140M Nuclear Physics

19. CC p-p Experiments: MOON Nuclear Physics

20. CC p-p Experiments: MOON Issue: Double beta decay background! Nuclear Physics

21. ES p-p Experiments: HERON Spokesman: Bob Lanou ~ 5,000 events/yr (10 ton fid. Vol.) BP00 SSM Nuclear Physics

22. Low Energy Solar Neutrino Fluxes Bahcall, Gonzalez-Garcia, Pena-Garay, hep-ph/0204194 Ga  SNO  KamLAND  BOREXINO  BP00 Ga Ga CNO SNO KamLAND BOREXINO Exp’t X-Sect. SSM CC Exp’t Exp’t Sterile        +0.05 +0.01 +0.00 fpp = 1.05 (1 ± 0.11 ± 0.007 ± 0.05 ± 0.04 ) - 0.08 - 0.02 - 0.02 = 1.05 (1 ± 0.15)  Dedicated pp Experiments required to make Improvements. Flux Predictions for a pp Elastic Scattering Experiment 0.697 ± 0.023 (100 keV) 0.693 ± 0.024 ( 50 keV) Nuclear Physics

23. Low Energy Solar Neutrino Fluxes SAGE Results: 69.6 +4.4/-4.3 (stat) +3.7/-3.2 (syst) SNU GALLEX + GNO: 70.8  4.5 (stat)  3.8 (syst) SNU Progress in determining the flux of low-energy solar ne can only be achieved in the next decade by improved Ga measurements SAGE: 1990-2003 The Gallium experiments should continue to operate until they are systematics limited Nuclear Physics

24. The Russian-American Gallium Experiment It has been my experience that SAGE has proved to be a perfect example of the value of international scientific collaborations The SAGE collaboration has provided the means for achieving a significant scientific result It has been my privilege and honor to play a role in SAGE I am extremely grateful to the many people who have made SAGE a success - Without all of their support the success and recognition that we have received in the world scientific community would not have been possible. Nuclear Physics