APS Multi-Divisional Neutrino Study

1. APS Multi-Divisional Neutrino Study Boris Kayser Wine & Cheese February 4, 2005

3. The last seven years Compelling evidence that neutrinos have mass and mix Open questions about the neutrino world Need for a coherent strategy for getting answers

4. A year-long study of the future of neutrino physics, sponsored by the American Physical Society Divisions of – Nuclear Physics Particles and Fields Astrophysics Physics of Beams

5. WHOreally triggered this study??? One data point: HEPAP felt that, in view of the discoveries in neutrino physics, the roadmap for the particle physics future needs to have a neutrino component.

6. What Have We Learned?

7. We do not know how many neutrino mass eigenstates there are. If the Liquid Scintillator Neutrino Detector (LSND) experiment is confirmed, there are more than 3. Confirmation of LSND would show that our usual assumptions about the neutrino spectrum and neutrino mixing are wrong. If LSND is not confirmed, nature may contain only 3 neutrinos. Then, from the existing data, the neutrino spectrum looks like —

8. 3 3 m2atm 2 2 m2sol m2sol } } 1 1 Normal Inverted or (Mass)2 m2atm ~ ~ m2sol = 7.9 x 10–5 eV2, m2atm = 2.4 x 10–3 eV2

9. Generically, grand unified models (GUTS) favor — GUTS relate the Leptons to the Quarks. is un-quark-like, and would probably involve a lepton symmetry with no quark analogue.

10. The Unitary Leptonic Mixing Matrix U l(le e, ll) Ui i Detector The component of i that creates l is called , the neutrino of flavor . The  fraction of i is |Ui|2.

11. sin213 sin213 The spectrum, showing its approximate flavor content, is 2 3 } m2sol 1 m2atm or (Mass)2 m2atm 2 } m2sol 3 1 [|Ui|2] [|U i|2] e [|Uei|2]

12. The Mixing Matrix Solar Atmospheric Cross-Mixing cij cos ijsij sin ij Majorana CP phases 12 ≈ sol ≈ 32°,23 ≈ atm ≈ 36-54°, 13 < 15°  would lead to P() ≠ P().CP But note the crucial role of s13 sin 13. ~

13. Observing Oscillations

14. The Neutrino Study

15. To quote the Charge — • “The Study will lay scientific groundwork for the choices that must be made during the next few years.” • A grassroots study like this, co-sponsored by several APS Divisions, is unprecedented. • It aimed at consensus, which was not a trivial goal. But consensus on key recommendations was achieved!

16. The Structure of the Study • Over 200 Participants • Seven Working Groups • Solar and Atmospheric Neutrino Experiments • John Bahcall, Josh Klein • Reactor Neutrino Experiments • Gabriela Barenboim, Ed Blucher Superbeam Experiments and Development • Bill Marciano, Doug Michael

17. Neutrino Factory and Beta Beam Experiments and Development Stephen Geer, Michael Zisman • Neutrinoless Double Beta Decay and Direct Searches for Neutrino Mass Steve Elliott, Petr Vogel What Cosmology/Astrophysics and Neutrino Physics can Teach Each Other Steve Barwick, John Beacom • Theory Discussion Group Rabi Mohapatra

18. Writing Committee: Hamish Robertson (Chair), Janet Conrad, Andre de Gouvea, Steve Elliott, • Stuart Freedman, Maury Goodman, Boris Kayser, • Josh Klein, Doug Michael • Organizing Committee: Janet Conrad, Guido Drexlin, Belen Gavela, Takaaki Kajita, Paul Langacker, Keith Olive, Bob Palmer, Georg Raffelt, Hamish Robertson, Stan Wojcicki, Lincoln Wolfenstein • Co-Chairpersons: Stuart Freedman, Boris Kayser

19. Our Main Report,The Neutrino Matrix, and the reports of the Working Groups, may be found at – www.aps.org/neutrino

20. Forming a Neutrino Scientific Advisory Group (SAG) to respond • Briefings have been given for — • DOE • NSF • HEPAP • NSAC • P5 • FERMILAB PAC • OSTP • EPP 2010 (National Academy Committee)

21. The Open Questions

22. Neutrinos and the New Paradigm • What are the masses of the neutrinos? • What is the pattern of mixing among the different types of neutrinos? • Are neutrinos their own antiparticles? • Do neutrinos violate the symmetry CP?

23. Neutrinos and the Unexpected • Are there “sterile” neutrinos? • Do neutrinos have unexpected or exotic properties? • What can neutrinos tell us about the models of new physics beyond the Standard Model?

24. Neutrinos and the Cosmos • What is the role of neutrinos in shaping the universe? • Is CP violation by neutrinos the key to understanding the matter – antimatter asymmetry of the universe? • What can neutrinos reveal about the deep interior of the earth and sun, and about supernovae and other ultra high energy astrophysical phenomena?

25. Recommendations for Future Experiments

26. We recommend, as a high priority,acomprehensive U.S. program to — • Complete our understanding of neutrino mixing • Determine the character of the neutrino mass spectrum • Search for CP violation among neutrinos

27. Components of this Program An expeditiously– deployed reactor experiment with sensitivity down to sin2213 = 0.01 A timely accelerator experiment with comparable 13 sensitivity, and sensitivity to the mass hierarchy through matter effects A megawatt-class proton driver and neutrino superbeam with an appropriate very large detector capable of observing CP violation

28. In Pursuit of 13 If sin2213 < 0.01, a neutrino factory will be needed to study both of these issues. Both CP violation and our ability to tell whether the spectrum is normal or inverted depend on 13. How may 13 be measured?

29. sin213 3 m2atm (Mass)2 2 } m2sol 1 sin213 = Ue32 is the small e piece of 3. 3 is at one end of m2atm. We need an experiment with L/E sensitive to m2atm, and involving e.

30. Possibilities Reactore disappearance while traveling L ~ 1.5 km. L/E ~ 500 km/GeV. This process depends on 13 alone. Accelerator  e while traveling L > Several hundred km. L/E ~ 400 km/GeV. This process depends on 13, 23, the CP phase, and on whether the spectrum is normal or inverted.

31. cos213sin223 cos213cos223 e disappearance depends on sin2213.   e depends on sin2213sin223 .  disappearance depends essentially on sin223cos223. sin213 3 m2atm (Mass)2 2 } m2sol 1

32. 1. The Reactor Experiment A relatively modest-scale reactor experiment can cleanly determine whether sin2213 > 0.01, measure it if it is, and help break the2390º – 23degeneracy. Sensitivity: Experiment sin2 213 Present CHOOZ bound 0.2 Double CHOOZ 0.03 (In ~ 2011) Future US experiment 0.01(Detectors at ~200 m and ~ 1.5 km)

33. 2. The Accelerator Experiment • An accelerator  experiment can probe several neutrino properties: • 13 • 23 • Whether the spectrum is normal or inverted • CP violation • Only the U.S. can have baselines long enough to probe whether the spectrum is normal or inverted.

34. E6 GeV Why are long baselines needed? At superbeam energies, matter effects  sin2 2M = sin2 213 [ 1 + S ] . Sign[m2( ) - m2( )] At oscillation maximum, P(e) >1 ; P(e)<1 ; 30% ; E = 2 GeV (NOA) 10% ; E = 0.7 GeV (T2K) ~ (—) (—) { { The effect is

35. Larger E is better. But want L/E to correspond roughly to the peak of the oscillation. Therefore, larger E should be matched by larger L. Using larger L to determine whether the spectrum is normal or inverted could be a unique contribution of the U.S. program.

36. 3. The Proton Driver and Large Detector These facilities are needed if we are to be able to determine whether the spectrum is normal or inverted, and to observe CP violation, for any sin2213 > (0.01 – 0.02).

37. Why would CP in  oscillation be interesting? The most popular theory of why neutrinos are so light is the — See-Saw Mechanism { Familiar light neutrino  } Very heavy neutrino N The heavy neutrinos N would have been made in the hot Big Bang.

38. If neutrino oscillation violates CP, then quite likely so does N decay. Then, in the early universe, we would have had different rates for the CP-mirror-image decays – Nl + … andNl+ + … This would have led to unequal numbers of leptons and antileptons (Leptogenesis). Perhaps this was the original source of the present preponderance of Matter over Antimatter in the universe.

39. The Difference a Proton Driver Can Make

40. The spectral hierarchy without a proton driver (Feldman)

41. The spectral hierarchy with a proton driver (Feldman)

42. CP violation without a proton driver “… one cannot demonstrate CP violation for any delta without a proton driver.” (Feldman) “Without a proton driver, one cannot make a 3 sigma CP discovery.” (Shaevitz)

43. CP violation with a proton driver 90% CL contours for 5 yr  + 5 yr  running (BNL)

44. We recommend, as a high priority, that a phased program of increasingly sensitive searches for neutrinoless nuclear double beta decay (0) be initiated as soon as possible.

45. Observation of 0 would establish that – • Lepton number L is not conserved • Neutrinos are Majorana particles (  =  ) • Nature (but not the Standard Model) contains Majorana neutrino masses. These involve physics different from that which gives masses to the charged leptons, quarks, nucleons, humans, the earth, and galaxies. Then neutrinos and their masses are very distinctive.

46. The Quest for the Origin of Mass Neutrino experiments and the search for the Higgs boson both probe the origin of mass. The see-saw mechanism suggests that the physics behind neutrino mass resides at 1015 GeV, near where Grand Unified Theories say all the forces of nature, save gravity, become one.

47. We recommend the development of a solar neutrino experiment capable of measuring the energy spectrum of neutrinos from the primary pp fusion process in the sun. • Confirm the Mikheyev-Smirnov-Wolfenstein explanation of solar neutrino behavior • Test, at last, whether the pp fusion chain is the only source of solar energy

48. The Context Our recommendations for a strong future program are predicated on fully capitalizing on our investments in the current program: • Accelerator  experiments within the U.S. • American participation in experiments in Antarctica, Argentina, Canada, Germany, Italy, and Japan

49. The current/near-future program should include – • Determination of the 7Be solar neutrino flux to 5%. • Clear-cut confirmation or refutation of LSND. • R&D on techniques for detecting astrophysical neutrinos above 1015 eV. • Measurements of neutrino cross sections needed for the interpretation of neutrino experiments.

50. An Important Observation Future experiments that we feel are particularly important rely on suitable underground facilities. Having these facilities will be crucial.