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How Will We See Leptonic CP Violation?

How Will We See Leptonic CP Violation?. D. Casper University of California, Irvine. Will We See Leptonic CP Violation?. Matter asymmetry of the universe likely tied to CP-violation (and baryon number non-conservation) Hadronic CP violation seems too small to account for matter asymmetry

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How Will We See Leptonic CP Violation?

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  1. How Will We See Leptonic CP Violation? D. CasperUniversity of California, Irvine

  2. Will We See Leptonic CP Violation? • Matter asymmetry of the universe likely tied to CP-violation (and baryon number non-conservation) • Hadronic CP violation seems too small to account for matter asymmetry • Hadronic mixings and CP violation are small • Leptonic mixing angles are large… • …maybe leptonic CP violation is also large?

  3. The Prerequisite: 13 • CP violation requires three-flavor mixing • All three mixing angles enter the CP-violating term • All angles must be non-zero • 12 and 23 are large • Observing leptonic CP violation requires observing non-zero 13

  4. The Search for 13: CHOOZ • CHOOZ reactor experiment final results (1999) • Limit on 13 ~ 11° • sin2 213 < ~ 0.1 • Best current limit in atmospheric mass region

  5. The Search for 13: Atmospheric Inverted hierarchy Normal hierarchy Super-Kamiokande three-flavor analysis (Little prospect of reaching significantly beyond CHOOZ)

  6. sys=2.5% sys=0.6% Far detector only Far & Near detectors together 05/2007 05/2008 05/2009 05/2010 The Search for 13: Reactors • Several reactor experiments proposed to search for 13: • Double CHOOZ • Daya Bay • Braidwood • All hope to improve on CHOOZ (disappearance) sensitivity • Typical sensitivities:sin2 2 13 ~ 0.03 • Double CHOOZ hopes to reach this by December 2010 • No sensitivity to … Dazeley, NUFACT 2005

  7. The Search for 13: Superbeams • Exploit off-axis “trick” to create narrow-band beam without losing signal • T2K • Approved • Funded in Japan • Beam under construction • Detector (SuperK) exists • NOA • Approved by PAC • Not yet funded (~$200M?) • Beam exists • 50 kt liquid scintillator detector design • Begin construction in one year? • Fully operational July 2011? Yamada, NUFACT 2005 Nelson, NUFACT 2005

  8. CP Violation in Neutrino Oscillation • CP violation is manifest in differences between neutrino and anti-neutrino oscillation probabilities • Unfortunately matter effects are also CP violating • Matter effects in turn depend on the mass hierarchy • CP violation does not affect disappearance channels • These differences are typically a few percent

  9. Detector Challenges • Since CP violation causes small changes in probability, large data samples are required to measure them • Big detectors… • Expensive detectors…

  10. Matter Effects and Degeneracies • Observable oscillation probabilities may not uniquely determine the physical parameters • Parameter degeneracies • 13 -  • sgn(m232) • octant of 23

  11. Systematics • 1% measurements require careful control of systematics • To find CP violation, must compare neutrinos and anti-neutrinos (different cross-sections) • Anti-neutrino beams contain significant contamination from neutrino interactions • Conventional neutrino beams difficult to predict accurately • CC interactions and backgrounds are different in near and far detectors, due to oscillation • Your near detector cannot easily measure cross-sections for the appearance signal

  12. Superbeams? Nelson, NUFACT 2005

  13. A Neutrino Factory? • A neutrino factory (20-50 GeV muon storage ring) is the ultimate tool for studying neutrino oscillation • Wrong-sign muon appearance • Potential step toward muon collider • Serious technical and cost challenges… • Important R&D ramping up • MICE • MUCOOL • nTOF11 • … P. Huber, NUFACT 2005

  14. A Betabeam? • The idea: accelerate and store -unstable ions to create a pure electron-flavor beam • -: 6He • +: 18Ne • Shares many advantages of neutrino factory: • Spectrum is ~perfectly known • Flux is ~perfectly known • Muon appearance • Can in principle run neutrinos and anti-neutrinos simultaneously • Near and far spectra nearly identical • No secondary beam cooling/reacceleration • Technically, a much simpler problem P. Zucchelli, Phys.Lett.B 532, 166-172 (2002)

  15. Ion production Acceleration Neutrino source Experiment Proton DriverSPL Acceleration to final energy PS & SPS Ion production ISOL target & Ion source SPS Neutrino Source Decay Ring Decay ring Br = 1500 Tm B = 5 T C = 7000 m Lss = 2500 m Beam preparation Pulsed ECR PS Ion acceleration Linac Acceleration to medium energy RCS CERN Betabeam Concept M. Lindroos, NUFACT 2005

  16. Low-Energy Betabeam • Initial studies focused on low- scenario at 150 km baseline • Reduce backgrounds by sitting near  threshold • No energy dependence available • Counting experiment • Low boost reduces focusing and flux Sensitivity to distinguish =0° from =90°at 99% CL: betabeam and betabeam plussuperbeam, compared to NUFACT andand T2K M. Mezzetto, J.Phys.G 29, 1771-1776 (2003) [hep-ex/0302007]

  17. A Higher-Energy Betabeam • New approach: higher energy, longer baseline •  ~  • Exploit energy dependence • Increase flux with more focusing • More cross-section at higher energy • NC backgrounds still manageable =60/100, 150 km, 400 kt H2O =350/580, 730 km, 40 kt H2O =350/580, 730 km, 400 kt H2O Region where  can be distinguished from =0 and =90 at 99% CL J.Burguet-Castell, D. Casper, J.J. Gomez-Cadenas, P.Hernandez, F. Sanchez, Nucl.Phys.B 695, 217-240 (2004) [hep-ph/0312068]

  18. Optimizing the Betabeam • Relax baseline and boost constraints to maximize 13 and  sensitivity • Setup 0: • Original Frejus, low- • Setup 1: • Optimal Frejus (=120) • Setup 2: • Optimal SPS(L=350 km, =150) • Setup 3: • Optimal betabeam(L=730 km, =350) Region of the 13 -  plane where we can determine at 99% CL that 13  0 J. Burguet-Castell, D. Casper, E. Couce, J.J. Gomez-Cadenas, P. Hernandez,Nucl.Phys.B 725, 306-326 (2005) [hep-ph/0503021]

  19. Optimized Betabeam CP Sensitivity • For optimal betabeam •  sensitivity ~ 10° • 13 sensitivity ~ 10-4 • Also sensitive to sgn(m223) and octant of 23 • If T2K sees non-zero 13, measure  • If T2K sees no signal, extend 13 sensitivity by another factor of 10 • Proton decay sensitivity ~1035 years (e+ 0) Region of the 13 -  plane where wecan distinguish  from =0 and =180at 99% CL for any best-fit value of 13 (i.e. that there is leptonic CP violation)

  20. TeV? • Our optimization studies show that increasing the Lorentz boost optimizes the sensitivity of the beta-beam • Two feasible sites for  ~ few hundred: • CERN-SPS (possibly with upgrade) • Tevatron • Need Fermilab feasibility study to estimate realistic costs • Similar to neutrino factory study • An opportunity for the decisive neutrino oscillation experiment!

  21. A Mono-energetic Beam? • Accelerate an ion that decays by electron capture • Two-body final state • Monoenergetic  • A challenge • Ions cannot be completely stripped • Finite survival time in partially ionized state • Must decay rapidly • Must have small enough Q value • 150Dy • Short decay time (~7 minutes) • 1.4 MeV neutrino in rest frame • 0.1% -decay J. Bernabeu, J. Burguet-Castell, C. Espinoza, M. Lindroos, hep-ph/0505054

  22. Conclusion • Seems reasonable to expect leptonic CP violation • The most challenging neutrino physics measurement ever attempted • A betabeam at Fermilab could be the decisive, complementary follow-on to T2K

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