Resolving Neutrino Mass Hierarchy using Nuclear Reactor(s) Wei - PowerPoint PPT Presentation

Resolving Neutrino Mass Hierarchy using Nuclear Reactor(s) Wei Wang, College of William and Mary Invisibles’13, Lumley Castle, July 16, 2013 •A review of MH via reactors and key challenges •Sensitivity studies using JUNO as an example •Subtleties in statistics •Status of the field •Summary

Nuclear Reactors and Neutrinos courtesy : Karsten Heeger 0 Far / Near (weighted) 0 5 10 No Oscillation 2012 - Observation of short baseline 1.2 Best Fit reactor electron neutrino disappearance 1 0.8 0 5 10 Efficiency (%) Prompt Energy (MeV) Daya Bay 2008 - Precision measurement of Δ m 122 . Evidence for oscillation KamLAND data no oscillation 250 best-fit osci. accidental Events / 0.425 MeV 13 16 2003 - First observation of reactor 200 C( α ,n) O best-fit Geo ν e best-fit osci. + BG antineutrino disappearance 150 + best-fit Geo ν e 100 1995 - Nobel Prize to Fred 50 KamLAND Reines at UC Irvine 0 0 1 2 3 4 5 6 7 8 E (MeV) p 1980s & 1990s - Reactor neutrino flux Past Reactor Experiments measurements in U.S. and Europe Hanford Savannah River 1956 - First observation ILL, France of (anti)neutrinos Bugey, France Rovno, Russia Goesgen, Switzerland Chooz Krasnoyark, Russia Palo Verde Chooz Chooz, France KamLAND, Japan Double Chooz, France Reno, Korea 70 Karsten Heeger, Univ. of Wisconsin ORNL, July 5, 2012 Daya Bay, China Savannah River Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 2

The Gate to Mass Hierarchy is Open How to resolve neutrino mass hierarchy • using reactor neutrinos – KamLAND (long-baseline) measures the solar sector parameters – Short-baseline reactor neutrino experiments designed to utilize the oscillation of atmospheric scale ✓ Both scales can be probed by observing the spectrum of reactor neutrino flux Petcov&Piai, arXiv:0112074 70 ν e = 1 − cos 4 θ 13 sin 2 2 θ 12 sin 2 ∆ 21 L~20km 60 P ¯ N Ν � arb. units � ν e → ¯ 50 40 − sin 2 2 θ 13 (cos 2 θ 12 sin 2 ∆ 31 + sin 2 θ 12 sin 2 ∆ 32 ) 30 20 ✓ The mass hierarchy is contained in the spectrum 10 ✓ Independent of the unknown CP phase 2 3 4 6 7 8 5 E Ν � MeV � • the value of sin 2 θ , which controls the magnitude of the sub-leading effects due to ∆ m 2 31 on the � − driven oscillations: the effect of interest vanishes in the decoupling limit of sin 2 θ → 0 ; ∆ m 2 Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 3

Fourier Transformation to Extract Mass Hierarchy • Treating L/E as the time domain, the frequency domain simply corresponds to Δ m 2 • In the Δ m 2 domain, take Δ m 2 32 as the reference point, - NH: take “+” sign, the effective Δ m 2 peaks on the right of Δ m 2 32 , then a valley - IH: take “-” sign, the effective Δ m 2 peaks on the left of Δ m 2 32 , right to a valley • Δ m 2 spectra have very distinctive features for different hierarchies • In principle, no need for the absolute value of Δ m 2 L. Zhan et al., PRD78(2008)111103 32 J. Learned et al proposed the FT method 2006 Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 4

Reading the Signal in Another Way 12 sin 2 ∆ 21 ν e = 1 − 2 s 2 13 c 2 13 − 4 c 4 13 s 2 12 c 2 P ¯ ν e → ¯ q 12 sin 2 ∆ 21 cos(2 ∆ 32 ± φ ) +2 s 2 13 c 2 1 − 4 s 2 12 c 2 13 c 2 12 sin 2 ∆ 21 1 . 27 · E φ ∆ m 2 φ ( L, E ) = tan φ = c 2 12 cos 2 ∆ 21 + s 2 L 12 -3 Reading it from a different 10 • × 100 L (km) perspective gives us, the 0.16 experimentalists, a few 80 obvious catches 0.15 – Δ m 232 uncertainty is too big 60 0.14 for the small differences caused by different mass 40 0.13 hierarchies. The shift can be easily absorbed by the 0.12 20 uncertainty – Energy resolution squeeze 0.11 the “useful” part from the left 2 4 6 8 10 E (MeV) Q. Xin et al, arXiv:0112074 vis Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 5

The Energy Resolution Requirement In order to see the • L=50 km NH 6000 IH atmospheric scale oscillations Best Fit to NH data 5000 δ E vis /E vis = 0 in the survival spectrum, to 4000 dN / dE ν [1/MeV] the first order, the energy 3000 resolution should be at least 2000 the ratio between solar mass- 6000 squared difference and the 5000 δ E vis /E vis = 6%/ √ E vis atmospheric one is ~3% 4000 3000 r a 2 + b 2 E + c 2 2000 ∆ E 2 3 4 5 6 E = E 2 E ν [MeV] S.F. Ge et al, arXiv:1210.8141 at total visible energy E , a leakage & Photon Noise non-uniformity statistics (dominant). Needs <3% Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 6

Give The MH Signal a Closer Look 40000 It is obvious that the • S.F. Ge et al, arXiv:1210.8141 30 km NH 30000 IH baseline is better 20000 10000 beyond 30km 14000 40 km NH Practically speaking IH • dN / dE ν [1/MeV] 10000 6000 (for real experiments), 2000 the power lies in the 7000 50 km NH IH contrast between the 5000 3000 lower part and the 1000 higher part of the 60 km NH 4000 IH 3000 inverse beta decay 2000 spectrum 1000 0 2 3 4 5 6 7 8 E ν [MeV] At the energy where the effective mass-squared difference shift disappears, • NH and IH spectra are identical. Below and above this energy, the phase difference between NH and IH shift in different direction. Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 7

Energy Scale Places Another Challenge S.J. Parke et al, arXiv:0812.1879 Q. Xin et al, arXiv:1208.1551 2 2 NH: | ∆ m | = 2.43e-3 eV Ideal Spectrum 100 kTyear 1500 1500 Ideal Spectrum 100 kTyear 1500 32 Events per 0.08 MeV 100 kTyear 2 2 2 2 NH: | ∆ m | = 2.43e-3 eV NH: | m | = 2.43e-3 eV ∆ 32 32 2 2 IH: | ∆ m | = 2.55e-3 eV 32 2 2 2 2 IH: | ∆ m | = 2.55e-3 eV IH: | ∆ m | = 2.43e-3 eV 100 kTyear Ideal Spectrum 32 32 1000 1000 1000 500 500 500 0 0 0 2 4 6 8 2 4 6 8 2 4 6 8 E (MeV) E (MeV) E (MeV) vis vis vis Ratio of NH/IH Ratio of NH/IH Ratio of NH/IH 1.15 1.15 1.3 Events per 0.08 MeV 1.1 1.2 1.1 1.05 1.1 1.05 1 1 1 Figure 4. The percentage di ff erence between the 0.95 0.9 0.95 inverted hierarchy and the normal hierarchy. The 0.8 0.9 blue curve is assuming E obs = E true and max- 0.9 imum di ff erence is less than 2%. Whereas for 0.7 0.85 0.85 2 4 6 8 2 4 6 8 2 4 6 8 the red curve we have assumed that E obs = E (MeV) E (MeV) E (MeV) vis vis 1 . 015 E true − 0 . 07 MeV for the IH, so as to repre- vis sent a relative calibration uncertainty in the neu- Oscillation is governed by ~ Δ m 232 /E, thus they • trino energy. Here the maximum percentage dif- ference is less than 0.5%. have the same role Uncertainty in Δ m 232 causes nearly degenerated • spectra between NH and IH Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 8

Degenerated Spectrum • Recall the survival probability 12 sin 2 ∆ 21 ν e = 1 − 2 s 2 13 c 2 13 − 4 c 4 13 s 2 12 c 2 P ¯ ν e → ¯ Could there be identical q 12 sin 2 ∆ 21 cos(2 ∆ 32 ± φ ) +2 s 2 13 c 2 1 − 4 s 2 12 c 2 oscillation patterns? 13 2 | ∆ 0 m 2 32 | + ∆ m 2 φ ( E ¯ ν e , L ) E rec = ν e , L ) E real . 2 | ∆ m 2 32 | � ∆ m 2 φ ( E ¯ The current uncertainty in • atmospheric mass-squared real Q. Xin et al, arXiv:1208.1551 IH IH E /E difference, combined with /E rec real rec a non-linear energy 1.02 NH NH E E /E response, would create the rec real same survival spectrum for 1 both mass hierarchies. No way to resolve MH if • 0.98 the non-linear energy response allows such curves 0.96 2 4 6 8 10 E (MeV) vis Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 9

Practical Energy Scale Issues Related to Reactor MH Experiments ν e + p → e + + n Inverse beta decay: ¯ We need “free” protons and we need photons, the more the better • ➡ Liquid scintillator detector seems the ideal choice: protons (H), many photons, and cheap. It turned out to be this is the choice of all current proposals. ➡ But liquid scintillator has a notorious feature: energy non-linearity due to quenching and Cherenkov lights ➡ Based on past/current understanding, the “convenient” non-linearity curve which could cause degeneracy follows a similar shape to the liquid scintillator energy response. ➡ There could be difficulties in resolving MH due to the C. Zhang, Los Alamos seminar on Daya Bay non-linearity feature of LS Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 10

Challenges in Resolving MH using Reactor Flux Energy resolution • Y.F. Li et al, arXiv:1303.6733 Energy non-linearity • Statistics • Reactor distribution • – The mass hierarchy information is in the multiple atmospheric oscillation cycles in the survival spectrum. For the valuable part of the spectrum ~3.5MeV, the oscillation length is ~3.5km. – Thus, if two reactor cores with equal or close powers differ by half oscillation length, the mass hierarchy signal will get cancelled. • What is the status of the field? – JUNO (Jiangmen Underground Neutrino Observatory, previously dubbed as Daya Bay II) in China. Stealing slides from Yifang Wang et al from IHEP – RENO-50 in South Korea. Stealing slides from RENO-50 collaborators Wei Wang W&M Mass Hierarchy using Reactors, Invisibles’13 11