Neutrinoless Double Beta Decay – Status and Plans FNAL PAC 20 June 2016

Caveats I am not an expert in NLDBD. If I say something wrong or out of date, please speak up. As chair of the Nuclear Science Advisory Committee, I served on the last two NSAC Subcommittees on NLDBD and chaired the Long Range Plan. These reports are good summaries of the field and the issues. I don’t represent the agencies. I will summarize the general plans of DOE and NSF as reported to NSAC.

2015 LRP Recommendations – in brief The exact text is important! Recommendations – Follow the 2007 LRP – Lead a ton-scale neutrinoless double beta decay experiment – Build an Electron Ion Collider after FRIB construction is compete – Increase investment in small and mid scale projects and initiatives Initiatives – Theory and Theory Computing – R&D for the EIC and neutrinoless double beta decay Workforce, Education and Outreach 3

RECOMMENDATION II The excess of matter over antimatter in the universe is one of the most compelling mysteries in all of science. The observation of neutrinoless double beta decay in nuclei would immediately demonstrate that neutrinos are their own antiparticles and would have profound implications for our understanding of the matter-antimatter mystery. We recommend the timely development and deployment of a U.S.-led ton-scale neutrinoless double beta decay experiment. A ton-scale instrument designed to search for this as-yet unseen nuclear decay will provide the most powerful test of the particle-antiparticle nature of neutrinos ever performed. With recent experimental breakthroughs pioneered by U.S. physicists and the availability of deep underground laboratories, we are poised to make a major discovery. This recommendation flows out of the targeted investments of the third bullet in Recommendation I. It must be part of a broader program that includes U.S. participation in complementary experimental efforts leveraging international investments together with enhanced theoretical efforts to enable full realization of this opportunity. 4 Note there is a specific cost estimate associated with this recommendation.

Neutrinoless Double Beta Decay Observation of Neutrinoless Double Beta Decay would Demonstrate the lepton number is not conserved Prove that a neutrino is an elementary Majorana particle, that is, its own antiparticle. Suggest that a new mechanism for mass generation, not the Higgs mechanism, is at work. Provide evidence for one of the key ingredients that could explain the preponderance of matter over antimatter in the universe, leptogenesis. But not a direct connection. 5

Mechanisms Mediated by a light Majorana neutrino Additional emission of hypothetical bosons calls Majorons Exchange of heavy Majorana neutrino Exchange of sterile neutrino All four of these mechanisms require there is a Majorana component to the neutrino mass matrix, but it does not need to be dominant.

8 0  decay Experimental Issues Good energy resolution Low background Majorana Flip helicity: - RH coupling - m ≠ 0

9 NLDBD and Neutrino Mass Phase space Nuclear Matrix Element

This prize follows the 2002 Nobel prize winning work of Davis and Koshiba for detecting cosmic neutrinos. This work sets a minimum mass for the heaviest of the three neutrinos of 58 meV.

NLDBD is Obviously at the Intersection of Nuclear and Particle Physics It Is being done by groups working in both subfields that emphasize detector technologies from both subfields At DOE, neutrinoless double beta decay was declared Nuclear Physics and reactor and accelerator based oscillation neutrino experiments were declared High Energy Physics. NSF has now moved NLDBD out of Particle Astrophysics into Nuclear Physics Other countries divide the space differently

In the light neutrino exchange mechanism the decay rate depends directly on a weighted sum of the masses of light neutrinos. With data from neutrino oscillations which only measure mass differences, we know the expected weighted sum, subject to knowing the mass of the lightest neutrino, which hierarchy is realized in nature, and the new Majorana phases. Goal of ton- scale experiments 12

Other Information on Neutrino Masses Direct measurement in nuclear beta decay: KATRIN aiming for 200 meV Long-Baseline Neutrino Oscillations (HEP): NOVA expects 3 sigma determination of hierarchy in three years. First data release of T2K and Nova provide 1-2 sigma hints of normal hierarchy In both cases there are only a few events and the conclusion requires excellent control of backgrounds. Stay tuned. Cosmology sets limits on the sum of the masses: Current limit: Planck + BAO: < 230 meV implies m ββ < 80 meV Planck + SDSS + BAO +Lyman alpha forest < 208 meV CMB-S4 + DESI project < 15 meV (early 2020’s) These constraints rely on an underlying set of simplifying model assumptions [scale invariance, flatness, w = -1, etc.]. This introduces a level of theoretical model (systematic) uncertainty. Laboratory complementarity is essential. eV sterile neutrinos also can increase the weighted mass sum We need to have double beta decay results on the same time scale as these complementary results. Tension between them can point to other well motivated neutrinoless double beta decay mechanisms or issues in cosmology. 13

New Physics and LHC 15 arXiv: Type-II seesaw within left-right symmetric model

Neutrinoless Double Beta Decay Context We are aiming for U.S. leadership of the most promising ton- scale experiment, and expecting significant international and interagency (DOE, NSF) collaboration. For the highest cost options, we only projected about ~60% funding from U.S. Isotope costs: Ge $100 per gm, Te $17 per gm, Xe $9 per gm. If a positive result is seen, it needs to be confirmed on another isotope and with another technique. We expect secondary U.S. involvement in at least one other international effort would go ahead with a similar time scale. Total integrated U.S. budgets for two projects ~ $250M in $FY15 Ongoing NSAC Subcommittee activities 16

Cosmological Limits Within the ΛCDM model, cosmology probes the neutrino freestreaming scale (neutrino hot dark matter component), which depends on Σ≡Σ i m i and the relic neutrino energy spectra Current combined bound: Σ < 230 meV This corresponds to a smaller bound on m ν ~ meV Projected bounds ( <10 years): Σ < 100 meV (can tell ordering) Presentation to Subcommittee by K. Azerbajian (C Irvine) 17

Major Issue: Background 18 For “background-free” experiment, lifetime sensitivity goes as T 1/2 ~ M·t run (M= isotope mass)  factor of 50 in T 1/2 needs factor of 50 in M (for constant t run ) For experiment with background, as T 1/2 ~ (M·t run ) 1/2  factor of 50 in T 1/2 needs factor of 2500 in M (for constant t run ) Background reduction is the key to a successful program -deep underground -radiopurity -better E resolution -better event characterization  R&D will be crucial

Inverted Hierarchy Coverage19 Figure source: A. Dueck, W. Rodejohann, and K. Zuber, Phys. Rev. D83 (2011) y y y T½T½ 76 Ge 130 Te 136 Xe now 5 yrs ( =17.5meV)

Simple Background Estimate 20 NLDBD Rate = N x ln(2) / T 1/2 (assume T 1/2 ≈ yr) For 1 Tonne, N=10 6 g x 6x10 23 / MW (MW= 67, 130, 136  use MW≈100) So N≈ 6x10 27 NLDBD Rate = 0.4 /Tonne/yr Background free  Background < 0.1/Tonne/yr/ROI No one is close to this so far!

21 Project Isotope Isotope Mass (kg fiducial) Currently Achieved (10 26 yr) CUORE 130 Te206 >0.028 MAJORANA 76 Ge24.7 GERDA 76 Ge18-20 >0.21 EXO Xe79 >0.11 NEXT Xe10 SuperNEMO 82 Se+7 >0.001 KamLAND-Zen 136 Xe434 >1.1 SNO+ 130 Te160 Primary goals: Demonstrate background reduction for next generation experiment Extend sensitivity to T 1/2 ~10 26 years. Current Projects 2016 result

Updated Timeline 22 10/15/15 NSAC Meeting Construction Operation (not time until downselect) Today

Anticipated Plan With the NSAC Subcommittee report on R&D issues, NSF and DOE are planning to submit a joint Funding Opportunity Announcement for R&D, hopefully this summer. NSF has the lead in this. Issuing a joint FOA has taken longer than anticipated. Down-select to primary DOE/NSF funded experiment in 2 ½- 3 years. International competition will be an issue. There are plans, for example, in China to move aggressively.

Application of modern techniques to 0  and 2  - Ab initio methods 1.)Light nuclei to test g A quenching for 0  2.) Develop better effective interactions for heavier nuclei -Better approximations for heavy nuclei 1.)Larger model spaces 2.)Density Functional Theory 3.)Interacting Boson Model Larger and broader group of nuclear theorists interested in working on this problem. This has been approved as a DOE topical collaboration in nuclear theory starting in Nuclear Theory Developments 24

Summary The science is compelling We should not wait for the results from the oscillation experiments or cosmology surveys to move forward. I anticipate we can solve the nuclear matrix element issues to at least better than a factor of two with a focused effort in theory and experiment in the next few years. The backgrounds are extremely challenging. We need to move ahead rapidly, but that always depends on budgets.