Neutrinoless Double Beta Decay II

Michelle Dolinski Drexel University INSS, 16 August 2017 Liquid (organic) scintillators: Crystals: - KamLAND-ZEN (136Xe) - GERDA, Majorana - SNO+ (130Te) Demonstrator, LEGEND (76Ge) - CUORE, CUPID (130Te)

Pros: “Simple”, large detectors Pros: Superb energy resolution, exist, self-shielding possibly 2-parameter Cons: Poor energy resolution, measurement 2ν background Cons: Intrinsically fragmented Low density trackers: Liquid TPC: - NEXT, PandaX (136Xe gas - EXO-200, nEXO (136Xe) TPC) - SuperNEMO (foils and gas tracking, 82Se) Pros: Homogeneous with good Pros: Superb topological E resolution and topology information Cons: Does not excel in any Cons: Very large size single parameter Tour of Experiments, continued Cuoricino to CUORE

CUORE: ~200 kg 130Te CUORE first results

With 3 weeks of physics data, higher exposure than CUORE-0 and Cuoricino and surpassed their limit. Combined analysis of CUORE, CUORE- 0, and Cuoricino.

Cremonesi for the CUORE Collab., TAUP 2017 Beyond CUORE: CUPID

Pirro, TAUP 2017

24 Zn82Se bolometers, for a total mass ≈ 5.1 kg of 82Se

Qββ(82Se) = 2998 keV Challenges of the ton-scale

Shielding a detector from MeV gammas is difficult! Typical 0nββ Q-values Example: γ–ray interaction length in Ge is 4.6 cm, comparable to the size of a germanium detector. Gamma interaction cross section cross interaction Gamma

Shielding 0nββ decay detectors is much harder than shielding dark matter detectors We are entering the “golden era” of 0nbb decay experiments as detector sizes exceed interaction length 7 Monolithic detectors

2.5 MeV γ-ray attenuation length 8.5cm =

5kg 150kg

LXe mass (kg) Diameter or length (cm) 5000kg 5000 130 150 40 5 13 8 Background suppression All observables have a role in separating signal from background. A very large, homogeneous detector has great advantages but only if its energy resolution is sufficient to sufficiently suppress the 2νbb mode.

9 Scintillator-based detectors

KamLAND-Zen

3.9 t natural Te dissolved Up to 800 kg 90% enriched in liquid scintillator in the Xe dissolved in inner upgraded SNO detector volume of KamLAND KamLAND-Zen

Enriched xenon (90% 136Xe) dissolved in scintillator in the inner volume of the KamLAND detector in Japan. 0νββ 26 T1/2 > 1.07 x 10 yr

〈mββ〉 < 61 – 165 meV 11 Gando et al., Phys. Rev. Lett. 117, 082503 (2016) Beyond KamLAND-Zen 800 Higher energy resolution = lower 2n background: KamLAND2-ZEN

Light collection gain Winston cones x1.8 Higher q.e. PMTs x1.9 LAB-based liquid scint x1.4 Overall x4.8

expected s(2.6MeV)= 4% → ~2％ target sensitivity 20 meV

1000+ kg xenon

Super-KamLAND-Zen Beyond? in connection with Hyper-Kamiokande target sensitivity 8 meV But eventually 2n background becomes dominant Ba+ tagging: Overview 136Xe ® 136Ba++ + 2e-

Time projection chamber

1-10 tons Xe

Ba+ tagging would allow for the elimination of all backgrounds other than the background from 2nbb Ba+ tagging: Spectroscopy 136Xe ® 136Ba++ + 2e-

2 P1/2

650nm

493nm

metastable 4 D3/2

+ 2 Ba level structure S1/2

•Ba+ system is well studied. See H. Dehmelt et al. Phys. Rev. A22, 1137 (1980). •Very specific signature with laser induced fluorescence. 14 •Single ions can be detected from a photon rate of 107/s Ba+ tagging: Trap

~9σ discrimination in 5s integration

M.Green et al., Phys Rev A76 (2007) 023404 B.Flatt et al., NIM A578 (2007) 409 Ba+ tagging: RIS probe

Ba+ 5d • Resonant Ionization Spectroscopy uses lasers tuned to atomic 5d8d resonances to first excite and then ionize specific atoms. + 1 Ba 6s P1 • We use pulsed dye lasers at 553.5 nm and 389.7 nm. 1 • Autoionization: The 5d8d P1 state decays to a lower energy 389.7nm ionized state, allowing use of the high cross section of the resonance to achieve ionization.

1 6s6p P1

553.5nm

2 1 6s S0

Efficiency of ~10-3 in “bulk mode” setup. New setup will grab ions from liquid Xe cell. 16 Barium tagging: Solid xenon

Images of few Ba atoms in solid xenon in a laser beam NEXT Collaboration, too!

Ander Simón Estévez, TAUP 2017 NEXT-100 •15 bar high pressure gas Xe time projection chamber (TPC) with ~100 kg fiducial mass. SiPMs (MPPCs) for tracking and PMTs for energy. •Proportional electroluminescent amplification for large photon yield. •Tracking and event topology reconstruction. •Good energy resolution. Demonstrated <0.9% energy resolution achievable at 0nbb Q-value. •Will be sited at the Canfranc laboratory (LSC). Projected 3 year sensitivity of 5x1025 y. NEXT R&D •Multiple prototypes at the ~kg scale (IFIC, LBL, Canfranc). •Study of 60Co calibration data for event topology.

Ander Simón Estévez, TAUP 2017 EXO-200 Liquid Xe TPC

~100 kg fiducial mass Xe enriched to 80% in 136Xe, ultralow background construction. Readout plane is made up of LAAPDs + crossed wire grid. Operating with enriched Xe at the Waste Isolation 21 Pilot Plant since May 2011.

EXO-200 @ WIPP EXO-200 is sited at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, NM, a DOE facility for the disposal of radioactive waste. Provides ~1600 m.w.e. shielding and low U, Th, and Rn. •TPC housed in thin-walled copper vessel. •Vacuum insulated cryostat with HFE-7000 for shielding and thermal bath. •Lead shield. •Clean room environment. •Active muon veto. •Other support systems not shown here (refrigeration, gas handling, etc.). Operated with enriched Xe from May 2011 to Feb. 2014 (Phase I)

23 Upgraded detector running since June 2016 (Phase II) 2nbb precision measurement

2nbb signal to background ratio: 11: 1

Inner 40% fiducial volume signal to background ratio: 19: 1

Most precise measurement of the 2nbb half-life 2nbb 21 T1/2 = 2.165 ± 0.016(stat) ± 0.059(sys) × 10 yr [PRC 89, 015502 (2014)] Energy measurement

Scintillation vs. ionization, 228Th calibration: Reconstructed energy, 228Th calibration:

ALPHA CUT keV = 2458 2458 = ββ Q

• Anticorrelation between scintillation and ionization in LXe known since early EXO R&D [E.Conti et al. Phys Rev B 68 (2003) 054201] • Rotation angle determined weekly using 228Th source data, defined as angle which gives best rotated resolution • EXO-200 has achieved ~ 1.25% energy resolution at the double-beta decay Q value in Phase II. Position and multiplicity Allows for background measurement and reduction

Events with > 1 charge cluster: multi-site events Events with 1 charge cluster: single-site events.

0nbb: ~90% SS g-rays: ~20% SS at 0nbb Q-value

228Th calibration data, SS: 228Th calibration data, MS:

Data Monte Carlo Counts/20 keV Counts/20 keV

Rotated energy [keV] Rotated energy [keV] Improved g-background Rejection

Additional discrimination in SS using spatial distribution and cluster size

LXe self-shielding:

Entering g-rays are exponentially attenuated by LXe self-shielding, 2.5MeV γ providing an independent measurement attenuation of g-backgrounds. We call this standoff length: 8.5cm = distance. Single Site Events (SS) Multiple Site Events (MS) Single Site EventsLong rise (SS) time Multiple Site Events (MS)

γ Collection signalγ The size of individual events is in 2 wires estimated from pulse rise time γ γ (longitudinal direction) and the number of wires with a charge collection signal β (transverse). β Collection signal in 1 wire x/yu z/timet u t u Short rise timet u t Optimal 0nbb Discrimination • Optimize SS discriminators into a more powerful one

• Using a boosted decision tree (BDT) between 0νββ and main γ- ~35% γ-rejection Data (dot)backgrounds vs MC (line) ~90% signal efficiency

226Ra 2νββ 0νββ

• Fitting 0νββ discriminators SS-fraction SS events MS events • Energy

• SS/MS Qββ Energy Energy • BDT à ~15% sensitivity improvement Results • Background model + data à maximum likelihood fit • Combine Phase I + Phase II profiles Systematics Phase I Phase II (%) (%) Detection 82.4 ± 3.0 80.8 ± 2.9 efficiency Shape differences ±6.2 ±6.2 SS fraction ±5.0 ±8.8

• No statistically significant excess: combined p-value ~1.5σ Energy of Interest

Contributions Phase I (cts) Phase II (cts) to bkg ±2s

232Th 15.8 4.8 238U 9.4 4.2 137Xe 4.4 3.6 Total 30.7±6.0 13.2±1.4 Data 43 8

0n discriminator

• Background index ~ 1.5±0.2 x10-3 counts/(kg yr keV)

• Component contributions • 232Th reduction consistent with difference in resolution • 137Xe rejection ~25% Sensitivity & Limits • Combined analysis: • Total exposure = 177.6 kg.yr

Sensitivity of 3.7x1025 yr (90% CL) 0nbb 25 T1/2 > 1.8 x 10 yr

〈mbb〉 < 147 – 398 meV (90% C.L.)

• Individual phase limits

Livetime Exposure Limit (90% CL)

0nbb 25 Phase I 596.7 d 122.0 kg.yr T1/2 > 1.0x10 yr 0nbb 25 Phase II 271.8 d 55.6 kg.yr T1/2 > 4.4x10 yr

Caio Licciardi, TAUP 2017 and arXiv:1707.08707 Other recent papers

Rare event searches

• First Search for Lorentz and CPT Violation in Double Beta Decay with EXO-200, J.B. Albert, et al. Phys. Rev. D 93, 072001 (2016).

136 + 136 • Search for 2nbb decay of Xe to the 01 excited state of Ba with the EXO-200 liquid xenon detector, J.B. Albert, et al. Phys. Rev. C 93, 035501 (2016).

• Search for Majoron-emitting modes of double-beta decay of 136Xe with EXO-200, J.B. Albert, et al Phys. Rev. D 90, 092004 (2014).

Detector physics

• Measurement of the Drift Velocity and Transverse Diffusion of Electrons in Liquid Xenon with the EXO-200 Detector, J.B. Albert, et al. Phys. Rev. C 95, 025502 (2017).

• Measurements of the ion fraction and mobility of alpha and beta decay products in liquid xenon using EXO-200, J.B. Albert, et al. Phys. Rev. C 92, 045504 (2015). Majoron modes in Xe Majoron Search in EXO-200

Phys. Rev. D 90, 092004 (2014) Current best 0nbb sensitivities

Isotope Experiment Exposure Reference (kg yr)

76 GERDA 46.7 5.8 >8.0 <120-270 L. Pandola for Ge GERDA Collab, TAUP 2017 130 CUORE 38.1 >0.66* <210-590 O. Cremonesi for Te CUORE Collab, TAUP 2017 EXO-200 177.6 3.7 >1.8 <147-398 Albert et al. arXiv: 1707.08707 (2017) 136Xe KamLAND- 504** 4.9 >11 <60-161 Gando et al., PRL 117 (2016) 082503 ZEN (run 2)

Note that the range of “viable” NME is chosen by the experiments and uncertainties related to gA are not included ** All Xe. Fiducial Xe is more like ~150 kg yr Comparison across isotopes

Current limits, 76Ge vs. 136Xe Current limits, 130Te vs. 136Xe

EXO-200: this result, arXiv: 1707.08707 EXO-200: this result, arXiv: 1707.08707 New Results by GERDA: talk by L. Pandola New Results from CUORE: talk by O. Cremonesi KamLAND-Zen: PRL 117 (2016) 082503 Sensitivity in PRL 115 (2015) 102502 KK&K Claim: Mod. Phys. Lett., A21 (2006) 1547 EXO-200 to nEXO

130 Preliminary artist view of nEXO cm in the SNOlab Cryopit

nEXO TPC 46 EXO-200 cm A 5000 kg enriched LXe TPC, TPC based on success ofEXO-200 nEXO discovery potential

27 nEXO 10 year discovery potential at T1/2=5x10 yr nEXO sensitivity

Baseline design assumes: • Existing measured materials • 1% s/E energy resolution • Factor of two improvement in SS/MS discrimination The future? What does come next? Let’s assume we see it…

• The next generation of neutrinoless double beta decay experiments may detect 0nbb at mass scales of mbb ~100 meV. • If they do, precision experiments to nail down the nuclear physics and the mechanism! •First, precision measurements in multiple isotopes to nail down the nuclear physics. •Angular correlations to try to understand the mechanism. • Low pressure gas TPC or NEMO-like experiment to study angular correlations. Need lots of decays, so this is really hard. •Mechanism would be further constrained by observation of neutrinoless EC/b+ or double EC and complementary searches for other lepton number violating phenomena. •Finally, Majorana phases?? (Not likely!) But if we rule out the IH…

Accessing the normal hierarchy is really tough! • Need good energy resolution. • Need ~10 tons or more of enriched isotope, so scalability is essential. • Need heroic background rejection. Self shielding will help! Summary

• Neutrino masses are an open window into physics beyond the Standard Model. • Majorana neutrino masses may be the key to understanding the matter-antimatter asymmetry of the universe. • Neutrinoless double beta decay is the most sensitive experimental probe of whether neutrinos have Majorana masses. • There is a varied experimental program to search for neutrinoless double beta decay. • We need nuclear AND particle theorists, too!