Two Neutrino Double Beta (2νββ) Decays into Excited States

Two Neutrino Double Beta (2νββ) Decays into Excited States International School of Subnuclear Physics 54 th Course: The new physics frontiers in the LHC-2 era Erice, 17/06/2016 Björn Lehnert TU-Dresden, Germany Carleton University, Canada

Neutrinoless Double Beta (0νββ) Decay proton number Z 2 :(Z, A)! (Z +2,A)+2e +2 e 0 :(Z, A)! (Z +2,A)+2e 76 Ge: 2039 kev (T1/2 = 1.9 x 10 21 yr) (T1/2 > 2.1 x 10 25 yr) in 76 Ge www.nndc.bnl.gov neutron number N - Lepton Number Violation (LNV) - Neutrinos are Majorana particles - Indication of neutrino mass Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 2

T 0 1/2 M Standard Mechanism of 0νββ Decay 1 = G 0 M 0 2 m ee 2 measured half-life phase space factor nuclear matrix element effective Majorana neutrino mass atomic physics nuclear physics particle physics Nuclear Matrix Elements (NME) are model dependent and have large theoretical uncertainties (factor 2-3) mlightest [ev] arxiv:0812.0479v2 (2009) Conversion of T1/2 into neutrino mass dominated by uncertainty of NME calculation Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 3

Non-Standard Mechanism of 0νββ Decay Possible processes (not exhaustive ) PHYSICAL REVIEW D 83, 113003 (2011) light Majorana Higgs triplet SUSY particle right handed currents If 0νββ decay is discovered, the LNV mechanism is not clear Other approaches e.g. LHC 0νββ observation in multiple isotopes can help to disentangle the mechanism Strong motivation for different DBD experiments / isotopes NMEs have to be knows precisely Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 4

Improvement of Nuclear Matrix Elements 0 + 76 Ge Qββ=2039.1 kev 2-26.3 h 76 As 2039.1 kev 2 : 0 : 2 + 2 0 + 1 2 + 1 T 2 1/2 T 0 1/2 1 = G 2 M 2 2 1 = G 0 M 0 2 m ee 2 563.2 36% 64% 657.0 1216.1 kev 1122.3 kev 559.1 kev 0 + 76 Se 559.1 1216.1 0 kev Measurement of 2νββ decays allow direct test of NME calculations Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 5

Improvement of Nuclear Matrix Elements 0 + 76 Ge Qββ=2039.1 kev 2-26.3 h 76 As 822.0 kev 916.8 kev 1480.0 kev 2 : 0 : 2 + 2 0 + 1 2 + 1 T 2 1/2 T 0 1/2 1 = G 2 M 2 2 1 = G 0 M 0 2 m ee 2 563.2 36% 64% 657.0 1216.1 kev 1122.3 kev 559.1 kev 0 + 76 Se 559.1 1216.1 0 kev Decays into excited states give an additional possibility to test NME calculations and can constrain nuclear model parameters (ga, gpp in QRPA) So far 2νββ 0 + 1 transition discovered in 100 Mo (1995) and 150 Nd (2004) Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 6

2νββ Excited States: Overview Experimental approaches: Direct theoretical calculation with various nuclear models Often very inconsistent Use ratio of NME and PSF which cancels some uncertainties (e.g. ga) Then scale with measured ground state T1/2 Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 7

2νββ Excited States: Overview Experimental approaches: Direct theoretical calculation with various nuclear models Often very inconsistent Use ratio of NME and PSF which cancels some uncertainties (e.g. ga) Then scale with measured ground state T1/2 Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 8

GERDA = Germanium Detector Array Low background experiment: Reducing environmental radioactivity HPGe detectors in liquid argon (LAr): Shielding, cooling, active scintillation veto clean room lock system water tank HPGe detector array LAr cryostat Phase I: Nov. 2011 till May 2013 Phase II: Since Dec. 2015 Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 9

Laboratori Nazionali del Gran Sasso in Italy underground labs outside lab 1400m overburden (3500 m.w.e.) muon flux suppressed by factor 106 L Aquila GERDA Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 10

Analysis: 76 Ge 2νββ Decay into Excited States most probable transition experimental sensitivity reaches predictions Best previous experimental result: Theoretical predictions for 2νββ 0 + 1 2νββ 0 + 1: T1/2 > 6.2 10 21 yr nuclear model year T1/2 JETP Lett. 72 (2000) Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States HFB 1994 1.3 10 21 yr QRPA 1994 4.0 10 22 yr QRPA 1996 4.5 10 22 yr QRPA 1996 7.5 10 21 yr QRPA 1997 (1.0-3.1) 10 23 yr QRPA 2014 (1.2-5.8) 10 23 yr IBM-2 2014 6.4 10 24 yr ShM 2014 (2.3-2.6) 10 24 yr 11

Analysis: 76 Ge 2νββ Decay into Excited States most probable transition experimental sensitivity reaches predictions 2-Detector coincidences Phase I data set: 2700 2-detector events 2νββ Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 12

Analysis: 76 Ge 2νββ Decay into Excited States Signal Monte Carlo (Geant4) Background Monte Carlo Cut and count analysis (background from sidebands) Signal selection cuts: 1 of 2 detector has one of the ɣ-energies Other detector above certain threshold Background rejection cuts: Excluding background ɣ-lines ( 42 K, 214 Bi, 108m Ag) Select only detector-detector pairs which contribute to sensitivity Tuning ad-hoc cut parameters to maximize sensitivity Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 13

Analysis: 76 Ge 2νββ Decay into Excited States Signal Monte Carlo (Geant4) Background Monte Carlo Cut and count analysis (background from sidebands) Signal selection cuts: 1 of 2 detector has one of the ɣ-energies Other detector above certain threshold Background rejection cuts: Excluding background ɣ-lines ( 42 K, 214 Bi, 108m Ag) Select only detector-detector pairs which contribute to sensitivity Tuning ad-hoc cut parameters to maximize sensitivity Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 14

Analysis: 76 Ge 2νββ Decay into Excited States signal- region sideband region for background evaluation all 2- detectorevents background model 5 events in signal region, 8.5 events expected No signal observed Detection efficiency 0.91%, exposure 22.3 kg yr Lower half-life limit: T1.2 > 3.7 10 23 yr (90% CL) Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 15

Analysis: 76 Ge 2νββ Decay into Excited States Experimental results decay mode 2νββ 2 + 1 2νββ 0 + 1 2νββ 2 + 1 T1/2 (90% CL) >1.6 10 23 yr >3.7 10 23 yr >2.3 10 23 yr J. Phys. G: Nucl. Part. Phys. 42 (2015) 115201 Theoretical predictions for 2νββ 0 + 1 nuclear model year T1/2 HFB 1994 1.3 10 21 yr QRPA 1994 4.0 10 22 yr QRPA 1996 4.5 10 22 yr QRPA 1996 7.5 10 21 yr QRPA 1997 (1.0-3.1) 10 23 yr QRPA 2014 (1.2-5.8) 10 23 yr IBM-2 2014 6.4 10 24 yr ShM 2014 (2.3-2.6) 10 24 yr Half-life limits improved by 2 orders of magnitude Most old NME calculations excluded for 0 + 1 mode Sensitivity in Phase II of T1/2 > 10 24 yr for 0 + 1 mode Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 16

Conclusions 0νββ decay tests LNV and Majorana nature of neutrinos Standard interpretation: Dominant light Majorana neutrino exchange Access to neutrino mass (mee) Non-standard interpretation: Other LNV processes dominant or mixture NMEs crucial to connect T1/2 and mee (or LNV parameter) Currently large uncertainties for NMEs (factor 2-3) Improvement with 2νββ decay measurements (ground or excited states) Analysis for 76 Ge with GERDA Phase I data 2 orders of magnitude improvement Many older NME calculations excluded New generation of 0νββ will soon discover more 2νββ 0 + 1 transitions Erice, 17/06/16 Bjoern Lehnert 2νββ into Excited States 17

Backup 18

Experimental Overview of Neutrino Masses Beta-decay KATRIN, MARE, Echo Limit: m < 2.3 ev (model independent) Cosmology Planck, LSST,... Limit: m < 0.28 ev (model dependent) Neutrinoless DBD GERDA, EXO, KamLAND- ZEN,... Limit: m < 0.14-0.38 ev (model dependent) 19

Schechter Valle Standard process: Light Majorana neutrino exchange There are also other lepton number violating processes that can trigger 0νββ Schechter-Valle theorem: If 0νββ exists, it can always be interpreted as a neutrino Majorana mass term Contribution of Majorana mass terms to neutrino mass might be very small 20

NME only 1 + : only GT all states: GT + Fermi 21

Other DBD Isotopes decay final state particles candidate 2νβ - β - (Z,A) (Z+2,A) + 2e - + 2ν - 35 2νECEC (Z,A) + 2e - (Z-2,A) + 2ν 34 2νβ + EC (Z,A) + e - (Z-2,A) + e + + 2ν 19 2νβ + β + (Z,A) (Z-2,A) + 2e + + 2ν 6 Isobar A=76 Sr mass excess (MeV) Zn Ga Ge As Br Se Rb Kr protons Z 22

DBD Isotopes Effective neutrino mass: (only for dominant light Majorana neutrino exchange) G 0 phase space factor (atomic physics) matrix element (nuclear physics) Beyond SM process (particle physics) 0νββ half-life (mee = 1eV) PHYSICAL REVIEW C 87, 014315 (2013) 23

0νββ Global Picture 24

Sensitivity and Experimental Approaches Sensitivity: (for Gaussian background) Advantage 76 Ge: Well known HPGe detector technology Excellent energy resolution O(0.1%) Intrinsic low background (Semiconductor) Disadvantages 76 Ge: Expensive enrichment (7.8% to 87%) Difficult to scale Q-value < 2614 kev of 208 Tl 25

The$GERDA$Collabora0on$ h5p://www.mpi<hd.mpg.de/gerda/% INR% Moscow% ITEP% Moscow% Kurchatov%% Ins3tute% Laboratori Nazionali del Gran Sasso 16$ins0tu0ons$ ~100$members$ 26

GERDA Timeline Commissioning Phase I Transition to Phase II Phase II 2010 Nov 2011 May 2013 Dec 2015 very early GERDA spectrum mini-shrouds (MS) Larger 42 Ar contribution than expected 1.5 years of commissioning to understand and mitigate 42 K background Conclusion: 42 K is charged and attracted by HV Installation of mini-shroud in Phase I Test with HV potentials on MS 27

High Purity Germanium (HPGe) Detectors standard technology since 1980s 76 Ge intrinsic in germanium 7.8% 76 Ge enrichment to 87% very good energy resolution 0.1% 28

High Purity Germanium Detectors Detectors enriched to contain 87% 76 Ge Semi-coaxial Ge detector: Standard design for gamma spectroscopy Major detector for Phase I Large up to 3 kg Thick n + electrode 2 mm e - hole Broad Energy Ge (BEGe) detector: Major detector for Phase II Small around 600 g Thin n + electrode <1 mm Better energy resolution Superior pulse shape discrimination 29

100 Mo 0 + 1 Gammas: 539.5 kev and 590.8 kev [1] First discovered in 1995 by Barabash et al.: Phys. Lett. B, vol. 345, pp. 408 413 (1995) [2] Latest measurement by NEMO-3 Collaboration: Nucl. Phys. A, vol. 925, pp. 25 36 (2014) [2] 2.6 kg of enriched metallic 100 Mo 600 cm 3 HPGe detector 3.3% efficiency @ 540 kev Exp [2]: T1/2 = 7.5±1.2 10 20 yr QRPA: T1/2 = (1.6-2.2) 10 21 yr IBM-2: T1/2 = 2.2 10 22 yr 30

150 Nd 0 + 1 Gammas: 333.9 kev and 406.5 kev [1] First discovered in 2004 by Barabash et al.: JETP Lett., vol. 79, pp. 10 12 (2004) [2] Latest measurement by Barabash et al.: Phys. Rev. C, vol. 79, no. 4, p. 045501 (2009) [2] 3 kg of natural Nd2O3 (153 g 150 Nd) HPGe: 2.3% efficiency @ 334 kev [2]: T1/2 = 1.33 +0.63-0.36 10 20 yr IBM-2: T1/2 = 1.9 10 21 yr [2] 31

CUORICINO: 130 Te EXO-200: 136 Xe ϵ=0.5% ϵ=3.0% single site multi site ϵ=1.3% Future CUORE 19 towers x 13 floors 11.3 kg of TeO2 bolometers ɣ: 536 kev and 1257 kev; β <734 kev T1/2 > 1.3 10 23 yr [Phys. Rev. C 85 045503 (2012)] QRPA: T1/2 = (0.5-1.4) 10 23 yr IBM-2: T1/2 = 2.2 10 25 yr 80 kg liquid enriched Xe TPC Complicated analysis without segmentation ɣ: 761 kev and 819 kev; β: <879 kev T1/2 > 1.2 10 23 yr [PhD Thesis Yung-Ruey Yen (2013)] IBM-2: T1/2 = 2.5 10 25 yr 32