Measurement of the Double Beta Decay Halflife of

Measurement of the Double Beta Decay Halflife of 150 Nd and Search for Neutrinoless Decay Modes with the NEMO-3 Detector 狭間 俊介 @colloquium 10/15, 2008

Double beta decay Introduction Double beta decay physics E (A, Z+1) Qββ (A, Z) ββ (A, Z+2) (A, Z) (A, Z+2) +2 e- +2 ββ 2ν decay is allowed by the Standard Model and has been observed for 9 nuclei (7 studied by the NEMO experiments). (A, Z) (A, Z+2) +2 e. Beyond Standard Model ββ 0ν decay

Double beta decay ( 0 ) & neutrino physics (A, Z) (A, Z+2) + 2 e • L=2 lepton _ number conservation violated • = Majorana particle • m 0 Experimental signature: 2 e-, t~0 ns, Ee=Q Q few Me. V C 0 : phase space factor M : nuclear matrix element <m > : effective neutrino mass mi : mass of i Majorana field Uei: mixing parameters 0 Neutrino nature, absolute scale of neutrino mass

NEMO - Neutrino Ettore Majorana Observatory The NEMO 3 experiment modane The building of NEMO 3 detector at the Frejus Underground Laboratory(LSM) in Modane in 2001 and with the anti-radon tent since October 2004

The NEMO 3 experiment The NEMO 3 detector Sources: 10 kg of emitters wood 20 secteurs 6. 914 kg of 100 Mo and 0. 932 kg of 82 Se S = 20 m 2, 60 mg/cm 2 Tracking volume: Drift chamber (Geiger mode) Drift cells are 2. 7 m long 6180 cells resolution : σt=5 mm, σz≤ 1 cm (vertex) Gaz: He + 4% ethanol + 1% Ar + 0. 1% H 2 O Calorimeter & TOF: 3 m 1940 plastic scintillators(10 cm thick) Energy resolution : 14. 1 to 17. 7 % (for 1 Me. V e-) Timing resolution : 250 ps (for 1 Me. V e-) associated to low-background PMTs 214 Bi, 208 Tl, 40 K three orders of magnitude below B (25 G) Tank containing borated water 4 m. Iron shield Gamma shielding: Iron (18 cm) Neutron shielding: borated water + wood Magnetic field: 25 Gauss

The NEMO 3 experiment ββ emitters in NEMO 3 2 116 Cd 405 g Q = 2805 ke. V 96 Zr 9. 4 g Q = 3350 ke. V 150 Nd 37. 0 g Q = 3367 ke. V 48 Ca 7. 0 g Q = 4272 ke. V • It has the high Q compared to 130 value(Q=3. 367 Me. V) Te 454 g Q = 2529 ke. V 100 Mo 6. 914 kg Q = 3034 ke. V 82 Se, …) other double beta decay emitters (100 Mo, External background 214 Bi (3. 27 nat. Te • the expected background from Me. V, 82 Se 0. 932 491 g kg measurement radon) will have less impact in the energy region of Q = 2995 ke. V interest for neutrinoless Cu double 621 g beta decay 0

The NEMO 3 experiment ββ event selection Data taking : 924. 7 days (February 2003 to December 2006) Transversal view Run Number: 2040 Event Number: 9732 Date: 2003 -03 -20 Longitudinal view Emission vertex Deposited energy: E 1+E 2= 2088 ke. V Internal hypothesis: ( t)mes – ( t)theo = 0. 22 ns Common vertex: ( vertex) = 2. 1 mm Emission vertex ( vertex)// = 5. 7 mm Selection criteria • 2 tracks associated with negatively charged particles • 2 PMTs, energy deposit greater than 0. 2 Me. V • PMT-Track association • Common vertex • Internal hypothesis (external event rejection) • No other isolated PMT (γ rejection) • No delayed track (214 Bi rejection) After this selection 2789 events remain.

The NEMO 3 experiment Background sources BG sources are divided into two categories. • internal background → BG events originating from the radioactive impurities in the source foils a) electron conversion b) Møller scattering c) Compton scattering d) 207 Bi 207 207 Pb + 2 e. Bi Pb* E. C. electron conversion • external background → BG events originating from e- or γ anywhere outside the source foils a) 222 Rn 222 214 Rn Bi + 2α + e 214 Po + e- + γs Bi Radon purification facility was installed. → Radon induced BG for this analysis is reduced by about a factor of six.

BACKGROUND EVENTS OBSERVED BY NEMO-3 Electron + a delay track (164 ms) Electron + N g’s 208 Tl 214 Bi 214 Po 210 Pb (Eg = 2. 6 Me. V) Electron – positron pair B rejection

The NEMO 3 experiment Background sources The background activities are determined using high germanium(HPGe) measurement and using control channels : e- γ channel : e- channel : BG events from neighboring source foils ※ 228 Ac → 208 Tl 212 Bi → 208 Tl

The NEMO 3 experiment Background measurement Internal e- - γ analysis channel Energy of the electron Energy of the γ Single e- analysis channel Energy of the electron

ββ 2ν results for 150 Nd Energy sum of the electrons PRELIMINARY Angular distribution PRELIMINARY Efficiency of the 2νββ event selection : 7. 2 % T 1/2 (ββ 2ν) = (9. 11 +0. 25 -0. 22 (stat) ± 0. 63 (syst)) x 1018 y Uncertainty 150 • sum of internal & external BG : 4. 3 % • position of the Nd : 3 % • energy measurement : 2 % • Tracking efficiency : 5. 7 % • TOF : 1 %

ββ 2ν results Results of previous experiments ITEP TPC experiment 9 events observed, 38 days, natural Nd 2 O 3 (2. 5 g of 150 Nd) 36 events observed, 53 days, 51. 5 g of enriched Nd 2 O 3 (V. Artemiev et al. , Phys. Let. B. , 345, (1995) 564) University of California (Irvine) group TPC experiment 476 events observed, 262 days, 15. 5 g of Nd 2 O 3 enriched to 91 % in 150 Nd (A. De Silva et al. , Phys. Rev. C 56 (1997) 2451)

ββ 0ν results for 150 Nd LLR test statistic method above 2. 5 Me. V CLs = CLs+b / CLb → 1 - CLs = 0. 9 Detection efficiency: 19% T 1/2 (ββ 0ν) > 1. 8 x 1022 y 90 % CL <mν> < 4. 0 – 6. 3 e. V V. A. Rodin et al. , Nucl. Phys. A 766 (2006) 107 Previous result: T ½ > 1. 7 x 1021 y 90 % CL 150 Nd PRELIMINARY ββ 0ν MC (T 1/2 = 1. 45 1022 y) radioactive background MC ββ 2ν background + radioactive background MC A. A. Klimenko et al. , Nucl. Instr. Meth. B 17 (1986) 445 Emission of Majoron (M 1) : T 1/2 (ββ 0ν) > 1. 52 x 1021 y 90 % CL Above 2. 5 Me. V 28. 6 ± 2. 7 events expected from background 29 events observed (Qββ - Esum ) n ※

Conclusion and perspectives • The following value of the half-life for the ββ 2ν decay of 150 Nd was obtained: T 1/2 (ββ 2ν) = (9. 11 +0. 25 -0. 22 (stat) ± 0. 63 (syst)) x 1018 y • The limit on the half-life value for the ββ 0ν process has been improved. T 1/2 > 1. 8 x 1022 y 90% CL <mν> < 4. 0 – 6. 3 e. V • We have provided the first limits for several Majoron models using 150 Nd. • 150 Nd is a good candidate for future beta decay experiment: – high Qββ value – large phase space factor

Back up slides

NEMO 3 results Summary of NEMO 3 results Data T ½ (ββ 2ν) - years 389 days - Phase 1 7. 11 ± 0. 02 (stat) ± 0. 54 (syst) 1018 * 389 days - Phase 1 9. 6 ± 0. 3 (stat) ± 1. 0 (syst) 1018 116 Cd 365 days - Phase 1 2. 8 ± 0. 1 (stat) ± 0. 3 (syst) 1019 96 Zr 365 days - Phase 1 2. 0 ± 0. 3 (stat) ± 0. 2 (syst) 1019 48 Ca 365 days - Phase 1 3. 9 ± 0. 7 (stat) ± 0. 6 (syst) 1019 130 Te 534 days – Phases 1 + 2 7. 6 ± 1. 5 (stat) ± 0. 8 (syst) 1020 Isotope 100 Mo 82 Se * * R. Arnold et al. , Phys. Rev. Lett. 95 182302 (2005) Isotope Data T ½ (ββ 0ν) - years <mν> 100 Mo 693 days - Phases 1 + 2 > 5. 8 x 1023 < 0. 4 – 0. 7 e. V 82 Se 693 days - Phases 1 + 2 > 2. 1 x 1023 < 1. 4 – 2. 2 e. V