DOUBLE BETA DECAY TO THE EXCITED STATES (EXPERIMENTAL REVIEW) A.S. BARABASH ITEP, MOSCOW

OUTLINE  Introduction  2 - (2) -decay to the excited ststates Mo Nd  2 - (0) –decay to the excite states  ECEC to the excited states  Conclusion

I. Introduction

1.1. Historical introduction E. Fiorini (1977, NEUTRINO’77) – first time obtained limit on decay in Ge-76 (as by-product for transition) *): T 1/2 (0)> 3·10 21 y (68% C.L.) *) Of course, from geochemical experiments one can extract limits too.

1982 – first special experimental work to investigate 2- decay to the excited states was done: E. Bellotti et al., Lett. Nuov. Cim. 33 (1982) 273. ( 58 Ni, 76 Ge, 92 Mo, 100 Mo, 148 Nd, 150 Nd; Transitions to 2 + 1, O + 1, 2 + 2, … excited states) Limits on the level ( )·10 21 y for 76 Ge and ~ y for other nuclei were obtained.

 Many new results for 76 Ge ( ). And even “evidence” of the decay on 2.5 level (T 1/2  y) have been obtained.  3 experiments were done by E. Norman ( 50 Cr, 58 Ni, 64 Zn, 92 Mo, 94 Zr, 96 Zr, 96 Ru, 106 Cd, 116 Cd, 124 Sn; natural samples). Limits on the level ~ y were obtained.

 It was demonstrated that 2(2; )-decay can be detected in 100 Mo, 96 Zr and 150 Nd by existing low-background HPGe detectors and new experiments were proposed (A.S.B. preprint ITEP, , 1989; JETP Lett. 51 (1990) 207).  ITEP-USC-PNL-UM experiment with 1 kg of 100 Mo was started on 30 May 1990 (in Soudan mine).

Measurement was stopped in September’91  First “positive” result was reported at a few International Conferences (Moriond’91, TAUP’91, WEIN’92,…)  Final result was published in 1995 [PL B 345 (1995) 408]: T 1/2 = ·10 20 y

1999 – 2-nd positive result for 100 Mo; 2001 – 3-d positive result for 100 Mo (in ITEP-TUNL experiment); 2004 – 1-st positive result for 150 Nd; 2007 – 4-th positive result for 100 Mo (in NEMO-3 experiment).

ITEP-TUNL experiment

1.2. Present motivation  Nuclear spectroscopy  NME problem  Checking of some “creasy” ideas ( “bosonic” neutrino, …)  0-decay: - very nice signature (2 - ); - high sensitivity (ECEC; resonance conditions).

II. 2 - (2)-decay to the excited states Table 1. 2 transition to excited state. Nucle i E 2, keVT 1/2, y Exper. Theory, 2007[1] Theory, [2,3] 48 Ca3288.5>1.8· · Nd3033.6>9.1· Zr2572.2>7.9· ·10 25 ( )· Mo2494.5>1.6· · ·10 22 [4]

Table 1. (Continue) 82 Se2218.5>1.4· · · Te1992.7>2.8· ·10 26 (2.8-15)· Xe · · Cd1511.5>2.3·10 21 >3.4· · Ge1480>1.1·10 21 >5.8· · Pd1342 ->1.5·

Table 2. 2-decay to excited state Nucle i E 2, keVT 1/2, y Exper. Theory [2] Theory [4] 150 Nd2627.1=1.4· Zr2202.5>6.8· · · Mo1903.7=6.2· ·10 21 [5]2.1· Se1507.5>3.0·10 21 ( )· Ca1274.8>1.5·

Table 2. (Continue) 124 Sn Cd1048.2>2.0· · · Ge916.7>6.2· · · · Xe · · Te735.7>2.3· ·10 22 – 1.4·

References [1] A.A. Raduta and C.M. Raduta, Phys. Lett. B 647 (2007) 265. [2] M. Aunola and J. Suhonen, Nucl. Phys. A 602 (1996) 133. [3] J. Toivanen and J. Suhonen, Phys. Rev. C 55 (1997) [4] S. Stoica and I. Mihut, Nucl. Phys. A 602 (1996) 197. [5] J.G. Hirsch et al., Phys. Rev. C 51 (1995) 2252.

Table 3. Present “positive” results on 2  (2 ) decay of 100 Mo to the first 0 + excited state of 100 Ru T 1/2, y; x10 20 NS/BYear, method ± ± ± ~ 1/3 8/1 3/1 1995, HPGe [6] 1999, HPGe (many samples) [7] 2001, 2xHPGe (coincidence) [8,9] 2007, NEMO-3, (2e+2  ) [10] Average value: ·10 20 y

References [6] A.S. Barabash et al., Phys. Lett. B 345 (1995) 408. [7] A. S. Barabash et al., Phys. At. Nucl. 62 (1999) [8] L. De Braeckeleer et al., Phys. Rev. Lett. 86 (2001) [9] M.J. Hornish et al., Phys. Rev. C 74 (2006) [10] R. Arnold et al., Nucl. Phys. A 781 (2007) 209.

NEMO Mo: bb decay to exc. states 2  decay to the state: S/B = 3.0 T 1/2 =[ (stat) ± 0.8(syst)]  y 0  decay to the state: T 1/2 > 8.9  % C.L. 2  decay to the state: T 1/2 > 1.1  % C.L. 0  decay to the state: T 1/2 > 1.6  % C.L. Nucl. Phys. A 781 (2007)  decay to the state: S/B = 3.0 T 1/2 =[ (stat) ± 0.8(syst)]  y 0  decay to the state: T 1/2 > 8.9  % C.L. 2  decay to the state: T 1/2 > 1.1  % C.L. 0  decay to the state: T 1/2 > 1.6  % C.L. Nucl. Phys. A 781 (2007) 209 Clear topology: : 2e  in time & energy and TOF cuts : 2e  in time & energy and TOF cuts 100 Mo (540 keV) (1130 keV) (1227 keV) 0 + (g.s.) 100 Ru (1362 keV) 3034 keV 11 22 NEMO-3; days of data (Phase I)

TOP VIEWSIDE VIEW 11 22 11 22 e1e1 e2e2 e1e1 e2e2 100 Mo (NEMO-3) Reconstructed event-candidate

100 Mo (NEMO-3) The main characteristics of selected events

150 Nd Decay scheme 150 Nd (333.9keV) (740.4 keV) (773.3 keV) 0 + (g.s.) 150 Sm ( keV) 3367 keV 11 22 E  1 = keV E  2 = keV

SCHEME OF EXPERIMENT E = 1.9 keV (for 1332 keV) T = h Experiment is done in Modane Underground Laboratory, 4800 m w.e.

Energy spectrum in the range keV

Energy spectrum in the range keV

Analysis of events in the range of peaks under study Peak, keV333.9 ± ± 1.12 Number of events Continuous background Isotopes E, keV 214 Bi 227 Th 228 Ac 333.1, ; ; Bi 211 Pb Contributio n from background Excess of events 86 ± ± 25

150 Nd. Transition to the 0 + excited state T 1/2 = [ (stat) ± 0.3(syst)]·10 20 y [JETP. Lett. 79 (2004) 10]

Comparison of NME for transition to the 0 + ground and excited states (for 100 Mo and 150 Nd)  Conclusion is NME(g.s.)  NME(0 + 1 )  But, in fact, it looks like: NME(g.s.)  ( )xNME(0 + 1 )

III. 2(0)-decay to excited states Very nice signature: 2e - and two (one) gamma with fixed energy  Possibility to have “zero” background  Possibility to have high sensitivity (if registration efficiency and energy resolution are high enough)

3.1. 2(0)-decay to excited state Many years people thought that this decay is sensitive only to right-handed currents. T. Tomoda demonstrated that this is not through: “…relative sensitivity of decays to the neutrino mass and the right-handed currents are comparable to those of decays.” [T. Tomoda, Phys. Lett. B 474 (2000) 245] The decay is suppressed in ~ two order of magnitude in comparison to transition to 0 + g.s.

Table 4. Best present limits on 2  (0 )- decay to excited state NucleiE 2, keVT 1/2, y (90%CL) 76 Ge1480> 8.2·10 23 (G-M) 100 Mo2494.5> 1.6·10 23 (NEMO-3) 130 Te1992.7> 1.4·10 23 (MiBeta) 116 Cd1511.5> 2.9·10 22 (Solotvino) 136 Xe1649.4> 6.5·10 21 (Milan) 82 Se2218.5> 2.8·10 21 (HPGe)

Table 5. Best present limits on 2(0)-decay to the excited state NucleiE 2 , keVT 1/2, y (90%CL) Theory [11- 14] Theory [15] 150 Nd2627.1>1.0· Zr2202.5>6.8· ·10 24 *) Mo1903.7>8.9· ·10 26 *) 1.5·10 25 *) 82 Se1507.5>3.0· ·10 26 *) 4.5·10 25 *) 48 Ca1274.8>1.5·

Table 5. (Continue) 116 Cd >1.4· ·10 27 *) - 76 Ge 916.7>1.3· ·10 26 *) 2.4·10 26 *) 130 Te 735.3>3.1· · *) For = 1 eV [11] J. Suhonen, Phys. Lett. B 477 (2000) 99. [12] J. Suhonen, Phys. Rev. C 62 (2001) [13] J. Suhonen, Nucl. Phys. A 700 (2002) 649. [14] J. Suhonen and M. Aunola, Nucl. Phys. A 723 (2003) 271. [15] F. Simkovic et al., Phys. Rev. C 64 (2001)

2(0)-decay to excited state  Future possibilities: - MAJORANA and GERDA with 76 Ge  ~ y; - CUORE with 130 Te  ~ y; - SuperNEMO with 82 Se (or 150 Nd)  ~ y;

IV. ECEC to the excited states  1994 (A.S.B. JETP Lett. 59 (1994) 677). Main assumption: NME(2 ; 0 + g.s. )  NME(2 ; ): then T 1/2 (0 + 1 ) ~ y for 96 Ru, 106 Cd, 124 Xe, 136 Ce; ~ y for 78 Kr and 130 Ba Very nice signature: in addition to two X-rays we have here two gamma with strictly fixed energy (E ~ keV)  New experiments were proposed

Table 6. Best present limits on ECEC(2) to the excited state NucleiE ECEC, keVT 1/2, y (90%CL) Theory 130 Ba1236>1.5·10 21 *) ~ Cd1580>7.3·10 19 ~ Kr1343>1.2·10 21 **) ~ Mo230>8.1·10 20 ~ Ru1519>4.5·10 16 ~ *) Extracted from geochemical experiment **) Extracted from result for g.s. transition

IV.2. ECEC(0); resonance conditions Transition to the ground state. For the best candidates ( = 1 eV):  +  + (0) ~ y  + EC(0) ~ y ECEC(0) ~ y (One can compare these values with ~ y for 2 - -decay)

ECEC(0) to the ground state  2e b + (A,Z)  (A,Z-2) + 2X +  brem + 2 + e + e - + e - int E ,.. = M -  e1 - e2 Suppression factor is ~ 10 4 (in comparison with EC + (0)) – M. Doi and T. Kotani, Prog. Theor. Phys. 89 (1993)139.

Resonance conditions  In 1955 (R.Winter, Phys. Rev. 100 (1955) 142) it was mentioned that if there is excited level with “right” energy then decay rate can be very high. (Q’-E has to be close to zero. Q’-energy of decay, E- energy of excited state)  In 1982 the same idea for transition to ground and excited states was discussed (M. Voloshin, G. Mizelmacher, R. Eramzhan, JETP Lett. 35 (1982)).  In 1983 (J. Bernabeu, A. De Rujula, C. Jarlskog, Nucl. Phys. B 223 (1983) 15) this idea was discussed for 112 Sn (transition to 0 + excited state). It was shown that enhancement factor can be on the level ~ 10 6 !

J. Bernabeu, A. De Rujula, C. Jarlskog, Nucl. Phys. B 223 (1983) Sn  112 Cd [0 + (1871)] M = ±4.8 keV Q’(KK;0 + ) = M – E*(0 + ) – 2E K = = (-4.9 ± 4.8) keV T 1/2  3·10 24 y (for m = 1 eV) (if Q’ ~ 10 eV)

Resonance conditions  In 2004 the same conclusion was done by Z. Sujkowski and S. Wycech (Phys. Rev. C 70 (2004) ).  Resonance condition (using EC(2) arguments): E brems = Q’ res = E(1S,Z-2)-E(2P,Z-2) (i.e. when the photon energy becomes comparable to the 2P-1S level difference in the final atom) Q’-Q’res < 1 keV

Decay-scheme of 74 Se Here Q = M = keV Q’ = M - 2E b Q’(E*) = Q’ –

74 Se. (Nucl. Phys. A 785 (2007) 371)  400 cm 3 HPGe detector (Modane Underground Laboratory; 4800 m w.e.)  563 g of natural Se (4.69 g of 74 Se)  Measurement time is h

74 Se

74 Se (Nucl. Phys. 785 (2007) 371.)

74 Se. (Nucl. Phys. A 785 (2007) 371) Transition to ( keV) state (595.8 keV and keV lines were investigated): T 1/2 (0;L 1 L 2 ) > 0.55·10 19 y

74 Se. Possible improvements  1 kg of 74 Se  ~ 3·10 21 y  200 kg of 74 Se (using an installation such as GERDA or MAJORANA)  ~ y

Other isotope-candidates NucleiA,% M,keV E*,keVEKEK E L2 74 Se ± (2 + ) Kr ± (2 + ) Ru ± (?) Cd ± (1,2 + ) Sn ± (0 + ) Ba ± (?) Ce ± (1 +, (1 +, Er ± (1 + )

Table 7. Best present limits on ECEC(0) to the excited state (for isotope-candidates with possible resonance conditions) NucleiE*(J  f )T 1/2, y 106 Cd2741 (1,2 + )> 3·10 19 [16] > 5·10 19 *) [17] 74 Se1204.2(2 + )> 0.55·10 19 [18] 130 Ba2608.4(?)> 1.5·10 21 [19,20] (geochemical) 78 Kr2838.9(2 + )> 1.2·10 21 *) [21] *) Extracted from result for 2( g.s. ) transition

References [16] P. Belli et al., Astr. Phys. 10 (1999) 115. [17] N.I. Rukhadze et al., Phys. At. Nucl. 69 (2006) [18] A.S. Barabash et al., Nucl. Phys. A 785 (2007) 371. [19] A. P. Meshik et al., Phys. Rev. 64 (2001) [20] A.S. Barabash and R.R. Saakyan, Phys. At. Nucl. 59 (1996) 179. [21] Ju.M. Gavriljuk et al., Phys. At. Nucl. 69 (2006) 2124.

g.s.-g.s. transitions 152 Gd (0.2%), 164 Er (1.56%), 180 W(0.13%) (There are only X-rays in this case)

Problems  There is no good theoretical description  Accuracy of M (and Q as a result) is not very good (~ 2-10 keV) [It is possible to improve the accuracy of M to ~ 200 eV]

CONCLUSION  2(2)-decay to 0 + excited state was detected for 100 Mo and 150 Nd  There are good prospects to search for 2(2) and 2(0)-decay to 0 + excited states in future (large) experiments  ECEC(0) is a “new-old” possibility to search for neutrino mass (in case of resonance conditions)