Double beta decay — Physics at beyond accelerator energies

UCLEAR PHYSIC~ ELSEVIER

Nuclear Physics B (Proc. Suppl.) 48 (1996) 216-222

PROCEEDINGS SUPPLEMENTS

Double Beta D e c a y - Physics at Beyond Accelerator Energies H.V. Klapdor-Kleingrothaus *a

Max-Planck-Institut ffir Kernphysik, P.O.Box 10 39 80, D-69029 Heidelberg, Germany Double beta decay yields - besides proton decay - one of the most promising possibilities to probe beyond standard model physics at beyond accelerator energies. The possibilities include the neutrino mass, SUSY models, compositeness, leptoquarks, right-handed W bosons and others. We discuss the status and future perspectives of ~ research with enriched 76Ge detectors, in particular of the HEIDELBERG-MOSCOW experiment, including applications some double beta technology can find in the search for dark matter. 1. I n t r o d u c t i o n The potential of double beta decay includes investigation of the neutrino mass, of the parameter space of SUSY models, of right-handed W bosons, compositeness, leptoquarks, Majorons, and others. For these topics double beta decay is comfortably competitive to high-energy accelerators [1-5,19]. 2. T h e H E I D E L B E R G - M O S C O W experiment 2.1. S t a t u s The HEIDELBERG-MOSCOW experiment [17,19] is now exploring the sub-eV range for the mass of the electron neutrino. With five enriched (86% of 76Ge) detectors of a total mass of 11.5 kg taking data in the Gran Sasso underground laboratory the experiment has reached its final *Spokesman of the H E I D E L B E R G - M O S C O W cooperation 0920-5632/96/$15.00 O 1996 Elsevier ScienceB.V. All rights reserved. PIh S0920-5632(96)00243-5

setup. The experiment gives at present for most parameters the sharpest limits from double beta decay. They are discussed in detail by B.Maier, M. Hirsch, H. P£s, O.Panella, E. Takasugi and H.V. Klapdor-Kleingrothaus in [33]. These results set the scale of this type of experiments. They will be briefly listed up here. Fig. 1 shows the spectrum in the 0~,~fl region taken in a measuring time of 13.6 kg y [17,191. Half-life of neutrinoless double beta decay The deduced half-life limit for 0t~/~fl decay is T°/~2 > 7.4. 1024y (90%C.L.) > 12.7. 1024y (68%C.L.)

(1) (2)

Neutrino mass -Light neutrinos: The deduced upper limit of an (effective) electron neutrino Majorana mass is, with the matrix ele-

H. V. Klapdor-Kleingrothaus /Nuclear Physics B (Proc. Suppl.) 48 (1996) 216-222

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217

Right-handed W boson For the right-handed W boson we deduce [6] a lower limit of

mwR >_ 1.1TeV

0.6

(6)

SUS Y parameters

0 2000

2010

2020

2030

2040

2050

2060 2070 2080 energy [keV]

Figure 1. HEIDELBERG-MOSCOW ex-

periment: Region of interest for Ou/3~ decay after subtraction of the first 200 days of measurement of each detector, leaving 13.60 kg y of measuring time. The dotted curve corresponds to the signal excluded with 90%C.L. It corresponds to T°~2 >

New constraints on the parameters of the minimal supersymmetric standard model with explicit R-parity violation are deduced [3,5] from the 0u13/3 half-life limit, which are more stringent than those from other low-energy processes and from the largest high energy accelerators (Fig. 3).

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meat from

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[12]


(3)

< 0.43eV (68%C.L.)

(4)

This is the sharpest limit for a Majorana mass of the electron neutrino so far. -Superheavy neutrinos: For a superheavy left-handed neutrino we deduce ([17]) exploiting the mass dependence of the matrix element a lower limit

(rnH) >_5.1 • 107GeV

(5)

For a heavy right-handed neutrino the relation obtained to the mass of the righthanded W boson is shown in Fig. 2 (see

[6]).

0.10,~

j i

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0.5

i

2

"~

" t~.

20.

Log((m~)))[TeVl Area excluded from the HEIDELBERG-MOSCOW experiment (below the curves) in the plane of the righthanded W boson mass versus the mass of a heavy right-handed neutrino. The full line is the constraint from 0u/3/3 decay, the dotted line is the requirement of vacuum stability (from [6]) Figure 2.

Compositeness Evaluation of the 0u/3/3 half-life limit for exchange of excited Majorana neutrinos

218

H. V. Klapdor-Kleingrothaus ~Nuclear Physics B (Proc. SuppL ) 48 (1996) 216-222

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.

.

.

.

The experiment produced for the first time a high statistics 2~,/3/3 spectrum (,-, 20000 counts). The deduced half-life is

.

[19] T?/2 ~ (1.77_0a2). -t-0.14 1021y

oo1

/ /-- J 100

200

7" 500

1000

2000

m~

Figure 3. Comparison of limits on the R-parity violating MSSM parameters from different experiments in the ,~11-m~ plane. The dashed line is the limit from charged current universality according to [27]. The vertical line is the limit from the data of Tevatron [28]. The thick full line is the region which might be explored by HERA [29]. The two dash-dotted lines to the right are the limits obtained from the half-life limit for O,flfl decay of 76Ge, for gluino masses of (from left to right) mo = 1Te V and 100 Ge V, respectively. The regions to the upper left of the lines are forbidden ([3]).

u* yields under some assumptions [34] as lower mass bound of an excited neutrino m~. > 5.9. 104TeV

(7)

This is the most stringent bound so far. The bounds deduced on the compositeness scale in different models are roughly of the order of magnitude as those coming from high energy experiments (see Panella and Wakasugi [33]). Half-life of 2~fl~ decay

(8)

Majoron-accompanied decay Fitting simultaneously the 2u spectrum and one selected Majoron mode yields for the first time experimental limits for the half-lives of the decay modes of the newly introduced Majoron models (C. Burgess et hi. [14-16], P/is et al. [7]). The small matrix elements and phase spaces for these modes (see Pgs et al. [7]) already determined that these modes by far cannot be seen in experiments of the present sensitivity if we assume typical values for the neutrino-Majoron coupling constants around (g) = 10 -4. 2.2. P e r s p e c t i v e s The HEIDELBERG-MOSCOW experiment will probe the neutrino mass within 5 years down to the order of 0.1 eV (Fig. 5). This limit will be reached taking into account the current background of 0.1 counts/kg y keV in the 0u~/3 region and a further reduction by a factor of ~ 5 by digital pulse shape analysis (DPSA). The new DPSA method which we developed [20] allows for the first time in a very efficient, and reliable way to discriminate between multiple site (MSE) and single site events (SSE) (see [32]). Examples of the second class are the interaction of a beta particle, of the first class multiple Compton scattering events. Fig. 4 shows the result of the first application of this method with one of the enriched detectors

H. If Klapdor-Kleingrothaus/Nuclear Physics B (Proc. Suppl.) 48 (1996) 216-222

in the Gran Sasso for a measuring time of 256 kg d. The energy of the central count in the SSE spectrum is (2038.5 -t3.6) keV corresponding exactly to the Q ~ value. According to its shape the pulse is a clear single site event and thus a clear double beta candidate. The strong reduction of the background by the new DPSA method, with the potential of reducing the background in the 0u/3~ region to _< 0.02 counts/kg y keV, will be essential for the further experiment. 3. G e n e r a l P e r s p e c t i v e s

219

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Figs. 5a,b show the future perspectives of fl/3 decay experiments for the next decade. They show the present results and aims of the most promising double beta decay experiments in comparison with the HEIDELBERG-MOSCOW experiment. For a detailed discussion we refer to [19]. As pointed out recently by Raghavan [21], even use of an amount of about 200 kg of enriched 136Xe or 2 tons of natural Xe added to the scintillator of the KAMIOKANDE detector or similar amounts added to BOREXINO would hardly lead to a sensitivity larger than the present 76Ge experiment. It is obvious that the HEIDELBERGMOSCOW experiment will give the sharpest limit for the electron neutrino mass till the end of the decade and longer. 4. D a r k m a t t e r s e a r c h w i t h e n r i c h e d Ge d e t e c t o r s The best laboratory limits on dark matter (WIMPs) are obtained at present

0.6 0.4 0.2

2000 2010 2020 2030 2040 2050 2060 2070 2080 Energy [keV]

Figure 4. First test application of digital pulse shape analysis with an enriched 76Ge detector in the Gran Sasso laboratory in a measuring time of 166 kg d. a) MSE spectrum b) SSE spectrum, demonstrating a drastic reduction of the background (from

[20])

by search with Germanium detectors. The HEIDELBERG-MOSCOW experiment allowed for the first time a search for dark matter with isotopically enriched material [22]. The existing cross section limits for WIMP masses above -~ 150 GeV were improved compared to other recent

[1. V. Klapdor-Kleingrothaus /Nuclear Physics B (Proc. Suppl.) 48 (1996) 216-222

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Figure 6. Current detection bounds for WIMPs with Ge ionisation detectors (shaded regions) and anticipated increase in the sensitivity with Ge cryodetectors The figure shows also expected values for spin-independently and spin-dependently interacting WIMPs in various GUT models (after [25,30,18]).

48Ca 4Sea 76Ge USe t°°Mo nGCd '3°Te 136Xe'36Xe '3aXe t~Nd

Present situation, 1995, and expectation for the near future until the year 2000, of the most promising /?~experiments concerning accessible half-life (a) and neutrino mass limits (b). The filled bars correspond to the present status, open bars correspond to "safe" expectations and dashed lines correspond to long-term planned or hypothetical experiments. Figure 5.

work, and Dirac neutrinos could be excluded as the dominant component of the dark halo in the mass range 26 GeV to 4.7 TeV. The measured limit rules out also

heavy sneutrinos as dark matter in scenarios of a minimal supersymmetric standard model [23]. The potential of 76Ge dark matter detectors for search for neutralinos in relation to the non-zero spin 73Ge has been carefully investigated recently [24]. For dark matter search the progress obtained with enriched 76Ge is shown in Figs. 6,7. A major step in sensivity improvement is expected on long terms from cryogenic detectors (see Fig. 6 and [25]). Similarly interesting and realizable on shorter time scale could be the new HEIDELBERG project planning to use ionisation Ge detectors in a special new configuration (see Fig. 7 and [31]).

H. V. Klapdor-Kleingrothaus /Nuclear Physics B (Proc. Suppl.) 48 (1996) 216-222

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others. New classes of GUTs basing on degenerate neutrino mass scenarios [811] which could explain these observations, can be checked by double beta decay in near future. The HEIDELBERGMOSCOW experiment among the new ~fl experiments as the first now yields results in the sub-eV range.

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Figure 7. Current detection bounds for WIMPs from the HEIDELBERGMOSCOW experiment [22], and the UK experiment [26], the present claimed goal of the Berkeley cryo detector project and the expectation for the new HEIDELBERG project using ionisation Ge detectors in a special configuration [31]

5. Conclusion Double beta decay has a broad potential for providing important information on modern particle physics beyond present and future high energy accelerator energies which will be competitive for the next decade and more. This includes SUSY models, compositeness, left-right symmetric models, leptoquarks, and the neutrino mass. For the latter double beta decay now is particularly pushed into a key position by the recent possible indications of beyond standard model physics from the side of solar and atmospheric neutrinos, dark matter COBE results and

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