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Nuclear Physics A 678 (2000) 341–352
www.elsevier.nl/locate/npe
Limits on different Majoron decay modes of
100
Mo,
116
Cd,
82
Se and
96
Zr for neutrinoless double beta
decays in the NEMO-2 experiment
NEMO collaboration
R. Arnold
j
,C.Augier
h
, J. Baker
e
, A. Barabash
g,∗
,D.Blum
h
,
V. Brudanin
c
, A.J. Caffrey
e
, J.E. Campagne
h
, E. Caurier
j
,D.Dassié
a
,
V. Egorov
c
, T. Filipova
c,h
,R.Gurriaran
a
, J.L. Guyonnet
j
,F.Hubert
a
,
Ph. Hubert
a
, S. Jullian
h
,I.Kisel
c
, O. Kochetov
c
, V.N. Kornoukhov
g
,
V. K o va l en k o
c
, D. Lalanne
h
,F.Laplanche
h
, F. Leccia
a
,I.Linck
j
,
C. Longuemare
b
, Ch. Marquet
a
,F.Mauger
b
, H.W. Nicholson
i
,
I. Pilugin
g
, F. Piquemal
a
,J-L.Reyss
d
, X. Sarazin
h
,F.Scheibling
j
,
J. Suhonen
f
,C.S.Sutton
i
,G.Szklarz
h
,V.Timkin
c
,R.Torres
a
,
V. U ma t o v
g
, I. Vanyushin
g
, A. Vareille
a
, V. Vasilyev
g
,Ts.Vylov
c
a
CENBG, IN2P3-CNRS et Université de Bordeaux, 33170 Gradignan, France
b
LPC, IN2P3-CNRS et Université de Caen, 14032 Caen, France
c
JINR, 141980 Dubna, Russia
d
CFR, CNRS, 91190 Gif-sur-Yvette, France
e
INEEL, Idaho Falls, ID 83415, USA
f
JYVÄSKYLÄ University, 40351 Jyväskylä, Finland
g
ITEP, 117259 Moscow, Russia
h
LAL, IN2P3-CNRS et Université Paris-Sud, 91405 Orsay, France
i
MHC, South Hadley, MA 01075, USA
j
IReS, IN2P3-CNRS et Université Louis Pasteur, 67037 Strasbourg, France
Received 4 April 2000; revised 31 May 2000; accepted 5 June 2000
Abstract
The NEMO-2 tracking detector located in the Fréjus Underground Laboratory was designed
as a prototype for the NEMO-3 detector and to study different modes of double beta decay.
Measurements with
100
Mo,
116
Cd,
82
Se and
96
Zr were carried out. Presented here are the
experimental half-life limits on double beta decays for new Majoron emission modes and limits
on effective neutrino–Majoron coupling constants.
2000 Elsevier Science B.V. All rights reserved.
PACS: 23.40.-s; 14.80.Mz
Keywords: Majoron; Double-beta decay
∗
Corresponding author: barabash@vxitep.itep.ru
0375-9474/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.
PII:S0375-9474(00)00326-2
342 R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352
1. Introduction
Spontaneousviolationof global (B–L) symmetry in gaugetheories leads to the existence
of a massless Goldstone boson, the Majoron. At the beginning of the 1980’s there were
considered to be singlet [1], doublet [2] and triplet [3] Majoron models. All these models
resulted in the neutrinoless double beta (2β) decay with the emission of a Majoron (χ
0
):
(A, Z) → (A, Z + 2) + 2e
−
+ χ
0
. (1)
However, the interaction of the triplet (or doublet) Majorons with the Z
0
boson would
give a contribution to the width of the Z
0
decay, which corresponds to two (or 1/2)
additional massless neutrino types (see, for example [4–6]). LEP data gives 2.994± 0.012
neutrino types [7], thus triplet and some doublet Majorons are excluded. Nevertheless,
in Ref. [8] it is proposed, that a small gauge coupling constant does not eliminate the
possibility of a large Yukawa coupling with neutrinos. Thus, the singlet and doublet
Majorons can still contribute to neutrinoless 2β-decay [8,9].
Another possibility for neutrinoless 2β-decay with Majoron emission arises in super-
symmetry models with R-parity violation [9,10]. It was first stated in [10] that there is the
possibility of a 2βχ
0
χ
0
-decay with the emission of two Majorons:
(A, Z) → (A, Z + 2) + 2e
−
+ 2χ
0
. (2)
In the 1990’s several new “Majoron” models were suggested. The term “Majoron” here
denotes massless or light bosons with a coupling to neutrinos. In these models “Majoron”
can carry a lepton charge, but cannot be a Goldstone boson [11]. Additionally there can be
decays with the emission of two “Majorons” [12]. In the models with a vector “Majoron”
it is a longitudinal component of the massive gauge boson emitted in 2β-decay [13]. All
these new objects are called Majorons for simplicity.
In Table 1 there are nine Majoron models presented (following [12–14]), which are
considered in this work. It is divided into two sections, one for lepton number violation and
one for lepton number conservingmodels. The table also showswhether the corresponding
Table 1
Different Majoron models according to [12,14]. The mode IIF corresponds to the model of
Carone [13]
Case Decay mode Goldstone boson LnMatrix element
IB 2βχ
0
no 0 1 M
F
− M
GT
IC 2βχ
0
yes 0 1 M
F
− M
GT
ID 2βχ
0
χ
0
no 0 3 M
Fω
2
− M
GT ω
2
IE 2βχ
0
χ
0
yes 0 3 M
Fω
2
− M
GT ω
2
IIB 2βχ
0
no −21 M
F
− M
GT
IIC 2βχ
0
yes −23 M
CR
IID 2βχ
0
χ
0
no −13M
Fω
2
− M
GT ω
2
IIE 2βχ
0
χ
0
yes −17M
Fω
2
− M
GT ω
2
IIF 2βχ
0
gauge boson −23 M
CR
R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352 343
Fig. 1. Energy spectra of different modes of 2β2ν(n= 5),2βχ
0
(n = 1and3) and 2βχ
0
χ
0
(n = 3
and 7) decays of
100
Mo.
2β-decay is accompanied by the emission of one or two Majorons. The next three entries
list the main features of the models: the third column lists whether the Majoron is a
Goldstone boson or not (or a gauge boson in the case of vector Majorons, type IIF). In
column four the leptonic charge L is given. Column five gives the “spectral index” n of
the summed energy of the emitted electrons, which is defined by the phase space of the
emitted particles, G ∼ (Q
ββ
− T)
n
.HereQ
ββ
is the energy released in the decay and T
the energy of the two electrons. Energy spectra of different modes of 2β2ν(n= 5),2βχ
0
(n = 1and3) and 2βχ
0
χ
0
(n = 3and7) decays are presented in Fig. 1. The different
shapes can be used to distinguish the different Majoron decay modes from each other and
2β-decay with the emission of two neutrinos. In the last column of Table 1 the nuclear
matrix elements (NME) are listed.
Attempts to observe 2β-decay with Majoron emission have been undertaken for the
last 20 years. Consequently there now exist strong limits on the “standard” Majoron with
the “standard” electron energy spectrum shape (n = 1), see Table 2. The best limits on
the Majoron coupling constant (hg
ee
i) were obtained in experiments with
128
Te [15],
116
Cd [16],
100
Mo [17] and
136
Xe [27] yielding a limit on hg
ee
i on the level ∼ 10
−4
.
Sufficiently less information exists for “nonstandard” Majoron models. The most carefully
studied “nonstandard” models are being investigated with
76
Ge [18]. There are also limits
on decays with the emission of two Majorons in
100
Mo [19] and
116
Cd [20].
In this work a systematic search for 2β-decays with different Majoron types was carried
out for
100
Mo,
116
Cd,
82
Se and
96
Zr, using the experimental data obtained with the
NEMO-2 detector [21]. Limits on the standard Majoron (n = 1) were published earlier
[16,22–24].
344 R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352
Table 2
Summary of the best results on the 2βχ
0
-decay with n = 1. All limits are presented at the 90% CL.
The dispersion of hg
ee
i values is due to uncertainties in the NME calculation. The NME from the
following works were used:
48
Ca [29–31],
150
Nd [32–35], and others [16,24,31,33–37]
Nucleus T
1/2
,y hg
ee
i
48
Ca > 7.2 × 10
20
[25] <(5.3−8.8) × 10
−4
76
Ge > 7.9 × 10
21
[26] <(2.6−7.5) × 10
−4
82
Se > 2.4 × 10
21
[23] <(2.3−4.3) × 10
−4
96
Zr > 3.9 × 10
20
[24] <(2.6−4.9) × 10
−4
100
Mo > 3.1 × 10
21
[17] <(1−4.3) × 10
−4
116
Cd > 1.2 × 10
21
[16] <(1.2−4.4) × 10
−4
128
Te > 2 × 10
24
(geochemical) [15] <(0.7−1.4) × 10
−4
130
Te > 0.8 × 10
21
(geochemical) [15] <(2.8−6.8) × 10
−4
136
Xe > 7.2 × 10
21
[27] <(1.3−3.8) × 10
−4
150
Ne > 2.8 × 10
20
[28] <(1−5.4) × 10
−4
2. NEMO-2 detector
The NEMO-2 detector (Fig. 2) consists of a 1m
3
tracking volume filled with helium
gas and 4% ethyl alcohol. Vertically bisecting the detector is the plane of the source foil
(1 m× 1 m). Tracking is accomplished with long open Geiger cells with an octagonal cross
section defined by 100 µm nickel wires. On each side of the source foil there are 10 planes
of 32 cells which alternate between vertical and horizontal orientations. Collectively the
cells provide three-dimensional tracking of charged particles.
A calorimeter made of scintillators covers two vertical opposing sides of the tracking
volume. It consisted of two planes of 64 scintillators for the
100
Mo measurements and 25
scintillators for the
116
Cd,
82
Se and
96
Zr measurements (12 cm × 12 cm × 2.25 cm and
19 cm × 19 cm × 10 cm, respectively). In the last case low radioactivity photomultipliers
tubes (PMT) were used. Finally, the tracking volume and scintillators were surrounded by
a lead (5 cm) and iron (20 cm) shield.
2.1. Performance
Details of the performances and parameters of NEMO-2 are described elsewhere [16,
21–24]while the most salient characteristicsare briefly outlined here. As mentioned above,
the three-dimensionalmeasurements of charged particle tracks are provided by the array of
Geiger cells. The transverse position is given by the drift time and the longitudinal position
by the plasma propagation times. The transverse resolution is 500 µm and the longitudinal
resolution is 4.7 mm. Track reconstruction is accomplished with the tracking method based
on the Kalman filter [38]. The calorimeter energy resolution (FWHM) is 18% at 1 MeV
with a time resolution of 275 ps (550 ps at 0.2 MeV). A laser and fiber optics device is
used to check the stability of the scintillation detectors.
R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352 345
Fig. 2. The NEMO-2 detector without shielding: (1) central frame with the source plane capable of
supporting plural source foils, (2) ten frames of 2× 32 Geiger cells for tracking, (3) scintillator array.
2.2. Event definition
An electron is defined by a track linking the source foil and one scintillator. The
maximum scattering angle along the track has to be less than 20
◦
to reject hard scattering
situations. A photon is recognized as one or two adjacent fired scintillators without an
associated particle track. For photons and electrons an energy deposited greater than
200 keV is required in order to obtain sufficiently good time resolution. The two-electron
events are defined by two tracks which have a common vertex and are associated with two
fired scintillators with a deposited energy of at least 200 keV in each one. In the analysis
a two-electron event is identified as (2e), electron-photon event as (eγ ). A more detailed
description of the analysis procedure can be found in Refs. [16,22–24].
2.3. Source-foils parameters
Natural (163 g) and
100
Mo enriched (172 g) molybdenum metallic foils were manufac-
tured using a standard rolling technology. They were studied in the first experiment. The
enriched and natural foils each defined half of the central plane. The second experiment
used natural (143 g) and
116
Cd enriched (152 g) cadmium metallic foils. The third ex-
periment involved selenium and zirconium sources, which were composed of strips that
were produced using a special technique to deposit the material with a binder on Mylar
films. Masses of enriched materials were m
Se
= 157 g and m
Zr
= 20.5 g and natural were
m
Se
= 134 g and m
Zr
= 18.3 g. The Se was placed in the outer region of the central plane
346 R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352
and Zr foils in the inner portion of the central plane. The thicknesses of the foils were
approximately 40–50 mg/cm
2
for all foils.
Values of the different contaminations in the foils were obtained with the NEMO-2
detector by analysing electron–gamma and single-electron events, as explained in the
sections devoted to backgrounds. These results were compared with HPGe detector
measurements.
3. Backgrounds
Backgrounds for the NEMO-2 detector had “internal” and “external” origins. Events
connected with natural samples were used to estimate the external background in the
enriched samples.
The “external” background is due to photons coming from outside of the tracking
detector and interacting with the source foils or with the scintillators. Compton electrons
produced in the scintillators and crossing the tracking device were rejected by time-of-
flight analysis. Compton electrons produced in the source foils can generate a secondary
electron by Möller scattering. A double Compton effect or pair production is also seen
as a 2e event (NEMO-2 could not distinguish between e
+
and e
−
). These 2e background
events cannot be rejected by time-of-flight cuts. The dominant contribution to the external
background comes from the flux of photonsemitted by radon located between the tracking
detector and the shielding. Another source of background is due to the flux of photons
emitted by the PMTs.
Radioactive pollution in the source foils produces a background identified as “internal”.
An electron which gives rise to the Möller effect, or is associated with an internal
conversion electron, or a Compton electron can produce a 2e background event.
The main part of the 2e background events are due to Möller scattering which
leads mainly to small angles between the two electrons. This is not the case for 2β2ν
decay, where the angles are wide. To improve the signal-to-background ratio the cut
cos(θ
12
)<0.6 on the angle between two electrons, (θ
12
), was applied in the 2e event
selection for the measurements with
116
Cd,
82
Se and
96
Zr. Unfortunately the raw data
of
100
Mo experiment was not saved, and the data were analyzed without this cut.
Since the enriched samples of
100
Mo,
116
Cd and
82
Se were rather pure the major part
of the background in that experiments is of “external” origin. This is not the case with the
96
Zr where the background is most of “internal” origin. More thoroughly the problem of
the backgroundsis considered in previous works [16,22–24].
4. Experimental results
4.1. Analysis methods of experimental data
The experimental data from enriched samples are shown in Fig. 3 as solid line
histograms. The sums of external and internal backgrounds for the different experiments
R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352 347
Fig. 3. The 2e events (solid line) and estimated backgrounds (dashed line) for (a)
100
Mo, (b)
116
Cd,
(c)
82
Se and (d)
96
Zr.
are presented as dashed line histograms. The detection efficiencies for the decays depend
on the energy of the electrons and were calculated for all four nuclei, for all the Majoron
modes (spectral indices n = 1, 3 and 7) and for the double beta-decay (n = 5) by a Monte
Carlo simulations with the GEANT 3.21 code.
Obtaining limits on the different modes was performed by two methods. In the first
one we estimated T
2β2ν
1/2
from our measurements. Then one can get limits on the Majoron
mode if the 2β2ν and background are known and used as expected averages in the Helene
formula [39,40] for Poisson processes:
CL(N) = 1 − e
−(µ
b
+N)
n
0
X
n=0
(µ
b
+ N)
n
n!
,
e
−µ
b
n
0
X
n=0
µ
n
b
n!
, (3)
where µ
b
is the expected average number of eventsin an interval and is defined by the sum
of 2β2ν and background events, n
0
is the number of observed events in the same interval,
and N is the limit on the mean number of events from a signal. The dependent variable in
this equation is the parameter N while the CL(N) is fixed at 90%.
If one considers the existence of both 2β2ν and Majoron decay modes, then the T
2β2ν
1/2
estimation should not depend on the existence or absence of decays with the emission
of χ
0
.Thisisapplicablefor2βχ
0
with spectral index n = 1, where the 2β2ν and Majoron
spectra profiles peak in different energy regions (Fig. 1). This was done in previous works
[16,22–24]. Results for all nuclei are given in Table 3. Also shown, for comparison only,
are the calculations by the Helene formula method for modes with other spectral indices.
348 R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352
Table 3
Limits on T
1/2
(y) at 90% CL for decays with Majoron emission, estimated via the Helene formula
Nucleus
100
Mo
116
Cd
82
Se
96
Zr
n = 1 > 5.0 × 10
20
[22] > 1.2 × 10
21
[16] > 2.4 × 10
21
[23] > 3.5 × 10
20
[24]
n = 3 > 9.9 × 10
19
> 4.6 × 10
20
> 1.1 × 10
21
> 6.3 × 10
19
n = 7 > 1.7 × 10
20
> 2.0 × 10
20
> 3.7 × 10
20
> 5.1 × 10
19
In the case when shapes of the spectra are similar one cannot use the Helene formula,
and should follow another method. If one considers the Majoron modes as existing decay
channels similar to 2β2ν, then the experiment is the sum of two processes, 2β2ν decay
and decay with χ
0
emission. Thus, one cannot know the expected number of 2β2ν decays
and should set a limit on the decays with Majoron emission by analysing the deviation in
the shape of the experimental data calculated for 2β2ν decay. This can be done with the
likelihood function.
Here the experimentalspectrum was again treated as a histogram. One then needs to take
into account that the distribution of the events in each bin is a Poisson one and independent
of the others. Thus, one constructs the likelihood function as:
L(N
β
, N
χ
) =
n
2
Y
i=n
1
e
−(N
β
η
β i
+N
χ
η
χ i
+N
bgri
)
N
exp i
!
(N
β
η
β i
+ N
χ
η
χ i
+ N
bgri
)
N
exp i
, (4)
where n
1
and n
2
are the bin numbers of the energy interval, N
exp i
is the number of
experimental events in the i-th bin, N
bgri
is the expected number of backgroundevents, and
η
β i
and η
χ i
are the Monte Carlo simulated efficiencies of 2β2ν and Majoron decays in the
i-th bin. Finally, N
β
and N
χ
0
are the average numbers of decays and they are considered
as free parameters.
To find the confidence level for the upper limit on the mean number of decays with
Majoron emission (N
χup
) this function (4) has to be normalized and then integrated over
all possible values of N
β
and N
χ
from 0 to N
χup
:
CL(N
χup
) =
N
χup
Z
0
dN
χ
∞
Z
0
dN
β
L(N
β
, N
χ
)
,
∞
Z
0
dN
χ
∞
Z
0
dN
β
L(N
β
, N
χ
). (5)
Again, this is an equation for the free parameter N
χup
,whereCL(N
χup
) is fixed. To
simplify the calculation in the case of
100
Mo, for bins with a large number of the events
(> 14 events) the Poisson distribution was replaced by a Gaussian distribution. The results
are presented in Table 4.
4.2. Results and discussion
The half-life limits for different isotopes and decay modes are presented in Tables 3
and 4. Using the half-lives one can get limits on the coupling constants for different
Majoron models via the relations (6) and (7).
R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352 349
Table 4
Limits on T
1/2
at 90% CL for decays with Majoron emission, estimated with help of likelihood
function
Nucleus
100
Mo
116
Cd
82
Se
96
Zr
n = 1 > 6.0 × 10
20
> 9.2 × 10
20
> 2.3 × 10
21
> 3.1 × 10
20
n = 3 > 1.6 × 10
20
> 3.5 × 10
20
> 6.3 × 10
20
> 6.3 × 10
19
n = 7 > 4.1 × 10
19
> 4.1 × 10
19
> 1.1 × 10
20
> 2.4 × 10
19
T
−1
1/2
=
hg
ee
i
2
|M|
2
Gfor2βχ
0
, (6)
T
−1
1/2
=
hg
ee
i
4
|M|
2
Gfor2βχ
0
χ
0
. (7)
The relevant matrix elements M and values of phase spaces G are presented in Tables 5
and 6. Using the data from Table 4 the limits on the coupling constants are calculated and
presented in Table 7. In addition, the limits on Majoron-neutrino coupling constants ob-
tained in the
76
Ge experiment[18]are presented.Note that for
100
Mo and
116
Cd there were
also limits obtained on decays with two Majoron emission (n = 3) for which the limits are
> 5.3 × 10
19
years (68% CL) [19] and > 2.6 × 10
20
years (90% CL) [20], respectively.
To summarize the results reported here more thoroughlyone can note the following. For
100
Mo the limit on decays with n = 3 obtained here is three times higher than that in [19],
Table 5
The pn-QRPA nuclear matrix elements for different nuclei. For
82
Se,
100
Mo and
116
Cd NME are
taken from [18]. For
96
Zr the M
F
− M
GT
is presented in [24], the M
CR
value is the lowest among
the other nuclei which is taken as a conservative estimation, and for the M
Fω
2
− M
GT ω
2
used the
same estimate as for the other nuclei in [18]
Nucleus M
F
− M
GT
M
CR
M
Fω
2
− M
GT ω
2
82
Se 4.03 0.14 10
−3
100
Mo 4.86 0.16 10
−3
116
Cd 3.29 0.10 10
−3
96
Zr 5.58 0.10 10
−3
Table 6
Phase-space integrals (G [y
−1
]) for different nuclei and models of decay [18]. Zr phase space for
n = 1 is taken from [41], and for n = 3 and 7 it is calculated following the formulas of [14]
Nucleus 2βχ
0
, n = 12βχ
0
, n = 32βχ
0
χ
0
, n = 32βχ
0
χ
0
, n = 7
82
Se 1.03 × 10
−15
3.49 × 10
−18
1.01 × 10
−17
7.73 × 10
−17
100
Mo 1.80 × 10
−15
7.28 × 10
−18
1.85 × 10
−17
1.54 × 10
−16
116
Cd 1.75 × 10
−15
6.95 × 10
−18
1.60 × 10
−17
1.03 × 10
−16
96
Zr 1.24 × 10
−15
1.07 × 10
−17
2.81 × 10
−17
3.26 × 10
−16
350 R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352
Table 7
Limits on the Majoron coupling constant hg
ee
i at the 90% CL for
82
Se,
96
Zr,
100
Mo and
116
Cd.
76
Ge results are presented for comparison
Model Mode n
82
Se
96
Zr
100
Mo
116
Cd
76
Ge [18]
IB 2βχ
0
1 < 1.6 × 10
−4
< 2.6 × 10
−4
< 2.0 × 10
−4
< 2.1 × 10
−4
< 2.3 × 10
−4
IC 2βχ
0
1 < 1.6 × 10
−4
< 2.6 × 10
−4
< 2.0 × 10
−4
< 2.1 × 10
−4
< 2.3 × 10
−4
IIB 2βχ
0
1 < 1.6 × 10
−4
< 2.6 × 10
−4
< 2.0 × 10
−4
< 2.1 × 10
−4
< 2.3 × 10
−4
ID 2βχ
0
χ
0
3 < 3.5 < 4.7 < 4.3 < 3.6 < 4.1
IE 2βχ
0
χ
0
3 < 3.5 < 4.7 < 4.3 < 3.6 < 4.1
IIC 2βχ
0
3 < 0.15 < 0.36 < 0.19 < 0.20 < 0.18
IID 2βχ
0
χ
0
3 < 3.5 < 4.7 < 4.3 < 3.6 < 4.1
IIF 2βχ
0
3 < 0.15 < 0.36 < 0.19 < 0.20 < 0.18
IIE 2βχ
0
χ
0
7 < 3.3 < 3.2 < 3.6 < 3.9 < 3.3
while the limit on decays with n = 7 is given for the first time. The result for n = 1 [22] is
several times lower than in [17].
The limit on
116
Cd decays with n = 3 is two times higher than that in [20]. The limit
on decays with n = 7 is presented for the first time. The limit on decays with Majoron
emission for n = 1, obtained in [20], coincides with the results of our earlier work [16].
Next, for
96
Zr all the limits are presented for the first time in a direct counting
experiment. They can be compared with the geochemical experiments, which give a half-
life, T
1/2
= (3.9 ± 0.9) × 10
19
years. This result is treated as a half-life for 2β2ν, while
T
1/2
> 3 × 10
19
y should be treated as a limit on all possible transitions
96
Zr →
96
Mo,
such as those involving Majoron emission processes. The NEMO-2 limits exceed those
obtained from geochemical experiments for all types of decays with Majoron emission
(n = 1, 3and7).
Finally, the
82
Se results for n = 3 and 7 are presented here for the first time. Note that
the result for the transition with n = 1 [23] is also the most stringent for
82
Se. Analysis
of the results documented above shows that the best limits on the coupling constant for
all “nonstandard” decays with Majoron emission (n = 3and7) were obtained with the
NEMO-2 experiment with
82
Se.
5. Conclusion
Though NEMO-2 was developed as a prototype for NEMO-3 [42], the limits obtained
on 2β -decay processes with Majoron emission are good enough. In particular limits on
“nonstandard” Majoron with n = 3 and 7 are more stringent than the limits coming from
other experiments. The current plan is to start measurements with the NEMO-3 detector at
the end of the year 2000. The total mass of the 2β-sources will be increased to 10–15 kg,
and different isotopes (
100
Mo,
82
Se,
116
Cd,
130
Te,
150
Nd and
96
Zr) will be investigated.
The sensitivity to half-life measurements for processes with Majoron emission (n = 1, 3
R. Arnold et al. / Nuclear Physics A 678 (2000) 341–352 351
and 7) will be improved by 10 to 100 times, while the limits on the coupling constant will
be improved by 3 to 10 times, depending on the type of decay.
Acknowledgement
The authors would like to thank the Fréjus Underground Laboratory staff for their
technical assistance in running the experiment, and are very thankful to V.I. Tretyak for
coding new Majoron decay modes. Portions of this work were supported by the Russian
FoundationforBranchInvestigations(RFBI) undercontract97-02-17344,by INTAS under
grant 96-0589 and by US Department of Energy Grant DE-FG02-90ER40553.
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