SEARCH FOR MASSIVE NEUTRINO BRANCHES IN NUCLEAR BETA DECAY

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SEARCH FOR MASSIVE NEUTRINO BRANCHES IN NUCLEAR BETA DECAY

G. Colvin, K. Schreckenbach, F. von Feilitzsch

To cite this version:

G. Colvin, K. Schreckenbach, F. von Feilitzsch. SEARCH FOR MASSIVE NEUTRINO BRANCHES IN NUCLEAR BETA DECAY. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-139-C3-142.

�10.1051/jphyscol:1984326�. �jpa-00224040�

SEARCH FOR MASSIVE NEUTRINO B R A N C H E S IN N U C L E A R BETA DECAY G. Colvin, K. Schreckenbach and F. von ~ei~itzsch*

Zxtitut Laue-Langevin, 156X, F-38042 Grenoble Cedex, France

*physik Department, T U M iinchen, 8046 Garching, F.R .G.

RdsumG - La ddsintegration bBta de 6 4 ~ u a bt6 btudiee entre 30-460 KeV pour determi- ner le degr6 de m6lange des neutrinos massifs. Une limite de sin 2 O = 3,s x a 6t6 obtenue B m u = 350 KeV.

Abstract

-

The possibility of non-zero neutrino rest masses and mixing of massive neutrinos is still an open question both from the experimental and theoretical side. Experimen- tally neutrino masses m _w < 60 eV, m w < 500 KeV, mvT < 250 MeV /I/ are not excluded,

-

-

and only Lubimov et al. report a definite mass for ve 12.1. For the description of neutrinos as superpositions of neutrino mass eigenstates (and hence neutrino mixing) there is to our knowledge no firm experimental evidence.

In our work we have studied the question of neutrino mixing by searching for an admixture of a heavy neutrino mass eigenstate to the main light component of the electron neutrino. Most experiments on this problem have been performed by looking for oscillations with reactor or accelerator neutrinos / 3 / . However, if during a reaction the mass of a neutrino eigenstate becomes comparable with the mass and kinetic energy of a charged lepton, a massive neutrino branch should produce a signal in the momentum spectrum of the lepton. For example, the analysis of the electron in tritium decay and the muon in pion decay give best limits for the mass of v and v respectively.

P

For superposed mass eigenstates, the momentum spectrum of the charged lepton shows a series of "kinks" 141. In the present experiment we refer to the search for such kinks in the electron spectrum in nuclear beta decay for mass eigenstates in the range 30-500 KeV 151.

For the simplified case of two neutrino mixing, the weak interaction eigenstates of v can be described as

I

1

where 8 is the mixing angle and

I

A beta-decay comprising two branches with neutrinos vl and v2 would show a momentum spectrum N for the emitted beta particles of

B

which for the case of an allowed beta-decay and mvl 2 ⁰ has the form

2 2

N p f 6 COS 0

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984326

C3-140 JOURNAL DE PHYSIQUE

where f = p F' (pe is the electron momentum and F the Fermi function), and 6 = E - E (E is the end -point energy for m = 0)

o B o

In the Kurie representation (K =J [I$ /F]) and small admixtures, sin 2 8 , the observed Kurie plot will be

K 6 cos8 + (sin 812 cos8) 2 (rS2

-

"2 "2

Thus the Kurie plot shows a distinct deviation from linearity for 6 > m and thus Y '

provides a sensitive search for massive neutrino branches in the spectrai shape of beta decay.

To limit problems arising from background structure and to have a firm signal in the case of a massive branch, a comparative investigation of the positron and electron spectrum of the decay of 6 4 ~ u (t = 12.07 h) was undertaken with a magnetic spec-

112

trometer. The isotope 6 4 ~ u has the advantages that both decay modes have single end point energies and are allowed beta decays (see fig. (1)). Also the positron spec- trum will be free from lines and edges. Therefore a massive branch of mass mTn

-L

would be present as a kink in both spectra at energies EP = E6+ - my and EB- = Eo- - ^m

.

Vq

spectra would 6e a firm signature for a massive neutrino branch in the beta decay.

The instrument used for this measurement was the high resolution iron-core spectro- meter "BILL" situated at the HFR Grenoble /6/. A resolution of AppEp - = 6 x 10-4 was achieved. Approximately 3 x 10 8 events were recorded for each spectrum in the range 6 = 30 - 500 KeV. To determine the extent to which a massive neutrino branch occured in the experimental spectra, a derivative method was used which took into account the expected shape of a massive neutrino branch / 5 / .

.6%

f -

\

electron decay in 6 4 ~ u

of 40% EC

191 /C*

~ = 6 5 3 k e V

The exclusion plots based on both the B + and the B - spectrum analysis is shown in figure (3). The statistical accuracy of our experiment determined the limit of the sensitivity. The regions which are neither excluded by the 6' or B -

spectra lies below the solid line in figure (3), and thus defines the final limit achieved in the present work. It should be emphasised that these limits are valid for any kind of massive neutral particles which are emitted with the electron in the beta decay. In this sense the result is more general than appearance experiments at accelerators, which have achieved similar limits bi~t only for specific channels such as ve + v _!J

.

In conclusion, no evidence has been found for a massive neutrino branch in the beta decay of 6 4 ~ u in the range m, = 30-400 keV. We believe that with the present compa- rative method, an even higher sensitivity may be achieved. By use of irradiation facilities at a HFR, and off-line measurement (to reduce background) with an iron-

'(re/. _- u:its) - ^. - ^.

. _{. . -}

\

_, .

mv=360keV

-. ^._

-

_,

- ^. _{. -. .}

\

, ..

\ _\

--. .

- 2 0 0 3 0 0 400 500

I I 1

500 4 0 0 300 200 100 b(keV) (I

Fig. 3 - Exclusion plots for massive neutrino branches deduced from present measure- ments. The solid line shows the achieved limits of the combined analysis of the B + and 6 - spectra (90 % confidence). The region above this line is excluded by these measurements.

Fig. 2 - Straightened and horizontally turned Kurie plot of the B + spectrum with statistical error bars (per KeV window). A hypothetical sin20 = 0.03 branch with m, = 360 KeV and 100 KeV is shown.

L : I I I I

3 -

.... -

n

-

-. .. . .._

..

3 -

.-

*

-

$

i3

-

limit from p-spectrum :

~ ~ t J : ~ ~ a ~ r o m rl+B

comb~r~ed l ~ m l t @

I I I I

0 100 200 300 LO 0 500

C3-142 JOURNAL DE PHYSIQUE

free spectrometer (allows fast changing between 8' and 6-1, a sensitivity of 10 -3 should be easily achievable. A higher statistics may be possible using Si detectors in which a thin @+source is sandwiched between two Si detectors and the events mea- sured in coincidence with the 511 keV annihilation radiation. Thus the sum pulses of the two Si detectors should result in a high sensitivity being achieved.

/I/ BERGQUIST K.E., Nucl. Phys. B39 (1972) 317 ANLlERHUB H., et al., Phys. LX. (1982) 76 BACINO W., et al., Phys. Rev. Lett. 42 (1979) 749 / 2 / LUBIMOV V.A., et al., Phys. Lett.

94B

KWON H., et al., Phys. Rev. - D24 (1981T24 141 SHROCK R.E., Phys. Lett. 96B (1980) 159

McKELLER B.H.J., Phys. ~ e (1980) 93 x ~

/5/ SCHRECKENBACH K., et al., Phys. Lett.

129

154