A signature of solar antineutrinos in Superkamiokande

Progress in Particle and Nuclear Physics PERGAMON

Progress

in Particle

and Nuclear

Physics 40 (1998) 149-150

A Signature of Solar Antineutrinos in Superkamiokande G. FIORENTINI,

M. MORETTI

We propose to exploit the anisotropy p --t n + e+ for extracting background.

The sensitivity

A fraction

emitted in the reaction i7e +

signal from the Superkamiokande

to Y, + ~~ transition

probability

collected in the first hundred

this sensitivity

test of several theoretical

of the positrons

a possible antineutrino

already with the statistics of data taking,

and F. L. VILLANTE

days.

is at the 3% level Within three years

will reach the 1% level, thus providing

a stringent

models.

of the v, formed in the core of the Sun, as predicted in several theoretical model (see

[l] and references therein), could transform into Ve during their trip from Sun to Earth.

This work

shows that, by exploiting the anisotropy of the positrons emitted in the reaction Pi + p -+ n + e+, it is possible to disentangle a solar antineutrino signal from Superkamiokande (SK) background. As well known, the specific signature of antineutrinos in hydrogen containing materials is through the inverse beta decay (IPD), Pi +

p +

n + e+,

which produces almost isotropically distributed mo-

noenergetic positrons (&+ = ET- Am; Am = m, - mP). For energy above a few MeV, the differential cross section is:

~ da

= -741 Q(Ed

dcosb’

where:

2

- acos01

h/SV)” - i

a= _I

,

\^

S(gA/gV)‘+ gV (gA)

’

No

i

.-.

1

is the vector (axial) coupling of the neutron and o,-,(E?) is the total cross section for antineutrino

energy &. In the absence of a solar antineutrino flux, the SK background is expected to be isotropic. In the presence of solar antineutrinos, the positrons emitted by IPD contribute to the background.

As

a consequence, due to the angular dependence of IPD cross section, SK background should have a non-zero angular slope proportional to the antineutrino flux.

A linear fit to the counting yield,

C = C,-,- C1 cos 9 (in the angular region where events from the 1/- e interactions can be neglected, see Fig.

i j j, provides the antineutrino fiux Gi7 through the reiation:

where NP is the number of free protons, T is the exposure time, 6 is the (assumed constant) detection efficiency, EO the minimal detectable antineutrino energy and ire is the cross section averaged over the antineutrino spectrum for ET > Eo. 014&641O/~~/$19.00

i- 0.00 0

PII: SOl46-6410(98)00020-9

1998 Elsevier Science BV. All rights reserved.

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C. Fiormtini et al. /Prog. Pmt. Nuci. PhJs. 40 (1998) 149-150

,,I,

-1

,,,I

-0.5

IIII

IIll

0.5

0

1

00se

Figure 1: Sketch of the expected angular distribution of events in the presence of a solar T?, flux. In order to provide a quantitative illustration of the previous points we used data from the first 101.9 operational days of SK, as reported in fig.3 of [2], corresponding to Es = 8.3MeV.

The reported

background does not show an angular dependence. According to equation (3), to extract an upper limit on the solar antineutrino flux, we must know the average cross section ~0, which can be determined within two approches: a)Assuming

that the antineutrino spectrum has the same shape as that of sB solar neutrinos, one

has ~0 = 7.06 . 10-42cm2. This gives, as a final result, a,(&

> 8.3MeV)

< 6

I04cm-*s-i,

to the

95% C.L. This bound corresponds to a fraction x=3.5% of the solar neutrino flux (in the energy range E, > 8.3MeV)

predicted by the SSM [3].

b) As us is an increasing function of ET, one has Zs 2 us(&) gives a model independent

bound @,(Ep

> 8.3MeV)

< 9

= 4.5. 10-42cm2. This lower limit to Zs

I04cm-*s-l

to the 95% C.L.

In conclusion, we remark some important points of the method just presented: e The sensitivity to antineutrinos increases as statistics accumulates. the slope Ci is limited by statistical fluctuations, AC, A@,, c. l/m.

N fl

0: m

In fact the accuracy on and consequentely

W’tIh in three years of data taking, the sensitivity to v, -+ V, transition

probability will reach the 1% level, thus allowing for a definite test of several theoretical models. s The determination of the angular slope Ci provides a mean for detecting antineutrinos from the Sun (and not only for deriving upper bounds). A non vanishing slope for SK background would be, in fact, a clear signature of a solar antineutrino flux.

References [l] 6. Fiorentini, M. Moretti, F.L. Villante, Phys. Lett. B, in press, 1997 [2] Y. Tots&a,

“First result from Super-Kamiokande”,

presented at Texas Symposium (1996).

[3] J.N. Bahcall and M.H. Pinsonneault, Rev. Mod. Phys. 61 (1992) 885.