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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.
Printed
in Great
Britain
150
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.
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.
Printed
in Great
Britain
150
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.