The possibility of investigating neutrino oscillations in the proposed MACRO experiment are considered. Its sensitivity taking into account the theoretical uncertainties coming from flux calculations, geomagnetic effects and propagation through matter, and the experimental limitations

Musser, J.

English

NASA Technical Reports Server

128
HE 5.3 - lO
MEASUREMENTOF NEUTRINOOSCILLATIONS IN MACROEXPERIMENT
, The MACRO Collaboration
'Paperpresented by J. Musser
ABSTRACT: We considerthepossibilityofinvestigatingeutrinooscillations
in the proposedMACRO experiment.We calculateitssensitivityakingintoac-
count the theoreticaluncertaintiescoming from fluxcalculations,geomagneticef-
fectsand propagationthroughmatter,as wellasthe experimentalimitations.
I. The use of atmospheric cosmic ray neutrinos in underground experiments
for investigating neutrino oscillations could in principle allow to explore a region of
parameter space beyond that accessible in reactor or accelerator experiments.
Unfortunately, many limitations prevent to reach the nominal high sensitivity
deriving from the long oscillation path (i.e. the Earth diameter D _ 104Kin.)
First, the low statistics ineherent in this kind of experiments makes small
mixing angles not accessible. Furthermore, an average over the neutrino energy
spectrum andneutrino directions, i.e. over oscillation lengths, is unavoidable: in
particular, the effect of averaging over directions is much larger than the averaging
over source dimensions.
The last limitation arises because of the matter. The different indices of re-
fraction in matter for u_ and ue, generated by the charged current contributions to
elastic u_ - e scattering, lead to a decoherence of the u_ and uc components after a
characteristic length of ,-_9000 Km in the earth.
2. Considering only two neutrino flavours, namely u_ and u_, an underground
experiment can record the disappearence of u_ or the variation of the ratio u_/ue.
The disappearence method needs the knowledge of the expected flux and hence is
affected by uncertainties of the order of 10 - 20_, as confirmed also from results of
large nucleon decay experiments.
The influence of the Earth magnetic field, as well as that of solar wind, adds
further uncertainties to the calculation of the low energy" part of the spectrum.
https://ntrs.nasa.gov/search.jsp?R=19850027791 2020-03-20T17:03:27+00:00Z
129
HE 5.3 - 10
The angular distribution of the muons produced by v in the surrounding rock
J
and the ratio z/_,/ve are however less sensitive to systematic uncertainties. In fact,
the energy of parent neutrino for muons traversing an underground apparatus is
high enough ( Ev >_ IOGeV ) to allow neglecting geomaoo_etic effects, and the
comparison between different path lengths makes unnecessary an absolute monitor
on the flux, provided the angular distribution is known with sufficient accuracy.
The ratio g_]ve is obviously less dependent on theoretical uncertainties.
Two final remarks on experimental possibilities are in order.
A direct comparison of the downward and upward neutrino fluxes using muons
produced in the rock is difficult, since, at the deepness of the existing and proposed
large underground detectors the cosmic muon background is too high even near the
horizontal direction.
In addition, the energy of neutrinos that produce muonic events contained in
a detector of reasonable size is very low, so the angle between the produced muon
and the parent neutrino is large, making less efficient the distinction between up
and down directed neutrinos.
3. We evaluated the sensitivity of MACRO [1]using a complete Montecarlo
computation, including generation, transport and tracking of leptons. The v_, flux
is taken from ref. [2], assuming equal contribution for v_, and _, and _(ve) =
0.2 x 6(v_,). The interaction cross section includes quasielastic, A and inelastic
channels. The survival probability for vu is :
P(v u --+ v_) = 1 - (1 - p)sin_(2O)sin2(_rL/Iv) (1)
in vacuum, and
-+ = - - (2)
in matter.
130
In the above formulas, 0 is the vacuum mixing angle, p the ratio ve/vu, lv =
2.5E](m_ - rn_) ( oscillation length in vacuum ), 10 = 2zr/GN_ = 9 × 103 Km and
= t(1 + (l/1o)2 21/tocos20)-l/2 (3)
is the oscillation length in matter [3]. The signs refer to neutrino and antineutrino
respectively.
A p produced inside the detector must be fully contained to cut the background
of the entering stopping muons, and its verse of motion cannot be identified. Thus
we consider only the total number of contained events.
The maximum flux reduction for oscillations in such case is 20% requiring a
knowledge of the expected flux better than the present.
A g produced in the rock must traverse the detector with a minimum track
length to allow the determination of the verse of motion by time-of- flight measure-
meat. The median energy of parent neutrinos is, in this case, ,-- 60GeV and the
mean v - p angle is ,._3.5 °.
The modulation factor, defined as the ratio between the measured and the
expected angular distribution of the detected muons, is shown, for various Am 2
and at maximum mLxing, in Fig.1 and 2.
Matter effects reduce the sensitivity in Am 2 by about an order of magnitude.
The error bars indicated in Fig.2 correspond to about 3 years of exposure and
show that a 3 _r lower limit of Am 2 = 5 × 10-a is achievable. For Am 2 > 5 x 10-2
the modulation factor becomes again nearly flat, giving the upper limit for this
experiment.
REFERENCES
[l] See paper HE 6.1 - 4 in these proceedings.
[21 L.V. Volkova, Sov.J.Nucl.Phys. 31, 784 (1980)
[3] L. Wolfe,,stein,Phys. Rev. 917", 2369, (1978)
131
HE 5.3- I0
Figure Captlons
1) Modulation factor at MACRO, only vacuum oscillations.
2) Modulation factor with matter effects taken into account. Error bars refer
to 3 years of data taking.
• Am_=10:4... ......---.. I
I *eeeejoeeeeeeeee_eeea ee eeD ee • oe" • eeoeje eJe-
A m2-10 -3 ." ....
ee ee eeeee =e _'* "°e'e •
.9 ..-'" .."" .9 .......... -
• ee°• • .e
• • le
ee • • • e e*5x10 -4 • • .. ..
• eeoeel oe ee •
.8- ..' .8 _xlO-_'"
• eoe e
eee e oiiel eli
• eeo e°*ee
ee e oeoeo e°e
.7- .'" .7 ..'"
ee e e o
x 10-3... 5x10-2.. ."vacug m "''''''" matt er
,,eeee J _•
.6 .6
1 I 1 I
180° 150° 120° 90° 180° 150° 120° 90o
H
fig. 1 fig.2


Measurement of neutrino oscillations in MACRO experiment

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server