A large area underground detector with accurate muon tracking and directionality can be used for the search of extraterrestrial sources of high energy neutrinos. The sensitivity of the MACRO detector to possible sources of neutrinos was evaluated with a Monte-Carlo simulation of the neutrino interaction in the rock and and of the detection in the real apparatus. Two categories of possible neutrino sources are discussed in comparison with the detector sensitivity. Promising candidate objects for this search appear to be the two binary X-ray sources in the southern key Vela X1 and LMC X4, which are known to emit gamma rays up to the 10,000 TeV region

Auriemma, G.

English

NASA Technical Reports Server

HE 5.3-12
HIGH ENERGY NEUTRINO ASTRONOMY WITH MACRO
The MACRO Collaboration
Paper presented by G. Auriemma*
ABSTRACT
A large area underground detector with accurate muon tracking and directionality can be used
for the search of extraterrestrial sources of high energy _neutrinos. The sensitivity of the MACRO
detector to possible sources of neutrinos was evaluated with a Monte-Carlo simulation of the neutrino
interaction in the rock and of the detection in the real apparatus.
Two categories of possible neutrino sources are discussed in comparison with the detector sensi-
tivity. Promising candidate objects for this search appear to be the two binary X-ray sources in the
southern sky Vela X1 and LMC X4, which are known to emit _-rays up to the 104 TeV region.
A large flat underground muon detector can be used as a neutrino telescope because the horizontal
and upward going muons are generated only by neutrino interactions in the rock 1,2). The MACRO
apparatus 3) in the Gran Sasso Laboratory is particularly well suited to this application due to its
intrinsically very high tracking accuracy (/_0 <__1°), its large area (_ 1000 m 2) and its time-of-flight
resolution (_ 1 nsec). The depth of the Laboratory (_, 4000 m.w.e.) assures a good filtering of the
downward muons generated by the cosmic ray interactions above the Gran Sasso mountain. The
omnidirectional measured muon flux in the Laboratory 4) is N 1/_ m -2 hour -1. Therefore the time
of flight measurement given by the scintillator counters can distinguish the upward-going component
of the muon flux with negligible contamination. Under this circumstances the background to the
detection of extraterrestrial neutrino sources is given by the conversion of atmospheric neutrinos. The a
estimated rate of this background 5) is _ 1/_ day -1 over the entire MACRO apparatus.
An analytical computation of the response of underground detectors to extraterrestrial neutrinos
was reported by Gaisser & Stanev 6). This computation has shown that the detection efficiency for
neutrinos is a fast rising function of the neutrino energy. Therefore the detectability of extraterrestrial •
neutrinos is related to the hardness of the spectrum of the source.
We have computed the response of MACRO to extraterrestrial neutrino sources with a Monte-
Carlo program in order to derive that parameters, such as detection efficiency and the angular spread
of the muons respect to the neutrinos direction, which cannot be easily deduced from analitycal
computations.
The rate of muons in the MACRO apparatus can be factorized as follows :
/;T
where A(_, _)isthe time averagedexposed areaforthe pointinthe sky with equatorialcoordinates
_, 6, e the detection efficiency in the apparatus, Er the minimum energy of the muon which can
be detected in the apparatus, Pv the probability that a neutrino produces an upward muon at the
• I.N.F.N. Sezione di Roma and Dipartimento di Fisica ,Universit_ di Roma "La Sapienza ", Rome,
Italy
https://ntrs.nasa.gov/search.jsp?R=19850027793 2020-03-20T17:03:23+00:00Z
137 HE 5.3-12
detector (see Gaisser & Stanev 6)) and F v is the spectral distribution of the neutrinos. Assuming, as
costumary for astronomical high energy sources, a power-law spectral distribution for the neutrinos:
F_ = g_ x E_ -_ (2)
we have estimated both the efficiency and the average neutrino conversion probability Pv, defined :
-fiv= 'f_ F_× Pv dE_
and the average energy Ev of the neutrinos which have originated a muon detected in the apparatus:
......I_T _"x Pv dEv (4)
In addition from the Montecarlo we have obtained also the angular point-spread function of the muons.
Detailed description and results of the Montecarlo will be reported elsewhere 7). We summarize in
Table I the results obtained for spectral index from -2 to -3.8. We have included in the Monte-
Carlo code the angular spread originated by the neutrino-nucleus interaction as well as the effect of
transport in the rock. The differential cross-section of neutrino-nucleus scattering used takes into
account both the effect of the mass of the W boson and the nucleon structure functions according to
Duke & Owens parametlSzation s}. In the transport section of the program we have included only
the multiple Coulomb scattering of the muon . The distribution of the angle 0 between the muon
in the apparatus and the neutrino original direction shows a central dominating gaussian core and
a tail corresponding to relatively rare large angle scatterlngs.We have therefore quoted two angular
paranaeters in Table I. The one reported in column 6 is the r.m.s, angle wieh is dominated by the
Coulomb scattering and the one reported in column 5 is the angle wich includes 90 % of the total
muons. Comparison between the two angular parameters shows that steeper spectra can be easily
identified by the larger angular spread.
TABLE I. Summary of Montecarlo results
_/ Zls e fly 090 % OR.M. 8 M.D.F.
, (TeV) ET = 1GeV (ergs cm -2 s -x)
2.0 23.3 .75 6. x 10 -9 < 1 ° .47 ° 2. x 10 -s
2.2 7.8 .68 1. x 10 -9 1° .61 ° 1, x 10 -r6
2.6 .86 .56 7. X 10 -11 2°.25 10.2 1. X 10 -e
3.0 .095 .39 1.2 X 10 -11 6°.0 2°.0 1. × 10 -5
3.8 .01 .14 3.2x I0-12 11°.5 4°.6 1.7x 10-4
The detectionofextraterrestrialneutrino sourceswith MACRO islimitedby the sensitivityof
the experimentitselfand not by the atmosphericbackground . Infactthe resolutionofthe telescope
forhard neutrinosources(i.e.sourceswith spectralindex? ,-,-2) isbetterthen 1 degree.Therefore
HE 5.3-12
138
the expected atmospheric background is _, .05 #tyear -1. The detection of neutrino emission from an
identified celestial object can be unambiguously proved by the clustering of a small number of muons
in a cone of aperture -,, 1° around its direction. Therefore the minimum detectable flux (MDF) from
a source, reported in Table I corresponds to the neutrino flux which gives 10 muons in a cone of 1°
aperture in five years of exposure of the apparatus. The intrinsic neutrino luminosity of a source
detectable with MACRO is :
Lv > (M.D.F.)x 41rD2 (5)
where D is the distance of the source.
The localization of the Gran Sasso Laboratory at 38°N makes the MACRO experiment suitable
for the survey of the southern celestial emisphere. Infact the time averaged exposure area in formula
(1) is a function of the source celestial declination. We have reported in Fig. 1 a plot of the equal
exposure contours in a galactic coordinates plot. The sensitivity given in table 1 corresponds to
the maximum exposure in the direction of the celestial southern pole. For a given source the MDF
reported in Table I must be scaled to the actual exposed area
+90 o
V
+180 o
• G.C, _' I
_90o
Fig. 1 MACRO field-of-view in Galactic coordinates.Contours represent equal time-averaged exposed
area. The shaded region in the northern emisphere is inaccessible to MACRO survey.
It was shown that the MACRO survey will be sensitive mainly to high energy neutrinos. Several
classes of objects were proposed as strong neutrino sources 9). We will discuss briefly the detectability
with MACRO of two interesting classes of objects.
a) U.H.E. binaries
Recently U.H.E. '7-rays (E. 1 _> 104 TeV) emission was observed from the well known X-ray
binaries Cyg X3,Vela Xl and LMC X4 xo,n,12,1s). Very reasonable models of this objects suggest
that hard neutrino emission is expected it) in the TeV region, if the U.H.E. -_ emission is originated
from hadronic interaction of accelerated protons (or nuclei) on the non-degenerate companion star.
The spectral index of the neutrinos is predicted to be q < 2 from the observed "/-ray spectrum. In
HE 5.3-12
139
Fig. 1 we observe that two of this sources (viz. Vela Xl and LMC X4) are in a very good position for
the MACRO apparatus.In particular the source in the Large Magellanic cloud is predicted to have a
neutrino luminosity well above the detectability of MACRO.
b)Active Galactic Nuclei
Radio and X-ray observations of the compact nuclei of several peculiar galaxies (viz. Seyfert's
type I galaxies, BL-Lac objects and Quasars} indicate that extreme non-thermal emission originate the
huge luminosity of these"objects. Neutrino emission could be associated to this non-thermal emission
15,16}. In particular the neutrino radiation could be extremely relevant in the case of continous re-
acceleration of particles in the source. Recently high quality observations of the X-ray spectra have
shown that the non-thermal emission of a large sample of objects can be fitted with an _universal _
power law distributionZT). This could be the evidence that in all these sources the particle spectrum
has a spectral index q ,-, 2.3. In principle we can expect that the neutrino spectral index would be
the same. To be detectable with MACRO a source with spectral index -2.3 should have an intrinsic
neutrinoluminosityof]_ _>1045 X D_p cergscrn-2 8-I. Thisneutrinoluminosityisanyway too
much highalsoforthesetypeofobjects,accordingtothecurrentestimates.
We aregratefulltoT. Gaisser,T. Stanevand V. Stengerforveryhelpfulldiscussions.
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High energy neutrino astronomy with MACRO

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