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AMANDA Observations Constrain the Ultra-High Energy Neutrino Flux
Francis Halzen1 and Dan Hooper2
1Department of Physics, University of Wisconsin, Madison, WI
2Particle Astrophysics Center, Fermilab, Batavia, IL
A number of experimental techniques are currently being deployed in an effort to make the first
detection of ultra-high energy cosmic neutrinos. To accomplish this goal, techniques using radio and
acoustic detectors are being developed, which are optimally designed for studying neutrinos with
energies in the PeV-EeV range and above. Data from the AMANDA experiment, in contrast, has
been used to place limits on the cosmic neutrino flux at less extreme energies (up to ~10 PeV). In
this letter, we show that by adopting a different analysis strategy, optimized for much higher energy
neutrinos, the same AMANDA data can be used to place a limit competitive with radio techniques
at EeV energies. We also discuss the sensitivity of the IceCube experiment, in various stages of
deployment, to ultra-high energy neutrinos.
PACS numbers: 95.55.Vj, 95.85.Ry, 98.70.Sa; MADPH-06-1465; FERMILAB-PUB-06-097-A
As ultra-high energy protons and nuclei propagate over
cosmological distances, they interact with the cosmic mi-
crowave and infra-red backgrounds, producing pions .
These pions decay, generating neutrinos with typical en-
ergies in the range of 107 to 1010 GeV. These ultra-high
energy particles, known as cosmogenic or GZK neutrinos,
have not yet been observed.
A wide range of experimental efforts are currently un-
derway to detect ultra-high energy neutrinos . These
include experiments using radio antennas such as RICE
under the Antartic ice of the South Pole , and the
balloon mission ANITA (and its predecessor ANITA-
lite) . Techniques for observing ultra-high energy neu-
trinos using acoustic detectors are also being explored,
although the prospects for this technology are not yet
well understood . Cosmic ray experiments, such as
the Pierre Auger Observatory , are also capable of de-
tecting ultra-high energy neutrinos by observing slighly
upgoing showers produced by Earth-skimming tau neu-
trinos or deeply penetrating quasi-horizontal showers .
Each of these experimental techniques is specifically
suited to studying particles (ie. neutrinos or cosmic rays)
at extremely high energies. The effective volumes of
RICE, AUGER and ANITA become appreciable only
above roughly ~107 GeV, ~108 GeV and ~109 GeV,
respectively. Acoustic techniques are likely to have
energy thresholds comparable to or higher than these
other methods. In contrast, high-energy neutrino tele-
scopes using optical detectors, such as AMANDA ,
ANTARES , IceCube , and KM3 , while also
capable of observing ultra-high energy neutrinos , are
designed to be sensitive to neutrino induced cascades
with energies as low as a few TeV.
The strongest limit on contained neutrino-induced
cascades in the ~PeV energy region has been pub-
lished by the AMANDA collaboration . This limit
(E2dN /dE < 8.6 x 10-7 GeV s-1 sr- cm-2, at the
90% confidence level, in the 50 TeV to 6 PeV energy
range) was arrived at after making a number of exper-
imental cuts on the data to remove a large fraction of
the background from atmospheric neutrinos and muons.
Furthermore, the analysis was optimized for a spectrum
falling with dN /dE x E72
This approach is less than ideal for searching for cos-
mogenic neutrinos, which appear as isolated spectacular
events of much greater energy (~10 EeV). At these en-
ergies, the showers generated in an experiment such as
AMANDA are very large (approximate radii of ~ 300 me-
ters for a 107 GeV shower and ~ 500 meters for a 10l
GeV shower; considerably larger than the instrumented
width of AMANDA). Such enormous volume events are
expected to have a negligibly small background due to
the rapidly falling spectrum of atmospheric neutrinos and
muons. The information contained in these events is so
much superior to that of average AMANDA events, that
throughout this study we assume that showers and muons
can be efficiently separated. Given that the background
can be effectively eliminated, the optimal analysis strat-
egy for studying this class of events is very different from
that suited for neutrinos with TeV-PeV energies.
The largest shower reconstructed in the AMANDA ex-
periment has an energy close to 100 TeV, occupying a vol-
ume that triggered fewer than 300 of its 677 modules .
Motivated by the fact that no larger events have been
observed, we have considered an alternative cut designed
to retain as many potential ultra-high energy events as
possible. This cut is to simply consider only events in
which 300 or more of the modules trigger while omitting
the restrictive cuts of Ref. . Our simulation adopts
a simple geometric approach in which spherical showers
are overlayed against the AMANDA geometry (roughly
a cylinder with height and diameter of 600 and 200 me-
ters, respectively) . This method reflects the science
involved and can be considered a good approximation for
such extremely large events, even in the absence of a full
and detailed detector simulation. The physics of neutrino
interactions and detection at these high energies has been
established and confirmed though the calibration of the
In Fig. 1, we compare the effective volume (for elec-
tron neutrinos) of the AMANDA experiment, as calcu-
lated using the cuts adopted in the cascade analysis opti-
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Halzen, Francis; /Wisconsin U., Madison; Hooper, Dan & /Fermilab. AMANDA Observations Constrain the Ultrahigh Energy Neutrino Flux, article, May 1, 2006; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc882172/m1/1/: accessed June 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.