david hardtke- exploring the neutrino universe with amanda and icecube

8/3/2019 David Hardtke- Exploring the Neutrino Universe with AMANDA and IceCube

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May 17, 2005 1 1:36 P roceedings Trim Size: 9in x 6in LLWIHardtke

EXPLORING THE NEUTRINO UNIVERSE WITH

AMANDA AND ICECUBE

DAVID HARDTKE

FOR THE ICECUBE COLLABORATION

Department of Physics

University of California

Berkeley, CA 94720

High-energy neutrino flux measurements allow us to probe the extreme astrophys-

ical environments where hadronic acceleration may occur, and could provide newinsights on the origins of very high-energy cosmic-rays. Recent results from the

AMANDA telescope are presented. These results include searches for point sourcesof high energy neutrinos, and searches for diffuse fluxes of extra-terrestrial high-energy neutrinos. The IceCube observatory, currently being constructed at the

South Pole, will be the first cubic kilometer scale neutrino detector.

At this conference we saw many examples of how astrophysical obser-

vations motivate current experimental research. The baryon-antibaryon

asymmetry in the universe motivates us to look for CP violation in the

B-meson system. The observational success of the CDM model motivateus to look for supersymmetric particles at existing accelerators and the

LHC. The emerging field of neutrino astronomy, the subject of this contri-

bution, is motivated by the desire to find the origins of the highest energy

cosmic rays (E > 1019 eV). Do these ultra-high energy cosmic rays arise

from persistent sources (e.g Active Galactic Nuclei), transient sources (e.g.

the sources of gamma-ray bursts), or do they come from decays of exotic

particles with masses of order the Planck scale? By exploiting the unique

properties of the neutrino we hope to answer this question.

Extra-terrestrial neutrinos have to date been observed from the Sun and

from Supernova 1987A. In both cases, the observed neutrinos were of low

energies (few MeV) and were registered in deep underground experiments

exploiting the inverse beta decay process or elastic neutrino-electron scat-

* List of authors at http://icecube.wisc.edu/pub and doc/conferences/conference-

papers.shtml

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tering. The strategies for the observation of high energy neutrinos differdramatically. In the TeV - EeV range, the favored method is to use natural

sources of water as a neutrino target and Cerenkov radiator. The Cerenkov

radiation is generated either by long muon tracks created in + N charged

current interactions or by relativistic electrons produced during the elec-

tromagnetic and hadronic cascades produced at the neutrino interaction

vertex. Many experiments (DUMAND, Baikal, Antares, etc.) have used

liquid water as a detector. AMANDA (Antarctic Muon and Neutrino De-

tector Array) and IceCube are the first to use polar ice as the neutrino

target and detector. Polar ice is well suited for the construction of large

neutrino detectors due to the long absorption lengths for light ( 100 m at

400nm).

AMANDA consists of 677 optical modules arranged on 19 strings at

depths between 1500 m and 2300 m. Each optical module has a large pho-

totube housed in a spherical glass pressure vessel. The vertical spacing

between optical modules varies between 10 and 20 meters, and the hori-

zontal spacing between strings averages 50 m. Raw PMT signal are sent to

the surface via electrical cables or optical fibers. At the surface, the signals

are amplified and fed into a system of ADCs and TDCs. Prompt signals

from the phototubes are used to form a multiplicity trigger (typically 24

phototubes in the current detector configuration). Most of the triggers are

due to down-going muons.

IceCube, currently under construction, will utilize the same basic con-

cepts as AMANDA but with a much larger effective detection volume( 0.01 km3 for AMANDA versus 1 km3 for IceCube). With such a

large volume a different data acquisition and trigger system is required.

IceCube will have 4800 digital optical modules (DOMs) on 80 strings at

depths between 1500 m and 2500 m. The major technological upgrade that

allows IceCube to operate is the use of signal digitization in the ice. Each

DOM contains a phototube, on-board high voltage, and waveform digitiz-

ers. The DOMs are read out asynchronously via a DSL connection. The

surface DAQ will build events with common hit times and trigger the array.

The latency of the trigger can be as large as several minutes. Online recon-

struction will be done at the South Pole to eliminate most down-going muon

events. During the austral summer 2004-2005, the first IceCube string was

successfully installed, demonstrating both the performance of the new En-hanced Hot Water Drill and allowing in-situ tests of the DOM hardware.

In addition, the first few tanks of the IceTop surface air shower array have

been installed and tested.


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Due to the long lengths of the neutrino induced muon tracks, thesesparsely instrumented arrays are able to reconstruct neutrino directions

with a resolution of order 1-3. Muon energies (and thus inferred neutrino

energies) are measured via dE/dx. The resolution for muon reconstruction

is (log10E) 0.4. Neutrinos detected via their electromagnetic cascades

have better energy resolution ((log10 E) 0.15) but appreciably worse

angular resolution (of order 30).

Neutrino telescopes hope to observe extraterrestrial sources of high-

energy (> 1 TeV) neutrinos. Due to the good intrinsic pointing resolution

of AMANDA/IceCube, potential point sources of neutrinos can be stud-

ied. The catalog of possible neutrino point sources includes TeV emit-

ting blazars (e.g. Markarian 421), GeV emitting blazars, microquasars

(e.g.Cygnus X1), Pulsar Wind Nebulae (e.g. Crab Nebula), and gamma-ray

bursts.

Isotropically distributed astrophysical particle accelerators would lead

to a diffuse flux of neutrinos. Generic arguments suggest that diffuse extra-

galactic neutrino fluxes should have a considerably harder energy spectra

than atmospheric neutrinos. The atmospheric neutrino background comes

primarily from cosmic-ray induced pion production and decay. These neu-

trino follow a dN/dE E3.7 energy spectrum. The spectral index reflects

both the incoming cosmic-ray spectrum (dN/dE E2.7) and the energy

losses of pions in the atmosphere before they decay. There is also a small

atmospheric neutrino contribution due to prompt decay of kaons and charm

with dN/dE

E

2.7

.Extragalactic neutrinos, however, should have the same energy spec-

trum as the pre-cursor hadrons accelerated at the neutrino sources. If the

acceleration is due to the Fermi shock acceleration mechanism, the neu-

trino energy spectrum will be dN/dE E2. Analyses of the ultra-high

energy cosmic ray spectrum, accounting for propagation effects, are con-

sistent with a spectral index of approximately 2 at the cosmic-ray source.

Additionally, measured high energy gamma-ray spectra are consistent with

a source spectral index of about 2.

Figure 1 shows the up-going high-energy muon neutrino flux measured

by the AMANDA detector compared to lower energy FREJUS results. The

muon energies are estimated using a neural network energy reconstruction

trained on a full detector simulation. The muon neutrino energy spectrumis extracted from the measured muon energy spectrum using regularized un-

folding. Also shown in Figure 1 are the parameterized atmospheric muon

neutrino fluxes for horizontal (upper solid curved) and vertical (lower solid


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Figure 1. Preliminary unfolded neutrino spectrum from AMANDA compared to lowerenergy FREJUS results 1.

curve) neutrinos. As mentioned previously, one expects that extragalactic

neutrinos will have a considerably harder spectra than atmospheric neu-

trinos. For the highest energy bin (100 TeV< E


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of AMANDA data. The analysis cuts are optimized assuming an E2

signal neutrino spectrum, and the final sample contains 3329 up-going events. The most significant local excess is 3.35. By randomizing the

skymap, we find that such an excess is present in more than 90% of the

randomized skymaps and deduce that these data shows no evidence for

point sources of neutrinos. These data have also been used to test for

significant excesses from candidate neutrino emitters and no object shows

a statistically significant excess.

24h 0h

-90

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Figure 2. AMANDA neutrino skyplot for a 4-year (2000-2003) analysis.

While AMANDA has found no evidence for extragalactic high-energy

neutrinos, the field of neutrino astronomy is rapidly maturing. Gigaton

scale neutrino detectors such as IceCube and future underwater arrays will

lead to insights into the origins of high energy cosmic rays through the

observation of extragalactic neutrinos.

References

1. K. Daum et al., Z. Phys. C66, 417 (1995).2. M. Ackermann et al., Astroparticle Physics 22, 127 (2004).3. M. Ackermann et al., Astroparticle Physics 22, 339 (2005).4. F.W. Stecker et al., Phys. Rev. Lett. 66, 2697 (1991); F.W. Stecker and M.H.

Salamon, Space Sci. Rev.75

, 341, 1996; R.J. Protheroe, astro-ph/9607165.5. E. Waxmann and J. Bahcall, Phys. Rev. D59, 023002 (1998).6. J. Ahrens et al., Astrophysical Journal 583, 1040 (2003); J. Ahrens et al.,

Phys. Rev. Lett. 92, 071102 (2004); M. Ackermann et al., Phys. Rev D71,077102 (2005).

david hardtke- exploring the neutrino universe with amanda and icecube

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