Transcript Search for a Diffuse Flux of TeV to PeV Muon Neutrinos with AMANDA-II Why search for a diffuse flux of muon neutrinos? Current.

Search for a Diffuse Flux of TeV to PeV Muon Neutrinos with AMANDA-II
Why search for a diffuse
flux of muon neutrinos?
Current theories on cosmic particle acceleration
predict that neutrinos and gamma rays are among the
by-products of pp and pγ interactions in sources such
as AGNs (active galactic nuclei) or GRBs (gamma ray
bursts). Many extraterrestrial TeV gamma ray sources
have already been identified by other experiments, but
the missing link is the detection of an extraterrestrial
neutrino flux.
Neutrinos have no charge, and hence can travel in
straight lines directly from the source to detector. The
neutrino flux from individual cosmic sources is
expected to be very small on Earth. If an excess of
events was observed in a large sky region over the
expected atmospheric neutrino background, it would
be indicative of the presence of a cosmic diffuse flux
of neutrinos. The main neutrino source candidates are
expected to be isotropically distributed throughout the
Universe.
Separating
atmospheric
neutrinos from
extraterrestrial
neutrinos
Atmospheric neutrinos (dN/dE ~ E-3.7) have a softer
energy spectrum than the proposed extraterrestrial
neutrino signal (dN/dE ~ E-2). As a result, these two
event classes can be separated best by their energy
in the Monte Carlo. At high energy, the
extraterrestrial neutrino flux would dominate over
the atmospheric neutrinos.
Since the energy of an event is not directly
observable, the number of OMs hit during an event
was used as an energy-correlated parameter. Monte
Carlo optimization indicated that the best signal-tobackground region would be obtained by using
events with at least 100 OMs hit.
At the end of the
analysis, the number of
actual data events seen
in this high energy
window was compared to
the predicted
atmospheric neutrino
background.
Jessica Hodges, University of Wisconsin – Madison, for the IceCube Collaboration
Detecting Neutrinos with
AMANDA / IceCube
AMANDA-II detects light from high energy
charged particles traveling though the ice below
the South Pole. 677 optical modules (OMs) are
buried between 1500m and 2500m deep in the
polar ice [1]. Each OM contains a
photomultiplier tube (capable of detecting one
photon) encased in a glass sphere. The OMs
are attached along nineteen cables or strings.
Selecting High
Quality Events
Backgrounds for
the Diffuse Analysis
The analysis began with all events collected by
AMANDA-II during 2000 – 2003. The initial data
set was mainly comprised of downgoing
atmospheric muons.
The diffuse muon neutrino analysis looks only for UPGOING
neutrinos. The Earth is used as a filter to remove large
downgoing atmospheric muon backgrounds.
(CORSIKA [5])
Cosmic
ray
proton
Upgoing
Zenith = 180o
Downgoing
Zenith = 0o
South Pole
μ
upgoing
horizon
downgoing
Atmospheric
Muons
υ
180o
90o
0o
events removed
South Pole
Charged particles traveling faster than the speed of
light in ice emit Cherenkov light in a cone as they
travel through a transparent medium. When many OMs
detect light in a short time window, the particle’s track can
be reconstructed to within a few degrees.
Muons are a by-product of muon neutrino – matter
interactions. In the TeV to PeV energy range of this
analysis, these muons travel in the same direction as the
initial neutrino and are detectable by the Cherenkov light
they emit.
υ
υ
μ
Atmospheric
Neutrinos
Signal Neutrinos
(extraterrestrial)
horizon
upgoing
180o
90o
horizon
Cosmic ray proton
upgoing
180o
Systematic Uncertainties on the Predicted Background and Detector Response
Overall, the atmospheric neutrino background was best described by a range, not a single prediction. [4] Two different
atmospheric neutrino models were used, Barr et al. [2] and Honda et al. [3]. Uncertainty in the cosmic ray flux was also
considered. All of the atmospheric neutrino Monte Carlo was scaled so that the number of Monte Carlo events matched the
number of data events in the region 50 < Number of OMs triggered < 100.
All events between 0o
and 80o were removed.
However, many
downgoing muons were
misreconstructed and
remained in the
upgoing data sample.
90o
horizon
upgoing
To assess detector and Monte Carlo performance, an inverted analysis was performed. Downgoing events that were previously
eliminated (0o < zenith angle < 80o) were reintroduced. The characteristics of high energy events were studied without having to
reveal the high energy upgoing data events.
To remove
misreconstructed
downgoing backgrounds,
quality requirements
were established. Events
only survived if they had
long, smooth tracks with
many photons arriving
close to their expected
arrival times.
After progressively
tightening the
requirements, the
misreconstructed
backgrounds were
eliminated.
References
[1] J. Ahrens et al., Nucl. Instr. Meth. A 524, 169 (2004).
[2] G.D. Barr, T.K. Gaisser, P. Lipari, S. Robbins, and T.
Stanev, Phys. Rev. D 70, 023006 (2004).
Final Results
Six data events were observed on an average
predicted atmospheric neutrino background of 6.1
events. Since no excess of events was seen
indicating an extraterrestrial signal, a limit was set for
the region 15.8 TeV to 2.5 PeV (the energy region
covered by 90% of the simulated signal). The upper
limit on the diffuse flux of muon neutrinos from
AMANDA-II data from 2000 – 2003 is :
[3] M. Honda, T. Kajita, K. Kasahara, and S. Midorikawa, Phys.
Rev. D 70, 043008 (2004).
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spectrum to 1 TeV and beyond, in: Proceedings of the
27th International Cosmic Ray Conference, Hamburg,
Germany, 5, 1643 (2001).
[5] D. Heck, Tech. Rep. FZKA 6019 Forschungszentrum
Karlsruhe (1998).
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E2Φ90%c.l. < 8.8 x 10-8 GeV cm -2 s -1 sr -1
Signal models with other energy spectra were also tested and constrained.
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