Search for a Diffuse Flux of TeV to PeV Muon Neutrinos with AMANDA-II

Detecting Neutrinos with AMANDA / IceCube Backgrounds for

the Diffuse Analysis

Why search for a diffuse flux of muon

neutrinos?

Selecting High Quality Events

Separating atmospheric neutrinos from extraterrestrial neutrinos Systematic Uncertainties on the Predicted Background and

Detector Response

Jessica Hodges, University of Wisconsin – Madison, for the IceCube Collaboration

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 :

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-to-background region would be obtained by using events with at least 100 OMs hit.

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.

The diffuse muon neutrino analysis looks only for UPGOING neutrinos. The Earth is used as a filter to remove large downgoing atmospheric muon backgrounds.

UpgoingZenith = 180o

DowngoingZenith = 0o

υSouth Pole

Signal Neutrinos (extraterrestrial)

Atmospheric Neutrinos

Atmospheric Muons

Cosmic ray proton

Cosmic ray proton

μ

υμ

υ

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.

The analysis began with all events collected by AMANDA-II during 2000 – 2003. The initial data set was mainly comprised of downgoing atmospheric muons.

upgoing downgoing

All events between 0o and 80o were removed. However, many downgoing muons were misreconstructed and remained in the upgoing data sample.

events removed

horizon

0o90o180o

180o 90o

upgoinghorizon

upgoinghorizon

upgoing

horizon

After progressively tightening the requirements, the misreconstructed backgrounds were eliminated.

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.

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.

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.

South Pole

References

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[3] M. Honda, T. Kajita, K. Kasahara, and S. Midorikawa, Phys. Rev. D 70, 043008 (2004).

[4] T.K. Gaisser, M. Honda, P. Lipari, and T. Stanev, Primary spectrum to 1 TeV and beyond, in: Proceedings of the 27th International Cosmic Ray Conference, Hamburg, Germany, 5, 1643 (2001).

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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.

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.

[6,7]

[8]

[9]

[10]

[11]

[11]

[12,13]

[14]

90o180o

Final Results

Signal models with other energy spectra were also tested and constrained.

(CORSIKA [5])

E2Φ90%c.l. < 8.8 x 10-8 GeV cm -2 s -1 sr -1