[Paper Review] IceCube - status and recent results

A. Karle|arXiv (Cornell University)|Jan 18, 2014

Astrophysics and Cosmic Phenomena12 references3 citations

TL;DR

IceCube, a cubic kilometer neutrino observatory at the South Pole, reports high uptime (>98%) and stable operation since May 2011. Using veto techniques to suppress atmospheric backgrounds, it presents strong evidence for an astrophysical neutrino flux above 30 TeV, with a 4σ significance for a diffuse $E^{-2}$ spectrum, marking a key milestone in neutrino astronomy.

ABSTRACT

In May 2011, the IceCube neutrino observatory with one cubic kilometer instrumented volume started full operation with 5160 sensors on 86 strings and 324 sensors on 162 IceTop detectors. The fine-tuning of operation and calibration of the detector is still in progress while a very high uptime of well above $98\%$ is obtained. New analysis techniques rely on veto techniques for enhanced rejection of atmospheric muon and neutrino backgrounds. We will give an overview of recent results including the techniques of searching for starting tracks and some comments on the reported evidence of astrophysical neutrinos at energies above 30 TeV.

Motivation & Objective

• To establish and maintain a fully operational cubic kilometer neutrino telescope at the South Pole.
• To achieve high detector uptime and reliability for long-term physics data collection.
• To develop and implement advanced background rejection techniques, particularly muon and atmospheric neutrino vetoing.
• To search for and identify high-energy astrophysical neutrinos using starting track and cascade event analyses.
• To measure the diffuse astrophysical neutrino flux and constrain its energy spectrum and normalization.

Proposed method

• Deployment of 5160 optical sensors (DOMs) on 86 strings and 324 IceTop sensors across 162 stations in Antarctic ice.
• Use of Cherenkov radiation from charged particles in ice to detect neutrino interactions via photomultiplier tubes.
• Implementation of a hit-spooling system to buffer photomultiplier signals for hours, enabling post-trigger analysis of rare events.
• Application of angular and energy-based event selection to reject atmospheric muon and neutrino backgrounds.
• Use of veto techniques that exploit the coincidence of muons and neutrinos in air showers to suppress downgoing atmospheric neutrino backgrounds.
• Global fitting of data in the 60 TeV to 2 PeV range to determine the normalization and spectral index of the astrophysical flux.

Experimental results

Research questions

• RQ1 What is the flux and energy spectrum of high-energy astrophysical neutrinos above 30 TeV?
• RQ2 To what extent can atmospheric muon and neutrino backgrounds be suppressed using veto techniques?
• RQ3 Is there a statistically significant excess of high-energy neutrinos consistent with an astrophysical origin?
• RQ4 What is the angular distribution of high-energy neutrino events, and does it indicate a point source or isotropic flux?
• RQ5 How do the observed neutrino rates and energy spectra compare to atmospheric neutrino models and theoretical predictions like the Waxman-Bahcall bound?

Key findings

• IceCube achieved a data acquisition uptime of 98.5% during the 2012–2013 science run, with only 0.5% annual failure rate for deployed DOMs.
• The detector recorded over 200 neutrinos per day at full operation, with a 0.4° angular resolution at 100 TeV.
• A significant excess of 28 high-energy events with contained vertices was observed above the atmospheric background, particularly in the southern hemisphere.
• The zenith angle distribution shows strong suppression of atmospheric neutrinos below ~60° due to muon vetoing, confirming the effectiveness of background rejection.
• The data are well described by a diffuse astrophysical neutrino flux with a $E^{-2}$ spectrum, with a per-flavor normalization of $E^{2}\Phi(E) = (1.2 \pm 0.4) \cdot 10^{-8}\ \mathrm{GeV}\ \mathrm{cm}^{-2}\ \mathrm{s}^{-1}\ \mathrm{sr}^{-1}$.
• The evidence for an astrophysical flux is inconsistent with zero at the 4 sigma level, providing strong support for a cosmic origin of high-energy neutrinos.