[Paper Review] The IceCube Neutrino Observatory IV: Searches for Dark Matter and Exotic Particles

The IceCube Collaboration|arXiv (Cornell University)|Nov 11, 2011

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper presents updated searches for magnetic monopoles and dark matter using the IceCube Neutrino Observatory, leveraging data from the 40-string configuration. By analyzing Cherenkov light patterns from relativistic particles, the study improves sensitivity to magnetic monopoles with speeds β > 0.8, setting the most stringent experimental limits to date—improving previous bounds by an order of magnitude and projecting a further factor of four improvement with ongoing data.

ABSTRACT

Exotic particle searches: WIMPs annihilating in the Sun, in the galactic center, in nearby dwarf galaxies; magnetic monopoles; Submitted papers to the 32nd International Cosmic Ray Conference, Beijing 2011.

Motivation & Objective

• To search for relativistic magnetic monopoles using Cherenkov light signatures in the IceCube neutrino telescope.
• To improve experimental sensitivity to magnetic monopoles beyond previous limits from the 22-string configuration.
• To extend indirect dark matter searches to the Sun, Milky Way, and nearby dwarf spheroidal galaxies using high-energy neutrino data.
• To develop optimized analysis techniques for distinguishing monopole signals from atmospheric muon backgrounds.
• To set new upper limits on the flux of exotic particles such as Kaluza-Klein states and WIMPs via neutrino detection.

Proposed method

• Utilizes the IceCube Neutrino Observatory’s 40-string configuration, with 86 strings of photomultiplier tubes embedded in Antarctic ice.
• Applies a time-over-threshold (ToT) to total photo-electron (NPE) ratio as a key discriminant to separate monopole-like signals from muon backgrounds.
• Employs line-fitting reconstruction of particle tracks and uses the Nch/NPE ratio (number of hit DOMs over total photo-electrons) to enhance signal discrimination.
• Implements directional and speed-dependent cuts, with stronger suppression for vertical downgoing events to reduce background contamination.
• Uses Monte Carlo simulations to model monopole light emission and background muon bundles, validating the cut efficiency.
• Applies a final cut at ToT/NPE = 1.6, ensuring no data events survive, to set conservative upper limits at 90% confidence level.

Experimental results

Research questions

• RQ1 What is the sensitivity of the IceCube 40-string detector to relativistic magnetic monopoles with speeds β = 0.8, 0.9, and 0.995?
• RQ2 How do Cherenkov light patterns from monopoles differ from those of atmospheric muons, and can they be effectively discriminated?
• RQ3 What are the improved upper limits on magnetic monopole flux compared to previous IceCube and AMANDA searches?
• RQ4 How do the detection efficiencies and effective areas vary with monopole speed and direction?
• RQ5 To what extent can the analysis distinguish monopole signals from rare background events such as stochastic energy loss in single DOMs?

Key findings

• The analysis sets the most stringent experimental limits to date on relativistic magnetic monopoles with speeds β > 0.8, improving previous bounds by an order of magnitude.
• For monopoles with β = 0.995, the effective area is 185.57 km², with a 90% confidence level sensitivity of 1.1 × 10⁻¹⁸ cm⁻² s⁻¹ sr⁻¹.
• The sensitivity for β = 0.9 is 178.87 km² effective area and 1.1 × 10⁻¹⁸ cm⁻² s⁻¹ sr⁻¹ flux limit.
• For β = 0.8, the effective area is 134.61 km², with a slightly higher flux limit of 1.5 × 10⁻¹⁸ cm⁻² s⁻¹ sr⁻¹ due to reduced Cherenkov light emission near threshold.
• The analysis identifies two dominant background classes: single-DOM high-signal events and high-inclination muons grazing detector corners, which may require additional safety cuts.
• Future analysis of the 40-string data is expected to improve sensitivity by a factor of ~4 over the 22-string analysis, based on projected signal-to-noise performance.