[Paper Review] Study of High pT Muons in IceCube

L. Gerhardt, S. R. Klein|arXiv (Cornell University)|Sep 1, 2009

Astrophysics and Cosmic Phenomena4 references3 citations

TL;DR

This paper presents a method to detect high transverse momentum (pT) muons in the IceCube neutrino telescope using combined in-ice and surface array data, enabling measurement of the muon pT spectrum to probe cosmic ray composition. By reconstructing muon tracks separated by >150 m from the shower core and applying selection criteria, the study identifies candidate high-pT muon events, with a minimum resolvable pT of 6 GeV/c, offering a novel pQCD-sensitive probe of cosmic ray composition beyond traditional models.

ABSTRACT

Muons with a high transverse momentum (p_T) are produced in cosmic ray air showers via semileptonic decay of heavy quarks and the decay of high p_T kaons and pions. These high p_T muons have a large lateral separation from the shower core muon bundle. IceCube is well suited for the detection of high p_T muons. The surface shower array can determine the energy, core location and direction of the cosmic ray air shower while the in-ice array can reconstruct the energy and direction of the high p_T muon. This makes it possible to measure the decoherence function (lateral separation spectrum) at distances greater than 150 meters. The muon p_T can be determined from the muon energy (measured by dE/dx) and the lateral separation. The high p_T muon spectrum may also be calculated in a perturbative QCD framework; this spectrum is sensitive to the cosmic-ray composition.

Motivation & Objective

• To develop a method for identifying high pT muons in cosmic ray air showers using IceCube's dual in-ice and surface (IceTop) arrays.
• To measure the lateral separation spectrum (decoherence function) of high pT muons at distances >150 m from the shower core.
• To determine the muon transverse momentum (pT) from energy (dE/dx) and lateral separation, enabling pQCD-based composition analysis.
• To suppress backgrounds from single and double-coincident cosmic ray showers through topological and timing selection criteria.
• To explore horizontal events as a lower-background channel for high pT muon detection.

Proposed method

• Utilize IceTop surface array to reconstruct shower core position, direction, and energy with ~10 m core resolution and ~1.5° angular accuracy.
• Use the in-ice IceCube array to reconstruct high-energy muon tracks with sub-degree angular resolution and dE/dx measurements for energy.
• Calculate muon pT using the formula pT = (dEμ/hc), where d is lateral separation, Eμ is muon energy, and h is average interaction height (25 km).
• Apply a two-track reconstruction algorithm to identify events with a high-pT muon separated by ≥150 m from the shower core, exceeding inter-string spacing.
• Implement selection criteria: require tracks to be parallel and within 1 μs in time to suppress double-coincident cosmic ray backgrounds.
• Conduct visual scanning of 10% of data for horizontal events (within 30° of horizontal) to search for double-track topologies with minimal background.

Experimental results

Research questions

• RQ1 Can high pT muons (pT > 6 GeV/c) be resolved from the main muon bundle in IceCube using lateral separation and energy measurements?
• RQ2 To what extent can the decoherence function (lateral separation spectrum) of high pT muons be measured at separations >150 m?
• RQ3 How effective are topological and timing cuts in suppressing double-coincident cosmic ray backgrounds for high pT muon identification?
• RQ4 Can horizontal events with parallel double tracks provide a lower-background channel for detecting high pT muons?
• RQ5 How sensitive is the high pT muon spectrum to the composition of ultra-high-energy cosmic rays?

Key findings

• The minimum resolvable pT for high pT muons in IceCube is 6 GeV/c, based on a 150 m lateral separation threshold and 1 TeV muon energy.
• A two-track reconstruction algorithm successfully identifies candidate high pT muon events with lateral separations up to 400 m from the shower core.
• Several candidate high pT muon events were found after visual scanning of 10% of IceCube data, including one with a 5.6° angle between tracks and 400 ns time separation.
• Horizontal event searches revealed candidate double-track events with topologies consistent with high pT muons, suggesting a viable low-background channel.
• The background from double-coincident cosmic ray showers is estimated to be comparable to the expected number of high pT muon events, indicating a need for full simulation to distinguish them.
• The method enables a pQCD-sensitive probe of cosmic ray composition, particularly for heavy nuclei, via the high pT muon spectrum.