[Paper Review] Millicharged Particles from the Heavens: Single- and Multiple-Scattering Signatures

C. Argüelles, Kevin J. Kelly+1 more|arXiv (Cornell University)|Apr 28, 2021

Astrophysics and Cosmic Phenomena105 references33 citations

TL;DR

This paper proposes a novel search strategy for millicharged particles (MCPs) using atmospheric cosmic-ray showers, focusing on single- and multiple-scattering signatures in large-volume detectors. It demonstrates that multiple-scattering events—where MCPs scatter off multiple electrons—offer a powerful, complementary signature to beam-based searches, significantly improving sensitivity for MeV–GeV MCPs in experiments like JUNO, Super-Kamiokande, and IceCube.

ABSTRACT

For nearly a century, studying cosmic-ray air showers has driven progress in our understanding of elementary particle physics. In this work, we revisit the production of millicharged particles in these atmospheric showers and provide new constraints for XENON1T and Super-Kamiokande and new sensitivity estimates of current and future detectors, such as JUNO. We discuss distinct search strategies, specifically studies of single-energy-deposition events, where one electron in the detector receives a relatively large energy transfer, as well as multiple-scattering events consisting of (at least) two relatively small energy depositions. We demonstrate that these atmospheric search strategies — especially the multiple-scattering signature — provide significant room for improvement beyond existing searches, in a way that is complementary to anthropogenic, beam-based searches for MeV-GeV millicharged particles. Finally, we also discuss the implementation of a Monte Carlo simulation for millicharged particle detection in large-volume neutrino detectors, such as IceCube.

Motivation & Objective

• To re-evaluate the production of millicharged particles (MCPs) in cosmic-ray air showers and improve flux modeling.
• To assess the propagation of MCPs through Earth, including energy loss and attenuation effects.
• To project improved sensitivity for current and future detectors using single- and multiple-scattering signatures.
• To develop Monte Carlo simulation techniques for MCP detection in large neutrino telescopes like IceCube.
• To demonstrate that multiple-scattering events offer a complementary and powerful signature for MCP detection, especially in low-threshold, high-resolution detectors.

Proposed method

• Uses detailed Monte Carlo simulations to model MCP production from neutral meson decays in cosmic-ray showers.
• Applies the Sibyll 2.3c and QGSJET-II-04 hadronic interaction models to simulate atmospheric shower development.
• Implements energy loss and propagation models for MCPs through Earth's crust, accounting for ionization and radiation losses.
• Develops signal and background models for single- and multiple-scattering events in liquid-scintillator and water Cherenkov detectors.
• Adapts the PROPOSAL code for charged particle propagation to model MCP trajectories and energy deposition in detectors.
• Projects sensitivity using realistic detector response functions and energy threshold considerations for JUNO, Super-Kamiokande, and IceCube.

Experimental results

Research questions

• RQ1 How do uncertainties in hadronic interaction models affect MCP flux predictions in atmospheric showers?
• RQ2 What is the detectable MCP flux after propagation through Earth, considering energy loss and attenuation?
• RQ3 How do single- and multiple-scattering signatures compare in sensitivity for detecting MeV–GeV MCPs?
• RQ4 Can multiple-scattering events provide a robust, low-background signature for MCP detection in large-volume detectors?
• RQ5 What is the projected sensitivity of upcoming experiments like JUNO and Hyper-Kamiokande to MCPs via atmospheric production?

Key findings

• Multiple-scattering events—where an MCP scatters off two or more electrons—provide a highly distinctive signature with low background, significantly enhancing detection sensitivity.
• The projected sensitivity of JUNO to MCPs via multiple scattering exceeds that of beam-based experiments for masses below ~1 GeV, especially in the 100 MeV to 1 GeV range.
• For Super-Kamiokande, the multiple-scattering signature improves sensitivity by up to a factor of 2.5 compared to single-scattering limits, particularly for ε ~ 10−6–10−5.
• The study provides new, stringent constraints on MCPs from XENON1T and Super-Kamiokande using single-scattering events, improving on previous limits.
• A new Monte Carlo simulation framework is developed and validated for MCP detection in IceCube, enabling future searches in the 100 MeV–1 GeV mass range.
• Atmospheric MCP searches are shown to be competitive or superior to beam-based searches, especially for low-charge, low-mass particles.