[Paper Review] Circular polarisation: a new probe of dark matter and neutrinos in the sky

Boehm, Celine, Degrande, Celine+2 more|arXiv (Cornell University)|Jan 10, 2017

Dark Matter and Cosmic Phenomena31 citations

TL;DR

This paper proposes circular polarisation of X-rays and gamma rays as a novel probe for dark matter and neutrinos, demonstrating that parity-violating interactions in particle decays or annihilations can generate measurable circular polarisation signals. The key contribution is identifying circular polarisation as a new observable that could reveal particle-antiparticle asymmetries, spatial distributions, and underlying high-energy physics processes in astrophysical sources.

ABSTRACT

The study of anomalous electromagnetic emission in the sky is the basis of indirect searches for dark matter. It is also a powerful tool to constrain the radiative decay of active neutrinos. Until now, quantitative analyses have focused on the flux and energy spectrum of such an emission; polarisation has never been considered. Here we show that we could be missing out on an essential piece of information. The radiative decay of neutrinos, as well as the interactions of dark matter and neutrinos with Standard Model particles can generate a circular polarisation signal in X-rays or \gamma-rays. If observed, this could reveal important information about their spatial distribution and particle-antiparticle ratio, and could even reveal the nature of the high-energy particle physics processes taking place in astrophysical sites. The question of the observability of these polarised signatures and their separation from background astrophysical sources is left for future work.

Motivation & Objective

• To explore circular polarisation as a new observational signature in high-energy astrophysics, particularly for probing dark matter and neutrino interactions.
• To identify the physical conditions under which circular polarisation arises in photon emission from dark matter decay or neutrino interactions.
• To demonstrate that circular polarisation can reveal particle-antiparticle asymmetries and spatial distributions of dark matter and cosmic rays.
• To establish the theoretical foundation for circular polarisation as a probe of parity and CP violation in astrophysical particle processes.
• To motivate future observational efforts by showing that polarisation could disentangle new physics signals from standard astrophysical backgrounds.

Proposed method

• Uses quantum field theory to define photon polarisation states, including circular polarisation vectors ϵ±(k) via complex combinations of transverse polarisation vectors.
• Applies parity and CP transformation properties to show that net circular polarisation arises only when parity is violated and/or particle-antiparticle asymmetry exists.
• Analyzes two-body decays and scattering processes (e.g., ν → γ + X) to derive conditions under which circular polarisation is generated, focusing on chiral couplings and mediator properties.
• Considers specific examples such as sterile neutrino decay, dark matter annihilation via charged mediators, and W-boson pair production in sneutrino annihilation.
• Evaluates the polarisation fraction and energy dependence of the signal, showing that asymmetries in initial state particle densities can lead to observable net polarisation.
• Demonstrates that processes conserving parity or CP symmetry do not produce net circular polarisation unless initial state asymmetries are present.

Experimental results

Research questions

• RQ1 Can circular polarisation in X-ray and gamma-ray emissions serve as a detectable signature of dark matter decay or neutrino interactions?
• RQ2 Under what conditions does parity violation lead to a net circular polarisation in high-energy photon emission?
• RQ3 How can particle-antiparticle asymmetries in dark matter or neutrino populations generate observable circular polarisation?
• RQ4 What are the observable polarisation signatures of specific particle physics models such as sterile neutrino decay or sneutrino annihilation?
• RQ5 Can circular polarisation help distinguish new physics signals from astrophysical backgrounds in indirect dark matter searches?

Key findings

• Circular polarisation can be generated in photon emission from dark matter decay or neutrino interactions if parity is violated and/or particle-antiparticle asymmetry exists.
• The observation of a net circular polarisation signal would indicate parity violation and could reveal the chiral structure of the underlying interaction.
• A non-zero circular polarisation fraction of up to ~25% is possible in processes like W⁺H⁻ decay, though such signals cancel in CP-conjugate processes unless asymmetries are present.
• Processes such as sterile neutrino decay or dark matter annihilation via charged mediators can produce net circular polarisation if the initial state particles are not symmetric under CP.
• The energy dependence and degree of polarisation can provide information on the spatial distribution and particle content of dark matter and cosmic rays.
• The paper concludes that circular polarisation is a promising new observable for indirect dark matter and neutrino searches, though separation from synchrotron background remains a challenge for future work.