[Paper Review] Combined analyses of the antiproton production from cosmic-ray interactions and its possible dark matter origin

Pedro De la Torre Luque|arXiv (Cornell University)|Jul 14, 2021

Dark Matter and Cosmic PhenomenaPhysics and Astronomy110 references36 citations

TL;DR

This study combines Markov Chain Monte Carlo (MCMC) analyses of secondary cosmic-ray ratios (B/C, Be/B, Li/Be) and the antiproton-to-proton ratio (¯p/p) to constrain cosmic-ray propagation parameters and cross-section uncertainties. It finds that the energy dependence of the AMS-02 ¯p/p spectrum is well explained by a pure secondary origin, with a constant ∼10% excess best explained by a 10% rescaling of antiproton production cross sections rather than a dark matter WIMP signal.

ABSTRACT

Recent cosmic-ray (CR) studies have claimed the possibility of an excess on the antiproton flux over the predicted models at around $10$ GeV, which can be the signature of dark matter annihilating into hadronic final states that subsequently form antiprotons. However, this excess is subject to many uncertainties related to the evaluation of the antiproton spectrum produced from spallation interactions of CRs. In this work, we implement a combined Markov-Chain Monte Carlo analysis of the secondary ratios of B, Be and Li and the antiproton-to-proton ratio ($\bar{p}/p$), while also including nuisance parameters to consider the uncertainties related to the spallation cross sections. This study allows us to constrain the Galactic halo height and the rest of propagation parameters, evaluate the impact of cross sections uncertainties in the determination of the antiproton spectrum and test the origin of the excess of antiprotons. In this way, we provide a set of propagation parameters and scale factors for renormalizing the cross sections parametrizations that allow us to reproduce all the ratios of B, Be, Li and $\bar{p}$ simultaneously. We show that the energy dependence of the $\bar{p}/p$ ratio is compatible with a pure secondary origin. We find that the energy dependence of the evaluated $\bar{p}/p$ spectrum matches the AMS-02 data at energies above $\sim3$GeV, although there is still a nearly constant $\sim10\%$ excess of $\bar{p}$ over our prediction. We discuss that this discrepancy is more likely explained from a $\sim10\%$ scaling in the cross sections of antiproton production, rather than a component of dark matter leading to antiprotons. In particular, we find that the best-fit WIMP mass ($\sim 300$ GeV) needed to explain the discrepancy lies above the constraints from most indirect searches of dark matter and the resultant fit is poorer than with a cross sections scaling.

Motivation & Objective

• To resolve the long-standing discrepancy between predicted and observed antiproton fluxes at ~10 GeV.
• To test whether the observed antiproton excess is due to dark matter annihilation or systematic uncertainties in spallation cross sections.
• To simultaneously constrain propagation parameters (diffusion, halo height, etc.) and cross-section scale factors using multiple secondary cosmic-ray ratios.
• To evaluate the impact of cross-section uncertainties on antiproton spectrum predictions and the significance of the excess.

Proposed method

• Perform a combined MCMC analysis of B/C, Be/B, Li/Be ratios and the ¯p/p ratio using AMS-02 data.
• Incorporate nuisance parameters to renormalize spallation cross sections for B, Be, and Li production.
• Use the DRAGON2 code to simulate cosmic-ray propagation and antiproton production in the Galaxy.
• Model dark matter contributions via WIMP annihilation into hadronic final states, parameterized by mass and ⟨σv⟩.
• Compare fits with and without a WIMP component to assess significance and consistency with indirect detection constraints.
• Use the new AMS-02 ¯p/p dataset to improve constraints and assess the shape of residuals.

Experimental results

Research questions

• RQ1 Is the observed antiproton excess at ~10 GeV consistent with a secondary origin, or does it require a dark matter component?
• RQ2 How do uncertainties in spallation cross sections affect the prediction of the antiproton spectrum?
• RQ3 Can a simultaneous fit to B, Be, Li, and ¯p/p ratios constrain propagation parameters and cross-section scale factors more robustly?
• RQ4 What is the best-fitting WIMP mass and annihilation cross section needed to explain the excess, and is it consistent with other indirect searches?
• RQ5 Does the shape of the residual in the ¯p/p spectrum resemble a WIMP signal or a systematic offset?

Key findings

• The energy dependence of the ¯p/p spectrum is well reproduced by a pure secondary origin, with residuals below 12% at 10 GeV.
• A constant ∼10% excess in the antiproton flux over the prediction is observed above ∼3 GeV, which is best explained by a 10% rescaling of antiproton production cross sections.
• The best-fit WIMP mass required to explain the excess is ∼300 GeV, which lies above the constraints from most indirect dark matter searches and yields a poorer fit than the cross-section rescaling.
• The χ² value for the fit with a 10% cross-section scaling is nearly a factor of two lower than the fit including a WIMP component, indicating a significantly better statistical fit.
• The use of the new AMS-02 ¯p/p dataset leads to flatter residuals than previous datasets, reducing the apparent bump-like structure and weakening the case for a WIMP signal.
• The analysis provides a consistent set of propagation parameters and cross-section scale factors that simultaneously reproduce the B, Be, Li, and ¯p/p ratios within 1σ uncertainties.