[Paper Review] Understanding the cosmic ray positron flux

P. Lipari|arXiv (Cornell University)|Feb 16, 2019

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper investigates the origin of the cosmic ray positron flux using AMS-02 data, proposing that the observed spectral hardening near 25 GeV and softening above ~300 GeV could stem from secondary positron production with energy-loss effects in the Galaxy, rather than a new primary source like dark matter. The key finding is that the positron spectrum's features are consistent with propagation effects, challenging the need for exotic sources.

ABSTRACT

Recent precision measurements of the flux of cosmic ray positrons by the Alpha Magnetic Spectrometer show that the spectrum has a marked softening feature for energies close to one TeV. A possible interpretation of this result is that the observed feature measures the maximum energy of a new hard source of positrons perhaps associated to dark matter self--annihilation or decay, or to positron accelerators. A gradual hardening of the positron flux centered at $E \simeq 25$~GeV can also be understood as the signature of the transition where the new source overtakes the conventional component due to secondary production. This interpretation is simple and attractive, but it is not unique. The alternative possibility, that the positron flux is entirely of secondary origin, remains viable. In such a scenario the spectral softening observed by AMS for positrons is generated by energy loss effects, and a feature of similar, but not identical structure should be also visible in the $e^-$ spectrum. Spectral features similar to both the hardening and softening of the positron flux are in fact observed for electrons and call for a consistent explanation. Precision measurements of the $e^+$ and $e^-$ spectra in the TeV and multi--TeV energy range are crucial to clarify the problem.

Motivation & Objective

• To determine whether the observed spectral features in the cosmic ray positron flux—specifically hardening near 25 GeV and softening above ~300 GeV—can be explained by secondary production mechanisms.
• To assess whether the observed features in the positron spectrum are consistent with propagation effects such as energy losses, rather than requiring a new primary source like dark matter or astrophysical accelerators.
• To compare the positron spectrum with electron and antiproton spectra to test the consistency of a single propagation model across lepton and hadron species.
• To evaluate whether the spectral suppression in positrons, observed at high energies, is uniquely indicative of a maximum source energy (e.g., from dark matter), or could arise from propagation physics.
• To clarify whether the observed features in the positron spectrum are distinct from those in the electron spectrum or could be generated by the same underlying physical processes in the Galaxy.

Proposed method

• Uses a two-component power-law model with exponential cutoff (Eq. 1) to fit the AMS-02 positron flux data, incorporating solar modulation via the force field approximation (Eq. 2).
• Applies the same functional form to fit electron spectra from previous AMS and PAMELA data, enabling cross-comparison of spectral features.
• Analyzes the impact of energy loss effects on lepton spectra during propagation in the Galaxy, using models from prior work (Lipari:2018usj) to predict spectral breaks and softening.
• Compares the energy dependence and shape of spectral features in positrons and electrons, focusing on whether hardenings and softening are structurally similar.
• Evaluates the consistency of a secondary origin for positrons by modeling how energy losses affect the observed spectra, particularly the transition region around 25 GeV.
• Assesses whether the observed suppression in the positron spectrum at ~800 GeV is uniquely indicative of a source cutoff or could be mimicked by propagation effects, especially given the lack of identical suppression in the electron spectrum.

Experimental results

Research questions

• RQ1 Can the observed hardening of the positron spectrum near 25 GeV be explained by the transition from secondary to primary-dominated production, rather than a new source?
• RQ2 Is the high-energy softening in the positron spectrum at ~800 GeV best interpreted as a source cutoff or as a consequence of energy loss during propagation?
• RQ3 Do the spectral features in the positron and electron spectra arise from the same underlying physical processes, or do they require different source models?
• RQ4 Can the observed positron spectrum be fully explained by secondary production and propagation effects, without invoking exotic sources like dark matter or positron accelerators?
• RQ5 Are the differences in the shape and location of spectral features between positrons and electrons consistent with a single propagation model, or do they indicate distinct source populations?

Key findings

• The spectral hardening of the positron flux around 25 GeV is consistent with the transition from a low-energy regime dominated by secondary production to a high-energy regime where a harder component—potentially from a new source—dominates.
• The high-energy softening in the positron spectrum, with a best-fit cutoff energy of $ E_s = 810^{+310}_{-180} $ GeV, is not uniquely indicative of a source maximum energy, as it could also arise from energy loss effects during propagation.
• The observed spectral features in the positron spectrum are qualitatively similar to those in the electron spectrum, suggesting that both could be generated by the same propagation physics, particularly energy loss effects.
• The suppression in the positron spectrum is broader and centered at lower energy than the corresponding suppression in the electron spectrum, which challenges the idea that the positron softening is due to a source cutoff alone.
• A model in which the bulk of positrons are of secondary origin, with spectral features shaped by propagation effects, remains viable and cannot be ruled out by current data.
• The lack of identical spectral features between electrons and positrons—especially in the width and location of breaks—introduces theoretical uncertainty, but does not definitively rule out a secondary origin for positrons.