Where do the AMS-02 anti-helium events come from? (1808.08961v3)

Abstract: We discuss the origin of the anti-helium-3 and -4 events possibly detected by AMS-02. Using up-to-date semi-analytical tools, we show that spallation from primary hydrogen and helium nuclei onto the ISM predicts a $\overline{{}^3{\rm He}}$ flux typically one to two orders of magnitude below the sensitivity of AMS-02 after 5 years, and a $\overline{{}^4{\rm He}}$ flux roughly 5 orders of magnitude below the AMS-02 sensitivity. We argue that dark matter annihilations face similar difficulties in explaining this event. We then entertain the possibility that these events originate from anti-matter-dominated regions in the form of anti-clouds or anti-stars. In the case of anti-clouds, we show how the isotopic ratio of anti-helium nuclei might suggest that BBN has happened in an inhomogeneous manner, resulting in anti-regions with a anti-baryon-to-photon ratio $\bar{\eta}\simeq10^{-3}\eta$. We discuss properties of these regions, as well as relevant constraints on the presence of anti-clouds in our Galaxy. We present constraints from the survival of anti-clouds in the Milky-Way and in the early Universe, as well as from CMB, gamma-ray and cosmic-ray observations. In particular, these require the anti-clouds to be almost free of normal matter. We also discuss an alternative where anti-domains are dominated by surviving anti-stars. We suggest that part of the unindentified sources in the 3FGL catalog can originate from anti-clouds or anti-stars. AMS-02 and GAPS data could further probe this scenario.

Analysis of Anti-helium Events in AMS-02 Cosmic Ray Data

The paper explores the enigmatic events concerning anti-helium detected by the Alpha Magnetic Spectrometer - 02 (AMS-02), exploring potential sources and discussing the implication of these findings for our understanding of cosmic structures and nucleosynthesis processes. The authors have approached the paper with a robust analytical framework, proposing that neither secondary cosmic ray production nor dark matter (DM) annihilation convincingly accounts for the observed detection of anti-helium in cosmic ray data.

Secondary Anti-helium Production

The possibility of secondary anti-helium production through spallation processes, where primary cosmic rays such as protons and helium nuclei interact with the interstellar medium (ISM), is thoroughly revisited. Using advanced semi-analytical calculations, the authors assert that secondary sources fall significantly short of explaining the sensitivity required by AMS-02, proposing that the secondary production rate is one to two orders of magnitude below detection capacity for anti-helium-3 and five orders of magnitude below for anti-helium-4. This re-evaluation incorporates new insights into the coalescence model from contemporary high-energy physics data, undermining the likelihood of a purely secondary origin.

Dark Matter Annihilation

The paper reviews dark matter annihilation as another potential origin for these anti-helium events. However, even with the creative flexibility allowed in constructing DM models and parameters, the data does not easily accommodate such a scenario. The mismatch between expected yields and observed data remains substantial, raising questions around the nature and characteristics of possible DM interactions that could be involved.

Novel Hypotheses: Anti-clouds and Anti-stars

Given the above constraints, the authors entertain the hypothesis that AMS-02's observations may point to the existence of large-scale regions of antimatter, potentially in the form of anti-clouds or anti-stars. The paper meticulously analyzes this radical possibility, considering theoretical adjustments to Big Bang Nucleosynthesis (BBN) that could yield such isotopic ratios through a locally varied anti-matter-to-matter density. This scenario, though controversial, suggests that distributions rich in anti-helium might exist, pointing towards 'anti-world' pockets scattered throughout the universe that survived annihilation processes primarily through near-zero local hydrogen densities.

Implications and Constraints

The implications of anti-helium detection extend deeply into our understanding of galactic processes and cosmic structure. The survival of antimatter-dominated regions requires very specific conditions, particularly against annihilation with matter in regions like the Milky Way. The authors discuss constraints from current gamma-ray and cosmic-ray observations, as well as theoretical survival requirements that posit these regions must be nearly free of normal matter to remain extant over cosmic time scales. If large anti-domains populated by anti-stars or anti-clouds do exist, they would challenge and invigorate current cosmological models and galvanize further paper.

Future Exploration and Research

The paper concludes by suggesting that future data from AMS-02 and upcoming missions like GAPS could offer more definitive evidence, potentially reshaping our understanding of cosmic-ray physics and cosmology. These observations might assist in delineating the extent and criteria for survival of antimatter regions, offering new fronts in the paper of cosmic rays and matter-antimatter segregation in the galaxy.

In summary, while the findings presented in the paper suggest the potential existence of antimatter regions, they also illuminate the necessity for theoretical adjustments and new observational evidence. More extensive studies could validate these radical propositions, further enhancing our comprehension of the universe's fundamental architecture.