The Cosmic Ray Lepton Puzzle

Recent measurements of cosmic ray electrons and positrons by PAMELA, ATIC, Fermi and HESS have revealed interesting excesses and features in the GeV-TeV range. Many possible explanations have been suggested, invoking one or more nearby primary sources such as pulsars and supernova remnants, or dark matter. Based on the output of the TANGO in PARIS –Testing Astroparticle with the New GeV/TeV Observations in Positrons And electRons : Identifying the Sources– workshop held in Paris in May 2009, we review here the latest experimental results and we discuss some virtues and drawbacks of the many theoretical interpretations proposed so far.

💡 Research Summary

The paper “The Cosmic Ray Lepton Puzzle” reviews the surprising excesses observed in the high‑energy cosmic‑ray electron and positron spectra, as reported by the PAMELA, ATIC, Fermi‑LAT and HESS experiments. PAMELA has measured a clear rise in the positron fraction above ~10 GeV, confirming earlier hints. ATIC reported a sharp peak in the combined e⁻+e⁺ spectrum near 600 GeV, while Fermi‑LAT sees a smoother bump in the same energy range, and HESS observes a steepening of the spectrum around the TeV scale. Although systematic uncertainties and energy‑scale calibrations differ among the instruments, all data point to the presence of a nearby (≲ 1 kpc) source of high‑energy electron‑positron pairs.

The authors discuss two main approaches to modeling Galactic cosmic‑ray propagation. Semi‑analytical methods use average source distributions and diffusion parameters, allowing rapid scans of the propagation parameter space and straightforward estimation of secondary backgrounds. Numerical tools such as Galprop implement a full three‑dimensional model of gas, radiation fields, magnetic fields and source distributions, enabling consistent predictions of γ‑ray and synchrotron emission but at a higher computational cost. Both approaches agree that secondary production alone cannot reproduce the observed lepton spectra; an additional primary component is required.

Two classes of conventional astrophysical sources are examined. Pulsars, especially mature nearby objects such as Geminga, Monogem and Loop I, can inject e⁺e⁻ pairs through magnetospheric cascades and wind nebula acceleration. By adjusting the pulsar birth‑rate, distance, age and injection spectrum, the positron fraction rise and the total lepton bump can be fitted, though uncertainties remain in the conversion efficiency and in the expected γ‑ray counterparts. Supernova remnants (SNRs) may also contribute if secondary particles are produced and accelerated directly at the shock front, as proposed by Blasi. This mechanism predicts a correlated rise in the antiproton‑to‑proton ratio above ~100 GeV, providing a near‑future test with PAMELA’s antiproton data.

The paper then turns to dark‑matter interpretations. Standard weakly interacting massive particles (WIMPs) with the thermal relic cross‑section (⟨σv⟩≈3×10⁻²⁶ cm³ s⁻¹) underproduce the lepton excess by 2–3 orders of magnitude. To bridge the gap, the authors discuss Sommerfeld enhancement (velocity‑dependent cross‑section increase due to near‑threshold bound states) and possible boost from dense subhalos, though the latter is deemed unlikely. Even with an enhanced annihilation rate, the associated production of antiprotons, γ‑rays (especially from the Galactic Center) and synchrotron radio emission typically exceeds current observational limits. Consequently, “leptophilic” models are introduced, wherein dark‑matter couples preferentially to leptons via new force carriers or symmetries, suppressing hadronic channels. Decaying dark‑matter scenarios (lifetimes ≳10²⁶ s) are also considered; because the decay rate scales linearly with density, constraints from the bright Galactic Center are relaxed compared with annihilation models.

A comprehensive multi‑messenger analysis is presented, incorporating predictions for (i) positrons, (ii) antiprotons, (iii) Galactic‑Center γ‑rays, (iv) halo γ‑rays, (v) extragalactic γ‑rays from subhalos, and (vi) synchrotron radio from the inner Galaxy. The authors find that most dark‑matter models that fit the lepton data inevitably overproduce at least one of these channels, especially radio and γ‑ray emission from the Galactic Center and dwarf spheroidal galaxies. Only finely tuned leptophilic or decaying scenarios survive, and even then only within specific assumptions about the dark‑matter density profile and magnetic‑field configuration.

In conclusion, the current lepton data can be accommodated by conventional nearby astrophysical sources, but the required source parameters are not uniquely determined, and systematic uncertainties remain large. Dark‑matter explanations remain attractive but are heavily constrained by multi‑messenger observations; future high‑precision measurements of antiprotons, γ‑rays and radio emission will be decisive in discriminating between pulsar/SNR origins and exotic dark‑matter scenarios.