What can we really learn from positron flux ‘anomalies’?

Abstract

We present a critical analysis of the observational constraints on, and of the theoretical modelling of, aspects of cosmic ray (CR) generation and propagation in the Galaxy, which are relevant for the interpretation of recent positron and antiproton measurements. We give simple, analytic, model-independent expressions for the secondaryp flux, and an upper limit for the secondary e + flux, obtained by neglecting e + radiative losses, e + /(e + + e − ) < 0.2 ± 0.1 up to ∼300 GeV. These expressions are completely determined by the rigidity-dependent grammage, which is measured from stable CR secondaries up to ∼150 GeV nucleon −1 , and by nuclear cross-sections measured in the laboratory.p and e + measurements, available up to ∼100 GeV, are consistent with these estimates, implying that there is no need for new, non-secondary,p or e + sources. The radiative loss suppression factor f s,e + of the e + flux depends on the e + propagation in the Galaxy, which is not understood theoretically. A rough, model-independent estimate of f s,e + ∼ 1/3 can be obtained at a single energy, ε ∼ 20 GeV, from unstable secondary decay and is found to be consistent with e + measurements, including the positron fraction measured by Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA). We show that specific detailed models, that agree with compositional CR data, agree with our simple expressions for the e + andp flux, and that the claims that the positron fraction measured by PAMELA requires new primary e + sources are based on assumptions, that are not supported by observations. If PAMELA results are correct, they suggest that f s,e + (ε) is slightly increasing with energy, which provides an interesting constraint on CR propagation models. We argue that measurements of the e + top ratio are more useful for challenging secondary production models than the e + /(e + + e − ) fraction.

Astrophysical implications of a visible dark matter sector from a custodially warped GUT

“…with respect to what could be expected from standard astrophysics as follows: The actual positron intensity does not exhibit an excess when contrasted with model independent calculations [15], which successfully describe the observed abundance of other secondary CR particles, such as antiprotons. Moreover, since measurements of unstable CR isotopes can be used to infer the cooling suppression of positrons at an energy of around 20 GeV, a theoretical estimate for the corresponding positron flux can be derived at that energy [15].…”

“…Thus, the combination of the predicted positron flux and the available e − + e + data [2,3,5,7,8,9] yields an independent estimate of the background positron fraction for this energy range. The authors of [15] have compiled the above data and shown that it is, in fact, consistent with the PAMELA measurement, leaving little room for an anomaly. It is, therefore, conceivable that the rising positron fraction may just imply that the currently fashionable diffusion models for CR propagation in the Galaxy are incorrect.…”

“…We shall now construct a Pati-Salam model based on the 15 representation of SU(4) and show that it is compatible with full unification into SO(10) 15 . The model also has SU (5) normalization for 15 See also reference [63].…”

“…Moreover, since measurements of unstable CR isotopes can be used to infer the cooling suppression of positrons at an energy of around 20 GeV, a theoretical estimate for the corresponding positron flux can be derived at that energy [15]. Thus, the combination of the predicted positron flux and the available e − + e + data [2,3,5,7,8,9] yields an independent estimate of the background positron fraction for this energy range.…”

“…Furthermore, in wait of future data release by the PAMELA and upcoming missions [30,31], it is timely to consider theoretically clean observables for indirect detection, such as the antiproton to proton, and positron to antiproton flux ratios [15].…”