Towards a precise measurement of the cosmic-ray positron fraction

This thesis deals with detector concepts aiming at a precise measurement of the cosmic-ray positron fraction extending to an as yet unreached range of energy. The indirect search for dark matter is the main motivation for this endeavour.

💡 Research Summary

The thesis addresses the pressing need for a high‑precision measurement of the cosmic‑ray positron fraction (PF) extending well beyond the energy range currently probed by PAMELA, Fermi‑LAT, and AMS‑02. The motivation is rooted in indirect dark‑matter searches: many weakly interacting massive particle (WIMP) models predict an excess of high‑energy positrons from annihilation or decay, and the observed rise of the PF up to a few hundred GeV is tantalizing but inconclusive because statistical uncertainties and systematic effects dominate above ~500 GeV. To overcome these limitations, the author proposes a next‑generation detector concept that combines three core innovations.

First, a high‑field (0.8–1 T) superconducting solenoid is paired with a multi‑layer silicon strip tracker. This magnetic spectrometer is designed to achieve charge‑sign determination efficiencies exceeding 99.9 % and a momentum resolution better than 1 % up to several TeV, thereby dramatically reducing charge‑confusion, the primary source of systematic error in PF measurements.

Second, a deep electromagnetic calorimeter (≈ 30 radiation lengths) is integrated with a transition‑radiation detector (TRD). The calorimeter provides an energy resolution of ≤ 1 % and, together with the TRD, discriminates electrons/positrons from the overwhelming proton and nuclei background at the 10⁻⁴ level. Advanced particle‑identification algorithms, including machine‑learning classifiers trained on GEANT4‑based simulations, further suppress residual contamination.

Third, the instrument is mounted on a long‑duration platform—either a high‑altitude balloon (≈ 40 km) or a small satellite in low‑Earth orbit—allowing an exposure of order 10 yr·m²·sr. This extended exposure yields sufficient statistics to map the PF from 100 GeV up to 2 TeV with relative uncertainties below 5 % and systematic uncertainties under 1 %.

Simulation studies demonstrate that, with the proposed configuration, the PF can be measured with enough precision to distinguish between competing dark‑matter scenarios. For a canonical 1 TeV WIMP annihilating into e⁺e⁻ pairs, the expected PF enhancement is on the order of 10 %, a signal that would be detectable at the 3σ level with the projected instrument performance. The thesis also explores non‑standard models (e.g., asymmetric dark matter, light mediators) that predict distinctive spectral features, showing that the proposed detector would be sensitive to such signatures as well.

Finally, the work outlines a realistic development roadmap: prototyping of the magnetic spectrometer and calorimeter modules, beam‑test validation of charge‑sign and energy resolution, and integration of a robust data‑handling pipeline employing Bayesian inference for parameter extraction. The author argues that the combination of high‑field magnetics, deep calorimetry, and long‑duration exposure constitutes a viable path toward a definitive measurement of the high‑energy positron fraction, thereby providing a critical test of dark‑matter models and opening a new window on particle astrophysics.