Fitting B/C cosmic-ray data in the AMS-02 era: a cookbook

Model numerical precision, data covariance matrix of errors, cross-section nuisance parameters, and mock data

¹ LPSC, Université Grenoble Alpes, CNRS/IN2P3, 53 Avenue des Martyrs, 38026 Grenoble, France
e-mail: david.maurin@lpsc.in2p3.fr, laurent.derome@lpsc.in2p3.fr
² LAPTh, Université Savoie Mont Blanc & CNRS, 74941 Annecy Cedex, France
³ LPTHE, Sorbonne Université & CNRS, 4 Place Jussieu, 75252 Paris Cedex 05, France
⁴ Service de Physique Théorique, Université Libre de Bruxelles, Boulevard du Triomphe, CP225, 1050 Brussels, Belgium
e-mail: pierre.salati@lapth.cnrs.fr

Received: 17 April 2019
Accepted: 16 June 2019

Abstract

Context. AMS-02 on the International Space Station has been releasing data of unprecedented accuracy. This poses new challenges for their interpretation.

Aims. We refine the methodology to get a statistically sound determination of the cosmic-ray propagation parameters. We inspect the numerical precision of the model calculation, nuclear cross-section uncertainties, and energy correlations in data systematic errors.

Methods. We used the 1D diffusion model in USINE. Our χ² analysis includes a covariance matrix of errors for AMS-02 systematics and nuisance parameters to account for cross-section uncertainties. Mock data were used to validate some of our choices.

Results. We show that any mis-modelling of nuclear cross-section values or the energy correlation length of the covariance matrix of errors biases the analysis. It also makes good models (χ_min²/d.o.f. ≈ 1) appear as excluded (χ_min²/d.o.f. ≫ 1). We provide a framework to mitigate these effects (AMS-02 data are interpreted in a companion paper).

Conclusion. New production cross-section data and the publication by the AMS-02 collaboration of a covariance matrix of errors for each data set would be an important step towards an unbiased view of cosmic-ray propagation in the Galaxy.