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The B/C ratio is used in cosmic-ray physics to constrain the transportparameters. However, from the same set of data, the various published valuesshow a puzzling large scatter of these parameters. We investigate the impact ofusing different inputs (gas density and hydrogen fraction in the Galactic disc,source spectral shape, low-energy dependence of the diffusion coefficient, andnuclear fragmentation cross-sections) on the best-fit values of the transportparameters. We quantify the systematics produced when varying these inputs, andcompare them to statistical uncertainties. We discuss the consequences for theslope of the diffusion coefficient delta. The analysis relies on thepropagation code USINE interfaced with the Minuit minimisation routines. Wefind the typical systematic uncertainties to be larger than the statisticalones. The several published values of delta (from 0.3 to 0.8) can be recoveredwhen varying the low-energy shape of the diffusion coefficient and theconvective wind strength. Models including a convective wind are characterisedby delta > 0.6, which cannot be reconcile with the expected theoretical values(1/3 and 1/2). However, from a statistical point of view (chi^2 analysis),models with both reacceleration and convection-hence large delta-are favoured.The next favoured models in line yield delta that can be accommodated with 1/3and 1/2, but require a strong upturn of the diffusion coefficient at low energy(and no convection). To date, using the best statistical tools, the transportparameter determination is still plagued by many unknowns at low energy (~GeV/n). To disentangle between all these configurations, measurements of theB/C ratio at TeV/n energies and/or combination with other secondary-to-primaryratios is necessary.

Systematic uncertainties on the cosmic-ray transport parameters: Is it possible to reconcile B/C data with delta = 1/3 or delta = 1/2?