The exotic atom of muonic hydrogen 𝑝𝜇 has a relatively long lifetime allowing for precise
measurements of its spectra. The hyperfine splitting of its ground-state Δ𝐸
hfs ∼ 0.182 eV is in
the infra-red optical range that makes it convenient to be studied with laser spectroscopy. Several
experiments for the determination of the proton Zemach radius from measurements of Δ𝐸
hfs, are
currently underway. One of them is the FAMU experiment, aiming to determine the Zemach
radius of the proton by studying the time distribution of the characteristic X-rays signaling the
laser-stimulated spin-flip of the 𝑝𝜇 atoms. As the accurate knowledge of the collision energy
dependence of the muon transfer rate from the ground-state of 𝑝𝜇 to oxygen, 𝜆pO(𝐸), is crucial
for the efficiency of the FAMU method, we use the available data from recent measurements of
the muon transfer rate at temperatures in the range 70 K<𝑇 <336 K in fully thermalized gas to
obtain reliable estimates of 𝜆pO(𝐸) by fitting the experimental data with various trial functions.
The advantages and disadvantages of each type of trial functions are discussed in detail.