Kaon-proton strong interaction at low relative momentum via femtoscopy in Pb-Pb collisions at the LHC

In quantum scattering processes between two particles, aspects characterizing the strong and Coulomb forces can be observed in kinematic distributions of the particle pairs. The sensitivity to the interaction potential reaches a maximum at low relative momentum and vanishing distance between the two particles. Ultrarelativistic heavy-ion collisions at the LHC provide an abundant source of many hadron species and can be employed as a measurement method of scattering parameters that is complementary to scattering experiments. This study confirms that momentum correlations of particles produced in Pb-Pb collisions at the LHC provide an accurate measurement of kaon-proton scattering parameters at low relative momentum, allowing precise access to the $ {K}^{-} p\rightarrow {K}^{-} p$ process. This work also validates the femtoscopic measurement in ultrarelativistic heavy-ion collisions as an alternative to scattering experiments and a complementary tool to the study of exotic atoms with comparable precision. In this work, the first femtoscopic measurement of momentum correlations of ${K}^{-} p\ ({K}^{+}\overline{p})$ and ${K}^{+}p ({K}^{-}\overline{p})$ pairs in Pb-Pb collisions at centre-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}} = 5.02$ TeV registered by the ALICE experiment is reported. The components of the ${K}^{-} p$ complex scattering length are extracted and found to be $\Re f_0=-0.91\pm~{0.03}$(stat)$^{+0.17}_{-0.03}$(syst) and $\Im f_0 = 0.92\pm~{0.05}$(stat)$^{+0.12}_{-0.33}$(syst). The results are compared with chiral effective field theory predictions as well as with existing data from dedicated scattering and exotic kaonic atom experiments.


Phys. Lett. B 822 (2021) 136708
HEP Data
e-Print: arXiv:2105.05683 | PDF | inSPIRE

Figure 1

Schematic illustration of available experimental techniques for measuring the interaction among hadrons: a) scattering experiments, b) measurements of energy shifts from the X-ray de-excitation spectrum of exotic atoms, c) femtoscopy in small collision systems (pp) with coupled channel effects shown in green, and in large collision systems (Pb--Pb) with vanishing coupled channel contributions shown in blue.

Figure 2

The $\rm K^{-}p \oplus K^{+}\overline{p}$ correlation functions in the six centrality classes, with the corresponding Lednick\'y--Lyuboshitz fits (denoted as ``L--L'') and Kyoto model calculations shown as light cyan and orange bands, respectively. The width of the bands corresponds to the 1-$\sigma$ uncertainties. The inserts show the $\rm K^{+}p \oplus K^{-}\overline{p}$ correlation functions with Lednick\'y--Lyuboshitz fits as light cyan bands. The bottom panels show the difference between data and the fit (model) normalised by the statistical uncertainty of the data $\sigma_{\rm stat}$. The average pair transverse mass $\langle m_{\rm T}\rangle$ is $0.92\pm0.03$ GeV/$c^{2}$ for all centrality intervals The statistical and systematic uncertainties are added in quadrature and shown as vertical bars

Figure 3

Left: scattering parameters obtained from the Lednick\'y--Lyuboshitz fit compared with available world data and theoretical calculations. Statistical uncertainties are represented as bars and systematic uncertainties, if provided, as boxes. Right: experimental femtoscopic correlation function for $\kam p\oplus \kap \overline{p}$ pairs in the 30--40\% centrality interval, together with various Lednick\'y--Lyuboshitz calculations obtained using the scattering length parameters from Refs.  and the source radius from this analysis. The statistical and systematic uncertainties of the measured data points are added in quadrature and shown as vertical bars.