Identical pion intensity interferometry at $$\sqrt{s_{\mathrm{NN}}}=2.4~\hbox {GeV}: HADES collaboration

High-statistics $$\pi ^-\pi ^-$$ π - π - and $$\pi ^+\pi ^+$$ π + π + femtoscopy data are presented for Au + Au collisions at $$\sqrt{s_\mathrm{NN}} = 2.4~\hbox {GeV}$$ s NN = 2.4 GeV , measured with HADES at SIS18/GSI. The experimental correlation functions allow the determination of the space-time extent of the corresponding emission sources via a comparison to models. The emission source, parametrized as three-dimensional Gaussian distribution, is studied in dependence on pair transverse momentum, azimuthal emission angle with respect to the reaction plane, collision centrality and beam energy. For all centralities and transverse momenta, a geometrical distribution of ellipsoidal shape is found in the plane perpendicular to the beam direction with the larger extension perpendicular to the reaction plane. For large transverse momenta, the corresponding eccentricity approaches the initial eccentricity. The eccentricity is smallest for most central collisions, where the shape is almost circular. The magnitude of the tilt angle of the emission ellipsoid in the reaction plane decreases with increasing centrality and increasing transverse momentum. All source radii increase with centrality, largely exhibiting a linear rise with the cube root of the number of participants. A substantial charge-sign difference of the source radii is found, appearing most pronounced at low transverse momentum. The extracted source parameters are consistent with the extrapolation of their energy dependence down from higher energies.