Azimuthally-Differential Pion Femtoscopy Relative To The Second And Third Harmonic In Pb-Pb Collisions

Heavy-ion collisions at LHC energies create a hot and dense medium of deconned quarks and gluons, known as the quark-gluon plasma (QGP) [56]. The QGP reball rst expands, cools and then freezes out into a collection of nal-state hadrons. Correlations between the free particles carry information about the space-time extent of the emitting source, and are imprinted on the nal-state spectra due to a quantum-mechanical interference eect [18]. The correlation of two identical particles at small relative momentum, commonly known as intensity, or Hanbury Brown-Twiss (HBT), interferometry, is an eective tool to study the space-time structure of the emitting source in relativistic heavy-ion collisions [23]. Due to the position-momentum correlations in particle emission, the HBT radii become sensitive to the collective velocity fields, from which information about the dynamics of the system evolution can be extracted. The spatial anisotropies in the initial state are converted, via pressure gradients and interactions between constituents, to momentum anisotropies, leading to anisotropic particle flow. Anisotropic flow is usually characterized by the Fourier decomposition of the particle azimuthal distribution and quantied by the flow harmonic strength vn and the event plane angle [48]. Elliptic flow is dened by the second flow harmonic strength (n = 2), whereas triangular flow is dened by the third flow harmonic strength (n = 3) [48]. Azimuthally-dierential femtoscopic measurements can be performed relative to the di100 rection of dierent harmonic event planes [25]. The harmonic event planes are determined using the event plane method [48]. The measurements of HBT radii with respect to the first harmonic (directed) flow at the AGS [30] revealed that the source was tilted relative to the beam direction [31]. In particular, measurements of the HBT radii relative to the second harmonic event plane angle ( 2) provide information on the nal shape of the system, which is expected to become more spherical compared to the initial state due to stronger in-plane expansion [55]. In contrast, hydrodynamics model studies have shown that the azimuthal dependence of the HBT radii relative to the third harmonic event plane angle can originate from the anisotropies in collective velocity gradients or the initial spatial anisotropy (triangular) [33]. The signs of the HBT radii relative to the third harmonic event plane angle oscillations constrain the origin of these oscillation [33].