Exploring the Fluid Behavior in p+p Collisions at sqrt[s]=13  TeV with Viscous Anisotropic Hydrodynamics.

The applicability of hydrodynamics in small collision systems remains controversial due to the small size and short lifetime of the system. In this Letter, we employ viscous anisotropic hydrodynamics (VAH), which incorporates large pressure anisotropies, to study the collectivity in p+p collisions at sqrt[s]=13  TeV. VAH provides a good description for v_{2}{2} and v_{3}{2} over a wide range of multiplicities and correctly reproduces the experimentally observed negative c_{2}{4}. Traditional second-order viscous hydrodynamics (VH), on the other hand, can describe the measurements, in particular the negative c_{2}{4}, only with model parameters for which the bulk of the evolution is characterized by large values of the shear Knudsen number. It also cannot capture the large pressure anisotropy between the longitudinal and transverse directions during the early evolution. These demonstrate the failure of traditional viscous hydrodynamics in small collision systems and establish viscous anisotropic hydrodynamics as a more reliable framework to describe the bulk evolution and the observed anisotropic flow in p-p collisions at the LHC.