Relativistic heavy-ion collisions produce very large numbers of subatomic particles in all directions.
Elliptic flow is strong evidence for the existence of quark–gluon plasma, and has been described as one of the most important observations measured at the Relativistic Heavy Ion Collider (RHIC).
[2][3] Elliptic flow describes the azimuthal momentum space anisotropy of particle emission from non-central heavy-ion collisions in the plane transverse to the beam direction, and is defined as the second harmonic coefficient of the azimuthal Fourier decomposition of the momentum distribution.
[4] Elliptic flow is a fundamental observable since it directly reflects the initial spatial anisotropy, of the nuclear overlap region in the transverse plane, directly translated into the observed momentum distribution of identified particles.
[5] A measurement of elliptic flow thus provides access to the fundamental thermalization time scale and many more things in the early stages of a relativistic heavy-ion collision.