acoplanarity

Acoplanarity refers to a state where multiple particles or points do not lie within the same plane. In physics, it is a concept used to describe the spatial arrangement of particles, particularly in high-energy particle collisions. When particles are produced in a collision, their momentum vectors and positions should ideally lie in a single plane, often referred to as the event plane, if certain conservation laws are strictly obeyed and no external forces are acting.

Deviations from this planar arrangement are measured by acoplanarity. It is a quantitative measure of how "out of plane" the particles are. In experiments, acoplanarity is often observed and studied to understand fundamental interactions, test theoretical models, and search for new physics phenomena. For instance, in certain particle decays or interactions, acoplanarity can be a signature of processes that violate symmetries or involve the production of new, weakly interacting particles that escape detection.

The degree of acoplanarity can be calculated using various methods, typically involving the momentum vectors of the involved particles. A common approach is to define a plane based on the momentum vectors of a subset of particles and then calculate the angle or deviation of the remaining particle's momentum vector from this plane. Small values of acoplanarity suggest the particles are close to being coplanar, while larger values indicate a significant departure from a planar configuration. Understanding acoplanarity is crucial for precisely interpreting experimental results in particle physics.