In particle physics, the acoplanarity of a scattering experiment is the degree to which the paths of the scattered particles deviate from being coplanar. Measurements of acoplanarity provide a test of perturbative quantum chromodynamics, because QCD predicts that the emission of gluons can lead to acoplanar scattering events.^[1]

For a two-jet final state, a useful measure of acoplanarity is

${\displaystyle \varphi =\pi -(\phi _{2}-\phi _{1})}$

where ${\displaystyle \phi _{i))$ are the azimuthal angles of the final state jets with respect to the beam line.^[2] An alternative measure of acoplanarity which is infrared safe and which works for broad jets of many particles is given by

${\displaystyle A=4\min {\left({\frac {\sum _{i}|p_{out}^{i}|}{\sum _{i}|p_{i}|))\right)^{2))}$

where ${\displaystyle p_{i))$ are the momenta of the final state particles and ${\displaystyle p_{out}^{i))$ are the components of these momenta perpendicular to a plane chosen such that A is minimized.^[1] In the case of two coplanar final state particles, the plane which minimizes A would contain the paths of both particles and the beamline, and A would equal 0.

• Color confinement
• Asymptotic freedom
• Dijet event