Acoplanarity

In the context of experiments involving quantum chromodynamics, acoplanarity can arise from the emission of gluons from the scattered final state particles.

In particle physics, the acoplanarity of a scattering experiment measures 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]

Measures of acoplanarity

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

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

where $\phi _{i}$ 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 $p_{i}$ 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.

See also

• Color confinement
• Asymptotic freedom

References

1. De Rújula, A.; Ellis, J.; Floratos, E. G.; Gaillard, M. K. (1978). "QCD predictions for hadronic final states in e +e - annihilation". Nuclear Physics B. 138 (3): 387–429. Bibcode:1978NuPhB.138..387D. doi:10.1016/0550-3213(78)90388-7.
2. Bordes, G.; Nicolaidis, A. (1980). "Acoplanarity distributions at large transverse momenta". Physical Review D. 22 (9): 2152–2156. Bibcode:1980PhRvD..22.2152B. doi:10.1103/PhysRevD.22.2152.