Exchange interaction independently by physicists Werner Heisenberg and Paul Dirac in 1926. The exchange interaction is sometimes called the "exchange force". However, it is not a true force and should not be confused with the exchange forces produced by the exchange of force carriers, such as the electromagnetic force produced between two electrons by the exchange of a photon, or the strong force between two quarks produced by the exchange of a gluon. Although sometimes erroneously described as a force, the exchange interaction is a purely quantum mechanical "effect" unlike other forces. Quantum mechanical particles are classified as bosons or fermions. The
Higgs boson Supersymmetry ("SUSY") also predicts relations between the Higgs-boson masses and the masses of the gauge bosons, and could accommodate a neutral Higgs boson. The key method to distinguish between these different models involves study of the particles' interactions ("coupling") and exact decay processes ("branching ratios"), which can be measured and tested experimentally in particle collisions. In the Type-I 2HDM model one Higgs doublet couples to up and down quarks, while the second doublet does not couple to quarks. This model has two interesting limits, in which the lightest Higgs couples to just fermions ("gauge-phobic") or just gauge bosons ("fermiophobic"), but