The force carriers of gauge interactions such as the Z and W particles thus interact with each other by exchanging themselves. This self-interacting nature of the gauge particles is at the heart of the gauge principle and should be proved at JLC through precision measurements. Beyond the Standard Model Even if the standard model passes all of the above tests at JLC, it cannot be taken as the ultimate theory of nature. The model runs into serious trouble when applied to higher energy phenomena, because of unavoidable quantum fluctuations. Supersymmetry and compositeness scenarios represent such new physics.
Supersymmetry demands each fundamental particles to be accompanied by a supersymmetric partner.
In the compositeness scenario, some of the fundamental particles of the standard model are assumed to be composite. In order to observe the internal structures of these particles, experiments at high energy are inevitable. JLC provides opportunities to probe possible compositeness of leptons as well as quarks. At present, the gauge symmetry of the standard model is nothing more than an empirical fact. We can imagine a larger symmetry governing the world.