Frontiers of Elementary Particle Physics to be Opened Up by JLC

Key Points of the Standard Model

High energy physics (elementary particle physics) has made a remarkable progress after the invention of particle accelerators. Now we have the Standard Model which describes virtually all the experimental data we have at present.
The standard model dictates that fundamental matter particles are leptons and quarks. There are three generations of leptons and quarks, which interact with each other in a universal manner through exchange of force carrying particles (field quanta) called gauge particles.
The standard model is, however, not yet fully tested.

Top Quark
The standard model requires the existence of the top quark as the partner of the bottom quark.
The standard model gives, however, no reason why the top quark is so heavy compared to all the other quarks. We need JLC to study its properties in detail.

Higgs Particle
The standard model requires a hypothetical particle called Higgs boson which is responsible for mass generation of all the fundamental particles. The mass of the Higgs particle is, however, unpredictable from the model.
JLC is essential to understanding of this mysterious particle.

Forces among Gauge Particles
The standard model is based on the gauge principle which dictates the existence of a gauge symmetry to every fundamental force. 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.
This difficulty can only be got around by assuming new physics beyond the standard model to appear in the energy region to be explored by JLC.
Supersymmetry and compositeness scenarios represent such new physics.
The remarkable successes of the standard model also suggest the possibility of the unification of the three forces among the fundamental particles into a single gauge force corresponding to a higher gauge symmetry (Grand Unification).

Supersymmetric Particles
Supersymmetry (SUSY) is the only symmetry that can unify all the four forces including gravity. Supersymmetry demands each fundamental particles to be accompanied by a supersymmetric partner.
These superparticles are likely to be discovered at JLC.

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.

Totally New Fundamental Particles
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. The larger symmetry implies the existence of new matter particles and new gauge particles.
The extra-Z boson is such a particle and is easy to find, if it is in the energy region of JLC.

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