Electron-Positron Colliders vs Proton-Proton Colliders

Circular Proton-Proton Colliders

Colliders that allow higher energies and are suitable for discoveries of distinct new phenomena

Protons have a large mass compared to that of electrons and can be accelerated in a circular accelerator up to higher energies without synchrotron radiation loss. There is a proton-proton collider project called LHC in Europe, aiming at an energy above 10TeV.
Hadron colliders such as LHC are good at quickly surveying energy frontiers, though the signals have to be distinct from the huge background inherent in the collisions of strongly interacting composite particles such as protons. In addition, the collision energies of the constituents are uncontrollable, which complicates the experiments thereat. Nevertheless, when the signal is clean, hadron colliders can be very effective as has been demonstrated at CERN by discovering the W and Z particles.

Circular Electron-Positron Colliders

Circular electron-positron colliders can hardly compete with hadron colliders in terms of energy. They are, however, suitable for precision measurements and therefore stringent tests of fundamental principles.

Electrons and positrons are pointlike particles which interact with each other through electroweak force and therefore their collision energy is well controlled and the background is small. Because of this we can detect essentially all the new phenomena up to the total energy of the two beams.
It is, however, practically impossible to achieve an energy comparable to that of LHC with a circular electron-positron collider such as TRISTAN or LEP due to the synchrotron radiation loss which increases as the fourth power of the beam energy.

Linear Electron-Positron Colliders

Linear electron-positron colliders provide an effective collision energy of constituents similar to that of proton-proton colliders: The effective collision energy of a 10TeV proton-proton collider is equivalent to that of a 1TeV electron-positron collider. Moreover, being electron-positron colliders, they allow us to detect essentially all the new phenomena up to the kinematical limit and provide opportunities to study them in detail.

The linear collider is a new kind of collider which consist of an electron linac and a positron linac opposing to each other. The collider has no curved sections downstream of damping rings and are therefore free from the energy limitation due to synchrotron radiation.
JLC will enable us to study various new phenomena unambiguously and will lead us to pin down underlying new physics. It should be also emphasized that the polarized electron beam is essential to controlling reaction types to be studied.