Editing International Linear Collider (section)

== Background: linacs and synchrotrons ==

There are two basic shapes of accelerators. Linear accelerators ("linacs") accelerate [[elementary particle]]s along a straight path. Circular accelerators ("synchrotrons"), such as the [[Tevatron]], the [[Large Electron–Positron Collider|LEP]], and the [[Large Hadron Collider]] (LHC), use circular paths. Circular geometry has significant advantages at energies up to and including tens of [[GeV]]: With a circular design, [[particle physics|particle]]s can be effectively accelerated over longer distances. Also, only a fraction of the particles brought onto a collision course actually collide. In a linear accelerator, the remaining particles are lost; in a ring accelerator, they keep circulating and are available for future collisions. The disadvantage of circular accelerators is that charged particles moving along bent paths will necessarily emit electromagnetic radiation known as [[synchrotron radiation]]. Energy loss through synchrotron radiation is inversely proportional to the fourth power of the [[rest mass|mass]] of the particles in question. That is why it makes sense to build circular accelerators for heavy particles—hadron colliders such as the LHC for [[proton]]s or, alternatively, for [[lead]] [[atomic nucleus|nuclei]]. An electron–positron collider of the same size would never be able to achieve the same collision energies. In fact, energies at the LEP which used to occupy the tunnel now given over to the LHC, were limited to 209 GeV by energy loss via synchrotron radiation.

Even though the nominal collision energy at the LHC will be higher than the ILC collision energy (14,000&nbsp;[[GeV]] for the LHC<ref>Since the actual collisions happen between the constituent of protons—[[quark]]s, antiquarks and [[gluon]]s—the effective energy for collisions will be lower than 14,000&nbsp;GeV but still higher than 500&nbsp;GeV), a typical collision at the LHC will be of higher energy than a typical ILC collision.
</ref> vs. ~500&nbsp;GeV for the ILC), measurements could be made more accurately at the ILC. Collisions between electrons and positrons are much simpler to analyze than collisions in which the energy is distributed among the constituent [[quark]]s, [[antiquark]]s and [[gluon]]s of [[baryon]]ic particles. As such, one of the roles of the ILC would be making precision measurements of the properties of particles discovered at the LHC.