SLAC Linear Collider (SLC)

The Stanford Linear Collider (SLC) began construction in 1983 and was completed in 1987. The experimental physics program using the SLC started with the MarkII detector in 1989, which demonstrated that same year the first evidence that only three families of matter particles exist. Later, the SLD detector would observe over half a million Z⁰particle events and make many world-class measurements, including precise measurements of parity violation in electroweak interactions and precise studies of processes involving bottom and charm quarks. The SLC was a novel machine that served both as a test bed for new accelerator techniques and as a frontier physics facility studying the production and decay of the massive Z⁰particles.

SLC Operation: Polarized electrons are produced by photoemission from a Ti:sapphire laser and a GaAs photocathode at the electron gun. Two short 2-nanosecond bunches of electrons are produced spaced 60 nanoseconds apart. This time structure is repeated at 120 Hz. The first bunch of electrons is used for collisions, and the second electron bunch is used to make positrons. The two electron bunches are accelerated in the Linear Accelerator (Linac) to 1.2 Giga (billion) electron-Volts (GeV). They are then kicked by a pulsed magnet into the Damping Ring (DR), which stores the beam for 8 milliseconds to reduce its emittance (size). A pulsed magnet then kicks the beam back into the Linac. These two bunches of electrons are preceded down the Linac by a positron bunch which has been extracted from the positron DR. All three bunches are accelerated down the Linac. The trailing electron bunch is accelerated only to 30 GeV, and is sent to the positron production target. Positrons in the energy range 2-20 Mega (million) electron-Volts (MeV) are collected, then accelerated to 200 MeV and transported to near the start of the Linac for transport to the positron DR. There they are stored for 16 milliseconds to reduce their emittance. At the end of the Linac, the electron and positron energies are each 46.6 GeV. A magnet deflects the electron bunch into the north collider arc and the positron bunch into the south collider arc for transport to the Interaction Point (IP) at the center of the SLD Detector. In the arcs, the beams lose about 1 GeV in energy from synchrotron radiation so that the resulting center-of-mass collision energy is 91.2 GeV, chosen to match the Z0 rest mass.

The SLC was the world’s first and only linear collider. In circular machines, two beams of particles travel in opposite directions in storage rings, and are brought to a fiery collision in the middle of a large detector. But accelerating particles lose energy due to synchrotron radiation, and a particle that is bending in the magnetic field of a circular accelerator has a large acceleration towards the center of the ring. The amount of synchrotron radiation, P, emitted in a ring of fixed radius, r, scales as the 4^th power of the beam energy and inversely as the 3^rd power of the beam particle’s mass -- P ~ E⁴/(r²m³). For electrons, which are 2000 times lighter than protons, this becomes a severe constraint and dictates that high energy electron circular colliders must have a very large radius, with the radius scaling roughly as the square of the beam energy. The cost for such a machine would also scale as the square of the beam energy. The 100 GeV beams at CERN’s LEP ee collider travel around a circle, which is 27 kilometers in circumference. The LEP machine is the last of the high-energy circular colliders for electrons due to this limitation.

A Linear Collider is the natural solution to the scaling problems of a circular collider. At the SLC, electron and positron beams are accelerated in a single pass through a linear structure. At the end of the two-mile linear accelerator, they are bent into two arcs and then brought to a head-on collision at the center of a very large particle detector. The SLC presented many challenges. First, high acceleration gradients were needed to achieve the full beam energy in a single pass. In a circular machine, the beam energy can be ramped up as the beam circulates many times through the accelerating rf cavities. Second, the beams in a linear collider must have high intensity and be focused to very small spotsizes to achieve high collision rates, since the beams only collide once before they are dumped. For a circular machine, the beam particles can be reused many times for collisions, and so smaller beam intensities with less focusing are needed.

The accumulation of Z⁰ particles by the SLD experiment. The Z⁰ production rate increased steadily as improvements were made to the operation of this novel machine. In 1992-1995, 150 thousand Z⁰ events were accumulated. In 1996-1998, 380 thousand Z⁰ events were accumulated, including over 200 thousand events in less than 6 months of operation in 1998. Unfortunately, lack of federal funding for this program led to its termination in June of 1998. The numbers shown in red are the polarization of the electron beam.

The success of the SLC machine has led to designs for the next generation of electron colliders with beam energies of 250 GeV and collision energies of 500 GeV. Future energy upgrades of these machines to collision energies of 1 Tera (trillion) electron-Volts (TeV) and higher will also be possible. Design efforts for these Linear Colliders are being led by teams of physicists in the United States, Japan, Germany and at the international CERN laboratory in Geneva. The physics prospects at these machines is exciting and is the subject of a collaborative worldwide study.