What Are Colliding Beam Accelerators?

Colliding beam accelerators are particle physics facilities in which two beams of charged particles, traveling in opposite directions, are brought into collision at designated interaction points. Unlike fixed-target experiments, where a beam strikes a stationary sample, the head-on geometry of colliding beams makes the full kinetic energy of both beams available for producing new particles. When two 31.4-GeV proton beams collide head-on, for example, up to 62.8 GeV is available for the interaction, compared with roughly 27 GeV when a single 400-GeV beam strikes a proton at rest. This energy advantage makes colliding beam machines the dominant tool for high-energy particle physics research.

The concept was pioneered in the 1960s and reached maturity through a succession of increasingly powerful facilities. The underlying physics requires that the total incoming momentum sum to near zero, which allows collision products to emerge with low kinetic energy relative to their rest mass. Particles whose rest masses equal twice the beam energy can therefore be created, a condition that drove the construction of ever-larger rings throughout the late twentieth century.

Storage Rings and Beam Configuration

Most colliding beam accelerators use circular storage rings to maintain beams at constant energy while collisions accumulate over time. For collisions between identical particles such as two proton beams, two separate rings with magnets of opposite polarity are required to bend each beam along the same circular path in opposite directions. For collisions between particles and their antiparticles, such as electrons and positrons, a single vacuum chamber suffices because the opposite charge means the same magnetic field bends each species in opposite directions. The CERN Large Hadron Collider, operating in a 27-kilometer tunnel beneath the Franco-Swiss border, uses two interlocked proton rings and accelerates each beam to 6.5 TeV, yielding 13 TeV collision energies that enabled the 2012 discovery of the Higgs boson.

Klystrons and Radio-Frequency Acceleration

Particle beams gain energy through radio-frequency (RF) cavities placed around the ring. Klystrons are high-power microwave amplifiers that supply the RF drive signals: a klystron takes a low-power microwave input and produces a high-power output beam by bunching electrons through a series of resonant cavities, then extracting the amplified signal. This RF power is coupled into the accelerating cavities, where oscillating electric fields repeatedly push bunches of particles to higher energies on each revolution. Superconducting RF cavities, cooled to near absolute zero, reduce resistive losses and allow the very high accelerating gradients needed in modern colliders. The combination of klystron power sources and superconducting cavity technology underlies the performance of both the LHC and planned future colliders described in CERN's accelerator development program.

Synchrotrons and Magnet Systems

Virtually all large colliding beam machines are synchrotrons, a design in which the guiding magnetic field increases in synchrony with the rising momentum of the beam. As the Britannica account of synchrotron-based colliding beam storage rings explains, maintaining a constant orbital radius as beam energy climbs requires that dipole magnets throughout the ring ramp their field strength in a tightly coordinated sequence. Quadrupole magnets focus the beam transversely to keep particle densities high at the interaction points, a quantity called luminosity that determines how frequently collisions occur. The LEP electron-positron collider, which operated at CERN from 1989 to 2001 in the same tunnel now housing the LHC, achieved over 100 GeV per beam and established precision measurements of electroweak parameters before giving way to the proton collider.

Applications

Colliding beam accelerators have applications in a range of fields, including:

• High-energy particle physics, enabling discovery of the W and Z bosons, the top quark, and the Higgs boson
• Precision tests of the Standard Model of particle physics
• Synchrotron light production for materials science, biology, and chemistry when electron storage rings are used as radiation sources
• Medical isotope production using beams extracted from accelerator complex systems