Afixed-target experiment inparticle physics is an experiment in which a beam of accelerated particles is collided with a stationary target. The moving beam (also known as a projectile) consists of charged particles such aselectrons orprotons and is accelerated torelativistic speed. The fixed target can be a solid block or a liquid or a gaseous medium.[1][2] These experiments are distinct from thecollider-type experiments in which two moving particle beams are accelerated and collided. The famousRutherford gold foil experiment, performed between 1908 and 1913, was one of the first fixed-target experiments, in which thealpha particles were targeted at a thin gold foil.[1][3][4]
The energy involved in a fixed target experiment is 4 times smaller compared to that in collider with the dual beams of same energy.[5][6] More over in collider experiments energy of two beams is available to produce new particles, while in fixed target case a lot of energy is just expended in giving velocities to the newly created particles. This clearly implies that fixed target experiments are not helpful when it comes to increasing the energy scales of experiments.[3][7] The targeted source also wears down with number of strikes and usually require a regular replacement. Current day fixed-target experiments try to use highly resistant materials but the damage cannot be avoided entirely.[8]
The fixed target experiments have a significant advantage for experiments that require higherluminosity (rate of interaction).[5][9] TheHigh Luminosity Large Hadron Collider, which is an upcoming upgraded version of theLarge Hadron Collider (LHC) atCERN, will attain total integrated luminosity of around in its run.[10] While luminosity scale of about have already been approached by older fixed target experiments such at theE288 led byLeon Lederman atFermilab.[3][11] Another advantage for fixed-target experiments is that they are easier and cheaper to build compared to the collider accelerators.[5]
Rutherford's gold foil experiment that led to the discovery that mass and positive charge of an atom was concentrated in a small nucleus was probably the first fixed-target experiment. Later half of the 20th century saw the rise of particle and nuclear physics facilities such as CERN'sSuper Proton Synchrotron (SPS) andFermilab'sTevatron where number of fixed-target experiments led to new discoveries. 43 fixed-target experiments were conducted at the Tevatron during its run period from 1983 to 2000.[12] While proton and other beams from SPS are still used by fixed target experiments such asNA61/SHINE andCOMPASS collaboration. A fixed-target facility at theLHC, called AFTER@LHC, is also being planned.[13][14]
The fixed-target experiments are mainly implemented for the intensive studies of the rare processes, dynamics at high Bjorken x, diffractive physics, spin-correlations, and numerous nuclear phenomena.[13][14]
The experiments at Fermilab's Tevatron facility covered wide range of physics domains such as testing the theoretical predictions ofquantum chromodynamics theory, studies of structure ofproton,neutron andmesons, and studies of heavy quarks such ascharm andbottom. Several experiments looked intoCP symmetry tests. Few collaborations also studied thehyperons and theneutrinos created at fixed-target setups.[12][15]
NA61/SHINE at the SPS is studying thephase transitions in strongly interacting matter and physics related toonset of confinement.[16] While theCOMPASS experiment investigates the structure of thehadrons.[17]
AFTER@LHC aims at the studies ofgluon andquark distribution inside protons and neutrons using fixed-target facilities.[13] There are possibilities to observe theW and Z bosons as well.[18] Observation and studies of theDrell-Yan pair production andquarkonium are also being looked into.[14]
Thus the number of options available to explore extreme and rare physics at the fixed-target experiments are numerous.