What is an Antiproton and Why is it Important?
An antiproton is a subatomic particle that has the same mass and spin as a proton, but with a negative electric charge and an opposite magnetic moment. It is the antiparticle of the proton, which means that when an antiproton collides with a proton, they annihilate each other and release a burst of energy.
Antiprotons were predicted by Paul Dirac in 1933, based on his equation that described the behavior of electrons and positrons (the antiparticles of electrons). He suggested that there should be positive and negative solutions to Einstein’s energy equation (E=mc), and that every particle should have an antiparticle with opposite charge and spin.
The first experimental confirmation of antiprotons came in 1955, when Emilio SegrÃ¨, Owen Chamberlain and their colleagues at the University of California at Berkeley used the Bevatron accelerator to bombard a copper target with high-energy protons. They detected antiprotons among the products of the collisions, and received the Nobel Prize in Physics in 1959 for their discovery.
Why are Antiprotons Important?
Antiprotons are important for several reasons. First, they provide a way to test the fundamental symmetries of nature, such as charge-parity (CP) symmetry, which states that the laws of physics should be the same if we swap particles with antiparticles and reverse their spatial coordinates. If CP symmetry is violated, it could explain why there is more matter than antimatter in the universe, a mystery that remains unsolved.
Second, antiprotons can be used to study the structure and properties of matter at the subatomic level. By colliding antiprotons with protons or nuclei, physicists can probe the interactions between quarks and gluons, the elementary constituents of hadrons (particles made of quarks). Antiprotons can also be used to create exotic forms of matter, such as antihydrogen (an atom consisting of an antiproton and a positron) or antinuclei (nuclei made of antiprotons and antineutrons).
Third, antiprotons have potential applications in medicine and energy. Antiprotons can be used to deliver precise doses of radiation to cancer cells, by injecting them into tumors and triggering their annihilation with protons. This technique could minimize the damage to healthy tissues and reduce the side effects of conventional radiotherapy. Antiprotons could also be used to generate power from fusion reactions, by combining them with deuterium (a heavy isotope of hydrogen) or helium-3 (a rare isotope of helium). This could produce more energy than fission reactions, without producing radioactive waste or greenhouse gases.
How are Antiprotons Produced and Stored?
Antiprotons are produced by accelerating protons to high energies and smashing them into targets made of heavy elements, such as copper or lead. The collisions create a spray of particles, including antiprotons, which are separated from the rest by using magnets and electric fields. The production rate of antiprotons is very low, however, as only about one in a billion protons produces an antiproton.
Antiprotons are stored by using devices called storage rings or traps. Storage rings are circular accelerators that keep antiprotons circulating at high speeds by using magnets and electric fields. Traps are devices that confine antiprotons at low energies by using magnetic and electric fields. Both methods require ultra-high vacuum conditions to prevent antiprotons from annihilating with residual gas molecules.
The largest facility for producing and storing antiprotons is the Antiproton Decelerator (AD) at CERN, the European Organization for Nuclear Research in Geneva, Switzerland. The AD produces about 10 antiprotons per hour and can store them for several hours or days. The AD supplies antiprotons to several experiments that study antimatter physics, such as ALPHA, ASACUSA, ATRAP and BASE.