Antimatter
Chris Meierhans
Physics 4D
5/12/03
Antimatter
Physicists love to find symmetry in nature. It is believed that every particle has a corresponding antiparticle. Paul Dirac first thought up the concept of antimatter in 1928. He had come up with a strange mathematical equation that predicted the existence of a world, identical to ours, but made up of antimatter.
In the 1920’s Erwin Schrodinger and Werner Heisenberg had invented the new quantum theory of physics. There was only one big problem with this theory; it was not relativistic. Paul Dirac came along and solved this problem with an equation that combined quantum theory and special relativity to describe the behavior of an electron. Although Dirac’s equation solved one problem, physicists were presented with a new problem. Dirac’s equation had two solutions: one for an electron with positive energy, and one for an electron with negative energy. Dirac interpreted this to mean that for every particle there exists an antiparticle. He predicted this antiparticle would be exactly the same but have the opposite charge.
Physicists began searching for antiparticles. While studying cosmic rays in 1932, Carl Anderson found something very peculiar. Using a cloud chamber, he saw a track left by something with a positive charge but with the same mass as an electron. After a year of observation he decided the tracks were antielectrons.
By the 1950’s, aided with the invention of powerful accelerators, physicists discovered the antiproton and the antineutron. In 1965 they were able to observe the antideuteron, the nucleus of an antideuterium atom. This confirmed that antinuclei could be formed. The next step was to discover if antielectrons would stick to antinuclei. In 1995, using the Low Energy Antiproton Ring at CERN, antihydrogen was made.
Until now, antiparticles were studied using accelerators that would collide high-energy particles together. However, high-energy particles are no longer desired. In order to accurately study antiparticles and antimatter, they must be moving at slower speeds. This allows for more precise measurements of their properties and can allow for extremely accurate comparisons between the properties of hydrogen and antihydrogen.
CERN has built an antiproton decelerator, AD for short, in order to study antimatter. The AD is a ring with a circumference of 188m. Antiprotons are injected into the ring and circulate around a vacuum pipe. These particles enter at almost the speed of light with different energy levels (the energy spread), and scatters in different directions (the transverse oscillations). Several vacuum pumps are placed around the ring to make sure the antiprotons do not collide with air molecules and annihilate each other. Two different types of magnets are placed around the AD: dipole and quadrupole magnets. The dipole magnets change the direction of movement to make sure the particles stay within the circular track. The quadrupole magnets are used as lenses; these magnets focus the beam of particles to ensure the size of the beam is not larger than the diameter of the vacuum pipe. The energy of the beam is controlled by radio frequency cavities that create an electric field. These cavities are able to produce high voltages in synchronicity with the rotation of particles around the ring to decelerate the particles. The deceleration unfortunately has a negative side effect that increases the size of the transverse oscillations. To prevent the antiprotons from crashing into the vacuum walls, they are cooled using stochastic and electron cooling. This decreases the energy spread and transverse oscillations of the antiproton beam. When the speed is about 10% of the speed of light, the antiprotons are ejected into an awaiting experiment.
Physicists at CERN are currently working on three experiments. ATHENA and ATRAP are being conducted to simply create antihydrogen by combining antiprotons ejected from the AD with positrons emitted by a radioactive source. Hydrogen atoms have been one of the most crucial physical systems for different and fundamental measurements related to the behavior of matter. The production of antihydrogen opens the door to a systematic exploration of the properties of antimatter and to the test of fundamental physical principles.
Bibliography
Greene, Brian. The Elegant Universe. New York. W.W. Norton & Company: 1999.
Live From CERN. http://livefromcern.web.cern.ch/livefromcern/antimatter/index.html
Thornton, Stephen. Modern Physics for Scientists and Engineers. USA. Brooks/Cole:
2002.