Particle Physics | Research & Encyclopedia Articles

Particle Physics

Particle physics is the study of the basic elements of matter and the forces which act among these basic elements. The goal of particle physics is to discover the fundamental laws that govern how matter is constructed. From these fundamental laws, particle physics then attempts to expand its theories into the laws that describe the physical universe. In fact, it is often the case that the study of the smallest subatomic particles gives rise to laws that hold for the largest objects, such as stars and galaxies, and provide insight into cosmological processes.

Particle physics is concerned with the search for the fundamental building blocks of matter. Well over 150 particles have thus far been described. Some particles are extraterrestrial, coming through space as cosmic rays. Others have been manufactured in particle accelerators. Particles are catalogued by the Particle Data Group, an international group of particle physicists dedicated to the free exchange of information and standardization of particle physics throughout the globe.

Quantum mechanics is the set of laws that applies to the subatomic world. Such laws can be extended to the macroscopic world as in English physicist and mathematician Sir Isaac Newtonüs classical laws of mechanics, and Scottish physicist James Clerk Maxwell's equations combining electricity and magnetism. It is only in the world of the very smallest particles where the old laws of classical physics fail, and quantum mechanics is required.

The study of particle physics dates to the fifth century B.C. with argument advanced by the Greek philosopher Democritus (c. 420 B.C.) that matter is made up of very small indivisible particles called atoms. According to Democritus, one could keep dividing matter again and again until reaching the level of atoms that could be divided no further.

Knowledge of particle physics remained stagnate until the 19th century. In the interim, German astronomer and mathematician Johannes Kepler, Italian astronomer and physicist Galileo Galilei, Isaac Newton, Christiaan Huygens, Michael Faraday, Maxwell, and a host of other scientists had brought physics to point where many felt there was nothing new to be discovered. Only electricity seemed to require further investigation.

The British physicist William Crookes (1832-1919) studied the passage of electricity through tubes of gas connected to vacuum pumps which controlled the amount of gas present in the tubes. The electric current created strange and fascinating glows around the tubes. It was believed that rays called cathode rays caused the gas to glow. In 1897, however, J. J. Thompson professor of physics at Cambridge University, proved that the cathode rays were a stream of particles. The particles were named electrons, and Thompson found their mass to be 2,000 times less than the hydrogen atom, the lightest known atom. Nothing lighter than a hydrogen atom was thought to exist at the time. The first evidence that atoms had a complex inner structure had been found, and a new field of physics, particle physics, was opened for discovery. Thompson's discoveries showed that atoms were not solid like billiard balls but had a complicated internal structure.

In 1895, Wilhelm Roentgen (1845-1923) discovered x rays. In 1896, Henri Becquerel conducted research on the heavy elements. Becquerel found that uranium gave off rays that darkened photographic plates, and discovered radiation. Marie and Pierre Curie further explored radioactivity and radioactive substances, and discovered two new elements, polonium and radium. Rutherford and Frederick Soddy (1877-1956) discovered radioactive transmutations, that is the changing of one element into a different element through the mechanism of radioactive decay. Rutherford also came to the conclusion that atoms were like solar systems in miniature, with considerable space between the central nucleus and the orbiting electrons. Rutherford also discovered protons by firing alpha particles from radioactive substances at boron, fluorine, sodium, and other elements. As technology improved, new particles were found, and the atom slowly revealed its secrets. Hans Geiger (1882-1945) had invented his now famous particle counter ("Geiger counter"), but a better medium than photographic plates or counters was needed to detect particles.

In 1894, Charles Wilson invented the cloud chamber. With this device the tracks of charged particles could be seen. In 1910, Wilson began to use his new detector. From 1921 to 1924, Wilson allowed alpha particles to bombard nitrogen in the chamber, and captured nuclear transmutation on film. The cloud chamber was an effective detector of subatomic particles.

By the 1920s, the role of the protons carrying the positive charge of the nucleus had become firmly established. The negatively charged electrons were known to orbit the nucleus. A problem lingered, namely that the nucleus contained 99.95% of the atom's mass and the protons should have carried this mass. However, this was not the case. The protons accounted for less than half the nuclear mass. It was Ernest Rutherford who proposed the neutron, a neutrally charged particle, as making up the remainder of the mass. Based on the work of Walther Bothe in Germany, and Irene Curie and her husband Fredrick Joliot in France, James Chadwick discovered the neutron in 1932. The nucleus was ultimately considered to consist of protons and neutrons held together by the strong nuclear force. The electromagnetic force attracted the negatively charged electrons to the positively charged nucleus.

Physicists then considered the questions of particle physics, such as what particles make the atom, and what caused some elements to transmutate by radioactive decay. It was known that if a magnetic field were placed across a cloud chamber and a radioactive source placed near the chamber, three types of radiation would be seen. Electrons (beta radiation) were bent toward the positively charged side of the chamber, alpha particles (helium nuclei) were bent toward the negatively charged side, and gamma rays (found to be similar to Roentgen's x rays) traveled straight through. Occasionally, a particle from outside the system would fire through the chamber. These were cosmic rays, extraterrestrial particles from outer space flying through the atmosphere.

The year 1912 marked the discovery of cosmic rays by Victor Hess (1883-1963). Robert Millikan pushed the investigations further using high altitude balloons. His cohort, Carl Anderson in 1934 discovered a particle with the exact mass of an electron but with a positive charge. Called the positron, this particle was the antimatter equivalent of the electron. Hideki Yukawa, a Japanese theoretical physicist working in Kyoto predicted a mesotron, later shortened to meson, to be a middleweight particle between protons and electrons. Anderson observed such a particle, but it did not fulfill all criteria and was later found to be a muon. Yukawa's predicted particle was found a decade later with the discovery of the pi-meson, or pion.

Further investigations by Cecil Powell and his team used photographic emulsions on mountain peaks carried to high altitude by polyethylene balloons. Powellüs group discovered that primary cosmic rays are either photons or subatomic particles. These primary cosmic rays had enormous energies compared with alpha, beta, and gamma radioactive decay. Apollo astronauts were startled as they slept when a heavy particle cosmic ray would strike their retinas and cause a tiny flash of light.

Primary cosmic rays come predominantly from within our galaxy, the Milky Way, although the ultra-high-energy cosmic rays may originate from beyond our galaxy. When these primary rays strike our atmosphere they interact with the nuclei of atoms producing a shower of secondary particles consisting primarily of muons, protons, electrons, pions and neutrinos. Some cosmic rays are energetic enough that they come directly through the atmosphere. Before the development of particle accelerators, cosmic rays were the only source of high-energy particles.

Particle accelerators were the next accomplishment in particle physics. Rutherford had been the first to "smash the atom" but it was Ernst Lawrence's cyclotron in 1931 that lead to today's high-energy accelerators. Lawrence used a magnetic field to bend the path of the particles into a circular orbit so they would run the same accelerating gap many times, a circular accelerator operating in cycles. From this point onward it was only a matter of building larger, more powerful accelerators including the 1.8 mi (3 km) long non-circular Stanford Linear Accelerator at Stanford University and the giant circular rings at Fermilab near Chicago, and CERN near Geneva, which has a 27 km path for particle acceleration. CERN (a joint European nation project) has both a Super Proton Synchrotron and the Large Electron-Positron collider. All of these accelerators are seeking higher and higher energies at which to bombard or collide particles. They represent earthbound cosmic ray producers that can be controlled.

The Geiger counter and the cloud chamber led to the bubble chamber, a machine which used a superheated liquid to track the trails of particles, similar to the cloud chamber but with much longer lasting and larger tails than a cloud chamber which allowed photographs to be more easily taken.

Contemporary particle detectors reflect our most advanced technological innovations. Although occasionally a bubble chamber-like apparatus may still be used, most detectors consist of sophisticated electronic devices and computer systems for the control and analysis of data. As a result, there are now over 200 particles listed by the Particle Data Group. Although the place of many of these particles in the scheme of matter and antimatter remains to be discerned, the standard model has developed into an overall particle-based and dynamic atomic model.

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