The Elusive Neutrino
Introduction

Ever since the human race evolved from a hunter-gatherer to an agrarian society, people have had time to explore the world around them. This knowledge base was passed from one generation to the next, with each successive generation expanding on the work of the previous one. Although this is the way it works most of the time, there have been exceptions where a theory would last for a generation, century, or even millennia, only to be disproved later. One example of this is Aristotleís geocentric view of the universe that persisted for two thousand years before Galileo and Nicholas Copernicus disproved it.

Advances in technology help scientists to search for answers at further and further extremes. A few centuries ago the only thing known about the human body was how some of the various systems worked. Now scientists have peered into the cell, and even the DNA that is the basic genetic code that determines how a human develops. At the other end of the spectrum are astronomers who can see farther away from the earth by using stronger and different types of telescopes. These observations give us a better understanding of the universe, as well as a glimpse back in time.

One theory that endured for over two thousand years is that matter is made up of small, indivisible particles called atoms, and there is nothing smaller than an atom. Democritus even used the word "atom" for these particles, the same term used today. This theory remained undisputed until the 19th century when scientists started to determine the atomic structure using indirect results of experiments. With the invention of the electron microscope researchers have only recently been able to see the outlines of atoms, but still cannot see inside. Now, with the advent of particle accelerators, physicists are able to peer into the subatomic level of the atom, but still only indirectly.

The History of the Atom

The biggest obstacle to understanding the atom and its constituent particles is that physicists cannot see inside the atom. This is unlike studying other physical phenomena, such as gravity. In studying gravity one can see an object drop, measure its initial height, and then time the fall. With the atom though, researchers can only make a theory, design an experiment to test the theory, and then see if the theory seems to be correct, all the time not seeing the object they are studying. This problem forces them to look at indirect evidence to understand how an atom is structured.

One of the first modern theories was developed by J. J. Thomson in 1898, eleven years after the discovery of the electron. Thomson theorized that the electron was a negatively charged particle embedded in the positively charged proton. This is similar to a chocolate chip on the surface of a cookie.

This theory persisted until 1911 when Ernst Rutherford performed his famous experiment and formulated a new model for the atom. Rutherford directed a beam of alpha particles, which are helium nuclei with a mass of about 7000 times that of an electron, at a thin sheet of gold foil. The particles were sent at a high velocity, thus giving them a large kinetic energy. What Rutherford expected was that the particles would go straight through the foil and strike a screen of zinc sulfide positioned around the foil. The particles striking the screen would emit light, allowing him to see if the particles were deflected. Instead of going straight through, some of the particles were deflected when they came close to the nucleus, and some even bounced backward. After the experiment Rutherford likened this to firing "a 15-inch shell at a piece of tissue paper and it came back and hit you." From his results Rutherford determined that an atom is mostly empty space.

Two years after Rutherfordís experiment, Niels Bohr developed a model for the hydrogen atom. In the Bohr model the electron orbits the nucleus in a circular path and is attracted to the nucleus according to Coulombís Law. Also, only some of the electron orbits are stable, where they neither gain nor lose energy. If energy is absorbed by the atom, such as when an electric current flows through a tube filled with hydrogen, the electron jumps to a higher energy level. The electron then immediately returns to the original stable orbit, and emits a photon to compensate for the energy loss. While this theory explained the behavior of the hydrogen atom, it could not be generalized to explain more complex atoms.

In the 1930s, particle physics was relatively simple. The proton, the neutron, and the electron were considered the fundamental building blocks of matter, and the photon, positron, and neutrino had been theorized. Still, some of the concepts thought to be true at the time contradicted classical physics, such as how a nucleus could have more than one proton. Coulombís Law would predict repulsive forces too great to allow the protons to stay together. Research into resolving these contradictions led to a standard model for the atom.

The Standard Model
In the standard model, the atom is not considered the smallest unit of matter, nor are the proton or the neutron. Instead there are twelve fundamental building blocks that are divided into two sets of six particles, the sets being called quarks and leptons, and four forces, each with an associated force carrier.

Physicists have discovered six quarks, which have ended up with rather odd names. They are the up, down, charm, strange, top, and bottom quarks. Quarks are all known to be smaller then a proton (how small they actually are is unknown), but one, the top quark, has a mass that is greater than a gold atom. Two of these quarks, the up and down quarks, make up the protons and neutrons in atoms. One substantial difference between the proton and the quarks that make it up is the charge. While the proton has a charge of +1, quarks can have fractional charges.

The other set of building blocks is the leptons. The electron is the best know lepton and has a charge of -1. It, along with the up and down quarks, make up all of the matter around us. Two more of the leptons, the muon and the tau, have the same charge as the electron, but each has a greater mass. Each of these leptons has a neutrino associated with it. These are called the electron neutrino, the muon neutrino, and the tau neutrino.

The standard model also takes into consideration four elementary forces, and each force has a carrier called a boson. The bosons carry small amounts of energy from one particle to another. These packets of energy are called quanta. The first force is the strong force. The strong force is the force that holds quarks together to form protons and neutrons, and its boson is called the gluon. The second force is the electromagnetic force, which holds the electrons in orbit around the nucleus of the atom. The boson for the electromagnetic force is the photon, which also transmits light. The third force is the weak force, which aids in the decay of heavy particles as they become smaller particles. There are two bosons associated with the weak force, the W and Z bosons. The final force is the gravitational force. The gravitational force holds together massive objects and does not play a part at the subatomic level. The boson for the gravitational force is the graviton. The graviton is thus far only hypothetical though, since it has never been observed.

These twelve particles and four forces make up our universe, but this is not the entire story. When the universe began there was both matter and antimatter. For some unknown reason matter became dominant, and that is why everything around us is made of matter instead of antimatter. The reason antimatter does not exist in nature is that when matter and antimatter come together they annihilate each other. Antiparticles have characteristics that are similar, but opposite to, particles. An antiproton has the same mass as a proton but the charge is opposite. So electrons have positrons, quarks have antiquarks, and so on. Atoms also have counterparts, so hydrogen also has a counterpart called antihydrogen.

Even with the problems with antimatter and matter not mixing well, scientists have been able to create it for long enough to use it in experiments. One way they use these particles is in high energy particle accelerators, where they collide at speeds near the speed of light.

Particle Accelerators

Particle accelerators help to determine the nature of subatomic particles by colliding other particles together at velocities approaching the speed of light. These collisions are considered high energy because they release a small amount of energy in a very small area. This amount of energy is equivalent to the amount of energy a mosquito requires to fly. During the collision new particles are formed and detectors record the actions and characteristics of these new particles. Then the data is analyzed in computer farms to determine what particles resulted from the collision.

There are two types of particle accelerators: the linear accelerator and circular accelerator. Although they are different types they both work on the same principle. The particles, such as protons with a positive charge, travel through a hole in metal plates. As the proton approaches a plate the voltage of the plate is negative so it will attract the proton. As the proton reaches the plate the voltage changes to a positive value to repel the proton. In some high-energy circular accelerators the voltage can be switched several billion times each second, and each time the proton moves through a plate the kinetic energy of the proton is boosted. This increase in kinetic energy results in an increase in velocity.

Linear accelerators are used for three purposes. One is to collide particles into a fixed target. Another use is for linear collisions, with each particle coming from an opposite direction. The last use is in conjunction with circular accelerators. The linear accelerator is used to inject particles into, or extract them from, a circular accelerator.

Circular accelerators are used when high velocities are required before colliding the particles. The advantage is that the particles can go through the same plates as many times as necessary to achieve this velocity. For example, a particle going through a circular accelerator with a circumference of three miles and making one thousand revolutions is similar to a three thousand-mile long linear accelerator. In order to make the particle travel in a circular path magnets are placed around the accelerator. As the particle approaches a magnet, the magnetic force deflects the particleís path enough to make the path circular.

The collisions in accelerators take place inside detectors. The detector does many jobs, such as counting the number of particles created, determining the path of each particle, measuring the energy of the particles, recording how long each particle lives before decaying into another particle, and identifying each particle. In a typical event, about one million collisions take place, and approximately one hundred megabytes of data is recorded every second. Computer programming is used to eliminate most of the ordinary readings. These detectors can vary from the size of a computer chip to the size of an apartment building.

The final step is when the data is analyzed by other computers. An example of the magnitude of this analysis is shown when the tau neutrino was found in Fermilabís DONUT (Direct Observation of the NU Tau) experiment. During the event six million likely interactions were recorded. Of these all but one thousand were eliminated, and only four provided evidence of the tau neutrino.

Fermilabís Tevatron

Fermilabís Tevatron is the most powerful particle accelerator in the world. It uses a series of accelerators to accelerate and collide protons and antiprotons in its four-mile long underground circular accelerator.

The first step in a particleís journey to the Tevatron takes place in the Cockcroft-Walton pre-accelerator. Here negative ions are created by ionizing hydrogen gas, giving it a negative net charge by adding one electron to the atom. The ions are then accelerated and obtain an energy level of 750,000 electron volts. The ions then enter a 500-foot long linear accelerator called the LINAC, and are accelerated to 400 million electron volts. Before they leave this accelerator the ions pass through a carbon foil to remove the electrons. This leaves only the protons.

Next the protons are injected into the Booster, a circular accelerator, and are accelerated to 8 billion electron volts (8 GeV). This energy level is obtained as the protons travel around the Booster about 20,000 times. The protons then move into the Main Injector where they are accelerated to 150 GeV. When the energy level reaches 120 GeV some of the protons are removed to the Antiproton Source. Here the particles collide with a nickel target, which produces many types of secondary particles. Among these secondary particles are antiprotons, which are collected and stored in the Accumulator Ring. When there are enough antiprotons collected they are sent back into the Main Injector and accelerated to 150 GeV. The protons and antiprotons are then injected into the Tevatron going in opposite directions.

When the particles enter the Tevatron they are accelerated to 1 tera electron volt (TeV), which is 1000 GeV. This is where the Tevatron gets its name. At the energy level of 1 TeV the particles are moving at just 200 miles per hour less than the speed of light. When all is ready the beams cross in the two detectors, the CDF (Collider Detector at Fermilab) and the DZero.

The CFD and the DZero detectors are both three story tall structures weighing about 5000 tons. The various devices inside the detectors are arranged like the layers of an onion, with the pipe of the accelerator in the center. The devices closest to the pipe are made of silicon and gas-filled chambers. When the particles go through these devices electrons are torn from the media. This causes an electric impulse that the detector can use to track the path of the particles. The next layer of devices are calorimeters that measure the energy of the particles. As the particles pass through layers of absorbent and dense materials they lose energy. The particles then strike metal plates, where they cause a shower of light. The detector can use the intensity of the light to measure the energy of the particle. The outermost layer of the detector contains muon detectors. Muons are usually the only particles that can escape the calorimeter. This is why the muon detector is placed on the outside of the detector.

The Neutrino

One of the most elusive subatomic particles is the neutrino. In the 1930s scientists were having trouble explaining a loss of energy in radioactive decay. Since energy must be conserved, Wolfgang Pauli proposed the existence of a new particle to explain the loss, but he doubted anyone would ever find it. In 1931 Enrico Fermi proposed the name "neutrino" for Pauliís hypothetical particle. The reason that researchers did not think a neutrino would be found is that it does not interact with other particles. The only time scientists can see evidence of a particle is when it causes a reaction in another particle.

Most neutrinos are created in celestial bodies such as the sun and supernovae. These particles spread out from the source and pass through any matter they encounter. It is estimated that every cubic foot of space contains about one million neutrinos, and that one million million neutrinos pass through every person every minute.

Evidence of the existence of neutrinos has been found. In 1956 Clyde Cowan and Frederick Reines found evidence of the neutrino using a fission reactor for a neutrino source. Then in 1962 scientists at Brookhaven National Laboratory and Columbia University demonstrated the existence of two types of neutrinos, the electron neutrino and the muon neutrino. The last of the neutrinos, the tau neutrino, was not discovered until 2000 when scientists at Fermilab discovered it in the DONUT experiment.

One aspect that researchers are trying to determine is whether or not the neutrino has mass. To determine this they look for oscillations between different types of neutrinos. If they can find these oscillations it would mean that the neutrino has mass. If neutrinos do indeed have mass, it could help account for five percent of the dark matter in the universe.

Conclusion

Particle physics has developed at a fantastic rate in the last century. From determining the basic structure of the atom to finding out what makes up the individual elements of that atom. In the future many more questions may be answered. One concept that researchers hope to find proof of is the Higgs Boson, which would explain why some particles are massless. Finding the dark matter in the universe, which physicists have observed by gravitational forces from an unknown mass, may be accomplished. Also, scientists hope to determine what happened to the antimatter that was created when the universe was born. Finally, physicists hope to fulfill one of Albert Einsteinís dreams, unifying the gravitational and electromagnetic forces. With the advances in technology that are taking place, these mysteries may be resolved in the near future.

Bibliography

Serway, R. A., Beichner, R. J. (2000) . Physics for scientists and engineers: with modern physics. Fort Worth: Harcourt College Publishing.

"Inquiring Minds." 10 April 2001. World Wide Web. 26 May 2001. "http://www.fnal.gov/pub/inquiring"

"The Particle Adventure." World Wide Web. 26 May 2001. "http://particleadventure.org/frameless/"

"Nuclear Physics: Past, Present and Future." 28 October 1996. World Wide Web. 26 May 2001. "http://library.thinkquest.org/3471/"

"Welcome to CERN." 4 May 2001. World Wide Web. 26 May 2001. "http://public.web.cern.ch/Public/"

Written May 29, 2001

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