With the discovery of the antielectron, the search for other antimatter particles heated up. It seemed reasonable that if the electron had an antiparticle, so too should a proton and a neutron. The methods for probing the reality of subatomic particles began with experiments as simple as those with which the electron, proton, and neutron were discovered—firing beams of light or electrons at various substances and then making very precise observations and drawing as many conclusions as possible. Physicists of the early century were able to make some amazing discoveries about the structure of the atom. However, from our point of view, their technology was limited, but they did the best with what they had to work with. In order to discover these new particles, a way to produce controlled, reliable high - energy experiments was needed. This led to the creation of particle accelerators and detectors.
Cosmic Rays
With the discovery of radioactivity in the late 1800s, measurement and detection of this radiation became a driving force in physics. It was soon found that more radiation was being measured on the Earth than was predicted. In an effort to find the source of this radiation, Victor Hess in 1912 carried detectors with him in a hot air balloon to a height of meters (without the aid of a breathing apparatus). At this height he was able to discover “cosmic rays,” which shower Earth from all parts of the universe at incredibly high speeds. Others soon discovered that the rays were actually charged particles, such as alpha particles and protons.
Figure 4.10
Cosmic Ray Shower
As it turns out, these charged particles that zoom through space began their journeys from the Sun, supernovae, and distant stars. Most of the primary cosmic rays are protons or alpha particles traveling at very high speeds. When they hit another nucleus in our atmosphere and stop, many more particles are knocked downward, creating a cascading effect called a shower. When these reactions and the particles that they produced were first analyzed it was quickly discovered that nothing like this had been seen on Earth before. Thus began a flurry of research to discover more about these particles from outer space.
Up until the 1950s and the development of particle accelerators, cosmic rays were the primary source of high–energy particles for physicists to study. Carl Anderson not only discovered antimatter through his cosmic ray research, but he went on to discover a particle that had a unit charge with a mass between the electron and proton. Muons were later shown not to have any nuclear interactions and to be heavier versions of electrons. In 1947, Cecil Powell discovered another particle that did interact with nuclei. The mass of this new particle was greater than the muon and it was soon determined that the particle would decay into a muon. This new particle was given the name pi-meson, or pion. A few months later, new particles with masses in between the pion and the proton were discovered. The kaon was a strange new particle that was always produced in pairs, had a relatively long lifetime, and decayed into pions and muons.
Although a number of exciting new particles were discovered with the cosmic rays, there were limitations to this type of research. Interesting events happen very rarely and when they do it is very difficult to catch them in a particle detector. Researchers have no control over when or where the cosmic ray shower will occur, making it very difficult to perform experiments. The other problem that was quickly becoming apparent was that all the low energy events seemed to be well researched and that the interesting events were the high–energy events. The problem with the high–energy events was that they were incredibly rare. So, the lack of control over when and where these events would occur and the infrequent high–energy cosmic ray events posed a problem for researchers. Physicists needed to come up with a solution to these problems—namely, to create controlled high–energy experiments in a laboratory-type setting.
Particle Accelerators
Particle accelerators were designed to study objects at the atomic scale. Particle accelerators allow for millions of particle events to occur and to be studied without waiting for the events to come from the sky. Accelerators do for particle physicists what telescopes do for astronomers. These instruments reveal worlds that would otherwise be left unseen. Vacuum tubes and voltage differences accelerated the first electrons and then the Cockcroft-Walton and Van de Graaff machines were invented using the same principles only on a grander, more complex scale. A modern example of this type of device is the linear accelerator, such as the Stanford Linear Accelerator (SLAC). In order to achieve high energies, all linear accelerators must be very long. For example, the Stanford Accelerator is nearly miles long and actually crosses under a highway in California. SLAC is able to achieve energies of up to . An electron volt is a unit of energy that is equivalent to . A GeV is equal to . The need for such great length to achieve the high energy is a major limitation with this type of accelerator.
Figure 4.11
Stanford Linear Accelerator Center, Palo Alto, CA
The great breakthrough in accelerator technology came in the 1920s with Ernest O. Lawrence’s invention of the cyclotron. In the cyclotron, magnets guide the particles along a spiral path, allowing a single electric field to apply many cycles of acceleration. The first cyclotrons could actually fit in the palm of your hand and could accelerate protons to energies of . Over the next decade or two, unprecedented energies were achieved (up to ), but even the cyclotron had its limitations due to relativistic effects and magnet strength. Fortunately, the same type of technology that allows for a cyclotron to work also works in the next version of the accelerator, a synchrotron. The synchrotron’s circular path can accelerate protons by passing them millions of times through electric fields allowing them to obtain energies of well over . The first synchrotron to break the TeV energy level was at Fermilab National Accelerator Laboratory (Fermilab). The Tevatron at Fermilab is nearly miles in circumference and can accelerate particles to in each direction around the ring.
Figure 4.12
Fermi National Accelerator Laboratory, Batavia, IL
Figure 4.13
Jefferson National Accelerator Laboratory, Newport News, VA
The last advancement in accelerator technology involved the collision of the accelerated particles. Up until the 1970s, all accelerators were fixed target machines. This means that the very energetic particles collide with a stationary target and all the newly produced particles continue moving in the same direction as the debris, the new particles and energy, which comes from the collision. As a result, not all of the mass-energy that derives from the accelerated particles is available to be converted into new particles and new reactions. Some of the mass - energy is lost into the target and not all of it is transferred into the particle collisions. Early in the 1960s, physicists had learned enough about accelerators to build colliders. In a collider, two carefully controlled beams pass around the synchrotron in opposite directions until they are made to collide at a specific point. Although colliders are significantly more challenging to build, the benefits are great. In a collider, the accelerating particles moving in opposite directions are brought to a point for the collision and because they are traveling in opposite directions their collision energy is greater than a fixed target collision and the net momentum is zero. This means that all their energy is now available for new reactions and the creation of new particles. For example, although the Tevatron at Fermilab can only accelerate the protons and antiprotons to energies of , the energy that is involved in each proton-antiproton collision will approach .
Why the need to achieve such high energies? High–energy physicists know that it takes particles with energy about to probe the structure inside of a proton. In order to get to the even smaller parts of matter, higher energy is needed. Also, higher energies would allow for more “massive” particles to be created. Currently, the Fermilab's Tevatron has enough energy to produce the top quark . If particle physicists want to learn more about the building blocks of matter they need more energy. Over the past decade in Geneva, Switzerland, they have been trying to accomplish just that—to bu
ild the world’s largest particle accelerator. In 2009, the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) is scheduled to go online. The LHC is in circumference and will accelerate particles to energies approaching . This means that at the collision point the energy will be up to and the potential for new particle discoveries are endless.
Figure 4.14
Section of the Large Hadron Collider tunnel, CERN, Geneva, Switzerland
Particle Detectors
The first particle detectors resembled the ones used by Rutherford in his famous gold foil experiment. The detectors involved the emission of light when charged particles hit a coated screen. Other methods for detecting radiation were soon developed, such as electroscopes (that could tell if a charged particle was present) and Geiger counters (which counted how many charged particles were present). All of these detectors could only tell if a charged particle was present and/or provide a rough approximation to how many charged particles were present. They were all incapable of providing any specific information about the properties of the charged particles.
Then a breakthrough came in 1912 when the cloud chamber was invented. The cloud chamber involved producing a vapor that remained in a supersaturated state. C. T. R. Wilson, a Scottish physicist, developed a cloud chamber based on his studies of meteorology and his research into the atmosphere and cloud formation. It was well known that an electrical charge could cause condensation in this kind of supersaturated state. Wilson was eager to find out if he could produce a similar effect with rays. In 1896, he performed an experiment and found that, like electricity, rays could induce condensation in the supersaturated vapor. In 1912, he incorporated all of his ideas into a device that he called a cloud chamber. He found that radiation from a charged particle left an easily observable track when it passed through the cloud chamber. The track was a result of the interaction between the charged particles and the air and molecules within the container. This interaction resulted in the formation of ions on which condensation occurred. This provided a plain view of the path of the radiation and so gave a clear picture of what was happening. The events could then be viewed by taking a photograph of them. When used, the cloud chamber is placed between the poles of a magnet. The magnetic field causes particles to bend in one direction or another, depending on the electrical charge they carry. The magnetic field , the velocity , the radius of the circular orbit , the mass , and the charge are related by the formula: The kind of particles that have passed through the chamber can be determined by the types of tracks they leave. Although the cloud chamber had many useful applications, it was replaced by the bubble chamber that was invented in 1953 by Donald Glaser.
The bubble chamber is a more sophisticated version of the cloud chamber. Glaser's idea was to use a liquid, like liquid hydrogen, as a detecting medium because the particles in a liquid are much closer together than are those in a gas. Glaser's bubble chamber is essentially the opposite of a cloud chamber. It contains a liquid that is heated beyond its normal boiling point. If the liquid is kept under pressure it will not boil. Instead, it will remain in a superheated state. Particles released from the radioactive source will travel through the bubble chamber and interact with atoms and molecules in the liquid. This interaction will result in the formation of ions, atoms, or molecules that carry an electrical charge. The ions act as nuclei on which the liquid can begin to boil. The path taken by the particle as it moves through the bubble chamber is marked by the formation of many very tiny bubbles, formed where the liquid has changed into a gas. At this moment, the camera records the picture. Bubble chambers were widely used to study nuclear and particle events until the 1980s.
For a long time, bubble chambers were the most effective detectors in particle physics research. Bubble chambers were very effective, but they did require a picture to be taken and then analyzed. With the improvement in technology, it became desirable to have a detector with fast electronic read-out. Bubble chambers, thus, have largely been replaced by wire chambers, which allow particle numbers, particle energies, and particle paths to be measured all at the same time. The wire chamber consists of a very large number of parallel wires, where each wire acts as an individual detector. The detector is filled with carefully chosen gas, such that any charged particle that passes through the tube will ionize surrounding gaseous atoms. The resulting ions and electrons are accelerated by an electric potential on the wire, causing a cascade of ionization, which is collected on the wire and produces an electric current. This allows the experimenter to count particles and also determine the energy of the particle. For high - energy physics experiments, it is also valuable to observe the particle's path. When a particle passes through the many wires of a wire chamber it leaves a trace of ions and electrons, which drift toward the nearest wire. By noting which wires had a pulse of current, an experimenter can observe the particle’s path.
The wire chamber became one of the main types of detectors in modern particle accelerators. They were much more effective at collecting information about the particle events and in storing them to be analyzed at a later time. A bubble chamber could only produce one picture per second and that picture could not be stored in a computer. A typical wire chamber could record several hundred thousand events per second, which could then be immediately analyzed by a computer. The ability to collect hundreds of thousands of events and allow those events to be quickly analyzed and stored on a computer led to the creation of the magnificent modern particle detectors.
Figure 4.15
Schematic of the Compact Muon Solenoid Detector, CERN, Geneva, Switzerland
The Compact Muon Solenoid (CMS) is one of the two major detectors of the LHC (the other one is called ATLAS). Each of these detectors is quite similar in their general features and in their ability to collect and quickly analyze millions of particle events per second. CMS is long, wide, and high and it weighs tons. The huge solenoid magnet that surrounds the detector creates a magnetic field of Teslas, this is about 100,000 times the strength of the Earth’s magnetic field. CMS is an excellent example for illustrating the construction of a modern particle detector. The various parts are shown in Figure 13 with a brief description following.
Tracker
Purpose is to make a quick determination of particle momentum and charge.
The tracker consists of layers of pixels and silicon strips.
The pixels and strips cover an area the size of a tennis court.
million separate electronic read-out channels, connections per square centimeter.
The tracker records the particle paths without disturbing the energy or motion of the particle.
Each measurement that the tracker takes is accurate to , a fraction of the width of a human hair.
The tracker can re-create the paths of any charged particle; electrons, muons, hadrons, and short-lived decay particles.
Electromagnetic Calorimeter
Purpose is to identify electrons and photons and to do it very quickly ( between collisions).
Very special crystals are used that scintillate, momentarily fluoresce, when struck by an electron or photon.
These high-density crystals produce light in fast, short, well-defined photon bursts that is proportional to the particle’s energy.
The barrel and the endcap of the detector are made up of over crystals.
Hadron Calorimeter
Purpose is to detect particles made up of quarks and gluons, for example protons, neutrons, and kaons.
Finds a particle’s position, energy, and arrival time.
Uses alternating layers of brass absorber plates and scintillator that produce a rapid light pulse when the particle passes through.
The amount of light measured throughout the detector provides a very good measurement of the particle’s energy.
There are barrel “wedges,” each weighing tons.
Muon Detector
The purpose of the muon detector is to detect muons, one of the most important t
asks of CMS.
Muons can travel through several meters of iron without being stopped by the calorimeters, as a result the muon chambers are placed at the very edge of the detector.
Due to the placement of the muon chambers the only particles to register a signal will be a muon.
The muon chambers have a variety of detectors that help track these elusive particles.
Computing
One billion proton-proton interactions will take place inside the detector every second.
A very complex trigger system will be set up in the computers to eliminate many of the events that are not “interesting” to the physicists. Only less than percent of all interactions will be saved to a server.
Nearly petabytes, a million gigabytes, of data per year will be saved when running at peak performance.
CK-12 21st Century Physics: A Compilation of Contemporary and Emerging Technologies Page 9