BEFORE MODERN ACCELERATORS AND COLLIDERS
The idea of a particle in the modern sense began with Max Planck in 1900 and Albert Einstein five years later. Planck discovered that the light produced from the walls of a black-body cavity1 comes in the form of parcels of energy, the amount of which is equal to a constant h times the frequency of the light ν. This was a complete surprise in experimental physics at the time. It was widely accepted, and had been proved by James Clerk Maxwell back in the mid 19th century, that light existed as waves. In fact, Planck never accepted the idea that his radical discovery of packets of energy was true, and described nature accurately, nor did many of his colleagues. Einstein, the iconoclast, took Planck’s discovery a step further in 1905 by claiming not only that the black-body walls produced light in quantum packages, but that light itself consisted of quantum packages in the form of particles, which we now call photons.
This idea met with opposition even though Einstein figured out a way to reconcile the idea with known experiments, such as one conducted by Philipp Lenard in 1900. In this experiment, light was shone on a metal surface consisting of a cathode, a negatively charged electrode. It knocked out electrons from the metal; the ejected electrons were picked up by another electrode called an anode, which is positively charged. This produced a tiny but measureable current. These measurements led to an important observation: the energy and, in turn, the number of electrons knocked out of the metal were not proportional to the amplitude of the light wave hitting the metal, which is what one would expect from waves, but rather were proportional to the frequency of the light wave. Lenard and other physicists at the time could not understand the meaning of these results, because they fully believed that light consisted of waves. Moreover, the effect was color dependent. Light has colors like the rainbow, and only light of blue or violet seemed to have enough energy to eject the electrons. The red, yellow, and green parts of the spectrum were not able to do so.
Einstein figured out that there was an experimentally observed photon energy threshold of about 1.5 eV, at which the light was able to kick out the electrons. Indeed, the blue light had an energy of approximately 3.5 eV, and thus blue and violet light played a special role in this experiment. Einstein understood that the amount of energy left over from 3.5 less the 1.5 threshold energy was carried by the electrons knocked off the metal. From Einstein’s theoretical calculations, assuming that the light consisted of particles of energy equal to Planck’s E = hν, the electron energy should increase linearly with the increasing frequency of the light shining on the metal, an important prediction of his particle idea of light. Einstein published these ideas in a 1905 paper titled “On a Heuristic Viewpoint Concerning the Production and Transformation of Light.”2
It was not until 1914 and 1915 that Robert Millikan, after diligent effort on a series of experiments, proved that Einstein’s prediction was correct. Despite this experimental confirmation, Millikan, as well as Planck and Niels Bohr in Copenhagen, still did not believe in Einstein’s proposal that light consisted of particles. Since Maxwell, everyone knew that light was made up of waves. After all, light diffracts and goes around corners; light waves interfere with one another constructively and destructively. All of this was well-known experimentally. Did we actually have to go back to Isaac Newton’s idea that light consisted of “corpuscles”? Did we have to go back to the 17th century to understand the photoelectric effect?
Later, during the 1920s, much progress toward understanding the particle nature of matter was made experimentally with the invention of the photomultiplier and the advance of vacuum tube technology. Eventually, after the photomultiplier instrument was perfected, it was able to detect single photons for the first time. With the advance of this “single-photon counting,” experiments were able to prove Einstein’s principle of the photoelectric effect—that light did indeed consist of particles.
Despite the Millikan experiment and the photomultiplier results, physics luminaries such as Niels Bohr still refused to believe in the idea that light consisted of these photon particles. Indeed, Bohr steadfastly opposed the idea, suggesting that the photoelectric effect was the result of the violation of the conservation of energy. It was not until 1922 that Arthur Compton scattered photons off electrons and proved conclusively that the experimental result of the scattering could only be explained if light was made up of these photon particles. Einstein was awarded the 1921 Nobel Prize in Physics (given to him in 1922) for his ingenious explanation of the photoelectric effect. Eventually, Bohr grudgingly accepted the fact that light did indeed consist of photon particles.
Russian physicist Pavel Cherenkov made an important discovery in 1934, which came to be called Cherenkov radiation. What he found was that when he passed charged particles through certain solvents, he saw a blue flash of light when they interacted with the molecules of the material. This was no surprise, for such an effect, called fluorescence, had already been observed by, among others, Marie and Pierre Curie in their experiments with radium. However, Cherenkov went further, and discovered that when the charged electrons passed through water in a tank, the electrons were actually moving faster than light in that medium, because light travels slower in water than in a vacuum. As a result of the excessive speed of the electrons, they had to shed energy. This energy was in the form of photons being radiated, which produced a bluish cone the size of which depended on how many photons were produced as the electrons raced through the water. The theoretical understanding of Cherenkov’s experiments was achieved by Il’ia Frank and Igor Tamm. These two theorists, together with Cherenkov, won the Nobel Prize in Physics in 1958 for discovering and explaining Cherenkov radiation.
Photons can be detected in other ways. When a photon passes a nucleus with a sufficiently strong electric force (Coulomb force), it can produce spontaneously an electron and positron (positively charged electron) pair, which can be detected. If an electric field is strong enough, it can reverse the process and make an electron and a positron annihilate and produce a photon. This phenomenon is called spontaneous creation of either photons or pairs of electrons and positrons.
Another effect is that when an electron passes through the electric field of a nucleus, it produces a spray of photons from the interaction of the electron with the electric field, which can also be detected. This effect is called Bremsstrahlung, which in German means the slowing down or “braking” of the electrons by the electric field. The Bremsstrahlung effect can repeat over and over again with different nuclei and, eventually, with sufficiently high energy, a shower of photons is produced. These showers are characteristic effects caused by high-energy photons and electric fields.
The electrons and positrons produced by the photons near an electric field make “tracks” that can be observed in other experimental devices. The earliest instrument invented to follow and measure such movements of ionized particles was the “cloud chamber,” invented by Scottish physicist Charles Wilson in 1895. Wilson discovered that condensation occurs around a charged particle in a medium of water vapor. Indeed, the way to measure the charge of the ions is by counting the vapor droplets that surround the ions in the cloud chamber. Through diligent experimental research, Wilson eventually perfected his cloud chamber in 1911 by passing the charged particles through water, where the vapor condensing on them produced tracks of bubbles. However, Wilson’s cloud chamber was not able to detect the scattering of particles because water is not dense enough to serve as an effective target.
The next major advance in cloud chamber technology came 40 years later. In 1952, Donald Glaser invented the “bubble chamber” at the University of Michigan. He started with a medium of diethyl ether, the solvent and anesthetic, only 3 cm3 in volume, but soon produced chambers of much larger sizes. He sent beams of charged particles through the medium, which produced tracks in the form of bubbles. Glaser also used liquid hydrogen kept at a very low temperature of −253°C. The hydrogen atoms, of course, contained a proton nucleus, so the proton becam
e a target for the charged particles. Now experimentalists were able to produce significant scattering events using charged particles and protons. The bubble chamber played an important role in the development of accelerators, culminating in the giant European bubble chamber at CERN, called Gargamelle (Figures 2.1 and 2.2).
Bubble chambers produced clear tracks of charged particles, which made it possible for experimentalists to study nuclear scattering processes more efficiently (Figure 2.3). Bubble chambers served as physicists’ favorite particle detectors for many years. They were also filled with liquid freon, liquid propane, and other liquids that provide sufficiently dense targets for the scattering of particles. For his important contribution to experimental particle physics, Glaser was awarded the Nobel Prize for Physics in 1960.
Figure 2.1 Old Fermilab bubble chamber.
SOURCE: Wikipedia Commons, from Fermilab.
Figure 2.2 CERN’s Gargamelle bubble chamber. © CERN
Figure 2.3 CERN photograph of early bubble chamber tracks. They are produced in a small bubble chamber filled with liquid hydrogen. The photo shows a pion beam striking a proton target in the upper left, which creates a spray of new particles. © CERN
MODERN DETECTORS
Significant technological advances have been made in the instruments used to detect subatomic particles at the colliders and accelerators that smash particles together at high energies. One of the earliest such detectors was the spark chamber, invented by Shuji Fukui and Sigenore Miyamoto. It was important in particle physics research from the 1930s to the 1960s. The spark chamber consisted of a stack of metal plates immersed in a gas such as helium or neon. When a charged cosmic ray particle passed through the detector, it ionized the gas, and sparks were generated between the plates, showing the path of the cosmic ray particle.
Another type of detector was the proportional wire chamber, invented in 1968 by CERN experimentalist Georges Charpak, for which he won the Nobel Prize in 1992. The idea was based on the existing wire chamber detector that had been used at accelerators to detect particles of ionizing radiation, which itself had been a significant advance over the Geiger counter3 and proportional counter. Like the spark chamber, these detectors all use metal, high voltage, and a gas medium to track the path of ionized particles.
In high-energy accelerators, tracking the path of a particle is necessary to measure its energy or mass, charge, and other properties. This was done for a long time, up to the early 1960s, using bubble chambers and taking photographs. However, with the rapid development of electronics, it became possible to devise a system with a fast electronic read-out of results. By tracking the path of a particle in a magnetic field, one can measure the charge on the particle and its mass, and then infer other quantum numbers possessed by the particle to identify it.
In the case of the proportional wire chamber, and most other detectors of charged particles, the particles are led through a uniform magnetic field that turns their paths into spirals because of the electromagnetic force on the fast-moving charged particles. By measuring the curvature of the spirals, we can determine the charge and momentum of the particle. The accuracy of the reconstruction of the particle’s path can be greatly increased by observing that the ions need time to drift toward the nearest wire. The measurement of this drift time can determine where the particle passes the wire. This apparatus is called a drift chamber. By combining many drift chambers with wires orthogonal to one another, and by arranging the orthogonal wires to be orthogonal to the beam direction of the particles, a very accurate measurement can be made of the position of the particle, and from this we can determine its momentum. From the momentum of the particle we can tell what kind of particle we are dealing with.
One of the great advances that came with the proportional wire chamber is that it was now possible to be selective in identifying the particle one was searching for from the myriad of possibilities after a collision. This cuts down the number of events possible in a given measurement to a number of events that can be handled more easily by the computer systems used to analyze the data. Indeed, many millions of potential “events” occur when protons collide with protons or antiprotons, and a selective triggering mechanism is required to reduce the number of possible events to a manageable size. Richard Feynman once famously said that colliding protons together was like bashing two full garbage cans; one needed a method of sorting out the debris.
Another detection instrument, which is currently used at CERN, is the calorimeter. It is a composite detector that absorbs particles to measure their energy. The trillions of interactions generate showers of particles, which is why the name shower counter is also used for a calorimeter. Most of the particle energy in the showers will be converted into heat for this kind of detector, which explains the name calorimeter (calor means “heat” in Latin), but in practice, no temperature is measured in these detectors. Particles enter the calorimeter and initiate a particle shower. The particles’ energy is deposited in the calorimeter, where it is collected and measured. A total measure of the energy can be obtained by a complete containment of the particle shower, or the energy can be sampled. There are two kinds of calorimeter: the electromagnetic calorimeter and the hadronic calorimeter. The former is used to measure the energy of particles that interact electromagnetically whereas the latter is designed to measure particle energies that interact via the strong nuclear force. An important feature of the electromagnetic calorimeter is that it is the only practical way to detect and measure electrically neutral particles, such as photons, in a high-energy collision.
A calorimeter is made of multiple individual cells, over the volume of which the absorbed energy is measured. The cells are aligned to form towers typically along the direction of the incoming particles. Incoming electromagnetic particles, such as electrons and photons, are fully absorbed in the electromagnetic calorimeter, which takes advantage of the comparatively short and concentrated electromagnetic shower to measure the energies and positions of these particles (e.g., neutral pi mesons decaying into photons). In a hadronic calorimeter, the incident hadrons (such as protons) may start their showering in the electromagnetic calorimeter, but usually are absorbed fully only in later layers of the apparatus.
The compact muon solenoid (CMS) and a toroidal LHC apparatus (ATLAS) detectors at the LHC have special electromagnetic calorimeter detectors used to measure decays of particles into photons and electrically neutral bosons such as the Z boson (Figure 2.4). Experimentalists are able to detect a light neutral Higgs boson, which can decay into two photons. This particular decay is difficult to measure because the cross-section, or area of interaction, of the two-photon decay is very small. However, the electromagnetic detectors are amazingly sensitive to the amount of light or photons emitted by the Higgs boson in its decay. This experiment is one of the important means of detecting the elusive light Higgs particle, because there is very little background of strongly interacting hadrons such as bottom and antibottom quarks. This calorimeter detector was crucial for the presumed detection of the standard-model Higgs boson, announced on July 4, 2012, by the CMS and ATLAS groups at the LHC.
How do we know what kind of particle we are seeing in a detector? By determining the amount of energy and momentum of a particle, which can be measured in a detector, we can find out the quantum numbers of the particle, such as its charge, mass, and quantum spin, which identify it. Take, as an example, a particle with an energy of 200 GeV and zero momentum. This is a particle at rest. Except for a stable particle such as the proton and electron, a particle can decay into other particles, each individually of smaller energy. This whole process of decay must satisfy the conservation of energy and momentum.
Figure 2.4 Calorimeter at the ATLAS detector. © CERN for the benefit of the ATLAS Collaboration
Normally, the probability of a particle decaying into other particles is determined when the particle is at rest. Some particle decays are very rapid. For example, an electrically neutral Z boson can decay into a posi
tron and an electron with a decay time of about 10−24 seconds. This particle has a very short track in a bubble chamber compared with the much longer decay time of a muon, which takes about a millionth of a second (10−6 seconds) to decay. The muon, therefore, traverses a sufficient distance in a bubble chamber that its path can be identified as a muon. On the other hand, the distance traversed by the Z boson is so short that the paths in the bubble chamber cannot be used to identify the Z. Instead, we have to identify the specific decay products, the particles produced by the decay of the Z, and determine that the energy and momentum of the decay products equal those of the parent particle. This is possible because of the overall conservation of energy and momentum of the process.
When one is trying to discover a new particle, such as the Higgs boson, one determines the mass and momentum of the particles in the decay products and infers whether the measured results match the physical properties of known particles. If they don’t, then it is possible that one has discovered a new particle. If a particle such as the neutral Z boson (Z0) is being investigated, it will not leave the track of a charged particle. However, one sees a V shape either in the bubble chamber or in the data tracking from modern detectors, signifying that it has decayed into a positron and an electron; this V shape is a signature of the neutral Z boson.
EARLY ACCELERATORS AND COSMIC RAYS
The very first accelerator was used by J.J. Thomson in 1897 when he discovered the electron. This primitive apparatus accelerated particles between two electrodes with different electric potentials. Thomson used electromagnetic fields to determine the ratio of charge to mass of the accelerating particles that were in the form of cathode rays, or negatively charged particles. In contrast to physicists working at modern accelerators, Thomson studied the properties of the beams of particles, not the impact of the beams on a target. Although modern accelerators are huge in size and much more complex than Thomson’s accelerator, they work on much the same principles. Accelerators were found commonly in the living rooms of the 1950s and 1960s. Instead of today’s flat-screen liquid crystal display TV, the first television sets used a cathode ray tube. The TV tube accelerated electrons, which hit the screen and were deflected, thereby emitting light. The TV with its accelerator was, of course, a tiny version of modern-day accelerators. It produced, at most, a 10,000-eV energy during the acceleration of the electrons.4 Compare 10,000 eV with the LHC, which has a maximum energy of 14 TeV.
Cracking the Particle Code of the Universe Page 6
