by Frank Close
The idea is to have one linear accelerator of electrons and another accelerating positrons. With modern technology for the acceleration, and a length of several miles, it may be feasible to have collisions at a total energy of several hundred GeV. At such energies it could produce top quarks and antiquarks, and ultimately the Higgs boson (see Chapter 10).
To have a decent chance of a collision in a linear accelerator, where the beams meet once only, requires high-intensity beams that are less than a micron (10–6 m) across. In actuality it is more miss than hit. As the like charges within each beam repel one another, making and controlling such tightly focused beams is a technological challenge.
Colliders
In a linear accelerator aimed at a static target the debris of the collision is propelled forward, just as a stationary car is shunted forwards when another car crashes into its rear. When a beam hits a stationary target, the hard-won energy of the beam particles is being transferred largely into energy of motion – into moving particles in the target – and is effectively wasted. This problem is overcome if we can bring particles to collide head-on, so that their energy can be spent on the interaction between them. In such a collision the debris flies off in all directions, and the energy is redistributed with it – none is ‘wasted’ in setting stationary lumps in motion.
These arguments were clear to accelerator builders as long ago as the 1940s, but it took 20 years for particle colliders to take shape, and another 15 years for them to become the dominant form of particle accelerator, as they still are today. A problem is that the particles tend to miss one another and it is only in the last 30 years that the technique has become viable.
The major application has been to enable collisions between particles and antiparticles, principally protons and antiprotons, or electrons and positrons.
Protons are bunches of quarks, and antiprotons are likewise made of antiquarks. With a mass of nearly 2,000 times that of an electron, protons and antiprotons suffer less synchrotron radiation and also pack a bigger punch. Hence they are the prime choice when the aim is to reach out to previously unexplored higher energies. Such was the case in 1983 when head-on collisions between protons and antiprotons at CERN led to the discovery of the W± and Zo carriers of the weak forces (see Chapter 7). However, the collisions lead to lots of debris, and finding the W or Z is like looking for a needle in a haystack. The proton’s energy is shared among its quarks and it is chance whether the energy of a single quark that meets an antiquark matches that required to form a Zo or W±. Nonetheless they showed up as one in a million special cases in the collection of images of the collisions. The challenge was then to make a Zo regularly without the vast unwanted and confusing background. This could be done by tuning a beam of electrons and positrons to the required energy. This led to the Large Electron Positron (LEP) collider. The technical challenges of making experiments with such machines can be illustrated by reference to LEP.
When LEP began running, in the 1990s, needle-like bunches of electrons and positrons would pass through each other at the heart of the detectors every 22 microseconds (22 millionths of a second). Even though there were some million million particles in each bunch, the particles were thinly dispersed, so interactions between them were rare. An interesting collision, or ‘event’, only occurred about once every 40 times or so the bunches crossed. The challenge was to identify and collect the interesting events, and not to miss them while recording something more mundane. An electronic ‘trigger’ responded to the first signals from a collision to ‘decide’ within 10 microseconds whether something interesting had occurred. If it had, the process of reading out and combining the information from all the pieces of the detector would begin, and a display on a computer screen would recreate the pattern of particle tracks and show where energy had been deposited in the detector.
Currently a collider of protons and even of atomic nuclei is being constructed to replace LEP. This is the Large Hadron Collider (LHC), which is planned to start in 2007. It will accelerate the protons to energies of 8 million million electron volts (8 TeV) per beam, so that they will collide at a total energy of 16 TeV. This is nearly 100 times greater than the energy of LEP’s collisions, and nearly 10 times greater than the energy of proton-antiproton collisions at Fermilab.
At Hamburg there is a unique asymmetric collider where a beam of protons collides with a beam of electrons or of positrons. The resulting collisions enable the proton substructure, and that of its quarks, to be probed at distances down to 10–19 m.
Factories
The conundrum of how matter and antimatter differ has moved into focus in recent years. This has led to an intense interest in the properties of strange particles and antiparticles – the kaons – where a subtle asymmetry was discovered nearly 50 years ago, and their bottom analogues (see p. 98) where a large asymmetry has been predicted. This has led to the concept of particle ‘factories’, capable of producing as many kaons or B mesons as possible.
The idea is to make electrons and positrons collide at specific energies, ‘tuned’ to produce kaons or B mesons, respectively, in preference to other kinds of particles. At Frascati, near Rome, is DAFNE, a small machine in that it fits in a room little bigger than a gymnasium. Its electrons and positrons annihilate at a total energy of just 1 GeV, which is ideal for making kaons.
A ‘B factory’ makes electron-positron collisions at a total energy of around 10 GeV, optimized to produce B mesons and their antiparticles (, pronounced B-bar) together. So compelling is the challenge that two machines were built in the late 1990s – PEP2 at Stanford in California and KEKB at the KEK laboratory in Japan.
The B factories differ from previous electron-positron colliders in an intriguing way. In a standard electron-positron collider, the beams travel in opposite directions but with the same speed, so that when particles meet their motion exactly cancels out. The resulting ‘explosion’ when the electrons and positrons mutually annihilate is at rest, and newly created particles of matter and antimatter emerge rather uniformly in all directions. In the B factories, the colliding beams move with different speeds, so the resulting explosion is itself moving.
As a result of this asymmetric collision, the matter and antimatter that emerge tend to be ejected in the direction of the faster initial beam, and at higher speeds than from an annihilation at rest. This makes it easier to observe not only the particles created, but also the progeny they produce when they die – thanks to an effect of special relativity (time dilation) which means that particles survive longer and travel further (about 1 mm) when moving at high speed. These are essential tricks because a B meson, at rest, lives only for a picosecond, a millionth of a millionth of a second, and this is on the margins of measurability.
Plans are afoot to make neutrino factories, where intense sources of neutrinos will enable study of these enigmatic particles. Their masses are too small to measure, but indirect measures of their differences in mass can be obtained. There is even the possibility that neutrinos and antineutrinos might change into one another, a form of matter into antimatter that could have important implications for our understanding of this profound asymmetry. Such effects could be measured at suitable neutrino factories.
Finally, the anticipated discovery of the Higgs boson later this decade among the debris from collisions between protons and/or antiprotons, is creating interest in producing large numbers of them under more controlled conditions. To do so, the plan is to have electron-positron collisions at the optimum energy. As this is expected to be several hundred GeV, a linear accelerator will be required. Hence there is much talk about two linear accelerators, one of electrons and one of positrons, aligned so as to produce head-on collisions of the beams. This is how the future of experimental high-energy physics with accelerators is likely to be.
Chapter 6
Detectors: cameras and time machines
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A survey of a century of detectors. Bubble chambers – great 50 years ago but mo
dern electronics offer so much more. Spark chambers, and their descendants. Swiss rolls at LEP. Battleship-sized detectors for the LHC. The images – how they distinguish varieties of particle and enable us to decode the message of the collisions.
* * *
Early methods
Ways of detecting subatomic particles are more familiar than many people realize. The crackle of a Geiger counter, and the light emitted when electrically charged particles, such as electrons, hit specially prepared materials forming the picture on our television screen, are but two.
Rutherford discovered the atomic nucleus by its effect on beams of alpha particles; they had scattered through large angles. He had used scintillating materials to reveal them as they scattered from the atomic nucleus. Rutherford and his colleagues had to use their own eyes to see and count the flashes; by the 1950s electronic components had automated the process of counting the flashes from modern plastic scintillators.
When a charged particle travels through a gas, it leaves behind a trail of ionized atoms. A whole range of particle detectors, from the cloud chamber to the wire spark chamber, depends on sensing this trail of ionization in some way.
By such means, nearly a century ago, Rutherford was able to detect alpha particles that had been emitted by radium, one at a time.
The key feature was that the detector could greatly amplify the tiny amount of ionization caused by the passage of a single alpha particle. It consisted of a brass tube, which was pumped out to a low pressure, and had a thin wire passing along the centre. The wire and tube had 1,000 volts applied between them, which set up an electric field. Under this arrangement, when a charged particle passes through the rarefied gas, ions are created. They are attracted towards the wire, and as they speed up they ionize more gas, amplifying the initial effect. One ion could produce thousands of ions, which all end up at the central wire, producing a pulse of electric charge large enough to be detected by a sensitive electrometer connected to the wire.
In the modern ‘Geiger counter’ the electric field at the wire is so high that a single electron anywhere in the counter can trigger an avalanche of ionization, such that the tiniest amount of ionization produces a signal.
Although this reveals the presence of radiation, it is far removed from what is needed for detecting particles in modern high-energy experiments. They are used in conjunction with other detectors. To see how this is done, it is helpful to see how detection has developed.
The first detector capable of revealing trails of charged particles was the cloud chamber, which is a glass chamber fitted with a piston and filled with water vapour. When you quickly withdraw the piston, the sudden expansion will cool the gas and a mist forms in the cold, damp atmosphere. When alpha and beta particles from radioactivity pass through, they ionize atoms in the vapour and cloud drops form instantly around their trail. When illuminated, the tracks stand out like the dust motes in a sunbeam.
The cloud chamber was used to detect particles in cosmic rays, its efficiency improved by combining it with the Geiger counter. Put one Geiger counter above and another below the cloud chamber, then if both fire simultaneously it is very likely that a cosmic ray has passed through them and, by implication, through the chamber. Connect the Geiger counters to a relay mechanism so that the electrical impulse from their coincident discharges triggers the expansion of the cloud chamber and a flash of light allows the tracks to be captured on film.
The first example of an antiparticle, the positron, and also strange particles were discovered in cosmic rays by means of the cloud chamber. However, such techniques were superceded by the use of emulsions.
Emulsions
Photographic plates had figured in the very earliest work on radioactivity; indeed, it was through the darkening of plates that both X-rays and radioactivity were discovered.
In the late 1940s, high-quality photographic emulsions became available. When taken to high altitudes by balloons, they produced the first beautiful images of the interactions of cosmic rays.
These emulsions were especially sensitive to high-energy particles; just as intense light darkens photographic plates, so can the passage of charged particles. We can detect the path of a single particle by the line of dark specks that it forms on the developed emulsion. The particle literally takes its own photograph. A set of emulsion-covered plates is sufficient to collect particle tracks; a cloud chamber, on the other hand, is a complex piece of apparatus, needing moving parts so that the chamber can be continually expanded and recompressed. As a result emulsions became, and have continued to be, a useful way of detecting and recording the trails of charged particles.
Bubble chamber
The advent of accelerators produced high-energy particles, which created new challenges for detection. Energetic particles fly through a cloud chamber without interacting with the atoms in the chamber’s thin gas. For example, to record the whole life of a strange particle, from production to decay, at energies of a few GeV would have required a cloud chamber 100 metres long! In addition, cloud chambers are slow: the cycle of recompression after an expansion can take up to a minute; by the 1950s particle accelerators were delivering pulses of protons every two seconds.
What was needed was a detector that would capture the long tracks of high-energy particles and operate quickly. Gases were much too tenuous for the job whereas liquids were better, because their much greater density means they contain far more nuclei with which the high-energy particles could interact. This brings us to the bubble chamber. The basic idea develops from what happens when you keep a liquid under pressure, very close to its boiling point. If you lower the pressure in these circumstances, the liquid begins to boil, but if you lower the pressure very suddenly, the liquid will remain liquid even though it is now above its boiling point. This state is known as ‘superheated liquid’ and because it is unstable, it can be maintained only so long as no disturbance occurs in the liquid.
Release the pressure and then immediately restore it. Particles entering the liquid during the critical moments of low pressure create a disturbance and trigger the boiling process as they ionize the atoms of the liquid along their paths. For a fraction of a second, a trail of bubbles forms where a particle has passed, which can be photographed. The immediate restoration of pressure would mean that the liquid was once again just below boiling point, and the whole process could be repeated quite rapidly.
15. Cosmic rays provided physicists with the first glimpses of new subatomic particles, which were later studied in detail in experiments at particle accelerators. Positrons, muons, pions, and kaons, all figure in this photograph from the 2 m bubble chamber at the CERN laboratory.
The operation of a bubble chamber is always intimately tied to the operating cycle of the accelerator that feeds it. The particles enter the chamber when the piston is fully withdrawn, the pressure at its minimum, and the liquid superheated. Then, about one millisecond later, an arc light flashes, illuminating the trails of bubbles formed by charged particles. The delay between minimum pressure and the flash allows the bubbles to grow large enough to show up on the photographs. Meanwhile, the piston moves back in towards the chamber, increasing the pressure again, and the film in the cameras is automatically wound on to the next frame. It then takes about a second for the chamber to ‘recover’ and be ready for the next expansion. Thus the bubble chamber shows where the particles have been, enabling their behaviour to be studied at leisure.
In a magnetic field, a charged particle’s trajectory will curve, the direction revealing whether the particle was positively or negatively charged, and the radius of the curve revealing its momentum. So we can deduce the charge and momentum; if you know a particle’s momentum and velocity, you can calculate its mass and hence its identity.
One method of pinpointing the velocity used two scintillation ‘counters’, which produced a flash of light each time a charged particle passed through. Each tiny burst of light was converted to a pulse of electricity, which was then ampli
fied to produce a signal. In this way, two or more scintillation counters could reveal the flightpath of a particle as it produced flashes in each counter, and from the time taken to travel between the two counters, the particle’s speed could be determined.
However, such techniques did not help solve the identification puzzle in the case of a bubble chamber picture. Often the only way was to assign identities to the different tracks, and then to add up the energy and momentum of all the particles emerging from an interaction. If they did not balance the known values before the interaction, the assumed identities must be wrong, and others must be tested, until finally a consistent picture was found. This was time-consuming, but the state of the art around 1960. Identifying particles through such trial-and-error calculations is the kind of repetitive job at which computers excel, and today bubble chambers have been superceded by electronic detectors that lend themselves better to computer analysis.
16. The tracks of many charged particles are made visible in this image from the NA35 experiment at CERN, Geneva. The particles emerge from the collision of an oxygen ion with an atomic nucleus in a lead target at the lower edge of the image. Tiny luminous streamers reveal their tracks as they pass through the influence of a magnetic field, positive particles bending one way, negative particles the other. Most of the particles are very energetic, so their paths curve only slightly, but at least one particle has a much lower energy, and it curls round several times in the detector, mimicking the shell of an ammonite.