by Frank Close
From bubble chamber to spark chamber
A bubble chamber can provide a complete picture of an interaction, but it has some limitations. It is sensitive only when its contents are in the superheated state, after the rapid expansion. Particles must enter the chamber in this crucial period of a few milliseconds, before the pressure is reapplied to ‘freeze’ the bubble growth.
To study large numbers of rare interactions requires a more selective technique. In the 1960s, the spark chamber proved the ideal compromise.
The basic spark chamber consists of parallel sheets of metal separated by a few millimetres and immersed in an inert (less reactive) gas such as neon. When a charged particle passes through the chamber it leaves an ionized trail in the gas, just as in a cloud chamber. Once the particle has passed through, you apply a high voltage to alternate plates in the spark chamber. Under the stress of the electric field, sparks form along the ionized trails. The process is like lightning in an electric storm. The trails of sparks can be photographed, or their positions can even be recorded by timing the arrival of the accompanying crackles at electronic microphones. Either way, a picture of particle tracks can be built up for subsequent computer analysis.
The beauty of the spark chamber is that it has a ‘memory’ and can be triggered. Scintillation counters outside the chamber, which respond quickly, can be used to pinpoint charged particles passing through the chamber. Provided all this happens within a tenth of a microsecond, the ions in the spark chamber’s gaps will still be there, and the high-voltage pulse will reveal the tracks.
17. An image of one of the first observations of the W particle – the charged carrier of the weak force – captured in the UA1 detector at CERN in 1982. UA1 detected the head-on collisions of protons and antiprotons, which in this view came from the left and right to collide at the centre of the detector. The computer display shows the central part of the apparatus, which revealed the tracks of charged particles throughout the ionization picked up by thousands of wires. Each dot in the image corresponds to a wire that registered a pulse of ionization. As many as 65 tracks have been produced, only one of which reveals the decay of a W particle created fleetingly in the proton-antiproton collision. The track is due to a high-energy electron. Adding together the energies of all the other particles which reveals that a relatively large amount of energy had disappeared in the direction opposite to the electron, presumably spirited away by an invisible neutrino. Together, the neutrino and electron carry energy equivalent to the mass of the short-lived W particle.
Subdivide the plates of the spark chamber into sheets of parallel wires, a millimetre or so apart. The pulse of current associated with each spark is sensed only by the wire or two nearest to the spark, and so by recording which wires sensed the sparks you know to within a millimetre where the particle has passed. Notice how this enables the wire spark chamber to produce information ready for a computer to digest with little further processing.
Wire spark chambers can be operated up to 1,000 times faster than most bubble chambers and fitted in particularly well with the computer techniques for recording data that were developed in the 1960s. Signals from many detectors – scintillation counters, wire chambers – could be fed into a small ‘on-line’ computer, which would not only record the data on magnetic tape for further analysis ‘off-line’, but could also feed back information to the physicists while the experiment was in progress. Sets of chambers with wires running in three different directions provided enough information to build up a three-dimensional picture of the particle tracks. And the computer could calculate the energy and momentum of the particles and check their identification.
In the 1960s spark chambers allowed the rapid collection of data on specific interactions; bubble chambers, on the other hand, gave a far more complete picture of events, including the point of interaction or ‘vertex’. The ‘electronic’ and ‘visual’ detectors were complementary, and together they proved a happy hunting ground for the seekers of previously unknown particles.
Electronic bubble chambers
At modern particle accelerators, the number of interactions are huge compared with those in the days of bubble chambers and even early spark chambers. Modern developments include the multiwire proportional chamber and the drift chamber, which work much faster and more precisely than wire spark chambers. In particular, the drift chamber and its variations figure in tracking charged particles in almost every experiment today.
A multiwire proportional chamber is superficially rather similar to a spark chamber, being a sandwich of three planes of parallel wires fitted into a gas-filled structure, but differs in that the central plane of wires is held continuously at some 5,000 volts electrical potential relative to the two outer planes. Charged particles then trigger an avalanche of ionization electrons when they pass through the gas. A chamber with wires only 1–2 mm apart produces a signal within a few hundredths of a microsecond after a particle has passed by, and can handle as many as a million particles per second passing each wire – a thousand-fold improvement on the spark chamber.
The downside is that to track particles across a large volume, of a cubic metre say, requires a vast number of wires each with electronics to amplify the signals. Furthermore, it has limited precision. These problems are overcome with the ‘drift chamber’, whose basic idea is to measure time – which can be done very precisely with modern electronics – to reveal distance. The chamber again consists of parallel wires strung across a volume of gas, but some of the wires provide electric fields that in effect divide a large volume into smaller units or ‘cells’. Each cell acts like an individual detector, in which the electric field directs the ionization electrons from a charged particle’s track towards a central ‘sense’ wire. The time it takes for electrons to reach this wire gives a good measure of the distance of the track from the sense wire. This technique can locate particle tracks to an accuracy of some 50 micrometres.
Silicon microscopes
Several strange particles live for about 10–10 seconds, during which brief span they may be travelling near the speed of light and cover a few millimetres. Over such distances they leave measurable trails.
Particles containing charmed or bottom quarks live typically for no more than 10–13 s, and may travel only 300 micrometres. To see them one must ensure that the part of the detector closest to the collision point has as high a resolution as possible. Nowadays, almost every experiment has a silicon ‘vertex’ detector, which can reveal the short kinks where tracks diverge as short-lived particles decay to those with longer lifetimes.
When a charged particle passes through the silicon it ionizes the atoms, liberating electrons, which can then conduct electricity. The most common technique with silicon is to divide its surface during fabrication into fine parallel strips spaced some 20 microns (millionths of a metre) apart, yielding a precision on measuring particle tracks of better than 10 microns.
Silicon strip detectors have come into their own at colliders, providing high-resolution ‘microscopes’ to see back into the beam pipe, where the decay vertices of particles can occur close to the collision point. They have proved particularly important in identifying B particles, which contain the heavy bottom quark. The bottom quarks prefer to decay to charm quarks, which in turn like to decay to strange quarks. Particles containing either of these quarks decay within 10–12 s, and travel only a few millimetres, even when created at the highest energy machines. Yet the silicon ‘microscopes’ constructed at the heart of detectors can often pinpoint the sequence of decays, from bottom to charm to strange particles. At the Tevatron at Fermilab, the ability to ‘see’ bottom particles in this way was critical in the discovery of the long-sought top quark, which likes to decay to a bottom quark.
Detecting neutrinos
Any individual neutrino may be very unlikely to interact with matter in a detector, but with enough neutrinos, and large detectors, a few may be caught. The basic idea to detect those rare ones is to exploit
their tendency to turn into an electrically charged lepton, such as an electron, when they make a hit, and the electron, being charged, is easy to detect. This is how we have learned a lot about the neutrinos that stream down on us every second from the Sun.
When light passes through material, such as water, it travels slower than when in free space. So although nothing can travel faster than light in a vacuum, it is possible to travel faster than light does through a material. When a particle moves through a substance faster than light does, it can create a kind of shock wave of visible light, known as Cerenkov radiation. The Cerenkov radiation emerges at an angle to the particle’s path, and the greater the particle’s velocity, the larger this angle becomes. The SuperKamiokande experiment detects neutrinos when they interact in water to make either an electron or a muon, depending on the neutrino’s type. These particles, unlike the neutrino, are electrically charged and, moving faster than light through the water, can emit Cerenkov radiation. By carefully analysing the patterns of light, one can distinguish between muons and electrons created in the detector, and hence between muon- and electron-neutrinos.
The Sudbury Neutrino Observatory (SNO) is 2070 metres below ground in a nickel mine in Sudbury, Ontario. Its heart is an acrylic vessel filled with 1,000 tonnes of ‘heavy water’, called deuterium, in which a neutron joins the single proton of ordinary hydrogen. In SNO, electron-neutrinos interact with the neutrons in the deuterium to create protons and electrons, and the fast-moving electrons emit cones of Cerenkov radiation as they travel through the heavy water. The Cerenkov light forms patterns of rings on the inner surface of the water tank, where it is picked up by thousands of phototubes arrayed around the walls.
However, the key feature is that SNO can also detect all three types of neutrino (see p. 100) through a reaction unique to deuterium. A neutrino of any kind can split the deuterium nucleus, freeing the neutron, which can be captured by another nucleus. The capture is detected when the newly bloated nucleus gets rid of its excess energy by emitting gamma rays, which in turn make electrons and positrons that create characteristic patterns of Cerenkov light in the surrounding water.
18. Electrons – beta rays – have a much smaller mass than alpha particles and so have far higher velocities for the same energy. This means that fast electrons do not lose energy so rapidly in ionizing the atoms they pass. Here we see the intermittent track of a fast beta-ray electron. (The short thick tracks are not caused by the beta ray; they are knocked from atoms in the gas filling the chamber by invisible X-rays. Their tracks are thicker because they are moving more slowly than the beta ray and are therefore more ionizing; and they wiggle about because they are frequently knocked aside in elastic collisions with electrons in the atoms of the gas.)
By such experiments it has been possible to count the neutrinos from the Sun. They confirm that the Sun is indeed a nuclear fusion engine. That this is how stars, such as the Sun, burn had long been suspected, but it was proved finally in 2002.
Detectors at colliders
Electronic detectors have produced their most spectacular results in an environment that is inaccessible to bubble chambers – at colliding-beam machines where particles meet head-on within the beam pipe.
These individual pieces are today combined in cylindrical detectors that surround the interaction point at a collider accelerator. The collision happens on the central axis of the detector. As the debris streams out, it encounters a series of different pieces of detector, each with its own speciality in recognizing particles.
At the Large Hadron Collider bunches of particles will pass through each other 40 million times a second and each time they cross there will be up to 25 collisions, making nearly a billion collisions per second in all. The ensuing data collection rate demanded of the detectors is equivalent to the information processing for 20 simultaneous telephone conversations by every man, woman, and child on Earth.
Huge detectors will be housed at the collision points. CMS (Compact Muon Solenoid) and ATLAS (A Toroidal LHC ApparatuS) will explore the new energy region looking for all kinds of new effects – both expected and unexpected. The ATLAS detector will be five stories high (20 m) and yet able to measure particle tracks to a precision of 0.01 mm.
CMS and ATLAS each follow the time-honoured structure for modern particle detectors. First comes the logically named ‘inner tracker’, which records the positions of electrically charged particles to an accuracy of about one-hundredth of a millimetre, enabling computers to reconstruct their tracks as they curve in the intense magnetic fields. The next layer is a two-part calorimeter, designed to capture all the energy of many types of particle. The inner part is the electromagnetic calorimeter, which traps and records the energies of electrons and photons.
High-quality lead glass, like the crystal of cut-glass tableware, is often used as a detector because the lead in the glass makes electrons and positrons radiate photons and also causes photons to convert into electron-positron pairs. The net effect is a miniature avalanche of electrons, positrons, and photons, which proceeds until all the energy of the original particle has been dissipated. The electrons and positrons travel faster in the glass than light does, and emit Cerenkov light, which is picked up by light-sensitive phototubes. The amount of light collected bears testimony to the energy of the original particle that entered the block.
Thousands of tonnes of iron are interleaved with gas-filled tubes to pick up protons, pions, and other hadrons – particles built from quarks. This is the ‘hadron calorimeter’, so called because it measures the energy of hadrons, just as calorimeters in other branches of science measure heat energy. The iron in the calorimeter has a dual purpose: as well as slowing down and trapping the hadrons, it forms part of the electromagnet used to bend the paths of charged particles, revealing their charge and helping to identify them.
19. A LEP detector with four scientists setting the scale.
20. Trails of particles and antiparticles as revealed on the computer screen; compare the computer view with the end view of the detector in Fig. 19.
An outermost layer consists of special muon chambers, which track muons, the only electrically charged particles that can penetrate this far. The set of detector components form a hermetic system designed to trap as many particles as possible as they emerge from the collisions at the centre. In principle, only the elusive neutrinos could escape completely, leaving no trace at all in any of the detector components. Yet even the neutrinos left a ‘calling card’, for they escaped with energy and momentum, both of which must be conserved in any interaction.
This entire detector is designed to record the debris from collisions that occur a billion times each second. This is a far cry from the early days of cloud chambers, which could record only once a minute, or even bubble chambers at once a second. Among the debris produced in these collisions, at energies exceeding anything ever measured at an existing particle accelerator, the jewel will be some unexpected phenomenon. Among the hoped-for discoveries is the Higgs boson (Chapter 10), but this is expected to be produced on average only once in every 20 million million collisions. This means that with up to a billion collisions each second, a Higgs boson would appear about once a day in each experiment at the LHC. It has been suggested that finding a needle in a haystack is easier than sighting the one Higgs in every hundred thousand billion other events. A challenge for computation will be to recognize the Higgs and record only selected data onto magnetic tape.
21. Here we see the result of electron and positron annihilation where three jets of particles have emerged. First a quark and an antiquark were produced and almost immediately one of these radiated a gluon. The quark antiquark and gluon are the sources of the three jets of detected particles.
This all illustrates how our ability to learn about the origins and nature of matter have depended upon advances on two fronts: the construction of ever more powerful accelerators, and the development of sophisticated means of recording the collisions.
Cha
pter 7
The forces of Nature
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There are four fundamental forces: gravity, electromagnetic force, and weak and strong forces. Here, we discuss the idea that forces are due to the exchange of particles: photon, W, Z, and gluon; and that the different nature of the forces makes the world go round – if particles are Nature’s alphabet, the forces are its grammar; unification of the forces.
* * *
Four fundamental forces rule the universe: gravity, the electromagnetic force and then two that act in and around the atomic nucleus, known as strong and weak. The latter pair act over distances smaller than atoms and so are less familiar to our macroscopic senses than are the effects of gravity and magnets. However, they are critical to our existence, keeping the Sun burning and providing the essential warmth for life.
Gravity is the most familiar to us. Between individual atoms or their constituent particles, the effects of gravity are nugatory. The strength of gravity between individual particles is exceedingly small, so small that in particle physics experiments we safely ignore it. It is because gravity attracts everything to everything else that its effects add up until they are powerful, acting over cosmic distances.
