Passage IV

NATURAL SCIENCE: This passage is adapted from Frank Close, Michael Marten, and Christine Sutton's The Particle Explosion (©1987 by Frank Close, Michael Marten, and Christine Sutton).

The detector is a kind of ultimate microscope, which records what happens when a [subatomic] particle strikes another particle, either in a fixed target such as a lump of metal or a chamber filled with a gas or liquid, or in an on-coming beam in a collider. The 1950s and 60s were the age of the bubble chamber, so called because electrically charged particles moving through it produce trails of tiny bubbles in the liquid filling the chamber. [But today most] experiments are based on electronic detectors.

Detectors rarely record all the particle collisions that occur in a particular experiment. Usually collisions occur thousands of times a second and no equipment can respond quickly enough to record all the associated data. Moreover, many of the collisions may reveal mundane "events" that are relatively well understood. So the experimenters often define beforehand the types of event that may reveal the particles they are trying to find, and program the detector accordingly. This is what a major part of the electronics in a detector is all about. The electronics form a filter system, which decides within a split second whether a collision has produced the kind of event that the experimenters have defined as interesting and which should therefore be recorded by the computer. Of the thousands of collisions per second, only one may actually be recorded. One of the advantages of this approach is its flexibility: the filter system can always be reprogrammed to select different types of event.

Often, computer graphics enable the events to be displayed on computer monitors as images, which help the physicists to discover whether their detector is functioning in the correct way and to interpret complex or novel events.

Imaging has always played an important role in particle physics. In earlier days, much of the data was actually recorded in photographic form—in pictures of tracks through cloud chambers and bubble chambers, or even directly in the emulsion of special photographic film. Many of these images have a peculiar aesthetic appeal, resembling abstract art. Even at the subatomic level nature presents images of itself that reflect our own imaginings.

The essential clue to understanding the images of particle physics is that they show the tracks of the particles, not the particles themselves. What a pion, for instance, really looks like remains a mystery, but its passage through a substance solid, liquid, or gas—can be recorded. Particle physicists have become as adept at interpreting the types of track left by different particles as the American Indians were at interpreting the tracks of an enemy.

A number of simple clues immediately narrows down the possibilities. For instance, many detectors are based around a magnet. This is because the tracks of electrically-charged particles are bent in a magnetic field. A curving track is the signature of a charged particle. And if you know the direction of the magnetic field, then the way that the track curves—to left or right, say—tells you whether the particle is positively or negatively charged. The radius of curvature is also important, and depends on the particle's velocity and mass. Electrons, for instance, which are very light-weight particles, can curve so much in a magnetic field that their tracks form tight little spirals.

Most of the subatomic zoo of particles have brief lives, less than a billionth of a second. But this is often long enough for the particle to leave a measurable track. Relatively long-lived particles leave long tracks, which can pass right through a detector. Shorter-lived particles, on the other hand, usually decay visibly, giving birth to two or more new particles. These decays are often easily identified in images: a single track turns into several tracks.

Neutral particles present more of a headache to experimenters. Particles without an electric charge leave no tracks in a detector, so their presence can be deduced only from their interactions or their decay products. If you see two tracks starting at a common point, apparently arising from nowhere, you can be almost certain that this is where a neutral particle has decayed into two charged particles.

Our perception of nature has deepened not only because the accelerators have increased in power, but also because the detection techniques have grown more sophisticated. The quality of particle imagery and the range of information it provides have both improved over the years.