by M. Mitchell Waldrop

Cosmic rays, at energies future accelerators cant hope to match, still have much to teach physicists and astrophysicists.

Austrlan-born physicist Victor F. Hess discovered a remarkable phenomenon in 1912 when he carried detectors

aloft in a balloon. The higher he took his instruments, the more he found the molecules of air around him to be ionized, or electri­cally charged. To explain his observation, he argued that some form of ionizing radia­tion must be descending from the heavens.

Proving Hess's contention and elucidating the nature of that radiation occupied some of the world's foremost physicists over much of the next three decades. Among them was Nobel laureate Robert A. Millikan, who in the 1920s was the first to refer to the radia­tion as "cosmic rays ."

The o v e r w h e l m i n g majority of cosmic rays, we now know, are protons, the posi­tively charged nuclei of hydrogen atoms. Next most common are helium nuclei, or alpha particles, followed by the nuclei of carbon, nitrogen, oxygen and iron, all among the most abundan t elements in the universe. Except for particles having the very highest energies, cosmic rays appear to arrive at the earth's surface almost uniformly from all directions, their intensity modulated primarily by interaction with the magnetic field en­countered near the earth. They come infre­quently, however—just a few particles per square centimeter per second at the top of the atmosphere. And this is just the average

Air shower. A cosmic ray's collision with a molecule in the atmosphere results in a relatively narrow, intense core of heavy particles (hadrons) and a penumbra of others—all grist for the physicists' mill M. M. Waldrop, by permission.

MOSAIC November/December 1980

flux; the rates for individual particles fall off roughly as the inverse cube of their energy. At the extreme limit of the known spectrum, 1020 electron volts, there are only a few part icles per square ki lometer per century. Collectively, cosmic rays deliver no more total energy to the earth than does starlight.

Even after seven decades of research, it is still unclear where these particles come from. Theories abound, of course, but prov­ing them is more difficult. Most cosmic rays with energies below 1017 electron volts prob­ably originate somewhere in the Milky Way galaxy. Carbon, nitrogen and oxygen nuclei, heavily represented among them, for ex­ample, are components of what is called the CNO (carbon/nitrogen/oxygen) cycle of stellar nucleosynthesis. Iron, also represented, is formed in the cores of very massive stars. Presumably these elements are injected into space by the supernova explosions in which massive stars expire.

Energy source But wha t accelerates these particles to

cosmic ray energy? It used to be thought that all cosmic ray constituents, including hydrogen and helium nuclei, were accelerated in the supernova explosion itself, or per­haps by pu l sa r s , the r emnan t s of super­novas. Recently, however, astrophysicists have tended to favor a more indirect process, one in which the particles are accelerated by shock waves in the tenuous gas of the inter­stellar medium. Such shock waves could arise from a number of causes, including super­novas themselves or even s t rong flares given off by otherwise normal stars.

Beyond 1017 electron volts, however, the cosmic rays may well come from outside the galaxy entirely. And in that event, there is as yet no convincing case to be made for a candidate source.

Part of the problem is that it is extraordi­narily difficult to study these high-energy particles, largely because they are so rare. Nonetheless, researchers have been paying more and more attention to them in recent years. Not only are cosmic ray particles with energies around 1020 electron volts astro-physically interesting in their own right, but they offer particle physicists a window on subatomic interactions at energies far be­yond anything that can be achieved experi­mentally on earth in even the most powerful present-day particle accelerators.

In a sense this is a return to the old days of cosmic ray research, before the advances in accelerator technology moved the frontiers of particle physics out of the skies and into the laboratory. One of the early milestones in particle physics, for example, was the

discovery of the muon in 1937. A heavy cousin of the electron, the muon was the first particle ever found that was not asso­ciated wi th the e l ec t ron -p ro ton -neu t ron trinity of ordinary matter. And it was dis­covered in a cosmic ray shower.

But reaching out toward the highest -energy cosmic rays has strained the state of experimental art and has begun to foster some radical new approaches. At the lower cosmic ray energies, in the range up to, say, 1012 electron volts, where the particles are relatively abundant , scientists can still follow in the tradition of Hess by placing part icle detectors on bal loons , sound ing rockets and even satellites. But at higher energies, around 1013 to 1015 electron volts, the rapidly declining cosmic ray intensities dictate the use of detectors of much larger area. In some cases these can cover more than 50 square meters of the earth's surface. To ensure collection of an adequate sample of h igh -ene rgy cosmic ray part icles, the collectors are often placed on mountaintops.

At higher energies still, above about 1016

electron vol ts , p lummet ing occurrences make even this approach unfeasible. Instead, scientists working in this realm give up any hope of detecting the primary cosmic ray and any attempt to build a single detector. They go back down to the flatlands. There the idea is to set out huge arrays of widely spaced detectors encompassing some tens of square kilometers. With these vast arrays, scientists seek a statistical sampling of the shower of secondary particles set loose when a high-energy cosmic ray encounters atoms of atmospheric gases several kilometers over­head. Their aim is to generate data sufficient to enable the operators of the array to recon­struct the energy and path of the original cosmic ray.

Huge arrays

It is not easy. At these energies the air shower s , cascades of secondary particles produced by multiple collisions in the at­mosphere, are huge. Moving at a large frac­tion of the speed of light, the primary cosmic ray particle meets its first atmospheric nucleus at an altitude of about 20 kilometers. Ou t of that and successive collisions comes a spray of electrons, photons, muons, protons, neu­trons and heavier nuclear fragments, all cas­cading furiously along the path of the original particle into the denser air below. Moving essentially at the speed of light, this cas­cade of particles will bu ry itself in the g r o u n d less than a t e n - t h o u s a n d t h of a second after the initial contact, in a disk-shaped pa t t e rn extending out a h u n d red meters or so and having a tenuous penum­bra that may still be detectable out to a

radius of several kilometers. Although making sense out of one of

these showers may be difficult, it is not impossible; the detector arrays at Volcano Ranch near Albuquerque, New Mexico, at Haverah Park near Leeds in England, at the University of Sidney, at the University of Tokyo and at Yakutsk in the Soviet Union, among others, have been providing invalu­able data at cosmic ray energies between 1016 and 1020 electron volts for more than a decade.

Detailed features of the highest-energy cosmic ray events, however, can be only inferred from ground-level detection. N o direct sampling of the cascade in the at­mosphere is attainable. To overcome this handicap, cosmic ray physicists are turning to an optical cosmic ray detector—a device called the Fly's Eye—which, after more than two decades of gestation here and abroad and seven years of design and development, is now in operation on a hilltop in Utah.

The Fly's Eye A few miles east of the Great Salt Lake

Desert, in the sagebrush flats of the United States Army's Dugway Proving Ground, the Fly's Eye stands atop a steep, isolated hill called Little Granite Mountain. It gives the impression, from a distance, of a garland of garbage cans crowning the mountain 's summi t . D u r i n g the day, a visitor who makes it up the twisting gravel road to the crest sees the two-meter-tall cans hanging from individual mounts , their open ends d o w n to shield their interiors from the desert sun. After sunset, however, on those clear moonless nights when the Utah desert grows darker than most city dwellers can imagine, the cans pivot upwards until each is pointed at its own particular patch of sky. At the bottom of each can, barely visi­ble in the starlight, is the concave mirror of an optical telescope; at the mirror's focus is a cluster of photomultiplier tubes so sensi­tive that even moonlight overwhelms them.

It takes a bit of imagination, standing there among the garbage cans, to see where this setup gets its name. But the resemblance is there, in the overlapping, multifaceted way in which this array of telescopes images the heavens. It is indeed very much like the multifaceted eye of a fly.

The 67 telescopes are set in two roughly concentric rings, surrounding a rambling shed. The shed serves principal investigator George Cassiday and his colleagues at the Univers i ty of Utah as computer center , workshop and bunkshack. On nights around the new moon, this is often where they can be found.

The Fly's Eye is fundamentally different

14 MOSAIC November/December 1980

from any previous cosmic ray experiment. It almost has to be. Cosmic ray showers above 1019 electron volts are so rare that even the largest ground arrays can detect only a hand­ful in a decade. But the Fly's Eye should get around that limitation; rather than at­tempting to detect the air shower particles themselves, its multifaceted " e y e " watches for streamers of light moving across the sky, the scintillation light left by the air shower's passage. In effect, the Fly's Eye is simply a

giant optical scintillation counter that uses the atmosphere as its detector.

"An air shower is just an electric current passing through a g a s / ' Cassiday explains. " T h a t ' s exactly w h a t is happen ing in a f luorescent light f i x t u r e . " Scinti l lat ion is simply the technical name for light emitted when an excited atom—in this case of an at­mospheric molecule raised in energy by the passage of the air shower—recaptures an electron.

A light show Air showers would m a k e a wonder fu l

light show if humans could only see them. Unfortunately, the showers happen much too fast for human eyes to respond, and in any case they are much too faint, most of their light lying in the ultraviolet between 3,300 and 4,100 angstroms. But the Fly's Eye, with its sensitive photomultipl iers and its electronics that slice up time by the bil-lionths of a second, can detect them.

Besides being stored on computer tape for analysis, the air shower data show up on a representation of the sky in the form of a hemispherical bowl of blue plastic set into the wall of the computer room in the central shed. The bowl, about a meter across, is marked off with the field of view of each of the Fly's Eye's 67 telescopes. Within those fields fainter lines mark off the smaller fields seen by each telescope's 12 to 14 photo-multipliers. Finally, at the center of each of the photomultiplier fields is a tiny red elec­tric light bulb.

At the touch of a computer key, the Fly's Eye's computer can be ordered to display a series of recorded air shower events. First, one oi hie reu ouius iignts up , representing the point at which the pr imary cosmic ray first encountered an air molecule and initiated the shower. Rapidly, but still more slowly than in the real event by a factor of a thou­sand, the initial light is followed by two more adjacent to it, then others , forming a glowing, red teardrop shape that spreads across the model sky just as the shower descended and spread across the real sky. Then the first event vanishes, to be followed by others: skinny teardrops, fat teardrops, then streamers like lightning bolts and round blobs spreading outward from their centers (showers coming straight toward the ap­paratus).

The Fly's Eye finished its calibration and began taking data in June 1980. It is designed to detect showers up to 50 kilometers away, giving it a huge collection area. "That 's what makes the Fly's Eye unique in its ability to study cosmic rays above 1019 electron volts," says Cassiday. In all the previous years of cosmic ray research with g r o u n d ar rays , fewer than a hundred of these events had ever been seen. The Fly's Eye sees that many every few months .

On the other hand, it's not enough just to see the events, Cassiday says. You want to know the original paths of the cosmic ray primaries, so that you can develop a statistical plot of their arrival directions; and you want to know their energy and composit ion.

Finding a primary's path is fairly straight­forward. The Fly's Eye measures a shower 's

MOSAIC November/December 1980 15

rate of progression across the sky. Knowing that and the shower's true velocity-—essentially the speed of light—it is a simple matter of geometry for the computers to reconstruct the shower's true path in space. Extending that path backward gives the original path of the primary.

Determining the pr imary 's energy is also fairly easy. It turns out, explains Cassiday, that the total number of electrons in an air shower is proportional to the energy of the incident particle, regardless of the kind of particle it was. The nucleus of an iron atom, for example, tends to interact higher in the atmosphere than a proton does and to pro­duce more electrons at higher altitudes. But it will also produce fewer electrons at lower altitudes; there is essentially no difference in the total number of electrons produced.

Further, since intensity of emitted light is also proportional to the electron density, the Fly's Eye can record the electron profile directly by picking out the light emitted at each point along the shower 's path. And from the electron profile, both the primary's energy and identity can be derived.

The nuclei of hydrogen, helium, carbon, nitrogen, oxygen and iron are known to be by far the most common species at lower energies for which data exist. But data are scarce at higher energies. " A t 1020 electron volts we're at ground zero ," says Cassiday. It could take three to four years of good, hard data-running, he says, but ultimately the Fly's Eye will be able to fill in those higher energy numbers.

Waldrop, a West Coast science writer, is shifting his base to Washington, D.C.

Because it's there The source™-the point of origin—of the

highest-energy cosmic rays is not the mystery today that it was a few years ago. Never­theless, the question is far from closed. And even though the Fly's Eye does not detect cosmic ray particles directly, its ability to plot the direction of travel of original particles should provide important corroboration, at least, if not new ideas on where in the cosmos these energetic cosmic rays originate.

Above 1018 electron volts, Cassiday ex­plains (or maybe the cut-off is higher; John Linsley of the University of New Mexico says 2-to-5 X 1019 electron volts), charged particles travel very nearly in a straight line. They are far too energetic to be deflected by the galaxy's magnetic field, which causes their less energetic brethren to spiral off in random directions.

If these very h i g h - e n e r g y cosmic rays originate in the galaxy, a statistical plot of their directions will show them to be coming from the plane of the Milky Way, and only a few from the galactic poles. If they are coming from nearby galaxies, such as those in the Virgo cluster of galaxies, a larger proportion of them will be arriving from that direction. But if they originate in very remote sources, such as quasars or the Big Bang itself, they will be found to be arriving uniformly, from every direction.

Efforts to determine the origin of cosmic rays of the highest energies have occupied the operators of very large ground arrays for more than a decade. And recently, says Alan A. Watson, director of the array at Haverah Park in Leeds, England, enough events have accumulated for scientists to

begin to be fairly confident of their statistical distribution. "We've seen it at Leeds, at John Linsley's array at Volcano Ranch in New Mexico, and at Yakutsk in the Soviet Union," he says. " M o s t of these cosmic rays seem to be coming from the Nor th Galactic Pole." That happens to be almost exactly the direc­tion of the Virgo Cluster, some 30 million light-years away.

Virgo is the nearest really big cluster. It is also the center of the local supercluster of galaxies of which the Milky Way is part. And it is the home of the giant elliptical galaxy M 8 7 , which some astronomers sus­pect of harboring at its core an immense black hole. Could it also be generating the 1020-electron-volt cosmic rays bombarding the earth?

That is far from proved, cautions Watson. " M 8 7 is only suspect because it's there." On the other hand there is good reason to believe that cosmic rays with an energy of 1019 or 1020 electron volts must come from a source that is no farther away than M 8 7 or other objects in the Virgo cluster.

"It relates to the 3°K blackbody radia­tion," he explains. This radiation, the after­glow of the Big Bang, permeates the uni­verse with photons of microwave energy. It is something like a very dilute gas that cooled as the universe expanded. In 1966, a year after the discovery of the radiation, Kenneth Greisen of Cornell Universi ty and G. T. Zatsepin of the P. N. Lebedev Physics Institute in Moscow independently realized that this photon gas sets a kind of cosmic speed limit by slowing down anything that tries to move too rapidly th rough It. Spe­cifically, particles above about 1019 electron volts will Interact with the pho tons , radiate pi mesons and lose energy.

Another way to look at it, says Watson, is to imagine what the 3°K blackbody radia­tion looks like from the viewpoint of the cosmic ray particle careering wildly through space. From that perspective, the photons ahead, a long the par t ic le ' s d i rect ion of motion, are enormously blue-shifted by the Doppler effect. To the particle, the 3°K background isn't a gentle whisper of micro­waves; it's a head-on hurr icane of gamma rays.

Too little drag After Gre isen and Za t sep in pub l i shed

their ideas in 1966, Watson recalls, it be­came an exciting thing for experimenters to look for the predicted cutoff in the cosmic ray energy spectrum. The 3°K background should make the spectrum nose-dive in the range of 1019 to 1020 electron volts, he ex­plains. But that 's not what everybody saw. "Unless Linsley and I and the others who 've

16 MOSAIC November/December 1980

worked on it are totally wrong / 7 says Wat ­son, " the spectrum flattens/' falling off less rapidly in the range above 1019 than it does below.

The implications of this flattening out of the curve are still unclear, Watson says. But whatever it means, the spectrum does not dive toward zero, perhaps because the black-body photons haven' t had time enough to slow the particles down. And that, accord­ing to theorists' calculations, in turn means that the particles are coming from some­where inside a sphere roughly 30 million light-years in radius. Over a greater distance than that, the slowing effect would be able to take hold. Virgo lies within that sweep.

Theorists can model the rate at which the particles slow down in the 3°K radiation, and experimental measurements allow some estimate of the number of particles involved. Together , these efforts lead to estimates for the rate at which these far-distant sources might be pumping energy into cosmic rays.

The rate turns out to be quite high, on the same order as the intense radio emissions from objects such as the bizarre Seyfert galaxies— or, for that matter, M 8 7 . (See "Exotic Gal­axies," Mosaic, Volume 9, Number 3.) These high-energy cosmic rays could thus be a very important drain on the energy of such galaxies, says Watson. And it's one that hasn ' t yet been taken into account.

But all this theorizing rests on a very meager data set and flimsy statistics, Watson cautions. That ' s why he finds the Fly's Eye especially exciting from an as t rophys ica l standpoint. " In one year, with good weath­er, they expect to get four times the number of events the rest of us have gotten so far ." The device will be able to repeat and, he hopes, verify the older measurements with much higher statistical accuracy.

"If the Fly's Eye really works as well as we think," says Watson, "I hope somebody builds a second one and carts it off to the Southern Hemisphere. Because if the pic­ture of the Virgo Cluster origin is correct, we should find a very different picture down there."

Right the first time The notion that scintillation light caused

by the passage of an air shower through the atmosphere could be used to study large showers goes back at least into the 1950s. A. E. Chudakov of the Soviet Academy of Science's Institute for Nuclear Research in Moscow tried the idea without success in 1956. Independently, in the early 1960s, the idea was developed by K. Suga of T o k y o University and Cornell's Ken Greisen.

Greisen's efforts to build a detector in the

MOSAIC November/December 1980 17

mid-1960s were disappointing. " T h e exper­iment really wasn ' t feasible t h e n / ' says Cassiday. " W h a t you need is lots of fast, cheap electronics and lots of big, cheap mirrors ," the kind of equipment that be­came available only in the 1970s.

Cassiday himself first heard about Greisen's idea while he was a graduate s tudent at Cor­nell in the 1960s; he returned to it in 1972 when he was a postdoctoral assistant at the University of Utah in Salt Lake City. Wi th the encouragement of his boss, Utah ' s Jack Keuffel, Cassiday spent a year doing calcu­lations and computer simulations in wha t ul t imately became a full-fledged des ign study.

The Utah group applied for federal fund­ing of the Fly's Eye in 1974, with Keuffel as principal investigator. But in 1975, just as fund ing was approved for p r o t o t y p e studies, Keuffel died of a heart attack while climbing a mountain; Cassiday took over as team leader.

The team has drawn upon a wide variety of talents, says Cassiday. Joseph Boone, for example, is an astronomer who has designed a lot of the optics for the system. Jerry Elbert, a physicist, did the trajectory models and shower calculations. " N o b o d y is going to have the equipment to replicate our re­sults anytime soon," says Cassiday. "So we all feel a strong obligation to make sure we do things right."

To prove the Fly's Eye concept, Cassiday and his co-workers set up a prototype de­tector at John Linsley's Volcano Ranch array outside Albuquerque, New Mexico. " W e pointed our mirrors over the top of his ar­ray," Cassiday recalls. " T h e big quest ion was whether we could see the showers he saw." They did; the number of electrons measured by the two methods agreed to within 10 percent.

In 1977, with these preliminary studies finished, construction of the full Fly's Eye with all 67 telescopes could get underway . The Dugway site was chosen, says Cassiday, because it was remote from city lights, be­cause it already had a power line, because military security would protect the site from vandalism and because the weather over the desert was generally clear.

"The sun limits us to running only 50 percent of the time," Cassiday points out . "The moon limits us further to about 22 percent, and then the weather knocks ou t about half of that. So it's only about 10 percent of the time that we can run . " T h a t ' s why the Fly's Eye has backups for almost all its electronic equ ipment , he adds . A n y breakdown during that 10 percent of the time that the window is open would be catastrophic.

18 MOSAIC November/December 1980

Sometimes Dugway hasn' t been quite re­mote e n o u g h . Cassiday recalls the n ight they put in the bank of mirrors that looks along the nor thern horizon. "A t 1.5-second intervals, ' w h a m ' / three or four hundred tubes would go off. We went outside and looked for wha t was doing it. Couldn ' t see a th ing." It took all that night and the next, but by watching the shift in the mystery flash's intensity as they pivoted the tele­scopes, Cassiday and his colleagues were able to p inpoint its direction.

Later, back in Salt Lake City, he took out a map of Utah , put his pencil point at Little Granite Mounta in , and with a straightedge drew a line along that same heading. Then he took a drive. A few hours later, on the western shore of the Great Salt Lake, he found the smelters of the International Lead Company. Above the smelters rose a tall smokestack, and on that stack was a strobe light, warn­ing off aircraft with a flash every 1.5 seconds. At that moment , Cassiday was standing 100 kilometers nor th of Dugway.

"I 've had to build a special circuit that blanks out the signals from those mirrors for 200 microseconds every time that thing goes off," he sighs.

A second eye Even as the array on Little Granite Moun­

tain was nearing completion in the summer of 1980, construct ion started on a second, smaller Fly's Eye array four kilometers away. "We 've always had the second eye in the back of our minds , " says Cassiday. "For one thing, you really need stereo to do a good job of reconstructing the distant events. Also, even the clearest air is a little bit hazy, which throws our results off. It turns out that a second eye is one of the best ways to find out exactly how hazy the atmosphere is." From any given point in the air shower, he explains, the light received at one detector will have traveled farther through the at­mosphere than light received at the other. C o m p a r i n g the measured intensi t ies will then allow the Fly's Eye team to calibrate and compensate for atmospheric attenuation.

Meanwhile , many cosmic ray physicists in the United States and abroad are coming around to the view that, while the Fly's Eye will be the centerpiece of this coun t ry ' s cosmic ray research for many years to come, it should be supplemented in the next five years or so with a more traditional large ground array nearby. No one is more enthu­siastic about this idea than Alan Watson.

"Convent ional ground arrays still have an enormous amount to offer," he says. The most beautiful aspect of the Fly's Eye is its direct measurement of the primary's energy. But the Fly's Eye, an optical array, can operate

only some 10 percent of the time, whereas an array detecting actual particles can operate day and night, in any kind of weather. Ex­perience shows they can run about 95 per­cent of the time. Watson also points out that the Fly's Eye sees only the electrons in a shower and provides no information on the muons produced from the decay of charged pi mesons , someth ing a proper ly ins t ru­mented ground array can do. "So even the

VHE particle

Cosmic ray detector. A traditional instrument array for cosmic ray detection, similar to that at Volcano Ranch, depends on direct detection of the primary and its daughter particles by sensors at different levels.

Fly's Eye misses alotof the action," he says. The ratio of muons to electrons in the

shower is particularly interesting, he adds, because it is quite sensitive to the nature of the primary particle. If that ratio can be measured—and most important if the enercrA7

of the slower is also accurately known—then an array can clearly distinguish a proton, say, from an iron nucleus. This in turn would provide an important corroboration of the Fly's Eye determination of primary particle composition.

If an array could be calibrated against the Fly's Eye energy measurements , says Watson, the combination would be a very powerful instrument for tagging the pri­maries. With a decade or so of running time, he adds, a combined array could produce a whole series of energy spectra and arrival direction plots: one labeled "pro tons , " one labeled " a lpha" particles and so on through all the major cosmic ray constituents.

The physics, too Cosmic ray physicists not only have to

grapple with a largely stil l-unknown com­position at the top of the atmosphere; they also have generally unfamiliar nuclear physics events taking place in the air showers within the atmosphere. But that unfamiliar nuclear physics represents a unique opportunity for particle physicists trying to pursue their sci­ence at energies beyond the reach of their accelerators. It's a venerable pursuit—witness the 1937 discovery of the muon in an air shower. But that was a rare lucky shot. For the most part, cosmic ray particle physics at ultrahigh energies is a tough and frustrating business, with the chaos of a massive air shower masking all but the most general

MOSAIC November/December 1980 19

aspects of the initial interaction. "I t ' s a fundamental fact of life with cos­

mic rays that the kinds of experiments you can do are classical, old-fashioned particle exper iments / ' says Thomas K. Gaisser of the Bartol Research Foundation at the Uni­versity of Delaware. "At the modern accel­erators/7 he notes, "the fashionable research involves efforts to get at the pointlike struc­tures—the quarks-™inside the h a d r o n s . " These experiments typically involve hitting the target hadron with a projectile, such as an electron or proton, that has been ac­celerated to a precise energy, then watching for fragments coming off at very large but carefully measured angles. Such precision just is not possible with cosmic rays.

Instead, says Gaisser, a scientist is look­ing at very broad-brush things like the total interaction cross section (essentially the total probability of the incoming particle's inter­acting with an air molecule), or the average variety of particles produced in the colli­sion. These are global properties of what is called the s t rong subnuclear interaction— the energy also released in thermonuclear fusion—and they involve all the complexities of the forces binding the quarks.

Nonetheless, says Gaisser, there has lately been a quickening of interest in cosmic rays among particle physicists. The highest energy collision that can be produced by an exist­ing accelerator is equivalent to a cosmic ray of only 2 X 1012 electron volts. But a whole new generation of accelerators, coming into operation in the 1980s, will produce colli­sions equivalent to 1014 or 1015 electron volts. These include proton/ant iproton colliders at Fermilab and at the European Center for Nuclear Physics (CERN), and a proton/proton collider named Isabelle at the Brookhaven National Laboratory. Too impatient to wait, a number of physicists are looking to cosmic rays in this energy range for clues to what their new machines might reveal.

Gaurang B. Yodh, a particle physicist at the University of Maryland, has recently collaborated with Gaisser on a review of this very subject. "All the air shower work to date implies that there is something very interesting to be seen in the new machines," he says. The evidence is typically subtle: At high energies, air showers seem to start higher in the atmosphere than one would expect from a straightforward extrapolation from accelerator energies.

This extrapolation simply assumes that no elementary particles exist other than the ones that are already known, Yodh explains. With this assumption, standard mathematical techniques allow a prediction for the total interaction probability for a particle at cosmic

ray energies. The higher that probability, the higher in the atmosphere the cosmic ray will begin its shower.

But the data show that the probability ex­trapolated in this way isn't as high as the real probability, says Yodh. A simple way to eliminate the discrepancy is to assume the product ion of new, very heavy particles in the first instants of the cascade.

Because of Einstein's mass-energy rela-

Air showers j

For the most part, cosmic ray detectors don ' t see the primary cosmic ray at all, but rather the shower of secondary par­ticles set loose by its interactions with the atmosphere. First detected in 1938 by Pierre Auger and his colleagues at the Ecole Normale Superieure in Paris, air showers have been the objects of a good deal of research in themselves.

University of New Mexico physicist I John Linsley, who for years ran one of

the largest g round ar rays , at Volcano Ranch, New Mexico, says that the phys­ical structure of an air shower can be modeled in an approximate way on a scale

I of 1:20 000 hyT a half-centimeter card­board disk sliding down a meter-long string. A complex of factors in the at-

I mosphere keeps the disk roughly the same size. The string represents the tightly bunched core of protons, neutrons, mesons and heavier fragments of air nuclei; the ) disk represents the halo, or penumbra, of photons , electrons, positrons and muons | flanking the core as they descend through I the atmosphere at the speed of light.

The electromagnetic component of the shower includes the photons, electrons and positrons. All of these arise ulti­mately from the creation of neutral pi m e s o n s , pi -zeros . T h e pi-zero decays within 8 x 10"17 second into two gamma rays. These gamma rays then interact with air molecules to create an electron/ positron pair. The positron and electron then collide with atmospheric nuclei and radiate more gamma rays, which in turn create more pairs. The sequence repeats itself again and again, building up in a chain reaction.

The muon component of the shower is less spectacular. Along with the neutral pi mesons, the initial collision also pro­duces a roughly equal number of positively and negatively charged pi mesons. Much longer-lived than their neutral brothers, each has a good chance of striking another air molecule before it decays. When they do decay, however, they form not photons but muons (and also neutrinos, which are extremely difficult to detect). •

tion (E = mc2), he explains, a collision must reach a certain threshold of energy before a particle of a given mass can be produced. Both theory and experiment at accelerator energies show that the interaction probabil­ity, or more technically, the cross section, jumps sharply upward at such thresholds. So a cosmic ray with energy enough to produce such a particle would tend both to interact and to start its air shower higher in the atmosphere than expected.

What kind of particles might these be? The least radical view is that they are more or less ordinary hadronic particles contain­ing a new type of quark. Five types of quarks are now known: up, down, strange, charmed and bottom, each more massive than the one before. For a number of theoretical reasons there is also widespread speculation that at least one other exists—called top—that is more massive yet. The first five, although they never seem to exist as individuals, bind to­gether in groups of two and three to form the protons, neutrons, mesons and other hadronic particles observed in nature; pre­sumably the top quark would join in form­ing a similar but heavier family of particles that could explain the rise in the cosmic ray cross section. If this is the answer, then these new top particles will soon be found at one of the new colliding beam machines.

Centauro events But is it possible that the top quarks aren't

the whole answer, that something stranger is going on? There are some tantalizing hints to that effect in the cosmic ray data from what are being called the "Centauro" events.

There are several different stories about how the Centauro events got their name. One version holds that the tracks made in the detector by the first event looked a bit like the constellation Centaurus. However it happened, there are five events known, all observed during the 1970s by a collabora­tion of Brazilian and Japanese physicists at a detector on Mt. Chacaltaya in Bolivia.

Their device, as are many cosmic ray de­tectors, is an emulsion detector, and is one of the largest of its kind. Covering some 40 square meters, it consists essentially of lead sheets and X-ray film in alternating layers. A n y incoming cosmic ray or air shower particle blasts loose a tiny particle shower of its own, leaving a series of microscopic dark spots on successive layers of film. This allows the scientists to determine the trajec­tory of the particle. Also, since hadrons, in­cluding pro tons and neu t rons , penet ra te deeper into the chamber than do electrons or gamma rays, the experimenters can with

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good confidence distinguish one class of particle from the other.

The first Centauro event came to the physi­cists' attention in 1973, when they noticed that an air shower contained an abnormally low ratio of gamma rays to hadrons. Re­constructing the event by triangulation from the particle tracks, and knowing that air shower gamma rays come primarily from the decay of neutral pi mesons produced in the original cosmic ray collisons, they deter­mined: 1) that the pr imary had interacted some 50 meters above the chamber, 2) that it probably had an energy in excess of 5 X 1014

electron volts and 3) that at most only one neutral pi meson was produced among a swarm of up to 75 other hadrons .

Pi missing

Four other Centauro events have come along in the years since then. Gaurang Yodh, who himself has worked at understanding these events, notes that it has not been pos­sible to analyze the others in as much detail as the first. But to the extent they can be analyzed, all are consistent with the produc­tion of no pi mesons at all.

To physicists accustomed to the copious production of pi-zeros—pi mesons, or pions, with zero electric charge—in ordinary colli­sions, this kind of behavior is just bizarre. Conventional theories lead one to expect some 20 pi-zeros. For a collision of this energy to produce just one seems highly im­probable; for five events to yield a total of one borders on the statistically impossible. Yet there they are. And there are other anom­alies. For instance, the Centauro shower par­ticles seem to spread out unusual ly far from the original cosmic ray trajectory. So un­less the Centauros are some kind of experi­mental artifact—and no one has found any­thing obviously w r o n g wi th the d a t a -something strange is indeed going on.

Especially tantalizing to particle physi­cists is that the Centauros occur at energies that may be accessible to the new colliding beam accelerators. If they represent the on­set of some exotic new interaction, such as the production of free quarks , then the ac­celerators almost certainly will see them. If, on the other hand, they announce the ar­rival in earth's a tmosphere of some kind of exotic cosmic ray particle—an unconfined quark left over from the Big Bang, perhaps, or some superdense state of nuclear m a t t e r -then the Centauro events won ' t be seen in the accelerators, because the physicists will be using only conventional projectiles like protons and electrons. But the Centauros may well be seen at the Fly's Eye, at much higher energy and with considerably better

statistics than at Chacaltaya. Whichever explanation turns out to be

true, whether the Centauros are new inter­actions, new objects or experimental arti­facts, they represent the best that cosmic rays have to offer particle physics. A glimpse, a hint, a vague, frustrating vision of some­thing just beyond experimental reach, they are a s t imulus to the imaginat ion. T h e y challenge the theorists and entice the ex­perimentalists. They are a niggling, small voice in the back of the mind, murmur ing

that maybe, just maybe, despite all this talk about q u a r k s , gluons and grand unified particle theories, we don ' t quite have it all figured out yet. And that ' s why, on the mountaintops , in the galleries of the great accelerators and in the skies over Utah, the search goes on. •

The National Science Foundation contrib­utes to the support of the research discussed in the article principally through its Ele­mentary Particle Physics Program.

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