SALTA: Snowmass-Area Large Time-coincidence Array


Imagine the kinetic energy of a baseball, thrown 100 mph, concentrated in a single proton. On average, subatomic particles that energetic strike a 100 km2 area of the earth's surface once a year.

Radiation of all kinds reaches the top of the earth's atmosphere from outer space. Traditionally, the science of observing electromagnetic radiation of any energy, from radio waves through visible light to gamma rays, is called astronomy. When we observe particles or atomic nuclei, it is called cosmic ray physics, or in recent years, "particle astrophysics". Cosmic rays provide us with information on violent, high-energy processes in distant stars, allowing us to study the fundamental particles and forces of nature under conditions far beyond the reach of man-made particle accelerators.

Because cosmic rays with extremely high energy arrive so rarely, it is necessary to build a detector with huge collecting area if we want to study these particles and their sources. One type of detector is the Extensive Air Shower (EAS) array, where small "counters" (individual particle detectors about 1m in diameter) are distributed over an area of about a square kilometer and linked together electronically (fig. 1). Arriving high energy cosmic rays smash a nucleus in the upper atmosphere, causing a "shower" of secondary particles, some of which reach sea level. By observing the pattern of particles detected in the EAS array, and the relative time of arrival at different counters, we can reconstruct the arrival direction and the energy of the original cosmic ray particle. EAS detectors have been used to study cosmic rays up to about 1018 eV, but to have the collecting power required to observe still higher energies, the array must be spread over a region of many km2.

School Networks

While the individual particle counters and other components of EAS detectors are not particularly expensive, the construction of huge dedicated arrays has been hindered by the need to find a suitable site, in a remote area where passersby will not disturb the equipment, and to install all of the required infrastructure - especially cabling for electrical power and data network equipment needed to interconnect the counters and the central data processing facility. Furthermore, the need to constantly check the performance of widely scattered equipment made operation of a huge array very expensive.

Today, when almost every school has an Internet link, the connectivity problem is solved if we use schools as detector sites. Efforts in this direction are underway in several areas in the USA and Canada. The NALTA Consortium (North American Large-scale Time-coincidence Arrays) represents an ongoing effort to share experience and materials between these local projects. It should even be possible to link local networks into a continental super-network, allowing observations on an unprecedented scale.

As part of a public outreach effort associated with a particle physics conference to be held at Snowmass in July, 2001, we want to place counter modules at high schools in the Roaring Fork Valley area. Participating schools will be given the necessary specialized electronics and a PC computer to serve as a data collector and forwarder. Students and teachers will attend a weeklong workshop at Snowmass, where they will work with particle physicists from around the USA and Canada, who are engaged in similar projects in their home towns. During the workshop, the students and teachers will learn about the scientific goals, and the experimental techniques used in the project, including hands-on work with detector components. The workshop will culminate with detector stations being set up at participating schools.

Once in operation at a high school, data will be downloaded at regular intervals from the SALTA station to a data analysis center at the University of Nebraska/Lincoln, the nearest NALTA member institute. Meanwhile, students and teachers at the school site will be active collaborators on the research project. They will be able to analyze both the data they are contributing and the overall data pool, providing opportunities for learning experiences in physics, math, and computer applications. Faculty and graduate students from UNL, the University of Washington, and other particle physics research centers will be actively engaged in working with the students and teachers at the schools, to help integrate the opportunities provided by this project with the normal requirements and goals of the school curriculum. The University members of the collaboration will work with teachers on developing the classroom materials required to accomplish this goal.

By engaging high school and perhaps middle school students in leading edge scientific research, we address another issue of great importance, to the nation as a whole as well as specifically to the science community: the need to promote greater interest in science in our high school students. Experience has shown that hands-on participation is the best way to engage students, and the continuing decline in enrollments in physical sciences on college campuses is a clear indication that we are not adequately communicating the excitement and rewards of a career in science to young people.

The Highest Energy Cosmic Rays

The nearest source of cosmic rays is the sun, which is 8 light-minutes away. Therefore only particles and nuclei which are stable or have long radioactive half-lives can possibly reach us. It is safe to assume that any cosmic ray we observe is a neutrino, an electron, a proton, or a stable atomic nucleus.

The sun pours forth a huge amount of energy in the form of very low energy (KeV to MeV) cosmic rays. (Table 1 explains the terminology used to describe cosmic ray energies.) However, the nuclear reactions which power the sun, even combined with the most favorable arrangements of the solar magnetic field, cannot produce particles more energetic than a few 10s or perhaps 100s of MeV. Higher energy cosmic rays must come from astrophysical phenomena more violent than the normal behavior of stars.

The cosmic ray energy spectrum, a graph showing the intensity (number of particles observed per square meter per second) of cosmic rays versus energy, is remarkable for the huge range it covers. Since cosmic rays were first detected in 1911, we have collected observations over an energy range covering more than 17 orders of magnitude! Our observations are limited at the low end by practical issues of identifying very low energy particles, and at the high end by the tiny flux (intensity) of particles. Within this energy range, the intensity of cosmic rays drops by more than 30 orders of magnitude. Thus it is necessary to use logarithmic scales to even begin to encompass the breadth of the cosmic ray spectrum, shown in Fig. 1. The plot is characterized by a very steep drop-off of intensity with energy. In fact, the intensity is proportional to 1/Eg, where g is a number between 2 and 3 depending upon the part of the spectrum considered.

Because the spectrum drops so steeply, it is often useful to look instead at a plot in which the intensity has been multiplied by Eg, so that we can more easily see variations in the slope. A spectrum doctored in this way is shown as fig. 2 (where the intensity values have been multiplied by E2.5).

The cosmic ray spectrum shows a distinct steepening around 1015 eV, a feature termed the "knee". Similarly, there appears reasonable evidence for an "ankle" around 1019 eV, where the slope increases. Finally, we expect to find a "toe" around 1020 eV, because of a fascinating phenomenon known as the Greisen effect.

The "knee" in the spectrum has been interpreted to mean that the sources of cosmic rays differ above and below the knee. That hypothesis makes sense, because we have known for over 50 years that supernovae (the violent explosions that ends the lives of massive stars) are a likely source for the observed cosmic ray spectrum at lower energies. However, the supernova mechanism is unable to account for cosmic ray energies above 1015 eV. At present, the sources of cosmic rays with energy above the knee region are not known for certain. Several candidates, have been proposed, all known to be sites of massive energy release in various ways (compact binary star systems, Wolfe-Rayet stars).

Cosmic rays between about 109 and 1018 eV are called "galactic" because they are too energetic to come from any process in the solar system, and not energetic enough to avoid bending in our Galaxy's magnetic field. The galactic magnetic field is only a few microgauss (millionths of the earth's magnetic field strength), but it is sufficient to bend protons with energy below about 1019 eV into paths with radii of curvature smaller than the Galaxy's radius. Lower energy cosmic rays are trapped in our Galaxy as if it were a magnetic bottle. This means that any particle below this threshold has been thoroughly "stirred" by the Galaxy, and its direction of arrival bears no relation to its point of origin. Neutral particles, like neutrinos and gamma rays, always point directly back to their sources at any energy. Thus we cannot use cosmic ray protons or nuclei of energy below about 1019 eV to search for extragalactic sources of cosmic rays. Similarly, cosmic rays of such high energies which are produced in our Galaxy will escape, so we expect cosmic rays in this energy range to be predominantly extragalactic in origin.

Finally, we expect the cosmic ray spectrum to cut off at approximately 1020 eV. At about this energy, cosmic rays will interact with the 3K blackbody photons which fill the Universe (the remnant "whisper of the big bang"). These photons look like high energy gamma rays to protons and nuclei, when viewed from the fast-moving cosmic ray's rest frame. Thus from the cosmic ray's point of view, it is facing a massive flux of extremely energetic gamma rays. The probability of a cosmic ray interacting before it can travel the distance from its galaxy of origin to our own is overwhelming. This effect is called the GKZ (Greisen-Kamata-Zatsepin) cutoff, after the American, Japanese and Russian physicists who first described it.

When a cosmic ray detector located in Utah announced the observation of several particles of energy greater than 1020 eV several years ago, it created a puzzle. The GKZ effect is based on sound theoretical ideas and ought to be there. But the Utah detector could not have seen so many "events" at such high energies if the cutoff worked as expected - it should have taken them decades to collect even a few events. Nobody has been able to find anything wrong with the experimental observations, so other options have to be taken seriously. One is that the character of particle interactions changes drastically between 1012 eV, where the highest energy laboratory particle beams have been carefully studied, and 1020 eV, where the GKZ effect comes into play. In past, whenever experimental data contradict established theories, rapid progress has followed. One hundred years ago, physicists similarly faced several puzzles, the solution of which led to quantum theory and relativity. For this reason, we think it is essential to collect additional observations of the highest energy cosmic rays.

Air Shower Detectors

One technique for detecting high energy cosmic rays is called the Extensive Air Shower (EAS) detector. Cosmic rays approaching the earth soon interact in nuclei within the atmosphere. We live under a layer of air amounting to about 1000 grams of air covering every square cm of the earth's surface. Luckily for all of this, the atmosphere shields us from cosmic rays. Unluckily for cosmic ray physicists, this means we cannot make direct observations of cosmic rays at sea level. However, the subatomic particles produced when the cosmic ray smashes a nucleus in the upper atmosphere (primary interaction) reach the earth's surface, perhaps after several additional generations of secondary interactions. When the original cosmic ray is sufficiently energetic, the result is a huge shower of subatomic particles striking the earth. The pattern of arrival (both the density of particles and their arrival time sequence) indicates the direction of arrival of the original cosmic ray. The total number of particles in the shower is proportional to the primary particle's energy. Details of the shower data (such as whether the shower particles are distributed smoothly or clumped together) let us at least roughly determine whether the primary cosmic ray was a gamma ray, proton or heavy nucleus.

Details of air shower physics and existing or planned detectors can be found at the following websites:

R. Jeffrey Wilkes, University of Washington