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In Review

Three hundred giant tanks, each holding more than 50,000 gallons of purified water, perch on the side of the Sierra Negra volcano in central Mexico, standing 13,500 feet above sea level. Four photosensors lie at the bottom of each tank.

The array of tanks is part of the High-Altitude Water Cherenkov Gamma-Ray Observatory, or HAWC, a joint project of Mexico and the United States. While the technology is comparatively simple, the project is ambitious: observing gamma and cosmic rays, and contributing to the search for dark matter.

Segev BenZvi, an assistant professor of physics, and fellow scientists are looking for very energetic gamma rays and cosmic rays that enter Earth鈥檚 atmosphere. When the high-energy rays interact, they create a 鈥減article cascade鈥濃�攁 shower of high-energy particles鈥攖hat falls to Earth.

BenZvi and colleagues on the project are studying extremely high-energy particle acceleration, from supernova remnants, black holes, neutron stars, and pulsars鈥斺�渙bjects with very, very large amounts of energy, some of which is being dumped into accelerating charged particles out in space,鈥� he says. When they interact, they produce gamma rays and cosmic rays.

HAWC is a 鈥渟caled-up鈥� version of a classic physics student experiment, he says, that uses a water tank in the lab to measure for muons鈥攈eavy, unstable versions of electrons that are the ground-level remnants of particle cascades.

But the project is innovative in three ways: its high-altitude location brings better sensitivity to the particles, which get absorbed by the atmosphere as they descend; its 鈥渙ptically isolated, densely packed鈥� tanks; and the algorithms that let the scientists make use of the data they assemble.

Construction of the observatory began in 2011 at a site in the Parque Nacional Pico de Orizaba, a national park and home to the dormant volcano Pico de Orizaba, Mexico鈥檚 highest peak. HAWC was formally opened last spring. When the experiments are complete, in about 10 years, the scientists will restore the area to as close to its original condition as they can. The park is a 鈥渃loud forest,鈥� with one of the highest tree lines in the world. An environmentally sensitive site, the forest affects cloud formation and rainfall in areas south and west of the park. No trees were removed in creating the observatory.

The high-altitude location poses minor challenges for researchers, like shortness of breath, says BenZvi. 鈥淎nd shortness of temper, believe it or not,鈥� he says. 鈥淵ou just get really irritated. Your thinking isn鈥檛 very clear. I find that I get bad at doing basic arithmetic in my head when I鈥檓 up there.鈥�

The enormous tanks鈥�23 feet wide and more than 16 feet high鈥攁re the same kind of tanks used by Midwestern farmers to irrigate their fields. A military contractor who manufactures 鈥渓ight-tight鈥� tents for soldiers in hostile territory makes the tanks鈥� hemispherical domes.

Local workers assembled the tank array, and then made 4,000 trips by truck up and down the mountain to haul the water to fill them鈥攁 volume of 55 million liters, or the equivalent of a soda can鈥檚 worth of water for each person living in Mexico.

The tanks sample the air shower particles at ground level. There are about 100 million particles in a cascade at its peak. The number of particles decreases as the cascade descends.

鈥淚t鈥檚 like a pancake of high-energy particles that moves toward the ground,鈥� BenZvi says. When the particles hit the ground, they move through the tanks鈥攁nd when high-energy particles move through water, they produce ultraviolet light, known as the Cherenkov effect. The photosensors in the tank record the ultraviolet light. And from the pattern of times that the sensors in each tank are triggered, scientists can reconstruct the direction of the particle pancake.

The data they collect may also shed some light, as it were, on dark matter.

鈥淭here is very strong evidence, from all kinds of measurements in astrophysics, that there is something called dark matter,鈥� BenZvi says. 鈥淏ut it鈥檚 not clear what it is. We think it鈥檚 a fundamental particle or particles.鈥�

But scientists don鈥檛 know how massive it is or what its interactions are. 鈥淪o it鈥檚 entirely possible that some of the gamma rays and cosmic rays that we see are actually not produced by neutron stars and supernovae and things like that鈥攖hey鈥檙e actually produced when clumps of dark matter interact and decay. That鈥檚 the idea,鈥� he says.

When anomalies are found in astrophysical data, scientists consider whether the source of the anomaly is a mistake in their model or the influence of dark matter.

鈥淎nd that鈥檚 kind of the name of the game,鈥� says BenZvi. 鈥淚t鈥檚 a tough game, as you can imagine. It鈥檚 sort of like the joke about 鈥榰nknown unknowns鈥欌�攜ou don鈥檛 know what you鈥檙e not modeling.鈥�

Telescopes offer another way to measure gamma rays. But they have a narrow field of view, taking in only a few degrees of the sky at a time. HAWC records information from two-thirds of the sky every 24 hours.

鈥淥ver the course of one day, we can see essentially the entire northern hemisphere,鈥� says BenZvi, noting that the methods are complementary. 鈥淲e鈥檝e made the bet on more coverage, less sensitivity; they make the bet on more sensitivity, less coverage. If you have both types of instruments running, you can look for unexpected stuff with HAWC鈥攚e communicate with those guys through back channels: 鈥楬ey, we see something interesting. Point your telescope there.鈥� And that鈥檚 how a lot of the field works.鈥�

Scientists are now processing their first year鈥檚 worth of data from HAWC, which they began to make public this spring. And they are expanding the array with some additional tanks鈥攋ust a few, which will bring with them a four-fold increase in sensitivity.

And there is talk of creating a second observatory, possibly in Chile. The southern hemisphere provides the best vantage point for observing the center of the galaxy鈥攁nd it鈥檚 鈥渁 very strong candidate for observing dark matter, because we believe there鈥檚 a super-massive black hole in the center of the galaxy, and so there should be a gravitational well there where dark matter is concentrated,鈥� says BenZvi.

鈥淚f we have a HAWC in the southern hemisphere, literally the center of the galaxy will be right overhead,鈥� he says.

鈥擪athleen McGarvey