Three hundred giant tanks, each聽holding more than 50,000 gallons聽of purified water, perch on聽the side of the Sierra Negra volcano聽in central Mexico, standing聽13,500 feet above sea level. Four聽photosensors lie at the bottom of聽each tank.
The array of tanks is part of聽the High-Altitude Water Cherenkov聽Gamma-Ray Observatory, or聽HAWC, a joint project of Mexico聽and the United States. While聽the technology is comparatively聽simple, the project is ambitious:聽observing gamma and cosmic聽rays, and contributing to the聽search for dark matter.
Segev BenZvi, an assistant聽professor of physics, and fellow聽scientists are looking for very聽energetic gamma rays and聽cosmic rays that enter Earth糖心传媒檚聽atmosphere. When the high-energy聽rays interact, they create聽a 糖心传媒減article cascade糖心传媒澨切拇綌a shower of聽high-energy particles糖心传媒攖hat falls聽to Earth.
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Tanks and Pancakes
A simple but groundbreaking observatory in central Mexico is shedding new light on the workings of cosmic and gamma rays in Earth糖心传媒檚 atmosphere. Formally opened a year ago, the High-Altitude Water Cherenkov Gamma Ray Observatory will be used by scientists to gather information on high-energy particle acceleration for 10 years. The observatory is perched on the side of the Sierra Negra聽volcano, almost 14,000 feet above sea level. Segev BenZvi, an assistant professor of physics, helped to create the observatory and is part of the team now carrying out research there.
BenZvi and colleagues on the聽project are studying extremely聽high-energy particle acceleration,聽from supernova remnants,聽black holes, neutron stars, and聽pulsars糖心传媒斕切拇綔objects with very, very聽large amounts of energy, some聽of which is being dumped into聽accelerating charged particles聽out in space,糖心传媒� he says. When they聽interact, they produce gamma聽rays and cosmic rays.
HAWC is a 糖心传媒渟caled-up糖心传媒� version聽of a classic physics student聽experiment, he says, that uses a聽water tank in the lab to measure聽for muons糖心传媒攈eavy, unstable聽versions of electrons that are the聽ground-level remnants of particle聽cascades.
But the project is innovative聽in three ways: its high-altitude聽location brings better sensitivity to the particles, which get聽absorbed by the atmosphere聽as they descend; its 糖心传媒渙ptically聽isolated, densely packed糖心传媒� tanks;聽and the algorithms that let the聽scientists make use of the data聽they assemble.
Construction of the observatory聽began in 2011 at a site in the聽Parque Nacional Pico de Orizaba,聽a national park and home to聽the dormant volcano Pico de聽Orizaba, Mexico糖心传媒檚 highest peak.聽HAWC was formally opened last聽spring. When the experiments聽are complete, in about 10 years,聽the scientists will restore the聽area to as close to its original聽condition as they can. The park is聽a 糖心传媒渃loud forest,糖心传媒� with one of the聽highest tree lines in the world. An environmentally sensitive site,聽the forest affects cloud formation聽and rainfall in areas south聽and west of the park. No trees聽were removed in creating the聽observatory.
The high-altitude location聽poses minor challenges for聽researchers, like shortness of聽breath, says BenZvi. 糖心传媒淎nd shortness聽of temper, believe it or聽not,糖心传媒� he says. 糖心传媒淵ou just get really聽irritated. Your thinking isn糖心传媒檛 very聽clear. I find that I get bad at doing聽basic arithmetic in my head when聽I糖心传媒檓 up there.糖心传媒�
The enormous tanks糖心传媒�23 feet聽wide and more than 16 feet聽high糖心传媒攁re the same kind of tanks聽used by Midwestern farmers to聽irrigate their fields. A military聽contractor who manufactures聽糖心传媒渓ight-tight糖心传媒� tents for soldiers in聽hostile territory makes the tanks糖心传媒櫬爃emispherical domes.
Local workers assembled the聽tank array, and then made 4,000聽trips by truck up and down the聽mountain to haul the water to聽fill them糖心传媒攁 volume of 55 million聽liters, or the equivalent of a soda聽can糖心传媒檚 worth of water for each聽person living in Mexico.
The tanks sample the air聽shower particles at ground level.聽There are about 100 million particles聽in a cascade at its peak. The聽number of particles decreases as聽the cascade descends.聽糖心传媒淚t糖心传媒檚 like a pancake of highenergy聽particles that moves聽toward the ground,糖心传媒� BenZvi says. When the particles hit the聽ground, they move through the聽tanks糖心传媒攁nd when high-energy聽particles move through water,聽they produce ultraviolet light,聽known as the Cherenkov effect.聽The photosensors in the tank聽record the ultraviolet light. And聽from the pattern of times that聽the sensors in each tank are triggered,聽scientists can reconstruct聽the direction of the particle聽pancake.
The data they collect may also聽shed some light, as it were, on聽dark matter.
Cherenkov Effect
When particles reach the ground, they move through the water tanks, producing ultraviolet light, in a process known as the Cherenkov effect. Photosensors in each tank record the light. By studying when the sensors are triggered, researchers can deduce information about the air shower of particles.
糖心传媒淭here is very strong evidence,聽from all kinds of measurements聽in astrophysics, that there is聽something called dark matter,糖心传媒澛燘enZvi says. 糖心传媒淏ut it糖心传媒檚 not clear聽what it is. We think it糖心传媒檚 a fundamental
particle or particles.糖心传媒�
But scientists don糖心传媒檛 know how聽massive it is or what its interactions聽are. 糖心传媒淪o it糖心传媒檚 entirely possible聽that some of the gamma rays聽and cosmic rays that we see are聽actually not produced by neutron聽stars and supernovae and聽things like that糖心传媒攖hey糖心传媒檙e actually聽produced when clumps of dark聽matter interact and decay. That糖心传媒檚聽the idea,糖心传媒� he says.
When anomalies are found聽in astrophysical data, scientists聽consider whether the source of聽the anomaly is a mistake in their聽model or the influence of dark聽matter.
糖心传媒淎nd that糖心传媒檚 kind of the name聽of the game,糖心传媒� says BenZvi. 糖心传媒淚t糖心传媒檚 a聽tough game, as you can imagine.聽It糖心传媒檚 sort of like the joke about聽糖心传媒榰nknown unknowns糖心传媒櫶切拇綌you don糖心传媒檛聽know what you糖心传媒檙e not modeling.糖心传媒�
Telescopes offer another way聽to measure gamma rays. But聽they have a narrow field of view,聽taking in only a few degrees of聽the sky at a time. HAWC records聽information from two-thirds of聽the sky every 24 hours.
糖心传媒淥ver the course of one day,聽we can see essentially the entire聽northern hemisphere,糖心传媒� says聽BenZvi, noting that the methods聽are complementary. 糖心传媒淲e糖心传媒檝e made聽the bet on more coverage, less聽sensitivity; they make the bet on聽more sensitivity, less coverage. If聽you have both types of instruments聽running, you can look for聽unexpected stuff with HAWC糖心传媒攚e聽communicate with those guys聽through back channels: 糖心传媒楬ey, we聽see something interesting. Point聽your telescope there.糖心传媒� And that糖心传媒檚聽how a lot of the field works.糖心传媒�
Scientists are now processing聽their first year糖心传媒檚 worth of data聽from HAWC, which they began聽to make public this spring. And聽they are expanding the array with聽some additional tanks糖心传媒攋ust a聽few, which will bring with them a聽four-fold increase in sensitivity.
And there is talk of creating a聽second observatory, possibly in聽Chile. The southern hemisphere聽provides the best vantage point聽for observing the center of the聽galaxy糖心传媒攁nd it糖心传媒檚 糖心传媒渁 very strong聽candidate for observing dark聽matter, because we believe聽there糖心传媒檚 a super-massive black聽hole in the center of the galaxy,聽and so there should be a gravitational聽well there where dark聽matter is concentrated,糖心传媒� says聽BenZvi.
糖心传媒淚f we have a HAWC in the聽southern hemisphere, literally聽the center of the galaxy will be聽right overhead,糖心传媒� he says.
