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Underground Science: The US Effort to Create a Deep Underground Laboratory
By Wick Haxton
Schematic showing the proposed layout for NUSEL-Homestake. The main development is on the 7400 ft. level, but certain experiments (including the megadetector) will be sited along the drift at 4850 ft. The geomicrobiologists will work at 8000 ft, drilling from there to 13,500 ft, the expected limit for microbial life (120? C)
The ross head frame, Homestake Gold Mine, which provides access to the 4850 ft. level of the mine.
The effort has had its full share of color and twists, and the final scenes remain to be played out. The main stage is an historic gold mine with connections to General Custer and the birth of solar neutrino astronomy. The players include a Canadian mining company, Barrick Gold, the state of South Dakota, and a group of physicists and earth scientists who have "self organized" to preserve a unique US asset. The resulting interactions have at times produced interesting theater: in one scene reminiscent of the '60s, senior US physicists demonstrated at the mine entrance for continued operation of the pumps.
Physicists have long appreciated that extremely sensitive, low-energy experiments offer a window on subatomic physics quite complementary to that provided by accelerators. Such searches can reveal subtle violations of symmetries and conservation laws, reflecting new physics hidden at energy scales beyond direct reach. Important examples-proton decay searches to probe the stability of matter, tests of the standard-model prediction of massless neutrinos-probe phenomena at GeV, the strong and electroweak unification scale.
Sensitive experiments to measure the spectrum of solar neutrinos or the lifetime of the proton cannot be done on the earth's surface. Unless the detector is placed thousands of feet below ground, the cosmic ray background rate is too high.
Around 1980, as physicists began to contemplate a new generation of massive underground experiments, the community recognized that a new model for such experiments was needed. The parasitic mode of the past-persuading a supportive mining company to provide space and hoist access-would be increasingly problematic as the scale and complexity of experiments increased. In quick succession the USSR, Italy, and Japan established multipurpose underground laboratories. The first of these, Baksan, was created in the late 1970s as the first deep laboratory dedicated to physics.
Under the direction of Chudakov and Zatsepin, a tunnel was excavated in Mt. Andyrchi in the Caucasus. Baksan was built for the study of the penetrating components of cosmic rays-the muons and neutrinos-and was the site of the SAGE solar neutrino experiment, mounted to measure the lowest energy branch of the solar neutrino flux.
In 1981 Zichichi proposed the world's largest multipurpose facility, the Gran Sasso National Laboratory. Built as part of a highway tunneling project during the early 1980s, the laboratory provides about 3800 meters water equivalent (mwe).
Gran Sasso has hosted (or is preparing to host) the GALLEX/GNO and Borexino solar neutrino experiments, two long-baseline experiments to detect neutrinos from CERN, the MACRO cosmic ray experiment, the kiloton supernova detector LVD, and a rich program of double beta decay, dark matter, and nuclear astrophysics studies.
At about the same time Japan began an underground science program within an operating mine, Kamioka, that produced exceptional results. Kamioka housed Koshiba's proton decay experiment, which also made crucial measurements of the neutrinos from Supernova 1987A and of the atmospheric neutrino flux. Its successor, the 50-kiloton Super- Kamiokande, resolved the atmospheric neutrino puzzle, established new bounds on proton decay, and made a precise measurement of the high-energy portion of the solar neutrino flux.
While Gran Sasso was being planned, a very serious effort to create a US facility was also underway, lead by Al Mann of the University of Pennsylvania and Bob Sharpe of Los Alamos. The US, lacking the deep road and railway tunnels common in Europe, has fewer opportunities for creating such a laboratory. The Mann/Sharpe proposal, as well as one put forth by the UC Irvine group, were "greenfield" projects. The former would have created a vertical shaft at a site near Yucca Mountain, Nevada, while the latter required excavation of a tunnel beneath Mt. San Jacinto, near Palm Springs. Despite considerable advocacy by the community, neither proposal was funded. Nearly twenty years earlier Luis Alvarez, Fred Reines, Aihud Pevsner, and others had advocated a US underground laboratory.
Arguably the lack of a national laboratory has had a profound impact on US underground science. The IMB proton decay experiment, the US effort that paralleled Kamioka, provided early evidence of the atmospheric neutrino anomaly that was later traced to oscillations and the discovery of neutrino mass. Yet no US follow-up experiment was approved. The field of solar neutrinos began with the Davis experiment at the Homestake Mine in Lead, South Dakota. Although the GALLEX pilot experiment was performed at Brookhaven, the chemistry of the GALLEX and SAGE experiments largely developed in the US, and the Sudbury Neutrino Observatory (SNO), these experiments were mounted elsewhere. US scientists have played an important but supporting role as the major experiments were hosted by Kamioka, Gran Sasso, Baksan, and most recently, Sudbury.
However, a new window of opportunity may be opening for US scientists. Recent discoveries by SNO and Super-Kamiokande leave open a host of questions about neutrino mass. We know two mass splittings, but not the overall scale of the masses, a parameter crucial to cosmology and astrophysics. The surprising strength of neutrino mixing-the mass eigenstates and weak interaction eigenstates appear to differ almost maximally-opens the possibility of testing neutrino CP violation in very long baseline oscillation experiments. There is a growing suspicion that related symmetry violation-leptogenesis-may explain why our universe has an excess of matter over antimatter. The US has two major laboratories, FermiLab and Brookhaven, capable of producing the neutrino "superbeams'' needed for precision studies of neutrino parameters, including CP violation. What is required to complete the equation is a underground laboratory, 2000-4000 km away, housing a megadetector ten times the size of Super-Kamiokande.
This same detector, if placed a mile underground, could extend searches for proton decay by more than an order of magnitude. It would allow experimenters to measure in great detail neutrino emission from the next galactic supernova, determining the structure of the nascent neutron star.
We now know, from measurements of large-scale structure, the cosmic microwave background, and distant supernovae, that the evolution of our universe is governed by unseen sources of matter and energy. The cold, dark matter is many times more plentiful than the ordinary matter visible in stars and gas clouds. Its nature is unclear, and its origin is outside the standard model. Underground experiments provide our best hope for identifying this matter.
These and many other underground physics efforts?double beta decay, nuclear astrophysics, low-level counting for industry and national security-have one aspect in common: increasingly great depths are needed. While experimentalists have become very adept at eliminating many natural radioactivity backgrounds, often the only solution to cosmic ray-induced activities is great depth. Furthermore, the experiments described above are governed by a Moore's law: sensitivities increase by about a factor of two every two years. This implies a simple rule of thumb: each decade experiments must move another 1500 feet deeper, to achieve a proportional reduction in cosmic ray muon backgrounds.
The urgency of the science and the need for depth were under discussion at a community Town Meeting on neutrinos, September 2000, in Seattle, when Ken Lande of the University of Pennsylvania made an announcement and proposal. The Homestake Gold Mine, the deepest in the US, was about to cease operations, after 125 years of mining. Its massive infrastructure-hoists and shafts reaching 8000 ft below ground, phone and fiber optics communications, a sophisticated ventilation and air conditioning system, its own hydroelectric power-might soon be available to science. Lande proposed the National Underground Science and Engineering Laboratory (NUSEL), a deeper next-generation Kamioka/Gran Sasso.
This proposal became the #1 recommendation of the 200 physicists in Seattle. The National Science Foundation and Department of Energy responded by funding a community study, headed by John Bahcall, to consider creation of a deep US laboratory. The Bahcall Committee's influential report was followed by NSAC and HEPAP (High Energy Physics Advisory Panel) recommendations and by two National Research Council studies. A Homestake proposal to create NUSEL-Homestake was submitted to the NSF and has been under review for the past two years.
Two other proposals have also been made. One is a modern version of the Mt. San Jacinto proposal to create a horizontal access laboratory by tunneling. There is a strong preference in the community for horizontal access, if a good site can be found. The other proposes deepening the Soudan Laboratory.
But the path to NUSEL has been difficult. While the Homestake site was recently designated by a distinguished NSF engineering panel as the best proposal, by far, from a geotechnical standpoint (the rock integrity is outstanding, allowing even the largest suggested detector cavities to be built with confidence), the land transfer has proven complicated. South Dakota and Barrick Gold, the Homestake owner, have negotiated for over two years, without resolution. Worse, Barrick turned off the mine pumps in June, arguing that they could not accept federal funds to continue maintaining the mine without a positive NSF decision. This was done despite strong objections from the physicists and earth scientists involved in the project: in addition to physics, NUSEL will host efforts in fields like geomicrobiology. The geomicro- biologists are deeply concerned that flooding will alter biological conditions in the mine, confusing any subsequent studies they might do.
While the US process slowly moved along, a Canadian proposal to expand the SNO laboratory was written, approved, and funded. Preliminary construction is under way. The SNOLab addition is considerably smaller than NUSEL and access is shared with miners. Still, SNOLab is now the one laboratory clearly deep enough to accommodate difficult experiments, like those designed to measure the lowest energy branch of the solar neutrino flux.
Where will this lead? The last act is about to play out. The interest in the science community, in the media, and in Washington is intense. Is the ending tragic or triumphant? If we only knew what the script holds for us.
Wick Haxton is professor of physics at the University of Washington in Seattle.