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The Sanford Underground Research Facility at Homestake–an Opportunity for the United States to Lead Profound Physics Experiments

By Kevin T. Lesko

The Department of Energy’s (DOE) Office of Science has the opportunity to enhance the prospects for major scientific discoveries in the US in the coming decade by supporting underground physics experiments that will profoundly advance our understanding of the physical universe.  

Last December, the National Science Board (NSB), in its role as the oversight body for the National Science Foundation (NSF), unexpectedly decided to deny further NSF funding for the Deep Underground Science and Engineering Laboratory (DUSEL) [1] As it did so, the NSB nevertheless expressed its interest in the scientific programs moving forward. [2,3]

These programs have been thoroughly vetted by the High Energy Physics (HEP) and Nuclear Physics (NP) communities and are essential elements to advance these disciplines. The US and international communities have been actively engaged with the DUSEL Project team. With NSF and DOE sponsorship, about 25 collaborations with over 700 researchers are developing experiments. The DUSEL Project, including NSF and DOE, have spent ten years forging the path for creating these experiments and providing the facilities necessary to lead the worldwide effort.

The DUSEL Project and the entire US underground-science community are hopeful for a successful evolution in the DOE and NSF stewardship of these efforts. DOE leadership will empower the HEP and NP communities to not just participate in, but lead world-class experiments. We applaud the Office of Science for their leadership in seeking solutions in the midst of such uncertain funding times.  

Following the NSB’s decision, three significant events occurred: 1) DOE established a committee to assess options for underground physics experiments–efforts that were underway: the Long Baseline Neutrino program (LBNE), searches for Dark Matter (DM), and for Neutrinoless Double Beta Decay (0νββ); 2) The DUSEL Preliminary Design was completed; and 3) the National Research Council’s report assessing DUSEL’s science opportunities is anticipated shortly [4].  

Fortunately there is a path forward that preserves US leadership roles, leverages the existing efforts designing the facility, capitalizes on South Dakota’s inspirational investments, and maintains the existing momentum.

In February, William Brinkman, Director of DOE’s Office of Science, announced the formation of the Independent Review of Options for Underground Science Committee to assess the costs, as well as siting and staging alternatives to achieve cost-effective options for implementing a world-class program of underground science [5].

The Compelling Science has been Identified and Prioritized
In 2008 the Particle Physics Project Prioritization Panel Roadmap of the HEP Advisory Panel (HEPAP) and in 2007 the Nuclear Science Advisory Committee Long Range Plan assessed these high priority research topics as the DOE and NSF jointly pursued concepts for an underground facility. The full suite of scientific experiments has been critically assessed by APS reports, including The Neutrino Matrix (2004); National Academy Reports–Connecting Quarks to the Cosmos (2002), Neutrinos and Beyond (2003); NSF reports–Deep Science (2007); and Joint DOE and NSF scientific assessments–Discovering the Quantum Universe (2004), the Dark Matter Scientific Assessment Group (2007), and the Particle Astrophysics Scientific Assessment Group (2009). Internationally, underground physics is the focus of the OECD Global Science Forum in its Report of the Working Group on Astroparticle Physics (2011). In 2010 the DUSEL Program Advisory Committee summarized: [6]

We are impressed by the breadth and depth of the DUSEL science. The envisioned program in physics and astrophysics will address fundamental questions about the Universe and its fundamental laws, such as the question of why the universe contains matter but no antimatter, the nature of dark matter, the origin of neutrino mass, and the genesis of the chemical elements. … In addition, the Committee felt that the interdisciplinary laboratory, with sustained support, will provide unique scientific opportunities that engage and educate the next generation of scientists and engineers.

Direct Detection of Dark Matter: There is compelling evidence that most of the matter in the universe consists of non-Standard Model particles subject to gravitational forces. This material directly influences large-scale cosmology, galactic formation, and evolution, and provides convincing evidence for new physics beyond the Standard Model. DM experiments have made impressive advances in sensitivity, pursuing multiple technologies and techniques. The Sanford Underground Research Facility is poised to provide excellent facilities for DM programs on a competitive time scale. The rock is an order of magnitude lower in U and Th than found at other proposed sites. A DM experiment is currently being deployed at Homestake and Generation-2 (~ 1 tonne) experiments can be installed in advance of new construction to support the Generation-3 (~ 10 tonne) experiments. These experiments are necessary to complement the LHC experiments in seeking to identify DM and capitalize on the current US leadership.

Neutrinoless Double-Beta Decay Searches: Much of underground physics focuses on completing our understanding of neutrino properties. While oscillation experiments have presented compelling evidence that neutrinos oscillate between massive families, there remain significant challenges to completing our understanding: the absolute neutrino mass, the ordering of the neutrino families (mass hierarchy), the full mixing matrix describing the oscillations among the three families, and possible charge and parity (CP) symmetry violating phases and/or Majorana phases. The 0νββ experiments will address the determination of the absolute neutrino mass, mass hierarchy, and Majorana phases which would indicate that the neutrino is its own antiparticle. When coupled with other experiments, even null 0νββ results are extremely valuable. The deployment of tonne-scale experiments would: capitalize on the US investment while developing this essential component of the US program; exploit the unique opportunity for a low-background experiment; and launch a world-leading effort with high discovery potential.

Long Baseline Neutrino Experiment and Proton Decay Searches: There is an abundance of evidence that neutrinos oscillate among the three known flavors ν_e, ν_μ, ν_τ, indicating that they have masses and mix with one another. Indeed, modulo an anomaly in the LSND and MiniBooNE experiments, which report excess candidates, all observed neutrino oscillation phenomena are well described by 3-generation mixing, which is described by two mass squared differences Δm²₁₂, Δm²₂₃ three mixing angles (θ₁₂, θ₂₃, θ₁₃) and a phase (δ_CP). As yet, the sign of Δm²₂₃ is undetermined. Resolving the sign of the mass hierarchy is an extremely important issue. Δm²₁₂ is large enough, compared, to Δm²₂₃, to make long baseline neutrino oscillation searches for CP violation feasible and likely to yield positive results. Currently, we know nothing about the value of δ_CP and only have an upper bound on θ₁₃: sin²2θ₁₃< 0.2. Knowledge of θ₁₃ and δ would complete our determination of the lepton-mixing matrix and provide a measure of leptonic CP violation. LBNE provides the clear path to obtaining the best sensitivity to all these parameters. NB: on 15 June the T2K collaboration reported the observation of six electron-like events. At 90% C.L., the data are consistent with 0.03(0.04)< sin2 2θ13 < 0.28(0.34) for δCP = 0 and normal (inverted) hierarchy. While requiring additional confirmation, T2K’s measurements of a relatively large θ13 indicate that the LBNE goals are well within the capability of the experiment design parameters. [7]

CP violation has only been observed in the quark sector. Discovery in leptons should shed light on the role of CP violation in nature. Most important, unveiling leptonic CP violation is compelling because of its potential connection with the observed matter-antimatter asymmetry of our universe, a fundamental problem at the heart of our existence. These studies will provide additional, sensitive probes for “New Physics” deviations from 3-generation oscillations.

LBNE’s large detector offers a rich field of physics discovery by pushing the limits on proton decay into modes suggested by supersymmetric GUTS. Establishing baryon-number conservation violation would have profound implications for cosmology and particle physics. The same detector can pursue astrophysical neutrino observations including measurements of supernova neutrinos.

To update the science the NSF and DOE requested a National Research Council assessment. This assessment and the Office of Science Report are anticipated by the end of June (shortly after APS News goes to press).

Facility Preliminary Design
The DUSEL Project completed its Preliminary Design in March. The design proposes the former Homestake Mine as the site. The Berkeley team is a collaborative effort working with South Dakota government and university entities. South Dakota established its Science and Technology Authority (SDSTA) to facilitate the development of DUSEL and to advance higher education and technology activities.

The SDSTA received title to the site in 2006. The property includes 186 surface and >7,000 subsurface acres with 600 km of existing shafts and tunnels. The SDSTA, using a HUD grant, state funding, and $70M of philanthropic funding, stabilized and re-established access to the underground, and re-established pumping of the accumulated underground water. The impacts of flooding the 4850 ft. level have been mitigated. Significant infrastructure and safety enhancements have been installed. The Davis Laboratory, which housed Davis’ Nobel Prize-winning solar neutrino experiment, has been expanded and a new hall excavated. Both are being outfitted to support physics experiments.

Geotechnical investigations affirm that the 100 ktonne cavity design poses few problems. Recent analyses indicate that 200 kt class excavations are well within existing excavation and ground support technologies.

The Facility Design was critically reviewed by a 23-member committee, who report: The costs are mature, well supported and well documented. Many sections of the report are well beyond the Preliminary Design Report (PDR) stage. There is a strong core team that understands the issues and knows how to address them. The project continued to do very high quality work on the PDR, despite the very chaotic environment and reductions in staff. The PDR is of very high quality and is at the level expected for a CD-2 review in the DOE system.

Advancing Underground Research with DOE Leadership
The Project team, working from the PDR, created reduced-scope options for consideration by the Office of Science Committee. One option for the newly named Sanford Underground Research Facility at Homestake (SURF) is shown in Figure 1. The designs are tailored to the DOE’s science goals, while maintaining flexibility to develop new areas. The options accommodate LBNE’s Water Cherenkov and/or Liquid Argon (LAr) detectors at the 4850 ft. level, and/or shallower depths for the LAr. In addition to LBNE’s detectors we propose a laboratory module capable of supporting two to three experiments. The 4850 ft. level option proposes experiments share a 100m long module, while at the 7400 ft. level we propose a 75 m long module. An independent construction management firm validated cost and schedule estimates. These options support all of DOE’s world-leading science programs in a single facility, while maintaining on-going efforts in DM and 0νββ:  LUX and MAJORANA DEMONSTRATOR in the Davis Campus. The aggressive schedule is supported by the state’s efforts. The design benefits from the substantial synergisms of a single site with coordinated design, construction, and operations.  

The transition to DOE leadership introduces the opportunity for additional funding paths for the variety of project scales which may benefit the science. Experiments can develop and be integrated as their plans sufficiently mature, rather than being funded “all at once” as required by an NSF Major Research Equipment and
Facility Construction account Project.  

While some efforts more clearly align with one agency, all involve both DOE- and NSF-supported scientists. It is among our highest priorities to maintain NSF’s engagement. Through the involvement of both agencies we foresee the greatest benefits to the physics program, the maximum realization of synergistic benefits, and greatest reduction of overall costs through the increased sharing of facilities.

We strongly encourage the DOE to assume leadership of SURF, to work with the scientific collaborations to maintain NSF’s engagement in the science, and to take advantage of the existing Project Team to produce a facility that will propel the US into world-leadership with efforts in neutrino studies and dark matter searches. All the essential elements for success exist now.

Kevin T. Lesko of UC Berkeley is the DUSEL Principal Investigator.

(1) http://news.sciencemag.org/scienceinsider/2011/05/chu-calls-nsfs-decision-to-aband.html?ref=ra (2) http://www.aps.org/publications/apsnews/201101/nsffunding.cfm (3) http://sites.nationalacademies.org/BPA/BPA_060036 (4)http://lbne2-docdb.fnal.gov/0035/003501/001/DUSEL_Charge_Letter.pdf (5)http://sites.nationalacademies.org/BPA/BPA_058955 (6) http://www.dusel.org/html/pac.html (7) http://arxiv.org/abs/1106.2822