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Particles and interactions

Particles and interactions

Fermilab told to rein in planned neutrino experiment

26 Mar 2012 Hamish Johnston
Back to the drawing board

Physicists in the US working on a major new experiment to study the properties of neutrinos have suffered a major setback with the announcement that the US Department of Energy (DOE) will not fund their current plan. Known as the Long-Baseline Neutrino Experiment (LBNE), it would involve creating a beam of neutrinos at Fermilab and detecting them 1300 km away in a new detector to be built deep underground in South Dakota’s Homestake mine. But writing to Fermilab director Pier Oddone last week, William Brinkman, who is director of the DOE’s Office of Science, says that the DOE “cannot support the LBNE project as it is currently configured”.

Brinkman adds that the decision has not been based on the scientific merits of the LBNE proposal, but rather because the full cost of the project (expected to be more than $1bn) cannot be accommodated by the current US budget or by projected budgets over the next decade. The LBNE would also involve building a new neutrino detector at Fermilab, which is located just outside Chicago, and physicists had hoped to have the experiment running by 2020. The LBNE is supposed to be the successor to the NOvA experiment, which should start sending neutrinos 810 km from Fermilab to an underground detector in Minnesota in 2014.

In a separate letter to Fermilab staff, Oddone insists that the laboratory remains committed to achieving the scientific goals of the LBNE. “We will work closely with the DOE and the particle-physics community over the coming months to outline options for a phased approach to long-baseline neutrino experiments,” he says. Fermilab now plans to work with the DOE and neutrino physicists to develop a new strategy for building the LBNE.

Phased approach

Neutrinos come in three “flavours” – electron, muon and tau – with the neutrinos able to change, or “oscillate”, between the different types. However, physicists also believe that neutrinos can be described in terms of combinations of three mass states – m1, m2 and m3. Interference between these mass states gives rise to the observed oscillations of neutrino flavour – a muon neutrino could change into an electron or tau neutrino, for example, as it travels through the Earth.

Under Fermilab’s plans for the LBNE, a pure beam of muon neutrinos would be created at Fermilab, and then when reached the Homestake neutrino detector it would be a combination of all three flavours. Measuring the rates at which these oscillations occur would therefore provide new information about neutrino mixing angles. The LBNE would also allow physicists to compare the oscillation rates of neutrinos and antineutrinos, and look for differences. Any such differences would violate the Standard Model of particle physics, which incorporates a fundamental symmetry between particles and antiparticles known as CP (“charge–parity”) symmetry.

Neutrinos from elsewhere

Fermilab physicists also hope to use the underground detector to study neutrinos produced naturally in a number of places, including the Earth’s atmosphere and distant supernovae. Brinkman now suggests that, instead of building the LBNE all at once, Fermilab should take “an affordable and phased approach that will enable important science results at each phase”. He also says that the plans for Homestake facility should be revised to include the option of co-locating other physicists experiments – such as a facility for doing dark-matter experiments.

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