The decision to develop the LBNF/DUNE megaproject in two parts would add to the already exorbitant expenditures.
Due to cost overruns, the Department of Energy (DOE) has opted to develop the United States’ next big particle physics experiment in two stages, officials said this month. The decision means that the megaproject, which is actually two intertwined efforts called the Long-Baseline Neutrino Facility (LBNF) and the Deep Underground Neutrino Experiment (DUNE), will not be completed to its original specifications until the mid-2030s, rather than late this decade, and that its already exorbitant cost will rise even further.
The revised design, however, should see a slimmed-down version of LBNF/DUNE operating as early as 2029, according to the researchers. With that timeline, the experiment still has a chance to beat a rival in Japan called Hyper-Kamiokande (Hyper-K) to a key neutrino measurement, according to Jim Siegrist, DOE’s associate director for high energy physics, who laid out the plan via video on March 16 during the Snowmass process, a community planning exercise. “DOE feels LBNF/DUNE still has a chance to come first,” Siegrist stated in an email.
According to Regina Rameika, a neutrino physicist at Fermi National Accelerator Laboratory (Fermilab) and co-spokesperson for the 1400-member DUNE team, the change in strategy was recommended by physicists working in the project. “We’re all on the same page when it comes to what comes first and what comes second,” she explains. However, the compressed timeframe poses a hurdle, according to Inés Gil-Botella, a DUNE physicist at Spain’s Center for Energy, Environmental, and Technological Research. “We need to work harder to minimise any delays between phases one and two as much as feasible,” she adds. The updated plan makes it clear that the current $3 billion cost estimate, which is roughly double the original, now only pertains to the first phase.
Neutrinos, which are almost massless and seldom interact with other matter, are classified into three categories based on how they are produced: electrons, muons, and taus. As the particles move at near–light speed, one kind might change into another. To investigate such neutrino oscillations, researchers may direct a beam of muon neutrinos produced by a particle accelerator toward a massive detector hundreds of kilometers distant, which counts the incoming muon neutrinos as well as electron neutrinos that have emerged along the route.
The LBNF/DUNE experiment would send a powerful beam of muon neutrinos from Fermilab to a detector 1300 kilometers distant at the abandoned Homestake gold mine in Lead, South Dakota. The revolutionary DUNE detector, filled with cold liquid argon, would record neutrino-atomic nuclei collisions in unparalleled detail. It would not only put scientists’ hypothesis of neutrino oscillations to the test, but it would also look for charge-parity (CP) violation, an imbalance between neutrino and antineutrino oscillations that might help explain how the cosmos generated more matter than antimatter.
After bickering for a decade, physicists in the United States coalesced behind the Snowmass experiment in 2013. A year later, the DOE’s Particle Physics Project Prioritization Panel (P5) designated it as the new national flagship program. DOE accepted the P5 concept in 2015, envisioning a globally sponsored experiment with a liquid argon detector with a target mass of at least 40,000 tons, a neutrino beam fueled by a multimegawatt proton beam, and a smaller detector at Fermilab to monitor the outgoing beam. DOE anticipated that it would cost less than $1.9 billion and be finished by 2028. However, mine renovation and excavation have increased the cost and slowed development.
Siegrist says the project will now start modest, with a detector in South Dakota holding just two of four planned argon modules, with a total target mass of little more than 24,000 tons. Fermilab’s proton beam power will be restricted to 1.2 megawatts, and the detector will be simpler. Following the completion of the first phase, DOE might pursue the larger detector and better beam in an upgrade that would most certainly cost hundreds of millions of dollars.
LBNF/DUNE should be able to measure CP violation with only the initial phase. However, scientists with Japan’s Hyper-K claim they will begin collecting data in 2028, a year before LBNF/DUNE. Hyper-K will be a 70-meter-tall underground cylindrical tank filled with 260,000 tons of ultrapure water. Approximately 40,000 phototubes will be used to photograph the different flashes of light produced as neutrinos launched from the Japan Proton Accelerator Research Complex in Tokai, 295 kilometers to the east, contact with the water.
Excavation has already started in Japan, according to Matthew Malek, a Hyper-K physicist at the University of Sheffield. Given Japan’s rather cautious attitude to scheduling, he believes that date will hold. “At this point, there’s almost no possibility that DUNE will activate before Hyper-K,” he explains. But Marvin Marshak, a DUNE physicist at the University of Minnesota, Twin Cities, isn’t going to give up the race just yet. “One thing I’ve learned in the 50-plus years I’ve been doing this is not to overestimate your competitors,” he adds.
Hyper-K and DUNE will also look for neutrinos emitted by supernova explosions as well as evidence that protons may decay. DUNE’s ultimate objective, according to Gil-Botella, is to test physicists’ concept of three neutrino kinds with enough accuracy to see whether it breaks down at some level, indicating the existence of additional particles and forces. She claims that Hyper-K will struggle to do so due to technological constraints.
DOE has requested physicists to examine the scientific argument for the second phase of the new Snowmass process and future P5 priority, perhaps providing naysayers with an opportunity to challenge whether the whole project is worth the additional expense. Tao Han, a theorist at the University of Pittsburgh who has been active in Snowmass organization, thinks that US physicists will continue to support it. “We will have community support to achieve our ultimate aim,” he adds.
Malek wonders whether the new plan is DOE’s way of suggesting that the first stage would be the only one built. “When you create anything in phases, the chances of getting through the first stage are more improbable than likely,” he explains. According to Mary Bishai, a DUNE scientist at Brookhaven National Laboratory, “Jim Siegrist replied, ‘Go to Snowmass and come back with a design for the third and fourth detector modules.'” He never said, ‘We’re done.'”
The problem arises at a critical juncture in the program’s history in the United States. Fermilab, the United States’ sole specialized particle physics lab, is looking for a new director after Nigel Lockyer announced his resignation in September 2021, three months before DOE failed the facility in its annual performance evaluation, citing, in part, the problems with LBNF/DUNE. Siegrist, 69, will retire on March 31 after managing DOE’s high energy physics program for ten years. As particle physics in the United States enters turbulent seas, it is uncertain who will take the ship.