The troubled American neutrino project is facing an uncertain future – and new opportunities News and research

Engineers are blasting and boring their way through a number of tunnels in the abandoned Homestake gold mine in South Dakota, preparing for the installation of the Deep Underground Neutrino Experiment (DUNE), America’s latest, largest particulate matter project. But things are not going as planned: construction-related setbacks have delayed DUNE’s full-scale debut from sometime later this decade to at best the mid-2030s. And because DUNE’s schedule has slipped, so has its competitiveness with other neutrino experiments, which led the US Department of Energy (DOE) last fall to announce its controversial decision to split the mega-project into two phases.

The two-phase approach, particle physicists in the United States hope, will help this flagship project keep pace with rivals while creating respite to reshape the later stages of its design. “Real success may not be exactly the same as we thought 10 years ago,” says Kate Scholberg, an experimentalist at Duke University.

Together with the Long-Baseline Neutrino Facility (LBNF), DUNE seeks to interrogate the most elusive particles in the standard model of particle physics, which many suspect is a portal to any theory that comes next. But last year, the mega-project’s price tag was revalued to more than $ 3 billion just for the first phase – about twice as much as the original estimate for the entire endeavor.

Digging out 800,000 short tonnes of rock to make room for four real detection modules has proved more complicated than expected. “Incorrect assumptions or premature estimates” of the state of the mine meant that the infrastructure had to be renovated before excavation could begin, a spokesman for the DOE said. The cost of installing detectors was also underestimated. As a result, the involvement of LBNF / DUNE has been delayed by several years, and the project must be confirmed by Congress given the size of the budget overrun.

Now that particle physicists are gathering for Snowmass, a process of periodic meetings to spell out the next decade’s research priorities in the United States, some have speculated that LBNF / DUNE could come second, or even third, by setting some of its motivational science goals. . “How do we change the way we think about DUNE to achieve these goals and more, or make them even better?” asks Jonathan Asaadi, an experimentalist at the University of Texas at Arlington. “The whole point of the Snowmass process is to have these very tough discussions and then build consensus.”

From neutrinos to new physics

Neutrinos are without a doubt the strangest inhabitants of the enormous and varied bestiary of particle physics: They exist in three types, but somehow they oscillate between these different forms as they travel. This is surprising, since neutrinos can only oscillate if they have mass – a property that conflicts with predictions from the otherwise very successful standard model. Ever since the discovery of this shape-shifting behavior in 1998, physicists have struggled to determine exactly how neutrinos oscillate, and three missing measurements of crucial parameters remain.

The first and most well-known parameter, charge parity violation (CP), dictates whether neutrinos and their counterparts against particles oscillate in the same way, and can help explain why there is more matter than antimatter in the universe. The second determines which are the heaviest and which are the lightest types of neutrino. And the third is related to how likely it is that one type of neutrino will turn into another type. Physicists dreamed of LBNF / DUNE a decade ago, during a previous Snowmass jamboree, as a way to measure all this and more.

The motives were about the same for Japan’s competing neutrino experiment Hyper-K – an even larger underground chamber filled with water instead of DUNE’s liquid argon. Hyper-K aims to accurately measure CP violations in the late 2020s, before LBNF / DUNE’s now delayed neutrino beam would even be turned on. At the same time, combinations of several smaller neutrino experiments – namely IceCube, JUNO and KM3NeT – are expected to weigh in on these and other long-standing neutrino puzzles in the coming decade. “DUNE runs a very serious risk of not measuring these parameters first,” says Asaadi. “Historically, large projects in Japan have been able to stick to their schedule with much more fidelity than many large American projects.”

Nevertheless, Asaadi and many others emphasize that these experiments are as much partners as competitors. “We all root for each other and root for each other at the same time,” says Mark Messier, an experimentalist who worked on Super-K (Hyper-K’s predecessor) as a doctoral student and now works on DUNE and other neutrino detectors.

First, each discovery must be confirmed by another independent experiment in order to be taken seriously. “For the sake of science, we do not really care who gets the answer,” says André Luiz De Gouvêa of Northwestern University. “But you do not want a big time difference between the projects … One project can steal the thunder from another.” And while high-minded physicists may accept the importance of validation instead of being the first to make a discovery, the concept of “second place” is a more difficult sale for politicians who keep their wallets.

Once again, the new phasing of LBNF / DUNE may ultimately keep the project healthy and competitive. “You make sure you get results, rather than letting everything be delayed while you wait on a full scale,” says Gabriel Orebi Gann of the University of California, Berkeley. “The only way to guarantee you lose the race is to not even run it.”

Nevertheless, this approach comes with a new risk: funding for the second phase is not secured. “People are encouraged to think about phase two and what it brings to the table a little more carefully than before,” says de Gouvêa. “You want to make sure you have a good story to tell, to convince yourself and everyone else that Phase Two is a worthwhile investment.”

Sunk-Cost Fallacy or Golden Opportunity?

As large-scale projects such as LBNF / DUNE have increased over the past five years, Congress has increased the DOE’s total budget for high-energy physics by almost 30 percent. However, funding for nuclear research at universities and for R&D on new accelerator and detector technologies has decreased. The new phasing of LBNF / DUNE aims to rebalance the budget, said a spokesman for the DOE. “Society must really be serious about what are reasonable timelines and goals,” says Asaadi, otherwise the mega-project can “suck all the air out of the room.”

All this takes place during a turbulent period for LBNF / DUNE’s host, Fermi National Accelerator Laboratory (Fermilab). In the autumn, Fermilab’s director Nigel Lockyer resigned for unknown reasons. Shortly afterwards, Fermilab also received a rare “C” from the DOE in its report card for project management in science and technology.

Despite the uncertainty, it is still difficult for many to stick to the original scope of LBNF / DUNE. “It’s hard to imagine a single neutrinoscillation experiment that is better,” says Denton. Unlike other experiments, the neutrino beam would examine a wide range of energies and thus take a more complete picture of how neutrinos oscillate. Detectors filled with liquid argon can also track the paths of particles much more accurately than those containing water. “Liquid argon technology is difficult, but we get something for that difficulty,” says Denton. The hope is that this new and ambitious line-up will not only map high-resolution neutrino oscillations, but that signs of new particles and forces may emerge, and physicists can finally understand the puzzling origin of neutrino masses.

For these reasons, the particle physics community gathered in 2014 behind new liquid argon detectors over proven water detectors – and most neutrino physicists see no reason to change that. – There is a very strong support within society for [LBNF/DUNE] to happen, ”says Orebi Gann. Even in internal documents can be seen by Scientific Americannominated a current spokesman for DUNE in the election earlier this year on the grounds that “LBNF / DUNE is currently experiencing poor acceptance in [high-energy physics] community… seriously challenges the future of DUNE. ”

Most agree that with the billions of dollars already allocated to the mega-project – by the US and international partners – there is no turning back. “The ‘sunk cost’ error is always present when you are this far down the road,” says Asaadi. back funding after the budget increased and dubious spending on expensive parties and lunches was revealed.As a result, “particle physics moved to Europe,” says Francis Halzen, lead researcher on the IceCube neutrino experiment. not back to you, or even to science. “

Again, undoubtedly supporting a major project whose “world first” ambitions may no longer be possible also involves risks. “We are in a catch-22. Interrupting DUNE would be a black eye for the credibility of high-energy physics,” says an anonymous source and member of the DUNE collaboration. Scientific American. “We have to find a way out of this, and the way out is not obvious.”

All together now

At Snowmass meetings, a balancing act is now underway to gather particle physicists resolutely again around LBNF / DUNE, with some presenting the newly phased design as an opportunity to turn lemons into lemonade. In addition to upgrading the neutrino beam and detector size, the second phase leaves two of DUNE’s four modules undefined – and new ideas are welcome. “There is a lot of excitement about it, and many creative ideas for the new detectors,” says Scholberg, who is involved in convening the priority group for neutrino physics for the latest Snowmass, which continues into the summer.

Theoretical insights in recent years have opened the door to new types of dark matter searches at LBNF / DUNE, while those detecting detectors could also accommodate a “neutrino-free double-beta decay” experiment to look for evidence that neutrons are their own antiparticle. Improved supernova detectors and the hunt for mysterious sterile neutrinos are also on the table to fill the two open detector openings. “If you expand that collaboration [to other areas of physics]it takes in more people who can make this happen, ”says Jocelyn Monroe, an experimentalist in dark matter at Royal Holloway, University of London.

Others prefer to double DUNE’s world-best ambitions and prioritize advances in detector designs and analysis techniques that would not normally be considered in a race to come first. Many really good ideas tend to be rejected because DUNE is “on track”: it has to work, and it has to be done this way, says Asaadi. A new proposal, called THEIA, combines a water-based detector, such as Hyper-K, with a “scintillation” detector that is more akin to DUNE – to reap the benefits of both approaches.

Among physicists, there seems to be a universal agreement on one thing: the efforts to turn DUNE’s mistakes with sunk-cost into an opportunity are high. “What we’re really working very hard on right now is trying to establish all these connections with the rest of the Snowmass community,” says Scholberg. Discussions will continue over the summer and then be part of a decision by the Particle Physics Project Prioritization Panel, a federal advisory group in the United States, which next year will decide whether DUNE / LBNF remains a flagship project in the United States. “We have to make sure everyone is on board,” said de Gouvêa. “Uncertainty is bad for a very long-term project because people have to invest a large part of their time in it … You do not want the effort to be in vain.”

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