Physics poster sessions: a heady mix of science, networking and robots. That last one isn’t so common at in-person science conferences, but it was a regular sight at the recently concluded Neutrino 2020 conference, where virtual reality poster sessions hosted thousands of robotic avatars belonging to particle physicists from around the world. While most walked through the virtual world as a robot in vaguely human clothing, others donned panda suits, uploaded custom skins with gigantic human heads or particle tracks or—in one case—turned themselves into a jar of Jif peanut butter.

“It’s both more weird and more fun than I expected,” says Diana Parno, a physicist at Carnegie Mellon University who presented in one of the virtual poster sessions. “And one of the nice things about poster sessions is meeting with new people and colleagues, and making introductions—and we’ve still been able to do that.”

The 29th International Conference on Neutrino Physics is the largest gathering of neutrino scientists in the world. This year the conference was hosted jointly by the Department of Energy’s Fermilab and the University of Minnesota and, for the first time, held entirely online. The change brought many advantages to the conference. Notably, more than 3400 people from 67 countries and covering all seven continents tuned in to the 79 talks – more than four times the typical number of attendees. (Almost half of attendees were early in their career, identifying as undergraduate or graduate students.) Instead of full days of talks, half-days were spread over two weeks, allowing greater involvement from participants in different time zones. Going virtual also meant some individuals with monetary restrictions, travel restrictions, or obligations at home or work could participate.

The decision to move online about two months before the event also brought challenges. Organizers were faced with quickly transitioning one of the most valuable parts of the conference: the networking, serendipitous interactions and other engagement that happens outside of the scheduled talks. That’s where virtual reality poster sessions came in.

“What we really wanted from this platform is a way for people to interact with others as if they were in person,” says Marco Del Tutto, a Fermilab physicist who created the virtual poster sessions using open source software from Mozilla Hubs. “It’s hard to tell if this is going to work until you try. It made me proud when I went into a poster session and saw people explaining their poster to a crowd, or walking through a room and running into people.”

Over the course of four sessions, presenters showed off 532 posters on display in 30 different virtual reality rooms. (A standard list of posters and short videos from the presenters were also available on a webpage). While in-person posters are typically set up in one concentrated area, spreading the posters across virtual rooms gave avatars more space – and reduced the amount of bandwidth to run each interactive area. It also gave Del Tutto a chance to add a neutrino twist. Four different styles of room represented four different sources of neutrinos: Fermilab’s iconic bison and Wilson Hall were the backdrop for accelerator neutrinos, a nuclear power plant represented reactor neutrinos, an enormous sun hung low in the sky for solar neutrinos and an exploding star overhead for supernova neutrinos. To complete the conference vibe, there were tables, couches and cookies scattered around.

Participants could talk in real time, their avatars’ heads rapidly bouncing around. Directional sound in headphones gave a sense of where different people were in the room and allowed users to stumble upon conversations. Walking around, one overheard all the usual topics of discussion at a physics conference—results, experimental status, careers, life updates—as well as discussion of the oddities of virtual environment—the digital bison standing nearby, how to make your avatar wear a tuxedo, the underlying technology and the inevitable glitch here or there.

“It’s clearly the kind of thing that will get better and better,” says Kate Scholberg, a neutrino physicist at Duke University and a member of the International Advisory Committee for the conference. She also gathered together about 20 members of the COHERENT collaboration for an avatar group photo in Grant Park. “It’s different, and I think it’s been really fun. I’ve been having conversations with people, and it replicates many of the in-person aspects.”

Del Tutto and the organizers also set up social rooms for researchers to hang out and chat, as well as virtual tours of Fermilab locations and spots in downtown Chicago.

“The other part of a conference is that, if it’s really in Chicago, it’s a chance to see the city with your colleagues, get to know them better and improve your working relationship with them,” Del Tutto says.

Of course, virtual reality was not the only way to interact with colleagues. Attendees also connected through chat on a dedicated Neutrino 2020 Slack account. Organizers set up channels for subsets of neutrino research (such as sterile neutrinos or long-baseline neutrinos) where attendees could drop in additional questions for the speakers and discuss ideas from the talks. There were also specific channels for topics such as the future of conferences, posters, help from the organizers, virtual reality and job opportunities. The Neutrino 2020 Slack saw more than 23,000 messages over the two-week conference.

“We had to change our plans quickly and explore creative options for making the conference as interactive as possible,” says Tanaz Mohayai, a Fermilab neutrino physicist and the webpage lead for Neutrino 2020. “It’s been wonderful to see the excitement surrounding the latest results in our field, as well as the many meaningful conversations taking place on Slack and in the poster sessions in the virtual world.”

Many attendees expect to see the virtual reality and chat features of Neutrino 2020 replicated in future online physics conferences – but it may not stop there. As conferences return to having an in-person component, there’s potential to keep the best of what works from the online world. One can envision a hybrid conference that allows for participation from around the world, virtual interactions and greater discussion through chat. And maybe, just maybe, there will be people walking around poster sessions dressed as robots—or a jar of peanut butter.

The poster portal, videos and recordings of plenary talks will continue to be available online at the Neutrino 2020 website.

Editor's note: A version of this article was previously published by Fermilab.

It started with a blast.

On June 23, construction company Kiewit Alberici Joint Venture set off explosives 3,650 feet beneath the surface in Lead, South Dakota, to begin creating space for the international Deep Underground Neutrino Experiment, hosted by the Department of Energy’s Fermilab.

The blast is the start of underground excavation activity for the experiment, known as DUNE, and the infrastructure that powers and houses it, called the Long-Baseline Neutrino Facility, or LBNF.

Situated a mile deep in South Dakota rock at the Sanford Underground Research Facility, DUNE’s giant particle detector will track the behavior of fleeting particles called neutrinos. The plan for the next three years, is that workers will blast and drill to remove 800,000 tons of rock to make a home for the gigantic detector and its support systems.

“The start of underground blasting for these early excavation activities marks not only the initiation of the next major phase of this work, but significant progress on the construction already under way to prepare the site for the experiment,” says Fermilab Deputy Director for LBNF/DUNE-US Chris Mossey.

The excavation work begins with removing 3,000 tons of rock 3,650 feet below ground. This initial step carves out a station for a massive drill whose bore is as wide as a car is long, about four meters.

The machine will help create a 1,200-foot ventilation shaft down to what will be the much larger cavern for the DUNE particle detector and associated infrastructure. There, 4,850 feet below the surface—about 1.5 kilometers deep—the LBNF project will remove hundreds of thousands of tons of rock, roughly the weight of eight aircraft carriers.

The emptied space will eventually be filled with DUNE’s enormous and sophisticated detector, a neutrino hunter looking for interactions from one of the universe’s most elusive particles. Researchers will send an intense beam of neutrinos from Fermilab in Illinois to the underground detector in South Dakota—straight through the earth, no tunnel necessary—and measure how the particles change their identities. What they learn may answer one of the biggest questions in physics: Why does matter exist instead of nothing at all?

“The worldwide particle physics community is preparing in various ways for the day DUNE comes online, and this week, we take the material step of excavating rock to support the detector,” says DUNE co-spokesperson Stefan Söldner-Rembold of the University of Manchester. “It’s a wonderful example of collaboration: While excavation takes place in South Dakota, DUNE partners around the globe are designing and building the parts for the DUNE detector.”

A number of science experiments already take data at Sanford Underground Research Facility, but no activity takes place at the 3650 level. With nothing and no one in the vicinity, the initial excavation stage to create the cavern for the drill proceeds in an isolated environment. It’s also an opportunity for the LBNF construction project to gather information about matters such as air flow and the rock’s particular response to the drill-and-blast technique before moving on to the larger excavation at the 4850 level, where the experiment will be built.

“It was important for us to develop a plan that would allow the LBNF excavation to go forward without disrupting the experiments already going on in other parts of the 4850 level,” says Fermilab Long-Baseline Neutrino Facility Far-Site Conventional Facilities Manager Joshua Willhite. Following a period of excavation at the 3650 level, the project will initiate excavation at the 4850 level.

Every bit of the 800,000 tons of rock dislodged by the underground drill-and-blast operation must eventually be transported a mile back up to the surface. There, a conveyor is being built to transport the crushed rock over a stretch of 4,200 feet for final deposit in the Open Cut, an enormous open pit mining area excavated in the 1980s. As large as the LBNF excavation will be, the rock moved to the surface and deposited in the Open Cut will only fill less than one percent of it.

Excavation at the 3650 level will be completed over the next few months, with blasting at the 4850 level planned to begin immediately after.

Editor's note: A version of this article was originally published by Fermilab.

At the beginning of the 20th century, physicists were aware of a pervasive shower of particles that seemed to rain down from space. By filling glass chambers with highly condensed vapor, they could indirectly see tracks left by these highly energetic particles now known as cosmic rays. In doing so, they quickly discovered the subatomic world was more complex than initially suspected. 

The first new matter particle they discovered was the muon. It was a lot like an electron, just more massive. At first, no one knew what to make of it. 

Some thought it might be a particle theorized to hold protons and neutrons together in an atom. But a pair of Italians conducting experiments in Rome during World War II proved otherwise. 

After discarding a few alternative theories—including one that posited that this particle might be a new kind of electron—physicists were left with one conclusion: They had discovered a particle that nobody had predicted. As Nobel Laureate I.I. Rabi famously quipped, “Who ordered that?”

Although scientists hadn’t realized muons would be on the menu, the discovery of muons eventually led to a discovery about how that menu was set up: Particles can come in different versions, each alike in charge, spin and interactions but different in mass. The muon, for example, has the same charge, spin and electroweak interactions as the electron, but is about 200 times heavier, and there’s an even heavier version of the electron and muon, called the tau.  

Physicists built on this principle to predict the existence of generations of other particles, such as neutrinos, which with electrons, muons and taus round out the set of particles called leptons. Eventually, scientists would find that all of the matter particles in the Standard Model, including quarks, could be organized into three generations, though only the lightest are stable. 

Muons continue to be useful tools for discovery to this day. Two international experiments, one currently underway and the other slated to begin in the early 2020s, are using the previously perplexing particles to push the boundaries of physics.

Flavor physics and the Mu2e experiment

Each of the three generations is called a different “flavor” of particle. 

At first, scientists assumed that flavor was a property that, like mass or energy, had to be conserved when particles interacted with each other. That wasn’t quite right, but in their defense, they did find this to be true almost all of the time.

“When you have some kind of an interaction that involves charged leptons, such as nuclear or particle decay or some type of high-energy particle interaction, the number of a given flavor of charged leptons remains the same,” says Jim Miller, a professor of physics at Boston University. 

When muons decay, for example, they transform into an electron, an anti-electron neutrino, and a muon neutrino. The electron and anti-electron neutrino cancel each other out, flavor-wise, leaving just the muon neutrino, which has the same flavor as the original muon.

Flavor conservation was useful; it allowed physicists to predict the interactions they would observe in particle accelerators and nuclear reactions. And those predictions proved to be correct. 

But then physicists discovered that the group of (uncharged lepton) particles called neutrinos are unaware they are expected to follow the rules. On their long journey to Earth from the center of the sun, where they are created in fusion reactions, neutrinos freely oscillate between generations, transforming from electron neutrinos to muon neutrinos to tau neutrinos and back without releasing any additional particles.

This phenomenon, which won researchers Takaaki Kajita and Arthur B. McDonald the Nobel Prize for Physics in 2015, left scientists with a question: If neutrinos could violate flavor conservation, could other particles do it, too?

Physicists hope to answer that exact question with Mu2e, an experiment scheduled to start generating data in the next few years at the US Department of Energy’s Fermi National Accelerator Laboratory. The experiment is supported by funding from DOE’s Office of Science. 

Mu2e will search for muons converting into electrons without releasing other particles, a process that would clearly violate flavor conservation. 

But why use muons? It’s because they’re the just-right middle of the lepton family. Not too big or too small, muons are a sort of Goldilocks particle that are perfectly suited to aid physicists in their search for new physics.

Electrons, the least massive charged leptons, are small and stable. Taus, the most massive ones, are so massive and short-lived that they decay far too quickly for physicists to effectively study. Muons, however, are massive enough to decay but not massive enough to decay too quickly, making them the perfect tool in the search for new physics.

In the Mu2e experiment, physicists will accelerate a beam of low-energy muons toward a target made of aluminum. In the resulting collisions, muons will knock electrons out of their orbits around the aluminum nuclei and take their place, creating muonic atoms for a brief moment in time.

“Since the mass of the muon is 200 times greater than the mass of the electron, and its average distance from the nucleus is 200 times smaller, there’s an overlap between the muon’s position and the position of the aluminum nucleus, allowing them to interact,” Miller says.

As the muon decays into an electron, physicists predict that the extra energy that usually goes into creating two neutrinos in a typical muon decay will instead be transferred to the atom’s nucleus. This would allow the conversion from one flavor to another, muon to electron, without any neutrinos or antineutrinos to provide balance. If observed, this direct transition of a muon into an electron would be the hoped-for discovery of flavor violation among charged leptons.

Magnetic moment of fame

Mu2e is not the only experiment that will use muons to test our understanding of physics.

Eight years before the discovery of muons, physicist Paul Dirac was developing a theory to describe the motion of electrons. In a single, elegant equation, Dirac successfully described that motion—while simultaneously merging Albert Einstein’s special theory of relativity with quantum mechanics and predicting the existence of antimatter. 

It’s hard to overstate how important and incredibly accurate Dirac’s equation turned out to be. Physicists still act giddy whenever it’s mentioned. 

To understand why it’s important, take a look at the electron.

Dirac’s equation correctly described exactly how the electromagnetic force worked and gave the correct estimate for how an electron’s spin would shift—or “precess”—if placed in a magnetic field, a measurement known as g. (That prediction was later refined through calculations from the field of quantum electrodynamics.)

When muons were discovered in 1936, Dirac’s equation was used to calculate what their precession rate would be as well. The value g for muons was predicted to be equal to 2. 

But when physicists began generating muons in accelerators at CERN in the 1950s to test his predictions, the results were not quite what they expected. Had they found a discrepancy between observation and theory? Although physicists worked hard for the next 20 years, they couldn’t generate enough energy with their accelerators to obtain a conclusive answer.

Scientists at Brookhaven National Laboratory were able to test Dirac’s prediction at higher energies between 1999 and 2001 with an experiment meant to directly determine the anomalous part of the magnetic moment called Muon g-2 (pronounced “Muon g minus 2”). They found hints of the same anomalous measurement, but even with their improved technology, they lacked sufficient precision to prove a disagreement with theory.

Could Dirac’s equation turn out to be wrong? Physicists think it could be that their findings in muons are actually hinting at a deeper structure in physics that has yet to be discovered and that studying muons could once again lead to new revelations.

“The g-2 factor has been measured for other particles,” says Fermilab physicist Tammy Walton.  “It’s been very precisely measured for the electron. It’s also been measured for composite particles, like the proton and neutron. But the large mass of muons make them more sensitive to new physics.”

Fermilab recently began the next generation Muon g-2 experiment, which physicists hope along with J-PARC in Japan will unequivocally confirm whether or not theory agrees with nature. Funded by the DOE's Office of Science, the experiment at Fermilab has been taking data since 2017.

“We hope to get 20 times the number of muons, giving us a fourfold reduction in statistical uncertainty,” says Erik Swanson, a research engineer at the University of Washington. “If our central value stays the same as that generated at Brookhaven, then we will have confirmed without a doubt the discrepancy between theory and observation. Otherwise it might just be that theory was right all along.”

If the theory is broken, physicists will have a lot of explaining to do, which could lead them to a new understanding of the particles and forces that make up our universe and the forces that govern them. Not bad work for a particle nobody ordered.