The ICARUS detector, part of Fermilab’s Short-Baseline Neutrino Program, will officially start its hunt for elusive sterile neutrinos this fall. The international collaboration led by Nobel laureate Carlo Rubbia successfully brought the detector online and is now collecting test data and making final improvements.
When teams began cooling the ICARUS neutrino detector and filling it with 760 tons of liquid argon in early 2020, few people knew how much the world would change in the two months that the fill would take.
“In an ideal world, as soon as the filling is complete and the cryogenic plant is stabilized, then we can activate the detector and start looking for particle tracks basically immediately,” said Angela Fava, the ICARUS commissioning coordinator and deputy technical coordinator.
The ICARUS collaboration includes more than 150 scientists from 23 institutions in Italy, Mexico, Switzerland and the United States. The detector is located at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, located near Chicago.
Restrictions on international travel instituted last year due to the COVID-19 pandemic meant that many European experts could not come to Fermilab in person as planned to start up the detector components. Researchers restructured their plans to get the detector up and running with much of the team working remotely.
The collaboration successfully activated ICARUS in August 2020 and recorded the first particle tracks — from cosmic rays, particles from space that constantly bombard Earth — soon after. Exposed to both the Booster and NuMI neutrino beams at Fermilab, the ICARUS detector has recorded the first muon and electron neutrinos, demonstrating the high-level detection capabilities of the liquid-argon time projection chamber technique.
The team is now working on finishing the system to identify and exclude cosmic-ray signals. They are also making final improvements to the neutrino data acquisition system to prepare the detector for its official first data collection run in fall 2021.
“We’ve been able to do our jobs with most people not moving from their local offices or homes,” said Claudio Montanari, the ICARUS technical coordinator. “Everybody contributed to the best of their ability, which was key to the success of the operation.”
Searching for stealth particles
When the ICARUS detector was originally assembled at the laboratories of the Italian National Institute for Nuclear Physics in Pavia in the early 2000s, it was the largest liquid-argon detector in the world. It began its neutrino-hunting career at Italy’s Gran Sasso National Laboratory in an experiment that ran between 2010 and 2014.
After the experiment in Italy concluded, scientists realized that the ICARUS detector could have a second life at Fermilab, searching for a new type of particle: the sterile neutrino.
Scientists already know of three types, or flavors, of neutrinos. The particles are notoriously hard to catch because they interact through only two of the four known forces: gravity and the weak force. But this potential fourth kind of neutrino — if it exists — may not even be sensitive to weak interaction, making detection even trickier. Scientists will have to look carefully at how the different flavors of neutrinos morph into one another, a phenomenon called neutrino oscillation.
Previous experiments saw hints of unusual oscillation, but researchers need more data to determine if sterile neutrinos were responsible for the results. Finding evidence of sterile neutrinos would advance scientists’ knowledge about physics beyond the Standard Model, the theoretical framework that has accurately described almost all known subatomic particle interactions for over 50 years.
To make this happen the ICARUS detector’s two school-bus-size modules were shipped from Gran Sasso to CERN for upgrades. In 2017, the two modules traveled by truck and ship to Fermilab, where they will soon begin hunting for ultra-elusive sterile neutrinos.
ICARUS is one of three particle detectors at Fermilab that will look for indicators of sterile neutrinos as part of the laboratory’s Short-Baseline Neutrino Program, along with the Short-Baseline Neutrino Detector and MicroBooNE. Together, the detectors will analyze how neutrinos oscillate as they travel along their straight beamline path through these detectors.
SBND, situated 110 meters from the start of the neutrino beamline, will provide a snapshot of the neutrinos right after they’re produced. MicroBooNE, located 360 meters farther down the beamline, will provide a second look at the beam composition. The final checkpoint is ICARUS, 600 meters from the start of the beamline. If ICARUS picks up fewer muon neutrinos and more electron neutrinos than expected based on data from SBND and MicroBooNE, “the combination of these things would be the unique signature of the oscillation and therefore of the existence of the sterile neutrino,” said Fava.
Getting ICARUS ready to search for signs of sterile neutrinos at Fermilab has involved three distinct stages: installation, activation, and commissioning. Installation started in 2018 and included set up of the vacuum chambers, insulation, cryostats and various electronics used to power the detector and collect data.
After electrical safety checks, making sure the vacuum chambers were leak-free and testing the components’ basic functionality, it was time to get the detector ready for activation. Technicians started up the filters, pumps and condensers for the cryogenic systems and began adding the liquid argon in early 2020.
Collaborators from CERN and INFN with historical knowledge of the detector were present for the beginning of the fill. They left with plans to return to Fermilab in April 2020 to help wrap up the process and see the detector through to activation. While they were unable to return in person, the group successfully coordinated with the Fermilab branch of the team to complete the activation last summer.
“We were lucky enough not to have any showstoppers,” said Montanari.
With the detector activated, the international collaboration turned its attention to debugging and optimizing the equipment. For example: To capture good neutrino data, the liquid argon inside the detector has to be ultra-pure. When researchers found the argon was less pure than expected, they traced the problem back to slow gaseous argon movement through the recirculation system and took steps to address the flow.
“That’s the life of a physicist — dealing with problems and finding a way of overcoming them,“ Fava said.
Since last year, ICARUS has been in the commissioning phase. The team is testing all of the subsystems to make sure they are in sync and calibrated to collect quality data with minimal noise before the start of official data collection.
Getting ready for takeoff
ICARUS began taking test data from the booster neutrino beam in December 2020. That data is now being used to refine the triggers for deciding what type of signal constitutes a particle “event” worthy of recording.
“The trigger system is one of the most critical components to commission, because it brings together all the other subsystems,” said Fava.
The trigger rate — how frequently the system records an event — must be finely tuned. If it’s too high, the researchers end up sifting through more data than they need to, wasting time and computing power. Too low, and they might miss recording particle interactions that are crucial to making a discovery. The team plans to test the next iteration of trigger logic in May.
In addition to refining the trigger, the ICARUS team will install a final set of cosmic-ray trackers. Roughly 10 cosmic rays hit the detector during each 1.6-millisecond time window used to record a potential neutrino interaction. The cosmic-ray trackers are used to sort out which signal is which.
“If there is an external signal and the timing is correct, we can reject that event on the basis that it was induced by a particle that was coming from outside,” said Montanari. Trackers on the bottom and sides have already been installed — all that’s needed now is to finish the top.
With everything expected to be in place this fall, the experiment will move into the next exciting stage: collecting high-quality data that will be used in scientists’ search for sterile neutrinos.
“I’m really looking forward to making a nice data analysis and seeing what nature is willing to tell us,” Montanari said.
ICARUS is supported by the U.S. Department of Energy Office of Science, the Italian National Institute for Nuclear Physics (INFN) and CERN, the European Organization for Nuclear Research.
Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC.
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