From DOE’s Thomas Jefferson National Accelerator Facility (US): “Electrons Set the Stage for Neutrino Experiments”

From DOE’s Thomas Jefferson National Accelerator Facility (US)


Early-career nuclear physicists show that a better understanding of how neutrinos interact with matter is needed to make the most of upcoming experiments.

Neutrinos may be the key to finally solving a mystery of the origins of our matter-dominated universe, and preparations for two major, billion-dollar experiments are underway to reveal the particles’ secrets. Now, a team of nuclear physicists have turned to the humble electron to provide insight for how these experiments can better prepare to capture critical information. Their research, which was carried out at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility and recently published in Nature, reveals that major updates to neutrino models are needed for the experiments to achieve high-precision results.

Neutrinos are ubiquitous, generated in copious numbers by stars throughout our universe. Though prevalent, these shy particles rarely interact with matter, making them very difficult to study.

“There is this phenomenon of neutrinos changing from one type to another, and this phenomenon is called neutrino oscillation. It’s interesting to study this phenomenon, because it is not well understood,” said Mariana Khachatryan, a co-lead author on the study who was a graduate student at Old Dominion University (US) in Professor and Eminent Scholar Larry Weinstein’s research group when she contributed to the research. She is now a postdoctoral research associate at The Florida International University (US).

One way to study neutrino oscillation is to build gigantic, ultra-sensitive detectors to measure neutrinos deep underground. The detectors typically contain dense materials with large nuclei, so neutrinos are more likely to interact with them. Such interactions trigger a cascade of other particles that are recorded by the detectors. Physicists can use that data to tease out information about the neutrinos.

“The way that neutrino physicists are doing that is by measuring all particles coming out of the interaction of neutrinos with nuclei and reconstructing the incoming neutrino energy to learn more about the neutrino, its oscillations, and to measure them very, very precisely,” explained Adi Ashkenazi. Ashkenazi is the study’s contact author who worked on this project as a research scholar in Professor Or Hen’s research group at The Massachusetts Institute of Technology (US). She is now a senior lecturer at Tel Aviv University [ אוּנִיבֶרְסִיטַת תֵּל אָבִיב ](IL).

“The detectors are made of heavy nuclei, and the interactions of neutrinos with these nuclei are actually very complicated interactions,” Ashkenazi said. “Those neutrino energy reconstruction methods are still very challenging, and it is our work to improve the models we use to describe them.”

These methods include modeling the interactions with a theoretical simulation called GENIE, allowing physicists to infer the energies of the incoming neutrinos. GENIE is an amalgam of many models that each help physicists reproduce certain aspects of interactions between neutrinos and nuclei. Since so little is known about neutrinos, it’s difficult to directly test GENIE to ensure it will produce both accurate and high-precision results from the new data that will be provided by future neutrino experiments, such as the Deep Underground Neutrino Experiment (DUNE) or Hyper-Kamiokande.

FNAL DUNE LBNF (US) from FNAL to Sanford Underground Research Facility, Lead, South Dakota, USA.

FNAL DUNE LBNF (US) Caverns at Sanford Underground Research Facility.

Hyper-Kamiokande [(神岡宇宙素粒子研究施設](JP) a neutrino physics laboratory to be located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan.

To test GENIE, the team turned to a humble particle that nuclear physicists know a lot more about: the electron.

“This exploits the similarities between electrons and neutrinos. We are using electron studies to validate neutrino-nucleus interaction models,” said Khachatryan.

Neutrinos and electrons have many things in common. They both belong to the subatomic particle family called leptons, so they are both elementary particles that aren’t affected by the strong force.

In this study, the team used an electron-scattering version of GENIE, dubbed e-GENIE, to test the same incoming energy reconstruction algorithms that neutrino researchers will use. Instead of using neutrinos, they used recent electron results.

“Electrons have been studied for years, and the beams of the electrons have very precise energies,” said Ashkenazi. “We know their energies. And when we are trying to reconstruct that incoming energy, we can compare that to what we know. We can test how well our methods work for various energies, which is something you can’t do with neutrinos.”

The input data for the study came from experiments conducted with the CLAS detector at Jefferson Lab’s Continuous Electron Beam Accelerator Facility [below], a DOE user facility. CEBAF is the world’s most advanced electron accelerator for probing the nature of matter. The team used data that directly mirrored the simplest case to be studied in neutrino experiments: interactions that produced an electron and a proton (vs. a muon and a proton) from nuclei of helium, carbon and iron. These nuclei are similar to materials used in neutrino experiment detectors.

Further, the group worked to ensure that the electron version of GENIE was as parallel as possible to the neutrino version.

“We used the exact same simulation as used by neutrino experiments, and we used the same corrections,” explained Afroditi Papadopoulou, co-lead author on the study and a graduate student at MIT who is also in Hen’s research group. “If the model doesn’t work for electrons, where we are talking about the most simplified case, it will never work for neutrinos.”

Even in this simplest case, accurate modeling is crucial, because raw data from electron-nucleus interactions typically reconstruct to the correct incoming electron beam energy less than half the time. A good model can account for this effect and correct the data.

However, when GENIE was used to model these data events, it performed even worse.

“This can bias the neutrino oscillation results. Our simulations must be able to reproduce our electron data with its known beam energies before we can trust they will be accurate in neutrino experiments,” said Papadopoulou.

Khachatryan agreed.

“The result is actually to point out that there are aspects of these energy reconstruction methods and models that need to be improved,” said Khachatryan. “It also shows a pathway to achieve this for future experiments.”

The next step for this research is to test specific target nuclei of interest to neutrino researchers and at a broader spectrum of incoming electron energies. Having these specific results for direct comparison will assist neutrino researchers in fine-tuning their models.

According to the study team, the aim is to achieve broad agreement between data and models, which will help ensure DUNE and Hyper-Kamiokande can achieve their expected high-precision results.

See the full article here .


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DOE’s Thomas Jefferson National Accelerator Facility (US) is supported by the Office of Science of the U.S. Department of Energy. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit

Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE Applied Technologies, manages and operates the Thomas Jefferson National Accelerator Facility for the U.S. Department of Energy’s Office of Science.


DOE’s Thomas Jefferson National Accelerator Facility(US) was established in 1984 (first initial funding by DOE, Department of Energy) as the Continuous Electron Beam Accelerator Facility (CEBAF); the name was changed to Thomas Jefferson National Accelerator Facility in 1996. The full funding for construction was appropriated by US Congress in 1986 and on February 13, 1987, the construction of the main component, the CEBAF accelerator begun. First beam was delivered to experimental area on 1 July 1994. The design energy of 4 GeV for the beam was achieved during the year 1995. The laboratory dedication took place 24 May 1996 (at this event the name was also changed). Full initial operations with all three initial experiment areas online at the design energy was achieved on June 19, 1998. On August 6, 2000 the CEBAF reached “enhanced design energy” of 6 GeV. In 2001, plans for an energy upgrade to 12 GeV electron beam and plans to construct a fourth experimental hall area started. The plans progressed through various DOE Critical Decision-stages in the 2000s decade, with the final DOE acceptance in 2008 and the construction on the 12 GeV upgrade beginning in 2009. May 18, 2012 the original 6 GeV CEBAF accelerator shut down for the replacement of the accelerator components for the 12 GeV upgrade. 178 experiments were completed with the original CEBAF.

In addition to the accelerator, the laboratory has housed and continues to house a free electron laser (FEL) instrument. The construction of the FEL started 11 June 1996. It achieved first light on June 17, 1998. Since then, the FEL has been upgraded numerous times, increasing its power and capabilities substantially.

Jefferson Lab was also involved in the construction of the Spallation Neutron Source (SNS) at DOE’s Oak Ridge National Laboratory (US). Jefferson built the SNS superconducting accelerator and helium refrigeration system. The accelerator components were designed and produced 2000–2005.


The laboratory’s main research facility is the CEBAF accelerator, which consists of a polarized electron source and injector and a pair of superconducting RF linear accelerators that are 7/8-mile (1400 m) in length and connected to each other by two arc sections that contain steering magnets.

As the electron beam makes up to five successive orbits, its energy is increased up to a maximum of 6 GeV (the original CEBAF machine worked first in 1995 at the design energy of 4 GeV before reaching “enhanced design energy” of 6 GeV in 2000; since then the facility has been upgraded into 12 GeV energy). This leads to a design that appears similar to a racetrack when compared to the classical ring-shaped accelerators found at sites such as European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire](CH) or DOE’s Fermi National Accelerator Laboratory(US). Effectively, CEBAF is a linear accelerator, similar to DOE’s SLAC National Accelerator Laboratory (US) at Stanford University (US), that has been folded up to a tenth of its normal length.

The design of CEBAF allows the electron beam to be continuous rather than the pulsed beam typical of ring shaped accelerators. (There is some beam structure, but the pulses are very much shorter and closer together.) The electron beam is directed onto three potential targets (see below). One of the distinguishing features of Jefferson Lab is the continuous nature of the electron beam, with a bunch length of less than 1 picosecond. Another is Jefferson Lab’s use of superconducting Radio Frequency (SRF) technology, which uses liquid helium to cool niobium to approximately 4 K (−452.5 °F), removing electrical resistance and allowing the most efficient transfer of energy to an electron. To achieve this, Jefferson Lab houses the world’s largest liquid helium refrigerator, and it was one of the first large-scale implementations of SRF technology. The accelerator is built 8 meters below the Earth’s surface, or approximately 25 feet, and the walls of the accelerator tunnels are 2 feet thick.

The beam ends in four experimental halls, labelled Hall A, Hall B, Hall C, and Hall D. Each hall contains specialized spectrometers to record the products of collisions between the electron beam or with real photons and a stationary target. This allows physicists to study the structure of the atomic nucleus, specifically the interaction of the quarks that make up protons and neutrons of the nucleus.

With each revolution around the accelerator, the beam passes through each of the two LINAC accelerators, but through a different set of bending magnets in semi-circular arcs at the ends of the linacs. The electrons make up to five passes through the linear accelerators.

When a nucleus in the target is hit by an electron from the beam, an “interaction”, or “event”, occurs, scattering particles into the hall. Each hall contains an array of particle detectors that track the physical properties of the particles produced by the event. The detectors generate electrical pulses that are converted into digital values by analog-to-digital converters (ADCs), time to digital converters (TDCs) and pulse counters (scalers).

This digital data is gathered and stored so that the physicist can later analyze the data and reconstruct the physics that occurred. The system of electronics and computers that perform this task is called a data acquisition system.

12 GeV upgrade

As of June 2010, construction began on a $338 million upgrade to add an end station, Hall D, on the opposite end of the accelerator from the other three halls, as well as to double beam energy to 12 GeV. Concurrently, an addition to the Test Lab, (where the SRF cavities used in CEBAF and other accelerators used worldwide are manufactured) was constructed.

As of May 2014, the upgrade achieved a new record for beam energy, at 10.5 GeV, delivering beam to Hall D.

As of December 2016, the CEBAF accelerator delivered full-energy electrons as part of commissioning activities for the ongoing 12 GeV Upgrade project. Operators of the Continuous Electron Beam Accelerator Facility delivered the first batch of 12 GeV electrons (12.065 Giga electron Volts) to its newest experimental hall complex, Hall D.

In September 2017, the official notification from the DOE of the formal approval of the 12 GeV upgrade project completion and start of operations was issued. By spring 2018, all fours research areas were successfully receiving beam and performing experiments. On 2 May 2018 the CEBAF 12 GeV Upgrade Dedication Ceremony took place.

As of December 2018, the CEBAF accelerator delivered electron beams to all four experimental halls simultaneously for physics-quality production running.