Neutrino Physics Comes to JLab

The inner workings of the sun, the mysteries of dark matter and dark energy and the structure of the early universe all may be unlocked by one cosmic key: neutrinos. Now, new research carried out in Jefferson Lab's experimental Hall C may help provide insight into neutrinos, the force that governs their behavior and, surprisingly, the structure of the nucleus of the atom.

Geoff Mills takes a break from monitoring the experiment target to explain neutrino research.

Neutrinos are ghostly particles emitted by the sun as a byproduct of the nuclear fusion that powers it. These subatomic particles zip along through space, the Earth's atmosphere, and even the planet without ever interacting with any other matter they encounter. According to Geoff Mills, a JLab user from Los Alamos National Lab, "They're like little hermits; they don't want to have anything to do with anything else. They just want to go on their merry way." That's because neutrinos are chargeless particles that only interact with matter via a force called the weak force. "It's very weak, so that's why they don't interact very often," he says.

However, sometimes neutrinos do interact with other matter in the universe. Physicists have built massive detectors to monitor these rare events in hopes of learning more about the elusive particles. Some detectors are designed to catch glimpses of neutrinos streaming out of the sun or other sources. But others monitor precise beams of neutrinos created for experiments right here on Earth.

Now neutrino physicists, some of whom are also involved in the Long Baseline Neutrino Oscillation Experiment from KEK to Kamioka (K2K) in Japan, the Main Injector Experiment v-A (MINERvA), which has received preliminary approval for running at the Fermi National Accelerator Laboratory (FermiLab), and the MiniBooNE experiment, which is currently taking data at Fermilab, recently came to Jefferson Lab to help them better analyze these experiments.

In the K2K experiment, a beam of neutrinos was sent more than 150 miles, from the KEK facility near Tsukuba, Japan to the Super-Kamiokande facility in Kamioka, Japan. This experiment was aimed at confirming neutrino oscillations first seen by the Super-Kamiokande experiment in 1998. Makoto Sakuda is a professor of physics at Okayama University and a member of the K2K and Super-Kamiokande experiment teams. "There are three types, or flavors, of neutrinos: electron-neutrino, muon-neutrino and tau-neutrino. One type of neutrino changes into another as time passes. This is called neutrino oscillation," he says.

Early neutrino experiments, designed to measure neutrinos coming from the sun, found fewer neutrinos than expected. In 1998, Super-Kamiokande experiment observed electron- and muon-neutrinos produced in the Earth's atmosphere by cosmic rays. It found that there were far fewer muon-neutrinos than expected. Some researchers theorized that perhaps they weren't seeing as many muon-neutrinos as expected because the muon-neutrinos had changed, or oscillated, into a different form before reaching the Super-Kamiokande detector.

"This can happen only when neutrinos have non-zero mass," Sakuda explains. Therefore, this discovery helped confirm that neutrinos have mass. The K2K team recently announced the observation of neutrino oscillations in the KEK beam.

In the MINERvA experiment, physicists will use a beam of protons generated with FermiLab's 120 GeV proton main injector ring to ultimately produce a beam of neutrinos. The researchers hope to learn about neutrinos by watching how the beam interacts with nuclei in the atoms of various target materials, including carbon and iron.

To help analyze the results of neutrino experiments, the researchers need to do what they call "neutrino engineering," a term coined by one of Mills' MiniBooNE colleagues, neutrino researcher Gerald Garvey. "Neutrino engineering is all the work you have to do in basic particle physics to actually do neutrino experiments well," Mills says. It turns out that neutrino experiments are very tricky to do because they contain a lot of unknown variables that can skew the results.

Richard Gran, a JLab user from the University of Washington, says researchers can minimize these variables by studying them in advance. "There're all these things we want to know. But some of it is very hard to study and some of it is very easy to study. And so, the trick is to get all the easy parts, at least from our point of view, tacked down so that you can then attack the difficult parts," Gran explains.

In the end, there are four main variables for neutrino researchers to look at: the characteristics of neutrino beams, the different ways neutrinos can interact with nuclei, how often neutrinos interact with the nuclei in one way versus another, and the physics of how neutrinos change flavor. The MINERvA team has already tackled the first variable for their beam. "I have been working in the past two years on an experiment that we did at CERN in Geneva to measure proton interactions on beryllium," Mills says. MiniBooNE will aim a beam of protons at beryllium to create a neutrino beam.

Arie Bodek watches the data-gathering process intently.

The second variable can be narrowed down here at Jefferson Lab, where researchers aim to learn how neutrinos interact with nuclei. Arie Bodek, a JLab user from the University of Rochester and an experiment spokesperson, explains, "If you want to study the properties of neutrinos, you have to understand how neutrinos interact with protons and neutrons, because that's how we detect them."

To understand that, neutrino researchers manned shifts in Hall C side-by-side with Jefferson Lab nuclear physicists in January to make precise measurements of how electrons interact with nuclei in carbon, hydrogen, deuterium and iron. Electrons interact with nuclei via the electromagnetic force. Though neutrinos interact with nuclei via a different force, the weak force, precise information on the electron interaction provides information about the neutrino interaction.

According to Mills, "It turns out that because of the unification of electromagnetism and the weak force, we understand that the force is really the same phenomenon - the electroweak force. So we can relate what we see here in electron interactions with certain nuclei directly to neutrino interactions." With the JLab results in hand, that leaves only the last two variables for neutrino physicists to deal with.

What's more, the neutrino experiments will feed back into JLab's main mission of studying the nucleus of the atom. At their core, these experiments are analyzing how neutrinos interact with nucleons (protons and neutrons) and nuclei. Bodek says, "Experimenters in nuclear physics and neutrino physics want to measure the same thing: they want to know how neutrinos interact with matter. One wants to understand the structure of the nucleon, and the other wants to know the structure of the nucleon to understand neutrinos."

Experiment Spokesperson Cynthia Keppel, a Jefferson Lab staff scientist and a university-endowed professor of physics at Hampton University, says that ultimately the research will provide extra information on the structure of nucleons that can't be obtained with research at JLab alone. "Neutrino research is complementary to Jefferson Lab physics; both involve pointlike leptons (electrons or neutrinos) scattering off of nuclear targets. Lepton scattering is what we do here at JLab - with electrons - to understand the structure of nucleons and nuclei. Because neutrinos interact only via the weak force, they can provide different information from the electrons, which interact primarily via the electromagnetic force," she explains.

Sakuda says neutrino experiments may also advance JLab's goal to go beyond the Standard Model, the theory that describes elementary particles. He says, "The discovery of finite neutrino mass is the only experimental phenomenon which contradicts the Standard Model. Thus, particle physicists expect that neutrino experiments will explore the physics beyond the standard model."

Thomas Jefferson National Accelerator Facility’s (Jefferson Lab’s) basic mission is to provide forefront scientific facilities, opportunities and leadership essential for discovering the fundamental structure of nuclear matter; to partner in industry to apply its advanced technology; and to serve the nation and its communities through education and public outreach. Jefferson Lab, located at 12000 Jefferson Avenue, is a Department of Energy Office of Science research facility managed by the Southeastern Universities Research Association.