In this plot, a neutrino enters from the left, interacting with a nucleus in the MINERvA detector material. The interaction results in two photons, a negatively charged muon that exits the end of the detector on the right, and a proton. This process can give neutrino oscillation experiments such as Fermilab’s NOvA and DUNE insight into two mysteries that they must solve. It is critical to determine two things about each neutrino that leaves a trace in the detector: its flavor and its energy.
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Neutral-pion production is a major character in a story of mistaken identity worthy of an Agatha Christie novel.
First, the cast of characters: Neutrinos, the quarries of the MINERvA experiment, are subtle, difficult-to-capture particles. They come in three types, two of which play a prominent role in this story: the electron neutrino and the muon neutrino. Since neutrinos are hard to capture, scientists do the next best thing: study other types of particles resulting from the neutrino’s interaction with a nucleus in the detector.
One of these resulting particles is the pion. Pions can be electrically neutral or not. In this story, we focus on neutral pions, and they often transform into two photons, particles of light.
Then there’s the electron, a particle we all know and love, and its cousin the muon. An electron neutrino leaves behind electrons, and a muon neutrino leaves behind muons.
Finally, a twist: neutrinos can switch identities. An electron neutrino can flip and become a muon neutrino and vice versa. The behavior is called oscillation.
Oscillation experiments look for electron neutrinos (which leave electrons in their wake) that were originally muon neutrinos (which produce mostly muons). If they see an electron, they know their muon neutrino has oscillated into an electron neutrino. If they see a muon, they conclude the muon neutrino did not oscillate by the time it reached them.
Every good mystery has its red herrings, and in this case of mistaken identity, the culprit is the pion: Neutral pions can disguise themselves as electrons —the telltale sign that an electron neutrino has passed through — in a particle detector. So when scientists think they have identified electrons, they might actually be looking at pions.
Ozgur Altinok (left) and Trung Le (right), both of Tufts University, led this analysis.
Why does this happen? Pions decay into two photons. Let’s say that one photon escapes detection, and the other, as often happens, looks a lot like an electron. This can trick the experiment into counting what was really the arrival of two photons as an electron sighting —therefore counting an event as the arrival of an electron neutrino when it was really a muon neutrino. So it seem like the neutrino has oscillated when, really, it has not.
To avoid such misidentification, MINERvA scientists measured how often a neutrino interaction results in a muon and a neutral pion. This measurement gives us one handle on predicting the number of cases of mistaken identity that bigger Fermilab neutrino experiments, namely NOvA and DUNE, will see.
Measuring neutral-pion production can also help us understand the relationship between the neutrino’s energy and the resulting particles we see in our detectors. One of the biggest problems in oscillation experiments occurs when a neutrino of one energy masquerades as a neutrino of another energy. This happens frequently when some of the particles get stuck inside the nucleus of the detector material or transform into other particles. MINERvA’s measurement of neutral-pion production is sensitive to these kinds of effects and helps theorists improve their models of the nucleus.
The MINERvA collaboration is excited about these new results. The protons in our detector are also excited — they reach very high energy levels in the neutrino events studied in this analysis.
For the first time ever, we have a measurement where both a neutral pion and a proton are observed. These pion-proton events tell us about the behavior of a kind of excitation that occurs inside the nucleus. These measurements also contain information about the energy and angles of both the neutral pion and the muon that goes with it.
This level of detail is important both for oscillation experiments and for improving our theoretical models.
The MINERvA collaboration recently submitted for publication a paper on measurements of neutral pion production by neutrinos. This result was presented at the Fermilab wine and cheese seminar on July 7 by Ozgur Altinok of Tufts University, an analysis he performed along with Trung Le, also of Tufts.
Barbara Yaeggy is a physicist at Federico Santa María Technical University in Chile.
This article was translated from the Spanish by physicist Chris Marshall of Lawrence Berkeley National Laboratory.
In September, lab employees move to Weston, Colorado bison move to Illinois, and Nigel Lockyer moves to the United States. Read on for more September historical milestones.
This is the first lab bison, in 1970.
In September 1969, the first bison arrived on the lab site. The first six animals were purchased by the lab from a herd in Colorado, and the next year a larger group arrived from a herd in Illinois. The bison were part of Director Robert R. Wilson’s vision of the lab as a physical and intellectual frontier and contributed to his goal of restoring some of the the native ecosystems of the area.
Nigel Lockyer became Fermilab’s sixth director on Sept. 3, 2013. Lockyer spent 22 years as a researcher on Fermilab’s CDF experiment and served as the co-spokesperson from 2002 to 2004. He also won the 2006 W.K.H. Panofsky Prize. When he became Fermilab’s director, he was director of Canada’s TRIUMF Laboratory.
A patient sits in the Neutron Therapy Facility treatment room.
Director Robert Wilson is sometimes known as the “father of proton therapy” for his 1946 paper “Radiological Use of Fast Protons,” which first suggested the idea of using protons for medical treatment. Wilson helped organize Fermilab’s Cancer Therapy Facility, as it was originally known, using funding from the National Cancer Institute and beam from Fermilab’s Linac. The Neutron Therapy Facility treated its first patient on Sept. 7, 1976. In 1989, Fermilab would design and build a proton accelerator for Loma Linda University Medical Center in California, the first hospital-based proton treatment center.
The Dark Energy Camera took this photo.
Fermilab is a member of the Dark Energy Survey, an international collaboration formed in 2003 that uses a camera mounted on the Blanco Telescope in Chile to study the effects of dark energy. On Sept. 12, the Dark Energy Survey’s Dark Energy Camera received first light.
This building received its name in 1980.
Shortly after Fermilab’s first director, Robert R. Wilson, retired, the lab renamed the Central Laboratory Building to Wilson Hall in his honor.
Flag raising in front of the director’s office in Weston on Sept. 24, 1968.
NAL employees started moving to the Weston site during the summer of 1968 and completed the move at the end of September.
Fermilab’s first three directors were present for the dedication. From left: John Peoples, Leon Lederman and Robert Wilson.
Fermilab’s Lederman Science Education Center was named in honor of the lab’s second director, Leon Lederman, in recognition of his work to improve science education.
Left: The Bubble Chamber is under construction. Right: This picture shows an example of bubble chamber tracks.
Bubble chambers detect particles by recording tracks of bubbles the particles leave in liquid, and Fermilab was home to the largest liquid-hydrogen bubble chamber in the world. The 15-Foot Bubble Chamber photographed its first particle trails on Sept. 29, 1973. It would record its last tracks on Feb. 1, 1988.
Helen Edwards, one of the designers of the Tevatron, helps shut it down.
After the more powerful Large Hadron Collider at CERN began operating in 2008, Fermilab retired the Tevatron.
Paintbrush
Back in 1972 or so, while I was a graduate student at Cornell University, fellow graduate student Steve Herb joined Fermilab experiment E-26. On one of his regular visits back to Cornell, Steve said that he was called into Fermilab Director Bob Wilson’s office. “What for?” I asked. Apparently Fermilab artist Angela Gonzales was not very happy about the green color that Steve had painted his toroid magnets.
Steve said that he could remove the windings that they had laboriously placed on the steel toroids, repaint and restore the windings. However, that would probably kink the windings, which would require the purchase of new copper cables. So he asked Bob whether he should do that.
Steve said that Bob replied something like, “Oh, it’s OK. But next time, before you pick up a paint brush, check with Angela first.”
These are the magnets from E-26, which were mispainted green. Photo: Fermilab
Viewpoint
In June 1978, Fermilab Director Bob Wilson’s sculpture “Broken Symmetry” was erected at the Pine Street entrance to Fermilab. I had been away on the day that it was erected, so later that evening my wife Jean and I walked in from our nearby home in Batavia to check it out.
Just as we approached “Broken Symmetry” from the west, Bob Wilson drove in from the east. He asked, “What do you think?” I said, “Pretty interesting … why did you paint it black?” Bob replied, “It’s not black, it’s orange.” Startled, I said “Say what?” Bob replied, “Come over here,” at which point I realized what he had done. I guess you just shouldn’t ask an artist “Why?”.
“Broken Symmetry” is black on one side … Photo: Reidar Hahn
… and orange on the other. Photo: Reidar Hahn
“Broken Symmetry” was under construction in 1978. Photo: Fermilab
A paint job
In 1983, as we prepared for the first beam from the Fermilab Tevatron, we refurbished and upgraded all of our facilities in the Fixed Target Experimental Areas. Although the Tevatron was to be capable of running at an energy of 800 GeV, the first few months of operations were to be limited to 400 GeV in order to finish off some experiments that were not to be upgraded to 800-GeV capability. I added a second dipole magnet to the single-dipole magnet in the Proton East primary beamline (my version of the Energy Doubler). In November 1983, a colleague was trying to tune the first Tevatron 400-GeV beam through these two dipoles in Proton East, without luck. The beam somehow got lost in those 40 feet of magnets. I walked into the control room and, using the beam loss monitors, was able to thread the beam through the two dipoles, but at 50 percent greater electrical current than expected. This indicated some sort of electrical short, which meant lower magnetic field and less bending. Since we had designed for 800-GeV operations, this extra current was within our tuning range at 400 GeV, at least for the short term.
During the next interruption of accelerator operations, that typically occur while commissioning a new machine, I accessed the Proton East beamline to see if I could determine what the problem was. Everything looked normal until I noticed that that the beam pipe on the upstream B2 dipole was a little wider (5 inches) than for the downstream B2 dipole (4 inches). “What the…!?!” One of those baby blue (NAL Blue Light) dipole magnets was really a B1 dipole, which should have been dark blue (NAL Blue Dark). The inner coils of the B1 and B2 dipoles are wired exactly opposite, so when the electricians saw the baby blue color, they had hooked up the B1 magnet as if it were a B2 magnet, thereby setting them up so that the two outer coils produced magnetic fields pointing upwards and the one inner coil produced a magnetic field pointing downward. This reduced the net field in that magnet, requiring more current to get the required beam bending. Once this was realized, it was an easy fix to reconfigure the connections for proper operation. With a Sharpie pen, I wrote on that miscolored magnet, “This is really a B1 magnet.” Yes, I should have noticed that error before beam arrived, but that level of subtlety was a little beyond normal.
The next workday, I discussed this with the engineer in charge of the beamline installations who said that earlier, in the shop, they had given a summer student a can of baby blue paint and told him to “paint those magnets,” not realizing that there was a B1 in among the B2s.
You’d be hard-pressed to tell the difference between these two types of magnets (B1 upper, B2 lower), which were both painted light blue, installed in the beam tunnel simply from the widths of their beam pipes.