The particle physics of you

“It’s nine years since we proposed, designed, built, assembled and commissioned this experiment,” said Bonnie Fleming, MicroBooNE co-spokesperson and a professor of physics at Yale University. “That kind of investment makes seeing first neutrinos incredible.”

After months of hard work and improvements by the Fermilab Booster team, on Oct. 15, the Fermilab accelerator complex began delivering protons, which are used to make neutrinos, to one of the laboratory’s newest neutrino experiments, MicroBooNE. After the beam was turned on, scientists analyzed the data recorded by MicroBooNE’s particle detector to find evidence of its first neutrino interactions.

“This was a big team effort,” said Anne Schukraft, Fermilab postdoc working on MicroBooNE. “More than 100 people have been working very hard to make this happen. It’s exciting to see the first neutrinos.”

MicroBooNE is funded by the U.S. Department of Energy. Its detector is a liquid-argon time projection chamber. It resembles a silo lying on its side, but instead of grain, it’s filled with 170 tons of liquid argon.

Liquid argon is 40 percent denser than water, and hence neutrinos are more likely to interact with it. When an accelerator-born neutrino hits the nucleus of an argon atom in the detector, its collision creates a spray of subatomic particle debris. Tracking these particles allows scientists to reveal the type and properties of the neutrino that produced them.

Neutrinos have recently received quite a bit of media attention. The 2015 Nobel Prize in physics was awarded for neutrino oscillations, a phenomenon that is of great importance to the field of elementary particle physics. Intense activity is under way worldwide to capture neutrinos and examine their behavior of transforming from one type to another.

MicroBooNE is an example of a new liquid-argon detector being developed to further probe this phenomenon while reconstructing the particle tracks emerging from neutrino collisions as finely detailed three-dimensional images. Its findings will be relevant for the forthcoming Deep Underground Neutrino Experiment, known as DUNE, which plans to examine neutrino transitions over longer distances and a much broader energy range. Scientists are also using MicroBooNE as an R&D platform for the large DUNE liquid-argon detectors.

“Future neutrino experiments will use this technology,” said Sam Zeller, Fermilab physicist and MicroBooNE co-spokesperson. “We’re learning a lot from this detector. It’s important not just for us, but for the entire neutrino community.”

In August, MicroBooNE saw its first cosmic ray events, recording the tracks of cosmic ray muons. The recent neutrino sighting brings MicroBooNE researchers much closer to one of their scientific goals, determining whether the excess of low-energy events observed in a previous Fermilab experiment was the footprint of a sterile neutrino or a new type of background.

Before they can do that, however, MicroBooNE will have to collect data for several years.

During this time, MicroBooNE will also be the first liquid-argon detector to measure neutrino interactions from a beam of such low energy. At less than 800 MeV (megaelectronvolts), this beam produces the lowest-energy neutrinos to be observed with a liquid-argon detector yet.

MicroBooNE is part of Fermilab’s Short-Baseline Neutrino program, and scientists will eventually add two more detectors (ICARUS and the Short-Baseline Near Detector) to its neutrino beamline.

The MicroBooNE collaboration includes more than 150 scientists from 28 institutions in five countries.

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. Visit Fermilab’s website at www.fnal.gov, and follow us on Twitter at @Fermilab.

The DOE 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, please visit science.energy.gov.

Physicists looked at gobs of data on planetary orbits to look for tiny anomalies that couldn’t be explained by either Isaac Newton’s theory of gravity — in which gravity is a force between objects that depends on their masses — or Einstein’s general relativity theory, which says gravity is a warping of space-time itself.

In a landmark study, scientists at Delft University of Technology in the Netherlands reported that they had conducted an experiment that they say proved one of the most fundamental claims of quantum theory — that objects separated by great distance can instantaneously affect each other’s behavior.

The finding is another blow to one of the bedrock principles of standard physics known as “locality,” which states that an object is directly influenced only by its immediate surroundings. The Delft study, published Wednesday in the journal Nature, lends further credence to an idea that Einstein famously rejected. He said quantum theory necessitated “spooky action at a distance,” and he refused to accept the notion that the universe could behave in such a strange and apparently random fashion.

Editor’s note: The following news release about the proposed LBNF/DUNE project was issued by the U.S. Department of Energy and is being hosted on the Fermilab website on its behalf.

A U.S. Department of Energy (DOE) environmental study has determined that building and operating the proposed Long Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) will not have a significant impact on the environment. LBNF/DUNE would help to advance our understanding of the basic physics of the elementary particles called neutrinos and thereby help us to understand the physical nature of our universe.

DOE is following the National Environmental Policy Act of 1969, which requires that the environmental impacts of any federal project must be studied. DOE explored a number of potential impacts in the draft Environmental Assessment (EA), including impacts on people and the environment, from building and operating the research machine. Impact to floodplains and wetlands were also considered. None were considered major, so a Finding of No Significant Impact (FONSI) was issued. Additionally, a Programmatic Agreement (PA), prepared pursuant to the National Historic Preservation Act would put procedures in place to ensure the protection of historic properties. Copies of the final EA and FONSI are available.

DOE started the environmental study on the LBNF/DUNE project by holding informational meetings for the public in Illinois and South Dakota. A draft EA was then prepared and DOE once again conducted public meetings to hear comments on that document. An existing high-energy particle accelerator at Fermilab in Batavia, Illinois will generate neutrinos, analyze them, and then direct them at a detector to be constructed 800 miles away about a mile below ground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota.

Above-ground and below-ground facilities will be constructed at both locations, although at SURF nearly all construction and operations will be underground. In South Dakota, construction for the project could begin in 2017, if funding becomes available, with construction in Illinois to follow in subsequent years.

People interested in reviewing, downloading or requesting a copy of the final EA, which also includes the public comments, DOE responses, the FONSI, and the PA, should visit this url: http://lbnf.fnal.gov/env-assessment.html

For information on the LBNF/DUNE activities, contact:

Mr. Michael J. Weis, Manager
U.S. Department of Energy, Fermi Site Office
P.O. Box 2000, Batavia, IL 60510
Telephone: 630-840-3281
e-mail: michael.weis@science.doe.gov

For general information concerning DOE’s NEPA process, contact:

Mr. Peter R. Siebach, NEPA Compliance Officer
U.S. Department of Energy (STS)
9800 S. Cass Avenue, Argonne, IL 60439
Telephone: 630-252-2007
e-mail: peter.siebach@science.doe.gov.

DOE support of this project comes from its Office of Science’s Office of High Energy Physics.

The search for physics beyond the Standard Model involves kidnapping. Seeking a never-before-seen phenomenon — the direct conversion of a muon into an electron — the forthcoming Mu2e experiment at Fermilab will kidnap muons and trap them in aluminum atoms. But what exactly happens when you shoot a muon at an aluminum foil?

While Mu2e is under construction, its scientists are already getting some valuable answers from a smaller accomplice: AlCap.

AlCap’s name is a smashup of “aluminum capture,” the kidnapping process the experiment studies. Hosted at the Paul Scherrer Institute in Switzerland, AlCap is currently measuring muon interactions in aluminum. It is a joint collaboration of members from both Mu2e and COMET, a muon experiment in Japan.

“It’s fostered cooperation, and there’s been some exchange of information,” said Jim Miller, Mu2e co-spokesperson, AlCap collaborator and professor at Boston University.

With a relatively small crew, all AlCap members have their hands in almost every part of the experiment, creating an excellent learning environment for younger members of the team.

“We started AlCap with a really green crew. In our first run, which was in December of 2013, we had to set up the entire experiment and do it in a period of about six weeks,” Miller said. We had an eager but inexperienced group of graduate students. They’d never been in a situation like that before, but they really learned a lot in a hurry, and now they’re experienced people.”

One of those students, John Quirk, is writing his thesis with AlCap data. Quirk, a research assistant at Boston University, has been involved in every step of AlCap, from preparation to data analysis. So are postdoctoral research scientists Anthony Palladino, at Boston University, and Andy Edmonds, at Lawrence Berkeley National Laboratory. All three of these early-career scientists also work on Mu2e with Miller.

In both AlCap and Mu2e, researchers shoot muons — a heavier cousin of the electron — at thin sheets of aluminum. Since a negatively charged muon is attracted to a positively charged aluminum nucleus, it will begin to orbit the aluminum nucleus as if it were an electron. AlCap shoots and stops a single muon at a time; Mu2e will stop about 10 billion muons every second.

Once it’s inside an aluminum atom, a muon can do one of three things. It might decay into an electron and two neutrinos, a common, pretty well-understood, process.

Or an aluminum atom’s nucleus could capture the muon — nuclear capture. In nuclear capture, a muon changes one proton in the aluminum’s nucleus into a neutron and produces a neutrino. AlCap is primarily investigating this process because it’s a potential source of background for Mu2e.

Scientists predict that a muon could do a third thing, something that’s never been seen before: convert directly into an electron without making any neutrinos. Mu2e seeks this rare event.

The electron produced directly from a muon captured by aluminum would have a telltale signature of 105 million electronvolts of energy. Some theorists predict this phenomenon happens frequently enough to be seen with a very sensitive detector. If the Mu2e detectors catch such signal electrons, it will be the discovery of physics beyond the Standard Model.

“The problem is that along with this signal electron, which may or may not be there, will come tons of protons and neutrons as other muons interact with aluminum nuclei during nuclear capture. These protons, for example, make very strong hits in the tracker that can blind us to looking for the signal,” Quirk said.

“Other backgrounds won’t blind the detector but could have such similar energies that they may mimic the 105-MeV signal,” Palladino said.

To ensure they’ll be able to pick out a 105-MeV electron from background, Mu2e researchers will use AlCap data to familiarize themselves with the pesky protons and other signal-fogging byproducts of nuclear capture, a process that is not well understood.

“We’re trying to update the numbers and get a better measurement with AlCap, because this process hasn’t been measured on aluminum for 40 years,” Edmonds said.

AlCap’s last data-taking run is this November. Scientists hope the data will also help determine the best way to count the number of muons stopped in Mu2e. When aluminum stops a muon, the interaction emits photons of different frequencies. AlCap hopes to figure out which frequency of photons Mu2e researchers should count as a proxy for the number of stopped muons.

Although the Mu2e crew is eager to find evidence of physics beyond the Standard Model, not finding any signal electrons may be just as exciting.

“That would slash away huge chunks of some of these beyond-the-Standard-Model models. Some models it will probably eliminate completely,” Miller said. “That will still be very interesting.”