Symmetry gets a new look

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.”

The Nobel Prize-winning discovery of neutrino oscillations by the Super-Kamiokande and SNO experiments put a big crack in the highly successful Standard Model of elementary particles and their forces. The historic discovery showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe, and many questions remain: Why are neutrinos so much lighter than all other matter particles? How do neutrinos get their mass? What is the neutrino mass ordering? How are neutrinos related to dark matter? Do neutrinos and antineutrinos behave differently? And, ultimately, scientists want to know: Are neutrinos the reason matter exists?

Experiments at Fermilab and other laboratories are investigating neutrino oscillations in detail to discover the physics beyond the Standard Model. Using neutrinos created by particle accelerators and nuclear reactors, scientists make measurements that go beyond the original neutrino oscillation results based on cosmic and solar neutrinos.

At Fermilab, the Main Injector Neutrino Oscillation Search began taking data in 2005. A year later, the first MINOS result corroborated earlier observations of muon neutrino disappearance, made by the Japanese Super-Kamiokande and K2K experiments. In the following years, MINOS made detailed measurements of neutrino oscillation parameters.

The NOvA experiment at Fermilab, which began taking data in 2014, released its first neutrino oscillation results earlier this year. While researchers know that neutrinos come in three types, they don’t know which is the heaviest and which is the lightest. Figuring out this ordering — one of the goals of the NOvA experiment — would be a great litmus test for theories about how the neutrino gets its mass. While the famed Higgs boson helps explain how some particles obtain their masses, scientists don’t know yet how it is connected to neutrinos, if at all. The measurement of the neutrino mass hierarchy is also crucial information for neutrino experiments trying to see if the neutrino is its own antiparticle.

The planned Deep Underground Neutrino Experiment aims to determine whether neutrinos could be the reason that matter exists. It will study neutrinos as they pass 800 miles through the earth and measure whether neutrinos adn antineutrinos behave differently, and help unravel the mystery of why the early universe created more matter than antimatter. The DUNE detectors also will look for neutrinos from a core-collapse supernova. The data would allow scientists to peer inside a newly formed neutron star and potentially witness the birth of a black hole. About 800 scientists from 26 countries are collaborating on DUNE.

The long-baseline neutrino experiments at Fermilab are complemented by a suite of other neutrino experiments dedicated to the search for additional types of neutrinos and studying their interactions with matter.

Fermilab’s research on neutrinos is as old as the lab itself. Its first experiment, E1A, was designed to study the weak interaction using neutrinos and was one of the first experiments to see evidence of the weak neutral current. (See the CERN Courier for more details.)

While much of the progress of particle physics has come by making proton beams of higher and higher energies, the most recent progress at Fermilab has come from making neutrino beams of high intensity. Fermilab’s flagship accelerator recently set a high-energy neutrino beam world record when it reached 521 kilowatts. The laboratory is working on improving neutrino beam intensities even further for NOvA, DUNE and other neutrino experiments.