Fermilab Today

The scientist who first detected the neutrino called the strange new particle "the most tiny quantity of reality ever imagined by a human being." They are so absurdly small and interact with other matter so weakly that about 100 trillion of them pass unnoticed through your body every second, most of them streaming down on us from the sun.

And yet, new experiments to hunt for dark matter are becoming so sensitive that these ephemeral particles will soon show up as background. It's a phenomenon some physicists are calling the "neutrino floor," and we may reach it in as little as five years.

The neutrino floor applies only to direct detection experiments, which search for the scattering of a dark matter particle off of a nucleus. Many of these experiments look for WIMPs, or weakly interacting massive particles. If dark matter is indeed made of WIMPs, it will interact in the detector in nearly the same way as solar neutrinos.

We don't know what dark matter is made of. Experiments around the world are working toward detecting a wide range of particles.

"What's amazing is now the experimenters are trying to measure dark matter interactions that are at the same strength or even smaller than the strength of neutrino interactions," says Thomas Rizzo, a theoretical physicist at SLAC National Accelerator Laboratory. "Neutrinos hardly interact at all, and yet we're trying to measure something even weaker than that in the hunt for dark matter."

This isn't the first time the hunt for dark matter has been linked to the detection of solar neutrinos. In the 1980s, physicists stumped by what appeared to be missing solar neutrinos envisioned massive detectors that could fix the discrepancy. They eventually solved the solar neutrino problem using different methods (discovering that the neutrinos weren't missing; they were just changing as they traveled to the Earth), and instead put the technology to work hunting dark matter.

In recent years, as the dark matter program has grown in size and scope, scientists realized the neutrino floor was no longer an abstract problem for future researchers to handle. In 2009, Louis Strigari, an astrophysicist at Texas A&M University, published the first specific predictions of when detectors would reach the floor. His work was widely discussed at a 2013 planning meeting for the US particle physics community, turning the neutrino floor into an active dilemma for dark matter physicists.

"At some point these things are going to appear," Strigari says, "and the question is, how big do these detectors have to be in order for the solar neutrinos to show up?"

—Laura Dattaro

Photo of the Day

Common buckeye

The common buckeye has an uncommon beauty. Photo: Leticia Shaddix, PPD

In the News

Pentaquarks make their debut

From Scientific American, Sept. 15, 2015

A veritable zoo of never-before-seen particles, including the famed Higgs boson, was generated in recent years inside the Large Hadron Collider (LHC) at CERN near Geneva. Hiding amid the data, another new particle has recently made itself known: the pentaquark, a composite of five quarks, the fundamental bits that make up protons and neutrons. The long-awaited discovery — pentaquarks were first predicted more than 50 years ago — provides insight into how matter's building blocks stick together to organize the universe as we know it.

Frontier Science Result: MINERvA

The one-percenters and those who fake it

This is an event from the MINERvA data that shows the signal when an electron neutrino hits a neutron in MINERvA's plastic scintillator, becomes an electron and changes the neutron into a proton. The color in each triangle represents how much energy is deposited in each of MINERvA's triangular scintillator bars.

Para una versión en español, haga clic aquí. Para a versão em português, clique aqui. Pour une version en français, cliquez ici.

In today's Wine and Cheese seminar, MINERvA will present a measurement of the probability that an electron neutrino interacts with a nucleus inside the MINERvA detector and produces an electron and no other particles besides protons and neutrons. This new result is the first high-statistics measurement of this process at energies comparable to a few times the proton mass.

Neutrino beams made by accelerators produce mostly muon neutrinos, not electron neutrinos. This makes it very difficult to study electron neutrino interactions. The NuMI beam at Fermilab, which is used by MINERvA, MINOS+ and NOvA, is made up of 99 percent muon neutrinos and only 1 percent electron neutrinos. (However, it is so intense that MINERvA still records thousands of electron neutrino interactions each year.)

Because of the relative lack of electron neutrino data, simulation programs assume that the only difference in the interaction probability between muon and electron neutrinos is due to the mass of the muon or electron that is produced. Today's result is the most in-depth look at electron neutrino interaction probabilities ever, and an important check on that assumption.

When we compare our electron neutrino result to the analogous measurement for muon neutrinos that was published in 2013, we find that they are consistent with one another.

This measurement is a very important input for experiments such as NOvA, which measures the probability for muon neutrinos to change into electron neutrinos as they travel from Fermilab to Ash River, Minnesota. To do that, they need to know how many muon neutrinos are produced at Fermilab and how many electron neutrinos reach their detector in Ash River.

The number of neutrinos that reach the NOvA detector is equal to the number of interactions they see divided by the probability that the neutrino will interact. The probability for a neutrino interaction is really tiny and difficult to measure accurately. To get around that, oscillation experiments like NOvA use two detectors, one near where the beam is produced and another far away to give the neutrinos a chance to change flavor. If you take the ratio of interactions at the near and far detectors, the interaction probability cancels out as long as it is the same in both detectors.

Neutrino interactions must be the same in Illinois and in Minnesota, right? The problem is that the neutrinos detected at Fermilab are muon-type, while the far detector sees mostly electron-type neutrino interactions. This means it is important to understand any differences in the interactions between the two types of neutrino.

In the course of searching for electron neutrino interactions, we found an unexpected background of events that look more like photons than electrons but were otherwise consistent with our signal. MINERvA can separate photons and electrons well, so this background has a tiny effect on our electron neutrino measurement. This kind of event is important for oscillation experiments because muon neutrinos that produce photons can be mistaken for electron neutrinos. We have characterized these background events, and believe they are similar to what we would expect from a process called diffractive scattering, where a single neutral pion is produced by a soft collision with the hydrogen in our scintillator target. Our observation and characterization is a first step towards development of a model to predict this process in other experiments.

—Chris Marshall, University of Rochester

Jeremy Wolcott of the University of Rochester, who is neither a one-percenter nor someone trying to look like one, will describe this result at today's Wine and Cheese Seminar.