In one second, an estimated 100 billion neutrinos pass through every square centimeter of your hand.
These subatomic particles traveling to Earth are a byproduct of nuclear reactions that occur in the sun, but their abundance has not made neutrinos any easier for scientists to understand.
Relatively straightforward questions about neutrinos - such as whether or not they have mass - are currently being addressed by researchers around the world and at Tufts, and they have important implications for the way that we understand the universe.
"Because they're the most abundant form of matter in the universe, our knowledge of their properties has important ramifications for a lot of different areas of physics, whether it's astronomy or astrophysics or cosmology," Assistant Professor of Physics and Astronomy Hugh Gallagher said.
Gallagher works in the particle physics branch of the department and does research on elements of our universe that are smaller than atoms.
Within an atom, particles called quarks make up the protons and neutrons that form the atom's nucleus. These quarks are subject to what physicists call the "strong force," which holds protons together despite their identical charge. It is one of four fundamental forces in the universe, along with the "weak force," the "electromagnetic force" and the "gravitational force."
Neutrinos do not experience the strong force, and because they carry no charge and are so light (or massless), they feel neither the electromagnetic force nor the gravitational force.
The only force that neutrinos experience is the weak force, making it difficult for scientists to conduct simple experiments and determine the mass of the particle.
"[Take] an electron for instance, or a proton: You give it some energy and you put it in a magnetic field and its trajectory is going to bend, and from how much it bends you can figure out some combination of its charge and its mass," Gallagher said. "But for particles that don't have electric charge, it gets a bit more complicated. And for particles like the neutrino that don't interact very often, it gets even more complicated."
The sort of questions that particle physicists address are part theory, part data-driven.
For instance, the answers to questions about what happened in the very early universe may hinge on neutrino properties and neutrino mass.
Many theories exist to try to answer these questions. Beyond the Standard Model of Physics, scientists are proposing the String Theory, the Grand Unified Theory and the Theory of Everything to try and tie humans' observations of the universe into one coherent explanation.
"And we're at a point right now where there are a lot of great theories out there, but we're struggling to get more and more data that allows us to say, 'These theories are right and these theories are not,'" Gallagher said.
He is part of an enormous, multinational experiment which aims to provide some of this much-needed data. Specifically, Gallagher works on the problem of neutrino mass.
By studying a phenomenon known as neutrino oscillations - when neutrino particles change what is known as their "flavor" and go from one state to another - researchers are trying to resolve the question of neutrino mass. If neutrinos do have mass, then they should be able to switch between flavors.
A large beam of neutrinos is generated at a facility called Fermilab outside of Chicago, where scientists study the particles before they are shot off to a detector in Northern Minnesota, and the beam is again monitored for signs of neutrino oscillations.
The probability of neutrino oscillations occurring increases over time, so scientists must wait a few milliseconds if they want to observe the phenomenon.
Because neutrinos travel almost at the speed of light, the detector (in Minnesota) must be set at some distance from the neutrino source.
"If you can measure how often that [neutrino oscillations] happens, you have a way of measuring what the neutrino mass is," Gallagher said.
Gallagher is also trying to understand what happens in the rare event that neutrinos interact with matter. Due to the infrequency of such interactions, it would take approximately a light year of lead to stop a beam of neutrinos.
So how do physicists study the interaction between neutrinos and the nucleus of iron atoms?
"The name of this game is getting the most intense neutrino beam that you can," Gallagher said.
Currently, research is focused on computer simulations that try to predict what neutrinos will do when they interact with matter. One of the outcomes of neutrino interaction is the scattering of particles called pions.
"In intra-nuclear re-scattering, the pions change a little bit because they're interacting so they get a little bit slower ... sometimes they change charge and sometimes they get slower and lose momentum and things like that. And sometimes they're absorbed, too," said junior and physics major Pauli Kehayias, who works in Gallagher's lab to help develop these models.
In addition to the complexity of the experiments, the generation of neutrino beams and the materials required to detect neutrino activity require huge investments of money. Not every physicist can have a particle accelerator next to his lab bench, so a unique collaborative element is introduced into the research done in particle physics.
"The experiments are expensive so the costs are shared over lots of different institutions and a lot of different countries," Gallagher said. He is currently working with scientists from the United States, England, Brazil, France and Russia.
Gallagher said that the study of particle physics today is simply the continuation of a long inquiry into our world which goes back thousands of years.
"Basically it's a tradition in physics, or in science, that goes back to the ancient Greeks and Chinese cultures trying to understand everything in the world in terms of basic kinds of objects and the way that they interact with each other," he said.
