A New Mexico State University researcher will send a scientific balloon aloft
on a mission next spring that could help unravel some of the mystery of
neutrinos — wispy subatomic particles that are passing through your body by
the billions as you read this.

Steve Stochaj, director of NMSU’s Particle Astrophysics Laboratory, has been
awarded a National Science Foundation grant of about $535,000 for the project,
which will provide scientists with new data in their quest to understand
neutrinos and their role in the cosmos.

“Of all the subatomic particles, neutrinos are among the most interesting,”
Stochaj said. “They don’t behave the way you would expect.”

Scientists know that neutrinos are produced in a specific kind of particle
interaction called “weak interaction.” The decay of a variety of subatomic
particles — muons, pions kaons, etc. — produce neutrinos. But the numbers
of neutrinos detected by instruments on the Earth’s surface fall far short
of the numbers predicted by current theories of physics, Stochaj said.

A key to solving the discrepancy could be a better understanding of what
happens to particles from space as they strike and pass through the Earth’s
atmosphere. Stochaj and a team of collaborators from Italy, Germany and
Sweden will send a helium-filled balloon with a payload of sensitive
particle detectors to the outer reaches of the atmosphere and take
measurements at various altitudes as they bring the balloon down in stages.

They won’t be measuring neutrinos directly, however.

“You have to have huge detectors to detect neutrinos, much bigger than we
could lift with a balloon,” he said. “But you can detect muons, one step
back in the decay chain, and if you know the number of muons you can
determine the number of neutrinos that will come from them.”

The goal of the project is to provide statistics on atmospheric muons that
are 10 times better than currently available data, Stochaj said. “Hopefully
this will shed some light on the problem. This will give the people who do
the models something to compare to.”

Ultimately, understanding neutrinos better will mean understanding the
makeup of the universe better, he said.

The existence of neutrinos was postulated in the 1930s, and for years
the tiny particles were presumed to have no mass, Stochaj said. Recent
experiments suggest they do have some mass, and because they are so
numerous, even with the slightest amount of mass they could amount to a
big part of the total mass of the universe.

Scientists now believe there are several types of neutrinos, and there is
evidence that they can “oscillate” or change from one type to another, he
said. Oscillation would be another sign that neutrinos have mass, he said,
and it also could be one reason the numbers of neutrinos detected on the
ground don’t meet scientists’ expectations of the numbers that should come
from the sun and cosmic rays. Some of the neutrinos could be changing, as
they pass through the atmosphere, into a type of neutrino the detectors
aren’t sensitive to, he said.

Stochaj and his international collaborators have sent their instruments
aloft numerous times in the past 12 years, but this mission is different.
Previous flights, funded by NASA and the collaborators’ scientific agencies,
have been used primarily to look for antimatter in cosmic rays at the edge
of Earth’s atmosphere.

The muon flight, which will be launched from Fort Sumner in eastern New
Mexico next spring, will carry instruments initially to nearly the same
altitude as the earlier flights — about 120,000 feet — but it will then
descend and stop at seven different altitudes to measure atmospheric muons
down to 60,000 feet.

“It’s the first time we’ve done a flight this complicated,” Stochaj said.
“Balloon flights are tricky anyway, and this one will be even harder
because winds can vary so much at different altitudes.”

The Particle Astrophysics Laboratory is a unit of the NMSU College of
Engineering’s Klipsch School of Electrical and Computer Engineering.