For the first time, scientists with an international experimental group say they have uncovered a new method of studying the components within the nucleus of atoms, using a novel method involving mysterious “ghost particles” known for the rarity of their interactions with matter.
Once considered impossible, the achievement was made by
physicists at the University of Rochester in association with the MINERvA
neutrino experiment, who now report their successful studies of the structure
of protons by employing a beam of neutrinos in the journal Nature.
As far back as the 1950s, Stanford University physicists
were able to measure the size of protons by using beams of electrons produced
by an accelerator. However, the new research by the Rochester team focused on a
similar approach, albeit one that relied on beams of neutrinos instead of
electrons.
With no electrical charge and almost no detectable mass,
neutrinos have garnered the nickname “ghost particles” because of their minimal
interactions with atoms. Due to their unique properties, neutrinos are also
renowned for being difficult to detect, despite being among the most abundant
particles known to exist in the universe.
It had been the research team’s study of neutrinos in
conjunction with the MINERvA experiment that led them to stumble onto a unique
method of analyzing the structure of protons, according to Tejin Cai, a York
University postdoctoral research associate and Ph.D. student with the
University of Rochester’s Neutrino Group.
“Scattering weakly interacting neutrinos gives the
opportunity to measure both vector and axial vector form factors of the
nucleon, providing an additional, complementary probe of their structure,” Cai
and his coauthors write in their recent paper.
“We weren’t sure at first if it would work,” Cai said in a
statement, “but we ultimately discovered we could use neutrinos to measure the
size and shape of the protons that make up the nuclei of atoms.”
“It’s like using a ghost ruler to make a measurement,” says
Cai, who was also lead author of the new paper published in Nature.
Kevin McFarland, the Dr. Steven Chu Professor in Physics at
Rochester University, called the new method a “very indirect way of measuring
something.” However, he says that the team’s approach “allows us to relate the
structure of an object—in this case, a proton—to how many deflections we see in
different angles.”
The Rochester team is quick to point out that although their
new technique involving the use of neutrinos offers no clearer imagery of
proton structure than past efforts relying on electron beams, the benefit to
their new methodology is that it allows physicists a unique opportunity to
gauge interactions that occur between the “ghost particles” and protons.
“The interaction between neutrinos and protons (or neutrons)
has two main ingredients,” McFarland told The Debrief in an email. “One is
exactly the information that you can get from scattering electrons from
protons, which is how the original measurements of the proton’s “size” were
made beginning in the 1950s and have been continually refined since with better
and better experiments involving the interaction of protons and electrons.”
McFarland told The Debrief that the second ingredient,
involving what is known as axial vector form factor, can’t be measured in the
same way.
“Previous inferences of this form factor relied on
calculations in some way or another,” McaFarland explained, “either to correct
for a measurement being done on a bound nucleon, or to relate some other
process to the axial vector form factor, or to just calculate it from first
principles without any data at all.”
“Our measurement is the first to measure it directly in the
most straightforward way by scattering neutrinos against protons,” McFarland
says.
Previously, such information could only be inferred
indirectly through theoretical models combined with other measurements. Cai,
McFarland, and the team hope that the new technique will help to facilitate
future studies where the effects involving neutrino scattering on protons can
be separated from similar phenomena related to neutrino scattering that occurs
at the atomic level.
As the authors conclude in their paper, “the tools developed
for this analysis and the result presented are substantial advancements in our
capabilities to understand the nucleon structure in the weak sector, and also
help the current and future neutrino oscillation experiments to better
constrain neutrino interaction models. Specifically, the team now hopes to use
the technique to separate the effects related to neutrino scattering on protons
from the effects related to neutrino scattering on atomic nuclei, which
involves bound collections of protons and neutrons.
“Because our measurement is unambiguously a measurement of
this other description of the proton’s shape that the ‘axial vector form
factor’ provides, it can now be used to predict other processes,” McFarland
told The Debrief, which includes the results of neutrinos scattering from
neutrons, or possibly even neutrinos or antineutrinos scattering against the
bound protons and neutrons within an atom’s nucleus.
“The latter requires models of how those protons and
neutrons are bound to work,” McFarland says, “but at least now data on those
effects cannot be convoluted by the possibility that what we actually have
wrong is the part that comes from the scattering on free protons or neutrons.”
“By using our new measurement to improve our understanding
of these nuclear effects,” McFarland added in a recent statement, “we will
better be able to carry out future measurements of neutrino properties.”
The team’s paper, “Measurement of the axial vector form factor from antineutrino–proton scattering,” was published in Nature on February 1, 2023.
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