Two types of neutron decay produce a proton, an electron and an electron antineutrino but eject them in different configurations, The experiments at NIST detected no imbalance, but the improved sensitivity could help place limits on competing theories about the matter-antimatter imbalance in the universe.Credit: emiT team |
Why
there is stuff in the universe—more properly, why there is an imbalance
between matter and antimatter—is one of the long-standing mysteries of
cosmology. A team of researchers working at the National Institute of
Standards and Technology (NIST) has just concluded a 10-year-long study
of the fate of neutrons in an attempt to resolve the question, the most
sensitive such measurement ever made. The universe, they concede, has
managed to keep its secret for the time being, but they’ve succeeded in
significantly narrowing the number of possible answers.
Though
the word itself evokes science fiction, antimatter is an ordinary—if
highly uncommon—material that cosmologists believe once made up almost
exactly half of the substance of the universe. When particles and their
antiparticles come into contact, they instantly annihilate one another
in a flash of light. Billions of years ago, most of the matter and all
of the antimatter vanished in this fashion, leaving behind a tiny bit of
matter awash in cosmic energy. What we see around us today, from stars
to rocks to living things, is made up of that excess matter, which
survived because a bit more of it existed.
“The
question is, why was there an excess of one over the other in the first
place?” says Pieter Mumm, a physicist at NIST’s Physical Measurements
Lab. “There are lots of theories attempting to explain the imbalance,
but there’s no experimental evidence to show that any of them can
account for it. It’s a huge mystery on the level of asking why the
universe is here. Accepted physics can’t explain it.”
An
answer might be found by examining radioactivity in neutrons, which
decay in two different ways that can be distinguished by a specially
configured detector. Though all observations thus far have invariably
shown these two ways occur with equal frequency in nature, finding a
slight imbalance between the two would imply that nature favors
conditions that would create a bit more matter than antimatter,
resulting in the universe we recognize.
Physicists including Pieter Mumm (shown) used the emiT detector they built at NIST to investigate any potential statistical imbalance between the two natural types of neutron decay. Credit: emiT team |
Mumm
and his collaborators from several institutions used a detector at the
NIST Center for Neutron Research to explore this aspect of neutron decay
with greater sensitivity than was ever possible before. For the moment,
the larger answer has eluded them—several years of observation and data
analysis once again turned up no imbalance between the two decay paths.
But the improved sensitivity of their approach means that they can
severely limit some of the numerous theories about the universe’s
matter-antimatter imbalance, and with future improvements to the
detector, their approach may help constrain the possibilities far more
dramatically.
“We
have placed very tight constraints on what these theories can say,”
Mumm says. “We have given theory something to work with. And if we can
modify our detector successfully, we can envision limiting large classes
of theories. It will help ensure the physics community avoids traveling
down blind alleys.”
The
research team also includes scientists from the University of
Washington, the University of Michigan, the University of California at
Berkeley, the University of Notre Dame, Hamilton College and the
University of North Carolina at Chapel Hill. Funding was provided by the
U.S. Department of Energy and the National Science Foundation.
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