how fast neutrons die. Without the neutron lifetime, it’s hard to test current
theories describing the early universe.
What’s more, the average neutron lifetime helps reveal how much ordinary
matter was generated in the Big Bang.
Knowing the amount of ordinary matter is an important factor in determining
how much mysterious dark matter lurks
in the universe. Quantifying dark matter
“is important in understanding how
galaxies form and the evolution of the
universe,” says Rocky Kolb, a cosmologist
at the University of Chicago.
Knowing the neutron lifetime would
also provide physicists with a better
understanding of the weak nuclear force,
possibly leading the way to insights
beyond the current standard model of
particles and forces.
Of bottles and beams
Those seeking the neutron lifetime share
a common goal: to better understand
what the universe is made of and how
all of its constituents interact. But the
teams use different strategies. After huge
batches of neutrons are created from
nuclear reactors or particle-accelerating
facilities, one approach determines the
portion that die in just a few milliseconds.
At the National Institute of Standards
and Technology in Maryland, Greene and
Honing survival Precision in measuring
the neutron lifetime has increased (error bars
in blue). But scientists need corroborating
measurements to within a second if they want
to answer key questions about nature’s forces
and the early universe.
Neutron lifetime through time
Average lifetime in seconds
1970 1980 1990 2000 2010
SOURCE: PARTICLE DATA GROUP/LBNL
colleague Fred Wietfeldt, a physicist
from Tulane University in New Orleans,
generate a beam of cold neutrons flying at
speeds of about 1,000 meters per second.
Some of these particles will decay while
they are still in the beam. By applying
magnetic and electric fields, scientists
can shepherd the positively charged
remnant protons and count them.
To calculate the average neutron
lifetime, scientists also need to know
the number of neutrons that were in
the beam to begin with, determined by
counting reactions between neutrons
and a thin piece of lithium in the beam.
In their last set of experiments, published in 2005, Greene and Wietfeldt set
the neutron lifetime at 886.3 seconds,
give or take more than three seconds.
“What prevented us from getting a
precise measurement 10 years ago was
associated with counting the neutrons,”
says NIST researcher Jeffrey Nico, who
works with the beam team. “We pushed
the state of the art in counting neutrons,
but it still wasn’t good enough.”
Meanwhile, across the globe at the
Institut Laue-Langevin facility in
Grenoble, France, physicist Anatolii
Serebrov and collaborators are using an
opposite strategy: Instead of determin-
ing the number of protons left behind by
neutrons that die, the team is bottling
neutrons and counting the particles that
Serebrov, of the Petersburg Nuclear
Physics Institute in Russia, and colleagues cool the neutrons to temperatures of 2 millikelvins, just above absolute
zero, in order to contain the particles. As
the ultracold neutrons are poured into
and emptied out of a bottle, they pass
through a detector. By tracking the number of neutrons at the beginning and end
of an experiment, the team can calculate
how many particles have decayed.
“If you have no losses in your bottle
besides beta decay, then you know exactly
the lifetime,” says Peter Geltenbort, who
worked with Serebrov at the Institut
But that’s a big if. Though the bottle
is made of metals that reflect neutrons,
residual gases or impurities can cause
This trap captures protons emitted by
neutron decay. Comparing the number
of protons after a certain time to the
initial number of neutrons helps scientists determine the neutron’s lifetime.
neutrons to be absorbed. Also, bouncing
particles can sometimes acquire enough
energy to escape out of the bottle’s top.
In 2005 Serebrov and his colleagues
reported that the neutron’s lifetime was
approximately 878.5 seconds. The team’s
measurement was precise to within a
second. But this number is nearly eight
seconds shorter than the time measured
by Greene and Wietfeldt, and about three
seconds less than the current average
reported by the Particle Data Group.
Another precise measurement could
help settle the dispute, but not if it
doesn’t match the beam team’s results.
“If they disagree, you know that at least
one of them is wrong,” Wietfeldt says.
For now, the two tribes are committed
to perfecting their experiments. Greene
says his team has recently recalibrated
its system to estimate the number of
neutrons in a beam with six times the
accuracy. And while Serebrov focuses on
updating his existing project, Geltenbort
and colleagues in France are working on
a magnetic bottling setup. Because neutrons respond to magnetism, scientists
think that magnetic fields will keep
Both teams plan on presenting new
data precisely pinning down the neutron’s lifetime within the next few
years — perhaps giving physicists the
number they’ve been waiting for.
“This is the decade of precision cosmology,” Kolb says. “Estimates no longer
cut it.” s
s Stephan Paul. “The puzzle of neutron lifetime.” Online at arxiv.org/
May 19, 2012 | SCIENCE NEWS | 21