Science News - May 19, 2012

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

1100

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

1050

1000

950

900

850

1960

1970 1980 1990 2000 2010

Year

NIST 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
survive.

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 Laue-Langevin.

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 neutrons well-contained.

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.