In the nuclear family, the neutron is clearly the black sheep.
Unlike its sibling the proton, the neutron is eccentrically — and irritatingly — neutral. Because they have no electrical charge, neutrons are hard to guide and focus using electric fields. And unlike protons, which can be liberated by igniting hydrogen gas, the neutron is stubbornly sequestered within the atomic nucleus, making it especially hard to interact with one-on-one.
What’s most annoying is that neutrons are steadfastly secretive. Unlike charged particles, neutrons are hard to detect — making it hard to measure how long they live. Pinning down the length of the neutron’s life span could help physicists better understand many atomic and cosmic processes, from the nature of forces acting on subatomic particles to the distribution of matter in the moments following the Big Bang. More than 60 years of research has gone into determining the neutron’s average lifetime precisely.
But despite all those decades of study, physicists still can’t agree on the length of the neutron’s life. The best measurements done in the last 20 years differ by more than 10 seconds. And most report the measurement with an error of a few seconds. Without an agreed on value that’s also more precise, scientists can’t answer key physics questions for which the neutron lifetime matters.
“It’s unsound to have measurements that are in such statistical disagreement,” says Geoffrey Greene, a physicist at the University of Tennessee, Knoxville and Oak Ridge National Laboratory who has been working on determining the neutron’s lifetime for three decades.
Right now, teams with two very different approaches — one working with beams of neutrons and another that uses bottles to trap them — are tweaking their experiments in hopes of determining this elusive property.
Dying to know
Neutrons are usually bound in a nucleus, where they can stay as long as the nucleus remains intact. But neutrons free from nuclear confinement are unstable: The weak nuclear force compels them to break apart and die.
At the end of its life, a neutron turns into a proton, a process known as beta decay. When the proton appears, two more particles fly away: the small, negatively charged electron (the beta particle) and a ghostlike particle known as an antineutrino. This decay process is the same one that occurs when carbon-14, the radioactive isotope used in carbon dating, decays into the stable nitrogen-14.
Typically when people talk about the timescale of radioactive decay, they refer to the half-life. Carbon-14’s half-life is 5,730 years, so after 5,730 years the initial amount of that form of the element is reduced by half. But hunters of the neutron lifetime are instead looking for the time that elapses from the moment a free neutron is born to the moment it turns into a proton.
What’s tricky is that not all neutrons die at the same age. Neutron decay, like all radioactive decay, is a matter of probabilities. So scientists are looking for the average lifetime.
In an ideal experiment, scientists would have a bunch of free neutrons that could be studied closely. If all these neutrons were born at once, researchers could record how long it took for each individual neutron to turn into a proton. Assuming the sample of neutrons was large enough, averaging those time spans would give a precise average lifetime.
But measuring the neutron lifetime is much more complicated. For one thing, free neutrons have lots of energy and tend to fly around, limiting the neutron-tracking ability of even the best detectors.
The Particle Data Group, an international collaboration that acts as an authority on subatomic particle properties, currently puts the neutron lifetime at 881.5 seconds (14 minutes, 41.5 seconds). This number is an average of the best seven measurements in the last two decades, weighted based on precision. But the values included range from 878.5 to 889.2 seconds. To say something new and interesting about physics, scientists need consistent independent experiments that pin down the neutron lifetime to within a second.
Getting such a value could help scientists test their theories about the raw materials for star formation. Within the first three minutes following the Big Bang, neutrons and protons came together to cook up those ingredients — mostly hydrogen, but also helium and a trace of lithium. The exact ratio of the ingredients created in this coalescence, called Big Bang nucleosynthesis, depends on 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 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 determining 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 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.
“This is the decade of precision cosmology,” Kolb says. “Estimates no longer cut it.”