Neutron Decay Thriller Baffles Physicists
Despite a long time of taking measurements, scientists can’t agree on how lengthy neutrons live. Neutrons are stable inside atoms, but on their very own they decay in about 15 minutes, kind of, into a few different particles. Precisely how much roughly is the sticking point. Every experiment seems to yield a distinct answer.
The lack of resolution is irritating. Understanding the lifetime of the neutron is essential not only for knowledge’s sake but in addition to reply other more fundamental questions about new physics beyond the recognized particles and processes in the universe, says Jeffrey Nico, who leads a neutron lifetime experiment on the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. “We can’t leave this disagreement simply hanging out there.”
To measure how lengthy neutrons final before decaying, scientists look for either the disappearance of neutrons or the looks of their decay products. Neutrons disappear via a course of referred to as neutron beta decay, transforming themselves into protons by emitting an electron (shedding a adverse cost to change into positively charged) and an antineutrino. One technique of measuring how lengthy neutrons final earlier than decaying entails trapping a group of neutrons inside a bodily bottle and counting how many stay after varied lengths of time. One other, referred to as the beam methodology, is to provide a tightly confined spray, or beam, of neutrons and encompass it with a “proton trap” to depend the protons created when neutrons decay.
Beam experiments have been going on for greater than 30 years, with the leaders in the field located at NIST. Their greatest, latest beam results, revealed in 2013, measured the neutron lifetime at 887.7 seconds, plus or minus three.1 seconds. Bottle experiments, in contrast, go back solely about 15 years however have already posted results with increased precision than beam findings. The perfect consequence so far from a bottle experiment got here in 2008 from a collaboration between the Petersburg Nuclear Physics Institute and the Joint Institute for Nuclear Analysis in Russia, together with the Institut Max von Laue Paul Langevin (Ill) in France. That crew measured the neutron lifetime at 878.5 seconds, plus or minus one second.
A distinction of about 9 seconds may not sound like much, however it is larger than the estimated error bars on the experiments, which means these error bars are wrong on some—maybe all—of the outcomes. “The discrepancy is embarrassing,” says College of Tennessee physicist Geoffrey Greene, who labored on the NIST venture. “One or extra of us screwed up, and we simply have to search out out which of them.”
And though 9 seconds shouldn’t be lengthy, it is enough to make a difference in some calculations that rely on the neutron lifetime, similar to predictions of how the first atomic nuclei formed. Protons and neutrons began out as free particles in the recent, dense child universe. Solely after the cosmos had cooled sufficiently—within in the primary 20 minutes after the massive bang—did they combine to type nuclei in a course of called nucleosynthesis. “Essentially all of the helium within the universe was made at that epoch,” Nico says. To know how many neutrons were accessible to kind nuclei, scientists should know how long neutrons last earlier than decaying. “For large bang nucleosynthesis, right now the neutron lifetime is the enter with the largest uncertainty on it, so by improving the neutron lifetime, you’ll be able to improve these predictions,” Nico provides.
If nucleosynthesis predictions prove not to match proof from astrophysical observations of the abundance of helium, for instance, exotic physics simply might be at work. One possibility is dark matter—the invisible, however plentiful, substance within the universe thought to be made from some forms of undiscovered particles. “All kinds of darkish matter candidates would possibly actually play a role in huge bang nucleosynthesis,” says theoretical physicist Susan Gardner of the University of Kentucky. These particles might have interacted with protons and neutrons or participated in reactions somehow to alter the number of nuclei that formed.
Understanding neutron beta decay can be important for understanding one of the 4 fundamental forces of nature—the weak drive. This drive is liable for nuclear fusion as well as radioactive decay—such as neutron beta decay. “Neutron decay is certainly one of the best examples of the weak interplay between mild particles like electrons and heavy particles like quarks [the components of neutrons],” Greene says. “That’s what we’re trying to get at by studying neutron decay.” The usual model of particle physics describes neutron decay very well, however scientists wonder if it is the total description. If measurements of neutron decay ever deviated from commonplace model t shirt joker batman predictions, they might level us toward a brand new, deeper stage of physics.
One other possibility is that new physics is behind the trouble scientists are having with neutron lifetimes. The discrepancy between bottle and beam measurements is curious, scientists level out. Bottle experiments are inclined to find a shorter neutron lifetime than the beam assessments do. Perhaps neutrons sometimes decay by a course of aside from beta decay, which means they might turn into one thing aside from protons. That may create various “missing” protons within the beam experiments, which would make them measure an extended lifetime than bottle experiments. (It would appear as if fewer neutrons are decaying within the allotted time, when actually they’re simply t shirt joker batman decaying into extra, unseen products.) “Is there some form of new physics that could clarify getting a distinct value for these two methods ” Greene asks. “The fact that we’re seeing an extended lifetime is according to the notion, and that would be tremendously exciting, but I think it’s more probably somebody’s screwed up their experiment.”
Neutron decay experiments are painstakingly complicated. For beam experiments, the primary challenge is to ensure to accurately tally each the neutrons in the beam, and the protons that end result from neutron decays. “Conceptually it’s fairly simple, however the trick is counting these particles absolutely,” Nico says. Additionally, the researchers should measure the length of the proton trap precisely to get correct measurements.
The issue in bottle experiments comes from the bottle—particularly the chance that neutrons work together somehow with the bottle walls , which will be made of assorted supplies, reminiscent of oil-coated copper. “When the neutron touches the wall, in the perfect case they needs to be totally mirrored with no losses, but there might be contaminations on the wall,” says Peter Geltenbort of Ailing, a member of the leading bottle experiment group. That group is engaged on building a t shirt joker batman larger bottle, and comparing the outcomes with experiments using smaller bottles. “The idea is for those who evaluate completely different bottles, you may then do an extrapolation to an infinitely giant bottle and convey the precision right down to an order of zero.Three or 0.Four seconds.”
One other tack is to get rid of the partitions of the bottle altogether. A number of teams are engaged on “bottles” made out of magnetic and gravitational fields, versus tangible supplies, to trap neutrons. Although neutrons don’t have an electrical charge, their spin offers them what’s known as a magnetic moment, making them act like tiny magnets inside a magnetic field.