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from the or-they-are-being-eaten-by-a-grue dept.
At the April 13th and 14th meeting of the American Physical Society in Denver, Co. Physicists debated new ways to determine how long neutrons actually live. While neutrons are typically bound up with protons in the nucleus of atoms, and are perfectly stable there, they don't last long on their own.
Depending on the approach taken to measure it, the average lifetime of a neutron returns different values.
Using the bottle method (put a bunch of Neutrons in a 'bottle' and count how many are left after a period of time), the average lifetime is 14 minutes, 39 seconds.
Using the 'beam' method (count the protons given off in a detector as neutrons decay), the average lifetime is 14 minutes, 47 seconds.
These two methods are so precise that they do not overlap even taking the worst possible margins of error of both. It is a puzzler.
"The discrepancy has bedevilled researchers for nearly 15 years."
One possibility is that one of the two methods is doing something wrong. In that case, researchers might want to combine beam and bottle in a single device. At the meeting, physicist Zhaowen Tang of the Los Alamos lab described how researchers could put a particle detector inside a bottle neutron trap and count neutrons using both methods. His team has acquired funding to start building the device.
Another possibility is that the beam and bottle approaches have been measuring the neutron lifetime correctly, but that some unseen factor accounts for the discrepancy between the two. A leading idea is that neutrons might occasionally decay into not just protons but also dark matter, the mysterious unseen material that makes up much of the Universe's matter.
Interesting that plain old neutrons might be the key to opening the door on dark matter.
Pinpointing the lifetime of a neutron is important for understanding how much hydrogen, helium and other light elements formed in the first few minutes after the Universe was born in the Big Bang, 13.8 billion years ago. Scientists also think they can hunt for new types of physics if they can better pin down the neutron's lifetime, because that would help to constrain measurements of other subatomic particles.
A few seconds goes a long way in physics.
(Score: 3, Insightful) by DrkShadow on Wednesday April 17 2019, @04:06AM (8 children)
Why not take a super computer or two and _calculate_ the statistical half-life of a neutron?
Seriously. Don't we know the attributes of the fundamental forces and particles sufficiently well that, given some subatomic particles in a particular starting configuration, given a particular noise input (or none), we can determine the life of that particle (neutron)? Repeat with a great many different starting conditions (charges in different place, particles in different places, momentums slightly different) and take the average. This seems like it would be an easy, solved problem. Verify with tests.
Though the summary. Neutrons are stable when combine with protons. (Curious on its own.) Then, "We'll shove a lot of unstable neutrons in a jar and count 'em" vs "We'll leave a neutron bound to a proton until they react in a particular way and a neutron shifts off" -- these seem like fundamentally different measurements. One, the neutron is stable, by itself. The other, the neutron is fresh out of a reaction that stripped it from a proton -- what energy has that reaction imparted unto the neutron?. That strikes me as different.
(Score: 5, Informative) by PiMuNu on Wednesday April 17 2019, @05:38AM (5 children)
> Why not take a super computer or two and _calculate_ the statistical half-life of a neutron?
It is quite hard because you have to calculate the sum of a series that doesn't converge very well. The quarks are bound by gluon interactions, but the gluons themselves are also bound by gluon interactions (and so on). Each step in the calculation is already quite lengthy, and it turns out it is rather a difficult problem to solve computationally. There is a summary on wikipedia which probably makes a better explanation.
https://en.wikipedia.org/wiki/Quantum_chromodynamics [wikipedia.org]
> Though the summary
You misunderstood the beam method. A beam of neutrons is created using neutron spallation. Many of the neutrons decay to a proton, electron and electron anitneutrino. The resultant decay protons are counted and, by comparing the initial number of neutrons to the number that decayed one can infer a half life.
Reference for "bottle method"
https://arxiv.org/pdf/1707.01817.pdf [arxiv.org]
Reference for "beam method"
https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.111.222501 [aps.org]
(annoyingly the original article is paywalled, I thought PRL was open access journal but maybe not)
(Score: -1, Troll) by Anonymous Coward on Wednesday April 17 2019, @08:43AM
ah! the electron "anti" neutrino.
most curious: the universe is flooded
with neutrinos and hydrogen is most abundant but
but "somehow" the universe doesnt cogel into a neutron
soup that turns everything unstable and radioactif.
srsly, those neutrons are a biiiig secriit ^_^
(Score: 2) by OrugTor on Wednesday April 17 2019, @04:08PM (1 child)
In the beam method are the particles travelling at relativistic speeds, requiring time dilation adjustment?
(Score: 2) by PiMuNu on Wednesday April 17 2019, @09:19PM
> In the beam method are the particles travelling at relativistic speeds, requiring time dilation adjustment?
I assume they thought of that. Other sources of bias - what is the efficiency with which the incident neutrons are counted? what is the efficiency with which incident protons are counted? Again, I am sure they thought of this stuff e.g. calibrated with a known source.
(Score: 2) by Osamabobama on Wednesday April 17 2019, @11:03PM
This is the first reference to half life I have seen on this page. The other references are to 'average lifetime". Is this article dumbing it down for us? Is the measurement method using a half life of something else to determine the lifetime of a neutron?
The article says the two measurements don't overlap, but we are talking about the calculation of the average. Does the distribution of lifetimes overlap? What sort of distribution is it?
Appended to the end of comments you post. Max: 120 chars.
(Score: 0) by Anonymous Coward on Thursday April 18 2019, @05:03PM
STOP being sensible.
You're implying that a guy on the internet who gave it 3 minutes thought can't blow through what hundreds of people specializing in physics have been pondering for decades.
(Score: 5, Informative) by stormwyrm on Wednesday April 17 2019, @09:00AM (1 child)
The strong interaction is notoriously difficult to computationally simulate, given that the interaction is highly non-linear and has a large coupling constant at low energies. The perturbation theory that was so successful with quantum electrodynamics does not work for quantum chromodynamics (the quantum field theory of the strong interaction) as a result. The most successful approach so far to computationally modelling the strong interaction is Lattice QCD [wikipedia.org], and it is so computationally intensive that it's been used as a benchmark for high-performance supercomputers [arxiv.org]. The main problem is that even today's supercomputing systems still don't have enough memory bandwidth to do these kinds of calculations efficiently. Lattice QCD can't even be applied for some classes of problems involving the strong interaction, such as those for which there is a numerical sign problem [wikipedia.org]. A general-purpose quantum computer would probably be able to help greatly in simulating these kinds of things, if we can build one.
Numquam ponenda est pluralitas sine necessitate.
(Score: 0) by Anonymous Coward on Wednesday April 17 2019, @08:24PM
As a physicist with a different focus area than this, I wanted to thank you for your informative post.