The mystery of the neutron lifetime:
Nine seconds. An eternity in some scientific experiments; an unimaginably small amount in the grand scheme of the universe. And just long enough to confound nuclear physicists studying the lifetime of the neutron.
The neutron is one of the building blocks of matter, the neutral counterpart to the positive proton. Like many other subatomic particles, the neutron doesn't last long outside of the nucleus. Over the course of about 15 minutes, it breaks apart into a proton, an electron, and a tiny particle called an anti-neutrino.
But how long the neutron takes to fall apart presents a bit of a mystery. One method measures it as 887.7 seconds, plus or minus 2.2 seconds. Another method measures it as 878.5 seconds, plus or minus 0.8 second. At first, this difference seemed to be a matter of measurement sensitivity. It may be just that. But as scientists continue to perform a series of ever-more-precise experiments to evaluate possible issues, the discrepancy remains.
This persistence leads to the possibility that the difference is pointing to some type of unknown physics. It could be revealing an unknown process in neutron decay. Or it could be pointing to science beyond the Standard Model scientists currently use to explain all of particle physics. There are a number of phenomena that the Standard Model doesn't fully explain and this difference could point the way towards answering those questions.
[...] Whatever results this experiment delivers, the work to understand the neutron lifetime will continue. "It's very telling that there are so many attempts to precisely measure the neutron lifetime. That tells you the emotional reaction of scientists to a discrepancy in the field – "I want to explore this!'" said Broussard. "Every scientist is motivated by the desire to learn, the desire to understand."
Journal Reference:
L.J. Broussard, K.M. Bailey, W.B. Bailey, et al. New search for mirror neutron regeneration [open], EPJ Web of Conferences (DOI: 10.1051/epjconf/201921907002)
(Score: 0) by Anonymous Coward on Monday September 14 2020, @11:05AM (24 children)
Why would the decay process be independent of the environment? Protons, electrons and antineutrinos must come together to create a neutron every now and then too right?
(Score: 2) by PiMuNu on Monday September 14 2020, @11:19AM (2 children)
Sure, but the probability of a conjunction is vanishingly small.
(Score: 5, Informative) by Non Sequor on Monday September 14 2020, @01:36PM (1 child)
Two processes that invert the proton->neutron conversion are electron capture and positron emission. Neither one involves collision of three particles. For electron capture, a proton absorbs an electron and emits a neutrino and changes into a neutron. For positron emission, a proton emits a positron and a neutrino and changes into a neutron. Both of these obey the same conservation equations as the beta decay (spontaneous conversion of a neutron to a proton and emission of an electron and an antineutrino) because absorption of an anti-particle has the same impact on the conserved quantities as emitting a particle.
Write your congressman. Tell him he sucks.
(Score: 2) by PiMuNu on Monday September 14 2020, @02:05PM
Sure, but the probability is still small, unless there is an incident electron beam - the nuclear cross section is not so big.
(Score: 1, Insightful) by Anonymous Coward on Monday September 14 2020, @11:49AM (4 children)
Sure, but if the decay process were dominated by random factors in the environment, you would expect something like a gaussian distribution to appear from all the measurements, and no one would have bat an eye.
But these results hint to a bimodal distribution, which means it's not clearly dominated by external factors. The question isn't whether the process is or isn't independent of the environment, but what internal physics gives rise to this effect.
(Score: 0) by Anonymous Coward on Monday September 14 2020, @12:24PM (3 children)
They mention two different methods of measurement. That isn't random, it is systematically different so a bimodal outcome is expected. If they also used a third set of circumstances wed expect trimodal.
(Score: 3, Insightful) by sjames on Monday September 14 2020, @02:11PM (2 children)
Imagine you have a ruler and a caliper. For everything you measure, they agree within their respective accuracies. Except this one type of wire where they consistently disagree for some reason.
That's why this is interesting.
(Score: 0) by Anonymous Coward on Monday September 14 2020, @03:44PM (1 child)
It says one method measures hot neutrons moving very fast in a beam and the other when they are ultra cold trapped in a container.
I would expect two different rates in those very different environments.
(Score: 3, Interesting) by sjames on Tuesday September 15 2020, @01:55AM
Other than relativistic time dilation (not sufficient to account for the difference), what would be the physics behind the difference? I've certainly never heard of any other form of particle decay that cares if it's in a container or if it's cold.
(Score: 2) by ikanreed on Monday September 14 2020, @01:21PM (10 children)
You misunderstand.
Both methods find a non-constant lifetime. The plus-or-minus 2.2 seconds or plus-or-minus 0.8 seconds listed in the summary? Those represent the variations found through thousands of replications each.
And as to the coming back together thing, yes, it's theoretically possible to unscamble an egg like that, but piles of "useful" subatomic particles all close enough together to randomly do that hasn't happened much since the big bang.
(Score: 2) by Immerman on Monday September 14 2020, @02:00PM (9 children)
I don't think that's quite right.
Yes, there's variation in the lifespan, but I suspect it's a LOT larger than +/-2.2 seconds. Because I'm pretty sure what they're ultimately measuring is the _half-life_ of a neutron before it spontaneously decays, and since you can't measure half-life directly, they're directly measuring the mean lifespan, and the +/- amount does in fact indicate the uncertainty of their measurements.
And of course any time you're measuring half-lives, you're going to have a huge window of decay times - some particles will decay almost instantly, while some last for millions of half-lives or more.
(Score: 2) by ikanreed on Monday September 14 2020, @02:49PM (8 children)
I didn't read the discussion section of the paper before posting this time, which is a bad habit to develop.
I have now, and they are not talking about half-life, but "expected lifetime". The graph looks like this [imgur.com]. A half-life graph looks like this [priyamstudycentre.com]
(Score: 0) by Anonymous Coward on Monday September 14 2020, @03:11PM (1 child)
Welcome to online discussions. It's been a long road and we're glad to see that therapy has finally worked, you've come to realize that resistance is futile, and acceptance is tranquility.
You'll find bathrooms and a break room down that corridor. HR will help you with all the forms and paperwork. Try to enjoy your stay because there's no way out.
(Score: 0) by Anonymous Coward on Tuesday September 15 2020, @12:23AM
What paper? I didn't know there was a paper.
(Score: 2) by Immerman on Monday September 14 2020, @03:38PM (4 children)
They're (potentially) two ways to talk about the same thing - expected life = 1.44*half life (expected life can also be used to discuss things that don't demonstrate exponential decay = don't have a half life. But neutrons do decay exponentially)
Meanwhile those two graphs are showing very different things (notice that the time axis is vertical on the first, and horizontal on the second?) - the radioactive decay shows a the non-decayed population size over time, while the expected life graph shows how many individual particles "lived" to each specific age.
(Score: 2) by ikanreed on Monday September 14 2020, @03:55PM (3 children)
Be that as it may, it still seems as though the characteristic behavior or decaying neutrons is not similar to decaying radioactive isotopes and does not follow the half-life pattern.
(Score: 2) by Immerman on Monday September 14 2020, @05:53PM (2 children)
Source? The second sentence on Wikipedia says otherwise: https://en.wikipedia.org/wiki/Free_neutron_decay [wikipedia.org]
So does the first random page on neutron decay I pulled from DDG: https://www.sciencedirect.com/topics/physics-and-astronomy/neutron-decay [sciencedirect.com]
where they say " One normally assumes that the neutron density will decay exponentially"
They do then immediately say that their experiment didn't find that to be the case - but the phrasing strongly suggests that exponential neutron decay is the accepted assumption.
(Score: 3, Insightful) by ikanreed on Monday September 14 2020, @06:07PM (1 child)
Hm. Looks like you're right. I followed the paper's citations, and the formula used in underlying papers is indeed dN/dt=−N/τn, a fairly standard representation of half life calculations.
That'll teach me to think I know what a graph means just by looking at it. Thank you.
(Score: 2) by Immerman on Monday September 14 2020, @06:34PM
You're welcome. I've been thrown off badly by misinterpreting graphs myself.
Looking at the original paper it looks like your graph was actually sensitivity of the characteristic oscillation time versus mirror magnetic field strength? ...I honestly have no idea what that even means.
(Score: 0) by Anonymous Coward on Tuesday September 15 2020, @01:53PM
I was expecting the second link to be www.steam.com/halflife or something...
(Score: 2) by Immerman on Monday September 14 2020, @01:41PM (4 children)
Same reason nuclear decay times are unaffected by the environment - "the environment" doesn't really exist at the subatomic level. The forces that hold together an atomic nucleus are so large that the electromagnetic forces between atoms are basically irrelevant. And the forces within a nucleon dwarf the nuclear forces.
(Score: 2) by HiThere on Monday September 14 2020, @01:48PM (3 children)
This, however, is talking about neutrons OUTSIDE of the nucleus.
Still, the environmental factors shouldn't produce the observed distribution. Something interesting is probably happening...unless it really *is* experimental error.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 3, Interesting) by Immerman on Monday September 14 2020, @02:19PM
Yes, but the forces within a neutron are still larger than the forces within a nucleus, and thus dwarf the forces between atoms. About the only way "the environment" can have any impact on the internals of nucleon (under known physics) is through, well, impact. Slam one nucleon into another, and suddenly their internal forces are interacting with each other and you're not actually talking decay anymore, more something akin to fission.
Unfortunately they don't mention which experiment gives which value - I wouldn't be incredibly surprised to find that the trapped beam measured a shorted lifespan due to neutrons traveling at 10Mm/s occasionally resulted in impacts - but unless they always perform the experiment with the same density of trapped of neutrons, you'd expect the discrepancy to increase with density. That also seems obvious enough that the researchers would have thought of it already.
In the interest of considering obvious alternatives, I also calculated the amount of time dilation they'd experience at 10Mm/s - but that only stretch 880s to 880.5s for an outside observer, not nearly enough to explain the discrepancy. Of course there's also time dilation due to acceleration - and I suspect that keeping something trapped when traveling at 10Mm/s would involve some truly staggering sustained accelerations - but to be honest I have no idea how to compute that. I'd assume though that particle physicists would be intimately familiar with the effects of time dilation, and have already corrected their measurements for it.
(Score: 2) by RS3 on Monday September 14 2020, @03:14PM
Interestingly (maybe?) the error itself is quantum.
(Score: 0) by Anonymous Coward on Tuesday September 15 2020, @12:26AM
Or maybe it's an inches centimeters thing.
(Score: 0) by Anonymous Coward on Monday September 14 2020, @11:43AM (8 children)
I'll read this on my lunch break, but you gotta love the writeup. "It could be systematic measurement bias between the two experiments, and it probably is. BUT IT COULD BE THE STRAW THAT TEARS DOWN THE WHOLE STANDARD MODEL AND OVERTURNS ALL OF PHYSICS AS WE KNOW IT!"
(Score: 2) by inertnet on Monday September 14 2020, @01:17PM
Or we could just blame it on very subtle dark matter interference.
(Score: 5, Insightful) by ikanreed on Monday September 14 2020, @01:26PM (2 children)
Sure, but systematic differences of 9 seconds aren't very likely with modern equipment. We aren't exactly using human fingers on a spring wound stopwatch anymore.
(Score: 0) by Anonymous Coward on Monday September 14 2020, @03:27PM (1 child)
This isn't unusual when you are talking about making tough measurements of an absolute value. Within an experiment they can be very stable and consistent, which gives them small error bars, but across experiments it isn't always so easy. They can be in dead nuts agreement in measured relative differences, like the slope of a curve fit, but if experiments or observations are very different from each other, you can have potential systematic offsets. Measurement of the Hubble Constant are very good [soylentnews.org] examples of this [soylentnews.org]. I don't know the details given forth in the paper, which I will read later, but I do know that wrangling neutrons is tough, particularly getting them to stay around for any appreciable amount of time.
(Score: 2) by Immerman on Monday September 14 2020, @06:50PM
I'm not sure measurement of the Hubble Constant is a good example since that's another case where evidence is building that something genuinely strange is going on, rather than there being a simple systematic bias in the measuring techniques.
Usually, when you're using multiple different techniques to measure something, all of your measurements are at least pretty close to agreeing with your margin of error, and and you get independent verification. Which is what was assumed was happening here until recently. When measurements don't agree it's usually because of either experimental error (which is ruled out by independent confirmation) or one or more methods aren't actually measuring what you think they're measuring, likely indicating a flaw in the theory on which the experiment is based.
In this case, as with the Hubble constant, many groups have been independently performing both kinds of measurements with ever-increasing precision, and the results agree closely with others of the same type of measurement. That the results between different measuring techniques now diverge by several multiples of the margin of error strongly suggests that there is an invalid assumption within the theoretical basis of at least one of the experiments.
(Score: 2, Informative) by Anonymous Coward on Monday September 14 2020, @02:19PM (3 children)
(Score: 2) by RS3 on Monday September 14 2020, @03:19PM (2 children)
That and "watch this".
(Score: 2) by Immerman on Monday September 14 2020, @06:52PM
Nah - then you're usually lucky to get something even as interesting as an explosion, which is something that's so dull and easy to create on demand that most scientists rarely even bother.
(Score: 0) by Anonymous Coward on Tuesday September 15 2020, @12:29AM
Watch this *flops out penis*
(Score: 0) by Anonymous Coward on Monday September 14 2020, @03:17PM (1 child)
887.7 +- 2.2 == 878.5 +- 0.8
What's the problem here? The statement is true. There is literally nothing to see here. And at some time there will be something to see, it will just come down to some systematic error in the measurements. And if that is not true, then MAYBE you have something to write about. Until then, this is literally like seeing boobs in the clouds and calling God perverse.
(Score: 2) by ikanreed on Monday September 14 2020, @03:58PM
878 not 887. Now that math doesn't work.
However, even if it did, the discrepancy would still be hard to explain because those are normalized 95% CI numbers, and with enough repetitions should still be the same in something as robustly studied as particle physics for "normal" subatomic particles.
(Score: 4, Insightful) by dltaylor on Monday September 14 2020, @04:01PM (5 children)
My first thought, since the beam time is longer than the trap time, was that the beam neutrons were not actually decaying slower, but that by moving at near-relativistic speed, we were observing time dilation. 10 million m/s (according to the article) is about 3% c, so the dilation "effect" is only about 0.5 seconds, not even the same order of magnitude as 9 seconds, but enough to make me wonder whether the 3% "increase" in mass has to show up, in our reference frame, as additional binding energy, or some such.
(Score: 2) by hendrikboom on Monday September 14 2020, @05:37PM
One of the sensible things to wonder.
(Score: 2) by Immerman on Monday September 14 2020, @06:04PM (2 children)
Another possible factor is that apparently acceleration also causes time dilation - and a trapped beam traveling at 10Mm/s (presumably in a confined circle) is going to be subjected to pretty intense accelerations.
Unfortunately while searching returns lots of references to the phenomena, I can't find a nice straightforward formula like there is for velocity-based time dilation.
That said, I would certainly hope that particle physicists would be well versed in both kinds of time dilation, and have already corrected their results to compensate.
(Score: 0) by Anonymous Coward on Tuesday September 15 2020, @01:39PM (1 child)
(Score: 2) by Immerman on Tuesday September 15 2020, @04:55PM
I want to say they should at least be able to plug in the acceleration of a classical body following the same average path and get at least a first-order approximation easily enough - except my inability to find a straightforward formula suggests that the reality is more complicated even for classical bodies.
(Score: 0) by Anonymous Coward on Tuesday September 15 2020, @02:08PM
This is an obvious thing to ask, and something I am assuming would have been ruled out well before they started complaining.
(Score: 0) by Anonymous Coward on Monday September 14 2020, @04:43PM (3 children)
Oh, I see this is some dark matter bs now. That is basically the "could it be aliens?" of physics.
(Score: 2) by Immerman on Monday September 14 2020, @06:06PM (1 child)
The only place Dark Matter is mentioned is in the context of "other, even more radical theories" to explain the discrepancy.
(Score: 0) by Anonymous Coward on Monday September 14 2020, @06:58PM
It is the first sentence of the paper:
> The astrophysical evidence for dark matter is strong.
(Score: 0) by Anonymous Coward on Tuesday September 15 2020, @12:31AM
Dark Lives Matter.