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Physicists make most precise measurement of neutron’s lifetime

nature.com

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Re: Physicists make most precise measurement of neutron’s lifetime

#51

Post by akomtu »

ben_w wrote: post #45
akomtu wrote: post #41

Earlier quoted context omitted.

I'm wary of such absolute statements. I'd put photons near protons in terms of stability: they just don't show signs of decaying.

I understand the wariness. However, in relativity a photon cannot decay: because it travels at the speed of light and has infinite time dilation, it does not subjectively experience the passage of time in which the possibly of decay could exist.

That's the model of photon, not the photon itself. A mathematical photon cannot decay. What prevents a photon from hitting a quantum bump on its way between galaxies and become an electron?

Re: Physicists make most precise measurement of neutron’s lifetime

#52

Post by adrian_b »

hilbert42 wrote: post #50
cozzyd wrote: post #27

Earlier quoted context omitted.

Generally decay rates scale with the energy release (Q) which is relatively small here compared to the W decay. Decay is a tunneling process so less Q generally means a slower decay since there are fewer final states available (but the details are complicated). Fermi's Golden Rule in principle allows you to calculate the decay rate. In practice we dont know how to calculate all the relevant quantities since QCD is ha…

A question I've never given much thought about until now. When the neutron begins to decay [i.e. at or around ~14.63 min] then is the duration of that decay process essentially equivalent to the time taken for the W- boson to decay into an electron and antineutrino?

There is no way to measure the "duration of the decay process".

When you look for it, you either find a neutron that has not decayed yet, or you find the decay products.

The quoted decay time is just the average time. A free neutron might decay after 1 millisecond or after 5 hours.

Like for any other decay process, it is unpredictable when an individual neutron will decay. Nevertheless the decay probability in a time interval is predictable so if you start with a large number of free neutrons, you can predict very accurately how many will remain after 1 minute or after 10 minutes or after an hour, because the number will decrease exponentially with a known time constant, which was measured more precisely in this new experiment.

Re: Physicists make most precise measurement of neutron’s lifetime

#53

Post by ncmncm »

ISL wrote: post #30
ncmncm wrote: post #29

It seems like the difference in lifetimes demands new physics. If the magnetically trapped neutron lifetimes match Standard Model predictions, then something involved in getting them into the beam must be changing them, or selecting out longer-lived individuals, both of which seem bonkers. There is probably a Nobel for whoever solves this. Capturing some from a beam into a magnetic trap seems like a good start.

The most-likely explanation for the discrepancy between the methods is systematic uncertainty. I think it will be a rather spectacular surprise if the refined beam measurements continue to disagree with the bottle measurements a decade from now. If the discrepancy persists, then there really will be a problem. There is plenty of theoretical speculation, especially in the last couple of years, about what such a discre…

Discovering a new decay pathway would be quite something. Then you would need to figure out why the beta pathway does not match what the Standard Model wants, and also why it does appear now to match what the Standard Model wants.

Nature seems to love tricks like this: Ha, you thought you understood how something is? Wrong, it just pretends to be like that! It is really much more eldritch, and you are damned. (I.e., the maths are too hard for actual people to work.) Good luck figuring out how it is only pretending to be that, too.

Re: Physicists make most precise measurement of neutron’s lifetime

#54

Post by ncmncm »

melbourne_mat wrote: post #36
ncmncm wrote: post #29

It seems like the difference in lifetimes demands new physics. If the magnetically trapped neutron lifetimes match Standard Model predictions, then something involved in getting them into the beam must be changing them, or selecting out longer-lived individuals, both of which seem bonkers. There is probably a Nobel for whoever solves this. Capturing some from a beam into a magnetic trap seems like a good start.

As a physics layman my first thought when reading that is special relativity: things happen more slowly (ie. decay) the faster something is moving. Not sure how fast the beams are moving though.

In a word, slowly. You may reasonably assume that particle physicists are familiar with relativistic time dilation, and how to adjust for it if that were needed.

That is not a universally reasonable assumption. Cosmologists apparently never really tried adjusting their expectations for galaxy rotation for general relativity. When a plasma fluid dynamicist facile with the maths looked, the rotation anomaly evaporated. (Cosmologists were assiduously ignoring this, last I checked.) GR maths are considered hard, except by plasma fluid dynamicists, who have to do actually hard maths just to graduate.

Re: Physicists make most precise measurement of neutron’s lifetime

#55

Post by kzrdude »

gus_massa wrote: post #46
kzrdude wrote: post #15

Earlier quoted context omitted.

How does the scattering affect the neutrons? When do we start the clock for their lifetime anyway? It sounds like they could absorb + reemit sometimes when being scattered.

It's one of the nice properties of exponential decay. They are measuring the mean time of life (~15 minutes) but it's easier to explain with half life (~10 minutes). If you have a bunch of neutrons and put them in a box, and look again 10 minutes later, you will see that you have only half of them. If pick all the neutrons that survived for 3 minutes, and put them in a box, and look again 10 minutes later, you will s…

Thanks for the reminder - we can start the clock at any time :)

Re: Physicists make most precise measurement of neutron’s lifetime

#56

Post by cozzyd »

adrian_b wrote: post #52
hilbert42 wrote: post #50

Earlier quoted context omitted.

A question I've never given much thought about until now. When the neutron begins to decay [i.e. at or around ~14.63 min] then is the duration of that decay process essentially equivalent to the time taken for the W- boson to decay into an electron and antineutrino?

There is no way to measure the "duration of the decay process". When you look for it, you either find a neutron that has not decayed yet, or you find the decay products. The quoted decay time is just the average time. A free neutron might decay after 1 millisecond or after 5 hours. Like for any other decay process, it is unpredictable when an individual neutron will decay. Nevertheless the decay probability in a time…

I think parent was asking about how long the virtual W "exists" for in the decay, but the virtual W is virtual so I don't think it has a "lifetime," but maybe I'm not thinking about this correctly.

Re: Physicists make most precise measurement of neutron’s lifetime

#57

Post by cozzyd »

akomtu wrote: post #51
ben_w wrote: post #45

Earlier quoted context omitted.

I understand the wariness. However, in relativity a photon cannot decay: because it travels at the speed of light and has infinite time dilation, it does not subjectively experience the passage of time in which the possibly of decay could exist.

That's the model of photon, not the photon itself. A mathematical photon cannot decay. What prevents a photon from hitting a quantum bump on its way between galaxies and become an electron?

Indeed ultra high energy photons "decay" by interacting with the CMB.

Re: Physicists make most precise measurement of neutron’s lifetime

#58

Post by kadoban »

JumpCrisscross wrote: post #42
kadoban wrote: post #18

Earlier quoted context omitted.

This is the best answer I've seen: https://physics.stackexchange.com/a/63387 which to my layman eyes basically seems to match your theory.

Does this mean the neutrons are continuously degrading? Their decay products continuously recombining into new neutrons? (Though that would imply "evaporation" at the surface?)

I _think_ not, it's just energenically unfavorable so it should ~never happen.

https://en.m.wikipedia.org/wiki/Urca_process this does seem to be going on though, which tbh I just don't understand so it may contradict what I just said, or not.

Re: Physicists make most precise measurement of neutron’s lifetime

#59

Post by ben_w »

cozzyd wrote: post #57
akomtu wrote: post #51

Earlier quoted context omitted.

That's the model of photon, not the photon itself. A mathematical photon cannot decay. What prevents a photon from hitting a quantum bump on its way between galaxies and become an electron?

Indeed ultra high energy photons "decay" by interacting with the CMB.

Using the word “decay” to include that feels like it can’t be right.

But I’m not a proper physicist, just an amateur.

Re: Physicists make most precise measurement of neutron’s lifetime

#60

Post by colechristensen »

supernova87a wrote: post #17

Here's a question I was always curious to hear a good intuitive explanation for -- Why does a neutron star not decay, as it is composed of neutrons, and free neutrons should decay in 15 minutes? Is it because the neutrons are in an energetic "well" and to decay out would actually require energy? In collapsing under gravity to neutron degeneracy, did the neutrons say, radiate away their ability to decay any more?

It seems there are several things going on. Protons absorbing electrons becoming neutrons doing the opposite of decay, a sort of connective cycle that radiates away neutrinos while enabling decay and reverse decay, and plain old Pauli exclusion where there is no room for electrons.

But it all comes down to lots of things being possible in such a crowded place and neutrons not being “free” in a neutron star. Nuclear chemistry is full of crazy stuff happening constantly in stable situations that all kind of balances out.

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