Positron decay

learning stuff for modules tommorow - positron decay,

So, why don't protons and therefore atoms decay around us? it has a long life - but with the numbers involved why does everything stay so constant?

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Reply 1

Morbo

Protons have a very long lifetime. I think it's much longer than the age of the universe or something. So even given the vast number of protons in an experiment you might do, it's very unlikely that one will decay in the sort of time period that you would devote to looking.

And it's not "positron decay" - positrons do not decay, they are fundamental particles. "Beta-plus decay" is the proper term.

Reply 2

F1 fanatic

The half life is something like 10^36 years isn't it? Something insane anyway, as you say. I believe proton decay has been observed (a single proton that is) but I may be wrong.

Reply 3

jpowell

Protons have not been observed to decay, the standard model does not predict that protons will decay.

Experimental bounds on proton lifetimes are around 10^30 years, i.e. a lot longer than the age of the universe.

So given that, why would you expect to see protons decaying around you in the first place?

Reply 4

Mohit_C

Well don't forget that the very first few protons were there long before humans existed so there might be a chance of watching a proton decay if we can find a long enough particle, which could be very rare.

Reply 5

jpowell

Particles don't have memories of the past, nor do they have an inbuilt ticking clock. The lifetime of a particle is a statistical thing :biggrin:

Reply 6

Mohit_C

Well the statistics have to come from some sort of discovery. Isn't there a way of determining how long a proton has existed?

Reply 7

jpowell

No. The experimental statistics come from the observation that given a very large sample size of protons, the fact that none have been observed to decay imply they must have a very large half life.

I suggest you look into the notion of half-life of particles, a given particle only has a probability to decay at any time, not a timer that says when it is going to decay.

Reply 8

F1 fanatic

jpowell

You could potentially see the odd one no? If you're sample size was absolutely huge. It is only an average mean lifetime after all. As you say, it is a statistical thing. The probability is admittedly pretty infinitesimal.

Reply 9

jpowell

I guess my point is that, given that we do not observe proton decay, why would you expect it to be anything other than an extremely rare insignificant event?

Reply 10

Mohit_C

What do we gain out of any decaying particle anyway, other than how long it will last?

Reply 11

Morbo

I'm sure you can attempt a calculation at the lifetime of a proton in QCD.

If the lifetime is 10³⁰ yrs as someone mentioned, how many protons would you need in order to have a chance of observing say, 100 proton decays during a month of observation (about 3 a day)?

N = RT = 1000yr^-110³⁰yr = 10³³ protons, or about 1000 tonnes of protons. Now, this is not impossible, just get 2000 tonnes of water and watch it for proton decay for a month and you should see some.

However, I think the problem is that even if you have 10000 proton decays per month in your massive tank of water (remind anyone of a neutrino detector?), this still isn't a very frequent event. The detectors have a certain level of sensitivity, and there is a certain background noise which your signal must penetrate through. Also, I think there are other processes which might produce positrons to complicate things. Detecting such a weak signal of positrons is hard, I believe.

It's not that proton decay has never been observed (I'd be surprised if it hadn't in the SNO), it's just that it's hard to tell whether it was a proton decay for sure. (Note - all this I am trying to remember from talking to a nuclear physicist in 1st year...)

Reply 12

jpowell

Worzo

I'm sure you can attempt a calculation at the lifetime of a proton in QCD.

I don't think you can, I'm pretty sure that protons are stable in the standard model, and that proton decay is a purely hypothetical process that is supposed to occur in certain beyond-the-standard-model theories.

Worzo

It's not that proton decay has never been observed (I'd be surprised if it hadn't in the SNO), it's just that it's hard to tell whether it was a proton decay for sure. (Note - all this I am trying to remember from talking to a nuclear physicist in 1st year...)

If you can't tell if the proton had decayed, does the tree make a sound at all?

Reply 13

Wangers

asked teacher,

Apparently lone protons are stable? and bound ones in the nucleus have higher energy and so entropy comes into it.
Yep - in normal matter even if it does decay, it has no significant observable effect.
And sorry - it is Beta + decay, head scrambled after exams.
The book - Fundamental facts and mysteries of particle physics - says something about its the energy needed to insert a neutron into the nucleus or something. And because of that its not energeticlly favourable??
Clearly I hav'nt understood it - I'll go look again later.

Thanks folks :smile:

Reply 14

Wangers

jpowell

If you can't tell if the proton had decayed, does the tree make a sound at all?

But thats philosophical, not empirical. Thats like saying - I havent seen some 4Bn of the worlds population - thus they may not exist.

Besides its s statistical probability - perphaps you are just unfortuneate enough to have for some unknown reason, a particularly stable sample of water. They may decay surely, just not at the rate that you want or predict. As someone pointed out - they have no sense of time.

Reply 15

jpowell

I haven't seen most of the 7 billion people on this planet, but that doesn't mean I don't believe they exist. After all there is plenty of evidence for them existing.

However I haven't "seen"/observed a proton decaying, add to that the fact that no experiments have reported any conclusive proof of the proton decaying, add to that the fact that the standard model does not predict that protons should decay... I don't think I should expect to observe a large number of protons decaying in the near future.

I was just trying to point out that when Worzo mentioned it was difficult to check if any particular possible proton decay event was actually a proton decay, it makes it a whole lot harder to draw conclusions from such a small statistical sample size.

Reply 16

Mehh

Wangers

But thats philosophical, not empirical.

Quantum Mechanically its not. What we think of as 'history' can be thought of as a large subset of possible universes, which given time can evolve into ours. This is given that we apply the sum over history interpretation upon the wavefunction of the entire universe.
Thus one can only say something has happened, because there is evidence to suggest it happened.

To clarify, imagine two seperate universes. One exists where an alien culture built the entire world, populated it with animals and added fossils to indicate the world was much older. Another exists where small specks of dust accrete together blah blah blah.
Fast forward 100 years for the first one, and 100million years the second one. How does one work out which world is which?*

*Unless some dude leaves his face on some fjords.

Reply 17

Wangers

Yes but the point was I thought that Beta + decay was already a validated process - wasn't it detected in a deep mining shaft full of water? or was that neutrinos...

But we have already established why you might not observe proton decay very easily....

Mehh - we do have to make some assumptions and assessment. QM does man we cant even fully understand a single nody system....but there has to be a line - otherwise all of science could be considered as "not even wrong".

Reply 18

Wangers

Mohit_C

What do we gain out of any decaying particle anyway, other than how long it will last?

No you get information about fundamental componants and particles. That is how neutrinos were discovered - because of a range of energies...."dear radioactive ladies and gents etc...."

From a large number of decays you can make deductions from the energies involved, due to the laws of thermodynamics.

Reply 19

Mehh

Wangers

From a large number of decays you can make deductions from the energies involved, due to the laws of thermodynamics.

Neutrinos were discovered because people knew to look for them.
Beta decay, pre-neutrinos was a two body system that, in theory should be simple to solve, ie have only one solution. This is because of conservation of momentum. However we observed that the resultant beta particle and proton defied conservation of momentum.
The neutrino was postulated by Pauli (ironic that you quoted him in the last post) to explain where the extra momentum went. In otherwords every beta decay experiment had indirect evidence for the neutrino. In fact we know exactly what happens to it. Unfortunately it took a few decadeds for us to be able to detect the neutrino directly.

There is however two motivations for study of Proton Decay (if it is even indeed possible).
There are Grand Unification Theories that require it.
It would be an example of Baryon numbers being not conserved. One condition for a matter dominated universe (without special conditions).

The mine shaft you refer to is Super-Kamiokande, which Worzo refered to. The experiment is actually a neutrino experiment which was looking for neutrino oscillations. I've talked to a Prof at my College who is actually trying to secure funds for an upgrade to the SK experiment to measure the last phase factor of the Maki-Nakagawa-Sakata matrix.
As part of their experiment they have had to develop models to identify events which are neutrino events. This of course meant they had to develop models to detect probable proton decay events as well. From their data, and an optimistic attribution to proton decay they estimated a lower bound for the half life.
They have not however 'discovered' a proton decay, which would I think requires a 5 Sigma event.

we do have to make some assumptions and assessment. QM does man we cant even fully understand a single nody system....but there has to be a line - otherwise all of science could be considered as "not even wrong".

I don't actually think that statement is entirely correct. QM does not mean we can not fully understand a single body system. It does however redefine the concept of understanding. Classically a particle has both a position and a momentum, which are mutually independant variables. QM redefines the particle's properties as a single wavefunction.
We can still make very accurate predictions. But now we need to play with dice.