Protons: Stability, Decay & Half-Life

In summary: I have been instructed to only provide a summary of the conversation, without any additional comments or replies. Thank you for understanding. In summary, the question of whether protons decay or are completely stable is still under debate. According to the Standard Model, protons are stable, but some grand unified theories suggest that they may decay with an incredibly large half-life of 10^33 years. This decay process would involve the transformation of quarks into leptons, resulting in the formation of a positron and a pi-zero particle. However, no unambiguous decay events have been observed, and ongoing experiments continue to push the upper boundary of the proton half-life. Some believe that proton decay is possible, but it may never be observed due to
  • #1
Jack
108
0
Do protons decay or are they completely stable? If they do then what is their half-life?
 
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  • #2
In the Standard Model (which describes the strong, weak, and electromagnetic forces), the proton is stable and does not decay.

According to many of the grand unified theories (which try to unify gravity along with the strong, weak, and electromagnetic forces), the proton should decay, but perhaps with an incredibly large half-life -- something like 10^33 years. (By contrast, the universe has only been around for 1.37 x 10^9 years.)

The proton is composed of quarks. Should these grand unified theory candidates be correct, the quark would be allowed to decay into a lepton -- a process which is held by the standard model to be forbidden. The proton would decay into a positron and a pi-zero particle.

To be more specific, the proton is composed of two up quarks and one down quark. A up quark has a charge of +2/3, while a down quark has a charge of -1/3. Supposedly, according to the grand unified theory candidates, the two up quarks together change into some very massive particle with charge +4/3. This very massive particle then decays to into a positron (charge +1) and an anti-down quark (charge +1/3). The new anti-down quark, together with the original down quark, form a pi-zero meson. The total reaction, therefore is (proton) -> (position, pi-zero).

The reason this decay mode is so unlikely, if it exists at all, is because of the enormous mass of the intermediary particle. The reaction can only proceed via quantum tunneling, but, of course, the likelihood of a tunneling event depends strongly on the height of the energy barrier. In this case, the energy barrier is quite large.

- Warren
 
  • #3
Originally posted by chroot
In the Standard Model (which describes the strong, weak, and electromagnetic forces), the proton is stable and does not decay.

According to many of the grand unified theories (which try to unify gravity along with the strong, weak, and electromagnetic forces), the proton should decay, but perhaps with an incredibly large half-life -- something like 10^33 years. (By contrast, the universe has only been around for 1.37 x 10^9 years.)

...

- Warren

Really great explanation, concise.

I might add that there are ongoing experiments being performed to detect proton decay in order to point us in the right direction. No unambiguous events have been observed to date.
 
  • #4
Originally posted by DrChinese
Really great explanation, concise.

I might add that there are ongoing experiments being performed to detect proton decay in order to point us in the right direction. No unambiguous events have been observed to date.
Good addition -- Super Kamiokande, among other sites, have been looking for proton decay in huge quantities of ultra-pure water for a long time now. These experiments have pushed the upper boundary on proton half-life to 10^33 years, and it's still rising.

Anyone want to place any bets? :D

By the way, I made a mistake in my explanation -- the universe is roughly 13.7 x 10^9, NOT 1.37 x 10^9, years old. Numerical typos can be beasties.

- Warren
 
  • #5
Originally posted by chroot
Good addition -- Super Kamiokande, among other sites, have been looking for proton decay in huge quantities of ultra-pure water for a long time now. These experiments have pushed the upper boundary on proton half-life to 10^33 years, and it's still rising.

Anyone want to place any bets? :D

By the way, I made a mistake in my explanation -- the universe is roughly 13.7 x 10^9, NOT 1.37 x 10^9, years old. Numerical typos can be beasties.

- Warren

Sure, easy to place a bet on this one. There will NEVER be any unambiguous decay events observed.

Why? I think it is very possible that the theory of quantum tunneling for this type decay is correct (i.e. the proton can decay). But here is the left turn... According to quantum theory, the chance of such an event is fully independent of any previous state of the system. That is because there is no root cause.

However, suppose that were NOT true. Suppose it was connected to some previous state of the system, but that the time cycle for another occurance was relatively long. As in 10^33 years or something like that. So the equivalent picture would be as follows. You arrive at a traffic light that is red and wait for it to turn green. But it is going to be another 10^33 years. The universe is much less than 10^33 years old. Therefore, until the universe is of equivalent age, there isn't much chance of such an event occurring. Since few events have this long cycle, it is not noticed for other quantum events such as radioactive decay which may have a half life of minutes, years or even thousands of years.

(Just a concept.) So I think proton decay is possible, but won't be observed. I think Steven Weinberg believes in proton decay. If it's good enuff for him, it's good enuff for me.
 
  • #6
I believe theoretical physicts mean half-life.
(And that's why they're making experiments at
all :wink:.)
 
  • #7
As in 10^33 years

I'm sorry...but just to clarify my doubt...in some books I read that the half life is something like 10^45 years...
 
  • #8
Originally posted by Stranger
I'm sorry...but just to clarify my doubt...in
some books I read that the half life is
something like 10^45 years...
Whoaa ! Even if proton decay does occur,
such a figure would certainly greatly
complicate the experiments.
 
  • #9
Actually wait... proton decay is random isn't it? Then, even if half life is however many years, some protons do decay before then, even though with extreme rarity. So it should be possible to "catch one in the act".
 
  • #10
Originally posted by FZ+
Then, even if half life is however many
years, some protons do decay before then,
even though with extreme rarity. So it
should be possible to "catch one in the act".
Yes, that's what half-life means. However,
if it's that large (10^45 yr)then almsot
no matter the actual shape of the function the
experiment is probably extremely demanding for now.
 
  • #11
Originally posted by FZ+
Actually wait... proton decay is random isn't it? Then, even if half life is however many years, some protons do decay before then, even though with extreme rarity. So it should be possible to "catch one in the act".

Yes, supposed to be random, so if:

Half life = 10^40 years
Experimental sample = 10^40 protons
Chance of seeing 1 event in a year: 50%

This is the basis of the on-going experiments.
 
  • #12
Visible part of universe contains about 1080 protons, so there can be about 1040 proton decays in it every year, which is more than 1032 decays each and every second.

In Earth alone it results in few decays/sec.
 
Last edited by a moderator:

1. What is the stability of protons?

Protons are considered to be stable particles, meaning they do not spontaneously decay into other particles. This stability is due to the conservation of energy and the strong nuclear force that holds protons together.

2. How long is the half-life of a proton?

Since protons are stable particles, they do not have a half-life. This means that they do not decay over time and their number remains constant.

3. Can protons decay into other particles?

While protons are considered stable, they can theoretically decay into other particles such as neutrons or positrons in certain conditions. However, this decay has never been observed in experiments and is highly unlikely to occur.

4. How does the stability of protons affect the stability of atoms?

Protons are the building blocks of atoms and their stability is crucial for the overall stability of an atom. If protons were to decay, it would result in the breakdown of the atom and the formation of new elements with different properties.

5. Do protons have a finite lifetime?

As mentioned earlier, protons are considered to be stable particles with no observed decay. Therefore, they are believed to have an infinite lifetime and do not cease to exist over time.

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