Protons: Stability, Decay & Half-Life

[Jack]

Do protons decay or are they completely stable? If they do then what is their half-life?

 

[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.)

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

 

[DrChinese]

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.

 

[chroot]

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

 

[DrChinese]

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 occurrence 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.

 

[drag]

I believe theoretical physicts mean half-life.
(And that's why they're making experiments at
all .)

 

[Stranger]

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...

 

[drag]

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.

 

[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".

 

[drag]

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.

 

[DrChinese]

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.

 

[Alexander]

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.