Is the Proton's Half Life Really 10 to the Power of 32 Years?

[quantumdude]

taxman,

According to the standard model, it is absolutely impossible for protons to decay. The argument "everything changes over time" is not a strong one at all.

To that I would add only that "everything changes over time" does not imply "everything decays". The proton does constantly change--internally. There are gluons running around, virtual quark-antiquark pairs constantly popping into and out of existence, etc. Also, if struck with a sufficiently energetic particle, the proton will be exicted into a resonance, and then deexcite back down.

The inside of a proton is a hive of activity. It just so happens that it goes through all that change without changing its asymptotic identity (that is, it don't decay ).

 

[arivero]

As for the question of artifitial production of protons... Machines as the LEP get hadrons, ie mesons and barions, from the product of electron positron colisions. This should include protons, shouldn't it?

 

[bkfizz02]

The original question was about the stability of the proton. I agree that the proton does not decay, but I do not believe that we can "prove" this. One could imagine a scenario the the original post presented - proton --> positron + photon. Certainly this is allowed by energy considerations. But it is not allowed by conservation of lepton number. In fact, for every proton decay that you can imagine that does not violate conservation of energy (a physics axiom), a conservation law is broken.
I think that this fact is what makes the original question really important. If we could observe a proton decay, we would be observing the breaking of a conservation law. Since we believe in many conservation laws, the direct experimental contradiction of them would be crucial for retooling our thinking.

Another example of a process which "cannot" happen is neutrinoless beta decay. People look for this process, which would violate lepton number conservation. If they found it, the proton decay question would almost certainly need to be revisited.

 

[Terry Giblin]

More to the point...

Unless the laws of statistics have changed since I left school.

Has anyone simply calculated the probability that we have not seen a proton decay already, by now - what's the definition of a "half-life"?

Regards

Terry Giblin

 

[marlon]

The only way a proton would disintegrate is if it were to have a tremendously high speeds. Asymptotic freedom then states that the strong force becomes smaller so the fluxtube between the constituent quarks will "decay" into gluons and the quarks would move away from each other.

At least this is the picture of the dual abelian higgs model using the concept of dual superconductivity and the S-duality of coupling constants + charge quantization from Dirac.

Ahhh, and you also need magnetic monopoles, responsible for this quantization.

regards
marlon

 

[humanino]

I really don't get it Marlon. The proton is more likely to decay at higher speed !? Usually, at higher speed, decay is just lowered by lorentz contraction of time. Or : the fastest decay should occur in the rest frame !

I could concieve that there is a dynamical process occurring during the acceleration process.

for every proton decay that you can imagine that does not violate conservation of energy (a physics axiom), a conservation law is broken.

Barionic number conserved through other conservation laws !?
Oh, by the way : is barionic number conservation not much better experimentally tested than the other conservation laws (with regards to proton's lifetime. I have been checking PDG online, and I am not quite sure.) ?

 

[vanesch]

The original question was about the stability of the proton. I agree that the proton does not decay, but I do not believe that we can "prove" this. One could imagine a scenario the the original post presented - proton --> positron + photon. .

The idea of proton decay comes from GUT (Grand Unifying theories). In the standard model, we have the gauge group SU(3)xSU(2)xU(1), where SU(3) describes a (non-broken) gauge symmetry group between the colors of the quarks (and the relevant bosons are the gluons of the strong interaction), while SU(2)xU(1) describes the (broken) symmetry of the electroweak charges (essentially chirality, hypercharge and electrical charge) and the relevant bosons are W+, W-, Z0 and the photon.

As such, there is no interaction possible which turns a quark into a lepton and from this follows baryon conservation. In the standard model, the 3 lepton families, and the 3 quark families, have a priori not much to do with each other, but the very fact that there are 3 of each cries out for a deeper structure.
Indeed, if you make a large tuple of all the fermion fields in one family (the 2 quarks - righthand, the two quarks - lefthand, the electron and neutrino - lefthand, and the electron, righthand) this big tuple transforms under a composite representation of SU(3)xSU(2)xU(1) which is really put in there by hand. On the other hand, it fits into only 2 different representations of SU(5), which contains SU(3)xSU(2)xU(1), namely the so-called 5* and the 10 representation. But SU(5) is not equal to SU(3)xSU(2)xU(1), it contains in fac t 24 gauge bosons, so there are EXTRA INTERACTIONS, and some of these link quarks and leptons, which comes down to say that a quark could, through this interaction, change into a lepton. Of course one assumes that this symmetry is broken, and that the relevant bosons have a big mass (the "GUT" scale), which decreases this interaction rate. If one estimates this scale (that's where the 3 coupling constants of the standard model should unify) one arrives at something of 10^15 GeV. Using this value, the proton decay rate should be of the order of a life time of 10^31 years, which has been falsified by experiment. So that's where all the hassle came from.
But people have not given up: other groups than SU(5) are possible ; this was simply the "smallest" group that could contain the standard model. People have been working on SO(10), for example.

cheers,
Patrick.

 

[Nereid]

IIRC, the experimentally observed limit on the proton's half-life is ~10³⁵ years, at least for several (one?) of the more 'likely' decay modes.

 

[Terry Giblin]

What's the probability of not seeing one decay

the proton decay rate should be of the order of a life time of 10^31 years, which has been falsified by experiment.

Lets assume the half-life on an electron, as estimated is 10^31 years, based on our current on going testing for detection of the first ever observed proton decay.

Is it not time to start considering alternative ideas and models

Regards

Terry Giblin

 

[Nereid]

Lets assume the half-life on an electron, as estimated is 10^31 years, based on our current on going testing for detection of the first ever observed proton decay.

Why not simply do an experiment to measure the half-life of the electron? How do experimental results showing that the half-life of the proton is ~>10³⁵ years lead to an estimate of half-life of the electron?

Is it not time to start considering alternative ideas and models

Of course! It's been obvious for some time that the Standard Model is in need of replacement/extension/etc! I doubt that there are any researchers active in particle physics who feel that the Standard Model is the end.

 

[vanesch]

Of course one assumes that this symmetry is broken, and that the relevant bosons have a big mass (the "GUT" scale), which decreases this interaction rate. If one estimates this scale (that's where the 3 coupling constants of the standard model should unify) one arrives at something of 10^15 GeV. Using this value, the proton decay rate should be of the order of a life time of 10^31 years, which has been falsified by experiment. So that's where all the hassle came from.

I should add a few things. The GUT scale is in fact already build into the standard model (together with the experimental values of the coupling constants), as we know, at "low" energies what are the coupling constants, and we can calculate how they evolve (running of the coupling constants in renormalization) when we go to higher energies (a few low-order Feynman graphs are sufficient). The nice thing is that they all cross (the electroweak couplings rise, and the strong coupling falls) at about 10^15 GeV. They ALMOST cross, but not really. If they are the results of a broken symmetry, then at the scale of the symmetry, they should become equal to the one and only coupling constant of the grand unifying interaction. And now, the precision on experimental parameters is such that we know that they do not cross exactly at the same point.
Comes in supersymmetry. It is possible to twiddle a bit here, and then one can make the running constants cross exactly. I'm not an expert on this, but apparently, supersymmetry can also "inhibit" proton decay, so that its life time becomes longer than naively estimated. So proponents of supersymmetry say that SUSY can save GUT and that the long proton decay life time is an indirect indication. Others say that this is a bit too artificial twiddling to get out what people wanted to get out.

cheers,
Patrick.

 

[jtolliver]

The mystery of the asymmetry of matter-antimatter remains to be resolved. At the present time, no such a theory of resolution exist.

We don't know that there is an asymmetry between matter and antimatter, except in this portion of the universe. Most astronomical data is from photons emitted by far away sources. Antiparticles interactions with other antiparticles are the same as particles interactions with other particles. If some region of the universe consists of antimatter, astronomical measurements would not be able to determine that, since there is no matter interacting with it for us to compare it with.

 

[Nereid]

We don't know that there is an asymmetry between matter and antimatter, except in this portion of the universe. Most astronomical data is from photons emitted by far away sources. Antiparticles interactions with other antiparticles are the same as particles interactions with other particles. If some region of the universe consists of antimatter, astronomical measurements would not be able to determine that, since there is no matter interacting with it for us to compare it with.

a) there'd be all kinds of fireworks from a zone of contact ... even the inter-galactic medium isn't empty, and cosmic rays pervade everything (no such fireworks observed); b) while not impossible for there to be regions of matter and regions of anti-matter in both of which there are stars, galaxies etc ... which don't ever get to an opposite region, there will be stars, planets, even galaxies that travel far from their parent cluster (no such megafireworks observed); c) cosmological models with this slight imbalance between matter and anti-matter fit observations well (I'm not sure models in which the matter and anti-matter are equal would fit at all).

 

[marlon]

I really don't get it Marlon. The proton is more likely to decay at higher speed !? Usually, at higher speed, decay is just lowered by lorentz contraction of time. Or : the fastest decay should occur in the rest frame !

I could concieve that there is a dynamical process occurring during the acceleration process.




Barionic number conserved through other conservation laws !?
Oh, by the way : is barionic number conservation not much better experimentally tested than the other conservation laws (with regards to proton's lifetime. I have been checking PDG online, and I am not quite sure.) ?

This is the principle of asymptotic freedom coming from renormalization. It states that the strong force gets weaker when the energyscale gets higher. The strong force gets stronger when the energyscale gets lower. This is why quarks are never allone in the QCD-vacuum. Wellcome to the world of Quarkconfinement

marlon

 

[humanino]

I don't see how asymptotic freedom could affect the stability of the proton.

Asymptotic freedom tells you that quarks look like free particles at high energy scales. Which scale ? In the context I am working in, Deep Inelastic Scattering (DIS), one typically has an lepton beam incident on an hydrogen target. The lepton exchange a virtual boson with (one of the quarks inside) the target. The energy scale at which one probes the hadronic structure is typically given by the invariant mass of the virtual particle. For instance, the mass of the photon exchanged. Upon selecting highly virtual exchanged bosons, one observes free quarks.

I am not working on experiments involving hadronic beams. However, I am still convinced that the decay of any particle is the fastest in the rest frame.

 

[Haelfix]

Again to nitpick, its possible that the proton can decay, even in the minimal standard model. Its just vanishingly small in principle, and again its only a theoretical argument and never been observed.

It has to do with baryon and Lepton # nonconservation from the electroweak sector first discovered by T'Hooft when he was studying topological effects on gauge theories. Its also really hard to quantify, particularly when you add in GUT interactions (that also imply a proton decay) as you end up with monopole contributions that makes things a nice mess.

Despite many attempts to find a proton decay signature, it has still remained elusive. Perhaps the best indirect evidence that this is true lies in the theory and models of Baryogenesis and the emerging field of Leptogenesis.

But if the world was pure QCD, you would never find a single quark anywhere. The force would grow to infinity the further you pull them apart, and our naive guess would be satisfied.

 

[zefram_c]

Again to nitpick, its possible that the proton can decay, even in the minimal standard model... It has to do with baryon and Lepton # nonconservation from the electroweak sector first discovered by T'Hooft when he was studying topological effects on gauge theories. Its also really hard to quantify, particularly when you add in GUT interactions (that also imply a proton decay) as you end up with monopole contributions that makes things a nice mess.

This is new to me; would you care to elaborate? My understanding of the standard electroweak model is that quarks couple only to other quarks and leptons to leptons. How would this break down?

 

[Haelfix]

I couldn't find T' Hoofts remarkable paper on SLAC (its in physical review somewhere) upon cursory browsing..

But I did manage to find a few papers that talk a little about these effects.

Briefly:

http://arxiv.org/PS_cache/hep-ph/pdf/9608/9608456.pdf

For more emphasis on astrophysics:

http://arxiv.org/PS_cache/hep-ph/pdf/9603/9603208.pdf