The ever-increasing proton lifetime

[swampwiz]

I was reading this article discussing how experiments have been able to observe proton decay:

https://www.economist.com/news/scie...ry-fundamental-physics-frustrating-physicists

It states that after concluding that there has been any evidence of proton decay in certain experiment, the lower bound of the average lifetime of a proton must be increased. Is this simply a calculation that given a certain number of protons being observed for a certain amount of time, the statistical spread of lifetimes must be such that it would be *probable* (perhaps 50%?) that a proton decay would have been observed if the average lifetime were a certain value (i.e., that becomes the minimum lifetime)?

It would seem to me that the answer all along is that protons don't decay, but I suppose that it is impossible to prove a negative. What gives physicists the notion that protons are supposed to decay in the first place?

 

[Nik_2213]

Did you mean, "... how experiments have not been able to observe proton decay..." ??

 

[Urs Schreiber]

By the way, for whatever it's worth, recently claims have appeared of GUT models without proton decay:

• Andreas Mütter, Michael Ratz, Patrick K.S. Vaudrevange,
  "Grand Unification without Proton Decay"
  (arXiv:1606.02303)
• Bartosz Fornal, Benjamin Grinstein,
  "SU(5) Unification without Proton Decay",
  Phys. Rev. Lett. 119, 241801 (2017)
  (arXiv:1706.08535)

 

[Vanadium 50]

Is this an A-level thread? i.e. do you have a graduate-level education in physics? I don't want to start writing and then find out it's all at the wrong level.

 

[MathematicalPhysicist]

Long live the proton!

 

[swampwiz]

Is this an A-level thread? i.e. do you have a graduate-level education in physics? I don't want to start writing and then find out it's all at the wrong level.

I have the 2 semester sequence in calculus-based physics for technical students (degree in mechanical engineering and then graduate study in engineering mechanics), LOL. That said, I am on the path of discovery. If this thread should be at a lower level, I would have no problem in changing it.

 

[swampwiz]

Did you mean, "... how experiments have not been able to observe proton decay..." ??

Yes, that is what I had meant.

I guess what I am having grokking is why should a proton be seen as something that decays? IIUIC, there has never been such an observation.

 

[protonsarecool]

Well, according to the standard model, the proton does not decay. However, it is impossible to proof that experimentally, so the best we can do is say: "We have observed no proton decay in time T, so we know that the half life has to be at least τ to be consistent with this observation".

On the other hand, we know that the standard model has its flaws, and some theories beyond the standard model do have decaying protons, and a positive result on decaying protons would strengthen the validity of these theories.

 

[Vanadium 50]

hy should a proton be seen as something that decays?

Why shouldn't it?

A good theory would explain why and how fast the proton decays if it is unstable, or why it does not if it is stable.

 

[mfb]

We live in a universe with matter and nearly no antimatter. Nearly all ideas where this asymmetry could come from imply that protons can decay, and some models predict a lifetime somewhere in the range of our experimental limits. The tests can either confirm proton decays or rule out some models.

 

[Urs Schreiber]

Nearly all ideas where this asymmetry could come from imply that protons can decay, and some models predict a lifetime somewhere in the range of our experimental limits.

What would be a good source for discussion of observational proton decay limits in relation to models of baryogenesis via the chiral anomaly?

 

[epenguin]

Is this an A-level thread? i.e. do you have a graduate-level education in physics? I don't want to start writing and then find out it's all at the wrong level.

With my knowledge background I have little occasion to visit this section of PF, but I had been intending for the last week to get around to bringing this article to the attention of members, hether here or in a more general section. .And, yes, hope that the priesthood could talk in simplified parables that convey something to us illiterate though worshipful peasants.

(I am one of those who has happily read quite a lot over time about this kind of physics. In a popular science journal an article on it will usually be the first thing I read. A lot of people are like that as a look at the popular science shelves in any bookshop will show - more books on this than on more concrete things of public concern like, say genetic engineering advances. We can't really understand the theories the books and articles talk about, I sometimes call them explanations without explaining or vice versa, but we have a pleasure in the patter and being bamboozled, and it's information about what's going on in this fundamental sphere. Okay it looks like there is some stuff I might try on this very site, must get around to that.)

At my level this article is well written and informative. I have long known of the hierarchy problem (gross disparity between the fundamental forces) but to the layman more telling than a number is just the comparison of a fridge magnet exerting more force electromagnetcally than the whole Earth gravitationally. I'd heard of most of the big science and big theories stuff, new to me and interesting were some small science endeavours. Trying to deform an electron in a field of 100GV/cm wow! that apparently exists in thorium monoxide, I imagine not a lot of people know that, whoever first did must have been really into some speciality? And I wonder why and whether it is that unique?

I wonder whether you physics people detect a slightly mischievous tone in the article and what your reactions are? It notes how the main theories have been around a long time. That they have a few successful predictions, seemingly about one each, but also predict lots of things that have not been found when looked for. That some of the theories can be tweaked endlessly. That “With every fudge applied, though, what were once elegant theories get less so”. That maybe they are trying to explain things that do not need to be explained? Quoting that “ideas became institutionalised. People stopped thinking of them as speculative.” (Also is it my lack of knowledge that makes me seem to see that there are breakthroughs in the theoretical field every now and then - but they are like breakthroughs in WW! - the follow-through of the initial promise of success seems always to get bogged down? Is only I who wondered whether it is a branch of the fashion industry? I have to say that the reports in PF by the sadly departed Marcus which we all enjoyed did a good job of making the subject bright, but at the same time quite far from dispelled this impression. I have also heard sometimes sceptical tones from physicists of more down-to-earth branches.)

The article concludes that Yes, it is all worthwhile, and should continue - but i's a sceptical undercurrent surfaces a bit with the funding question. It points out that the physics has for decades had a privileged relation with politics and funding. (I have had occasion to witness how physicists were just that much more effective, successful and better organized at chasing funding and at collaborating, thinking big and working like armies where other scientists were organised at the level of Boy Scout patrols.)

There is a companion article https://www.economist.com/news/lead...r-has-pushed-frontiers-knowledge-further-ever English they propose in which the economist proposes the next big facility after CERN should be in China. One can see that China's hunger for prestige technology hunger might bring full funding that others now would find it difficult to justify. I don't know whether this is a mischievous proposal of an influential Journal dabbling in things outside its field, Just good to know its own wheeze or whether reflects anything else e.g. does reflect something moving in the higher reaches of the profession.

I thought some issues came up for general discussion.

 

[Vanadium 50]

And, yes, hope that the priesthood could talk in simplified parables that convey something to us illiterate though worshipful peasants.

Snark does not become you. And it sure doesn't make me want to participate and answer your questions.

 

[mfb]

What would be a good source for discussion of observational proton decay limits in relation to models of baryogenesis via the chiral anomaly?

I'm not a theorist, I just see that from the side of what would be interesting experimentally.

There is a companion article https://www.economist.com/news/lead...r-has-pushed-frontiers-knowledge-further-ever English they propose in which the economist proposes the next big facility after CERN should be in China.

China is interested in that, but it is unclear if they really want to spend so much money, and it is unclear where the expertise would come from. You need many experts, including many foreign experts, and China is not the most popular place in that aspect.

 

[Urs Schreiber]

I'm not a theorist, I just see that from the side of what would be interesting experimentally.

Sure. What would be a reference for your statement from your favorite point of view.

 

[mfb]

Talks at the usual conferences (Moriond, EPS, ICHEP, ...). I remember the conclusion, but not the individual talks.

 

[Urs Schreiber]

I remember the conclusion, but not the individual talks.

I am just wondering,since its common to discuss proton decay for GUT models, but for some reason I never saw it discussed in the context of models for baryogenesis. I tried to look around with the evident keywords, but no luck so far. Good to hear from you that people at least say the words from time to time. :-)

 

[mfb]

Baryogenesis is a strong argument the baryon number can be violated, and that number is the only thing that protects protons from a quick decay.

 

[Urs Schreiber]

Baryogenesis is a strong argument the baryon number can be violated, and that number is the only thing that protects protons from a quick decay.

Sure, I know, that's why it seems curious that one never sees discussion of proton decay issues in the context of baryogenesis. At least I never saw it discussed in that context. It seems you are saying that you heard people speak about it, but that there is nothing tangible in print that one could point to. (But I'll leave it at that now, unless you have more to say.)

 

[ChrisVer]

on the title: If the proton's lifetime keeps on increasing as fast as our limits, then we will never observe it...

Most of the limits set some confidence level (XX% CL) for the non-observation of proton decay.

What gives us the motivation to look for such decays: the fact that some GUTs predict these decays.
E.g. a GUT which could potentially have some kind of mechanism that breaks baryon symmetry (e.g. couplings to quarks+leptons) through the mediation of heavy bosons is such an analogy -like the SU(5)- (which is also used in baryogenesis if you somehow introduce some CP-violation https://arxiv.org/abs/hep-ph/9801306 )...

 

[mfb]

Sure, I know, that's why it seems curious that one never sees discussion of proton decay issues in the context of baryogenesis. At least I never saw it discussed in that context. It seems you are saying that you heard people speak about it, but that there is nothing tangible in print that one could point to. (But I'll leave it at that now, unless you have more to say.)

A quick search for "proton decay baryogenesis" finds various results.
https://arxiv.org/abs/hep-ph/0005095
https://arxiv.org/abs/1207.5771
http://www.physics.mcgill.ca/~guymoore/research/baryogenesis.html
It even has its own section at Wikipedia: https://en.wikipedia.org/wiki/Proton_decay#Baryogenesis

 

[swampwiz]

Why shouldn't it?

Occam's Razor.

 

[Vanadium 50]

Occam's Razor.

Is not a very good tool to judge experimental results. And experimentally we know that decays that are possible occur at some rate.

 

[Urs Schreiber]

A quick search

I understand the general idea that baryogenesis, by definition, involves baryon non-conservation. What I was hoping to see was some more concrete anlysis with maybe numerical exclusion bounds on what the implication of experimental bounds on proton decay is for models of baryogenesis, similar to the detailed discussion one sees for GUT models, where experimental results have led to some of these models being effectively ruled out.

But I gather that models of baryogenesis just aren't detailed enough in themselves to admit any of this? I gather there is just Sakharov's conditions justifying the general possibility of baryogenesis, without any quantitative ideas of the process. Is that right?

I suppose quantitative understanding of baryogenesis via chiral anomay $\mathrm{div} J_{\mathrm{quark}} \propto tr(F \wedge F)$ all depends on having some idea of $tr(F \wedge F)$ in the early universe, or maybe at least its local fluctuations or something? Maybe there is some indirect (probably very indirect?) experimental bounds on what that could have been?

Anyway, it's these concrete implications of experimental bounds on proton-decay to models of baryogenesis that I never saw discussed, also not in the references that you googled, or else I missed them. Probably they just don't exist. That's fine, I just wanted to know.

 

[snorkack]

Occam's Razor.

Why would Occam´s razor favour baryogenesis?
What is wrong with baryon number being initial parametre of universe? Why are models with primordial baryon number against mainstream and why does Occam´s razor favour models with zero primordial baryon number PLUS baryogenesis imbalance that somehow produces just the observed number of baryons, PLUS predicted proton lifetime that always seems to be not observed at the rate expected?

 

[ohwilleke]

Why would Occam´s razor favour baryogenesis?
What is wrong with baryon number being initial parametre of universe? Why are models with primordial baryon number against mainstream and why does Occam´s razor favour models with zero primordial baryon number PLUS baryogenesis imbalance that somehow produces just the observed number of baryons, PLUS predicted proton lifetime that always seems to be not observed at the rate expected?

I'm with you on this. A positive finite baryon number of the universe as one of its initial conditions is no more problematic than a positive finite mass-energy of the universe at its inception and I have yet to see anyone argue for a Big Bang mass-energy in the universe that is either zero or infinite.

A positive baryon number of the universe as an initial condition is the only possibility that is consistent with the SM.

A vague desire for an initial condition of the universe to be different because it looks pretty if that is the case is not a very compelling reason to go head to head with overwhelming evidence that there are no observed cases of either baryon number violation in general by any means, or proton decay, or lepton number violation by any means, up to very, very stringent limits. It also goes up against the theoretical reality that the only possible baryon number violating process in the SM, the sphaeleron (forgive me if I've spelled it incorrectly), cannot account for baryon number asymmetry in the universe.

Also, keep in mind that the energy levels to which the SM has been experimentally tested are higher than those present in any natural phenomena in the universe for something like 13.5 billion years +/-. We haven't (and never will be able to) experimentally tested the SM at the energy scales of the Big Bang and a brief period of time immediately thereafter, but, any baryon number violating process has to be confined to a very short period of time. Any process that takes even hundreds of millions of years to produce the observed matter-antimatter asymmetry of the universe is too slow to be consistent with the experimental proof of the SM at energy scales we have tested. And, now that the Higgs boson mass has been determined, we know that the SM is theoretically consistent up to the GUT scale.

 

[Urs Schreiber]

the energy levels to which the SM has been experimentally tested are higher than those present in any natural phenomena

Maybe it doesn't matter for your argument, but ultra-high energy cosmic ray particles way beyond LHC energies are rare, but routinely seen by the Pierre Auger observatory.

 

[Urs Schreiber]

A positive baryon number of the universe as an initial condition is the only possibility that is consistent with the SM.

I am wondering if this is true. The chiral anomaly in the standard model says that baryon current conservation is violated by instantons, via $\partial_\mu J^\mu_{B} \propto tr(F \wedge F)$. While it is true that this violation is not seen in perturbation theory, it is present non-perturbatively in the standard model.

Thus it seems that just as with GUTs, already in the plain SM the question is not "why would it violate baryon number conservation at a measurable rate" but "why would it not?".

I have been trying to discuss this with "mfb" above. For some reason much less seems to be known about this for the plain SM baryon number violation, than for hypothetical GUT extensions.

 

[ohwilleke]

Maybe it doesn't matter for your argument, but ultra-high energy cosmic ray particles way beyond LHC energies are rare, but routinely seen by the Pierre Auger observatory.

Fair point. But, it doesn't really affect the argument as these show no sign of SM violations.

 

[ohwilleke]

I am wondering if this is true. The chiral anomaly in the standard model says that baryon current conservation is violated by instantons, via $\partial_\mu J^\mu_{B} \propto tr(F \wedge F)$. While it is true that this violation is not seen in perturbation theory, it is present non-perturbatively in the standard model.

I have seen several papers that calculated this and found the numbers to be insufficient. I'll try to find a cite to one or two of them.