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I The ever-increasing proton lifetime

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  1. Jan 17, 2018 #1
    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?
     
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  3. Jan 17, 2018 #2
    Did you mean, "... how experiments have not been able to observe proton decay..." ??
     
  4. Jan 18, 2018 #3

    Urs Schreiber

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    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)
     
    Last edited: Jan 18, 2018
  5. Jan 18, 2018 #4

    Vanadium 50

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    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.
     
  6. Jan 18, 2018 #5

    MathematicalPhysicist

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    Long live the proton!
     
  7. Jan 18, 2018 #6
    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.
     
  8. Jan 18, 2018 #7
    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.
     
    Last edited: Jan 18, 2018
  9. Jan 18, 2018 #8
    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.
     
  10. Jan 18, 2018 #9

    Vanadium 50

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    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.
     
  11. Jan 18, 2018 #10

    mfb

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    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.
     
  12. Jan 19, 2018 #11

    Urs Schreiber

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    What would be a good source for discussion of observational proton decay limits in relation to models of baryogenesis via the chiral anomaly?
     
  13. Jan 19, 2018 #12

    epenguin

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    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.
     
    Last edited: Jan 19, 2018
  14. Jan 19, 2018 #13

    Vanadium 50

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    Snark does not become you. And it sure doesn't make me want to participate and answer your questions.
     
  15. Jan 19, 2018 #14

    mfb

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    I'm not a theorist, I just see that from the side of what would be interesting experimentally.
    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.
     
  16. Jan 20, 2018 #15

    Urs Schreiber

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    Sure. What would be a reference for your statement from your favorite point of view.
     
  17. Jan 20, 2018 #16

    mfb

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    Talks at the usual conferences (Moriond, EPS, ICHEP, ...). I remember the conclusion, but not the individual talks.
     
  18. Jan 20, 2018 #17

    Urs Schreiber

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    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. :-)
     
  19. Jan 20, 2018 #18

    mfb

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    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.
     
  20. Jan 20, 2018 #19

    Urs Schreiber

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    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.)
     
  21. Jan 20, 2018 #20

    ChrisVer

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    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 )...
     
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