I’ve reached a point in the book-in-progress where I find myself needing to talk a little about particle physics. As this is very much not my field, this quickly led to a situation where the dog asked a question I can’t answer. But, hey, that’s why I have a blog with lots of smart readers…

The question is this:

What are all these extra particles for?

Or, to put it in slightly more physics-y terms: The Standard Model contains twelve material particles: six leptons (the electron, muon, and tau, plus associated neutrinos) and six quarks (up-down, strange-charm, top-bottom). The observable universe only uses four, though: every material object we see is made up of electrons and up and down quarks, and electron neutrinos are generated in nuclear reactions that move between different arrangements of electrons and up and down quarks. The other eight turn up only in high-energy physics experiments (whether in man-made accelerators, or natural cosmic ray collisions), and don’t stick around for very long before they decay into the four common types. So, to the casual observer, there doesn’t seem to be an obvious purpose to them. So why are they there?

Presumably, there is some good reason why the universe as we know it has to have twelve particles rather than just four. Something like “Without the second and third generations of quarks and leptons, it’s impossible to generate enough CP violation to explain the matter-antimatter asymmetry we observe.” Only probably not that exact thing, because as far as I know, there isn’t any way to explain the matter-antimatter asymmetry we observe within the Standard Model. But something along those lines– some fundamental feature of our universe that requires the existence of muons and strange quarks and all the rest, and would prevent a universe with only electrons and up and down quarks.

If there is such an explanation of the “extra” generations, though, I don’t think I’ve run across it. Or else I’ve completely forgotten it. So if you know of a good explanation of why the universe we live in couldn’t be made with only up and down quarks, electrons, and electron neutrinos, leave it (or a pointer to it) in the comments. The dog and I will thank you, both on-line and in print.

Comments

  1. #1 Eric Lund
    January 6, 2011

    Particle physics isn’t my field either, so the best answer I can come up with is, “Because we can observe these other particles experimentally.” Not just in particle accelerators, either; the muon is observed in cosmic ray showers, and we know that neutrinos can oscillate among the different flavors. I have vague recollections of somebody predicting that there should be exactly three families because (a) parts of three families had already been observed and (b) some aspects of the universe wouldn’t work the way they do if there were more than three, but that’s about it.

    Your question is not a new one. I. I. Rabi is said to have remarked about the muon, “Who ordered that?”

  2. #2 Nik Berger
    January 6, 2011

    As the previous commenter remarked, first of all because it is an observed fact. It is also the case that three generations are needed for CP violation. CP violation in the quark sector is not big enough to explain the matter-antimatter asymmetry, but there is some hope for the lepton sector (neutrino mixing matrix), but that is not well measured (yet). There are also some constraints on the maximum number of families, e.g. we know from Z0 decays that there are only three kinds of light (less than 45 GeV) neutrinos.

  3. #3 tim eisele
    January 6, 2011

    Well, I suppose the flip answer to why they exist, is “Why not?” There are lots of things whose non-existence would barely affect the universe (most of the higher trans-uranic elements come to mind), and yet they are there, at least briefly.

    I’m not a particle theorist either, but isn’t that the goal of things like string theory? To identify one fundamental entity with different “vibration modes”, some of which correspond to the stable particles while others correspond to the unstable particles? And in order to get the stable modes, there has to be the possibility of the unstable modes as well?

  4. #4 abb3w
    January 6, 2011

    “Why does it have to be FOR anything?”

  5. #5 Alex
    January 6, 2011

    I think what Chad is asking is whether the universe would be all that much different (after the first few seconds) without the other leptons and quarks. Would fusion in the hearts of stars work all that differently without the heavier quarks and leptons? Would interstellar or planetary matter behave any differently? Would nuclei be less stable?

    Nik Berger’s responses gets toward what Chad is asking.

  6. #6 Rob Knop
    January 6, 2011

    “The muon — who ordered that?” (Or quote very close to that.)

    Back when I was in college, there was a constraint on the number of generations of… neutrinos, I think… based on astrophysics. That is, if the degrees of freedom available from the number of families of particles were different, then we’d see different ratios of the elements. I don’t remember how different, though, but I believe that this wouldn’t make gross differences in the Universe we observe. Let me know if you want me to dig into this more, and I can try to pull out more information about it.

    I do have this fear that without something like string theory, the answer may be no more than “because that’s the way it is”.

  7. #7 Chad Orzel
    January 6, 2011

    I think what Chad is asking is whether the universe would be all that much different (after the first few seconds) without the other leptons and quarks. Would fusion in the hearts of stars work all that differently without the heavier quarks and leptons? Would interstellar or planetary matter behave any differently? Would nuclei be less stable?

    That’s exactly the sort of thing I’m after. As far as I know, the usual reactions people talk about in astrophysics and the like don’t use any of these particles as intermediate states. But are there processes where, for example, the strange quark plays a crucial role? Or are the higher generations mostly incidental to the functioning of the universe after the first few seconds after the Big Bang?

    I do have this fear that without something like string theory, the answer may be no more than “because that’s the way it is”.

    Which is fine as far as it goes– I mean, obviously we have these particles, otherwise a whole lot of particle physicists wouldn’t have jobs– but that doesn’t really satisfy the dog, you know?

  8. #8 Rhett
    January 6, 2011

    Have you ever put together a LEGO set? When you finish, you always have some left over pieces even though you put the model together correctly.

    Similar thing here with the particles.

  9. #9 rob
    January 6, 2011

    Luke and Yoda dicussed this very question.

    Luke(referring to the Standard Model):What’s in there?
    Yoda: Only what you take with you.

  10. #10 andy
    January 6, 2011

    What about “neutron” stars, which may or may not contain strange matter. Not sure how much of an effect the interior state of a neutron star would have on the majority of the universe though…

  11. #11 jim
    January 6, 2011

    A question to a question: Why are you privileging stability?

    I trained as a mathematician, so my comparison is that most functions aren’t continuous, most continuous functions aren’t differentiable, most differentiable functions aren’t infinitely differentiable, but we tend to like to talk about smooth functions — we have a bunch of tools that we can use to analyse them — and in the 19th C, the discovery of continuous non-differentiable functions elicited a reaction like Rabi’s: “who ordered that?”

    It seems to me that most entities aren’t stable. Tim Eisele above mentioned the transuranic elements, but there’s a regular element none of whose isotopes is stable and most (all?) elements have at least one unstable isotope. So most possible nuclei aren’t stable. We like to talk about the stable ones because we have a bunch of tools we can use to analyse them.

    Now it turns out that most elementary particles aren’t stable. We’d like to talk about the stable ones because we …. Nu?

  12. #12 Jolyon Bloomfield
    January 6, 2011

    No good reason why they’re there, other than we see them. They have to come in complete families though, in order to cancel the anomalies of QFT (quantities that are conserved classically but not quantum mechanically). I’m fairly sure that once the universe has cooled down to the point that protons and neutrons have formed, (a minute or two at most, don’t ask me exactly ;-), the extra particles don’t play any particularly meaningful role in the formation of the universe.

  13. #13 Alex
    January 6, 2011

    What about “neutron” stars, which may or may not contain strange matter. Not sure how much of an effect the interior state of a neutron star would have on the majority of the universe though…

    Do the processes leading up to formation of neutron stars involve heavier leptons and quarks? If the heavier particles didn’t exist (or interacted differently) would supernovas not happen, or happen differently, and would this have implications for how the heavy elements fused in the stellar interiors get sent out and made available for subsequent planet formation processes?

  14. #14 ppnl
    January 6, 2011

    I don’t really think of particles as being things at all.They are more like processes, little subroutines in the vast structure of the universe. In any vast complex program it isn’t surprising that you can pull out a subroutine that appears to make little sense in isolation.

    Any explanation for why there are so many different kinds of quarks can only be answered at a level that explains why there any quarks at all. And then we will look for an explanation for the ontological entities that appear on that level.

    This is just a child’s “why” question. At some point you just have to take “just because” as an answer for now.

  15. #15 rxa
    January 6, 2011

    We have plenty of reason to want a more fundamental theory, with fewer free parameters, including gravity, in which all the particles derive from something more basic. We have no idea what that is, or even if it exists, but Emmy is just a dog – you can lie to her. Pretend there is a Theory of Everything, the four common particles are the stable modes and everything else is a higher order vibration. For a “why” question, is that a plausible story?

  16. #16 Andrew Foland
    January 6, 2011

    To rephrase the question: in the last 10 billion years (to safely exclude anything from the Big Bang with plenty of margin), have the higher generations contributed to any process or effect which was found other than in a dedicated physics experiment?

    I suspect the answer is no. But I can think of two possible exceptions; you could follow them up with your local friendly nuclear theorist:

    1. The strangeness content of the proton is not completely negligible. As I recall, this may also affect the proton nuclear spin, which is important for, say, (a) MRI. But I might be wrong about that effect (on the nuclear spin; the content is definitely non-negligible.) Also, the strange quark likely contributes something to the proton mass (you can find numbers all over the map, from 2% to 30% of the proton mass.) I’ve never seen any calculation evaluating whether the mass effect is exactly the same for protons and neutrons. If not, it might have an effect on (b) the proton-neutron mass difference, which is very important for our universe. (At first order you’d expect it to be the same, but the mass difference is small enough you’d also want to check higher-order effects.) Finally, there does seem to be some non-negligible role in (c) neutron star equations of state. But an EoS might fall under the heading of a dedicated physics search.

    2. It’s not completely implausible (to me, a non-theorist) that there are virtual strange-quark loop contributions to important nuclear interactions. I believe Segre once said that there were a dozen cross sections that, had they been just a little different, would have kept an atomic weaopn from working. I don’t know if any of them have virtual strange loop contributions of the relevant size (though honestly I doubt it.) Again, a nuclear theorist would be the person who might know.

  17. #17 Andrew Foland
    January 6, 2011

    One other term you might find useful to Google: familons . The Goldstone bosons of a spontaneously broken local continuous generational gauge symmetry.

    (It’s not known that generations obey a gauge symmetry, let alone a local continuous one; so this is a purely conjectural boson.)

  18. #18 onymous
    January 6, 2011

    It’s a good question. The obvious thing that happens at 3 generations and not fewer is that CP violation in the quark sector can happen. But this source of CP violation isn’t good enough, and one could imagine that theories with fewer families have CP violation somewhere else that leads to a universe with matter and no antimatter.

    Another thing that might be true is that the large coupling of the top quark to the Higgs is needed to drive renormalization-group evolution of the Higgs mass to tachyonic values, so that the Higgs is forced to get a vacuum expectation value and break electroweak symmetry. (This “radiative electroweak symmetry breaking” mechanism is a standard part of supersymmetric theories, for instance.)

    But that only needs one heavy quark like the top, so it might explain who ordered the third generation but not the second.

    Offhand, no better anthropic just-so story comes to mind for the number of generations. Flavor structure in the Standard Model is generally quite mysterious; one always hopes that there’s deeper structure lurking in the background that we’ll understand someday….

  19. #19 prasad
    January 6, 2011

    Is it interesting that QCD’s confining property depends on nf and nc? Doesn’t tell you what nf should be of course.

  20. #20 sd
    January 7, 2011

    Also with only one generation, there is only one type of neutrino, so there are no neutrino oscillations – not that I think that this effects anyone except experimentalists trying to figure out the neutrino flux from the sun, for example.

    You might get some tweaking of the weak decay rates of nuclei, since there’s no more CKM matrix, but off-the-cuff I don’t think the effect would be significant.

    However, having one generation could cause some different initial conditions for the universe that could impact the current day. For instance, if you believe that current-day dark matter was formed in some thermalizing process during the early parts of the universe, having only generation might mean that you end up with a different modern day DM density, which affects all sorts of processes, like galaxy formation.

  21. #21 Kea
    January 7, 2011

    Disclaimer: I am a qualified particle physicist, but many prefer to think of me as a crackpot.

    OK, so the first problem is the tendency to think of particles (in the low E spectrum) as fundamental. Heisenberg explained this well. What is fundamental is some concept of observable. For example: if we create quantum gravitational observables from categorical diagrams, such as the Bilson-Thompson triple braid diagrams, then these have an interpretation in post quantum logic (which we need, because QM is clearly not good enough to explain the generations). In fact, these braids neatly list the SM particles, including generations.

    The question then is: WTF? Why three strand ribbon/braid diagrams?

  22. #22 Evan Berkowitz
    January 7, 2011

    Something interesting but generally unknown is that it is likely that the biological diversity of Earth depends on muons.

    The sea-level “high” (meaning high enough to bother a nucleotide) energy muon flux is ~200/m^2/s, descending from a higher flux in the upper atmosphere. The energy imparted by a muon colliding with a nucleotide can spur errors in DNA duplication. That’s not to say that diversity depends on muons: evolution would still happen one way or the other. But my understanding is that the error rate is non-negligibly faster because of muon events.

    So if you like an anthropic reason and think that this point in time is privileged, then that’s what muons are for.

  23. #23 scramton
    January 8, 2011

    To the poster above, excellent job bringing biology to a physics blog. DNA has to adapt to high speed charged particles “bothering a nucleotide”.

  24. #24 madarab
    January 8, 2011

    There is no good answer to that question… yet. Some of the candidates for a theory of everything have that many generations of quarks and leptons as a consequence of those theories. Still, there are some effects that the number of quark generations would have on the primordial distribution of elements.

  25. #25 Kaleberg
    January 8, 2011

    Right now, most of physics can be done with four basic particles, but the universe wasn’t always like this. Way back when, shortly after the big bang, it is quite likely that the other eight particles played more visible roles, and it is also quite likely that there were all sorts of particles with extremely high energies that we are likely never to observe again.

    Sure, I’m speculating, but biology is loaded with historical stuff. Why not physics?

  26. #26 Fakrudeen
    January 9, 2011

    I have no qualification in physics – so possibly stupid answer!

    I agree with Kaleberg.
    I think it is more like our grand fathers breaking their head over why 5 planets [and some weird theories like platonic solids of Aristotle/Kepler]. Now if we find one more planet after Pluto, we won’t ask why 10. Already we have artificially changed it from 9 to 8.

    I think there may be more than 3 generations and even if there isn’t, it might be just an accidental fact like no. of planets around our sun or eccentricity of earth [0.0167 now] etc. Just like some planets support life and some don’t, some particles support higher structures like proton, atom, people some don’t.

    So only relevant question to me is what is special about those specific particles? This is analogous to saying earth is at optimal distance from earth, had an atmosphere with methane, ammonia and CO2 due to volcanic activity and so on.

  27. #27 crd2
    January 9, 2011

    http://scienceblogs.com/startswithabang/2010/12/whats_in_the_universe.php

    Read this as it may be some what helpful. Also I would consider asking Ethan (writer of above link). I think he would be able to tell you a few different ways the universe could have turned out with different particle variations.

  28. #28 Torbjörn Larsson, OM
    January 9, 2011

    I’m betting on early physics being most important as well.

    I do have this fear that without something like string theory, the answer may be no more than “because that’s the way it is”.

    In environmental theories it’s “because that’s the way it works”. (To maximize dust production, say.) The other way reads like the catholic church head Ratzinger trying to explain physics (as “meaningless”). :-D

    Now if we find one more planet after Pluto, we won’t ask why 10. Already we have artificially changed it from 9 to 8.

    More like changed it from 10 to 8, since Ceres’ “planethood” was revoked long before Pluto’s had to be rejected for the same reason. (Because no one wants to count every dust grain out there as a planet, makes classification rather meaningless.)

  29. #29 ppnl
    January 9, 2011

    I don’t really buy the idea that the muon is important to evolution. First, evolution is not very dependent on error rate. It is true that you need errors but the rate is just not critical. Second, there are plenty of errors from other sources from environmental mutagens to free radicals. And finally a more important source of genetic variability than point mutation is gene duplication events. This probably cannot be caused by the muon at all.

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