What if no muons, taus, heavier quarks

Factual Questions
1

Suppose only the first generation of particles existed; that is, only electrons and electron neutrinos and up and down quarks plus the various force particles, but no muons, strange, charm, etc.

How would the world differ from what we see? Based on what we know is there anything these particles do that have a substantive difference? I know that weak interactions allow a change of quark type and that the weak interaction is partially responsible for nuclear fusion in the sun, but I do not think I’m asking the same thing as not having the weak interaction. I’d still let up and down quarks interchange by the weak interaction.

So could stars shine? Could life as we know it exist? I’m thinking perhaps there’s some subtle stability or resonance that allows nucleosynthesis above carbon, but maybe there’s nothing we couldn’t have we don’t have now.

2

I have this half-memory of hearing, back in my grad school days, that the Universe would look very different if the number of particle generations was different from three. The idea was that the number of different types of particles that were running around shortly after the big bang makes a big difference in how quickly the Universe cools off after the Big Bang. This might consequently affect some things you might not otherwise expect to be affected, like the relative abundances of hydrogen vs. helium once things cooled down more.

It’s possible, though, that I’m conflating one or more concepts here. Perhaps someone more knowledgeable than I will happen by with a more detailed answer (or tell me that I’m all wet.)

3

What about any current effects? If the universe (somehow) matched ours just after inflation, or 100,000 years after the Big Bang, or something, but didn’t have the higher particle generations?

4

Even if the hydrogen-helium-lithium balance shortly after the Big Bang had been affected, would this matter at all today? The temperature of the early universe does affect how much helium is produced, but there would still be neutrons (important as otherwise we have nothing but hydrogen) as deuterons are formed before He is. And even if the the universe cools faster or slower than it did, that may just mean we get our deuterons or helium faster or slower (and this all takes place in about the first 100 seconds as I recall) Unless there’s a different end result, I can’t think the time it took would matter billions of years later.

5

Having been summoned via PM, I come, but I must say that the reason I didn’t reply earlier is that I really don’t know. There are some experiments that are sensitive to the number of quark generations (and it’s really amusing seeing error bars much less than 1 on a quantity that inherently must be an integer), but I don’t know enough of the details of those experiments to say if they’re relevant to anything observed on a daily basis by laymen.

6

Thanks for the reply Chronos per my request

– of course you have shaken my faith that you know everything physics. :slight_smile:

7

I’ve often wondered why nature comes up with the beautiful, intricate structure of the first generation of elementary particles – and then just repeats it two times with slightly more oomph, like a washed-up out-of-ideas Hollywood director hoping to coast on the success of his last hit…

Less facetiously, I can think of three possible effects, none of which I know for certain has any role to play:

[ul][li]CP violation: it leads to an asymmetric behaviour of particles and antiparticles, and thus, plays a role in proposed explanations of why there’s apparently more matter than antimatter in the universe; it only occurs in particles of higher generations, such as kaons, that contain strange quarks[/li][li]Higher-order effects: in the calculations of any given interaction in QFT, higher generation particles play a role, even if they’re not among the primary interaction partners; maybe a theory lacking the additional terms has trouble, though I can’t conceive of any off-hand argument for this[/li][li]Neutrino mixing: there’s been the suggestion that the intergenerational mixing of neutrinos might contribute to the existence of a positive cosmological constant (dark energy) – but of course, this begs the question: why do we need a positive cosmological constant?[/ul][/li]
At least, these are directions I’d look in to find an answer to the question at hand – however, I’m not at all certain that any of these bear out.

Of course, one might be exceedingly optimistic, and hope that there is some fundamental theory, which perhaps only is consistent if the number of generations is exactly three. A possible place to look is theories with a higher number of spacetime dimensions. Generically, one single particle in a higher-dimensional spacetime can look like a number of different particles in a spacetime where some dimensions are ‘rolled up’ so as to be unobservable.

So if we live in a spacetime of sufficiently high dimension (say, D = 11), then it might be the case that one fermion propagating through these dimensions looks like all of the fermions we observe, generational structure and all, from our 4-D viewpoint (I’ve explained roughly how this works in this post).

(Actually, though, the mechanism by which you get generations in string theory is somewhat different – it’s related to the ‘shape’ of the extra dimensions, or actually to their Euler characteristic; but since you can construct compactifications of almost any Euler number, string theory offers little help here. If one could find a reason why some compactification should be preferred, then things would improve – but so far, there doesn’t seem to be any convincing proposal on that front.)

8

From that post:

Wouldn’t this approach lead to an infinite set of particles of higher mass, not just three?

9

Well, if there’s little hydrogen left then we won’t have long burning stars that can allow for the evolution of intelligent life, and we wouldn’t be here.

10

Well, it’s of course possible that there are more than three generations (though one has to cope with some experimental limits, making the fourth generation, especially its neutrino, necessarily immensely much heavier – otherwise, there’d be observable effects in Z boson decay). But I’m not really certain this is a good explanation for the existence of generations, too.

11

Am I wrong that the precise binding energy of an atomic nucleus depends on the resonances of kaons and more massive mesons? Thus, if there were two or four quark families instead of three, the stability of specific isotopes would change slightly.

And such atomic properties determine radioactivities and the life cycle of stars. If so, even with no way to predict what the new distribution of elements would be, we know it would be different. The actual values may yield a “Goldilocks” universe … or perhaps different values would make it even more Goldilocksian.

12

First, a quick fun fact: we currently don’t have any good reason to call the up/down quark generation the same thing as the electron/electron-neutrino generation. That is, the leptons and quarks both seem to have three generations, and each generation seems to have different masses, but there’s nothing that lines them up with each other. Thus, they are always ordered according to mass (due to a lack of anything better to go on at present).

As for any layman-noticeable differences due to a lack of heavier generations: absolutely! A few off the top of my head –

Since MikeS mentioned the early universe above, I’ll start with a follow up there. Yes, the number of particles species present, and their masses, affects cooling processed as well as (and perhaps more importantly) the clumpiness of the resulting matter distribution. Killing the heavier generations would have huge effects on galaxy size and formation rate. In turn, this would affect star size and lifetime.

But, if we quietly assume we make it through the first dozen billion years or so for the sake of discussion, then…

Without the heavier quarks, the masses of the proton and neutron would decrease by something around 1%. (This is a remarkably difficult thing to calculate, and predicting hadron masses from first principles is an active area of research. But 1% is the ballpark.) This means that everything would be lighter by 1%. Your weight as read by a standard scale would drop by 2%, since the earth would be lighter, too, making gravity at the surface weaker by 1%.

Relatedly, the mass differences would alter the energy levels of nuclei. Things that were stable could become unstable; things that were unstable could have different daughters; decay times would change; etc. It would take some effort to figure out exactly which changes would matter, but I suspect many would be significant to life.

The absence of heavy quarks (especially strange quarks) would alter nuclear magnetic moments, which would in turn have similar affects to the mass change on nuclear energy levels.

There would be drastic changes caused from altering the cosmic ray flux. At present, there are lots of high energy particles (mostly protons) hitting nuclei in the upper atmosphere. These collisions produce pions and kaons, and these particles subsequently decay, producing a variety of things but in particular copious muons. The muons (and some of the un-decayed parent particles) pass through the atmosphere and rain down on the earth’s surface.

This process would change in two catastrophic ways. First, there would be no muons, so any muon-driven process (see next paragraph) would cease. Second, the pion lifetime would grow tremendously. I estimate around a factor of 8000 increase, since its primary decay mode (to a muon and muon neutrino) would be disallowed to to non-existence and the alternative decay mode (to an electron and electron neutrino) would still be heavily “helicity suppressed” (which is outside of the scope of this post). In any case, this would mean pions would be able to survive all the way down to the surface instead of decaying in the upper atmosphere. (The more obvious issue that there could be no kaons produced in the primary interactions is dwarfed in importance by the above items.)

Two major effects (there would be numerous minor ones) –

(1) It is speculated that the muon flux at the surface is a driving force for genetic mutation, both from direct nuclear rearragnement and from the radionuclides externally produced and subsequently incorporated into the body. While that process would cease, the new (and greater) bombardment from pions would bring havoc. A pion is hundreds of times worse, radiation wise, than a muon at the energies of relevance here, as it tends to interact via the strong force, inducing showers of daughter particles each of which does its own nuclear damage. I suspect that a lot of life would just die out, actually, if it was a sudden change. Evolution might have been able to handle it just fine over the eons, but who can say?

(2) Cloud seeding is greatly enhanced by cosmic rays, especially pions. Things would become a lot cloudier and, thus, colder (easily cold enough to freeze the planet, I’d suspect).

Supernovae: isotopic abundances on earth would be affected by changes in past supernova explosions induced from the lack of multiple neutrino flavors. Neutrinos produced in the initial nuclear reactions would be unable to change flavor and, thus, would have a net higher interaction rate with the stellar material. (These interactions are a principle driver of the explosion process.)

It is perhaps worth noting explicitly that a layman would notice changes in isotopic abundances via their effects on industry and the economy.

13

Ah, good point, it had slipped my mind that the pion and the muon are different generations (as the generations are usually counted).

14

The difference in generation isn’t directly relevant, as there isn’t any Cabibbo suppression or anything here. The dominant decay mode is to the muon / muon-neutrino because the heavier muon more readily tolerates the “wrong” spin, as the rate for a reversed helicity final state goes at (m/E)[sup]2[/sup]. But perhaps your “ah” was about a different point.

ETA: On edit, perhaps you were just saying that it had slipped your mind that pion decay would be affected in the first place…

15

I vaguely remember something about the top quark being directly related to the weak force bosons Z and W. So the weak force would be messed up if the top quark didn’t exist?

16

I think my thought processes were in a superposition of the states “pion decay would be the same because pions and muons are both first generation, and so would both still exist” and “changes in pion decay would be irrelevant because the pion wouldn’t exist either, being second generation”. Both thought processes are muddled, of course.

17

If you did the top quark removal carefully, you could still have a Standard Model with the same W and Z masses and the same underlying interaction strengths with the remaining particles. However, the top quark contributes to so-called “radiative corrections” to the boson masses and to certain processes’ rates, so if you just plucked the top quark out of the Standard Model without any other adjustments, then yes, processes governed by the weak force would change.

18

I believe the term for this is “Kaluza-Klein partners.” However, I’ve seen it argued (here) that the particles in heavier generations can’t actually be Kaluza-Klien partners of the lighter ones.