Throwback Thursday: Have we found our last fundamental particle? (Synopsis)

“The particle and the planet are subject to the same laws and what is learned of one will be known of the other.” -James Smithson

Now that the Higgs has been discovered, the Standard Model is complete. But are there any other new particles?

Image retrieved from Fermilab, modified by me. Image retrieved from Fermilab, modified by me.

If asymptotic safety is right -- and it's looking like the Standard Model might be stable up to energies far beyond the reach of accelerators the size of the entire Earth -- there might not be anything at all accessible to humanity as far as experimental particle physics is concerned. And a Higgs mass of 126 GeV might be just the thing to seal that deal.

Image credit: Mikhail Shaposhnikov & Christof Wetterich. Image credit: Mikhail Shaposhnikov & Christof Wetterich.

Go read the whole thing here.

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Does the BICEP2 result have any effect on the idea of asymptotic safety? I had heard that the gravitational wave detection basically requires quantized gravity, but was wondering if that also means that asymptotic safety can't be right. Unfortunately the wikipedia article on asymptotic safety is basically unintelligible to non-QFT experts...


I have a quick question that's always bugged me a bit. How do we know that what we now call fundamental particles are really fundamental? After all, looking at the historical development of physics, we once thought that atoms were fundamental. After discovery of the electron, neutron and proton, we realized that atoms were not fundamental. However we did think that these subatomic particles were. Development of the quarks and the standard model showed that this was mistaken with regard to neutrons and protons. How do we know that there are not more fundamental particles that compose at least some of what we are now regarding as fundamental?

@ Sean T

think the answer is in QFT itself. In it, particles are actually quantum fields, not point particles per say, there isn't a sub structure as such.

But in reality, who knows.. couple of years I read that someone managed to split an electron in some orbitons or something. Don't even know if it's true. So many particles... :D

By Sinisa Lazarek (not verified) on 28 Mar 2014 #permalink

@Sean T #2: The short answer is, "We don't know." What we have are experimental limits on deviations from being pointlike. For example, the angular distributions of electron-positron scattering indicate that they are pointlike with a radius less than 10^-22 m. From electron-proton scattering at HERA (called "deep inelastic scattering") we get a limit on the quark radius of about 10^-17 m. These are just high-energy variations on Rutherford's experiment.

You may also wish to look up the "technicolor" model (not to be confused with the cinematic film system :-). This was an alternative to SUSY which proposed constituents for leptons and quarks.

By Michael Kelsey (not verified) on 28 Mar 2014 #permalink

@Sinisa #3: Regarding "orbitons", spin waves, etc. The word you're looking for is "quasiparticle." In solid-state or other many-body systems, we can very often observe collective behaviour which is quantised, and we can treat those quantisations as effective particles of their own, the so-called quasiparticles.

The simplest example are just the conduction electrons, which have a different mass than free electrons. But there are many more complex quasiparticles, with surprising and counterintuitive behaviours.

By Michael Kelsey (not verified) on 28 Mar 2014 #permalink

Hey, hadn't there been a whole pile of predictions about the mass of the Higgs in the years leading up to the discovery? I don't know that we can read that much into this one correct guess when there were so many incorrect guesses before, and no one paid particular interest to this one at the time--why not conclude that they were probably just lucky?

And Ethan did I miss an announcement to the effect that you don't engage with comments anymore? I've seen a number of good questions in comments recently that would be very straightforward and informative if you could shoot a quick answer. (If it's just that you're busy, forgive me--I think you're extraordinarily productive in general.)

By uncleMonty (not verified) on 28 Mar 2014 #permalink

@Sean #2: we can create and destroy electrons and positrons in pair production and annihilation, so the electron isn't really fundamental, not like energy. You could call it elementary in that they aren't made of other particles. But fundamental isn't the right word.

@Michael #5: I must protest. It's quantum field theory, not quantum point-particle theory. The electron is described as a field excitation. Its field is what it is. You can make electrons and positrons out of light waves in pair production. You can diffract an electron. You can refract it as per Ehrenberg & Siday. It's got a magnetic moment. It's a spinor. In atomic orbitals electrons "exist as standing waves". The Einstein-de Haas effect "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". And electron-positron annihilation results in light waves again. The evidence for the wave nature of the electron is overwhelming. I put it to you that the pointlike inference from scattering is the wrong inference. Like you're throwing rocks at an elastic band. Some come right back at you, but there's no billiard ball in there.

By John Duffield (not verified) on 28 Mar 2014 #permalink

John Duffield,

Then consider my query modified to "noncomposed" or "elementary" if you wish. My question was meant to be "how do we know that the electron, quarks, etc. are not composed of smaller entities?".

Thanks for your answers Michael and Sinisa. It seems, then, that it is not really impossible that what we now regard as fundamental particles might be found to be composed of other entities. I would doubt that we would find this to be the case, but each time we've found more fundamental constituents of matter, it has had a fairly profound effect of our understanding of the universe. Pretty much all of chemistry developed once we understood that the atom had constituent parts. The standard model developed as a result of the realization that baryons and mesons had structure. I just wonder what insights would be gained if we found structure in one or more fundamental particles. (I realize that there's really no way to speculate about this without concrete information about the properties of such structures; it's just something I thought interesting.)

In your article, where does this claim come from: "if the top quark... were twice the mass it actually is, every proton in the Universe would be 20% heavier than the protons that actually exist!"

I found hep-ph/9707508, which claims that the proton mass scales like the top mass to the 2/27 power -- assuming that a GUT is true. But that would mean a 10x increase in top mass to get a 20% increase in proton mass.

By Ambitwistor (not verified) on 28 Mar 2014 #permalink

@Ambitwistor #10,
I don't really know what I'm talking about here, but I'd say that the derivation of that 2/27 exponent somewhat ignores the effects of changing the top quark mass to other measurables that would also affect the proton rest mass.

For example, the M(proton)≈CΛ(QCD) correspondence used in the derivation explicitly ignores the masses of up and down quarks (setting them to zero), which could be considered kind of a bad idea if one starts to think about the mechanisms that would actually allow changing the top mass value.

Still, I wouldn't know if "twice" is correct either.

@John Duffield #7: You're fairly correct of course, but incomplete. At high energies, hard elastic scattering measures the effective spread of the wavefunction of the target (e.g., my far too long ago thesis on D_s muonic decays, equivalent to c-sbar scattering to mu nu_mu, measured f(Ds), which is the spread of the c-sbar wavefunction, or in pseudoclassical terms, the "diameter" of the meson).

If electrons, for example, were composite entities like protons, then their constituents would have to be bound in some potential, and that bound state would have a characteristic radius.

At intermediate energies, a scattered projectile would see the system as a blob of charge with that characteristic radius, and we would get an angular scattering distribution which reflect that finite radius. We do not. Instead, we get scattering which is perfectly consistently with a simple theoretical calculation for a zero-radius charge, and we can set an upper limit (< 10^-22 m) for the charge radius of the electron.

At very high energies, the projectile would be able to probe the electron's constituents directly, in just the same way that deep inelastic scattering, instead of seeing the proton as a blob of charge, actually probes the constituent quarks (and gluons!). Again, we do not see any such constituency, and can set limits on what they could possibly be.

By Michael Kelsey (not verified) on 28 Mar 2014 #permalink

@ Sean T

Of course it's not impossible. Even today, string theorists think they know what the substructure of "fundamental" particles are.

IMO, the fact that there are so many different "fundamental" particles, seems to point to something more fundamental. But this is just a guess, no proof.

@ Michael Kelsey

Thank you for info on quasiparticles. Will have to check on that. Didn't know there was another category :)

By Sinisa Lazarek (not verified) on 28 Mar 2014 #permalink

@ Michael Kelsey

learned a lot of new things. Thank you. Now I understand there is a huge difference between quasiparticle electron and a real one. Guess I was prey to another news title sensation.

The elementary electron hasn't been split. Solid state electron is an approximation of n-body movements of electron within a solid.

Michael, am I correct in understanding that what is actually happening in case of "splitting" of electron is that properties of spin and orbit are actually carried to joining atoms (electrons) in a lattice? Like a wave would propagate, but in a straight line instead of spherically?
One electron would move to a higher orbit, fall back, emit photon, some other electron would absorb it, move to the higher orbital, then a another one, then some other would drop again, emit photons and so on.. A cascade reaction in a way.. approximated to one electron spliting it's properties while traveling through a solid?

By Sinisa Lazarek (not verified) on 28 Mar 2014 #permalink

@Sean #8: we know that electrons aren't composed of smaller entities because in all the years we've been creating and destroying electrons, we've never seen any such smaller entities.

@Michael #12. All points noted, thanks. See this TQFT webpage? See those blue trefoil knots at the top? Pick one, start at the bottom left, and trace around it anticlockwise calling out the crossing-over directions: up down up. Now imagine it's made of elastic like the bag model, and start throwing rocks!

By John Duffield (not verified) on 29 Mar 2014 #permalink

@John Duffield #15: You must have issues with length scales. At accelerator energies (which is how we set _experimental_ limits, rather than philosophizing limits), those pretty flowers will look just like points. If they were large enough to measure, then we would see deviations from point-like behaviour, which we don't.

By Michael Kelsey (not verified) on 29 Mar 2014 #permalink

How does the result that we could know all of the particles fit with the fact that a large fraction of the unverse is made of dark matter?

By Ramon Crehuet (not verified) on 13 Apr 2014 #permalink


I am not an expert in the field, so take this for what it's worth, but there are two possible answers.

1. We have indeed found all FUNDAMENTAL particles. That does not rule out the possibility that dark matter consists of composed particles.

2. We have found all fundamental particles predicted by the standard model. The particles that make up dark matter are ones that are outside of the standard model. (AFAIK, nobody has ever claimed that the standard model is necessarily a complete model, just an accurate one).

The truth is, we really don't know what dark matter is actually made of; we just know what properties it must have to yield the observations that we can make.