Compositeness of quarks and leptons

  • Thread starter welatiger
  • Start date
  • Tags
    Quarks
In summary, the conversation discusses the idea of compositeness in particle physics and whether the discovery of the Higgs boson contradicts this concept. The compositeness model, first proposed in 1970, suggests that particles like quarks and leptons are made up of smaller particles known as preons. However, there is no evidence to support this model and the physics community generally agrees that fundamental particles are point-like. The conversation also touches on the concept of a finite radius for composite particles and how this affects their mass and binding energy. Finally, the conversation mentions the role of string theory in this discussion and how the finite size of strings is important for the theory to be a good candidate for a quantum theory of gravity.
  • #1
welatiger
85
0
Is the discovery of Higgs boson contradict the Compositeness model and the preon existence ?!
 
Physics news on Phys.org
  • #2
welatiger said:
Is the discovery of Higgs boson contradict the Compositeness model and the preon existence ?!

The Higgs boson was considered part of the Standard Model, so nothing has changed.

Note: Your sentence is unclear - could you define the terms?
 
  • #3
mathman said:
The Higgs boson was considered part of the Standard Model, so nothing has changed.

Note: Your sentence is unclear - could you define the terms?


You Know that in the compositeness model, the spontaneous symmetry breaking is due to the preon but not the Higgs boson.
 
  • #4
welatiger said:
You Know that in the compositeness model, the spontaneous symmetry breaking is due to the preon but not the Higgs boson.
To be honest, I never heard of the compositeness model.
 
  • #5
mathman said:
To be honest, I never heard of the compositeness model.

Do you mean, you have never heard of _any_ compositeness model? Fascinating.
 
  • #6
Dear all
The compositness model was first proposed by A.Salam 1970, mainly due to the hairechary problem, you can read about it in wiki pages.
 
  • #7
welatiger said:
Dear all
The compositness model was first proposed by A.Salam 1970, mainly due to the hairechary problem, you can read about it in wiki pages.

Your discussion style is obtuse

To move this discussion along:
http://en.wikipedia.org/wiki/Preon
"In particle physics, preons are postulated "point-like" particles, conceived to be subcomponents of quarks and leptons.[1] The word was coined by Jogesh Pati and Abdus Salam in 1974. Interest in preon models peaked in the 1980s but has slowed as the Standard Model of particle physics continues to describe the physics mostly successfully, and no direct experimental evidence for lepton and quark compositeness has been found, although in the hadronic sector there are some intriguing open questions and some effects considered as anomalies within the Standard Model."
 
  • #8
arivero said:
Do you mean, you have never heard of _any_ compositeness model? Fascinating.

Devils gave an answer to my question. The assertion that leptons or quarks are composite is so far completely unproven.
 
  • #9
Perhaps the point is if there is someone here in the forum.who is interested on composites. If nobody can comment beyond the wikipedia, it is probably not worth to raise the topic here.
 
  • #10
arivero said:
Perhaps the point is if there is someone here in the forum.who is interested on composites. If nobody can comment beyond the wikipedia, it is probably not worth to raise the topic here.
It is not just Wikipedia. It is the overwhelming consensus of the physics community that there is no evidence for compositeness of leptons and quarks.

Aside: spellcheck doesn't think "compositeness" is a word.
 
  • #11
mathman said:
It is not just Wikipedia. It is the overwhelming consensus of the physics community that there is no evidence for compositeness of leptons and quarks

There is a consensus on the no evidence of substructure. I fact even my esoteric sBootstrap is in the consensus, as it only ask for compositeliness (hey, that is worse!) for the scalar partners of the fermions in a susy theory.

But generically, I wonder if absence of substructure (ie, to be point-like in 4D) implies absence of compositeness. I am not sure if the fermionic states of a superstring theory are pointlike or not. And even if they are not pointlike, an open string in a 4D brane should be.
 
  • #12
I feel like I should point this out:

If a particle is composite, it has to have a finite radius (because at a small enough scale it is more than one particle). But we have experimental bounds on the radius of fundamental particles (I'm not sure if we have done this for all or just some). These bounds are very small, the radius of these particles are tiny. In order to have a composite particle with a very short radius, you need a very strong interaction, which raises the potential energy of the composite particle. But raising the energy like this also increases the mass of the composite particle.

So right now to make composite particles have a radius and mass consistent with what we have observed, is very difficult. As far as I know this is the main reason that preon models (and general composite models) are disfavored.
 
  • #13
DimReg said:
If a particle is composite, it has to have a finite radius

I am not sure, I have never seen a proof, besides the *classical* intuition that you mention.

Particularly, what does it happen with the fermionic states of the superstring? In the worldsheet, is there some rule telling that the bosonic coordinates X_mu, which are the actual location of the string in space time, need to be as a particle of finite radius?
 
  • #14
The finite radius of a string is a completely different thing. The string is not taken to be a composite particle, as far as I know.

A composite particle is a bound state of two particles, at least as far as they are usually discussed. In that case, there is ALWAYS a finite radius, there is the average separation of the particles from each other. If this distance were 0, it would imply an enormous binding energy, which most likely would be too large (creates a black hole).

I'm not sure how I would rigorously prove this for a general case. However, as a model you can use the infinite spherical well potential, so inside the radius the particle is free to move but a very strong restoring force holds the particle in if it tries to leave that radius (also, you do the usual trick of reducing a two body problem to a one body problem). The ground state energy (actually all energy levels) goes as r^-2, where r is the radius of the well. Clearly this diverges when r goes to 0.

For string theory, the finite size of the string is ideal because it "spreads out" the gravitational interaction, making the perturbation series renormalizable. This is why it's considered a promising candidate for a quantum theory of gravity. If the string wasn't extended, it wouldn't be a good theory of gravity. But most importantly, there isn't some derived rule that says that the string has to be an extended object, it is the postulated form a particle takes, and thus by construction the string in string theory is extended.
 
  • #15
DimReg said:
I'm not sure how I would rigorously prove this for a general case. However, as a model you can use the infinite spherical well potential, so inside the radius the particle is free to move but a very strong restoring force holds the particle in if it tries to leave that radius (also, you do the usual trick of reducing a two body problem to a one body problem). The ground state energy (actually all energy levels) goes as r^-2, where r is the radius of the well. Clearly this diverges when r goes to 0.

Fine example, but that is not all the history. For 1D potentials, we are granted that there is always at least a bound state inside a well, so what happens in the limit of "point-supported potentials" is a single state, if the potential is only supported in one point. Actually the classification of the possible states is equal to the possible boundary conditions, some of then can be produced as limits of potentials going to dirac-delta shapes, some others have other origins. A classic textbook on the topic, by the way, is wrong about the naming of the classification.

For 3D well, the case is as you describe, but Gosdzinsky-Tarrach and and Manuel-Tarrach found a way to "renormalise" the interaction, again producing a single bound state in the limit of point-like potential.

So I had never though of it, but these interactions could be described, via the two body to one body reduction, as examples of composite states with point-like properties.
But most importantly, there isn't some derived rule that says that the string has to be an extended object, it is the postulated form a particle takes, and thus by construction the string in string theory is extended.

The "construction" is that a world-sheet has a bosonic field [itex]X_\mu(\sigma)[/itex] with sigma taking values from a one-dimensional segment, but it does not tell that the projection in the target space, the one where the X maps to, is an extended object. I think that it is clear that for bosonic states it is, with the restriction that it could be "pointlike" in some subset of the coordinates (a "D-brane"). And for fermionic states, I have never seen a clear discussion.
 
  • #16
Looking at history, putatively elementary entities were discovered to be composite when one pounds them hard enough -- they start to break apart. That was true of atoms, that was true of nuclei, and that was true of hadrons.

But that is not true of either quarks or leptons, to within experimental limits, despite pounding them with much more energy than their rest masses.

You can find some tests of compositeness at Particle Data Group:
"Summary Data" > "Searches (Monopoles, SUSY, Technicolor, Compositeness, ...)"
"Reviews, Tables, Plots" > "Searches for Quark and Lepton Compositeness"

These links give a thumbnail history:
Deep Inelastic Scattering of Electrons
Survey of Scattering Investigations

-

Let's see what the binding energies look like for atoms, nuclei, and hadrons.


For atoms, one can add up the ionization potentials, the energy necessary to remove each electron. Ionization energies of the elements (data page) - Wikipedia has complete ones up to copper (Z = 29). I find a good fit to Z2.4 * 14 eV.

However, even these elements' innermost electrons are nonrelativistic, so this fit may not carry over very well to the heaviest elements. But using it for the heaviest element with a cosmological mean life, uranium (Z = 92), gives 0.72 MeV, a little more than the rest mass of an electron. The total rest masses of the electrons is 47 MeV, and the total rest mass of a uranium-238 atom is 222 GeV. That gives ratios 0.015 for the electrons and 3.2*10^(-6) for the entire atom.

For hydrogen-1, however, the ratios are 2.7*10^(-5) and 1.4*10^(-8).


Turning to nuclei, let us check on Nuclear binding energy - Wikipedia. The lowest mass per nucleon is for iron-56. It has a binding energy of 8.79 MeV per nucleon, or 0.0094 relative to the unbound protons and neutrons.


Hadron binding energy is poorly defined, since quarks can't be free. But for light hadrons, interaction energies are close to the valence quarks' kinetic energies, so we can say ~ 1 here. However, due to a quirk of QCD called chiral symmetry breaking, pions' masses are about sqrt(EQCD{/sub]*(mu+md)) -- they would be massless if the up and down quarks were massless. Is that due to cancellation?


But let's consider the experimental limits' ratios to the rest masses of the particles studied.

For electrons, the clearest results are from LEP, which went up to 104.4 GeV. Electrons' behavior was in close agreement with the Standard Model up to that energy, meaning that electrons' compositeness energy scale has to be at least that energy.

For its just-completed run, the LHC may be able to get results for energies up to about 1 TeV, at least for the up and down quarks.

So rest mass / compositeness scale:
LEP electrons: 5*10^(-6)
LEP up/down: 3*10^(-5)
LHC electrons: 5*10^(-7)
LHC up/down: 3*10^(-6)

(up and down quark masses ~ 3 MeV)

If the electron's compositeness scale is 500 GeV, then if it gets its mass in pion-like fashion, its constituents must have res masses of about 0.5 eV. This requires something like color confinement, because no such free particles are known to exist.
 

1. What is the evidence for the compositeness of quarks and leptons?

There is currently no direct experimental evidence for the compositeness of quarks and leptons. However, theoretical models such as the Standard Model of particle physics suggest that these particles may be made up of even smaller building blocks called subquarks or preons.

2. How can we observe the compositeness of quarks and leptons?

Observing the compositeness of quarks and leptons would require experiments with extremely high energies, which are currently beyond our technological capabilities. However, indirect evidence for compositeness can be obtained through precision measurements of particle properties and interactions.

3. What are the potential implications of discovering the compositeness of quarks and leptons?

If quarks and leptons are found to be composite particles, it would revolutionize our understanding of the fundamental building blocks of matter. It would also have implications for the development of new theories and models in particle physics.

4. Are there any alternative theories to the compositeness of quarks and leptons?

Yes, there are alternative theories that suggest quarks and leptons are not composite particles. Some theories propose that they may be made up of smaller, indivisible units called primons, while others suggest they may be fundamental particles with no internal structure.

5. What experiments are currently being conducted to study the compositeness of quarks and leptons?

Scientists are currently conducting experiments at particle accelerators such as the Large Hadron Collider (LHC) to search for evidence of compositeness. These experiments involve studying the properties and interactions of particles at high energies to look for any deviations from the predictions of the Standard Model.

Similar threads

  • Beyond the Standard Models
Replies
6
Views
2K
  • Beyond the Standard Models
Replies
7
Views
2K
  • Beyond the Standard Models
Replies
4
Views
1K
Replies
1
Views
1K
Replies
4
Views
2K
  • Beyond the Standard Models
Replies
4
Views
2K
  • Beyond the Standard Models
Replies
9
Views
3K
  • Beyond the Standard Models
Replies
9
Views
2K
  • Beyond the Standard Models
Replies
31
Views
4K
  • High Energy, Nuclear, Particle Physics
2
Replies
46
Views
3K
Back
Top