I Compositeness Limits -- Have We Reached Rock Bottom?

  • Thread starter lpetrich
  • Start date
983
173
Particle Data Group - 2017 Review has some strong lower limits for the mass scales of possible quark and lepton compositeness, or at least the compositeness of the easier-to-study ones, like up and down quarks and also electrons. The limits are well into the TeV range, though they are somewhat model-dependent.

This means that electrons, up quarks, and down quarks do not start to disintegrate even after applying energies a million times their rest masses.

Here are the maximum ratios of disintegration energy to rest mass for entities previously discovered to have been composite:
  • Atoms: 10^(-8) (ionization of hydrogen atoms)
  • Nuclei: 10^(-3) (dissociation of deuterons)
  • Hadrons: 1 (deep inelastic scattering off of nucleons)
That makes it very difficult for Standard-Model elementary fermions to be composite: they are much less massive than their compositeness energy scales.

But there is a theoretical analogy: light mesons, like pions. Their mass is roughly sqrt(mq*mc) where mq is the quarks' average mass and mc = QCD energy scale. For mq << mc, m(meson) << mc also. So if the electron has a compositeness scale of about 1 TeV, then the electron's constituents would have to have masses around 1 eV.
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
23,333
5,698
Pions are special because they are goldstones. That means there is an inexact symmetry that is driving their masses near zero. (If the symmetry were exact, they would be massless)
 
983
173
Pions are special because they are goldstones. That means there is an inexact symmetry that is driving their masses near zero. (If the symmetry were exact, they would be massless)
So in the scenario that I'd mentioned, the Standard Model's leptons and quarks are all pseudo-Goldstone particles, like the pion.
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
23,333
5,698
That's starting to sound like a personal theory. So far as I know, this is impossible without supersymmetry and virtually impossible with it.
 
983
173
That's starting to sound like a personal theory. So far as I know, this is impossible without supersymmetry and virtually impossible with it.
Looks like that mechanism cannot work. So it'll be very hard for electrons and up and down quarks to be composite.
 

Related Threads for: Compositeness Limits -- Have We Reached Rock Bottom?

  • Last Post
2
Replies
26
Views
4K
  • Last Post
Replies
8
Views
3K
  • Last Post
Replies
2
Views
799
  • Last Post
Replies
0
Views
835
  • Last Post
Replies
15
Views
4K
  • Last Post
Replies
0
Views
2K
Replies
6
Views
1K
  • Last Post
Replies
1
Views
545

Hot Threads

Top