Could second and third generation SM particles NOT be fundamental?

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Discussion Overview

The discussion explores the nature of second and third generation Standard Model (SM) particles, questioning whether they could be composite rather than fundamental. It examines the implications of such a hypothesis, particularly in relation to their instability and rapid decay, as well as the potential existence of supersymmetric (SUSY) partners.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant suggests that second and third generation SM particles might not be fundamental and could instead be bound states of more fundamental first generation particles.
  • Another participant argues against this idea, stating it is very unlikely due to the nature of particle decay, which is influenced by the mass of the particles and the available momentum states.
  • It is noted that decay rates can be estimated using dimensional analysis, with the decay rate being proportional to the mass of the parent particle under certain conditions.
  • Some participants point out the lack of evidence for compositeness, referencing data that supports the Standard Model's predictions for electron-positron collision rates.
  • There is a question raised about the possibility of bound states that could still conform to the Standard Model predictions, or if second and third generation particles could be quantized excited states of first generation particles.

Areas of Agreement / Disagreement

Participants express disagreement regarding the fundamental nature of second and third generation SM particles, with some supporting the idea of compositeness and others firmly opposing it based on current evidence and theoretical considerations.

Contextual Notes

The discussion highlights the complexity of particle decay mechanisms and the dependence on various factors such as mass and energy. There are unresolved questions regarding the implications of potential compositeness and the nature of particle interactions.

ensabah6
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could second and third generation SM particles NOT be fundamental?

since they are unstable and rapidly decay, could second and third generation SM particles NOT be fundamental? perhaps as bound states of more fundamental first generation particles?

if they are not fundamental, then do we have to worry about SUSY partners of these?
 
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Very, very, very, very unlikely. Their decay happens because it can, and it happens faster for more massive particles because there are more possible momenta that the resulting particles can have.

You can ask "How can that be possible, since momentum is a continuous quantity and not a discrete one"? The answer is that the possible momenta will be discrete for a finite volume, and that there are thus more possible momenta if the total energy is large.

With a large enough volume, the sums become integrals, making the calculation somewhat easier. The volume also drops out of the final results for decay rates and cross sections.

One can also use dimensional analysis to estimate the behavior of decay rates. If a particle's decays involve highly-relativistic and/or massless particles, then the most significant mass scale will be the parent particle's mass. Since in quantum mechanics with hbar = c = 1, mass, momentum, and energy have units of reciprocal length and time, the decay rate will be proportional to the particle's mass.

Violation of these conditions can lead to much greater dependence on mass. Weak interactions, for m << mW, give

m5/v4

m = particle's mass, v = Higgs vacuum expectation value
 
lpetrich said:
Furthermore, there isn't any evidence of such compositeness.

Figure 41.6 in Plots of cross sections and related quantities (rev.) at the Particle Data Group site shows how well electron-positron collision rates agree with the Standard Model.

Can't there be a bound state that also agrees electron-positron collision rates agree with the Standard Model? Or could second/third generation be a quantized excited version of first generation, etc?
 

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