Gluons turning into quarks and antiquarks

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Discussion Overview

The discussion revolves around the process of gluons transforming into quark and antiquark pairs, particularly focusing on the energy dynamics involved in such particle decays. Participants explore the implications of mass and energy conservation in high-energy particle interactions, with references to experimental contexts like those at the LHC.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions how a gluon can turn into a quark and antiquark pair when the final state has more mass than the initial state, suggesting a need for an interaction that allows this transformation.
  • Another participant argues that it is not the mass that matters, but the total energy, using the example of a photon producing an electron-positron pair if it has sufficient energy.
  • There is a mention of the LHC's purpose to produce heavier particles from lighter ones through high-energy collisions.
  • Participants discuss the necessity of a recoiling particle to conserve energy and momentum during such transformations, indicating that gluons behave similarly to photons in this respect.
  • A later reply highlights that while energy considerations may suggest pair production is possible, momentum conservation complicates the scenario, particularly in free space.
  • One participant notes the frame-dependence of energy and momentum in relativity, emphasizing that one cannot separate them in a straightforward manner.
  • Another participant acknowledges the complexity of discussing energy and momentum, indicating a preference to avoid overly technical details in the context of the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between mass, energy, and momentum in particle transformations. There is no consensus on the implications of these interactions, and the discussion remains unresolved regarding the specifics of how gluons can produce quark-antiquark pairs.

Contextual Notes

Participants highlight the importance of considering both energy and momentum in particle interactions, with some suggesting that the discussion could benefit from a more nuanced understanding of these concepts in the context of relativity. The complexity of the interactions and the conditions under which they occur are acknowledged but not fully resolved.

PotatoMan
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When looking into how particles decay, it seems that it will happen if there exists an interaction between the initial and final state, and if the final state has lower potential energy than the initial. (i.e. turning mass into kinetic energy) If this is true, how does a gluon turn into a quark and antiquark pair if the final state here has more mass than the initial? What interaction is even happening here?
 
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PotatoMan said:
how does a gluon turn into a quark and antiquark pair if the final state here has more mass than the initial
It's not the mass that matters, it's the total energy of which the mass is one component. For a more prosaic example, consider how a massless photon can produce an electron-positron pair if sufficiently energetic.
 
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Also, the purpose of things like the LHC is to produce heavier particles from lighter particles colliding at high energy.
 
Nugatory said:
It's not the mass that matters, it's the total energy of which the mass is one component. For a more prosaic example, consider how a massless photon can produce an electron-positron pair if sufficiently energetic.
Thanks for the clarification. So the total energy of the system goes down? Or are you referring to just the total potential energy of it?
 
Nugatory said:
consider how a massless photon can produce an electron-positron pair if sufficiently energetic.
Note that this does not happen in free space. You need a particle to recoil to conserve energy and momentum. Gluons are the same way.
 
Vanadium 50 said:
Note that this does not happen in free space. You need a particle to recoil to conserve energy and momentum. Gluons are the same way.
Yes, thought about mentioning that, decided that it went beyond the scope of the B-level question. Of course I could have avoided the issue completely by considering the two-photon case, but it felt wrong to use the way less probable interaction as the example.

A quibble: the heavy particle recoil is needed to conserve momentum - if we were to only consider energy the single-photon process would appear possible in vacuum?
 
I was also trying not to get lost in the weeds and said "energy and momentum" instead of "4-momentum". :smile:
 
Nugatory said:
if we were to only consider energy
You can't; in relativity the split between "energy" and "momentum" is frame-dependent.
 
PeterDonis said:
You can't; in relativity the split between "energy" and "momentum" is frame-dependent.
Of course, but that doesn’t stop me from working in the lab frame in which if the (frame-dependent) energy of a photon is greater than 1.022 MeV pair production appears to be energetically permitted but cannot be reconciled with momentum conservation.
 

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