I understand that mass can be created from energy, this follows the equation $E= mc^2$, but can nucleons be created?

If I gave you a number of nucleons in whatever form you'd like and as much energy as you'd like, is there a way for you to give me back more nucleons then I originally gave you?

The reason this question occured to me is because it is my understanding that in a star hydrogen is turned to helium (and sometimes heavier elements) and that releases energy but at no point does the total number of nucleons change. Neutrons can turn into protons and vice versa but the total number of neutrons and protons does not change during any of these reactions.

If any of this is incorrect please correct me.

In general, is there a way to create or destroy nucleons?


  • $\begingroup$ It's probably clear to you from the two answers below, ( both written by people that know far more than I do), but your question could also be phrased as : can quarks / gluons be created. as they are more fundamental, and as John says, it's an unholy mess, so there is a range of not just protons and neutrons, but mesons as well. $\endgroup$ – user108787 Sep 8 '16 at 6:45
  • $\begingroup$ Take note that there is such a thing as an anti-proton beam. Anti-protons are (anti-)nucleons, and you starting without zero of them. So... $\endgroup$ – dmckee Sep 8 '16 at 20:20

It is the baryon number that is conserved, thus nucleons can be created but the total baryon number does not change in the standard model of particle physics.

See here the creation of proton antiproton pairs in LEP. Generally if there is enough energy proton antiproton pairs are created or proton antineutron, neutron antiproton, still conserving total baryon number.

This creates a problem in the cosmological narrative of the Big Bang, because the universe we find ourselves in is predominantly baryons, and any antiprotons come from secondary interactions of large energy. Thus there should be an asymmetry in the model of elementary particles and various proposals exist for this.

CP violation is a good candidate, except the probability of its happening is very small, and that is why we can talk of a baryon conservation number. Its maginitude is too small to describe the observed asymmetry in the universe.

So ultimately we expect that the total number of nucleons is not a conserved number, but we need an extension of the standard model, otherwise our theoretical models of how the universe appeared cannot work.


Yes nucleons can be created, though this doesn't routinely happen in stars as the energies involved are not high enough.

In a high energy collision, for example at the LHC, an unholy mess of fundamental particles are created and this will include gluons. Those gluons hadronise very quickly to form particles like pions and protons. Offhand I don't know the typical production ratios, though I believe pions are the most common particles formed simply because it's easier to form a two particle bound state than a three particle bound state.

But you need the collision energies to be significantly greater than the rest mass of the particles you want to create. Protons and neutrons are around a GeV in mass so you need collision energies of at least a GeV to form them. However the highest temperature in the core of a star is of order $10^8$ K and this corresponds to an energy of only around $10$ keV. So while stars can rearrange existing nucleons they cannot create new ones.

Note that baryon number is a conserved quantity (under normal circumstances) so any process that creates baryons will create an equal number of anti-baryons. So even if a star got hot enough to create nucleons (possibly in a supernova?) it cannot change the net number of nucleons.


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