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I Are new nucleons formed in heavy-ion collisions?

  1. Dec 12, 2017 #1
    The subject heading says it all. In heavy-ion high energy collisions that form a quark gluon plasma (QGP), is it known whether new nucleons are formed from the added energy, or are all of the QGP's quarks ones that existed previously in the colliding heavy ion nuclei? More specifically, are there more nucleons (and not just their component quarks) after the collision--and after the QGP cools--than before? I'd like to know if stable nucleons are formed de novo in such experiments. I'm not familiar with the experimental details.
     
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  3. Dec 12, 2017 #2

    mfb

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    Yes - many. Central collisions at LHC energies produce thousands of new hadrons. Most of them are pions and kaons but there are also many protons and neutrons (and their antiparticles) and various other particles.
    The total number of baryons is conserved, for every new baryon there is also a new antibaryon.
     
  4. Dec 12, 2017 #3
    Thanks, mfb! I'm curious how this is known. Is it assumed that any anti-proton or anti-neutron they see flying away out of such collisions must be new, and therefore the accompanying, ejected protons or neutrons must also be new (due to baryon number conservation)? Or are they able to do actual "housekeeping" on the numbers of nucleons present before and after?
     
  5. Dec 12, 2017 #4

    Orodruin

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    All protons are the same. You can never point to a proton and say ”that one existed before the collision” or ”this proton is the partner of that antiproton”. There simply is no possible test that could verify this. What you can do is to check the number of protons (and other nucleons) before and after collision.
     
  6. Dec 12, 2017 #5

    mfb

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    Lead ions colliding in the LHC have 82 protons and 126 neutrons each, or combined 164 protons and 252 neutrons.
    If you see more than that, then new nucleons were created, and all the antinucleons were created as well of course (where else would they come from?).

    ALICE cannot measure particles that fly away close to the incoming beams (because there has to be space for the beams entering and leaving the detector, so counting everything doesn't work, but even the number of particles that hit the detector is much larger already.

    This is not limited to heavy ion collisions, by the way. Proton proton collisions can produce new nucleons as well.
     
  7. Dec 12, 2017 #6
    A beautiful meson has rest mass sufficient to produce a pair of a baryon and antibaryon. Indeed, sufficient to produce two pairs.
    And the lifetime of a beautiful meson is so long that t*c is 0.5 mm for a charged beautiful meson and 0.45 mm for neutral ones.
    If a heavy ion collision is observed to produce, besides a number of antibaryons and baryons of which it is not clear which specific ones existed before, some beautiful mesons which can be tracked to decay to baryons and antibaryons, can you confidently specify that a baryon formed in decay of a beautiful meson did not exist before formation of that beautiful meson?
     
  8. Dec 13, 2017 #7

    mfb

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    Sure. Even more, the baryons only form when the B mesons decay.

    This is easier to observe in proton-proton collisions where there are fewer other particles, but it works with lead-lead collisions as well.
     
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