It is known that an electron and a positron are created from the energy of gamma ray, is it possible to create heavier particles like proton and neutron from the energy of gamma ray? (if so what is the anti-particle for neutron?)
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2$\begingroup$ Protons and neutrons are not elementary particles but they consist of quarks and gluons. The necessary conditions to create these baryons are very different from those of a pair production event that creates an electron positron pair like the $\gamma \to e^−e^+$. It can be somewhat approximated in heavy ion collisions (quark-gluon-plasma) and processes like $pp\to 3p\bar p$, but those are already using baryons to begin with. I do not think that the kind of conditions that led to baryogenesis in the early universe can be achieved at a current accelerator facility. I might be wrong, though. $\endgroup$– CuriousOneCommented May 26, 2015 at 7:03
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$\begingroup$ The anti-particle for a Neutron is just the Antineutron. en.wikipedia.org/wiki/Antineutron. It's the corresponding 3 anti-quarks to the Neutron's 3 quarks. And I think Curiousone is right, Protons and Neutrons emerged primarily from the young universe full of hot quark soup, not gamma rays - uh, I think. $\endgroup$– userLTKCommented May 26, 2015 at 7:45
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2 Answers
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The reaction $$ \gamma\to e^+ e^-$$ is actually forbidden on kinematic grounds - one can show that momentum and energy cannot be simultaneously conserved if there is not something else to take "excess" momentum, e.g. a heavy nucleus nearby - pair production from gamma rays happens in metals, for example, but not in vacuum.
Of course, given enough energy, it is in principle possible to produce arbitrary particle-antiparticle pairs from a gamma ray interacting with something that can take the excess momentum.
answered May 26, 2015 at 10:06
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$\begingroup$ when there is a very high energy contains in a field(whatever this field is) in the vacuum real particles can spontaneously pop into existence, how do momentum come into this picture? $\endgroup$– user6760Commented May 26, 2015 at 10:44
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$\begingroup$ see my answer for references that pair creation happened already at LEP $\endgroup$– anna vCommented May 26, 2015 at 10:47
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$\begingroup$ @user6760: No, actual particles do not "pop into existence spontaneously", and I don't know what it means for "very high energy" to be "contained" in a quantum field. All reactions must obey the conservation of energy-momentum, and $\gamma\to e^+ e^-$ simply can't do that. In principle, $\gamma\gamma\to X^+ X^-$ is possible, but as the reference by annav shows, even there it is actually done by having some other things around and extracting the two-photon collision from a more complicated reaction. $\endgroup$– ACuriousMind ♦Commented May 26, 2015 at 11:00
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There are already antiproton proton pairs experimentally created at LEP , Proton–antiproton pair production in two-photon collisions at LEP
The reaction is studied with the L3 detector at LEP. The analysis is based on data collected at e+e− center-of-mass energies from 183 GeV to 209 GeV, corresponding to an integrated luminosity of 667 pb^−1. The differential cross section is measured in the range of the two-photon center-of-mass energy from 2.1 to 4.5 GeV. The results are compared to the predictions of the three-quark and quark–diquark models.
So yes, we can create even heavier pairs at higher energies.
