In beta decay neutrons convert into proton, in that process they'll release an electron (called beta particle) but mass still remains same
$$_zX^A \longrightarrow\; _{z+1}Y^{A}+ e^- $$ The question is where did that beta particle came out from no where like matter can't be created or destroyed?
Or am I misinterpreting something?

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    $\begingroup$ Matter can be created and destroyed - that's what radioactivity is. $\endgroup$ Jul 24, 2021 at 10:36
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    $\begingroup$ It should be noted that the neutron is more massive than the proton, so nucleus X is more massive than nucleus Y, though this difference is very small. $\endgroup$
    – Triatticus
    Jul 24, 2021 at 13:42
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    $\begingroup$ E=mc^2, so energy can be converted to mass (and vice versa) $\endgroup$
    – KingLogic
    Jul 25, 2021 at 3:51

1 Answer 1


In beta decay a neutron decays into a proton, an electron and an antineutrino. More precisely, one of the down quarks within the neutron changes into an up quark and emits a short-lived $W^-$ boson, which quickly decays into an electron and an antineutrino. The change of flavour by the quark turns the neutron into a proton.

The electron and the antineutrino do not have to "come" from anywhere. Transformations of one fundamental particle into another (or a set of others) can happen provided certain conservation conditions are met. In the case of beta decay we have

  • Conservation of charge - the charge of the neutron is $0$ and the net charge of the proton, the electron and the (uncharged) neutrino is $0$.
  • Conservation of energy - the neutron is more massive than the decay products, but the mass difference is accounted for by the kinetic energy of the decay products.
  • Conservation of momentum
  • Conservation of baryon number/quark number - the transformation leaves the number of quarks minus the number of antiquarks unchanged.
  • Conservation of lepton number - the lepton number of the neutron and the proton are $0$. The lepton number of the electron is $+1$, but this is balanced by the lepton number of the antineutrino, which is $-1$.
  • $\begingroup$ "Conservation of boson number - ..." You probably mean conservation of quark number (quarks are not bosons). $\endgroup$ Jul 24, 2021 at 11:41
  • $\begingroup$ @ThomasFritsch Yes indeed - fixed. $\endgroup$
    – gandalf61
    Jul 24, 2021 at 12:03
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    $\begingroup$ "the proton is more massive than the decay products" ... did you mean "neutron" instead of "proton"? $\endgroup$
    – Michael
    Jul 24, 2021 at 20:06
  • $\begingroup$ @Michael Yes I did. Fixed. $\endgroup$
    – gandalf61
    Jul 24, 2021 at 20:50
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    $\begingroup$ Thanks, @ThomasFritsch. The OP probably meant baryon number. $\endgroup$
    – J.G.
    Jul 24, 2021 at 21:08

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