A proton is made up, they say, by 2 up and 1 down quark, drowned in a sea of virual paricles: enter image description here

when an electron is captured

this process thereby changes a nuclear proton to a neutron and simultaneously causes the emission of an electron neutrino.

p  +  e−  →  n  +  ν

...and a neutron is made up by 2 down quarks and 1 up quarks enter image description here

Can you clarify some aspects of this "process" which I couldn't find anywhere?

  • 1) what happens to the electron? we know that it is an elementary particle, that means it has no internal structure and it can only be transformed into a photon when joined to a positron, is it first turned into energy? but there is no positron at hand in the proton. What are the steps that lead from 2u+1d+1e to 2d+1d+v?

  • 2) is there any qualitative difference between the charge of a down quark and an electron? can electric charge emerge from two different types of particle or a d-quark is just an electron with restmass= energy =1.2356*10^20Hz/3?


2 Answers 2


The process is called electron capture, and the Feynmann diagram for it is:

Electron capture

The up quark emits a virtual W$^+$ boson and changes to a down quark. The electron interacts with the W$^+$ and converts to an electron neutrino.

This is a weak force interaction, and the weak force can change the flavour of particles i.e. it can change quarks to a quark of a different type and likewise interchange between leptons and neutrinos.

Particles can change into other particles because all particles are excitation in a quantum field. For example there is an electron quantum field that pervades all of spacetime. If we add a quantum of energy to this field it appears as an electron. Likewise we can remove a quantum of energy from the electron field and this makes an electron disappear.

An electron can change into an electron neutrino because energy can be transferred from the electron field to the neutrino field making one electron disappear and one neutrino appear. Likewise for the up to down quark change.

  • $\begingroup$ We can describe the emission of W+ also in terms of field excitations, like the electron. The up quark is an excitation in up-quark field. This excitation disappears and gives its energy to the down quark field, where a down quark is created. And also W+ field is excited, to conserve charge. $\endgroup$
    – mpv
    Feb 23, 2016 at 8:54
  • $\begingroup$ Do quarks inside a nucleon feel Coulomb's law? Also, isn't same law somewhat unbalanced between adjoining protons : an up-quark repel repels another one andattracts a down, doesnt'it. The spherical outside elecric fields is nou uniform in all direction,is it? $\endgroup$
    – user104372
    Feb 23, 2016 at 16:20
  • $\begingroup$ @user104: Quarks in a nucleon feel all three forces (plus gravity as well I suppose). The unbalanced force you're referring to is the strong force. $\endgroup$ Feb 23, 2016 at 16:50

You may think about all processes involved elementary particles rather as about processes in which quantities called charges are conserved. So that, in weak reaction of electron capturing there are initially baryon charge $1$, electric charge $0$ and lepton charge $1$. Since weak interaction up to small corrections preserves these numbers, then the final state also must be characterized by these values. One of possible final states is neutron and electron neutrino. Microscopically it is explained by the weak interactions of $u$ and $d$ quarks through $W$ boson through the reaction $$ u + e \to \nu_{e} + d $$