When a proton combines with an electron, it does not directly result in the conversion of the electron into a down quark. The interaction you are describing is not a direct transformation of a proton into a neutron. Instead, it involves a more complex process called beta decay or beta-minus decay.
In beta decay, a neutron within an atomic nucleus can be converted into a proton, an electron, and an electron antineutrino. The process is mediated by the weak nuclear force, one of the fundamental forces of nature. Here's how it occurs:
A down quark within the neutron undergoes a transformation called a weak decay, where it is converted into an up quark while emitting a W- boson.
The W- boson then decays into an electron and an electron antineutrino.
The up quark produced in the first step combines with the other two up quarks in the proton, forming a new proton.
So, the net result is that a neutron is transformed into a proton, emitting an electron and an electron antineutrino in the process.
The electron is not turned into a down quark during this process. The electron is a lepton, which is a different type of fundamental particle than quarks. Leptons (such as electrons) and quarks (such as up and down quarks) are distinct categories of particles, and they do not transform into each other in these types of nuclear processes.