Question

A neutron decays to form a proton. Which particle is not involved in the decay process?
A) Antineutrino
B) Down quark
C) Positron
D) Up quark

Answer 1

Hint: A neutron decaying into a proton is a type of beta decay. In most decay processes there are two or more by-products. An antineutrino is such a byproduct of that particular beta decay.

Complete step by step answer:
Beta-decay is a radioactive decay process in which a beta particle is emitted. Beta particle refers to a fast electron or a positron (anti-particle of an electron, not to be confused with a proton).
So the decay of a neutron into a proton is a beta decay process, a beta minus process to be exact. So in this beta minus $\left( {{\beta }^{-}} \right)$process the neutron is converted into a proton accompanied by an electron and an antineutrino. We can express that decay process for an element as,
${}_{Z}^{A}X\to {}_{Z+1}^{A}X+{{e}^{-}}+\overline{{{\nu }_{e}}}$
$X$ is an element with mass number A and atomic number Z.
${{X}^{'}}$ is a decay product element with mass number A and atomic number Z+1
${{e}^{-}}$ is the electron produced in the decay process.
$\overline{{{\nu }_{e}}}$ is the antineutrino produced in the decay process.
In the options, there is a mention of quarks. So, What are quarks? Quarks are fundamental particles that make up a proton and a proton. There are different types of quarks. The quarks involved in the formation of neutrons and protons are made up of UP quarks and DOWN quarks.
So a proton consists of two UP quarks and one down quark while a neutron consists of two DOWN quarks and one UP quarks. So in the product side of the beta decay, a proton is present which contains both these quarks.

So, the only particle which is not present in the decay process is the positron from the options.
So the answer to the question is option (C)- Positron.

Note: So the decay of a proton into a neutron is a beta plus process. So in this beta plus $\left( {{\beta }^{+}} \right)$process the proton is converted into a neutron accompanied by a positron and an electron neutrino. We can express that decay process for an element X as,
${}_{Z}^{A}X\to {}_{Z-1}^{A}{{X}^{'}}+{{e}^{+}}+{{\upsilon }_{e}}$
$X$ is an element with mass number A and atomic number Z.
${{X}^{'}}$ is a decay product element with mass number A and atomic number Z-1
${{e}^{-}}$ is the electron produced in the decay process.
$\overline{{{\nu }_{e}}}$ is the antineutrino produced in the decay process.
The mass number remains constant during beta decay. The atomic number can increase or decrease by one depending on the decay process.