It
is now believed that protons and neutrons (which constitute nuclei of
ordinary matter) are themselves built out of more elementary units
called quarks. A proton and a neutron consist of three quarks each.
Two types of quarks, the so called ‘up’ quark (denoted by
u) of charge (+2/3) e,
and the ‘down’ quark (denoted by d) of charge (−1/3)
e,
together with electrons build up ordinary matter. (Quarks of other
types have also been found which give rise to different unusual
varieties of matter.) Suggest a possible quark composition of a
proton and neutron.