Question

Particles which are not composite and hence truly elementary are
A) Mesons
B) Protons
C) Neutrons
D) Leptons

Answer 1

Hint: Elementary particles are nothing but the fundamental particles, which make up the world around us. These particles do not have sub-particles. On the other hand, composite particles are made up of different elementary particles.

Complete step-by-step solution:
Elementary particles are the fundamental building blocks of particles in nature, which do not have sub-particles. Clearly, these particles cannot be broken down into other smaller particles. On the other hand, composite particles are made up of elementary particles, combined together. These particles can be broken down into sub-particles.
There are 12 elementary particles in nature. They are given as follows:
a) Six quarks – up, down, strange, charm, bottom and top
b) Six leptons – electron, electron neutrino, muon, muon neutrino, tau, and tau neutrino
Here, leptons are elementary particles of half-integral spin (spin=1/2). These fundamental particles do not involve strong interactions in nature. Leptons can be categorized into three charged leptons (electron, muon, tau) and three neutral leptons (electron neutrino, muon neutrino, tau neutrino)
On the other hand, quarks combine to form composite particles. Unlike leptons, quarks do involve strong interactions as well as weak interactions to form composite particles. They also have other intrinsic properties like charge, mass, and spin. In quarks, up quark and down quark have the lowest masses of all quarks. The composite particles formed by quarks are termed as hadrons. The two most stable particles of hadrons are protons and neutrons, which constitute the nucleus of an atom. A proton is made up of two up quarks and one down quark whereas a neutron is made up of two down quarks and one up quark.
Clearly, in the provided options for the given question, leptons are the particles, which are truly elementary.
Therefore, option D is the correct answer.

Additional information: Mesons are composite particles which are made up of one quark and one antiquark, held closer by strong interactions. Unlike protons and neutrons, mesons are unstable in nature.

Note: Out of all the six leptons in nature, electrons are the best-known. Charged leptons (electron, muon, tau) to combine with other particles to form composite particles like atoms and positronium. On the other hand, neutral leptons (electron neutrino, muon neutrino, tau neutrino) do not interact with any other particles. They rarely occur in nature.