Question

Antiparticle of proton is
A. Electron
B. Antiproton
C. Positron
D. Neutron

Answer 1

Hint: You can start by explaining which particle is the opposite of a proton (i.e. antiproton). Then move on to explain who predicted its existence and when it was first actually discovered. Then write its composition and properties. Then explain how we detect antiprotons in our universe.

Complete step by step answer:
The antiparticle of a proton is called an antiproton.

The existence of antiproton was predicted by Paul Dirac in a lecture he gave in 1933. Paul received a Nobel Prize for predicting the existence of a positive and negative solution to Einstein's energy equation. He also predicted the existence of an antimatter counterpart of an electron, with charge and spin opposite to that of the electron.

In 1955 with the help of Bevatron particle accelerator by Berkeley based physicists Emilio Segre, Owen Chamberlain and University of California. For their efforts they received the Nobel Prize in Physics in 1959.

Proton is made up of three quarks two of which are “up” quarks and one “down” quarks. Antiproton is also made up of three quarks two of which are “up” antiquarks and one “down” antiquarks.

Antiprotons have properties that correspond to the properties of the proton, but the electric charge and magnetic moment of the antiproton are opposite to that of the proton. Antiprotons are stable, but they normally have short lives. Antiprotons collide with protons, this will lead to the annihilation of both the particles and a release of burst of energy.

We have known the existence of antiprotons in our universe for over 25 years now. We use balloon-borne experiments and satellite based detectors to detect antiprotons that are present in the cosmic rays that reach us from very distant lands. The assumption is that the antiprotons in the cosmic rays are produced when protons in cosmic rays collide with the nuclei in the interstellar medium.
Hence, the correct answer is option B.

Note:
You may have noticed two rather strange things in the solution. First, that proton and antiproton collide and this leads to the annihilation of both of them. Second, cosmic rays carry antiprotons. So we don't see all the protons in the universe vanish once and for all. This is because the number of antiprotons in our universe is very less compared to the number of protons.