Answer to Question #137049 in Atomic and Nuclear Physics for Sydney

Question

Dear sir it say measons combine with protons and form neutrons please explain the mechanism

Answer 1

To understand how this is possible, you need to learn about quarks. By now, we know that some particles consist out of quarks. Some - because there are indeed elementary particles (electrons, neutrinos, photons, quarks and gauge bosons). But particles like neutrons, protons and mesons are compound particles. There are 6 quarks - up (u), down (d), strange (s), charm (c), beauty (b) and top(t), and corresponding to them antiquarks.

Quark composition of proton: p=uud; neutron: n=udd. Mesons are particles that consists of only 2 quarks. The lightest mesons are pions.

As an example, we can analyse reaction

$\pi^- + p \rightarrow \pi^0 + n$

$(\bar u d) + (uud) \rightarrow (u\bar u -d \bar d) + (udd)$

As every particle-antiparticle pair, quarks can annihilate, and as opposite quark-antiquark pairs can be born when there is enough energy for creation of its bounded state. Neutral pion is a mixture of two states. As you can see, quark composition remain the same except pair $d \bar d$ that appears because the total energy of $\pi^- + p$ is enough to create it and, therefore, enough to born heavier particles $\pi^0 + n$.