# Elementary particles

The elementary particles or subatomic particles discovered till date and are more than 200in number so far. They are called elementary because they are structureless and cannot be explained as a system of other particles. The classification of elementary particles are as follows

1. Baryons: Baryons are also known as heavy particles and they comprise of proton and other heavier particles. Proton and neutron of this group are called nucleons and the other are termed as hyperons. All baryons has anti-particle. We assign a number +1 for baryon and -1 for antibaryon. So, in any closed system, the baryon number is conserved in interaction or decay. This is called as the law of conservation of baryons.

Hyperons are very special and are known by a time decay of the order of $10^{-10}$ seconds and mass between that of neutron and deuteron. Their decay time is greater than the time of formation $(10^{-3})$. It is this unsolved problem that these particles along with K-mesons are called strange particles. There are four kinds of hyperons: Lambda, Sigma, Xi and Omega.

2. Leptons: This group contains electron, photon, neutrino and muon.

Mesons: The rest mass of these particle lie between 250me and 1000 me. It acts as an agent for interaction between particles inside the nucleus. Baryons and mesons are combinely called as hadrons and are particle of strong interaction.

 Leptons Name Symbol Photon $\gamma$ Neutrion $\nu_e \nu_{\mu}$ Electron $e^{\pm}$ Muon $\mu^{\pm}$ Mesons Pion $\pi^{\pm} \pi^0$ Kaon $K^{\pm} K^0 K_1 K_2$ Eta $\eta^0$ Baryons Proton $p^{\pm}$ Neutron $n$ Lambda $\Lambda^0$ Sigma $\Sigma^+ \Sigma^0 \Sigma^-$ Xi $\Xi^0 \Xi^-$ Omega $\Omega^-$