FAMILIES OF PARTICLES

 The earliest classification scheme for particles was based on their masses. 

1. Leptons 

The lightest particles, including the electron (m_ec = 0.511 MeV), were called leptons (from the Greek word for “small”). 

2. Baryons

The heaviest particles, including the proton (m_pc = 938 MeV), were called baryons (from the Greek word for “heavy’’). 

3. Mesons

In between were particles, including the pion (m_πc = 140 MeV), called mesons، (from the Greek word for “middle”). 

Today these classifications based on mass are no longer valid; for example, one lepton and many mesons are more massive than the proton.

However, we retain these three names as descriptive of particles with similar properties, even though the classification based only on mass is no longer valid. Table summarizes these three families of particles and some of their properties.

Family	Structure	Interactions	spin	Examples
Leptons	Fundamental	Weak, electromagnetic	Half Integral	e, v
Mesons	Fundamental	Weak, electromagnetic, strong	Integral	π, K
Baryons	Composite	Weak, electromagnetic, strong	Half Integral	P, n

Standard Model of Elementary Particles