Hadrons are particles made of quarks. Because they contain quarks, they feel the strong force. There are two families:Baryons are made of three quarks. Each has a baryon number B = +1 (antibaryons, made of three antiquarks, have B = –1). Examples: the proton and the neutron. The proton is the only stable baryon; a free neutron decays with a half-life of about 10 minutes, and other baryons eventually decay into protons.Mesons are made of a quark and an antiquark. Their baryon number is 1/3 + (–1/3) = 0. Examples: pions (π+, π0, π-) and kaons. All mesons are unstable.
Summary:
• hadrons (quarks, feel the strong force): baryons (3 quarks, B = 1) and mesons (quark + antiquark, B = 0)
• leptons (fundamental, no strong force, B = 0)Total baryon number is conserved in every interaction, so it can be used to decide whether a proposed reaction is possible.Worked example: can these proton collisions happen?
p + p → p + p + π0: B before = 1 + 1 = 2; B after = 1 + 1 + 0 = 2. Allowed by baryon number.
p + p → p + π+: B before = 2; B after = 1 + 0 = 1. Baryon number is not conserved, so this reaction cannot happen.
Summary:• hadrons (quarks, feel the strong force): baryons (3 quarks, B = 1) and mesons (quark + antiquark, B = 0)
• leptons (fundamental, no strong force, B = 0)Total baryon number is conserved in every interaction, so it can be used to decide whether a proposed reaction is possible.Worked example: can these proton collisions happen?
p + p → p + p + π0: B before = 1 + 1 = 2; B after = 1 + 1 + 0 = 2. Allowed by baryon number.
p + p → p + π+: B before = 2; B after = 1 + 0 = 1. Baryon number is not conserved, so this reaction cannot happen.