Proton Classification in Particle Physics

Steps:

• Recall that hadrons are composite particles made of quarks held by the strong force.
• Identify baryons as hadrons composed of three quarks (e.g., proton: uud quarks).
• Identify mesons as hadrons composed of a quark-antiquark pair.
• Classify proton: it is a baryon (thus a hadron) but lacks quark-antiquark structure, so not a meson.

Why A is correct:

• A proton is defined as a baryon with three valence quarks (two up, one down), and baryons are a subset of hadrons excluding mesons.

Why the others are wrong:

• B: Protons are not mesons; they are baryons, and all mesons are hadrons.
• C: Protons are hadrons but not mesons.
• D: Protons are baryons but not mesons.

Final answer: A