QB_PH_SEC9_002: Particle Physics & Fundamental Interactions

In the quark model of hadrons, what is the quark composition of a neutron?

Correct Answer

### Step-by-Step Derivation & Verification: 1. **Baryon Constitution**: Baryons are color-singlet bound states composed of three valence quarks ($qqq$), each possessing baryon number $B = 1/3$. 2. **Charge Conservation**: - $u$ quark: $Q_u = +\frac{2}{3}e, \; I_3 = +\frac{1}{2}$ - $d$ quark: $Q_d = -\frac{1}{3}e, \; I_3 = -\frac{1}{2}$ 3. **Neutron Properties**: - Charge $Q = 0$ - Isospin component $I_3 = -1/2$ $Q(u d d) = +\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0$ $I_3(u d d) = +\frac{1}{2} - \frac{1}{2} - \frac{1}{2} = -\frac{1}{2}$ 4. **Conclusion**: Composition is $udd$. Option (B) is correct.