Why is the proton (uud) lighter than the $Δ^0$ (uud) baryon? Neutron has quark composition udd with spin $\frac 12$. $\Delta^0$ baryon has quark composition udd with spin $3 \over 2$. 
On Wikipedia it says that $\Delta$ baryons have mass of approximately $1232 \frac{\mathrm{MeV}}{c^2}$ while the neutron has mass of approximately $~939 \frac{\mathrm{MeV}}{c^2}$. 
Why is the neutron lighter than the $\Delta$ baryon when they have the same quark composition? Is it because of the difference in spin?
 A: In QCD, much as in E&M, there is a chromo-magnetic dipole-dipole contact interaction $\propto S\cdot {S}'$, which will obviously differ between spin 1/2 and 3/2 baryons.  Bag-modelers have used it to estimate the mass difference between nucleons and deltas with tolerable accuracy.  
A: First note that it's the $\Delta^+$ particle that is $uud$. The $\Delta^0$ is $udd$ like the neutron.
The $\Delta^+$ particle is an excited state of the proton, that is the proton is the ground state of two up and one down quarks and the $\Delta^+$ particle is the first excited state. Likewise the $\Delta^0$ particle is the first excited state of the neutron.
To create a $\Delta^+$ particle from a proton we have to add the energy to excite the proton to a $\Delta^+$, and when we add that energy we are adding an equivalent mass related to the energy by Einstein's famous equation $E=mc^2$. The mass difference between the proton and $\Delta^+$ particle is simply due to this.
This is a general property of bound states. For example when we excite a hydrogen atom from the $1s$ to the $2p$ state we are adding an energy of $10.2$ eV and the mass increases by $10.2/c^2$.
A: The quark content is not a good predictor of baryonic mass. Only a small part stems from quark mass, hence from the Higgs mechanism. 98% or so is contributed by the energy of the massless gluons.
