# Do hadrons only interact via strong interaction?

According to my revision guide baryon and mesons always interact via the strong interaction.

Does this hold for baryon-baryon interactions? meson-meson?

Thanks

• Maybe there's some issue here with the distinction between always and only. – John Duffield May 9 '15 at 15:03

Charged hadrons, and neutral hadrons with nonzero magnetic moment, interact electromagnetically. A spinless, neutral hadron would not couple to the electromagnetic field at tree level, but the most obvious example of such a particle is the $\pi^0$, which decays electromagnetically to two photons.
All particles with flavor participate in the weak interaction. You mostly hear about this in terms of decays, because for ordinary interaction energies the weak interaction is too feeble to contribute much to the dynamics. You can consider the weak interaction between strongly- or electromagnetically-interacting particles as a Yukawa-type force, $$V \propto \frac{e^{-r/r_0}}{r},$$ where the length scale is set by the mass of the weak boson, $r_0 \approx (\hbar c) / (m_Wc^2)$.
However, the weak interaction has a different set of symmetries than the strong and electromagnetic forces. Specifically, the weak interaction is broken under parity transformations, while the strong and E&M transitions are not. You can therefore peer down into the short-distance physics of low-energy interactions by looking for parity-violating observables. The most common method is to look for an asymmetry in a scalar quantity, like reaction rate, that depends on the angle between a spin and a momentum, $\vec\sigma\cdot\vec p$.
The purely hardronic weak interaction (by which I mean, without any leptonic decays involved) is a hard thing to suss out theoretically, because the strong force is both (a) strong, and (b) complicated. In many-body systems, you may have opposite-parity excited states which happen to be nearly degenerate in energy and are mixed by the weak interaction. The largest known enhancement of this type, to my knowledge, occurs in some of the excited states probed by neutron capture on lanthanum: there is a correlation between the incoming neutron's spin $\vec\sigma_\text{n}$ and the outgoing photon's direction $\vec k_\gamma$ that turns out to be a 10% asymmetry. But lanthanum is an enormously complicated nucleus. In neutron capture on hydrogen the same asymmetry is about ten parts per billion.