Scattering particles

It turns out that the $\Delta^{++}$ particle (capital delta with a double '+' superscript, pronounced "Delta plus plus"), which is typically discussed as a composition of three up quarks, is actually just a resonance in scattering experiments. That is, we know it exists because if we're shooting pions at protons, for certain energy pions, we see way more scattering than for nearby energies. This is explained as the pion and the proton combining to create a new particle, the $\Delta^{++}$, which almost instantaneously decays into a proton and a pion. The new proton and pion can head off in basically whatever direction they want, so scattering cross section is very high. But the decay happens so fast that it's impossible to directly observe the $\Delta^{++}$ particle; it really is just a resonance!
This same resonance is observed in a variety of experiments involving different scatterers, and based on the width of the scattering peak, we can find out the mass and lifetime of the particle.

On a related note, the $\Delta^{++}$ is a member of the delta family, which consists of four baryons (particles composed of three quarks) involving only up and down quarks, the same basic ingredients as protons and neutrons. That includes the $\Delta^{+}$ (uud) and the $\Delta^0$ (udd) baryons, which have the same quark composition as protons and neutrons, respectively. The distinction is that in the delta family, the spins of all three constituent quarks are aligned, for a net spin of 3/2, rather than 1/2 for the proton and neutron. This spin alignment is a higher-energy state than the 'ground-state' nucleons, which is manifested in the delta family's higher masses and shorter lifetimes.