Pentaquark ($qqqq\bar{q}$)

A baryon with constituent content four quarks and one antiquark, whose net baryon number is one. Such content admits more flavour representations than three quarks alone. A representation in the tensor product is not evidence by itself of a bound state.

Exotic flavour weights can decay into a baryon and a meson while preserving every strong quantum number: $uudd\bar{s}\to p{K}^0$ or $n{K}^{+}$, $ddss\bar{u}\to {\Xi }^{-}{\pi }^{-}$, and $uuss\bar{d}\to {\Xi }^0{\pi }^{+}$. These are quark rearrangements. If above threshold with unsuppressed couplings, such states are short-lived strong resonances. Flavour symmetry alone fixes neither masses nor widths.

Two antisymmetric flavour quark pairs can each transform in $\overline{3}$, combine into $\overline{6}$, and with an antiquark give $\overline{6}\otimes \overline{3}=\overline{10}\oplus 8$. The antidecuplet rows are $(Y,I)=(2,0),(1,1/2),(0,1),(-1,3/2)$. Exactly three weights are absent from all three-quark states: $(I_3,Y)=(0,2)$ and $(\pm 3/2,-1)$.