  We present the first calculation of the scattering amplitude in the singlet
channel beyond QCD. The calculation is performed in $SU(2)$ gauge theory with
$N_f=2$ fundamental Dirac fermions and based on a finite-volume scattering
formalism. The theory exhibits a $SU(4) \to Sp(4)$ chiral symmetry breaking
pattern that is used to design minimal composite Higgs models currently tested
at the LHC. Our results show that, for the range of underlying fermion mass
considered, the lowest flavour singlet state is stable.
