NN($^1S_0$) pairs in $^3$He and in p$^3$He backward elastic scattering

It is shown that the shoulder observed in the cross section of $p^3He$ backward elastic scattering at proton beam energies of 0.4 -- 0.7 GeV can be explained by the mechanism of virtual $\pi$-meson production with a dominant contribution from the subprocesses $pd^*\to ^3$He$\pi^0$ and $p(pp)\to^3$He$\pi^+$, where $d^*$ denotes the singlet deuteron and the $pp$ pair is in the $^1S_0$ state. At high energies, 1-3 GeV, the mechanism of $np$-pair transfer dominates and probes high-momentum $NN(^1S_0$) correlations in $^3$He.