Parallel momentum distributions as a probe of halo wave functions

Distributions of the parallel momentum of $^{10}\mathrm{Be}$ fragments from the breakup of $63A$ MeV $^{11}\mathrm{Be}$ have been measured for $^{9}\mathrm{Be}$, $^{93}\mathrm{Nb}$, $^{181}\mathrm{Ta}$, and $^{238}\mathrm{U}$ targets. The distributions have similar narrow widths with a mean value of 43.6 \ifmmode\pm\else\textpm\fi{} 1.1 MeV/c FWHM, and agree with a theoretical momentum distribution for the valence neutron in $^{11}\mathrm{Be}$. The breakup mechanisms do not appear to distort the parallel momentum distribution of the $^{10}\mathrm{Be}$ core. These findings support the existence of a one-neutron halo in $^{11}\mathrm{Be}$ and the use of the parallel momenta of the heavy breakup products as a reliable probe of halo neutron momenta.