Determination of the radius of the 11Li neutron halo from pion double charge exchange

We have analyzed the pion-induced double-charge-exchange data for the direct population of the ground state of 11Li by the 11B(π-,π+)11Li reaction. We find that the measured cross section determines the rms radius of the last two neutrons in 11Li to be 5.1-0.9+0.6 fm, which corresponds to a total neutron rms radius for the nucleus of 3.28-0.330.24 fm. We show that for large assumed neutron-halo radii the cross section decreases as the radius of the halo to the inverse sixth power and that a radius of 12 fm leads to a cross section which is 3 orders of magnitude smaller than that measured