Long lived states in antiprotonic helium at low densities

It is now well established that a fraction of about 3% of the p's stopped in a helium target survives for a surprisingly long time (tens of microseconds). As suggested by Condo in 1964 the long lifetime of p's in helium is explained by the existence of metastable states of the exotic atom pHe/sup +/. Theoretical calculations predict an increase of such a fraction up to about 20% when lowering the pressure of the helium target to around 1 mbar. An analysis of data from p's stopped in helium targets at pressures from 150 mbar down to 0.2 mbar, collected by the Obelix apparatus at the LEAR decelerator at CERN, is presented. Evidence of a noticeable fraction of long lived states at helium target pressures lower than some millibar is reported. Moreover a dependence of this fraction on the p momentum distribution is observed. (16 refs).