XP Decay mode of radioactive nucleus

The decay mode of Carbon-8 is listed as 'XP' in this table. None of the references I looked at listed XP as a decay mode. What is it>

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half-life in keV is also suspicious – Pygmalion May 23 '12 at 15:35
@Pygmalion: That's a width. It means a exceedingly short halflife, determined by $\Delta t \Delta E > \hbar/2$. Nothing surprising there as the isotope is very far from stability. – dmckee May 23 '12 at 15:37
@dmckee Yes, this was my guess too, but all other halflifes are specified in second (including on Wikipedia). – Pygmalion May 23 '12 at 15:52
@dmckee Is that notation used because the half life is so short (2*10^-21s) that measuring it is impractical at rest energy? – Dan Neely May 23 '12 at 15:55
Dan, Two things. Firstly it is the width of the reconstructed energy that we use to measure these decays, and secondly the uncertainty in the width is roughly symmetric (i.e. you can safely write $\pm$), but when you invert it you get an asymmetric range unless the width uncertainty is very small. – dmckee May 23 '12 at 16:11