The full typographical representation of the $\overline{\textrm{P}}\textrm{ANDA}$-acronym features a bar on top of the 1st character 'P', so $\overline{\textrm{P}}$, which in particle physics indicates an antiproton. Many font systems, in particular for web content, don't allow for such character modifications, thus the bar is often missing.

The acronym itself stands for:

$\overline{\textrm{P}}\textrm{ANDA}$ = antiProton ANihilation at DArmstadt