[Hps] Second round of collaboration review of our first physics paper

[Alessandra Filippi]

Hi Omar,
thank you for the nice paper, I approve it.
I just would like to send a couple more remarks you might want to consider
before submission:
- line 4 of the institute list (Catania): Universit\`a (a slash is missing)
- page 4: do we have a reference for the LEE effect that we might include?

Last, do we mention why the search stops at 81 MeV, while we show in the 
figure a e+e- invariant mass spectrum that goes up to 150 MeV? (probably I missed
the information in reading fast).
Thanks, cheers
     Alessandra