For information:
Owing to the geographical position of the Organization on the border between France and Switzerland, we strongly recommend that you enquire at the relevant consulates to find out the conditions of admission applicable to you in Switzerland and in France, specifying the duration of your visit at CERN. If necessary, you are kindly asked to take the necessary steps to obtain the papers required, in particular Swiss and/or French multiple-entry visas.
Please consult the proper official web sites.
Entry requirements for Switzerland
Entry requirements for France