Is it possible to visit the European Organization for Nuclear Research (CERN) as a tourist to Geneva? Are there any guided tours? Which parts can be seen?
Yes - CERN has an outreach program which does tours along with other activities. From their Visit CERN page:
CERN welcomes the general public to visit and tour its exhibitions, experimental areas and other facilities. The admission is free. The permanent exhibition “Universe of particles” and the Microcosm exhibition can be visited from Monday to Saturdays from 10h00-17h00, with no reservation needed. Visitor groups who book in advance can also enjoy a guided tour of some of the surface installations of CERN's experimental areas and learn about the fundamental research done at the worldís largest particle physics laboratory. For individual visitors or small groups, we offer guided tours to the ATLAS visitor centre and the permanent exhibition “Universe of Particles”.
The exhibitions are well worth spending time in.
CERN Individual Tours
CERN organises both individual tours as well as group tours. The advantage of individual tours is that group tours can have very long waiting lists (sometimes even for months). Individual tours include (quoting from the CERN webpage on individual tours):
A tour for individuals offers an introduction to CERN, allowing visitors to discover the history of the Laboratory and the latest scientific developments. The tour consists of a visit to a historic area of CERN and to the operations centre of one of the experiments. Immersive audio-visual presentations complement the information provided by the tour guide. The tour is in English or French and lasts around two hours.
If you want to have a chance to visit the underground facilities at CERN you must book a group tour. Quoting from the CERN webpage on individual tours:
Please note that this tour does not include an underground visit.
CERN Group Tours
Group tours are available for parties of 8 to 48 people. These tours can include an underground visit, although this is not guaranteed. The CERN webpage on group tours provides a reservation form for group tours. Quoting from the same page:
Tours are organized from Monday to Saturday mornings and afternoons. Each tour lasts about 3 hours and comprises an introduction followed by a film, visits to experimental areas and/or to an accelerator. Please note that underground visits are not guaranteed. For safety reasons children under 13 years of age are not admitted.
I visited CERN years ago and toured the underground installations. That was when the LHC was still under construction. Seeing the ATLAS instrument up and close was quite impressive. Now that the LHC is shut down for maintenance underground tours are supposed to start again, but no mention of them is made on the CERN website. I would suggest you keep checking.
If CERN start does indeed start giving tours of the underground experimental areas expect to have to book them at least half a year in advance. More information will be on their website (see the linke mentioned in the previous answer)