The Laboratoire de l'Accelerateur Lineaire (LAL) is located about 25 km south of the center of Paris and is easily accessible via the RER B (Metro). The LAL website contains detailed information about how to get to LAL from the Paris airports and train stations. The closest airport to LAL is Orly.
Please make reservations directly with your hotel. The list of hotels on LAL's website contains hotels in the 5th, 6th, 13th and 14th "arrondissements" as well as a few near the Orsay campus. The above neighborhoods are the most convenient for catching the RER B train to Orsay.