Generally, non-U.S. citizens/residents must obtain a nonimmigrant B-1/B-2 visa to enter the United States for a temporary visit.
Under the Visa Waiver Program (VWP), citizens of certain countries may travel to the United States for stays of 90 days or less without a visa. Anyone traveling under VWP must apply for authorization through the Electronic System for Travel Authorization (ESTA). https://esta.cbp.dhs.gov/esta/
If you are not eligible to travel under the Visa Waiver Program, you may be required to appear at a U.S. embassy or consulate for an in-person interview prior to visa approval. There may be long waits to schedule an interview, so start the application process as early as possible. The SCCM-2015 attendees applying for a B-1/B-2 visa can obtain the SCCM-2015 letter of invitation by contacting Dr. Tony Zocher (zocher AT lanl.gov), the SCCM-2015 International Advisor.
Additional information about the visa application process can be found at the APS web site http://www.aps.org/programs/international/visa/visitors.cfm