Sigma Force is a division in DARPA.
The Defense Advanced Research Projects Agency is a real agency of the United States Department of Defense "established in 1958 to prevent strategic surprise from negatively impacting U.S. national security and create strategic surprise for U.S. adversaries by maintaining the technological superiority of the U.S. military", according to its official website. This means that it seeks out any scientific development to see how it might be used to help the country or, if it is dangerous, how it might be countered.
The SIGMA Force division is referred to by the author as "killer scientists". Each member of the Force is a highly trained operative who also happens to be a skilled scientist, specializing in various disciplines to have the expertise to handle whatever comes up.
The rationale behind the Force is the ever-expanding scientific breakthroughs, many of which could have devastating effects on the world is they fell into malevolent hands. It is the task of the members to try to keep that from happening and, when it invariably does, to find a way to resolve matters.
As the series starts, the lead agent for the group is Painter Crowe and he is the star of the first book. At its conclusion, he is promoted to running the division and from that point, his colleague, Grayson Pierce, is the primary agent in the field. Crowe is definitely management material as he has a keen analytical mind that sees beyond the obvious facts and puts the pieces together quickly. Pierce, no slouch in the brain department, is more of a man of action and seems to have more an emotional attachment to his work, or perhaps because he is the star for more adventures, it just seems that way.