From the Document: "The Sergeant at Arms and Doorkeeper (Sergeant at Arms) is an elected officer of the Senate and is the chamber's primary law enforcement and protocol official, responsible for the protection, security, decorum, and other administrative aspects of the Senate floor, Senate wing of the Capitol, Senate office buildings, and adjacent grounds. The office was initially established during the First Congress, when the Senate elected James Mathers as Senate Doorkeeper. The Senate Sergeant at Arms performs duties according to law, Senate rules, and custom."
CRS In Focus, IF11728
Congressional Research Service: https://crsreports.congress.gov/