Atrius Health, the largest independent physicians network in Massachusetts, has reached a deal to become part of Optum, the latest in a succession of consolidations in the state’s health care market. Under the deal, which needs approval from regulators, the nonprofit Newton-based physicians group would become a division of a national for-profit behemoth and significantly expand Optum’s footprint in Massachusetts. Optum includes the health care provider arm of UnitedHealth Group. “They will enable us to be a better version of ourselves,” Dr. Steven Strongwater, chief executive of Atrius, said Tuesday. Optum “will make investments in the kinds of things that will improve patient experience. . . . And we believe it will be very helpful for the Commonwealth because they will help us continue to drive down the total cost of care.”