Swartz's research has involved many aspects of experimental particle physics. From 1977 until 1983, he worked on several fixed target experiments at the Fermi National Accelerator Laboratory. These included a proposal to measure the cross sections for neutrino- and antineutrino-electron elastic scattering, a measurement of the transverse momentum spectra of final state hadrons produced in pion-nucleon interactions, and a study of large mass muon pairs produced in pion-nucleon collisions. His thesis on the atomic mass dependence of the muon pair cross section was supervised by Melvin Shochet.

From 1983 through 1986, Swartz was a member of the UA2 Collaboration at the CERN SPS Collider where he studied various properties of the recently discovered W and Z bosons. He also searched for several exotic particle states in the decays W and Z bosons.

In late 1986, he moved to SLAC, where he joined the Mark II Collaboration working at the SLAC Linear Collider (SLC) and an informal collaboration of Mark II and SLD physicists working to polarize the electron beam of the SLC (SLCPOL Collaboration). As a member of the Mark II Collaboration, he became interested in lepton flavor violation and searched for doubly-charged Higgs scalars in Z data and elsewhere. As a member of the SLCPOL Collaboration, he worked on the production, transport, and monitoring of a polarized electron beam for the SLC. When the Mark II experiment ended in 1991, he joined the SLD Collaboration which was preparing a new experiment for the SLC. He became the co-leader of the SLD Electroweak Working Group which has performed several measurements of the left-right Z cross section asymmetry ALR. This series of progressively more precise measurements constitutes the most precise test of the electroweak Standard Model yet performed.