The predictions of the inflationary LCDM paradigm match todays high-precision measurements of the cosmic microwave background anisotropy extremely well. The same data put tight limits on other sources of anisotropy. Cosmic strings are a particularly interesting alternate source to constrain. Strings are topological defects, remnants of inflationary-era physics that persist after the big bang. They are formed in a variety of models of inflation, including string theory models such as brane inflation. We assume a Nambu-Goto model for strings, approximated by a collection of unconnected segments with zero width, and show that measurements of temperature anisotropy by the South Pole Telescope break a parameter degeneracy in the WMAP data, permitting us to place a strong upper limit on the possible string contribution to the CMB anisotropy: the power sourced by zero-width strings must be <1.75% (95% CL) of the total or the string tension Gmu <1.7x10^{-7}. These limits imply that the best hope for detecting strings in the CMB will come from B-mode polarization measurements at arcminute scales rather than the degree scale measurements pursued for gravitational wave detection.
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