Draft version May 17, 2011 Preprint typeset using L A T E X style emulateapj v. 11/10/09 A MEASUREMENT OF THE DAMPING TAIL OF THE COSMIC MICROWAVE BACKGROUND POWER SPECTRUM WITH THE SOUTH POLE TELESCOPE R. Keisler, 1,2 C. L. Reichardt, 3 K. A. Aird, 4 B. A. Benson, 1,5 L. E. Bleem, 1,2 J. E. Carlstrom, 1,2,5,6,7 C. L. Chang, 1,5,7 H. M. Cho, 8 T. M. Crawford, 1,6 A. T. Crites, 1,6 T. de Haan, 9 M. A. Dobbs, 9 J. Dudley, 9 E. M. George, 3 N. W. Halverson, 10 G. P. Holder, 9 W. L. Holzapfel, 3 S. Hoover, 1,2 Z. Hou, 11 J. D. Hrubes, 4 M. Joy, 12 L. Knox, 11 A. T. Lee, 3,13 E. M. Leitch, 1,6 M. Lueker, 14 D. Luong-Van, 4 J. J. McMahon, 15 J. Mehl, 1 S. S. Meyer, 1,2,5,6 M. Millea, 11 J. J. Mohr, 16,17,18 T. E. Montroy, 19 T. Natoli, 1,2 S. Padin, 1,6,14 T. Plagge, 1,6 C. Pryke, 1,5,6,20 J. E. Ruhl, 19 K. K. Schaffer, 1,5,21 L. Shaw, 22 E. Shirokoff, 3 H. G. Spieler, 13 Z. Staniszewski, 19 A. A. Stark, 23 K. Story, 1,2 A. van Engelen, 9 K. Vanderlinde, 9 J. D. Vieira, 14 R. Williamson, 1,6 and O. Zahn 24 Draft version May 17, 2011 ABSTRACT We present a measurement of the angular power spectrum of the cosmic microwave background (CMB) using data from the South Pole Telescope (SPT). The data consist of 790 square degrees of sky observed at 150 GHz during 2008 and 2009. Here we present the power spectrum over the multipole range 650 <ℓ< 3000, where it is dominated by primary CMB anisotropy. We combine this power spectrum with the power spectra from the seven-year Wilkinson Microwave Anisotropy Probe (WMAP) data release to constrain cosmological models. We find that the SPT and WMAP data are consistent with each other and, when combined, are well fit by a spatially flat, ΛCDM cosmological model. The SPT+WMAP constraint on the spectral index of scalar fluctuations is n s =0.9663 ± 0.0112. We detect, at ∼5σ significance, the effect of gravitational lensing on the CMB power spectrum, and find its amplitude to be consistent with the ΛCDM cosmological model. We explore a number of extensions beyond the ΛCDM model. Each extension is tested independently, although there are degeneracies between some of the extension parameters. We constrain the tensor- to-scalar ratio to be r< 0.21 (95% CL) and constrain the running of the scalar spectral index to be dn s /d ln k = −0.024 ± 0.013. We strongly detect the effects of primordial helium and neutrinos on the CMB; a model without helium is rejected at 7.7σ, while a model without neutrinos is rejected at 7.5σ. The primordial helium abundance is measured to be Y p =0.296 ± 0.030, and the effective number of relativistic species is measured to be N eff =3.85 ± 0.62. The constraints on these models are strengthened when the CMB data are combined with measurements of the Hubble constant and the baryon acoustic oscillation feature. Notable improvements include n s =0.9668 ± 0.0093, r< 0.17 (95% CL), and N eff =3.86 ± 0.42. The SPT+WMAP data show a mild preference for low power in the CMB damping tail, and while this preference may be accommodated by models that have a negative spectral running, a high primordial helium abundance, or a high effective number of relativistic species, such models are disfavored by the abundance of low-redshift galaxy clusters. Subject headings: cosmology – cosmology:cosmic microwave background – cosmology: observations – large-scale structure of universe rkeisler@uchicago.edu 1 Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL, USA 60637 2 Department of Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL, USA 60637 3 Department of Physics, University of California, Berkeley, CA, USA 94720 4 University of Chicago, 5640 South Ellis Avenue, Chicago, IL, USA 60637 5 Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL, USA 60637 6 Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL, USA 60637 7 Argonne National Laboratory, 9700 S. Cass Avenue, Ar- gonne, IL, USA 60439 8 NIST Quantum Devices Group, 325 Broadway Mailcode 817.03, Boulder, CO, USA 80305 9 Department of Physics, McGill University, 3600 Rue Univer- sity, Montreal, Quebec H3A 2T8, Canada 10 Department of Astrophysical and Planetary Sciences and Department of Physics, University of Colorado, Boulder, CO, USA 80309 11 Department of Physics, University of California, One Shields Avenue, Davis, CA, USA 95616 12 Department of Space Science, VP62, NASA Marshall Space Flight Center, Huntsville, AL, USA 35812 13 Physics Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 94720 14 California Institute of Technology, MS 249-17, 1216 E. Cal- ifornia Blvd., Pasadena, CA, USA 91125 15 Department of Physics, University of Michigan, 450 Church Street, Ann Arbor, MI, USA 48109 16 Department of Physics, Ludwig-Maximilians-Universit¨at, Scheinerstr. 1, 81679 M¨ unchen, Germany 17 Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garch- ing, Germany 18 Max-Planck-Institut f¨ ur extraterrestrische Physik, Giessen- bachstr. 85748 Garching, Germany 19 Physics Department, Center for Education and Research in Cosmology and Astrophysics, Case Western Reserve University, Cleveland, OH, USA 44106 20 Department of Physics, University of Minnesota, 116 Church Street S.E. Minneapolis, MN, USA 55455 21 Liberal Arts Department, School of the Art Institute of Chicago, 112 S Michigan Ave, Chicago, IL, USA 60603 22 Department of Physics, Yale University, P.O. Box 208210, New Haven, CT, USA 06520-8120 23 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, USA 02138 24 Berkeley Center for Cosmological Physics, Department of arXiv:1105.3182v1 [astro-ph.CO] 16 May 2011