A Search for Dark Matter in the Monophoton Final State at CMS

The final state of a photon and missing transverse energy in pp collisions can be used as a valuable test of the Standard Model. The Standard Model production of Z(nu nu) + photon is a precise process with no dependence on tuned parameters, so it is sensitive to contributions from new physics events. At the same time, this constitutes an irreducible background for new physics searches with the same final state. This dissertation discusses a measurement of associated Z-gamma production, where Z decays into neutrinos. The data are collected in pp collisions at sqrt(s) = 7 TeV by the CMS experiment and correspond to an integrated luminosity of 5.0 fb^-1. Descriptions are provided for the methods of reducing and estimating the sources of background. A total number of 73 events are observed, which agrees well with the Standard Model expectation of 75.1 +/- 9:5 events. These results are interpreted in terms of a search for dark matter, which is widely accepted as the nonbaryonic dominant contribution to the matter density of the Universe. Models of production of dark-matter particles are used to set 90% confidence level upper limits of 13.6 +/- 15.4 fb on production in the photon-plus-missing-transverse-energy final state. At the time of publication, these provided the most sensitive upper limits for spin-dependent -nucleon scattering for masses between 1 and 100 GeV. For spin-independent contributions, the present limits are extended to M < 3.5 GeV.