Search for signatures of large extra dimensions in high-mass diphoton events from proton-proton collisions at √s = 13 TeV with CMS

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A search is performed for a nonresonant excess of high-mass diphoton events over the Standard Model background prediction. This type of excess could be a signature of new physics, such as from the existence of extra spatial dimensions in the universe. The presence of these extra dimensions could modify the fundamental scale of gravity in such a way as to solve the Standard Model hierarchy problem. The sample of diphoton events were produced from proton-proton collisions using the Large Hadron Collider at a center-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 35.9 fb−1 and were recorded in 2016 with the CMS detector. The dominant, irreducible diphoton background comes from Standard Model diphoton production and is determined using a next-to-next-to-leading order Monte Carlo calculation. The subdominant, reducible photon+jet and dijet backgrounds arise when a jet fakes a photon signature in the detector and one or two become misidentified as photons. A technique using control samples in data is used to estimate this contribution. The data are consistent with the background-only hypothesis and the results are interpreted in the context of the large extra-dimensional model of Arkani-Hamed, Dimopoulos, and Dvali. At 95% confidence level, lower exclusion limits are set on the string mass scale MS ranging from 5.6 to 9.7 TeV, depending on the model parameters. This result provides the current most stringent limits on searches for large extra dimensions in the diphoton channel.

Electronic Thesis or Dissertation
Physics, Particle physics