Derivation of a transfer function to estimate the Z(vv)+jets background from photon+jets in a search for dark matter at the ATLAS detector

Abstract: It has for long been known that dark matter must exist in our universe. Despite massive efforts to detect dark matter, its constituents remain a mystery. One strategy to unveil this mystery is to produce dark matter by colliding Standard Model particles. The most common signature of dark matter production is an excess of missing transverse energy, MET. Such an event would be very rare, which is why it is important to have a precise background estimation. One irreducible background in the search for dark matter is a Z boson decaying to two neutrinos. In this thesis, events with a photon are used to create a transfer function that can be used in the estimation of this background. First, events are selected with the same characteristics as the Z(vv) background events, but with a photon instead of the Z. The two samples are compared by adding the photon pT to the missing energy, to replace the missing transverse energy from the Z boson decaying to two neutrinos. The two samples show similar distributions for all studied variables. Next, the transfer function is created, by fitting the ratio of MC samples for the two bosons. The transfer function reflects the relation between the production cross sections and selection differences for the Z and and changes slightly with boson pT. It is for the range 500-2000 GeV determined to 0.2895 e-5 ' MET [GeV], with an uncertainty of 18%. To make a Z(vv) background estimate the transfer function can be multiplied by photon + jets data. After further validation, this transfer function will be used in the background estimation of the Z(vv) background for the mono-Higgs(bb) ATLAS analysis, aiming to improve its sensitivity in the full run II.