Measurement of Collinear $W$ Boson Emission from High Transverse Momenta Jets Using the ATLAS Detector

The high centre-of-mass energy and luminosity of the Large Hadron Collider offers a unique opportunity to probe the Standard Model in extreme and often unexplored regions of phase space. Processes involving gauge bosons and other massive particles are of particular interest given their potential connection to physics beyond the Standard Model. This thesis presents a measurement of the cross-section of $W \rightarrow \mu \nu$ in association with high transverse momenta jets using data collected by the ATLAS experiment from proton-proton collisions at a centre-of-mass energy of 8 TeV at the LHC, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The cross-section is expressed as a function of the angular separation between the muon and the closest jet. The focus is on the contributions to $W +$ jets processes from a high transverse momentum jet emitting a real $W$ boson, which are expected to be large at small angular separation. Collinear $W$ emission is an irreducible background to searches for new physics that involves Lorentz-boosted top quarks and is an important probe of both quantum chromodynamics corrections and electroweak corrections to $W +$ jets production. Theoretical predictions of this process, including several novel predictions, are compared to the data in terms of both the absolute and differential cross-sections.