Probing new physics using initial state radiation jets at the Large Hadron Collider

The existence of new particles and interactions could potentially address fundamental questions about our universe, for example, the nature of dark matter. If dark matter couples, even feebly, to the Standard Model, then new particles mediating this interaction could be produced in accelerator-based experiments. This dissertation describes the search for such mediators in a proton-proton collider, the LHC. The search is performed in a low-mass regime that has not been explored before, where these new mediator particles would couple weakly to the Standard Model quarks. Signal candidates will be recoiling against initial state radiation or ISR. The presence of ISR ensures that events in data will have enough energy to satisfy the trigger requirements that prevent saturation of the data band width. The ISR also gives the resonance a large Lorentz boost, so that its decay products are highly collimated inside a single jet of hadrons. The distribution of the jet mass is probed for a potential narrow peaking signal over a smoothly falling background. No evidence for such dark matter mediator resonances is observed within the mass range of 50-450 GeV and the most stringent constraints to date are placed within 50-300 GeV.