Searches for new physics phenomena with jet final states in the ATLAS detector

Collimated sprays of particles, known as $jets$, are abundantly produced in high-energy proton-proton ($pp$) collisions, and predicted by many theories of physics beyond the Standard Model. This thesis reports first on the calibration of jets with focus on the data-driven insitu calibration. Two searches for new physics phenomena in events with jets in the final state are then presented. The analyses use $\sqrt s = 13$ TeV $pp$ collision data collected by the ATLAS experiment at the LHC. The response to small- and large-radius jets is measured insitu using data collected between 2015 and 2017. The jet energy scales are measured in events where a jet recoils against a calibrated reference object. The mass scale of large-radius jets is measured with two methods: one uses track jets to provide an independent measurement of the jet mass, the other uses $t\overline{t}$-events with boosted, hadronically decaying $W$-bosons and top quarks. The first analysis is a search for massive coloured objects which are pair-produced and decay to two jets. The data was recorded between 2015 and 2016, corresponding to an integrated luminosity of 36.7 fb$^{-1}$ of data. Two selections of four-jet events are defined; one with two $b$-tagged jets and one inclusive. No significant deviation from the expected background of Standard Model processes is observed in either of the event selections. The results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is a stop squark, which decays to two quarks through $R$-parity violating couplings. Limits are derived on the the stop squark mass and on the mass of pair -produced colour-octet resonances. The scenario where the two squarks decays to a $b$-quark and a lighter quark is considered separately using the $b$-tagged event selection. The second analysis is still ongoing and using data collected between 2015 and 2018, which corresponds to an integrated luminosity of 139 fb$^{-1}$. The search looks for a resonance, which could be a mediator between a confined hidden sector and the Standard Model sector. The mediator decays to two dark partons which shower and hadronise according to the dynamics of the hidden sector. The search targets a selection of model parameters where all dark hadrons decay promptly to visible particle. The signature is a pair of ``dark jets'' with different characteristics than Standard Model QCD-jets. An mass-decorrelated tagger is defined based on the number of tracks associated with the jet. The tagger is designed to distinguish dark jets from the QCD background, without distorting the smoothly falling background distribution.