Search For New Phenomena In Jets Plus Missing Transverse Energy Final States At The Lhc

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of 3.2 fb–1 at √s=13 TeV collected in 2015 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons. Several signal regions are considered with increasing missing-transverse-momentum requirements between EmissT > 250 GeV and EmissT > 700 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. Here, the results are translated into exclusion limits in models with large extra spatial dimensions, pair production of weakly interacting dark-matter candidates, and the production of supersymmetric particles in several compressed scenarios.

A dedicated search is presented for new phenomena in inclusive eight- and ten-jet final states with low missing transverse momentum, with and without identification of jets originating from b quarks. The analysis is based on data from proton-proton collisions corresponding to an integrated luminosity of 19.6 inverse femtobarns collected with the CMS detector at the LHC at sqrt(s) = 8 TeV. The dominant multijet background expectations are obtained from low jet multiplicity control samples. Data agree well with the standard model background predictions, and limits are set in several benchmark models. Colorons (axigluons) with masses between 0.6 and 0.75 (up to 1.15) TeV are excluded at 95% confidence level. Similar exclusion limits for gluinos in R-parity violating supersymmetric scenarios are from 0.6 up to 1.1TeV. These results comprise the first experimental probe of the coloron and axigluon models in multijet final states.

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb-1 of \(\sqrt{s{\mathrm{}}} = 8\) TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT>120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between EmissT>150 GeV and EmissT>700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. Limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presented.

This thesis provides a detailed and comprehensive description of the search for New Physics at the Large Hadron Collider (LHC) in the mono-jet final state, using the first 3.2 fb-1 of data collected at the centre of mass energy of colliding protons of 13~TeV recorded in the ATLAS experiment at LHC. The results are interpreted as limits in different theoretical contexts such as compressed supersymmetric models, theories that foresee extra-spatial dimensions and in the dark matter scenario. In the latter the limits are then compared with those obtained by other ATLAS analyses and by experiments based on completely different experimental techniques, highlighting the role of the mono-jet results in the context of dark matter searches.Lastly, a set of possible analysis improvements are proposed to reduce the main uncertainties that affect the signal region and to increase the discovery potential by further exploiting the information on the final state.

Two projects are presented in this thesis, the first pertaining to a physics analysis using the data from the CMS Experiment at the LHC, and the second pertaining to the development of a new parallelizable algorithm for reconstructing particle tracks in the outer tracker of the CMS Detector at the upcoming High Luminosity LHC. A search for supersymmetric phenomena beyond the standard model in a final state containing an on-shell Z boson, jets and missing transverse energy is performed using a data sample of proton-proton collisions at [square root][s] = 13 TeV corresponding to an integrated luminosity of 137 fb[superscript -1] collected by the CMS Experiment at the LHC between 2016 and 2018. The observed event yields are consistent with standard model predictions in the signal regions. These results are then interpreted to constrain the masses of supersymmetric particles in the context of the signal models. Gluino masses up to 1870 GeV, and chargino (neutralino) masses up to 750 (800) GeV are excluded at the 9-% confidence level, which extends the reach over the previous results by a few hundred GeV. The High Luminosity LHC (HL-LHC) will increase the instantaneous luminosity by a factor of five larger than the current levels achieved by the LHC. This high pile-up environment requires efficient and fast reconstruction of charged particles. A new algorithm called Line Segment Tracking takes a fundamentally different approach from existing iterative Kalman Filter based algorithms by doing a bottom-up reconstruction of tracks. Track stubs from adjoining detector regions are constructed, and stubs that are consistent with typical track trajectories are hierarchically linked to reconstruct complete tracks. Since the track stubs are produced locally and only require information from neighboring regions, they can be made in parallel. This motivates using architectures like GPUs to take advantage of the parallelism. The algorithm is currently implemented in the context of the CMS Phase-2 Outer Tracker in the HL-LHC, and targets NVIDIA Tesla V100 GPUs. Good physics and timing performance are obtained which paves way for further developments.

A search is performed in final states with Z$\rightarrow\ell\ell$, jets, and missing transverse momentum using data collected with the CMS detector at the CERN LHC in 2015 corresponding to an integrated luminosity of 2.3~\fbinv of pp collisions at $\sqrt{s}=13$\TeV. Signal regions are defined to maximize sensitivity to new physics including one to search where ATLAS reported a 3.0~$\sigma$ excess at 8~TeV. The observations in all regions are consistent with the predicted standard model backgrounds, and the results are interpreted in the context of simplified models of supersymmetry.

This thesis describes in detail the search for new phenomena in mono-jet final states with the ATLAS experiment at the LHC. The final state is considered the golden channel in the searches for large extra dimensions (LED) but also allows access to a very rich SUSY-related phenomenology pertaining to the production of weakly interacting massive particles (WIMPS), SUSY Dark Matter candidates, GMSB SUSY models with very light gravitino masses, as well as stop an sbottom pair production in compressed scenarios (with nearly degenerated squarks and the lightest neutralino), and also invisible Higgs searches, among others. Here, a number of these scenarios are explored. The measurements presented yield new powerful constraints on the existence of extra spatial dimensions, the pair production of WIMPs, and also provide the best limit to date on the gravitino mass.