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When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
In this PhD thesis we investigate various aspects of phenomenology of new physics beyond the Standard Model (SM) in the context of extensions of supersymmetric & non-supersymmetric realisations.
First, we study the low scale predictions of the supersymmetric standard model extended by U(1)B-L Ⓧ U(1)R symmetry, obtained from SO(10) breaking via a left-right supersymmetric model, imposing universal boundary conditions. Two singlet Higgs fields are responsible for the radiative U(1)B-L Ⓧ U(1)R symmetry breaking, and a singlet fermion S is introduced to generate neutrino masses through an inverse seesaw mechanism. The lightest neutralino or sneutrino emerge as dark matter candidates, with different low scale implications. We find that the composition of the neutralino lightest supersymmetric particle (LSP) changes considerably depending on the neutralino LSP mass, from roughly half U(1)R bino, half minimal supersymmetric model (MSSM) bino, to a singlet higgsino, or completely dominated by the MSSM higgsino. The sneutrino LSP is statistically much less likely, and when it occurs it is a 50-50 mixture of right-handed sneutrino and the scalar $\tilde S$. Most of the solutions consistent with the relic density constraint survive the XENON 1T exclusion curve for both LSP cases. We compare the two scenarios and investigate parameter space points and find consistency with the muon anomalous magnetic moment only at the edge of a 2𝜎 deviation from the measured value. However, we find that the sneutrino LSP solutions could be ruled out completely by the strict reinforcement of the Z' mass bounds. We finally discuss collider prospects for testing the model.
Secondly, we perform a comprehensive analysis of the secluded UMSSM model, consistent with present experimental constraints. We find that in this model the additional Z' gauge boson can be leptophobic without resorting to gauge kinetic mixing and, consequently, also d-quark-phobic, thus lowering the LHC bounds on its mass. The model can accommodate very light singlinos as DM candidates, consistent with present day cosmological and collider constraints. Light charginos and neutralinos are responsible for muon anomalous magnetic predictions within 1𝜎 of the measured experimental value. Finally, we look at the possibility that a lighter Z', expected to decay mainly into chargino pairs and followed by the decay into lepton pairs, could be observed at 27 TeV.
Thirdly, we test E6 realisations of a generic U(1)' extended Minimal Supersymmetric Standard Model (UMSSM), parametrised in terms of the mixing angle pertaining to the new U(1)' sector, ϴE6, against all currently available data, from space to ground experiments, from low to high energies. We find that experimental constraints are very restrictive and indicate that large gauge kinetic mixing and ϴE6 ≅ -pi/3 are required within this theoretical construct to achieve compliance with current data. The consequences are twofold. On the one hand, large gauge kinetic mixing implies that the Z' boson emerging from the breaking of the additional U(1)' symmetry is rather wide since it decays mainly into WW pairs. On the other hand, the preferred ϴE6 value calls for a rather specific E6 breaking pattern different from those commonly studied. We finally delineate potential signatures of the emerging UMSSM scenario in both Large Hadron Collider (LHC) and in Dark Matter (DM) experiments.
Furthermore, we study mass bounds of the WR gauge boson in generic left-right symmetric models. Assuming that the gauge bosons couple universally to quarks and leptons, we allow different gauge couplings gR ≠ gL and mass mixing, VCKML ≠ VCKMR in the left and right sectors. Imposing constraints from collider experiments and K0, Bd, Bs physics, we investigate scenarios where WR is lighter, or heavier than the right handed neutrino 𝝼R. In these scenarios, WR mass bounds can be considerably relaxed, while ZR mass bounds are much more stringent. In the case where M_WR ≤ M𝝼R, the experimental constraints come from WR → tb and WR → jj channels, while if M_WR ≥ M𝝼R, the dominant constraints come from WR \to \ell \ell jj. The observed (expected) limits in the two-dimensional (M_WR, M𝝼R) mass plane excluded at 95% confidence level extend to approximately M_WR= 3.1 (3.3) TeV in the ee channel and 3.3 (3.4) TeV in the 𝜇𝜇 channel, for a large range of right-handed neutrino masses up to M𝝼R= 2.1 (2.1) TeV in the ee channel and 2.6 (2.5) in the 𝜇𝜇 channel, representing a significant relaxation of the mass bounds.
Finally, we perform a consistent analysis of the alternative left-right symmetric model emerging from E6 grand unification. We include a large set of theoretical and experimental constraints, with a particular emphasis on dark matter observables and collider signals. We show that the exotic neutrino inherent to this class of models, the scotino, is a viable candidate for dark matter satisfying relic density and direct detection constraints. This has strong implications on the scotino mass restricting it to lie in a narrow window, as well as on the spectrum of Higgs bosons, rendering it predictable, with a few light scalar, pseudoscalar and charged states. Moreover, we also show that the extra charged W' gauge boson can be light, and investigate the most promising signals at the future high-luminosity upgrade of the LHC. Our findings show that the most optimistic cosmologically-favoured scenarios should be observable at 5𝜎, whilst others could leave visible hints provided the background is under good control at the systematical level.