Anomalies, U(1)' and the MSSM

TL;DR

This thesis explores an extension of the MSSM with an anomalous U(1)' gauge symmetry, analyzing its phenomenology, decay processes, and dark matter implications, especially focusing on the anomalous Z' boson and axino as dark matter.

Contribution

It introduces a novel MSSM extension with an anomalous U(1)' and Green-Schwarz anomaly cancellation, allowing arbitrary quantum numbers and analyzing its phenomenological consequences.

Findings

Decay rates of Z' to Z0 gamma and Z0 Z0 are around 10^{-4} GeV.

Expected LHC events for these decays are approximately 10 per year.

The model's relic density predictions align with experimental data, featuring axino as dark matter.

Abstract

This Thesis reviews an extension of the MSSM by the addition of an anomalous abelian vector multiplet and contains some original results concerning the phenomenology of an anomalous $Z'$. The review part covers an introduction of the MSSM focusing on its main features, a discussion on the chiral anomalies and how to cancel them in the Standard Model and by the Green-Schwarz mechanism. Then, the original results are presented. We build the Lagrangian for the Minimal Anomalous $U(1)'$ Extension of the MSSM where the anomalies are cancelled by the Green-Schwarz mechanism and the addition of Chern-Simons terms, stressing the main differences between our model and the MSSM. The advantage of this choice over the standard one is that it allows for arbitrary values of the quantum numbers of the extra U(1). As a first step towards the study of hadron annihilations producing four leptons in the final state (a clean signal which might be studied at LHC) we then compute the decays $Z'\to Z_0 \g$ and $Z'\to Z_0 Z_0$. We find that the largest values of the decay rate are $\sim 10^{-4}$ GeV, while the expected number of events per year at LHC is at most of the order of 10. Then we compute the relic density predicted by our model with a new dark matter candidate, the axino, which is the LSP of the theory. We find agreement with experimental data admitting a bino-higgsino NLSP or a wino-like NLSP, almost degenerate in mass to the axino.