Phenomenology of Standard Model Effective Field Theory with polarizations and spin correlations

Subba, Amir (2025) Phenomenology of Standard Model Effective Field Theory with polarizations and spin correlations. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Amir Subba (19RS008)) 19RS008.pdf - Submitted Version Restricted to Repository staff only Download (6MB)

Abstract

This thesis explores the role of polarization and spin-correlation observables as sensitive probes of deviations from the Standard Model (SM) in high-energy processes. These observables are extracted as asymmetries derived from the angular distributions of the final-state particles. Angular functions related to vector polarization and its correlation with tensorial parameters require final-state particle identification to avoid averaging effects, addressed in this thesis using artificial neural networks (ANN) and boosted decision trees (BDT) for reconstruction. We focus on probing anomalous triple gauge boson couplings (W⁻W⁺γ/Z) in e⁻e⁺ → W⁻W⁺ at √s = 250 GeV across three decay channels, parameterized within the SM effective field theory (SMEFT) framework. We consider three CP-even and two CP-odd dimension-6 operators to encode the effects of ultraviolet physics in anomalous W⁻W⁺γ/Z couplings. In the hadronic decay of W boson, the identities of the two jets are determined as up/down-type jets using ANN and BDT. The fully hadronic channel e⁻e⁺ → 4j was studied to leverage large number of sensitive spin observables leading to tighter constraints on anomalous couplings. The reconstruction of two charged W bosons were done using jet charge variable. In the fully leptonic channel, ANN was employed to reconstruct missing neutrino momenta, framing the problem as a six-variable regression. In the neutral sector, we probed anomalous ZZZ/γ couplings in e⁻e⁺ → ZZ/Zγ process at √s = 250 GeV in presence of four dimension-8 operators. In both charged and neutral sectors, we obtained the marginalized constraints on all five couplings with Markov-Chain-Monte-Carlo methods using combinations of cross section and asymmetries. The asymmetries could distinguish between CP-even and CP-odd couplings and help to put tighter constraints on anomalous couplings. The thesis also deals with anomalous chromo-electric and magnetic dipole moments (CEMDM) of the top quark in tW(bl⁺l⁻ /E) process at 13 TeV LHC. The two missing neutrinos were reconstructed using mT2 assisted on-shell reconstruction algorithm. The asymmetries are found to be sensitive to anomalous top quark moments, giving tighter limits compared to experimental ones. Lastly, entanglement phenomena in high-energy scattering were investigated for bipartite systems (t¯t, tW, and V V ) in both lepton and hadron colliders, with detailed examination of beam polarization effects. For tripartite entanglement, the t¯tZ process was analyzed in the presence of dimension-8 operators, where genuine tripartite entanglement was observed at high t¯tZ invariant masses (√ˆs ≥ 1.5 TeV) under both SM and BSM scenario.

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