Lepton Number Violation in the Standard Model Effective Field Theory

Datta, Soumyajit (2025) Lepton Number Violation in the Standard Model Effective Field Theory. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Soumyajit Datta (20MS163)) 20MS163_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (14MB)

Abstract

The discovery of non-zero neutrino masses and the observed matter-antimatter asymmetry in the Universe strongly suggest the presence of physics beyond the Standard Model (SM), particularly mechanisms that violate lepton number. In this thesis, we investigate lepton number violation (LNV) in the Standard Model Effective Field Theory (SMEFT), a powerful model-independent extension of the SM that parametrizes possible effects of heavy new physics through higher-dimensional operators. We review the two complementary approaches of EFT – top-down and bottom-up approach, and discuss how the dimension-6 SMEFT operators modify different SM couplings. We also briefly discuss the validity range of EFT, and highlight that we must make some assumptions on NP to assess the validity of EFT. We particularly discuss the lepton number violation in dimension-5 and dimension-7 operators, and show that lepton number and/or baryon number must be violated in odd dimensional operators. We revisit the dimension-5 Weinberg operator in the context of Seesaw mechanism that gives mass to the SM neutrinos, and show that in the low energy limit, SMEFT can successfully explain the neutrino mass generation at the leading order. For the major part of thesis, we explore the lepton number violating dimension 7 operators in the context of same-sign muon colliders. Specifically, we study μ+μ+ ! W+W+/ W+qq0 production at the μTRISTAN at ps = 2 TeV energy with an integrated luminosity of 1 ab−1, using 2J final state signature. This process is sensitive to 8 distinct dimension-7 SMEFT operators, which provides a unique avenue for testing LNV beyond the Standard Model. We determine the maximal sensitivity to these operators and compare our results with existing LHC constraints and future projections from the FCC. Our results highlight the potential of same sign muon colliders to serve as powerful probes of LNV and New Physics (NP) at the TeV scale.

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