Effective Field Theory Interpretation of Four Top Quark Production in the CMS Experiment

Abstract

Multi-top quark production at the LHC provides a sensitive probe of the Standard Model (SM) of particle physics and of possible new interactions involving the top quark. This thesis studies such processes in the framework of the Standard Model Effective Field Theory (SMEFT), using proton-proton collision data collected with the CMS detector at √s = 13 TeV and corresponding to an integrated luminosity of 138 fb−1.The study of four top quark (tt̄tt̄) production is performed in multilepton final states using a multivariate analysis strategy, with three top quark (ttt) production included as a complementary signal process. A simultaneous fit to tt̄tt̄ and ttt production yields an observed (expected) signal significance relative to the background-only hypothesis of 6.6σ (5.4σ), consistent with the SM expectation. At a 95% confidence level, the profiled limits on the Wilson coefficients, for Λ = 1 TeV, are −1.15 ≤ ctt ≤ 1.22, −1.09 ≤ cQQ1 ≤ 1.22, −1.86 ≤ cQt1 ≤ 1.83, −3.35 ≤ cQt8 ≤ 4.17, −9.02 ≤ ctHRe ≤ 8.65, and −26.0 ≤ ctHIm ≤ 22.7. No significant deviation from the SM prediction is observed. These results provide the first simultaneous constraint on four-heavy-quark operators together with the top quark-Higgs boson interaction in tt̄tt̄ production.A complementary study is performed in the opposite-sign dilepton channel, which is sensitive to both tt̄tt̄ and tt̄bb̄ production. This channel enhances sensitivity to SMEFT effects and helps probe different operator structures. However, no independent measurement of the tt̄bb̄ production cross section is performed in this work. The current simulation setup and the limited availability of dedicated NLO SMEFT tt̄bb̄ samples do not allow a sufficiently robust extraction of a standalone tt̄bb̄ signal.The results extend existing constraints on SMEFT operators in the top quark sector and demonstrate the sensitivity of multi-top production processes to potential new physics at the TeV scale.