Search for a spin-zero particle in Higgs boson decays or top-associated production using the ATLAS detector

by Paula Martínez (IFAE)

Tuesday 11 Mar 2025, 11:00 → 14:00 Europe/Madrid

IFAE Seminar Room + Zoom (Hybrid)

This thesis contains two searches for light pseudoscalar particles using 140~fb$^{-1}$ proton-proton ($pp$) collision data at a centre-of-mass energy of 13~TeV collected by the ATLAS detector at the Large Hadron Collider during Run~2. Both searches focus on multi-$b$ final states, using novel techniques for $b$-quark identification. Only events with two leptons are considered, in order to reduce contributions from difficult-to-model hadronic background processes. The first search explores light pseudoscalar production in Higgs boson decays, $H\rightarrow aa$, with an $a$-boson mass between 12 and 60~GeV. Each pseudoscalar decays to two $b$-quarks, $a\rightarrow b\bar{b}$, which translates into a final state with four $b$-quarks in total. The Higgs boson is produced in association with a $Z$ boson that decays to either electrons or muons. No significant excess of events above the expected Standard Model background is observed. Upper limits at 95\% confidence level are set for the branching ratio of the Higgs boson decaying to two light pseudoscalars, $\text{BR}(H\rightarrow aa\rightarrow 4b)$, between 3\% and 25\%. The second search targets light pseudoscalar production in association with top quarks, $t\bar{t}a$. This production mechanism allows to explore the pseudoscalar mass range from 12 to 100~GeV. The $t\bar{t}$ pair is required to decay leptonically to electrons, muons or both. The light pseudoscalar is studied via its decay to $b$-quarks.  As no significant excess is observed, upper limits for the production cross section of a light pseudoscalar in association with top quarks, $\sigma(pp\rightarrow t\bar{t}a)\times\text{BR}(a\rightarrow b\bar{b})$, are presented. They range between 0.1 and 0.9~pb.

Thesis Supervisor: Imma Riu Dachs

Thesis Committee: Lluïsa-María Mir i Martínez, Arely Cortés Gonzalez, Juan Antonio Aguilar Saavedra

Zoom link: https://us02web.zoom.us/j/89787514064?pwd=SkRaOElqanZRNFZXM2d2SE9PN1d0Zz09

PMS_Thesis_20250311.pdf