Search for Beyond Standard Model Physics at BaBar

Citation

Li, Yunxuan (2022) Search for Beyond Standard Model Physics at BaBar. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tz7n-d662. https://resolver.caltech.edu/CaltechTHESIS:05232022-144829107

Abstract

This thesis reports searches for beyond Standard Model physics in e⁺e⁻ collisions in two directions.

We report the first search for a dark matter bound state. The existence of dark matter bound states could arise in a simple dark sector model in which a dark photon (A′) is light enough to generate an attractive force between dark fermions. We report herein a search for a J^PC = 1^− − darkonium state, the Υ_D, produced in the reaction e⁺e⁻ → γΥ_D, Υ_D → A′A′A′, where the dark photons subsequently decay into pairs of leptons or pions, using data collected with the BaBar detector. No significant signal is observed, and we derive limits on the γ − A′ kinetic mixing (ϵ) as a function of the dark sector coupling constant for 0.001 < m_A′ < 3.16 GeV and 0.05 < m_ΥD < 9.5 GeV. Bounds on the mixing strength ϵ down to 5 × 10^− 5 − 10^− 3 are set for a large fraction of the parameter space.

We also report a measurement of R(D) = B(B → D̄τν̄_τ)/B(B → D̄ℓν̄_ℓ) and R(D^*) = B(B → D̄^*τν̄_τ)/B(B → D̄^*ℓν̄_ℓ), where ℓ refers to either an electron or muon. We select samples by reconstructing tag-side B mesons in semileptonic decays and signal-side τ in a purely leptonic decay. Using data collected with the BaBar detector, we measure R(D) = 0.316 ± 0.062(stat) ± 0.019(syst) and R(D^*) = 0.226 ± 0.022(stat) ± 0.012(syst), which agree with the Standard Model expectations by 0.26σ and 1.10σ, respectively. Taken together, the results are in agreement with the Standard Model within 1.51σ level.

Thesis Files