Search for Beyond Standard Model Physics at BaBar

Author: Li, Yunxuan

Year: 2022

Degree: Dissertation (Ph.D.)

Advisor: Hitlin, David G.

Committee Members: Porter, Frank C.; Hitlin, David G.; Patterson, Ryan B.; Wise, Mark B.; Echenard, Bertrand

Option: Physics; Computer Science

DOI: 10.7907/tz7n-d662

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 JPC = 1 − − darkonium state, the ΥD, produced in the reaction e+e− → γΥD, ΥD → 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 < mA′ < 3.16 GeV and 0.05 < mΥD < 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 → D̄τν̄τ)/B(B → D̄ℓν̄ℓ) and R(D*) = B(B → D̄*τν̄τ)/B(B → D̄*ℓν̄ℓ), 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.

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