AbstractsPhysics

Search for a light Higgs boson with the BaBar detector : with proposals to improve drift chamber aging studies and particle identification

by Rocky Yat So




Institution: University of British Columbia
Department:
Year: 2015
Posted: 02/05/2017
Record ID: 2122649
Full text PDF: http://hdl.handle.net/2429/54188


Abstract

In the Standard Model of particle physics, the Higgs boson is an elementary particle required to explain the origin of mass. One extension, the next-to-minimal supersymmetric Standard Model, predicts the existence of a GeV/c² scale Higgs boson, denoted A0. This can be produced at B meson factories such as the BaBar experiment at the Stanford Linear Accelerator Center. We study e⁺e- → Υ(2S)→ π⁺π-Y(1S), Y(1S)→ γA0, A0 → gg, ss-bar, cc-bar decays using data collected by the BaBar detector at a centre-of-mass energy of 10.02 GeV/c². The search for A0 → gg or ss-bar is performed by fully reconstructing the entire decay chain. We search for an excess of events relative to expected backgrounds in the reconstructed A0 mass spectra. No significant excess is found. We set upper limits on product branching fraction B(Y(1S) → γA0)xB(A0 → gg) from 1x10-⁶ to 2x10-² for A0 masses from 0.50 to 9.00 GeV/c², and B(Y(1S) → γA0)x B(A0 → ss-bar) from 5x10-⁶ to 1x10-³ for A0 masses from 1.50 to 9.00 GeV/c². The search for A0 → cc-bar is done by reconstructing the dipion transition, the radiative photon, and a D meson as a tag for cc-bar decays. We infer the mass of an A0 candidate from the recoil of the dipion transition and the radiative photon. No significant signal is seen in the data and we set upper limits on product branching fraction B(Y(1S) → γA0)xB(A0 → cc-bar) from 7x10-⁵ to 2x10-³ for A0 masses from 4.00 to 8.95 GeV/c² and 9.10 to 9.25 GeV/c². These are the first measurements to date and set constraints on the next-to-minimal supersymmetric Standard Model. Proposals to improve drift chamber aging studies and particle identification are included as part of the thesis. We propose to improve aging studies with prototypes that better resemble a full-scale drift chamber. Using TRIUMF beam data, we establish that at a particle identification efficiency of 95%, misidentification rates may be reduced from 2.2% to 1.9% by including likelihood tests and Kolmogorov-Smirnov tests and to 0.5% by including cluster counting.