The branching fraction and CP asymmetry of B±→Ψπ± and B±→π±μ+μ− decays

Thesis

Two analyses are performed using data collected by the LHCb experiment during 2011. Both consider decays of charged B mesons reconstructed in the π^±μ⁺μ⁻ final state. Decays involving dimuons provide an experimentally clean signature, even in the high-background environment of the √s = 7 TeV proton-proton collisions at the LHC. The first analysis measures the CP asymmetry of B^±→Ψπ^± decays using 0.37 fb^-1 of data, where the dimuon decays of two resonances are considered, J/ψ→μ⁺μ⁻ and ψ(2S)→μ⁺μ⁻. The branching fraction is measured relative to the Cabibbo favoured B^±→ΨK^± mode. The second analysis uses 1 fb^-1 of data to make the first observation of the non-resonant B^±→π^±μ⁺μ⁻ decay. The branching fraction is measured relative to that of B^±→K^±μ⁺μ⁻, and measurements of the CP asymmetry and the ratio of CKM matrix elements Vtd/Vts are obtained.

The branching fractions of the decays of interest are found to be

B(B^±→J/ψ π^±) = (3.88 ± 0.11 ± 0.15) x 10^-5, B(B^±→ψ(2S)π^±) = (2.52 ± 0.26 ± 0.15) x 10^-5 and B(B^±→π^±μ⁺μ⁻) = (2.48 + 0.57 −0.52 ± 0.17) x 10^-8,

where the first uncertainty is related to the statistical size of the sample and the second quantifies systematic effects. The measured CP asymmetries in these modes are

A_CP^(J/ψπ) = 0.005 ± 0.027 ± 0.011, A_CP (ψ(2S)π) = 0.048 ± 0.090 ± 0.011 and A_CP (μμπ) = -0.045 ± 0.220 ± 0.066,

with no evidence of direct CP violation seen. The ratio of matrix elements is measured as

Vtd/Vts = 0.274 + 0.031 − 0.028 ± 0.008, which is in agreement with previous results.

Authors: Redford, S

• Publication date: 2012
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: CP violation; flavour physics; LHCb; rare decays
• Subjects: Physics; Particle physics