Precizno merenje narusenja $CP$ simetrije u raspadu $B_s^0→J/ψϕ$ u ATLAS eksperimentu

The violation of charge-parity $(CP)$ symmetry is one of the most interesting phenomena and the greatest puzzles of modern particle physics. In the Standard Model, $CP$ violation arises from a single complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. Thus, testing the validity of the CKM mechanism as the only source of $CP$ violation in the Standard Model is one of the major experimental challenges in particle physics today. This approach neglects $CP$ violation in the lepton sector as well as that in the strong interactions. Therefore, precise measurement of the CKM parameters constrains the Standard Model and may reveal effects beyond the Standard Model. The interference between $B_s^0$ decays to $J/ψϕ$ either directly or via $B_s^0 – B ̅_s^0$ mixing gives rise to a $CP$ violating phase $ϕ_s$. In the Standard Model the CP violating phase in this decay is predicted to be small and global CKM fit to experimental data gives $ϕ_s=–0.0365_{–0.0012}^{+0.0013}$. However new contributions to $B_s^0 –B ̅_s^0$ mixing may alter the expected value of $ϕ_s$. In this thesis, a measurement of the weak mixing phase $ϕ_s$ in $B_s^0→J/ψϕ$ decay is presented. The other parameters characterizing the time-dependent evolution of the decay $B_s^0→J/ψϕ$ are also shown. The measurement was performed with the ATLAS detector at LHC using the data from proton-proton collisions corresponding to the integrated luminosity of $4.9$ fb$^{-1}$ and $14.3$ fb$^{-1}$. The data were collected during the LHC operation in 2011 and 2012 (i.e. Run 1 period) at the proton-proton collision energies of $\sqrt{s}=7$ and $\sqrt{s}=8$ TeV respectively. To determine the physics parameters maximum likelihood fit is applied to the measured $B_s^0$ meson lifetime, various angular distributions of $B_s^0→J/ψ(μ^+ μ^- )ϕ(K^+ K^-)$ decay and invariant $B_s^0$ meson mass distribution. The fit also includes information about the $B_s^0$ meson flavour at the time of production, obtained by using the flavour tagging methods. In addition to the $CP$–violating phase $ϕ_s$ several other parameters of $B_s^0→J/ψ(μ^+ μ^- )ϕ(K^+ K^-)$ decay were also extracted. These include the $B_s^0$ lifetime $1/Γ_s$, the decay width difference $ΔΓ_s$ between the heavy $(Γ_H)$ and the light mass $(Γ_L)$ eigenstates, and the transversity amplitudes $|A_0 (0)|$ and $|A_∥ (0)|$. The result of the $CP$–violating weak mixing phase measurement at ATLAS experiment, $ϕ_s= –0.096±0.084±0.038$, is compatible with the Standard Model prediction. Within the statistical and systematic uncertainties, the results are consistent with the measurements of other LHC experiments.