Measurements of direct CP violation of B hardon decays at the LHCb experiment
Symmetry,particularly transformations involving charge conjugation(C)and parity(P),plays a fundamental role in modern physics and is indispensable in the development of the Standard Model.While strong,electromagnetic,and gravitational forces remain invariant under C and P transformation,weak interactions violate them,leading to CP violation.This phenomenon was initially observed in K meson decays during the 1960s,a pivotal discovery that greatly contributed to our understanding of the matter-antimatter asymmetry in the universe.Within the Standard Model,CP violation is introduced by the CKM matrix,a 3×3 unitary matrix defining the transformation between quark mass and weak interaction eigenstates.This matrix,determined by three rotation angles and one complex phase,accounts for CP violations but falls short in explaining the observed magnitude of matter-antimatter asymmetry,prompting the search for new sources of CP violation.Weak decays of quarks play a crucial role in investigating these fundamental issues.Research focuses on testing the CKM matrix's unitarity and measuring CP violation in decays sensitive to new physics.The LHCb experiment at CERN's Large Hadron Collider stands at the forefront of this research,amassing high statistics data on b-hadron decays.It provides optimal conditions for studying CP violation.In this review,recent results from the LHCb experiment on two key aspects are discussed.Firstly,it explores the measurements of CKM angle y,one of the main bottlenecks in the unitarity test of the CKM matrix.Secondly,it addresses charmless b-decays,which are important for searching for new sources of CP violation.The CKM angle y is directly measured through interference in tree-level b → u and b→ c processes.Various experimental methods have been developed,each with unique approaches and challenges.Strong parameters of D meson decays are crucial in these measurements,obtained through quantum-correlated D0(D)0 meson pairs produced in electron-positron collisions.LHCb's measurements have significantly contributed to the global average of the y angle.The experiment continues to improve the precision of the y angle through new measurements and exploration of innovative methods.This review discusses recent progress,including new measurements of the angle y in B+→ D*h+and B0s→D±sK(±)decays,discovering of potential decays for future y measurements.CP violation in two-body and three-body charmless b decays is sensitive to potential new physics contributions,which can be probed with the help of predictions using precise measurements of the CKM matrix elements such as angle y.In two-body decays like B0(s)→K±π(±),experiments,including LHCb,have observed significant CP violation,challenging theory predictions.Three-body charmless b decays provide a broader perspective for understanding CP violation due to rich structures over phase space.The LHCb experiment employs a model-independent approach to measure the direct CP violation on B+→ h+h'-h"+phase space,revealing notable CP violations,especially in regions with low and high invariant masses.Further amplitude analyses have identified substantial CP violation arising from interference between S-and P-waves.Large contributions from re-scattering process may also play a role in shaping the CP violation pattern,contradicted to previous understanding.Given the intricacies of the system,collaborative efforts from both theoretical and experimental physicists are essential to unraveling its underlying nature.Over the next two decades,the LHCb experiment will persist in accumulating more data from proton-proton collisions,offering the potential for enhanced prevision in CP violation studies.This ongoing effort is instrumental in extending our exploration beyond the Standard Model.
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