Z polarization at an e+e- collider and properties of decay-lepton angular asymmetries

• Published in: Proceedings of the Indian National Science Academy 2024-09, Vol.90 (3), p.664-677
• Main Authors: Rao, Kumar, Rindani, Saurabh D., Sarmah, Priyanka, Singh, Balbeer
• Format: Article
• Language: English
• Subjects: Chemistry and Materials Science; Materials Science; Research Paper
• ISSN: 0370-0046; 2454-9983
• DOI: 10.1007/s43538-024-00233-w

Z production at an e + e - collider, associated with production of other particles, can be an accurate source of information of details of electroweak interactions, including possible interactions beyond the standard model. This is because of the inherent low background at an e + e - collider as well as the fact that the Z state can be fully reconstructed. If the Z decays into a lepton pair, such a final state in spite of the lower branching ratio, can be studied with great accuracy. One such process is e + e - → H Z , where is H is the Higgs boson, the most important process for Higgs production at an e + e - collider. Apart from the angular distribution, the polarization of the Z , fully characterized by its spin density matrix in the production process, can give detailed information about the production process. The vector and tensor polarization parameters of the Z density matrix can be related to angular asymmetries of the decay leptons in the Z rest frame. Thus, an experimental study of these lepton asymmetries can give information on the underlying interactions in Z production. We discuss from a general physical point of view the properties of the density matrix as well as lepton angular asymmetries. While most considerations will be applicable to processes of Z production associated with any other particle or particles, for some discussions, we specialize to a HZ final state. While many of the results can be found in earlier literature, especially for the process e + e - → H Z , we give details of the reasoning, which are not always found. We discuss the properties of the spin density matrix under C, P and T transformations, and combinations thereof, as also the predictions of these for the corresponding leptonic asymmetries. The specific transformations P, CP, T and CPT are of special importance and we discuss the consequences of these symmetries or their absence for the leptonic asymmetries. A specific issue which has been given attention to is the role of beam polarization, and how one can infer on general grounds which asymmetries get enhanced by the use of beam polarization. Similarly, we also discuss which asymmetries would be sensitive to the measurement of tau polarization in Z decay in to τ + τ - .