Radiative corrections and threshold resummed predictions to pseudoscalar Higgs boson production in QCD

This thesis studies the pseudoscalar Higgs boson production via gluon fusion in the EFT framework in a CP-conserving model. First, it presents the di-pseudoscalar Higgs boson production cross-section via gluon fusion till NNLO with the results valid for the pseudoscalar Higgs boson of MSSM and 2HDM with small tan by adjusting the top Yukawa coupling. We have used dimensional regularization to regulate the UV and IR divergences while carefully treating the Levi-Civita tensor and ₅. Unlike the amplitudes involving a pair of scalar Higgs bosons, we do not need UV contact counter terms. We used Catani's predictions to factorize the IR singularities, and our IR poles agreed with Catani's predictions. Our results are essential for studies on producing a pair of pseudoscalar Higgs bosons at the LHC up to NNLO accuracy. This thesis also includes the next-to-soft-virtual (NSV) resummed corrections to NNLL accuracy, which are also matched to the NNLO cross-sections for a pseudoscalar Higgs boson production via gluon fusion. These NSV corrections are potentially significant compared to the conventional soft-virtual (SV) logarithms. We have estimated the uncertainties due to the choice of various PDFs and those due to the renormalization and factorization scales. The scale uncertainties show improvement for renormalization scale variation only, which suggests that NSV contributions from other parton channels and beyond NSV contributions in the gluon fusion channel are necessary for improved stability. We also studied the production cross-sections for mixed scalar-pseudoscalar states and their impact on QCD cross-sections for different values of the mixing angle. The study indicates the necessity of improving the precision results for the pseudoscalar Higgs boson up to an order comparable to that of the scalar Higgs boson.