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Exploring Extensions of the Scalar Sector of the Standard Model Ph.D. Thesis 7 1 0 2 n a J 9 by ] h p Najimuddin Khan - p e h [ 1 v 5 0 2 2 0 . 1 0 7 1 : v i X r a Discipline of Physics, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore - 453552, India i ii Exploring Extensions of the Scalar Sector of the Standard Model A THESIS Submitted in partial fulfillment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY by Najimuddin Khan Discipline of Physics, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore - 453552, India iii iv Dedicated to My Family v vi Acknowledgements First and foremost, I gratefully acknowledge the continual guidance and support of my adviser, Dr. Subhendu Rakshit. His dedication to research and pursuit of physics has been an invaluable source of inspiration and encouragement to me. I wouldalsoliketothankmyPSPCcommitteemembers, Dr. ManavendraNMahato, Dr. Antony Vijesh for serving as my committee members even at hardship. I would like to thank Prof. Amitava Raychaudhuri, Prof. Amitava Datta, Prof. Biswarup Mukhopadhyaya, Dr. Avirup Shaw, Avinanda Chaudhuri, Nabanita Gan- guly for their wonderful collaboration. You supported me greatly and were always willing to help me. A special thanks to my family. Words cannot express how grateful I am to my MAA-ABBA for all of the sacrifices that you’ve made on my behalf. Your prayer for me was what sustained me thus far. I would also like to thank all of my relatives who supported, and incented me to strive towards my goal. “There are some people in life that make you laugh a little louder, smile a little bigger and live just a little bit better”....FRIENDS. My jolly friends Minarul Islam, Lalsha Khan, Imran Ali. Thanks dude for helping to live. I would like to thank my JAJABOR friend at IIT Indore Mr. Sudeep Ghosh. You are the best person at IIT Indore who always fight with me and gave a lot of love to me. It is also unforgettable moments with VAI-BON: Sidharta Karmakar and Sujata. vii viii Abstract The Standard Model is one of the most successful theories which describes strong, weak and electromagnetic forces and interactions between the elementary particles. The scalar boson has been found at the Large Hadron Collider (LHC) on 4th July 2012 that has confirmed the Higgs mechanism. Although, the properties of this scalar are consistent with the Higgs as predicted by the standard model (SM), the experimental data still allow an extension of the scalar sector. The standard model fails to explain few physical phenomena in Nature, for example, the presence of dark matter, existence of massive neutrino, the observed matter-antimatter asymmetry, inflation of the Universe, etc. These phenomena indicate the presence of new physics beyond the standard model. In this thesis, various extensions of the scalar sector have been considered comprising of different SU(2) multiplets. The main purpose L for writing this thesis is to explore properties of these new scalars using the weak vectorbosonscatteringprocessesandfromthe(meta)stabilityofthescalarpotential. If the extended scalar sector participates in the electroweak symmetry breaking then these extra scalars need to couple with the known standard model particles. In this work, it has been shown that the vector boson scattering involving scalar boson exchanges provide a complimentary way to direct search methods to probe into the scalar sector. As different extended scalar sectors have similar types of scalar fields, e.g., an extra CP-even Higgs, charged Higgs etc., these new physics models can give rise to the similar types of experimental signatures. In this work, it has been shown that it is possible to distinguish between such models at various vector boson scattering processes by looking at the resonances. Also, the shapes of the resonances can provide further insight to the relevant parameter space of these models. As in the SM, the electroweak vacuum is metastable, it is important to explore if ix an extended scalar has an answer in its reserve. As the scalar weakly interacting massive scalar particles protected by Z symmetry can serve as viable dark matter 2 candidates, it is interesting to explore if they help prolong the lifetime of the Uni- verse. The effective Higgs potential gets modified in the presence of these new extra scalars, improving the stability of electroweak vacuum. Such an exercise has been undertaken in various kinds of extended scalar sectors. In order to show the explicit dependence of the electroweak stability on different parameters of these extended sectors, various kinds of phase diagrams have been presented. Graphical demonstra- tions have been provided to illustrate how the confidence level, at which stability of electroweak vacuum is excluded, depends on such new physics parameters. This study will help to estimate the lifetime of the electroweak vacuum, especially if it still remains in the metastable state in the extended scalar sectors. x