A Study of Five-Dimensional Small Black Rings and Closed String Tachyon Potential

Acknowledgment Whose hand and tongue is capable To fulfil the obligations of thanks to Him? – Sa'di I would like to express my gratitude to all those who helped me in the completion of this work. Especially I am highly indebted to my advisor, Atish Dabholkar, for his guidance and stimulating suggestions throughout the work. I would also like to thank all my collaborators, Ashoke Sen, Norihiro Iizuka, Masaki Shigemori and Joris Raeymakaer. I am grateful to Ashoke Sen and Norihiro Iizuka for many inspiring ideas and discussions. Two of our papers have been accepted for publication in Phys. Rev. Lett. and JHEP respectively. I acknowledge the financial support from Kanval Rekhi Scholarship and Sarojini Damodaran Scholarship during the course of my work. I thank all the faculty of our department for their kind hospitality which made my stay at TIFR memorable. especially for many stimulating discussions covering a wide range of subjects, not necessarily related to physics. I would also like to thank Atique, Anver, Amir, Mahbub and Kadir for their kind help and support. I express my deepest gratitude to my grandfather, Shaikh Mujibar Rahman, who has always been the guiding light of my life. I would like to thank my mother Nurun Nahar for single handedly supporting me. I also thank my uncle Mohd. Shahab Uddin and my wife Nursheva Salahuddin for their kind support. Declaration This thesis is a presentation of my original research work. Wherever contributions of others are involved, every effort is made to indicate this clearly, with due reference to the literature, and acknowledgment of collaborative research and discussions. In my capacity as supervisor of the candidate's thesis, I certify that the above statements are true to the best of my knowledge.