Observation and Measurement of a Standard Model Higgs Boson-like Diphoton Resonance with the CMS Detector by Mingming Yang Submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2015 c 2015 Mingming Yang. All rights reserved. The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Author.................................................................... Department of Physics June 15, 2015 Certified by . Christoph M. E. Paus Professor Thesis Supervisor Accepted by............................................................... Professor Nergis Mavalvala Associate Department Head for Education 2 Observation and Measurement of a Standard Model Higgs Boson-like Diphoton Resonance with the CMS Detector by Mingming Yang Submitted to the Department of Physics on June 15, 2015, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract This thesis concerns the observation of a new particle and the measurements of its prop- erties, from the search of the Higgs boson through its decay into two photons at the CMS experiment at CERN’s Large Hadron Collider (LHC), on the full LHC “Run I” data col- lected by√ the CMS detector during 2011 and 2012,√ consisting of proton-proton collision events at s = 7 TeV with L = 5.1 fb−1 and at s = 8 TeV with L = 19.7 fb−1, with the final calibration. In particular, an excess of events above the background expectation is observed, with a local significance of 5.7 standard deviations at a mass of 124.7 GeV, which constitutes the observation of a new particle through the two photon decay channel. +0.35 A further measurement provides the precise mass of this new particle as 124.72−0.36 GeV = +0.31 +0.16 124.72−0.32(stat)−0.16(syst) GeV. Its total production cross section times two photon decay +0.26 branching ratio relative to that of the Standard Model Higgs boson is determined as 1.12−0.23 +0.21 +0.15 = 1.12−0.21(stat)−0.09(syst), compatible with the Higgs boson expectation. Further extrac- tions of its properties relative to the Higgs boson, including the production cross section times decay branching ratios for separate Higgs production processes, couplings to bosons and to fermions, and effective couplings to the photon and to the gluon, are all compatible with the expectations for the Standard Model Higgs boson. Thesis Supervisor: Christoph M. E. Paus Title: Professor 3 4 Acknowledgments Looking back on the journey of searching for the Higgs boson in its decay into two photons at the CMS experiment at CERN’s Large Hadron Collider, I would like to thank first my adviser Christoph Paus, a genuine man with a warm soul, a passionate physicist, and an adviser I would choose again. His unwavering confidence in me has motivated me to pass through this monumental journey and to arrive here. I would also like to thank my other two MIT companions and passionate physicists: Fabian Stoeckli and Josh Bendavid. Starting as their apprentice and growing into their tenured team member has been my great honor. These three men are not only my teachers and colleagues, but also “Damon and Pythias”- like friends in my heart. Their guidance, encouragement, support, and collaboration have given me infinite strength and courage, while their devotion has driven me to devote my entire life for this journey as well. The “24-hour” discussions with them about physics and technical issues in person, online, or on the phone (Josh only), the many late nights working together with them on the analysis, the secret competitions with them in drinking more coffee and sleeping less, the extreme difficulties and pressures we faced together, and the incomparable excitement we shared when we were getting remarkable results compose an important part of my memory of the past few years. Their unconditional delivery of their knowledge, skills, experiences, and wisdom have nurtured my mind, while their passions, integrities, lively characters, genuine hearts, and warm souls have resonated with my heart. I have embedded my deep gratitude and respect to them into all my work, which I want to express now in these limited words. I would also like to express my sincere gratitude to all my colleagues from the CMS Higgs to Two Photons Working Group. Without them, this journey would not have been as exciting as it was, or simply even not have existed. I am grateful to their constructive competition, great trust, and also strong collaboration, and I treasure the days working extremely hard together with them to produce and cross-check several rounds of analysis results. It was my deep honor, representing them, to unblind our search result to the entire CMS collaboration, on June 15, 2012, which provided the first convincing evidence for the existence of a new particle. And it is my great pleasure to achieve the end of the journey 5 together with them, with the final result—using the full LHC Run I data with the best calibration—which confirms our results in 2012 and provides the standalone observation of this new particle, with properties consistent with the Higgs boson, from the two photon channel. I want to thank the CMS colleagues who have designed, constructed, and calibrated the Electromagnetic Calorimeter. Their huge efforts have provided the great resolution of photon energy measurement, crucial for the Higgs-to-two-photons analysis. And I also want to thank the colleagues for having worked on the tracker which allow us to identify electrons from photons and to use the electrons for validating the signal model. Moreover, I thank all the CMS colleagues for having worked on the different stages and aspects of the experiment during the past 20 years and enabled the final data analysis for the Higgs search. I am grateful for all the advice, help, and encouragements from the colleagues whom I had the opportunity to meet or work with. I would like to thank all the ATLAS colleagues as well for the competition and cross-check. And I also thank all the LHC colleagues for providing the most energetic and intense proton-proton collisions, essential for the creation and observation of the new particle. Furthermore, I thank all the people across the world who have provided support in one way or another to enable the search of the Higgs boson. I also treasure very much the time that I have spent together with all my MIT colleagues, whose strong support is the irreplaceable source of strength for me during the past few years. The numerous valuable comments and suggestions I having received from them through the group meetings and the MIT analysis email list are integral to this journey. In addition: — I am grateful to sit in the office with Guillelmo Gomez Ceballos Retuerto, Marco Zanetti, and Erik Butz. They are not only the experts on analysis, accelerator, and detector to learn from, but also very caring office mates. — I have also learned a lot from the colleagues sitting in the office in front of mine. Si Xie has answered tons of my questions on physics, detector, and computing, with enormous patience and crystal clear explanations. I owe him a big “Thank You”. I appreciate the advice from Gerry Bauer and Sham Sumorok, who have experienced the progress of high energy physics over a period longer than my life. It has also been my pleasure to work with Jan Veverka, who moved to the office later and became my new Higgs-to-two-photons 6 partner. — I would also like to thank Steve Nahn and Markus Klute, sitting next to my office, for organizing summer BBQs and cheese fondue dinners, which forced the group to stop working and to start talking about topics like favorite novels. — I also want to deliver my many thanks to all my other colleagues for their great help and company: Aram Apyan, Andrew Levin, Duncan Ralph, Katharina Bierwagen, Kristian Hahn, Kevin Sung, Lavinia Darlea, Leonardo Di Matteo, Matthew Chan, Max Goncharov, Olivier Raginel, Phil Harris, Pieter Everaerts, Roger Wolf, Stephanie Brandt, Valentina Dutta, and Xinmei Niu. I miss the afternoon ice cream, the explorations of Geneva restaurants, and the interesting conversations. It is also my pleasure to meet Brandon Allen, Daniel Abercrombie, Dylan Hsu, Sid Narayanan, and Yutaro Iiyama when I am writing this thesis at MIT. There are more people I need to thank during my PhD period. I want to thank my academic adviser Bolek Wyslouch for all the advice. I would also like to thank all the friends at MIT or CERN, for their help in both academic work and life. I should thank especially Jianbei Liu and Lulu Liu for letting me live on their “balcony”for free, Hai Chen and Wei Sun for introducing me to the wonderful books in the CERN library, and Lu Feng for her steady friendship and support through the entire period. I would also like to thank all the staff working at CERN providing various services. I appreciate the services from the warm staff at the CERN restaurants, and I enjoyed watching Mont Blanc while drinking coffee at Restaurant 1. I also thank the staff at the CERN Fire Brigade for giving me a ride home or helping me open the door of my office. I also want to give a great thanks to the staff at the CERN hostels, for being patient under my many unexpected interruptions near midnight and being able to find me a room.