Springer Theses Recognizing Outstanding Ph.D. Research Giuliano Gustavino Search for New Physics in Mono- jet Final States in pp Collisions at √s = 13 TeV with the ATLAS Experiment at the LHC Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. 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More information about this series at http://www.springer.com/series/8790 Giuliano Gustavino Search for New Physics in Mono-jet Final States in pp Collisions √ at s = 13 TeV with the ATLAS Experiment at the LHC Doctoral Thesis accepted by Sapienza University of Rome, Italy 123 Author Supervisor Dr. Giuliano Gustavino Prof. StefanoGiagu Sapienza University of Rome Sapienza University of Rome Rome Rome Italy Italy ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-319-58870-4 ISBN978-3-319-58871-1 (eBook) DOI 10.1007/978-3-319-58871-1 LibraryofCongressControlNumber:2017941490 ©SpringerInternationalPublishingAG2017 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. 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Printedonacid-freepaper ThisSpringerimprintispublishedbySpringerNature TheregisteredcompanyisSpringerInternationalPublishingAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland A zia Maria, al suo sorriso ed al suo esempio di una vita vissuta sempre con gioia ed allegria. ’ Supervisor s Foreword Astronomical and cosmological observations support the existence of invisible matterthatcanonlybedetectedthroughitsgravitationaleffects,thusmakingitvery difficult to study. Thismysterious matter known as “Dark Matter” makes up about 27%oftheknownuniverse.Asamatteroffact,oneofthemaingoalsofthephysics programme of the experiments at the Large Hadron Collider (LHC) of the CERN laboratory, is the search of new particles that can explain the Dark Matter. In this context, Giuliano Gustavino’s thesis documents his original work per- formed in searching for Dark Matter effects in the data by collected the ATLAS experimentatLHCatthehighestcentreofmassenergy,13TeV,everreachedata particle collider. The results of Giuliano’s work provide an outstanding picture of Dark Matter limits at low Dark Matter masses from new physics models. These limits are complementary to the ones obtained with direct detection experiments. Moreover, they expand our knowledge of one of the most important and chal- lenging questions facing physicists today. Rome, Italy Prof. Stefano Giagu March 2017 vii Abstract Manycosmologicalobservationsindicatethatthematterpredictedanddescribedby the Standard Model constitutes only a small fraction of the entire known universe. These astrophysics measurements infer the existence of Dark Matter which is constituted by beyond the Standard Model particles. Among the possible different approachestosearchthiskindofparticles,inthisthesisadetaileddescriptionofthe mono-jetanalysisisaddressedinwhichfinalstateswithhightransversemomentum and an energetic hadronic jet needed to tag the events are considered. The results presented are based on the full dataset recorded in 2015 in the ATLASexperimentatthecentreofmassenergy ofcollidingprotonsof13TeVat LHC. The level of agreement observed between data and Standard Model predic- tions is interpreted as limits in different theoretical contexts, such as compressed supersymmetricmodels,theorieswhichforeseeextra-spatialdimensions,andinthe Dark Matter scenario. In the latter, the limits are then compared with the ones obtained by other ATLAS analyses and by experiments based on completely dif- ferent experimental techniques. Finally,asetofpossibleanalysisimprovementsisintroducedinordertoreduce the main uncertainties which affect the signal region and to increase the discovery potential by exploiting further the information of the final state. ix Acknowledgements Iwouldprobablyneedanothercompletesectiontothankthepeoplethat,withtheir support, patience, and love, have allowed the successful completion of this thesis and my Ph.D. AspecialthanksisaddressedtoValerioIppolitoformakingmeintoaphysicist, for believing in me, and being a friend. When I was deciding on a thesis project, I chosethemono-jetanalysisforthepossibilitytoworkwithyouagainandifIcould go back in time I would make definitely the same decision. Thanks to Stefano Giagu for being my supervisor and for giving me the pos- sibilityand responsibility ofchoosing my way and cultivatingmy own interests in physics. Thank you for teaching to be clever, pushing me to do better, and encouragingmetothinkoutsidethebox.Iwillremembertheselessonsfortherest of my career and life. ThankstoCarloDionisiforsupportingmeandsimplyforbeinganinspirational example of a physicist. Thanks to Francesco, Francesco, Elvira, and Marco for your stimulating sug- gestions and company during these past three years. It was a pleasure to share this experience with you, always combining duty with a dash of mirth. Thanks to Emma, Cristiano, and Veronica for your cheerfulness and pleasant- ness. Our collaboration, teamwork, and friendship have been essential for my scientific training and my Ph.D. I hope to work again with you in the future. GrazieaGizzaePeter,alnostrospecialeefraternorapporto.Grazienonsoloper ilsostegnofondamentaleperlariuscitaelastesuradiquestatesi,maperesserestati duepilastri sucuipotermisempreappoggiareneimomentidibisognoeveriamici con cui poter condividere le gioie ed i successi. Grazie a Giulia e alla sua gioia di vivere che è stata sempre una spinta ed una carica nei momenti più felici come in quelli più bui e incerti. Grazie ad Arianna, Daniele, Mastro, Gianluca, Silvia e tutti i compagni di avventurediquest’ultimo annoaGinevra. Lavostraamiciziaebuonumorehanno reso speciale ed indimenticabile questa esperienza in una città ricca di opportunità xi xii Acknowledgements per gente che vuole ‘vivere la vita da protagonista’. I baccanali passano ma gli amici restano! Grazie a Francesca per aver creduto sempre in me, anche più di quanto abbia creduto io in me stesso. GrazieaDaniele,Frakkia,Mol,Mattizza,Nello,YakeglialtriamicidiL’Aquila sui quali ho potuto sempre contare anche dopo lunghi periodi di assenza e che quando li rincontro sembra mi siano stati sempre accanto. Grazieallamiafamigliaedinparticolareamiamadre,miopadreemiofratello, che sono i miei più grandi fan e che mi hanno permesso di diventare quello che sono. Finalmente gracias a Cris que por mí fue todo y con quien pasé tres años maravillosos.Graciasparacreerenmi,paracomprenderme,paraserpaciente,para apoyarme y simplemente gracias por como eres. Te quiero.