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Luminosity Determination and Searches for Supersymmetric Sleptons and Gauginos at the ATLAS Experiment Anders Floderus

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Luminosity Determination and Searches for Supersymmetric Sleptons and Gauginos at the ATLAS Experiment Anders Floderus Luminosity determination and searches for supersymmetric sleptons and gauginos at the ATLAS experiment Thesis submitted for the degree of Doctor of Philosophy by Anders Floderus CERN-THESIS-2014-241 30/01/2015 DEPARTMENT OF PHYSICS LUND, 2014 Abstract This thesis documents my work in the luminosity and supersymmetry groups of the ATLAS experiment at the Large Hadron Collider. The theory of supersymmetry and the concept of luminosity are introduced. The ATLAS experiment is described with special focus on a luminosity monitor called LUCID. A data- driven luminosity calibration method is presented and evaluated using the LUCID detector. This method allows the luminosity measurement to be calibrated for arbitrary inputs. A study of particle counting using LUCID is then presented. The charge deposited by particles passing through the detector is shown to be directly proportional to the luminosity. Finally, a search for sleptons and gauginos in final states −1 with exactly two oppositely charged leptons is presented. The search is based onp 20.3 fb of pp collision data recorded with the ATLAS detector in 2012 at a center-of-mass energy of s = 8 TeV. No significant excess over the Standard Model expectation is observed. Instead, limits are set on the slepton and gaugino masses. ii Populärvetenskaplig sammanfattning Partikelfysiken är studien av naturens minsta beståndsdelar — De så kallade elementarpartiklarna. All materia i universum består av elementarpartiklar. Den teori som beskriver vilka partiklar som finns och hur de uppför sig heter Standardmodellen. Teorin har historiskt sett varit mycket framgångsrik. Den har gång på gång förutspått existensen av nya partiklar innan de kunnat påvisas experimentellt, och klarar av att beskriva många experimentella resultat med imponerande precision. Men Standardmodellen är inte hela sanningen. Det finns fenomen som den inte kan förklara, och partikelfysiker bygger därför experi- ment för att testa olika utökningar av modellen. En av de mest lovande kallas för supersymmetri. Denna teori postulerar att den finns en ny, ännu oupptäckt, partikel för varje partikel inom Standardmodellen. De nya partiklarna tros vara mycket tyngre än de partiklar som hittills har observerats, och skulle till exempel kunna förklara förekomsten av fenomenet mörk materia. För att skapa tunga partiklar krävs det stora mängder energi. I CERN-laboratoriet utanför Geneve har man byggt världens största partikelaccelerator, vars syfte är just att förse partiklar med hög kinetisk energi. Acceleratorn kallas för LHC (Large Hadron Collider). Den accelererar två protonstrålar till en energi på 7 TeV, eller teraelektronvolt per proton, och låter sedan protonerna kollidera i mitten av en partikeldetektor. I dessa kollisioner skapas nya partiklar ur protonernas kinetiska energi. Detektorns upp- gift är att mäta egenskaperna hos de partiklar som bildas. Exempel på sådana egenskaper är massa och elektrisk laddning. Genom att studera data från detektorn kan partikelfysiker bestämma huruvida några fenomen bortom Standardmodellen har ägt rum. En viktig pusselbit i jakten på ny fysik är mätningen av protonstrålens luminositet. Detta är ett mått på det totala antalet kollisioner som äger rum. Det är nämligen inte tillräckligt att bara observera en ny typ av partikel — Man vill även mäta hur ofta den produceras! Det är inte ovanligt att olika utökningar av Standardmodellen gör snarlika förutsägelser i detta avseende, eller har fria parametrar som gör att antalet producerade partiklar inte kan bestämmas i förväg. För att testa en modell jämförs den teoretiska san- nolikheten (även känt som tvärsnittet) för att producera en viss partikel med en experimentell mätning. Alla modeller som inte stämmer överens med den experimentella mätningen kan sedan uteslutas. För att mäta ett tvärsnitt divideras helt enkelt antalet kollisioner som gav upphov till partikeln i fråga med det totala antalet kollisioner — Det vill säga luminositeten! Förmågan att mäta luminositet med hög preci- sion är också viktig för att kunna göra meningsfulla simuleringar av experimentet. Innan en simulering kan jämföras med riktig data så måste antalet simulerade kollisioner normaliseras till antalet kollisioner som faktiskt ägt rum. Denna avhandling beskriver den allra största partikeldetektorn vid LHC — Det så kallade ATLAS- experimentet. Därefter diskuteras olika metoder för att mäta luminositeten i ATLAS med hjälp av en av experimentets subdetektorer vid namn LUCID. En metod använder data från protonstrålar som tagits under speciella förutsättningar, och korrigerar med hjälp av denna data för det icke-linjära beteende som observeras hos LUCID-detektorn vid höga luminositetsnivåer. En annan metod är baserad på att mäta den totala energin hos partiklarna som passerar genom LUCID. Denna energi visas vara proportionell mot luminositeten, och metoden visas ha vissa egenskaper som är att föredra framför mer konventionella metoder vad gäller mätningar när luminositeten är hög. Slutligen beskrivs en analys vars mål är att hitta två olika typer av supersymmetriska partiklar i de kollisioner som ägt rum inom ATLAS-detektorn. Inga nya partiklar observeras, och resultatet av analysen är därför gränsvärden på vilka modeller av supersymmetri som kan uteslutas. iii To my family iv Contents Preface 1 Acknowledgements 3 1 The Standard Model4 1.1 Overview..........................................4 1.2 Gauge theories.......................................5 1.2.1 Lagrangians.....................................5 1.2.2 Gauge invariance..................................6 1.3 Quantum electrodynamics.................................7 1.3.1 The QED Lagrangian................................7 1.3.2 Vacuum polarization................................9 1.4 Quantum chromodynamics.................................9 1.4.1 The quark model.................................. 10 1.4.2 The QCD Lagrangian................................ 10 1.4.3 Long range behavior................................ 12 1.5 Weak interactions...................................... 13 1.5.1 Chiral fermions................................... 14 1.5.2 Weak iso-doublets................................. 14 1.5.3 A weak Lagrangian................................. 15 1.5.4 Electroweak unification.............................. 15 1.5.5 Spontaneous symmetry breaking.......................... 16 1.5.6 The Higgs mechanism............................... 19 1.5.7 Yukawa couplings................................. 22 1.5.8 The electroweak Lagrangian............................ 23 1.6 An incomplete theory.................................... 24 1.6.1 Dark matter..................................... 24 1.6.2 The hierarchy problem............................... 24 1.6.3 Grand unification.................................. 24 1.6.4 Neutrino masses.................................. 24 2 Supersymmetry 27 2.1 Overview.......................................... 27 2.2 Solving the hierarchy problem............................... 27 2.3 Particles in the MSSM................................... 29 2.3.1 Sparticle mixing.................................. 30 2.4 R-parity........................................... 31 2.5 Supersymmetry breaking.................................. 31 2.5.1 Gravity mediated.................................. 32 2.5.2 Gauge mediated.................................. 33 2.6 Sparticle decays....................................... 34 2.7 The pMSSM......................................... 36 v 3 The Large Hadron Collider 37 3.1 Overview.......................................... 37 3.2 Accelerator system..................................... 38 3.3 Experiments......................................... 39 4 The ATLAS experiment 41 4.1 Overview.......................................... 41 4.2 Coordinate system..................................... 43 4.3 Magnet system....................................... 45 4.4 Inner detector........................................ 46 4.4.1 Pixel detector.................................... 47 4.4.2 Semiconductor tracker............................... 48 4.4.3 Transition radiation tracker............................. 48 4.5 Calorimeters......................................... 48 4.5.1 Electromagnetic calorimeter............................ 50 4.5.2 Hadronic calorimeter................................ 50 4.6 Muon spectrometer..................................... 53 4.7 Forward detectors...................................... 54 4.7.1 BCM........................................ 55 4.7.2 MBTS........................................ 55 4.7.3 LUCID....................................... 55 4.7.4 ZDC........................................ 56 4.7.5 ALFA........................................ 56 4.8 Trigger and Data Acquisition................................ 56 5 The LUCID detector 58 5.1 Overview.......................................... 58 5.2 Detector design....................................... 59 5.3 Readout electronics..................................... 62 5.3.1 The local stream.................................. 63 5.3.2 LUMAT....................................... 64 5.4 Calibration......................................... 65 5.5 Upgrade........................................... 66 6 Luminosity 70 6.1 Overview.......................................... 70 6.2 Luminosity monitoring................................... 71 6.3 Calibration......................................... 72 6.4 Algorithms......................................... 73 6.4.1 EventOR...................................... 74 6.4.2 EventAND....................................
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