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Phenomenology of Supersymmetric Particle Production Processes at the LHC

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Phenomenology of Supersymmetric Particle Production Processes at the LHC Technische Universität München Max-Planck-Institut für Physik (Werner-Heisenberg-Institut) Phenomenology of Supersymmetric Particle Production Processes at the LHC Maike Kristina Trenkel Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigten Dissertation. Vorsitzender: Univ.-Prof. Dr. L. Oberauer Prüfer der Dissertation: 1. Hon.-Prof. Dr. W. F. L. Hollik 2. Univ.-Prof. Dr. A. J. Buras Die Dissertation wurde am 25. Juni 2009 bei der Technischen Universität München eingereicht und durch die Fakultät für Physik am 20. Juli 2009 angenommen. Abstract We study the hadronic production of strongly interacting SUSY particles in the framework of the MSSM. In particular, we consider top-squark pair, gluino– squark pair, and same sign squark-squark pair production processes. Aiming at precise theoretical predictions, we calculate the cross section contributions of electroweak origin up to the one-loop level. We find sizable effects both from tree-level electroweak subprocesses and next-to-leading order electroweak cor- rections, reaching the 20% level in kinematical distributions. In a second part of this thesis, we investigate the phenomenology of -parity vio- R lating B3 SUSY models with the lightest stau (τ˜1) being the LSP. We analyze the possible τ˜1 decay modes, taking into account the dynamical generation of non-zero -parity violating couplings at lower scales. As an application of our R studies which is interesting for experiments at particle accelators, we discuss sin- gle slepton production at the LHC and give numerical results for single smuon production. Zusammenfassung Wir beschäftigen uns mit der hadronischen Paarerzeugung stark wechselwir- kender SUSY-Teilchen im Rahmen des MSSM und betrachten insbesondere die Produktion von Top-Squark Paaren, Gluino–Squark Paaren und gleichgelade- nen Squark–Squark Paaren. Um möglichst präzise theoretische Vorhersagen zu erreichen, berechnen wir die elektroschwachen Beiträge zu den Produktionswir- kungsquerschnitten. Dabei ziehen wir sowohl elektroschwach-induzierte Prozes- se auf Born-Niveau als auch elektroschwache Quantenkorrekturen nächstfüh- render Ordnung in Betracht. Die größte Bedeutung erreichen diese Beiträge in kinematischen Verteilungen, wo sie bis auf 20% anwachsen können. In einem zweiten Teil der Arbeit untersuchen wir die Phänomenologie -Pari- R tät-verletzender B3 supersymmetrischer Modelle, in denen das leichteste Stau (τ˜1) das leichteste SUSY Teilchen ist. Wir analysieren die möglichen τ˜1-Zerfalls- moden und berücksichtigen dabei auch weitere, dynamisch erzeugte -parität- R verletzende Kopplungen. Als eine für Experimente an Teilchenbeschleunigern interessante Anwendung unserer Studien diskutieren wir die Resonanzproduk- tion von Sleptonen am LHC und werten die Ergebnisse für die Resonanzpro- duktion von Smyonen numerisch aus. i ii Contents 1. Introduction 1 2. Theoretical framework 5 2.1. TheStandardModel ............................... 5 2.2. Supersymmetry.................................. 10 2.2.1. Basic ideas and motivation . 10 2.2.2. Theoretical concepts of supersymmetry. ... 13 2.3. The minimal supersymmetric extension of the Standard Model (MSSM) . 21 2.3.1. FieldcontentoftheMSSM . 22 2.3.2. -parity.................................. 23 R 2.3.3. TheMSSMLagrangian ......................... 25 2.3.4. TheparticlespectrumoftheMSSM . 29 3. Production of colored SUSY particles at hadron colliders 41 3.1. Experimentalsearches . 42 3.1.1. Light-flavor squarks and gluinos . 43 3.1.2. Top-squarks(stops) ........................... 44 3.1.3. ProspectsforLHC ............................ 46 3.2. Hadroniccrosssections. 47 3.3. Classification of processes . 50 3.3.1. Squark and gluino production at LO . 50 3.3.2. Higher-orderQCDcorrections. 53 3.3.3. Electroweak contributions . 54 4. How to obtain a finite result at (α2α) 59 O s 4.1. Handling ultraviolet singularities . 59 4.1.1. Regularization .............................. 60 4.1.2. Renormalization ............................. 61 4.1.3. Renormalization for squark and gluino pair production at (α2α) . 63 O s 4.2. Handling infrared singularities . 75 4.2.1. Realphotonbremsstrahlung. 77 iii Contents 4.2.2. Realgluonbremsstrahlung . 85 4.2.3. Realquarkbremsstrahlung . 89 5. Stop–anti-stop production 91 5.1. LOcrosssectionsandnotations . 92 5.2. Electroweakcontributions . 93 5.2.1. Tree-level EW contributions . 94 5.2.2. Virtualcorrections ............................ 95 5.2.3. Realphotonandrealgluoncorrections . 100 5.2.4. Realquarkradiation . 104 5.3. Numericalresults................................. 105 5.3.1. Inputparametersandconventions . 107 5.3.2. Hadroniccrosssections. 109 5.3.3. Differential distributions . 110 5.3.4. DependenceonSUSYparameters. 116 ˜ ˜ 5.3.5. Production of t2t2∗ pairs ......................... 121 6. Gluino–squark production 125 6.1. LOcrosssectionandconventions . 126 6.2. Electroweakcontributions . 127 6.2.1. Tree-level EW contributions . 128 6.2.2. Virtualcorrections . 128 6.2.3. Realphotoncorrections . 130 6.2.4. Realquarkradiation . 132 6.3. Numericalresults................................. 134 6.3.1. Hadroniccrosssections. 135 6.3.2. Differential distributions . 137 6.3.3. DependenceonSUSYparameters. 142 7. Diagonal squark–squark production 147 7.1. LOcrosssectionsandnotations . 148 7.2. Electroweakcontributions . 149 7.2.1. Tree-level EW contributions . 150 7.2.2. Virtualcorrections . 150 7.2.3. Realphotonandrealgluoncorrections . 155 7.2.4. Realquarkradiation . 158 7.3. Numericalresults................................. 160 7.3.1. Hadroniccrosssections. 162 7.3.2. Differential distributions . 163 7.4. Outlook: non-diagonal and mixed-flavor squark–squark production . 165 iv Contents 8. SUSY with -parity violation and a τ˜ as lightest SUSY particle 169 R 1 8.1. The low-energy spectrum of the B3 mSUGRA model with a τ˜1 LSP. 172 8.1.1. SUSYparticlespectra . 173 8.1.2. Reference scenarios with a τ˜1 LSP ................... 174 8.1.3. Renormalization group equations . 175 8.2. τ˜1 LSP decays in B3 mSUGRAmodels ..................... 181 8.2.1. GeneralLSPdecaymodes . 181 8.2.2. Dependence of τ˜1 decaysonmSUGRAparameters . 183 8.3. Resonant single slepton production at the LHC . 188 8.3.1. Slepton production and slepton decays . 189 8.3.2. Single smuon production: An explicit numerical example . 193 9. Conclusions 203 A. Notations and definitions 207 A.1.Metricconventions ................................ 207 A.2.DiracandPaulimatrices............................. 208 A.3.Weylspinors ................................... 209 A.4. DiracandMajoranaspinors . 210 A.5.Grassmannnumbers ............................... 211 B. Input parameters for numerical cross section computations 213 B.1. StandardModelparameters . 213 B.2.MSSMparameters ................................ 213 B.3. SPSbenchmarkpoints .............................. 216 C. Slepton production and decay in specific B3 mSUGRA models 219 C.1. Crosssectionsandbranchingratios . 219 C.2. The B slepton decay ℓ˜− W ¯bd ....................... 221 3 i → − k Bibliography I Acknowledgments XVI v vi Chapter 1 Introduction The Standard Model of elementary particle physics (SM) [1–4] has been proven to success- fully describe all observed particles and their electroweak and strong interactions. Despite the excellent agreement between the theoretical predictions and experimental data [5], there remain unresolved issues such as the hierarchy problem, the non-unification of gauge cou- plings or the unknown source of dark matter in the universe, which point to new physics beyond the weak scale. It is thus widely believed that the SM is an effective theory, valid only in the low-energy limit of a more fundamental theory describing physics at arbitrarily high energies. Numerous candidate theories of physics beyond the SM have been elaborated in the past. In this thesis we delve into the possibility of extending the SM in a supersymmetric way. Imposing a symmetry between fermionic and bosonic states, supersymmetry (SUSY) [6,7] predicts new partner particles to every known particle that only differ in spin by half a unit. If SUSY was an exact symmetry, SM particles and their SUSY partners would be degenerate in mass. But no SUSY particle has been observed so far. In order to comply with present data, the possibly new particles have to be massive in comparison to their SM counterparts and supersymmetry has to be broken at low energies. Supersymmetry, and in particular if it is realized around the weak scale, is very attractive from the phenomenological point of view. Owing to the existence of new particles obeying opposite spin statistics, it allows for a stabilization of the large hierarchy between the Planck scale and the electroweak scale [8] and for a consistent unification of SM gauge couplings at high energies [9]. In addition, if the lightest SUSY particle is stable, SUSY provides a dark matter candidate and can account for the observed cold dark matter relic density [10,11]. The search for SUSY particles is one of the major topics in the experimental program of particle physics. In the low-energy region, electroweak and B-physics precision observables provide a powerful tool for testing the consistency of SM predictions with data. Since new particles would enter the theoretical evaluations via virtual quantum effects, it is also possible to discriminate between the SM and alternative theories. The comparison of mea- surements with computational
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