Gravitinos and hidden Supersymmetry at the LHC - UniversitÃ¤t ...

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CHAPTER 1. INTRODUCTION such that they cannot be produced at the LHC. The only accessible SUSY production channel is then direct electroweak production of higgsino states leading to negligible transverse momentum of the event. This fact, combined with the mass degeneracy between charged and neutral higgsino states, leads to the absence of visible signatures at the LHC, which we have confirmed in our study including the effects of detector simulation. The only hope for a LHC discovery is the search for monojet or monophoton signatures arising from initial state radiation giving rise to a transverse momentum kick and a jet or a photon. Taking into consideration only the results for the lower bound on the Higgs mass, the hybrid gauge-gravity mediation model allows for light stop states leading to a spectrum similar to the one obtained in natural SUSY proposals, see [77] and references therein. Stops decaying into higgsinos consequently play the most important role in one of our studies. Because of the usually large mass difference, this decay will give rise to very high-energetic jets, which provides a handle to distinguish signal events from Standard Model backgrounds. On the other hand, we can also discriminate between our light higgsino scenario and a generic MSSM with comparable squark and gluino masses. This is because in the latter one would expect to see also events with high-energetic isolated leptons from chargino and neutralino decays. Such events are absent in our scenario, since the higgsino-like chargino and neutralinos are nearly degenerate in mass; consequently, leptons in the final state are too soft to be detected. If R-parity is violated gravitino LSP and the NLSP will decay. Using a novel description of bilinear R-parity violation, which yields a trilinear coupling previously not discussed in the literature, we derive in a simple way all decay modes of the gravitino, the neutralino, for both bino and higgsino-like cases, and the stau. Then, we compute the gravitino decay into photon and neutrino as well as all neutralino and stau decays using the two-component spinor technique for fermions. Especially the computation of the gravitino decay is simplified compared to the usual four-component approach. Having the decay widths, we discuss cosmological constraints on the size of R-parity violation and compare them with the bounds obtained in direct searches and with indirect bounds from low-energy processes. Additionally, we review the bounds on the size of R-parity violation from the gravitino decays. An important observation of this work is the connection between the gravitino decays into photon and neutrino and the neutralino decays. Using the bounds on R-parity violating couplings from the Fermi-LAT data, we are able to make rather model independent predictions for the neutralino decay length at the LHC. If the NLSP is a stau, we obtain a lower bound on the stau decay length from the requirement that the baryon asymmetry is not washed out. Focusing on the results for neutralino NLSP, we observe that the predicted decay lengths are macroscopic. The neutralinos decay, therefore, throughout the detector volume and distort the missing transverse energy signature. We have evaluated this distortion on the generator level, since there is no disposable realistic detector simulation taking into account the final radial size of the detector. Nevertheless, we show that the distribution of missing transverse momentum is sufficiently different from the R-parity conserving case and SUSY can be hidden from searches relying on the usual signatures. In order to find the signal, we derive all signatures of decaying neutralinos. While a detailed study of a higgsino-like neutralino at the LHC is work in progress, we present a search for bino-like neutralino decays at the LHC using muons coming from secondary vertices. This study is based on our implementation of muon reconstruction process in cases where they are coming not from the primary interaction point. As a result, we find that for gluino and squark masses accessible at the LHC, values of the R-parity breaking parameter can be probed which are one to two orders of magnitude smaller than the present upper bound obtained from astrophysics and cosmology. Finally, we 6
also estimate the discovery reach of the LHC if all NLSP decays are taken into account. This thesis is organized as follows. In Chapter 2 we introduce the concepts of supersymmetry and supergravity and provide the main arguments why this theory is considered to be the best motivated extension of the Standard Model. The discussion of the supergravity Lagrangian uses modern two-component notation for fermions introduced in Appendix A, and the Lagrangian was rewritten from the notation of Wess and Bagger [78]. Having discussed the breaking of supersymmetry, we present a number of models leading to consistent cosmology. The supersymmetric particle spectra predicted by these models are used for the discussion of the LHC signatures. Finally, we provide a short review of the field-theoretical description of the gravitino in the two-component formalism and summarize its implications for cosmology. Chapter 3 motivates the breaking of R-parity and provides an introduction to this topic. After the presentation of consistent patterns of R-parity breaking, we focus on the bilinear R-parity breaking scenario. The phenomenological consequences of R-parity breaking are derived in Chapter 4. We also review the bounds on R-parity breaking couplings from various sources. The connection between consistent cosmology and hidden supersymmetry at the LHC is explored in Chapter 5. Finally, we will present our conclusions and a short outlook. The appendices contain supporting material on the calculations in this work: Appendix A introduces the two-component spinor technique extending the usual discussion to the case of gravitino decays. Appendix B presents the diagonalization of neutralino and chargino matrices, as well as the derivation of neutral and charged currents. The discussion of the bilinear R-parity violation as well as the presentation of the model for spontaneous R-parity breaking in Chapter 3 are based on the work published in [69]. Compared to the published results, the discussion is vastly expanded and includes the full supercurrent. The phenomenological discussion of neutralino and stau decays in Chapter 4 is also based on the publication [69]. However, the whole discussion of the neutralino decays was updated taking into account the recent LHC results as well as the results from [72]. Additionally, we have included the decay channel into the lightest Higgs boson and neutrino neglected in [69]. Furthermore, the discussion of the higgsino-like neutralino case is completely new, as well as the discussion of gravitino decays. The discussion of the Higgsino world scenario in Chapter 5 is based on the publication [79]. The search for bino-like neutralino NLSP decays via muons from secondary vertices has been published in [70]. Compared to the published result, we have updated the discussion in view of the recent LHC results. We have also analyzed a larger amount of Monte Carlo data for the backgrounds. The discussion of searches for higgsino-like neutralino is work in progress. 7
Page 1 and 2: Gravitinos and hidden Supersymmetry
Page 3: Abstract We investigate phenomenolo
Page 7: In memory of my father, Feliks Bobr
Page 10 and 11: CONTENTS 4.2.1 Gravitino Decays . .
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CHAPTER 4. BROKEN R-PARITY: FROM TH
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CHAPTER 6. CONCLUSIONS AND OUTLOOK
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CHAPTER 6. CONCLUSIONS AND OUTLOOK
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Appendix A Two Component Spinor Tec
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A.2. SPINOR REPRESENTATIONS OF SL(2
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A.3. PROPERTIES OF FERMION FIELDS w
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A.4. FEYNMAN RULES ( 1 2 , 0) fermi
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A.4. FEYNMAN RULES i, ˙α a, µ i,
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A.5. SUMMARY OF SPINOR ALGEBRA AND
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Appendix B Gauge and Mass Eigenstat
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B.2. NEUTRAL AND CHARGED CURRENTS w
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B.2. NEUTRAL AND CHARGED CURRENTS I
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B.2. NEUTRAL AND CHARGED CURRENTS T
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B.2. NEUTRAL AND CHARGED CURRENTS w
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B.2. NEUTRAL AND CHARGED CURRENTS +
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Bibliography [1] S. Weinberg, The m
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BIBLIOGRAPHY [30] C. Dappiaggi, T.-
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BIBLIOGRAPHY [226] S. A. Malik, Sea
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BIBLIOGRAPHY [259] G. Roepstorff, E
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