On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

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140 Chapter 3. Solving the Flavor Problem in Strongly Coupled Theories (a) (b) (c) (d) (e) (f) Figure 3.13: In the first row, the leading diagrams for the single and pair production of axigluon or scalar octet resonances are shown for gluon initial states. Coiled lines represent gluons, straight lines quarks and dashed lines can be either an axigluon KK mode or a scalar octet. The diagrams (b) and (c) exist also for an axigluon (octet) in the loop and a octet (axigluon) final state. The second line shows the favored decay channels for the pair production, with the straight lines in (e) denoting top quarks in case of a neutral octet and top-bottom pairs if the octets are charged. from gluon initial states [193]. However, even though they are color charged, the new scalars are electroweak doublets and can therefore only be produced from gluon initial states via loop suppressed gluon-gluon fusion, or in pairs, see Figure 3.13. The same holds true for the axigluon. From the second line of (3.84) it is clear that a GGA vertex is forbidden, as well as any vertex with an odd number of axigluons if θ = 45 ◦ , that is, if the new resonance couples purely axial. This is a result of parity conservation, since the axigluon must have negative parity. As a consequence the production channels from gluon initial states for a heavy axigluon resonance are also given by the diagrams in the first row of Figure 3.13. Regarding the decays one can say, that the color octet will decay primarily into tops, because its couplings are proportional to the Yukawa couplings. This is also true for the axigluon in the RS model, because it will be strongly IR localized and the RS-GIM mechanism suppresses couplings to the light flavors. Both will therefore also evade the usual dijet bounds, because a top pair final state will not look like two jets in the detector and is often not reconstructed in these analyses. Possible signatures would be seen in resonance searches in top pair final states for the gluon-fusion production channel, or in four top final states if the resonances are pair-produced at tree-level. There is also the diagram (f) in Figure 3.13, but this will not produce a narrow resonance in the dijet spectrum. Scalar octets can also radiate off an electroweak gauge boson before they decay, which is however suppressed if the decay in top pairs is kinematically accessible, see [200, Fig.4] and the analysis therein. In order to estimate whether the extended RS model is in conflict with the direct detection limits we will concentrate on the axigluon KK mode. For this resonance the
3.7. Flavor Observables and LHC Bounds 141 model predicts a mass which depends only on MKK, while the masses of the scalar octets are not connected to the KK scale but depend on a large number of parameters in the Higgs potential. We will therefore only refer to the literature on color octet electroweak doublets [197, 199, 200, 201] and note that a dedicated analysis using the LHC dataset is still to be undergone and the current upper limits from LEP searches are in the mO > 100 GeV range 15 . For the axigluon KK mode, we will compare the tree-level production cross section times the branching ratio into light flavors with the latest dijet bounds from the ATLAS collaboration based on 5.8fb −1 integrated luminosity at √ s = 8 TeV [202]. In order to do this we adapt the analysis done in [203, Sec. 5] to the case of a heavy axigluon with flavor non-universal couplings to quarks. The total cross section for tree-level production includes only q¯q → G, and reads σ = � q � (1) ffq¯q (m A )2 � CF /s, µf Nc 2π 2 αs s � A (gL ) 2 q + (g A R) 2� q , (3.112) where the sum extends over the light quark flavors q = u, d, s, c, b and √ s = 8 TeV. The parton luminosity functions, ffij(τ, µf ) = 2 1 + δij � 1 τ dx x f i/p(x, µf ) f j/p(τ/x, µf ) . (3.113) are evaluated at the parton center-of-mass energy corresponding to the resonant production of the axigluon, i.e. , τ = (m (1) A )2 /s. They are obtained from a convolution of the particle distribution functions (PDFs) fi/p(x, µf ), which describe the probability of finding the parton i in the proton with longitudinal momentum fraction x. We will employ MSTW2008LO PDFs with the renormalization and factorization scales set to µr = µf = m (1) A [228]. The branching ratio for the axigluon decay into light quarks reads B(A → q¯q) = Γq/ΓA , (3.114) with the total width denoted by ΓA. The partial decay rate can be computed to � Γq = αsTF 6 m(1) A × 1 − 4m2 q (m A 1 )2 � �(gA L ) 2 q + (g A R) 2 � q � 1 − m2q (mA 1 )2 � + 6 (g A L )q (g A m R)q 2 q (mA 1 )2 � (3.115) 15 These analysis refer to the Manohar Wise model [197] of a single scalar octet electroweak doublet, which has a considerably nicer potential then the one in the extended RS model. ,
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On the Flavor Problem in Strongly C
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UNIVERSITY OF MAINZ ABSTRACT THEORE
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Contents Acknowledgements vii Prefa
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Acknowledgements First and foremost
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x One of the most fascinating insig
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2 Chapter 1. Introduction: Problems
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10 Chapter 1. Introduction: Problem
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36 Chapter 2. The Randall Sundrum M
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Page 188 and 189: 178 Appendix A. Generating Paramete
Page 190 and 191: 180 Appendix A. Generating Paramete
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Page 194 and 195: 184 Appendix B. Neutral Meson Mixin
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Page 199: Parameter Value ± Error Reference
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192 Appendix D. Higgs Potential for
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194 Bibliography [13] S. Dimopoulos
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196 Bibliography [43] G. Nordström
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198 Bibliography [72] K. S. Babu,
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200 Bibliography [104] R. Contino a
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202 Bibliography [133] M. Baak, M.
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204 Bibliography [160] J. Charles,
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206 Bibliography [185] C. Csaki, G.
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208 Bibliography [213] E. Thomson e
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210 Bibliography [239] V. Ahrens, A
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