On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

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146 Chapter 4. The Asymmetry in Top Pair Production dσ/dy 0 y Figure 4.1: Rapidity distributions for top (red) and antitop (blue) quarks for the Tevatron in the left panel and the LHC in the right panel. The distributions are not to scale and represent only a qualitative illustration. which more recently has been confirmed by the LHC detectors ATLAS and CMS [208], dσ/dy m LHC t = 173.3 ± 0.5stat ± 1.3syst GeV . (4.2) Using the mass as an input, the theoretical predictions for the total inclusive cross section turns out to be in very good agreement with the data from the Tevatron [212, 207], � σ � t¯t = 7.56 DØ +0.63 −0.56 pb , (4.3) � σ �1.96 TeV t¯t = 7.06+0.27 pb , (4.4) NNLOapr −0.34 +0.69 −0.53 as well as with measurements at the LHC [209, 210], � σ � t¯t = 165.8 ± 2.2stat ± 10.6syst ± 7.8lumi pb , (4.5) CMS � σ �7 TeV t¯t = 161+12.3 pb . (4.6) NNLOapr −11.9 +15.2 −14.5 Concerning the theoretical values of the cross sections in (4.4) and (4.6), consult [211, Tab. 1] and the references therein. The quoted values correspond to approximative NNLO results and the first error is the total theoretical uncertainty, the second the PDF uncertainty. This agreement has to be contrasted with a ∼ 3σ discrepancy between SM prediction and measurements of both Tevatron detectors for the forward-backward asymmetry of top antitop pairs, which in the CM frame reads [239, Table 6], � � t AFB � � t AFB exp = (16.2 ± 4.7stat+syst) % , (4.7) SM = (7.3+1.1 −0.7 ) % , (4.8) where the experimental value corresponds to the first extraction from the full Tevatron dataset, recently done by CDF [213]. Any new physics that might explain this discrepancy should however be heavy or broad enough to have escaped detection in diject events and in the t¯t invariant mass spectrum analyzed at the end of Section 3.7. 0 y
4.1. Top-Antitop Pair Production and Observables at Tevatron and the LHC 147 We report therefore also the mass dependent asymmetry measured by CDF, which could be interpreted as a hint at heavy new resonances, since it shows a linear slope which is significantly larger than predicted in the SM, A t FB(m t¯t < 450GeV) = � 7.8 ± 5.4 � % , A t FB(m t¯t > 450GeV) = � 29.6 ± 6.7 � % , (4.9) although the SM prediction goes in the same direction [220] A t FB(mt¯t < 450GeV) = � 6.2 +0.4� −0.3 % , (4.10) A t FB(mt¯t > 450GeV) = � 12.9 +0.8� −0.6 % . One can understand the observable (4.8) as the number of top quarks going in forward direction minus the number of top quarks going in backwards direction relative to the interaction point in the CM frame, normalized to the total number of tops, A t FB = Nt(F ) − Nt(B) . (4.11) Nt(F ) + Nt(B) Because of charge conjugation invariance, the antitops scattered forwards (backwards) can be counted as tops scattered backwards (forwards), so that N¯t(F ) = Nt(B). Analogous to (4.11), one can therefore define the asymmetry as the number of tops scattered in forward direction minus the number of antitops scattered in forward direction, which then corresponds to a charge asymmetry A t C = Nt(F ) − N¯t(F ) . (4.12) Nt(F ) + N¯t(F ) A charge asymmetric initial state is required in order to generate contributions to such an asymmetry. This is the case at the Tevatron, as it is a proton antiproton collider, but not at the LHC, which collides protons with each other. This is illustrated by a sketch of the rapidity distributions for the Tevatron and LHC in Figure 4.1. Clearly, counting the tops versus the antitops in forward direction (y > 0) will only in the case of the Tevatron lead to a non-zero number. On a more fundamental level however, the asymmetry can be traced back to the fact, that the top quark likes to go in the direction of the initial state quark and the antitop in the direction of the initial state antiquark. These directions are well defined at the Tevatron as the proton consists mainly of quarks, which results in the shifts of the rapidity distributions in the left panel of Figure 4.1. At the LHC, one can infer that the initial antiquark must be a sea quark. Sea quarks carry less momentum fraction than valence quarks and consequentially the rapidity distributions at the LHC are symmetric, but have a different width, as shown in the right panel of Figure 4.1. On average, the top carries more momentum than the antitop. One can therefore define a charge asymmetry based on the difference in absolute rapidity, ∆|y| = |yt| − |y¯t|, A y N(∆|y| > 0) − N(∆|y| < 0) C = . (4.13) N(∆|y| > 0) + N(∆|y| < 0)
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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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92 Chapter 3. Solving the Flavor Pr
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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
Page 197 and 198: C Input Parameters This appendix is
Page 199: Parameter Value ± Error Reference
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Page 204 and 205: 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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