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Resolving the A_{FB}^b puzzle in an extra dimensional model with an extended gauge structure

Abdelhak Djouadi, Gregory Moreau, Francois Richard

Abstract

It is notorious that, contrary to all other precision electroweak data, the forward-backward asymmetry for b quarks $A_{FB}^b$ measured in Z decays at LEP1 is nearly three standard deviations away from the predicted value in the Standard Model; significant deviations also occur in measurements of the asymmetry off the Z pole. We show that these discrepancies can be resolved in a variant of the Randall-Sundrum extra-dimensional model in which the gauge structure is extended to $SU(2)_L \times SU(2)_R \times U(1)_X$ to allow for relatively light Kaluza-Klein excitations of the gauge bosons. In this scenario, the fermions are localized differently along the extra dimension, in order to generate the fermion mass hierarchies, so that the electroweak interactions for the heavy third generation fermions are naturally different from the light fermion ones. We show that the mixing between the Z boson with the Kaluza-Klein excitations allows to explain the $A_{FB}^b$ anomaly without affecting (and even improving) the agreement of the other precision observables, including the $Z \to bb$ partial decay width, with experimental data. Some implications of this scenario for the ILC are summarized.

Resolving the A_{FB}^b puzzle in an extra dimensional model with an extended gauge structure

Abstract

It is notorious that, contrary to all other precision electroweak data, the forward-backward asymmetry for b quarks measured in Z decays at LEP1 is nearly three standard deviations away from the predicted value in the Standard Model; significant deviations also occur in measurements of the asymmetry off the Z pole. We show that these discrepancies can be resolved in a variant of the Randall-Sundrum extra-dimensional model in which the gauge structure is extended to to allow for relatively light Kaluza-Klein excitations of the gauge bosons. In this scenario, the fermions are localized differently along the extra dimension, in order to generate the fermion mass hierarchies, so that the electroweak interactions for the heavy third generation fermions are naturally different from the light fermion ones. We show that the mixing between the Z boson with the Kaluza-Klein excitations allows to explain the anomaly without affecting (and even improving) the agreement of the other precision observables, including the partial decay width, with experimental data. Some implications of this scenario for the ILC are summarized.

Paper Structure

This paper contains 14 sections, 23 equations, 5 figures, 3 tables.

Table of Contents

  1. 1. Introduction
  2. 2. The physical set--up
  3. 3. Resolving the $\mathbf{A_{FB}^b(M_Z)}$ puzzle
  4. 4. $\mathbf{A_{FB}^b}$ outside the Z-pole
  5. 5. Expectations for the ILC
  6. 5. Conclusion

Figures (5)

  • Figure 1: Contour plots in the plane $[c_{b_L}, c_{b_R}]$ for $M_{KK}=3$ TeV and $\sin\theta'=0.1$, for which $A_{FB}^b$ and $R_b$ in the scenario RSa are equal to their experimental values (dotted lines) and are within their $\pm 1 \sigma$ bands; $c_{\rm light} \gg 0.5$ for leptons and light quarks.
  • Figure 2: Contour plots in the plane $[c_{b_L}, c_{b_R}]$ for $M_{KK}=3$ TeV and $\sin\theta'=0.1$, for which $A_{FB}^b$ and $R_b$ in the scenario RSb are equal to their experimental values (dotted lines) and are within their $\pm 1 \sigma$ bands; $c_{\rm light} \gg 0.5$ for leptons and light quarks.
  • Figure 3: The FB asymmetry $A_{FB}^b$ at LEP1 (left) and outside the $Z$ resonance (right) as a function of the c.m. energy. Shown are the predictions in the SM and in the two RS scenarios RSa [with $c_{b_L}=0.37, c_{b_R}=0.287, c_{\rm light} \gg 0.5$] and RSb [with $c_{b_L}=0.36, c_{b_R}=0.49, c_{\rm light} \gg 0.5$] for $M_{KK}=3$ TeV and $\sin\theta'=0.1$, as well as the various experimental measurements with their error bars.
  • Figure 4: Energy dependence of the Left--Right polarization asymmetry for $b$--quarks in the SM and the RSa (left) and RSb (right) case including the KK contributions and in the pure mixing case. The blue curve is for the SM, the green curve corresponds to mixing only while the red curve takes into account KK exchange.
  • Figure 5: Energy dependence of the Left--Right polarization asymmetry for top quarks in the SM and in the RSa and RSb scenarios. Conventions are as in the previous figure.