Implications of effective axial-vector coupling of gluon for $t\overline{t}$ spin polarizations at the LHC

We analyze the impact of effective axial-vector coupling of the gluon on spin polarization observables in $t\overline{t}$ pair production at the LHC. Working at leading order in QCD, we compute the $t\overline{t}$ spin correlation and left-right spin asymmetry coefficients in the helicity basis in the laboratory frame as functions of the new physics scale $\Lambda$ associated with this coupling. We found that the $t\overline{t}$ invariant mass dependent asymmetries are more sensitive to the scale $\Lambda$ than the corresponding inclusive ones, in particular when suitable cuts selecting high $t\overline{t}$ invariant mass regions are imposed. In the context of this scenario, we show that the LHC has potential either to confirm or to rule out the Tevatron forward-backward top asymmetry anomaly by analyzing the $t\overline{t}$ spin correlation and left-right polarization asymmetries. On the other hand, stringent lower bound on the new physics scale $\Lambda$ can be set in this scenario if no significant deviations from the standard model predictions for those observables will be measured.

Figure 1

Feynman diagrams (a)–(d) for the $q\overline{q}\to t\overline{t}$ process, with the contribution of the gluon effective axial-vector coupling to light quarks ($g_A^q$) and top quark ($g_A^t$).

Figure 2

Feynman diagrams (a)–(d) for the $gg\to t\overline{t}$ process, with the contribution of the gluon effective axial-vector coupling to the top quark $g_A^t$.

Figure 3

Top-antitop spin correlation $A_h$ (left plot) and corresponding significance $S[A_h]$ (right plot) in $t\overline{t}$ events, in the helicity basis and laboratory frame, for the LHC center of mass energy of 8 TeV and integrated luminosity $L=10{\mathrm{fb}}^{-1}$, as a function of the scale $\Lambda$ and for some ranges of $m_{tt}$. Dashed lines in the left plot correspond to the SM predictions at leading order in QCD.

Figure 4

Same as in Fig. 3 but for the LHC center of mass energy of 14 TeV and integrated luminosity $L=10{\mathrm{fb}}^{-1}$.

Figure 5

Top-antitop left-right polarization asymmetry $A_{\mathrm{LR}}$ (left plot) and corresponding significance $S[A_{\mathrm{LR}}]$ (right plot) in $t\overline{t}$ events, in the helicity basis and laboratory frame, for the LHC center of mass energy of 8 TeV and integrated luminosity $L=10{\mathrm{fb}}^{-1}$, as a function of the scale $\Lambda$ and for some ranges of $m_{tt}$.

Figure 6

Same as in Fig. 4 but for LHC center of mass energy of 14 TeV and integrated luminosity $L=10{\mathrm{fb}}^{-1}$.

Figure 7

The $G_F(x)$, evaluated at ${\overline{g}}_A=1$, and $G_{\theta }(x)$ versus $x=\widehat{s}/{\Lambda }^2$.

Figure 8

The $m_{tt}$ inclusive values for $A_h$ (left) and $A_{\mathrm{LR}}$ (right) respectively, for the LHC 7 TeV energy and $\Lambda =1\mathrm{TeV}$, versus the gluon axial-vector coupling cutoff ${\overline{g}}_A$. In the left plot, colored (dark blue) bands stand for the ATLAS (top region) and CMS (down region) $2\sigma$ regions, while the middle (light blue) band is the overlap between these two areas. The dashed dot and continuous (red) lines correspond to the SM prediction in Eq. (15) at the NLO in QCD and to the prediction of our scenario for the $G_F$ function respectively, all multiplied by the NLO rescaling factor.