Boosted top quark polarization

In top quark production, the polarization of top quarks, decided by the chiral structure of couplings, is likely to be modified in the presence of any new physics contribution to the production. Hence, it is a good discriminator for those new physics models wherein the couplings have a chiral structure different than that in the Standard Model. In this paper, we construct probes of the polarization of a top quark decaying hadronically, using easily accessible kinematic variables such as the energy fraction or angular correlations of the decay products. Tagging the boosted top quark using the jet substructure technique, we study the robustness of these observables for a benchmark process, $W^{\prime }\to tb$. We demonstrate that the energy fraction of $b$ jet in the laboratory frame and a new angular variable, constructed by us in the top rest frame, are both very powerful tools to discriminate between the left and right polarized top quarks. Based on the polarization-sensitive angular variables, we construct asymmetries that reflect the polarization. We study the efficacy of these variables for two new physics processes that give rise to boosted top quarks: (i) the decay of the top squark in the context of supersymmetry searches and (ii) decays of the Kaluza-Klein (KK) graviton and KK gluon, in the Randall-Sundrum model. Remarkably, it is found that the asymmetry can vary over a wide range about $+20\%$ to $-20\%$. The dependence of asymmetry on top quark couplings of the new particles present in these models beyond the SM is also investigated in detail.

Figure 1

Top tagging efficiency for left ($t_L$) and right chiral ($t_R$) top quark for the process $pp\to W^{\prime }\to tb$ with $m_{W^{\prime }}=3\mathrm{TeV}$.

Figure 2

The energy fraction $z_k$ [Eq. (6)] for the left- (red) and right-handed (blue) top quarks for $m_{W^{\prime }}=3\mathrm{TeV}$. The dashed lines correspond to the observable at the partonic level.

Figure 3

Energy fraction of $b$-like subjet for partonic (dashed) and reconstructed top jet using heptoptagger2 (solid).

Figure 4

The $\cos {\theta }^{*}$ distribution for $m_{W^{\prime }}=3\mathrm{TeV}$ for reconstructed top jet (solid) and partonic (dashed).

Figure 5

The asymmetry [Eq. (8)] for parton-level and tagged tops for $m_{W^{\prime }}=3\mathrm{TeV}$.

Figure 6

The $b$ quark energy fraction $Z_b$ for the event presented by Eq. (14). Lightest supersymmetric particle (LSP) is a pure Bino (${\tilde{\chi }}_1^0=\tilde{B}$), and ${\tilde{t}}_1$ is either pure ${\tilde{t}}_L$ (red) or ${\tilde{t}}_R$ (blue) giving rise to a $t_L$ or $t_R$ respectively in its decay.

Figure 7

Same as Fig. 6 but for $z_k$.

Figure 8

Same as Fig. 6 but for $\cos {\theta }^{*}$ [Eq. (3)].

Figure 9

Variation of asymmetry with the composition of ${\tilde{t}}_1$ for the cases of pure bino like (left) and equally mixed ${\tilde{\chi }}_1^0$ states (right).

Figure 10

Variation of asymmetry [Eq. (8)] with the composition of Bino- and wino like ${\tilde{\chi }}_1^0$ for the cases of ${\tilde{t}}_1={\tilde{t}}_L$ and ${\tilde{t}}_1={\tilde{t}}_R$.

Figure 11

The $z_k$ (top left), $Z_b$ (top right), and $\cos {\theta }^{*}$ (bottom center) distribution of reconstructed top jets for single top and $t\overline{t}$ pair production from a KK gluon of mass 4 TeV and $t\overline{t}$ pair from a KK graviton of mass 4 TeV.

Figure 12

The $A_{{\theta }^{*}}$ asymmetry for tagged tops for the KK graviton mass of 3 TeV.