Top quark polarization as a probe of charged Higgs bosons

We study the production and decay of a heavy charged Higgs boson in the $bg\to t{H}^{-}$ and $H^{-}\to b\overline{t}$ processes at the Large Hadron Collider (LHC). We show that the chiral structure of the $H^{-}t\overline{b}$ vertex entering both stages is sensitive to the underlying Higgs mechanism of electroweak symmetry breaking (EWSB) and we specifically demonstrate that one could distinguish between two popular realizations of a 2-Higgs doublet model (2HDM) embedding a new $H^{\pm }$ state, i.e., those with Type-I and -Y Yukawa couplings. The chiral structure of such a vertex, which is different in the two cases, in turn triggers a particular spin state of the top quark which is then transmitted to its decay products. Hence, both inclusive rates and exclusive observables can be used to extract the presence of such a charged Higgs boson state in LHC data.

Figure 1

Left: Scatter plot of $\tan \beta$ and $m_H^{\pm }$ showing the constraints from Higgs boson signal strengths and mass measurements using HiggsSignals. Right: The effect of the void direct searches for additional (pseudo)scalars on $\tan \beta$ and $m_{H^{\pm }}$ using HiggsBounds. The ${\chi }^2$ (from HiggsSignals) and $r_{95\%}$ (from HiggsBounds) values are shown as a color map. The horizontal black line on the right shows $r_{95\%}=1$ above which the parameter point is excluded at 95% CL. We illustrate here the case of the 2HDM-II.

Figure 2

Left: The chiral form factors of the $H^{+}\overline{t}b$ vertex as a function of $\tan \beta$ for the 2HDM-I (red) and 2HDM-Y (blue), where the overlapping solid(separate dashed) lines show the L(R)-handed components. Right: The $H^{\pm }$ production cross section as a function of $\tan \beta$ in the 2HDM-I (blue) and 2HDM-II (green), where the solid(dashed) lines show the cross sections for $m_{H^{\pm }}=200\mathrm{GeV}$($m_{H^{\pm }}=500\mathrm{GeV}$). Here, the error band is a result of the variation of the renormalization/factorization scale by a factor of 2 from the central scale choice defined in Eq. (12) (see below).

Figure 3

Left: The $\cos {\theta }_{\ell }^k$ distributions for the SM (solid black), 2HDM-I (solid red, green and blue) and 2HDM-Y (dashed red, green and blue) after applying all the basic selections. Right: the same distribution but for $H_T>1\mathrm{TeV}$. Data are for $\sqrt{s}=13\mathrm{TeV}$ and $\mathcal{L}=1000{\mathrm{fb}}^{-1}$. The color scheme identify different $H^{\pm }$ masses while the shading represents the statistical and luminosity error.

Figure 4

The $x_{\ell }$ (left panel) and $u$ (right panel) distributions for the SM (black), 2HDM-I (blue) and 2HDM-Y (red) applying the first set of cuts (denoted by Cuts1). Data are for $\sqrt{s}=13\mathrm{TeV}$ and $\mathcal{L}=1000{\mathrm{fb}}^{-1}$. The (almost invisible) shading represents the statistical and luminosity error. Here, $m_{H^{\pm }}=500\mathrm{GeV}$.

Figure 5

Left: The normalized distribution for the $\cos {\theta }_{\ell }^k$ variable. Right: The normalized distribution for the $u$ variable. The two distributions are shown for $m_{H^{\pm }}=300\mathrm{GeV}$ and $\tan \beta =1$ after passing the Cuts2 requirements. In both panels, predictions are shown for the 4FS (red), 5FS (green) and matched scheme (blue).