Graviton resonance phenomenology and a pseudo-Nambu-Goldstone boson Higgs at the LHC

We present an effective description of a spin two massive state and a pseudo-Nambu-Goldstone boson Higgs in a two site model. Using this framework, we model the spin-two state as a massive graviton and we study its phenomenology at the LHC. We find that a reduced set of parameters can describe the most important features of this scenario. We address the question of which channel is the most sensitive to detect this graviton. Instead of designing search strategies to estimate the significance in each channel, we compare the ratio of our theoretical predictions to the limits set by available experimental searches for all the decay channels and as a function of the free parameters in the model. We discuss the phenomenological details contained in the outcome of this simple procedure. The results indicate that, for the studied masses between 0.5 and 3 TeV, the channels to look for such a graviton resonance are mainly $ZZ$, $WW$, and $\gamma \gamma$. This is the case even though top and bottom quarks dominate the branching ratios, since their experimental sensitivity is not as good as the one of the electroweak gauge bosons. We find that as the graviton mass increases, the $ZZ$ and $WW$ channels become more important because of its relatively better enhancement over background, mainly due to fat jet techniques. We determine the region of the parameter space that has already been excluded and the reach for the LHC next stages. We also estimate the size of the loop-induced contributions to the production and decay of the graviton, and show in which region of the parameter space their effects are relevant for our analysis.

Figure 1

Moose diagram describing the model in terms of a two-site theory. Site-0 and site-1 have gauge symmetry groups ${\mathrm{G}}_0$ and ${\mathrm{G}}_1$. The Higgs arises from the spontaneous breaking of ${\mathrm{G}}_1$ down to ${\mathrm{H}}_1$ by the strong interactions on site-1. The link fields $\mathrm{\Omega }$, transforming nontrivially under the symmetries on both sites, allow us to connect the fields on site-0 and site-1.

Figure 2

Upper panel: Graviton branching ratios as a function of the two more relevant parameters, $C_H$ and $s_A$, within the universal coupling scenario. Lower panel: Sensitivity of graviton decay channels at 8 TeV and $20{\mathrm{fb}}^{-1}$ expressed in terms of $\mathcal{S}$ as a function of $C_H$ and $s_A$ within the universal scenario. All plots were generated with the following setting of graviton couplings to fermions: $s_q=0.7$, $s_t=0.8$ and $s_b=0.3$; the patterns depicted in the figure are a general feature of a massive graviton independently of these specific values though.

Figure 3

Upper panels: The values of $C_H$ and $s_A$ for the points corresponding to the most sensitive graviton decays channels for LHC at 8 TeV within the scanned parameter space (defined in the text), for $m_{X^{*}}=0.5\mathrm{TeV}$ and $m_{X^{*}}=1\mathrm{TeV}$, and according to the following color coding: green, red and gray represent $ZZ$, $\gamma \gamma$ and $t\overline{t}$ channels, respectively. Solid lines define regions excluded by present data. Dotted and dash lines correspond to strength values $\mathcal{S}=0.05$ and $\mathcal{S}=0.2$, respectively. Lower panels: Next-to-most sensitive graviton decay channels for LHC at 8 TeV, for $m_{X^{*}}=0.5\mathrm{TeV}$ and $m_{X^{*}}=1\mathrm{TeV}$. The numbers in parentheses next to a certain channel indicate its relative sensitivity (${\mathcal{S}}_{\mathrm{nMS}}/{\mathcal{S}}_{\mathrm{MS}})$ with respect to the most sensitive one for the central point of each rectangle of the grid.

Figure 4

Dominant channels in the plane $(s_W^2-s_B^2{)}^2/(s_W^2+s_B^2{)}^2$ vs. $C_H$ for LHC at 8 TeV. On the left we show the points that pass all the bounds for $m_{X^{*}}=0.5\mathrm{TeV}$ and on the right for $m_{X^{*}}=1\mathrm{TeV}$. The colors indicate which channel has the largest $\mathcal{S}$: red for $\gamma \gamma$, green for $ZZ$, black for $Z\gamma$, gray for $t\overline{t}$ and blue for $WW$.

Figure 5

Upper panels: The values of $C_H$ and $s_A$ for the points corresponding to the most sensitive graviton decays channels for LHC at 13 TeV within the scanned parameter space (defined in the text), for $m_{X^{*}}=1\mathrm{TeV}$ and $m_{X^{*}}=3\mathrm{TeV}$, and according to the following color coding: green, red and gray represent $VV$, $\gamma \gamma$ and $t\overline{t}$ channels, respectively. The black (blue) lines define exclusion regions for different luminosities, 30, 300, $3000{\mathrm{fb}}^{-1}$, and present data at 13 TeV (8 TeV). Lower panels: Next-to-most sensitive graviton decay channels for LHC at 13 TeV, for $m_{X^{*}}=1\mathrm{TeV}$ and $m_{X^{*}}=3\mathrm{TeV}$. The numbers in parentheses next to a certain channel indicate its relative sensitivity (${\mathcal{S}}_{\mathrm{nMS}}/{\mathcal{S}}_{\mathrm{MS}})$ with respect to the most sensitive one for the central point of each rectangle of the grid.

Figure 6

Dominant channels in the plane $(s_W^2-s_B^2{)}^2/(s_W^2+s_B^2{)}^2$ vs. $C_H$ for LHC at 13 TeV. We only show the points that pass the bounds on $\mathcal{S}$ at this center of mass energy. On the left we show the results for $m_{X^{*}}=1\mathrm{TeV}$ and on the right for $m_{X^{*}}=3\mathrm{TeV}$. The colors indicate which channel has the largest $\mathcal{S}$: red for $\gamma \gamma$, green for $VV$ and gray for $t\overline{t}$.

Figure 7

Exclusion regions for different luminosities, obtained at tree level (black) and including loop effects (orange), for $m_{X^{*}}=1$ (left) and 3 TeV (right).