Dedicated strategies for triboson signals from cascade decays of vector resonances

New colorless electroweak (EW) charged spin-1 particles with mass of a few TeV arise in numerous extensions of the standard model (SM). Decays of such a vector into a pair of SM particles, either fermions or EW bosons, are well studied. Many of these models have an additional scalar, which can lead to (and even dominate in certain parameter regions) a novel decay channel for the heavy vector particles instead—into a SM EW boson and the scalar, which subsequently decays into a SM EW boson pair. In this work, we focus on the scalar being relatively heavy, roughly a factor of 2 lighter than the vector particles, rendering its decay products well separated. Such a cascade decay results in a final state with three isolated bosons. We argue that for this “triboson” signal the existing diboson searches are not quite optimal due to combinatorial ambiguity for three identical bosons, and in addition, due to a relatively small signal cross section determined by the heaviness of the decaying vector particle. In order to isolate the signal, we demonstrate that tagging all three bosons, followed by use of the full triboson invariant mass distribution as well as that of appropriate subsets of dibosons, is well motivated. We develop these general strategies in detail within the context of a specific class of models that are based on extensions of the standard warped extradimensional scenario. We also point out that a similar analysis would apply to models with an enlarged EW gauge sector in four dimensions, even if they involve a different Lorentz structure for the relevant couplings.

Figure 1

Warped extradimensional model with SM fields in the same bulk (standard framework). Schematic shapes of extradimensional wave functions for various particles (zero-mode SM fermions and gauge particles, a radion, and a generic KK mode) are shown.

Figure 2

Warped extradimensional model with all SM gauge fields (and gravity) in the full bulk, but matter and Higgs fields in subspace (extended framework). Schematic shapes of extradimensional wave functions for various particles [zero-mode SM fermions and gauge bosons, an (IR) radion, and a generic KK mode] are shown.

Figure 3

Warped extradimensional model with only SM EW gauge fields (on the left) and only SM hypercharge field (on the right) in the full bulk, but matter, Higgs and other gauge fields in subspace (extended framework). Schematic shapes of extradimensional wave functions for various particles [zero-mode SM fermions and gauge bosons, an (IR) radion, and a generic KK mode] are shown.

Figure 4

Feynman diagram for the signal process from production to decay. Double (single) lines represent KK (SM) particles and $q/q^{\prime }$ denote light quarks inside the proton. The signal process is characterized by two resonance bumps given by $V_{\mathrm{KK}}$ and $\phi$.

Figure 5

Contours of the KK $W$ production at $\sqrt{s}=13\mathrm{TeV}$ of the LHC as a function of $g_{W_{\mathrm{KK}}}$ and $m_{W_{\mathrm{KK}}}$ with $m_{\phi }=1\mathrm{TeV}$. The numbers in boxes are the cross sections of producing KK $W$ in fb. The shaded region is ruled out by the search in Ref. [17] where $W^{\prime }$ decays to a single lepton associated with a large $E_T^{\mathrm{miss}}$. See the text for more detailed information.

Figure 6

The plot on the left shows the maximum of the $\mathrm{\Delta }{R}_{qq}$ from each $W$ and minimum of $\mathrm{\Delta }{R}_{WW}$. The plot on the right shows tagging efficiency as a function of jet radius. In both plots, the solid (dashed) lines show the result for the radion mass of 1 TeV (1.5 TeV).

Figure 7

Distribution of kinematic variables for $W\text{−}WWW\text{−}\mathrm{BP}1$ ($W\text{−}WWW\text{−}\mathrm{BP}2$) fully hadronic channel: $M_{JJJ}$ (top row, left), $M_{J_1{J}_2}$ (top row, right), $M_{J_1{J}_3}$ (second row, left), $M_{J_2{J}_3}$ (second row, right), $p_{T{J}_1}$ (third row, left), $p_{T{J}_2}$ (third row, middle), $p_{T{J}_3}$ (third row, right), ${\tau }_{21J_1}$ (bottom row, left), ${\tau }_{21J_2}$ (bottom row, middle), ${\tau }_{21J_3}$ (bottom row, right), for signal with 1 TeV radion (solid blue), signal with 1.5 TeV radion (solid purple) and backgrounds (solid red). We denote $p_T$-ordered jet as $J_{1,2,3}$, $J_1$ being the hardest jet.

Figure 8

Two-dimensional $M_{J_1{J}_3}\text{−}{M}_{J_2{J}_3}$ distributions for $W\text{−}WWW\text{−}\mathrm{BP}1$ fully hadronic channel: SG after preselection but before any other cuts (top row, left), BK after preselection but before any other cuts (top row, right), SG after all other cuts but two-dimensional $M_{J_1{J}_3}\text{−}{M}_{J_2{J}_3}$ cut (bottom row, left), and BK after all other cuts but two-dimensional $M_{J_1{J}_3}\text{−}{M}_{J_2{J}_3}$ cut (second row, right). The region surrounded by orange dashed line shows the signal selected by $\mathrm{the}+\text{cut used in our}$ three-dimensional analysis. We refer to the main text for more detailed information.

Figure 9

Two-dimensional $M_{J_1{J}_3}\text{−}{M}_{J_2{J}_3}$ distributions for $W\text{−}WWW\text{−}\mathrm{BP}2$ fully hadronic channel: SG after preselection but before any other cuts (top row, left), BK after preselection but before any other cuts (top row, right), SG after all other cuts but two-dimensional $M_{J_1{J}_3}\text{−}{M}_{J_2{J}_3}$ cut (bottom row, left), BK after all other cuts but the two-dimensional $M_{J_1{J}_3}\text{−}{M}_{J_2{J}_3}$ cut (second row, right). The region surrounded by an orange dashed line shows the signal selected by $\mathrm{the}+\text{cut used in our}$ three-dimensional analysis. We refer to the main text for more detailed information.

Figure 10

Distributions of new kinematic variables for the $W\text{−}WWW\text{−}\mathrm{BP}1(W\text{−}WWW\text{−}\mathrm{BP}2)$ fully hadronic channel: ${\tau }_{123}$ (left) and $p_{T,123}$ (right) for signal with 1 TeV radion (solid blue), signal with 1.5 TeV radion (solid purple) and backgrounds (solid red). We denote the $p_T$-ordered jet as $J_{1,2,3}$, $J_1$ being the hardest jet. These variables allow sharper distinction between the signal and the background compared with the standard $p_T$ and ${\tau }_{21}$ variables (see Fig. 7).

Figure 11

Distribution of kinematic variables for $W\text{−}WWW\text{−}\mathrm{BP}1(W\text{−}WWW\text{−}\mathrm{BP}2)$ semileptonic channel: $M_{WWW}$ (top row, left), $M_{W_1{W}_2}$ (top row, right), $M_{W_1{W}_3}$ (second row, left), $M_{W_2{W}_3}$ (second row, right), $p_{T{W}_1}$ (third row, left), $p_{T{W}_2}$ (third row, middle), $p_{T{W}_3}$ (third row, right), $p_{T_{W_{\ell }}}$ (bottom row, left), ${\tau }_{21J_1}$ (bottom row, middle), ${\tau }_{21J_2}$ (bottom row, right) for signal with 1 TeV radion (solid blue), signal with 1.5 TeV radion (solid purple) and backgrounds (solid red). We denote $p_T$-ordered $W$’s as $W_{1,2,3}$, $W_1$ being the hardest one.

Figure 12

Distribution of kinematic variables for $W\text{−}W\gamma \gamma \text{−}\mathrm{BP}1(W\text{−}W\gamma \gamma \text{−}\mathrm{BP}2)$ hadronic $W$ channel: $M_{J\gamma \gamma }$ (top row, left), $M_{\gamma \gamma }$ (top row, right), $p_{TJ}$ (second row, left), $p_{T{\gamma }_1}$ (bottom row, middle), $p_{T{\gamma }_2}$ (bottom row, right) for signal with 1 TeV radion (solid blue), signal with 1.5 TeV radion (solid purple) and backgrounds (solid red). We denote $p_T$-ordered $\gamma$’s as ${\gamma }_{1,2}$, ${\gamma }_1$ being the hardest one.

Figure 13

Distribution of kinematic variables for $W\text{−}W\gamma \gamma \text{−}\mathrm{BP}1(W\text{−}W\gamma \gamma \text{−}\mathrm{BP}2)$ leptonic $W$ channel: $M_{\mathrm{rec}}$ (top row, left), $M_{\gamma \gamma }$ (top row, right), $p_{T{\gamma }_1}$ (bottom row, left), $p_{T{\gamma }_2}$ (bottom row, right) for signal with 1 TeV radion (solid blue), signal with 1.5 TeV radion (solid purple), $W\gamma \gamma$ background (BK:$W\gamma \gamma$, solid red) and $Wj\gamma$ backgrounds (BK:$Wj\gamma$, solid orange). We denote $p_T$-ordered $\gamma$’s as ${\gamma }_{1,2}$, ${\gamma }_1$ being the hardest one. $M_{\mathrm{rec}}$ is the invariant mass of all particles including neutrino.

Figure 14

$E_T^{\mathrm{miss}}$ distribution for the same sign dilepton decay $W_{\mathrm{KK}}^{\pm }\to W^{\pm }\phi \to W^{\pm }{W}^{\pm }{W}^{\mp }\to {\ell }^{\pm }{\ell }^{\pm }\nu \nu q{\overline{q}}^{\prime }$ with $m_{\phi }=1\mathrm{TeV}$ using benchmark point $W\text{−}WWW\text{−}\mathrm{BP}1$.

Figure 15

Comparison of fixed $R$ and variable $R$ methods (blue and orange, respectively), for 1 and 1.5 TeV radion (left and right), corresponding to benchmark points $W\text{−}WWW\text{−}\mathrm{BP}1$ and $W\text{−}WWW\text{−}\mathrm{BP}2$, respectively. Some representative points are labeled by the $R$ ($\rho$) values to show their value near the optimal point, and the direction in which these parameters increase.