[Paper Review] WW Fusion in Higgsless Models

Rahul Malhotra|ArXiv.org|Nov 30, 2006

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper investigates the discovery potential of the first neutral Kaluza-Klein excitation, $V_1^0$, in Higgsless models at the LHC via $WW$ fusion, using the $e^\mu + \not{E}_T$ final state. It shows that with $300\ \text{fb}^{-1}$ luminosity and optimized cuts—especially a minijet veto and $P_T^{\text{max}}(e,\mu)$ threshold—$5\sigma$ discovery is achievable for $V_1^0$ masses above 1 TeV.

ABSTRACT

Recently several Higgsless models of electroweak symmetry breaking have been proposed in which unitarity of $W,Z$ scattering amplitudes is partially restored through a tower of massive vector gauge bosons. These massive states are expected to couple mainly to $W$ and $Z$ and should appear as resonances in $WW$ and $WZ$ fusion at the CERN Large Hadron Collider (LHC). We study the LHC discovery reach for the first neutral state, $V_1^0$, through the reaction $W^+W^- o V_1^0 o W^+W^- o e^\pm μ^\mp + \eslt$. The background from $t\bar{t}$, $τ^+τ^-$ and $W$ pair production is calculated and dual forward jet tagging as well as a mini-jet veto in the central detector region are applied. The maximal $P_T(e,μ)$ distribution is found to have a fat tail for large $P_T$ that rises above the backgrounds and allows $5σ$ discovery for $V_1^0$ masses above 1 TeV and total integrated luminosity of 300 fb$^{-1}$.

Motivation & Objective

• To assess the LHC discovery potential for the first neutral Kaluza-Klein state $V_1^0$ in Higgsless models, which arise from extra-dimensional or deconstructed theories.
• To address the challenge of identifying $V_1^0$ resonances in the $WW\to e\mu + \not{E}_T$ channel, which suffers from large backgrounds, particularly from $t\bar{t}$ and $WW+2j$ processes.
• To develop and apply a set of kinematic and topological cuts—including forward jet tagging, $b$-jet veto, $M_{JJ}$ selection, and minijet veto—to enhance signal significance.
• To optimize the $P_T^{\text{max}}(e,\mu)$ threshold for signal discrimination, leveraging the fat tail in the transverse momentum distribution that rises above background for high-mass $V_1^0$.
• To quantify the discovery reach across varying integrated luminosities, particularly demonstrating the feasibility of detecting $V_1^0$ up to 1.35 TeV with 300 fb⁻¹.

Proposed method

• The study models $WW$ fusion in Higgsless models where $V_1^0$ is produced via initial-state $W$ radiation and decays dominantly to $WW$, with subsequent leptonic decay to $e^\mu + \not{E}_T$.
• Key theoretical components include the sum rules (1) and (2) that relate $WWV_N^0$ couplings to $WWZ$ and $WW\gamma$ couplings, with the $N=1$ mode dominating due to convergence.
• The $V_1^0$ coupling is approximated as $g_{WWV}^1 \approx g_{WWZ} M_Z / (\sqrt{3} M_1^0)$, ensuring unitarity via cancellation of $E^2$ and $E^4$ growth in $WW$ scattering amplitudes.
• Backgrounds from $t\bar{t}$, $\tau^+\tau^-$, and $WW+2j$ are simulated and suppressed using: (1) $b$-jet veto in the central region, (2) $\tau\tau$ veto using $M_{\tau\tau} \in [M_Z-30, M_Z+30]$ GeV, (3) $M_{JJ} > 650$ GeV for forward jets, and (4) a minijet veto on $P_T > 20$ GeV central jets.
• Signal significance is maximized by selecting events with $P_T^{\text{max}}(e,\mu) > \text{threshold}$, where the threshold is tuned per $M_{V_1^0}$ to optimize signal-to-background ratio.
• The analysis uses Monte Carlo simulations to evaluate cut efficiencies and signal significance, with $\sqrt{s} = 14$ TeV and luminosities of 30, 100, and 300 fb⁻¹.

Experimental results

Research questions

• RQ1 Can the $V_1^0$ resonance in Higgsless models be discovered at the LHC via $WW$ fusion in the $e\mu + \not{E}_T$ channel despite large QCD and $t\bar{t}$ backgrounds?
• RQ2 How effective are forward jet tagging, $b$-jet veto, $M_{JJ}$ selection, and minijet veto in suppressing dominant $t\bar{t}$ and $WW+2j$ backgrounds?
• RQ3 Does the $P_T^{\text{max}}(e,\mu)$ distribution of the higher-transverse-momentum lepton exhibit a fat tail above background that enables discovery for high-mass $V_1^0$?
• RQ4 What is the $5\sigma$ discovery reach for $V_1^0$ as a function of mass and integrated luminosity, particularly for $\mathcal{L} = 300$ fb⁻¹?
• RQ5 How does the interplay between decreasing $VWW$ coupling ($\propto 1/M_{V_1^0}$) and increasing $P_T^{\text{max}}(e,\mu)$ for heavier $V_1^0$ affect the discovery potential?

Key findings

• With $300\ \text{fb}^{-1}$ of integrated luminosity, $5\sigma$ discovery of $V_1^0$ is possible for masses above 1 TeV, with optimal reach extending to 1.35 TeV.
• The $P_T^{\text{max}}(e,\mu)$ distribution exhibits a fat tail at high momenta that rises above the background, enabling signal enhancement for $M_{V_1^0} > 1$ TeV.
• The minijet veto improves signal significance significantly, reducing $t\bar{t}$ background survival from 90% to 32%, and is crucial for achieving high significance at high masses.
• For $\mathcal{L} = 300\ \text{fb}^{-1}$, the discovery potential improves between 800–1000 GeV due to enhanced $P_T^{\text{max}}(e,\mu)$ in high-energy decays, partially offsetting the $1/M_{V_1^0}$ coupling suppression.
• Without the minijet veto, the discovery reach remains substantial, extending to $M_{V_1^0} \approx 900$ GeV at $300\ \text{fb}^{-1}$, indicating robustness of the signal strategy.
• The requirement of at least 10 signal events limits the use of optimal $P_T^{\text{max}}(e,\mu)$ thresholds at low luminosities, causing a sharp drop in significance beyond certain masses.