[Paper Review] Dilepton + jet signature of Split-UED at the LHC

Swarup Kumar Majee, Seong Chan Park|arXiv (Cornell University)|Jan 28, 2013

Particle physics theoretical and experimental studies54 references3 citations

TL;DR

This paper proposes the dilepton + jet ($\ell^+\ell^-j$) signature as a powerful probe for new physics in the split-universal extra dimension (split-UED) model at the LHC. By leveraging a high $p_T^{\text{jet}}$ cut and invariant mass constraints, the signal from $Z_2$ gauge boson production with direct decay into $\ell^+\ell^-$ is efficiently separated from SM backgrounds, enabling discovery reach up to several TeV with $\sim100~\text{fb}^{-1}$ luminosity.

ABSTRACT

We study the signature of dilepton and a hard jet ($\ell^+\ell^-j$) via heavy new gauge boson production in split universal extra dimension scenario where the Kaluza-Klein parity is conserved but the Kaluza-Klein number is not. A hard cut to the jet energy effectively removes virtually all possible backgrounds and provides a handle to search of new physics involving new neutral heavy states as the Kaluza-Klein $Z$ boson. The signature can be more generically used in search of other new states such as graviton and radion in warped extra dimension models.

Motivation & Objective

• To identify a clean, high-significance signature for new physics in the split-UED model at the LHC.
• To address the challenge of distinguishing heavy gauge boson signals from dominant SM backgrounds in high-energy collider searches.
• To demonstrate that the $\ell^+\ell^-j$ final state with high jet transverse momentum can effectively suppress SM backgrounds due to the absence of missing energy and high-energy leptons.
• To extend the applicability of this signature to other models with heavy neutral states, such as warped extra dimension models with gravitons or radions.
• To provide a parton-level framework for future detailed detector-level analyses of $Z_2$ gauge boson production in split-UED.

Proposed method

• The study analyzes the $pp \to j\ell^+\ell^-$ process at parton level, focusing on the production of the second Kaluza-Klein excitation of the weak gauge boson ($\gamma_2$) in split-UED.
• A high transverse momentum cut ($p_T^{\text{jet}} > 200~\text{GeV}$) is applied to suppress SM backgrounds dominated by $Z$-boson and top-quark contributions.
• The dilepton invariant mass is constrained to be greater than 90% of the $\gamma_2$ mass to enhance signal purity.
• Additional kinematic cuts are applied: $|\eta| < 2.5$ for leptons and $\Delta R > 0.7$ between leptons and the jet to ensure isolation.
• SM backgrounds are estimated using MadGraph, with dominant contributions from $t\bar{t}$, $b\bar{b}$, $t\bar{b}$, $b\bar{t}$, $Z\gamma^*$, and $ZZ$ processes.
• Signal significance is evaluated using the $95\%$ confidence level criterion $\sigma = \sqrt{S/(S+B)}$ to determine required luminosity for discovery.

Experimental results

Research questions

• RQ1 Can the $\ell^+\ell^-j$ final state serve as a clean signature for new physics in split-UED with a conserved $\mathbb{Z}_2$ symmetry but non-conserved KK number?
• RQ2 To what extent can high $p_T^{\text{jet}}$ and dilepton invariant mass cuts suppress SM backgrounds in this channel?
• RQ3 What is the achievable discovery reach for the $\gamma_2$ gauge boson in terms of mass and luminosity in this signature?
• RQ4 How does the bulk mass parameter $\mu$ influence the signal cross section and detectability in this final state?
• RQ5 Can this signature be generalized to other models with heavy neutral states, such as gravitons or radions in warped extra dimensions?

Key findings

• The $\ell^+\ell^-j$ signature with high jet $p_T$ and invariant mass cuts effectively suppresses SM backgrounds, including $t\bar{t}$, $Z$-jets, and $ZZ$ processes, due to the absence of missing energy and high-energy final-state particles.
• Signal events increase with the bulk mass parameter $\mu$, as larger $\mu$ enhances the coupling strength, while they decrease with increasing $R^{-1}$ (i.e., heavier $\gamma_2$ mass).
• With $\sim100~\text{fb}^{-1}$ of integrated luminosity, the model can probe $\gamma_2$ masses up to several TeV, depending on the bulk mass parameter.
• The required luminosity for $95\%$ confidence level discovery decreases with increasing $\mu$, indicating better sensitivity for larger coupling strengths.
• The analysis shows that $\gamma_2$ production with direct decay into $\ell^+\ell^-$ is a viable and distinct signal in split-UED, differing from R-parity-conserving models that produce missing energy.
• The signature is generic and applicable to other models with heavy neutral states, such as the graviton or radion in warped extra dimension models, where similar final states arise from resonance decays.