Quark-Lepton Unification Signatures

TL;DR

The paper analyzes a minimal low-scale quark-lepton unification framework based on $SU(4)_C \otimes SU(2)_L \otimes U(1)_R$, predicting a vector leptoquark, two scalar leptoquarks, a color-octet scalar, and an extra Higgs doublet. Neutrino masses arise via the inverse seesaw with singlets $S_L$, enabling TeV-scale unification while keeping light neutrinos natural, and leading to heavy neutrino–leptoquark interactions that shape collider signatures. The authors compute production cross sections and branching fractions for the scalar LQs, emphasize decays to third-generation fermions (and to heavy neutrinos when allowed), and assess LHC constraints using the Contur framework, finding that current data exclude portions of parameter space (e.g., $M_{\Phi_3}, M_{\Phi_4} \sim 0.7$–$1.1$ TeV depending on couplings) while leaving ample room for discovery, particularly at the HL-LHC. The results highlight distinctive signatures such as $\tau$, top quark, and missing energy final states, and illustrate how heavy-neutrino decays can dilute MET signatures, guiding future searches for low-scale quark-lepton unification.

Abstract

We investigate the collider signatures of the minimal framework for quark-lepton unification at a scale not far from the electroweak symmetry breaking scale. This theory predicts a rich spectrum of new fields, including one vector leptoquark, two scalar leptoquarks, a color-octet scalar, and an additional Higgs doublet. Neutrino masses are generated via the inverse seesaw mechanism, facilitating viable matter unification at the low scale. We find that this theory predicts that in many cases the dominant leptoquark decays are to the third generation Standard Model quarks and leptons. We identify key experimental signatures at the Large Hadron Collider, evaluate and discuss the limits from current measurements, and outline potential strategies for probing this theory in the near future.

Figures (8)

• Figure 1: a) QCD Pair production cross section for the scalar LQ at $\sqrt{s}= 13$ TeV, as a function of LQ mass. b) Production cross section of a scalar LQ and SM lepton as a function of LQ mass when $y_4=1$. The cross-section was computed using our UFO model and MadGraph5Alwall_2014.
• Figure 2: Branching ratios of scalar LQ decays as a function of LQ mass. Fig. 2a) shows the Branching ratios of $\phi_3^{1/3}$ and Fig. 2b) shows the Branching ratios of $\phi_3^{-2/3}$. Fig. 2c) shows the branching ratios of $\phi_4^{2/3}$. Here, for illustration, we assumed $M_N = 500$ GeV, $y_4=1$, and $y_2=1$ in all scenerios.
• Figure 3: Branching ratios of 3rd generation right-handed neutrinos as a function of mass. Here, we assumed a) $y_2 = y_4 =1$ and b) $y_2 = y_4 = 0.01$. In both scenarios, we fix $M_{\phi^{1/3}_3}=M_{\phi^{-2/3}_3}=1$ TeV and assume the mixing between the SM neutrinos and right-handed neutrinos to be $V_{\nu N}=10^{-4}$.
• Figure 4: Contur exclusion plot of $M_{\Phi_3}$ against $y_4$ at $y_2=0$. The solid black line indicates the 95% exclusion and the dashed black line the 68% exclusion. The dotted black line shows the expected exclusion, where the data exactly coincide with the SM, and the dotted red line is an estimate of the eventual sensitivity after 3 ab$^{-1}$ of integrated luminosity.
• Figure 5: a) The di-$\tau$ invariant mass measurement (see text) showing the predicted signal for the point in Fig. \ref{['fig:m3_y4_y2is0']} with $y_4 = 0.13$ and $M_{\Phi_3} = 944$ GeV. b) The recoil $p_\mathrm{T}$ ratio (see text) showing the predicted signal for the point in Fig. \ref{['fig:m3_y4_y2is0']} with $y_4 = 0.13$ and $M_{\Phi_3} = 600$ GeV.