Updated Bounds on the Minimal Left-Right Symmetric Model from LHC Dilepton Resonance Searches

TL;DR

The paper refines bounds on the neutral gauge boson $Z_R$ in the minimal Left-Right Symmetric Model by recasting LHC Run II dilepton data at $\sqrt{s}=13$ TeV with $139~\mathrm{fb}^{-1}$, allowing the right-handed gauge coupling $g_R$ to vary. By computing the $pp\to Z_R\to\ell^+\ell^-$ cross section at leading order and comparing to ATLAS limits, it derives $M_{Z_R}$ lower bounds ranging roughly from $4.9$ to $6.1$ TeV across $g_R$, with a benchmark $g_R=g_L$ giving $M_{Z_R}\approx 5.4$ TeV. The results show complementary information to $W_R$ searches, including coverage of the region where the right-handed neutrino is heavier than $W_R$; they also relate $Z_R$ bounds to the $W_R$ mass via a $g_R$-dependent relation. The study highlights the importance of dilepton resonances as an independent probe of the LR symmetry and constrains the high-scale right-handed sector with collider data, contributing to a more complete mapping of LRSM parameter space.

Abstract

The Left-Right model is a popular extension of the Standard Model that features three new neutral gauge bosons, $W^{\pm}_R$ and $Z_R$. Collider searches for a Left-Right symmetry are often concentrated on the charged right-handed current, but in this work, we take advantage of the dilepton data at the LHC with center-of-mass energy of 13 TeV and 139 fb$^{-1}$ of integrated luminosity to place lower mass bounds in the $Z_R$ mass based on the $p\,p \rightarrow Z_{R} \rightarrow \ell^{+} \, \ell^{-}$ process. We vary the $SU(2)_R$ gauge coupling from $g_{R}=0.4$ to $g_{R}=1.0$, and impose $M_{Z_R}>4.9$ TeV and $M_{Z_{R}}>6.1$ TeV, respectively. Lastly, we put our findings into perspective with $W_R$ searches at the LHC and show that our limits cover an unexplored region of parameter space, where the right-handed neutrino is heavier than the $W_R$ boson.

Figures (4)

• Figure 1: Total decay Width of $Z_R$ divided by its mass $M_{Z_R}$ in percentage in terms of the coupling $g_R$.
• Figure 2: Cross section p p $\to \ell^+ \, \ell^- + X$ at the center-of-mass energy $\sqrt{s} = 13$ TeV in terms of the $Z_R$ mass on the left panel, and coupling $g_R$ on the right panel. The plot on the left panel shows the limits obtained from ATLAS data analysis on dileptons extracted from Figure 3 in reference ATLAS:2019erb as the full dark blue line, in comparison with the calculated cross section with $g_R = 0.55$, $g_{R}=0.65$ and $g_{R}=1.0$ as the magenta, green and orange lines, respectively. The dotted dark blue line is the conservatively extrapolated limit obtained by keeping the ATLAS cross section constant. The plot on the right panel shows the cross section as a function of the coupling $g_R$, with the masses $M_{Z_R} = 4$, $5$, and $6$ TeV plotted as the magenta, green, and orange lines, respectively.
• Figure 3: Exclusion region of the parameter space derived from $95\%$ C.L. dilepton cross section from ATLAS at $\sqrt{s}= 13$ TeV ATLAS:2019erb. The collaboration data range from $M_{Z_R} = 1$ to 6 TeV; we have extrapolated the cross section to derive the bounds at $M_{Z_R}> 6$ TeV.
• Figure 4: Dilepton bounds on the $M_{W_R} \times M_{N_R}$ mass plane. As the gauge boson masses are related (equation \ref{['eq:mwrmzr']}) the dilepton bound (orange) represents an orthogonal and independent limit on the $W_R$ mass, namely $M_{W_R}> 3.18$ TeV for $g_R = g_L = 0.65$.The excluded region in orange is compared with the $95\%$ C.L. limits from the $p p \to W_R \to \ell\ell jj$ search channel, represented by the green ($eejj$) and blue ($\mu\mu jj$) lines extracted from Frank:2023epx. The gray regions indicate constraints from neutrinoless double beta decay; solid and dashed lines represent the KamLAND-Zen and GERDA limits, respectively