Light neutralinos at LHC in cosmologically-inspired scenarios: new benchmarks in the search for supersymmetry

TL;DR

This work investigates light neutralinos with $m_{
} < 50$ GeV as LSP candidates in SUSY models with non-unified gaugino masses, analyzing the neutralino spectrum and identifying normal, inverted, and nearly degenerate spectroscopic schemes. It links cosmological relic-density constraints, via $\ \Omega_{
} h^2 \leq 0.124$, to two benchmark scenarios (A and B) with distinct parameter sets, and develops collider benchmarks to study squark-initiated decay chains at the LHC. The study computes branching ratios for sequential and branched decay chains, constructs representative benchmarks (A-seq1, A-seq2, B-seq, A-brc, B-brc1, B-brc2), and estimates event yields at $\sqrt{s}=14$ TeV with $L=200$ fb$^{-1}$, using ISASUSY and PROSPINO. The results indicate that Scenario A provides strong, accessible signals in both sequential and branched channels, while Scenario B requires higher statistics to observe branched decays; overall the work demonstrates the LHC’s potential to probe a cosmologically motivated region of SUSY parameter space and informs target benchmarks for experimental analyses.

Abstract

We study how the properties of the four neutralino states, chi_i (i = 1, 2, 3, 4), can be investigated at the Large Hadron Collider (LHC), in the case when the lightest one, chi_1, has a mass m_chi < 50 GeV and is stable. This situation arises naturally in supersymmetric models where gaugino masses are not unified at a Grand Unified (GUT) scale and R-parity is conserved. The main features of these neutralino states are established by analytical and numerical analyses, and two scenarios are singled out on the basis of the cosmological properties required for the relic neutralinos. Signals expected at LHC are discussed through the main chain processes started by a squark, produced in the initial proton-proton scattering. We motivate the selection of some convenient benchmarks, in the light of the spectroscopical properties (mass spectrum and transitions) of the four neutralino states. Branching ratios and the expected total number of events are derived in the various benchmarks, and their relevance for experimental determination of neutralino properties is finally discussed.

Figures (17)

• Figure 1: Neutralino masses as functions of $M_2$ for $M_1=25$ GeV, $\mu=300$ GeV and $\tan\beta=$10. .
• Figure 2: Compositions of the neutralino states $\chi_i$ as functions of $M_2$ for $M_1=25$ GeV, $\mu=300$ GeV and $\tan\beta=$10. Up--left panel: $\chi_1$, up--right panel: $\chi_2$, bottom--left panel: $\chi_3$, bottom--right panel: $\chi_4$. Solid lines denote $|a_1^{i}|$, dotted lines $|a_2^{i}|$, short--dashed lines $|a_3^{i}|$, long--dashed lines $|a_4^{i}|$. .
• Figure 3: Asymptotic spectroscopic schemes for the $\chi_i$ ($i=1,2,3,4$) neutralino states .
• Figure 4: Qualitative schemes for benchmarks in sequential decay chains: $\mathcal{A}$--seq1, $\mathcal{A}$--seq2, $\mathcal{B}$--seq. For each benchmark, extremes values for $M_2$ are considered: $M_2 \sim |\mu|$ and $M_2 > |\mu|$ for scenario $\mathcal{A}$; $M_2 < |\mu|$, $M_2 \sim |\mu|$ and $M_2 > |\mu|$ for scenario $\mathcal{B}$. .
• Figure 5: Branching ratios for the sequential process $\tilde{q} \rightarrow \bar{e} e \chi_1$ in benchmark $\mathcal{A}$--seq1 as functions of $M_2$. Each panel corresponds to a different value of $m_{\tilde{l}}$. The dashed lines show the branching ratio for the process $\tilde{q} \rightarrow q \chi_{4}$; the thin--dotted lines denote the branching ratio for the process $\chi_{4}\rightarrow e \tilde{e}_L\rightarrow e\bar{e}\chi_1$; the thick--dotted line in the top--left panel denotes the branching ratio for the process $\chi_{4}\rightarrow e \tilde{e}_R\rightarrow e\bar{e}\chi_1$ (the corresponding curves cannot be seen in the other panels because they are too low); the thick solid lines denote the branching ratio for the whole sequential decay chain $\tilde{q}\rightarrow q \chi_{4}\rightarrow q e \tilde{e}\rightarrow q e \bar{e}\chi_1$.