Neutralino Signatures of the Singlet Extended MSSM

TL;DR

The paper investigates singlet-extended MSSM variants (NMSSM, n/sMSSM, UMSSM, sMSSM) as solutions to the $\mu$ problem and analyzes how the enlarged neutralino sector reshapes mass spectra and cascade decays. Through parameter scans and decay calculations, it demonstrates that extra neutralinos can introduce additional cascade steps, leading to enhanced trilepton rates and the possibility of 5l or 7l final states, as well as potential displaced-vertex signatures. The n/sMSSM, in particular, features a very light singlino-like $\chi^0_1$ that modifies decay kinematics and enhances multilepton signals, while NMSSM and UMSSM can present heavy singlino or $Z'$-ino components that interplay with MSSM-like states. The results motivate targeted collider studies to distinguish these xMSSM scenarios from the MSSM by exploiting multilepton signatures and displaced vertices at the LHC/ILC.

Abstract

Extending the Higgs sector of the MSSM by the addition of a gauge singlet scalar field can remedy the mu problem. We explore the implications of extended models for both the spectrum of the neutralinos and the cascade decays of the neutralinos and charginos. Extra steps due to light decoupled neutralinos in the cascade decays of both neutralinos and charginos allow an excess of trilepton events compared to the MSSM and the existence of events with higher lepton multiplicity. Additionally, displaced vertices of the chi_2^0 due to small decay widths in some models may be observable.

Figures (6)

• Figure 1: Illustrative neutralino composition for the models in (a) a decoupled singlino scenario and (b) a strongly mixed singino scenario. Here, the MSSM contains a light Bino and Wino and heavy Higgsinos. The NMSSM has a similar spectrum, but contains an additional heavy neutralino, while the n/sMSSM has a very light extra neutralino. The UMSSM has two additional neutralinos that can intermix; their masses are strongly dependent on the singlet Higgs charge under the $U(1)'$ symmetry and the corresponding gaugino mass value. Common parameters used for this illustration are $\tan \beta = 3$, $\mu_{\rm eff} = 400$ GeV, $M_2 = 250$ GeV, and $\theta_{E_6} = {\pi\over 4}$ for the UMSSM. For (a) $\kappa=0.65$ in the NMSSM and $s = 2$ TeV for the extended models; while for (b) $\kappa=0.25$ in the NMSSM, $s = 800$ GeV for the NMSSM and n/sMSSM while the UMSSM illustration has $s= 1$ TeV to satisfy the $Z-Z'$ mixing constraints Barger:2005hb.
• Figure 2: Singlino and $Z'$ino composition of (a-c) the three lightest neutralinos and (d) the heaviest neutralino. The singlino composition is denoted with a black up triangle for the NMSSM, a red down triangle for the n/sMSSM and a green left triangle for the UMSSM, while the $Z'$ ino contribution is denoted with a blue x. The lightest neutralino is dominantly singlino in the n/sMSSM, while the UMSSM's heavier neutralinos are naturally singlino and $Z'$-ino. In the NMSSM, most of the parameter range yields a heavy neutralino that is dominantly singlino. The vertical line in (a) is the loose $\chi^0_1$ lower mass bound via constraints from $\Omega_{\chi^0_1} h^2$.
• Figure 3: Branching fractions of (a) $\chi^0_2 \to \chi^0_1 Z$ and (b)$\chi^0_2 \to \chi^0_1 H_1$ vs. the $\chi^0_2$ mass and (c) $\chi^0_3 \to \chi^0_2 Z$, (d)$\chi^0_3 \to \chi^0_2 H_1$, (e) $\chi^0_3 \to \chi^0_1 Z$, and (f)$\chi^0_3 \to \chi^0_1 H_1$ vs. the $\chi^0_3$ mass.
• Figure 4: Subprocesses that contribute to trilepton (a) and 5$l$ (b) events at hadron colliders. When the lightest neutralino is dominantly singlino, the trilepton process is enhanced kinematically since $\chi^0_2$ in the xMSSM has the same mass as $\chi^0_1$ in the MSSM with the same parameters. The 5$l$ events may be associated with $\chi^0_3$ production (or may also be due to $\chi^\pm_1 \to \chi^0_2 l \bar{\nu}_l$).
• Figure 5: Branching ratios of neutralinos and charginos to $3l,5l$ and $7l$ via on shell $Z$ and $W$ bosons vs. the neutralino mass-splitting $\Delta_{ij} = m_{\chi^0_i}-m_{\chi^0_j}$ where the leptons are summed over $e^\pm$ and $\mu^\pm$. Neutralino decays proceed through (a) $\chi^0_2 \to \chi^0_1 l \bar{l}$, (b) $\chi^0_3 \to \chi^0_1 l \bar{l}$, (c,d) $\chi^0_3 \to \chi^0_2 l \bar{l} \to \chi^0_1 + 4l$. Chargino decays occur via $\chi^\pm_1 \to \chi^0_1 W \to \chi^0_1 l \bar{\nu}_l$ in (a,b) and $\chi^\pm_1 \to \chi^0_2 W$ in (c,d). Note that $\chi^\pm_2 \to \chi^0_2 W$ can be alternate modes that contribute to a five lepton signal. Note that the branching fraction of the chargino and neutralino to trileptons in (a,b) are bounded by $Bf(Z \to l^+ l^-) \times Bf(W^\pm \to l \bar{\nu}) = 0.014$ where $l=e,\mu$.