Very Light Scalar Top Quarks at the LHC

TL;DR

The paper investigates very light stops within the MSSM framework, examining decay channels FV, 3B, 4B, and the conventional 2B, under MFV and for mass ranges where m_stop ≤ m_t + m_chi1^0. It combines indirect constraints from electroweak precision and flavour physics with collider studies, computing decay branching fractions and mapping phase-space dominance. Using 7 TeV LHC analyses in a simplified-model approach, it derives exclusions in the m_stop–m_chi1^0 plane and discusses how future 8 TeV data and more targeted searches could significantly enhance sensitivity. The work concludes that current LHC bounds challenge very light unmixed stops relevant for electroweak baryogenesis, and that MFV allows either FV or 4B decays to dominate, motivating dedicated searches for these channels and potential model extensions to reopen very light-stop scenarios.

Abstract

A very light scalar top (stop) superpartner is motivated by naturalness and electroweak baryogenesis. When the mass of the stop is less than the sum of the masses of the top quark and the lightest neutralino superpartner, as well as the of the masses of the lightest chargino and the bottom quark, the dominant decay channels of the stop will be three-body, four-body, or flavour violating. In this work, we investigate the direct and indirect constraints on a light stop, we compute the relative decay branching fractions to these channels, and we study the sensitivity of existing LHC searches to each of them.

Figures (5)

• Figure 1: Allowed light stop mass and mixing angles for $\tan\beta=10$.
• Figure 2: Dominant stop decay modes of a stop with mass $m_{\tilde{t}} = 225\,\, {\rm GeV}$ for $\tan\beta=10$, with $\cos\theta_{\tilde{t}} = 0.1$ (left) and $\cos\theta_{\tilde{t}} = 0.7$ (right). The four regions correspond to dominant decays by the 2B (gray), 3B (red), 4B (yellow) and FV (green) channels. We also show the kinematic boundaries between these decay channels by solid black lines.
• Figure 3: Dominant stop decay modes of a stop with mass $m_{\tilde{t}} = 225\,\, {\rm GeV}$ for $\tan\beta=30$, with $\cos\theta_{\tilde{t}} = 0.1$ (left) and $\cos\theta_{\tilde{t}} = 0.7$ (right). The four regions correspond to dominant decays by the 2B (gray), 3B (red), 4B (yellow) and FV (green) channels. We also show the kinematic boundaries between these decay channels by solid black lines.
• Figure 4: ATLAS and CMS exclusions on FV mode (left) and 4B mode (right). The two straight, diagonal lines correspond to $m_{\tilde{t}}=m_{\chi_{1}^{0}}$ (upper, FV), $m_{\tilde{t}}=m_{\chi_{1}^{0}}+m_b$ (upper, 4B), and $m_{\tilde{t}}=m_{\chi_{1}^{0}}+m_b+m_W$ (lower, both). The dashed/dotted lines correspond to varying the cross section by $\pm50\%$. The upper straight lines are the kinematic limits for these modes, while below the lower straight line we expect the 3B channel to take over.
• Figure 5: CMS exclusions on 3B mode. The dashed/dotted lines correspond to varying the cross section by $\pm50\%$. The upper solid diagonal line shows the kinematic limit for this decay, $m_{\tilde{t}}=m_{\chi_{1}^{0}}+m_{b}+m_{W}$, while the lower solid diagonal line has $m_{\tilde{t}}=m_{\chi_{1}^{0}}+m_t$ where the 2B decay channel is expected to take over.