`Stop' that ambulance! New physics at the LHC?

TL;DR

The paper investigates persistent excesses in $W^+W^-$ cross sections and SUSY-like multilepton final states at the LHC. It tests a simplified SUSY scenario with a light stop, wino-like electroweak states, and a bino LSP, performing a global fit to WW and SUSY search results using recasting tools. The fit yields a significant improvement over the SM, with best-fit masses around $m_{ ilde{t}_1}\approx 200$ GeV and $m_{ ilde{\chi}^0_1}\approx 140$ GeV, and indicates stop production and chargino–neutralino production can explain di-lepton and tri-lepton excesses. Predictions for 13 TeV runs and discriminants are discussed as tests for this explanation.

Abstract

A number of LHC searches now display intriguing excesses. Most prominently, the measurement of the $W^+W^-$ cross-section has been consistently $\sim 20\%$ higher than the theoretical prediction across both ATLAS and CMS for both 7 and 8 TeV runs. More recently, supersymmetric searches for final states containing two or three leptons have also seen more events than predicted in certain signal regions. We show that a supersymmetric model containing a light stop, winos and binos can consistently match the data. We perform a fit to all measurements and searches that may be sensitive to our model and find a reduction in the log-likelihood of 15.4 compared to the Standard Model which corresponds to 3.5-$σ$ once the extra degrees of freedom in the fit are considered.

Figures (4)

• Figure 1: The distribution of $-2\ln L$ as a function of the masses of the stop, $m_{\tilde{t}_1}$, and lightest neutralino, $m_{\tilde{\chi}^0_1}$. All of the signal regions given in the $W^+W^-$ measurements, any searches with di-lepton final states and the stop searches, table \ref{['tab:exp_analyses']} are included in the fit. Also shown are the 95% CLs exclusion lines given by the dedicated Atlas search for SUSY electroweak states Aad:2014vma and the dedicated Atlas stop search Aad:2014qaa.
• Figure 2: The distribution of $-2\ln L$ as a function of the masses of the lightest chargino, $m_{\tilde{\chi}_1^{\pm}}$, and lightest neutralino, $m_{\tilde{\chi}^0_1}$. All signal regions in the Atlas tri-lepton search for charginos and neutralinos are used in the fit Aad:2014nua. Also shown are the observed and expected 95% CLs exclusion lines for the same search.
• Figure 3: The distribution of $-2\ln L$ as a function of the masses of the stop, $m_{\tilde{t}_1}$, and lightest neutralino, $m_{\tilde{\chi}^0_1}$. All of the signal regions given in the $W^+W^-$ measurements, di-lepton and tri-lepton searches, table \ref{['tab:exp_analyses']} are included in the fit. Also shown are the 95% CLs exclusion lines given by the dedicated Atlas di-lepton Aad:2014vma, tri-lepton Aad:2014nua and stop Aad:2014qaa searches.
• Figure 4: (a) The distribution of $\cos\theta_{\ell\ell}^*$ for leptons produced from SM $W^+W^-$ and $t\bar{t}$ events, and from $\tilde{t}_1\tilde{t}_1^*$ events for our best fit point. (b) The significance of distinguishing the SM-only and SM+$\tilde{t}_1\tilde{t}_1^*$ case as a function of an integrated luminosity using the asymmetry eq. \ref{['eq:asymmetry']}. The red curve shows the significance with the cut eq. \ref{['eq:sqrtshat']}, while the black curve without the $\sqrt{\hat{s}}_\text{min}$ cut.