Light stops emerging in WW cross section measurements?

TL;DR

The paper investigates whether a light top squark pair production could account for the mild excess observed in WW cross-section measurements by ATLAS and CMS. Using a simplified stop–neutralino framework with a small stop–chargino mass splitting, it scans the stop parameter space and identifies regions that can fit the WW data while remaining consistent with direct-stop searches and Higgs/low-energy observables. It also proposes a discriminant based on angular correlations (cos_theta_ll) and a kinematic cut (sqrt_hat_s_min) to distinguish SUSY from SM WW production, projecting potential evidence with current and future LHC data. If validated, this scenario would link the WW anomaly to SUSY naturalness scales and motivate targeted analyses; if not, the findings translate into refined stop-search constraints and guidance for future colliders.

Abstract

Recent ATLAS and CMS measurements show a slight excess in the WW cross section measurement. While still consistent with the Standard Model within 1-2 sigma, the excess could be also a first hint of physics beyond the Standard Model. We argue that this effect could be attributed to the production of scalar top quarks within supersymmetric models. The stops of mstop_1 ~ 200 GeV has the right pair-production cross section and under some assumptions can significantly contribute to the final state of two leptons and missing energy. We scan this region of parameter space to identify stop mass range preferred by the WW cross section measurements. Taking one sample benchmark point we show that it can be consistent with low energy observables and Higgs sector measurements and propose a method to distinguish supersymmetric signal from the Standard Model contribution.

Figures (5)

• Figure 1: The $\chi^2$, eq. \ref{['eq:chi2']}, distributions in the $(m_{\tilde{t}_{1}}, m_{\tilde{\chi}^0_{1}})$ plane for each of the measurements, ATLAS7, CMS7 and CMS8. In panel (d), the sum of $\chi^2$s for the three measurements is shown. Blue areas represent the lowest values of $\chi^2$ and the region preferred by the experiments. A green dashed line indicates the kinematical threshold for $\tilde{\chi}^\pm_{1} \to W^\pm \tilde{\chi}^0_{1}$ decay. The shaded region below a black line is excluded by the ATLAS direct search ATLAS-CONF-2013-048. A dashed purple line shows a $68\%$ CL region.
• Figure 2: Distributions of: (a) the leading lepton transverse momentum $p_T^{\mathrm{max}}$, (b) the trailing lepton transverse momentum $p_T^{\mathrm{min}}$, (c) the dilepton system transverse momentum $p_T^{\ell\ell}$, and (d) the dilepton invariant mass $m_{\ell\ell}$. The SM, Higgs and stop contributions are shown separately. The genuine stop contribution is also depicted for comparison and multiplied by factor 5 for convenience. The SM event numbers, data points and uncertainties are taken from ref. Chatrchyan:2013oev. Note that we display the expected number of SM events, i.e. rescaled compared to figure 1 of ref. Chatrchyan:2013oev. We follow a convention proposed in ref. Curtin:2012nn in presenting this plot.
• Figure 3: (a) The polar angle, $\cos\theta^*$, of the initially produced $W^+W^-$, $t \bar{t}$ and stop pairs in the center-of-mass of the hard process frame. (b) The pseudorapidity difference of the lepton pair, $\cos\theta^*_{\ell\ell}$ eq. \ref{['eq:costhstar']}, for the $W^+W^-$, $t \bar{t}$ and stop events.
• Figure 4: (a) The $\sqrt{\hat{s}}_\text{min}$ distribution for the $W^+W^-$, $t\bar{t}$ and $\tilde{t}_1\tilde{t}_1^*$ events. (b) The pseudorapidity difference of the lepton pair, $\cos\theta^*_{\ell\ell}$ eq. \ref{['eq:costhstar']}, for the $W^+W^-$, $t \bar{t}$ and stops after the selection $\sqrt{\hat{s}}_\text{min} > 150 \ \mathrm{GeV}$.
• Figure 5: 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 $\sqrt{\hat{s}}_\text{min} > 150 \ \mathrm{GeV}$, while the black curve without the $\sqrt{\hat{s}}_\text{min}$ cut. Different $pp$ center-of-mass energies are shown for comparison.