Light Stop Searches at the LHC in Events with One Hard Photon or Jet and Missing Energy

TL;DR

The paper tackles the challenge of probing light stops in the MSSM, which are motivated by dark matter via stop–neutralino co-annihilation and by electroweak baryogenesis. It introduces stop production in association with a hard photon or jet at the LHC as a robust search strategy in the co-annihilation regime, and analyzes γ+MET and jet+MET channels using existing background studies and detector simulations. The results show that γ+MET can reach up to ~160 GeV in m_{̃t1} (with systematics considered) and jet+MET extends sensitivity beyond ~200 GeV, with Tevatron data providing complementary constraints; stops produced in gluino decays offer a further discovery path up to m_{̃g} ≈ 900 GeV at 30 fb⁻¹. The study also demonstrates that charm-tagging of the soft charm jets from ̃t1 decays can significantly enhance discovery reach and enable stop identification, though a full detector-level analysis is needed for robust mass determinations.

Abstract

Low energy supersymmetric models provide a solution to the hierarchy problem and also have the necessary ingredients to solve two of the most outstanding issues in cosmology: the origin of the baryon asymmetry and the source of dark matter. In the MSSM, weak scale generation of the baryon asymmetry may be achieved in the presence of light stops, with masses lower than about 130 GeV. Moreover, the proper dark matter density may be obtained in the stop-neutralino co-annihilation region, where the stop-neutralino mass difference is smaller than a few tens of GeV. Searches for scalar top quarks (stops) in pair production processes at the Tevatron and at the Large Hadron Collider (LHC) become very challenging in this region of parameters. At the LHC, however, light stops proceeding from the decay of gluino pairs may be identified, provided the gluino mass is smaller than about 900 GeV. In this article we propose an alternative method for stop searches in the co-annihilation region, based on the search for these particles in events with missing energy plus one hard photon or jet. We show that this method is quite efficient and, when complemented with ongoing Tevatron searches, allows to probe stop masses up to about 160 GeV, fully probing the region of parameters consistent with electroweak baryogenesis in the MSSM.

Figures (6)

• Figure 1: Distribution of the photon transverse energy, $E_{\rm T,\gamma}$, for the SM background (from Ref. cmsgemiss) and the stop signal with $m_{\tilde{t_{1}}}$=130 GeV and $m_{\tilde{\chi}^0_{1}}$=110 GeV. The fluctuations at the right end of the plot are due to Monte Carlo errors.
• Figure 2: Projected LHC 5$\sigma$ discovery reach in the $\gamma+E\space/\,_{\rm T}$ channel without (left) and with (right) systematic errors. For comparison the current and future Tevatron 95% C.$\,$L. exclusion bounds for light stops are also shown.
• Figure 3: Projected LHC 5$\sigma$ discovery reach in the jet$+E\space/\,_{\rm T}$ channel. For comparison the current and future Tevatron 95% C.$\,$L. exclusion bounds for light stops are also shown.
• Figure 4: Projected LHC reach in the $\tilde{g} \tilde{g} \to t t {\tilde{t}_1}^* {\tilde{t}_1}^* \; (\bar{t} \bar{t} {\tilde{t}_1} {\tilde{t}_1})$ channel. The errors bars indicate Monte Carlo errors.
• Figure 5: Jet mass distribution for charm jets from stop decays for different $\Delta m$, compared to light flavor jets from ISR.