[Paper Review] SUSY searches at CMS

A. Gaz|arXiv (Cornell University)|Nov 7, 2014

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper presents the results of supersymmetry (SUSY) searches conducted by the CMS Collaboration using the full 8 TeV proton-proton dataset from the LHC's Run 1, focusing on a variety of SUSY production and decay modes. Despite no significant excess over the Standard Model background, stringent exclusion limits were set across multiple simplified models and benchmark scenarios, particularly constraining stop, gluino, and electroweakino pair production.

ABSTRACT

Supersymmetry is one of the most popular theories that have been proposed to solve or alleviate the Hierarchy Problem and is one of the most active areas of research of Physics beyond the Standard Model at the LHC. In this contribution the results of recent SUSY searches performed by the CMS Collaboration on the 8 TeV dataset collected during 2012 are presented. Emphasis is given to R-parity conserving scenarios, where the Lightest Supersymmetric Particle is stable and may constitute a viable candidate for the Dark Matter in the Universe.

Motivation & Objective

• To search for supersymmetric particles in a wide range of final states using the full 8 TeV LHC dataset.
• To address blind spots in traditional stop pair searches, particularly in the region where $ m_{\tilde{t}} - m_{\tilde{\chi}^{0}_{1}} \sim m_{t} $, using dedicated analyses.
• To interpret results in both simplified models and realistic supersymmetric frameworks such as pMSSM and cMSSM.
• To improve sensitivity to electroweakino and Higgsino pair production via multilepton and multijet final states with large missing transverse energy.
• To set robust exclusion limits across diverse SUSY scenarios despite the absence of observed new physics signals.

Proposed method

• Employed the $ M_{T2} $ variable to reconstruct missing transverse momentum in multijet events, enhancing robustness against jet energy mismeasurements.
• Used signal regions defined by jet and $ b $-jet multiplicities, $ H_T $, and $ M_{T2} $ to separate signal from background.
• Applied the $ M_{CT} $ variable to identify $ \tilde{b}\tilde{b} $ final states via invariant mass reconstruction of two jets.
• Utilized topological variables such as $ E_{\mathrm{T}}^{\text{miss}} $, $ M_T(\ell, E_{\mathrm{T}}^{\text{miss}}) $, and lepton/b-jet multiplicity to suppress SM backgrounds.
• Combined multiple final states (e.g., $ \gamma\gamma $, $ bb $, $ \ell\ell $, $ jj $) in likelihood fits to improve sensitivity for Higgsino and electroweakino signals.
• Used data-driven control samples to estimate Standard Model backgrounds and validate predictions across all signal regions.

Experimental results

Research questions

• RQ1 What are the exclusion limits for stop pair production in the challenging region where $ m_{\tilde{t}} - m_{\tilde{\chi}^{0}_{1}} \sim m_{t} $, making events indistinguishable from SM $ t\bar{t} $?
• RQ2 How sensitive are inclusive $ M_{T2} $ analyses to gluino and squark pair production across different jet and $ b $-jet multiplicities?
• RQ3 What are the limits on direct $ \tilde{b}\tilde{b} $ pair production using the $ M_{CT} $ variable and $ b $-jet tagging?
• RQ4 What constraints can be placed on $ \tilde{t}_2 \to H/Z\tilde{t}_1 $ decays in simplified models with multilepton and $ b $-jet enriched final states?
• RQ5 What are the exclusion limits for higgsino pair production decaying to $ h\tilde{G} $ or $ Z\tilde{G} $, particularly in the $ \tilde{\chi}^{0}_{1} \to Z\tilde{G} $ mode?

Key findings

• No significant excess was observed over the Standard Model background in any of the analyzed final states.
• For the $ \tilde{\chi}^{0}_{1} \to Z\tilde{G} $ decay mode, higgsino pair production is excluded up to a mass of $ \sim 380 $ GeV.
• In the $ \tilde{\chi}^{0}_{1} \to h\tilde{G} $ mode, no exclusion is achieved even at 1 $ \sigma $, though multilepton and $ bb\,bb $ channels show sensitivity at low and intermediate masses.
• Exclusion limits for $ \tilde{t}_2 \to H/Z\tilde{t}_1 $ simplified models are set across various branching fractions, with the multilepton channel providing the highest sensitivity.
• The $ M_{T2} $ analysis excludes direct $ \tilde{q} $ pair production and gluino pair production with $ q\bar{q} $ decays up to masses above 1 TeV.
• The $ \tilde{b}\tilde{b} $ search sets limits consistent with the expected SM background, with no significant signal observed.