[Paper Review] A Closer Look at the 2011 cMSSM Results from CMS

S. Mrenna|arXiv (Cornell University)|Oct 18, 2011

Particle physics theoretical and experimental studies6 references3 citations

TL;DR

This paper analyzes the 2011 CMS supersymmetry search results within the constrained minimal supersymmetric standard model (cMSSM), identifying squark-squark and squark-gluino pair production as the dominant processes driving current exclusion limits. It reveals that the exclusion contours in the $m_0$-$m_{1/2}$ plane are shaped by the interplay between squark and gluino masses, with $M_{\tilde{q}} < 1$ TeV excluded regardless of gluino mass, while $M_{\tilde{g}} < 1$ TeV is only excluded for $M_{\tilde{q}} < 1.1$ TeV, and proposes new simplified models to better capture these dominant signatures.

ABSTRACT

We present a phenomenological appraisal of the results of several searches for Supersymmetry (SUSY) performed at the LHC by the CMS collaboration and interpreted in the context of the cMSSM. Part of the analysis focuses on which SUSY production processes are being probed. We observe that much of the current exclusion region is dominated by squark-squark and squark-gluino production, and explain the shape of the exclusion curves. Based on this analysis and an estimation of future reach, additional simplified models are suggested. Other phenomenological details are discussed, such as the effect of radiation on acceptance.

Motivation & Objective

• To understand which supersymmetry production processes dominate the exclusion limits in the cMSSM parameter space from the 2011 CMS data.
• To explain the shape of the exclusion curves in the $m_0$-$m_{1/2}$ plane, particularly the transition from squark-mass-dominated to gluino-mass-dominated exclusion.
• To assess the impact of initial state radiation and lepton veto on event selection efficiency in hadronic SUSY searches.
• To suggest new simplified models that better reflect the dominant production modes observed in the cMSSM interpretation.
• To evaluate the sensitivity of leptonic and hadronic searches to different sparticle decay modes and spectra.

Proposed method

• The study maps the cMSSM parameter space into simplified model spectra (SMS) to isolate dominant production processes.
• Fractional contributions of various SUSY pair production processes (e.g., $\tilde{q}\tilde{q}$, $\tilde{q}\tilde{g}$, $\tilde{g}\tilde{g}$) are computed across the $m_0$-$m_{1/2}$ plane for $\tan\beta=10$, $A_0=0$, $\text{sgn}(\mu)>0$.
• Monte Carlo simulations using Pythia6 are used to model sparticle production, decay, and parton showering, with jet reconstruction via FastJet.
• Event selection efficiency is evaluated by applying CMS-like cuts (e.g., Jets+mHT, $\alpha_T$, lepton veto) using Monte Carlo truth information.
• The role of initial state radiation is quantified by comparing event rates with and without ISR effects in the signal regions.
• Contour plots of physical masses ($M_{\tilde{q}}$, $M_{\tilde{g}}$) are overlaid on exclusion plots to correlate exclusion limits with physical masses.

Experimental results

Research questions

• RQ1 Which SUSY production processes—squark-squark, squark-gluino, gluino pair, etc.—dominate the exclusion limits in the cMSSM for the 2011 CMS data?
• RQ2 Why do the exclusion contours in the $m_0$-$m_{1/2}$ plane follow semi-circular shapes at low $m_0$ and flat contours at high $m_0$?
• RQ3 How does initial state radiation affect the acceptance of SUSY signals in the Jets+mHT and $\alpha_T$ analyses?
• RQ4 To what extent do current CMS leptonic searches probe the same regions of parameter space as hadronic searches?
• RQ5 What simplified model topologies should be prioritized in future searches to better reflect the dominant processes in the cMSSM?

Key findings

• Squark-squark and squark-gluino pair production are the dominant processes contributing to the current exclusion limits in the cMSSM, especially in the $m_0$-$m_{1/2}$ plane for $\tan\beta=10$, $A_0=0$, $\text{sgn}(\mu)>0$.
• The exclusion curve follows contours of fixed squark mass ($M_{\tilde{q}}$) for low $m_0 < 1$ TeV and fixed gluino mass ($M_{\tilde{g}}$) for high $m_0$, explaining the observed shape of the exclusion region.
• A squark mass below 1 TeV is excluded across all gluino masses, while a gluino mass below 1 TeV is only excluded when the squark mass is below 1.1 TeV.
• The Jets+mHT and $\alpha_T$ analyses are most sensitive to $\tilde{q}\tilde{q}$ and $\tilde{q}\tilde{g}$ production, with significant overlap (20–50%) between the two selections.
• The lepton veto reduces event acceptance by up to 50% in some regions, particularly affecting leptonic final states.
• Initial state radiation is more relevant than final state radiation for signal acceptance, except for gluino masses below 300–400 GeV.