[Paper Review] Supersymmetry production cross sections in pp collisions at sqrt{s} = 7 TeV

Michael Krämer, Anna Kulesza+5 more|arXiv (Cornell University)|Jun 13, 2012

Particle physics theoretical and experimental studiesPhysics and Astronomy38 references162 citations

TL;DR

This paper proposes a standardized framework for calculating supersymmetry (SUSY) production cross sections and their theoretical uncertainties in pp collisions at √s = 7 TeV, using next-to-leading order (NLO) and next-to-leading logarithmic (NLL) QCD corrections. It provides benchmark cross sections and uncertainty estimates for squark, gluino, stop, sbottom, and electroweak SUSY states, with uncertainties dominated by PDFs and scale variations, especially at higher masses.

ABSTRACT

This document emerged from work that started in January 2012 as a joint effort by the ATLAS, CMS and LPCC supersymmetry (SUSY) working groups to compile state-of-the-art cross section predictions for SUSY particle production at the LHC. We present cross sections for various SUSY processes in pp collisions at $\sqrt{s} =7$ TeV, including an estimate of the theoretical uncertainty due to scale variation and the parton distribution functions. Further results for higher LHC centre-of-mass energies will be collected at https://twiki.cern.ch/twiki/bin/view/LHCPhysics/SUSYCrossSections. For squark and gluino production, which dominate the inclusive SUSY cross section, we employ calculations which include the resummation of soft gluon emission at next-to-leading logarithmic accuracy, matched to next-to-leading order (NLO) SUSY-QCD. In all other cases we rely on NLO SUSY-QCD predictions.

Motivation & Objective

• To establish a consistent theoretical framework for SUSY production cross sections at the LHC, particularly for pp collisions at √s = 7 TeV.
• To reduce theoretical uncertainties in SUSY cross section predictions by employing NLO and NLL QCD corrections, improving accuracy for experimental interpretation.
• To quantify and standardize theoretical uncertainties—especially from scale variations and PDF sets—across different SUSY production channels.
• To provide a reference for ATLAS and CMS collaborations to interpret LHC data using consistent theoretical inputs.
• To extend the framework to higher center-of-mass energies (e.g., 8 TeV) as LHC data-taking progresses.

Proposed method

• Uses next-to-leading order (NLO) and next-to-leading logarithmic (NLL) QCD corrections for squark, gluino, stop, and sbottom pair production processes.
• Applies simultaneous variation of factorization and renormalization scales (0.5–2× reference scale μ, defined as average of final-state sparticle masses) to estimate scale uncertainties.
• Employs multiple PDF sets (CTEQ6.6 and MSTW2008) to assess PDF-related uncertainties, with error prescriptions applied consistently.
• Calculates cross sections for colored SUSY states (squarks, gluinos) and electroweak states (charginos, neutralinos, sleptons) in the CMSSM framework with tanβ = 10, A₀ = 0 GeV, μ > 0.
• Uses Prospino2 for handling decoupling limits in stop pair production and ensures consistency in counterterm and αs running for virtual corrections.
• Compares results across different PDF sets and scale variations to derive total theoretical uncertainties, with visualizations in the m₀–m₁/₂ mass plane.

Experimental results

Research questions

• RQ1 What are the NLO+NLL cross sections for squark and gluino pair production in pp collisions at √s = 7 TeV, and what are their dominant theoretical uncertainties?
• RQ2 How do scale and PDF uncertainties affect the interpretation of SUSY exclusion limits in the context of LHC data?
• RQ3 What are the cross sections and uncertainties for stop and sbottom pair production, and how do they differ from light-flavor squark production?
• RQ4 How do theoretical uncertainties in electroweak SUSY production (e.g., chargino/neutralino and slepton pair production) compare to those in strong production channels?
• RQ5 To what extent do PDF uncertainties dominate over scale uncertainties in high-mass SUSY scenarios?

Key findings

• Theoretical uncertainties for colored SUSY states (squarks and gluinos) are dominated by PDF uncertainties, especially at high masses, with scale uncertainties typically below 10%.
• For electroweak SUSY states such as chargino and slepton pairs, scale uncertainties are generally less than 10%, while associated production with colored states can have uncertainties up to 35%.
• PDF-related uncertainties for chargino pair production are less than 5% for both CTEQ6.6 and MSTW2008 sets, indicating good stability in these channels.
• The inclusion of NLO+NLL corrections significantly increases cross sections over leading-order predictions when scales are chosen near sparticle masses, reducing theoretical uncertainty bands.
• The framework successfully reduces theoretical uncertainties compared to earlier NLO-only estimates, enabling more precise exclusion limits and discovery potential in SUSY searches.
• A comprehensive collection of cross sections and uncertainties for various SUSY models and parameters is maintained at the SUSY cross section working group twiki page, with future extensions to 8 TeV and beyond.