Les Houches 2013: Physics at TeV Colliders: Standard Model Working Group Report

Abstract: This Report summarizes the proceedings of the 2013 Les Houches workshop on Physics at TeV Colliders. Session 1 dealt primarily with (1) the techniques for calculating standard model multi-leg NLO and NNLO QCD and NLO EW cross sections and (2) the comparison of those cross sections with LHC data from Run 1, and projections for future measurements in Run 2.

• The paper introduces an algorithm that incorporates electroweak Sudakov corrections into ALPGEN for Z/γ plus jets processes, refining background modeling in high-energy regimes.
• It utilizes a factorization approach to manage double and single logarithmic contributions, achieving percent-level precision in tree-level amplitude calculations.
• The results demonstrate significant corrections, with reductions up to 40% for Z + 2 jets and 20% for γ + 2 jets, which are essential for optimizing LHC Run II analyses.

Overview of Electroweak Sudakov Corrections to $Z/\gamma +$ Jets at the LHC

The production of vector bosons accompanied by jets ($Z/\gamma + n$ jets) is a significant background in searches for new physics phenomena at the LHC, particularly in studies involving missing transverse momentum. As the LHC seeks to explore energy scales well above the electroweak scale, electroweak (EW) corrections, especially those enhanced by Sudakov logarithms, become increasingly important. This prompts the need for precise theoretical predictions accounting for these corrections to maintain the sensitivity and accuracy of New Physics searches.

This paper addresses the electroweak Sudakov corrections to the processes involving $Z/\gamma$ bosons and jets at the LHC. The implementation leverages an algorithm that integrates EW Sudakov corrections into the ALPGEN simulation framework, focusing on the $Z(\to \nu \bar{\nu}) + n$ jets and $\gamma + n$ jets production channels.

Implementation Details

The approach taken relies on the factorization of EW corrections based on the external legs of the process, with a specific emphasis on managing double and single logarithmic contributions. Virtual corrections manifest as orders of magnitude larger in processes with substantial transverse momentum. The algorithm fits within the ALPGEN environment by calculating tree-level amplitudes in the unitary gauge, ensuring compatibility with the EW corrections. The accuracy, while targeting percent-level precision, successfully validates against available literature and cross-verifications with the GOSAM package.

Results and Impact

Numerical results demonstrate the significant impact of EW Sudakov corrections, particularly for the high transverse momentum regime. For instance, in the case of $Z(\to \nu \bar{\nu}) + 2$ jets and $\gamma + 2$ jets production, the electroweak corrections at high $m_{\text{eff}}$ values reach reductions of up to 40% and 20%, respectively. This differential impact reflects in the reduced ratio $R_\gamma^2$ and $R_\gamma^3$ for two and three-jet scenarios, aligning with configurations expected in LHC Run II analyses at 14 TeV.

The algorithm efficiently accounts for the asymmetries in the EW corrections across processes, thereby enhancing the theoretical uncertainty management essential for calibrating irreducible backgrounds within high-energy (TeV) searches. As the LHC progresses to higher luminosity and energy, precision in estimating these background processes becomes crucial for the accurate identification of signals indicative of Beyond Standard Model physics.

In conclusion, this study substantially advances the understanding and implementation of electroweak Sudakov corrections for vector boson plus jets events at the LHC, offering a robust framework that could be extended to broader electroweak processes. This enhances the theoretical tools available for ongoing and future collider analyses, directly impacting the search strategies for new physics at the energy frontier.