Inadequacy of zero-width approximation for a light Higgs boson signal

Abstract: In the Higgs search at the LHC, a light Higgs boson (115 GeV <~ M_H <~ 130 GeV) is not excluded by experimental data. In this mass range, the width of the Standard Model Higgs boson is more than four orders of magnitude smaller than its mass. The zero-width approximation is hence expected to be an excellent approximation. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalisation at the 1% precision level. For gg (-> H) -> VV, V= W,Z, O(10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to exclude this region in search channels where the Higgs invariant mass cannot be reconstructed. We note that the H -> VV decay modes in weak boson fusion are similarly affected.

• The paper demonstrates that off-shell contributions in gluon-gluon fusion produce corrections of about 10% to the Higgs signal, challenging the ZWA.
• It reveals a roughly 5% discrepancy in production cross-sections when including off-shell effects compared to the narrow-width approximation.
• The analysis recommends using transverse mass variables to adjust experimental cuts and improve the accuracy of Higgs boson measurements.

Inadequacy of the Zero-Width Approximation for Light Higgs Boson Signal

The paper by Kauer and Passarino critically evaluates the reliability of the zero-width approximation (ZWA) in describing light Higgs boson signals, particularly in the context of Higgs searches at the Large Hadron Collider (LHC). Traditionally, the narrow-width approximation has been deemed a valid simplification for the Standard Model (SM) Higgs boson, especially given its relatively small width compared to its mass. However, this work challenges the assumption that ZWA is invariably adequate across all scenarios, especially for theoretical predictions aimed at precision measurements.

Key Findings

• Off-shell Contributions: For gluon-gluon fusion leading to Higgs production and subsequent decay into vector bosons ($gg \to H \to VV$, where $V = W, Z$), off-shell effects have been shown to introduce corrections of order 10%, disrupting the expectation of ZWA validity. These include enhancements in regions where the invariant mass of the vector boson pairs surpasses twice their mass, generating significant Higgs-continuum interference.
• Impact on Cross-Sections: The paper reveals that for light Higgs boson masses, the production and decay cross-sections are particularly sensitive to the inclusion of such off-shell regions. Specifically, discrepancies between predictions using ZWA and those incorporating off-shell effects have been observed at the 5% level, a significant deviation in precision contexts.
• Experimental Implications: The analysis presented suggests that currently employed experimental cuts must be adapted to exclude regions contributing to the ZWA's inadequacies. During Higgs searches, especially where the Higgs invariant mass cannot be reconstructed directly, transverse mass variables should be used to effectively limit contributions from misleading off-shell regions.

Theoretical and Practical Implications

The insights from this study underscore the necessity to refine theoretical models used in experimental analyses of the Higgs boson. This includes the required precision in estimating Higgs signal strengths and decay widths. By adjusting selection criteria and incorporating off-shell regions cautiously, experimentalists can mitigate the overestimated signal strength due to misleading ZWA. This refinement is crucial not only for confirming the Higgs boson's properties but also for potential new physics searches beyond the SM, where exotic scalar resonance production might mimic similar kinematic features.

Future Directions

This paper paves the way for future theoretical refinement and experimental strategy adaptations. Ongoing efforts are needed to compute next-to-leading order (NLO) corrections and resummation effects with even greater accuracy, incorporating full off-shell dynamics and signal-background interference comprehensively. As LHC data accrues, leveraging high-statistics samples will allow experimentalists to probe discrepancies at finer scales, thereby robustly testing the theoretical models.

In conclusion, Kauer and Passarino highlight a critical aspect of high-energy physics at the LHC by revealing the limitations of conventional approximations and reinforcing the need for precision-tailored methodologies in understanding fundamental particle dynamics.