Higgs Boson Width Measurement Refines Standard Model Precision at LHC.

Measurements of the Higgs boson width, crucial for verifying the Standard Model, benefit from combined on and off-shell production analyses. This research compares theoretical predictions for off-shell production using Powheg, MadGraph, and Sherpa, assessing the impact of higher-order chromodynamics and jet merging techniques to refine future experimental measurements.

Precise determination of the Higgs boson’s properties remains a central goal of particle physics, offering stringent tests of the Standard Model and insights into the mechanism of electroweak symmetry breaking. While direct measurements of the Higgs boson width are hampered by experimental limitations, indirect measurements via analyses of both on-shell and, crucially, off-shell Higgs boson production offer a pathway to increased precision. Off-shell production, where the produced Higgs boson’s decay products do not precisely match its mass, presents a theoretical challenge due to the breakdown of simplifying approximations and interference with background processes. A collaborative effort led by Rafael Coelho Lopes de Sá from the University of Massachusetts Amherst, alongside Martina Javurkova of Matej Bel University, and Matteo Lazzeretti and Raoul Röntsch from the University of Milan and the Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Milano, addresses these challenges in a study titled ‘Theoretical modeling of QCD radiation in off-shell Higgs production through gluon fusion’. Their work compares predictions from several Monte Carlo event generators, including Powheg, MadGraph, and Sherpa, to assess the impact of higher-order quantum chromodynamics (QCD) effects and theoretical uncertainties on off-shell Higgs boson production at the Large Hadron Collider.

Precise theoretical predictions are essential for interpreting Higgs boson studies at the Large Hadron Collider, particularly for off-shell production, which allows for refined measurements of the Higgs boson’s width and rigorous tests of the Standard Model of particle physics. Researchers currently compare predictions generated by several Monte Carlo event generators, including Powheg, MadGraph, and Sherpa, at both leading order and next-to-leading order. These calculations incorporate parton shower effects, simulating the evolution of quarks and gluons produced in high-energy collisions, and reveal significant differences in predicted differential cross-sections for signal, background, and complete physical processes. These discrepancies highlight the sensitivity of results to the chosen theoretical framework and computational techniques, driving ongoing efforts to improve the accuracy and reliability of theoretical calculations.

Direct measurements of the Higgs boson width are currently limited by the resolution of particle detectors. Consequently, researchers explore combined analyses of both on-shell and off-shell production to enhance precision. The investigation demonstrates the impact of higher-order quantum chromodynamics (QCD) effects—the theory describing the strong force—on the accuracy of predictions. Next-to-leading order calculations consistently improve the modelling of off-shell Higgs production, addressing limitations inherent in leading-order approximations, which only consider the most dominant terms in a calculation.

Researchers examine the influence of jet merging algorithms on the overall accuracy and stability of predictions. Jet merging aims to combine the strengths of both fixed-order calculations—precise calculations of particle interactions—and shower-based simulations, which model the evolution of particle jets. These algorithms play a crucial role in bridging the gap between theoretical calculations and experimental observations, as they attempt to provide a more complete picture of the final state of a collision.

A key finding concerns the breakdown of the heavy-top approximation in off-shell Higgs production. This approximation simplifies calculations by assuming the top quark, a fundamental particle, is infinitely massive. However, accurately accounting for finite top quark mass effects becomes essential for achieving reliable predictions. Event generators handle these effects with varying degrees of sophistication, contributing to observed discrepancies. Researchers meticulously compare predictions obtained through jet merging with parton showers, a technique designed to improve the modelling of high-energy particle jets, with those derived from next-to-leading order calculations matched to parton showers, ensuring a comprehensive evaluation of theoretical uncertainties.

Researchers carefully evaluate the performance of these different approaches, recognising that jet merging aims to combine the precision of matrix element calculations—which describe the fundamental interactions—with the broad coverage of parton showers, providing a more complete description of the event. Future work should focus on reducing the theoretical uncertainties associated with off-shell Higgs boson production, including developing more accurate and efficient methods for calculating higher-order QCD corrections, improving the modelling of parton showers, and refining jet merging algorithms.