Herwig 7.0 / Herwig++ 3.0 Release Note (1512.01178v1)

Abstract: A major new release of the Monte Carlo event generator Herwig++ (version 3.0) is now available. This release marks the end of distinguishing Herwig++ and HERWIG development and therefore constitutes the first major release of version 7 of the Herwig event generator family. The new version features a number of significant improvements to the event simulation, including: built-in NLO hard process calculation for all Standard Model processes, with matching to both angular-ordered and dipole shower modules via both subtractive (MC@NLO-type) and multiplicative (Powheg-type) algorithms; QED radiation and spin correlations in the angular ordered shower; a consistent treatment of perturbative uncertainties within the hard process and parton showering. Several of the new features will be covered in detail in accompanying publications, and an update of the manual will follow in due course.

• The paper presents automated NLO corrections using MC@NLO and Powheg algorithms, significantly improving simulation accuracy for Standard Model processes.
• The paper introduces integrated QED radiation and spin correlations within angular-ordered parton showers to refine predictive capabilities.
• The paper demonstrates robust numerical results and streamlined usability that reduce uncertainties and align theoretical predictions with LHC data.

An Overview of Herwig 7.0: Advancements and Capabilities

The Herwig 7.0 release note represents the culmination of efforts to integrate and enhance Herwig and Herwig++ event generators, resulting in a sophisticated tool for particle physics simulations. This paper outlines the significant developments of the Herwig 7.0 release, with a focus on its extended capabilities that incorporate next-to-leading order (NLO) precision, improvements in perturbative uncertainties, QED radiation, spin correlations, and a novel approach to parton showering.

Key Features and Improvements

Herwig 7.0 introduces substantial advancements in simulation accuracy and functionality:

1. NLO Simulations: Herwig 7.0 automates NLO corrections for most Standard Model processes, offering precise simulations via subtractive (MC@NLO) and multiplicative (Powheg) algorithms. This enhancement, facilitated by the Matchbox module, streamlines NLO calculations without requiring external codes or intermediate file management.
2. QED Radiation and Spin Correlations: The addition of QED radiation for angular-ordered parton showers marks a new capability for Herwig, addressing prior limitations. Spin correlations are now incorporated throughout the entire parton shower, enabling more accurate predictions by linking azimuthal angles of branching to earlier emissions, accounting for both soft and spin effects.
3. Cluster Hadronization and Models: Herwig 7.0 retains its coherent parton showers and cluster hadronization model. The novel treatment of decays of unstable particles within the shower process ensures that the challenging aspects of parton-level interactions are more faithfully represented.

Strong Numerical Results and Claims

This release offers robust improvements, which are rigorously backed by comparisons to prior versions and competitors like Pythia and Sherpa, as well as experimental data. The results indicate that Herwig 7.0 delivers a more consistent description of data over a variety of distributions, including event-shape distributions from LEP data and the azimuthal separation in Z+jets events at LHC, showcasing its enhanced accuracy in simulating higher-order processes in hadron collisions.

Practical and Theoretical Implications

The improvements facilitate more reliable theoretical predictions that are critical for current and future experimental analyses, such as those at the LHC. These advances help bridge a gap between theoretical predictions and experimental findings by reducing the theoretical uncertainties and enhancing the precision of event simulators in collider experiments. On a practical level, the integration of new run modes and simplified input files significantly enhances usability and computational efficiency, benefiting the broader community involved in high-energy physics simulations.

Future Directions in AI and Particle Simulations

The release symbolizes an advancement towards comprehensive event simulation frameworks, foreshadowing future developments such as multi-jet merging at both leading and next-to-leading orders and incorporating electroweak corrections. The trajectory of Herwig's development hints at more automated, computationally efficient tools capable of further abating the discrepancy between simulated and actual events, potentially integrating AI algorithms to automate and optimize complex simulation tasks.

In conclusion, Herwig 7.0 sets a new standard in event simulation, blending increased precision with improved usability. These advancements underline the ongoing evolution in simulators, pivotal for both advancing fundamental particle physics and yielding deeper insights into the results derived from high-energy particle collisions.