[Paper Review] Top Quark Production and Decay in Herwig 7.1

Johannes Bellm, K. Cormier+5 more|arXiv (Cornell University)|Nov 30, 2017

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper presents major advancements in top quark simulation within the Herwig 7.1 Monte Carlo event generator, including improved angular-ordered and dipole showers with enhanced recoil handling and massive parton kinematics. Key contributions include NLO-matched top quark production and decay with a new shower-starting scale choice and a novel NLO multi-jet merging algorithm, significantly improving predictions for jet multiplicity and H_T distributions in t\bar{t} events at the LHC.

ABSTRACT

A summary of recent developments in the simulation of top quark production and decay in the Herwig Monte Carlo event generator.

Motivation & Objective

• Address deficiencies in top quark production and decay simulations in Herwig 7.0, particularly in the invariant mass of t\bar{t} pairs and B-fragmentation functions.
• Improve the treatment of recoil and momentum conservation in parton showers by leveraging color information from the hard process.
• Introduce a new optional shower-starting scale, μ_opt, for NLO-matched t\bar{t} events to reduce theoretical uncertainty from scale dependence.
• Implement a new NLO multi-jet merging algorithm based on the unitarised merging paradigm to better describe high-multiplicity jet final states.
• Enable consistent NLO QCD corrections in both production and decay of top quarks using the Powheg decay correction framework.

Proposed method

• The angular-ordered shower in Herwig 7.1 now uses color information more effectively during momentum reshuffling to restore energy-momentum conservation, reducing unphysical off-shell effects.
• The dipole shower has been fully reformulated for massive partons, including new kinematics and Jacobians for massive initial-state and final-state dipoles, improving accuracy in heavy quark fragmentation.
• The new shower-starting scale μ_opt is defined as the root mean square of transverse masses of outgoing particles in the hard process: μ_opt² = (1/n_out) Σ m_T,i².
• An NLO multi-jet merging algorithm based on unitarised merging is implemented, allowing merging of tree-level and one-loop matrix elements for increasing jet multiplicities.
• Top quark decays are now showered in the narrow-width approximation, with momentum conservation preserved between decay and subsequent showering, and NLO corrections applied via the built-in Powheg decay module.
• Both showers in Herwig 7.1 can now perform NLO-matched simulations of top quark production and decay using either subtractive or multiplicative matching schemes via the Matchbox module.

Experimental results

Research questions

• RQ1 How can the recoil treatment in the angular-ordered shower be improved to reduce unphysical off-shell effects in top quark pair production?
• RQ2 What impact does the new μ_opt shower-starting scale have on jet multiplicity distributions in t\bar{t} events at 7 TeV?
• RQ3 How does the reformulated dipole shower with improved massive parton kinematics affect B-fragmentation functions at LEP energies?
• RQ4 To what extent does the new NLO multi-jet merging algorithm improve the description of H_T distributions in t\bar{t} events at 8 TeV?
• RQ5 Can consistent NLO QCD corrections be applied to both top quark production and decay within a single event generator framework?

Key findings

• The new μ_opt shower-starting scale reduces jet multiplicity predictions across all bins, with a more pronounced effect in the dipole shower, indicating reduced scale dependence.
• The dipole shower in Herwig 7.1 shows a significant improvement in describing the SLD measurement of the B-fragmentation function, which was poorly reproduced in Herwig 7.0.
• Jet multiplicity distributions in 7 TeV pp → t\bar{t} events are better described with the new μ_opt scale, particularly in the dipole shower, reducing theoretical uncertainty from scale choice.
• The NLO multi-jet merged samples (t\bar{t}(0,1,2) and t\bar{t}(0*,1*,2)) show a clear improvement over the NLO-matched sample in describing the H_T distribution at 8 TeV, especially in the high-H_T region.
• Both the angular-ordered and dipole showers in Herwig 7.1 now correctly handle NLO-matched top quark production and decay, with momentum conservation preserved and NLO corrections applied consistently.
• The implementation of the Powheg decay correction in the dipole shower enables NLO-accurate description of the first emission in top quark decay, enhancing precision in top quark decay chain simulations.