Spin-Correlated Top Quark Decays in ttW Production

Abstract

This thesis investigates the modeling of top quark pair production in association with a W boson, focusing in particular on off-shell effects and spin correlations within the framework of the Narrow Width Approximation (NWA). Full NLO QCD computations for the complete ttW process, including finite-width and spin-correlation effects, offer the highest theoretical accuracy. However, they are computationally intensive, which limits their practical applicability and hinders extensions to higher-order simulations such as NNLO.

As a more efficient approach, we validate the feasibility of the NWA by analyzing its theoretical foundations and comparing leading-order (LO) simulations—including double-, single-, and non-resonant contributions—with full off-shell results. Through detailed cross-section and differential distribution studies, we confirm that the NWA introduces an acceptably small error.

To improve the modeling of spin correlations beyond conventional leading-order decay methods, we propose a new methodology by modifying matrix element generation within the MadGraph5_aMC@NLO framework. This allows us to fully preserve the spin correlations between the top quarks at LO under the NWA.

Our method successfully reproduces spin-correlation observables at the matrix-element level, showing improved agreement with theoretical expectations compared to standard LO decay treatments such as MadSpin. These results highlight the advantages of incorporating spin-correlated decays directly into the matrix-element generation, offering a robust and computationally efficient approach for precision simulations involving heavy particle decays.

Popular Abstract

The top quark is the heaviest particle we know in nature. It's so heavy and unstable that it decays almost instantly after being produced. But that's exactly what makes it special—since it decays so quickly, it doesn't have time to form other particles through strong interactions. That means we can still study its original properties, like its spin, by looking at the particles it decays into. By studying the top quark, scientists can test the Standard Model of particle physics, which is our best explanation of how the universe works at the smallest scales.

In this project, we look at a process where a top quark and an anti-top quark are produced together with another particle called a W boson. Although this kind of event is relatively rare, it plays an important role in many areas of research at the Large Hadron Collider, the world's most powerful particle accelerator.

To study this process, physicists use computer simulations called event generators to recreate what happens when particles collide. These simulations are based on complex equations from quantum physics and can be very demanding to compute. One common way to simplify the problem is to assume that unstable particles like the top quark are produced with a fixed mass. This is known as the narrow width approximation and helps break the simulation into two easier parts: the production of the top quark pair and their decay into other particles.

Many existing tools use this method, but they often include the spin information of the decaying top quark only in a simplified way. In this project, we developed a new method to better preserve this spin information in the simulation. We did this by modifying the structure of the simulation software at the code level, so that spin effects could be handled internally in a more consistent way. While this does not yet make the simulation more precise, it opens up the possibility for higher-accuracy studies and relatively low computational complexity in the future.

We tested our method by comparing it to existing tools and found that it gives very similar results when it comes to how the spin of the top quark affects the direction of its decay products. This means our approach works and could help improve future studies of top quarks and other heavy particles. It's a step toward more precise simulations that are also practical to use.

• author: Wang, Tingjie
• supervisor: Rikkert Frederix
• organization: Department of Physics; Particle and nuclear physics
• course: FYSM64 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Top quark, Spin correlation, Narrow Width Approximation (NWA), Off-shell effects, ttW production, MadGraph5_aMC@NLO, Matrix element, Madspin
• language: English
• id: 9196328
• date added to LUP: 2025-06-16 08:51:20
• date last changed: 2025-06-16 08:51:20