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Colour reconnection and its effects on precise measurements at the LHC

Torbjörn Sjöstrand

TL;DR

The paper surveys colour reconnection (CR) as a nonperturbative effect in high-density LHC events, highlighting how overlapping color strings and limited Nc=3 color degrees of freedom lead to reconnections that are not captured by standard hadronization alone. It reviews historical CR ideas, recent status at the LHC, and the potential impact on the masses of unstable coloured particles, particularly the top quark, illustrating how different MC generators implement CR and how experimental constraints are beginning to emerge. The author emphasizes that CR can influence precision measurements, such as the top mass, and may partially mimic collective flow phenomena, motivating broader model development and data-driven constraints. Looking ahead, the work calls for expanded CR modeling across generators, more extensive data comparisons, and opportunities for CR studies in future e+e- colliders to isolate and constrain the underlying mechanisms.

Abstract

There are experimental evidence for the occurrence of colour reconnection, but the mechanisms involved are far from understood. Previous reconnection studies are briefly summarized, and some potential implications for LHC physics are outlined.

Colour reconnection and its effects on precise measurements at the LHC

TL;DR

The paper surveys colour reconnection (CR) as a nonperturbative effect in high-density LHC events, highlighting how overlapping color strings and limited Nc=3 color degrees of freedom lead to reconnections that are not captured by standard hadronization alone. It reviews historical CR ideas, recent status at the LHC, and the potential impact on the masses of unstable coloured particles, particularly the top quark, illustrating how different MC generators implement CR and how experimental constraints are beginning to emerge. The author emphasizes that CR can influence precision measurements, such as the top mass, and may partially mimic collective flow phenomena, motivating broader model development and data-driven constraints. Looking ahead, the work calls for expanded CR modeling across generators, more extensive data comparisons, and opportunities for CR studies in future e+e- colliders to isolate and constrain the underlying mechanisms.

Abstract

There are experimental evidence for the occurrence of colour reconnection, but the mechanisms involved are far from understood. Previous reconnection studies are briefly summarized, and some potential implications for LHC physics are outlined.

Paper Structure

This paper contains 5 sections, 1 equation, 1 figure.

Table of Contents

  1. Introduction
  2. Historical overview
  3. Status at the LHC
  4. The mass of unstable coloured particles
  5. Summary and outlook

Figures (1)

  • Figure 1: $\langle p_{\perp} \rangle (n_{\mathrm{ch}})$ with the default Pythia 8 Tune 4C tuneCorke, and the same with CR switched off, compared with ATLAS data pTvsNchATLAS.