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On deglobalization in QCD

Mrinal Dasgupta

TL;DR

The paper identifies non-global logarithms as a critical missing piece in the resummation of perturbative QCD for observables with restricted phase space. It demonstrates that conventional independent-emission approaches fail to account for correlated soft-gluon emissions, necessitating all-order resummation in the large-$N_c$ limit via a dipole-based Monte Carlo evolution (or the BMS equation). A key insight, buffer dynamics, explains the geometry-insensitive suppression of emissions and provides a qualitative understanding of full QCD coherence. The findings have significant phenomenological impact on jet mass, energy-flow observables, and rapidity-gap studies, and the work discusses strategies to mitigate non-global effects and define observables less sensitive to them.

Abstract

The recent discovery and resummation of a class of single-logarithmic effects (non-global logs), has a significant impact on several QCD observables ranging from the classic Sterman-Weinberg jet definition to currently studied event shapes and rapidity gap observables. The discovery of the above effects overturns, for example, the common wisdom that hadronic energy flow in limited interjet regions is dictated primarily by the colour flow in the underlying hard partonic subprocess. We discuss some features of non-global logs and the rapid progress being made in estimating and controlling such corrections.

On deglobalization in QCD

TL;DR

The paper identifies non-global logarithms as a critical missing piece in the resummation of perturbative QCD for observables with restricted phase space. It demonstrates that conventional independent-emission approaches fail to account for correlated soft-gluon emissions, necessitating all-order resummation in the large- limit via a dipole-based Monte Carlo evolution (or the BMS equation). A key insight, buffer dynamics, explains the geometry-insensitive suppression of emissions and provides a qualitative understanding of full QCD coherence. The findings have significant phenomenological impact on jet mass, energy-flow observables, and rapidity-gap studies, and the work discusses strategies to mitigate non-global effects and define observables less sensitive to them.

Abstract

The recent discovery and resummation of a class of single-logarithmic effects (non-global logs), has a significant impact on several QCD observables ranging from the classic Sterman-Weinberg jet definition to currently studied event shapes and rapidity gap observables. The discovery of the above effects overturns, for example, the common wisdom that hadronic energy flow in limited interjet regions is dictated primarily by the colour flow in the underlying hard partonic subprocess. We discuss some features of non-global logs and the rapid progress being made in estimating and controlling such corrections.

Paper Structure

This paper contains 7 sections, 4 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Non global logs
  3. Resummation
  4. Buffer dynamics
  5. Phenomenological impact
  6. Damage assessment, limitation and control
  7. Conclusions

Figures (3)

  • Figure 1: Independent emission (Sudakov) from $J_1$ alongwith correlated (non-global) contribution from $J_2$
  • Figure 2: Resummed non-global contribution for energy flow into different regions in the $\eta,\phi$ plane.
  • Figure 3: Impact of NLL non global logs on the resummed current-jet--mass distribution.