[Paper Review] Back-to-back azimuthal correlations in $\mathrm {Z} +$jet events at high transverse momentum in the TMD parton branching method at next-to-leading order

H. Yang, A. Bermúdez Martínez+10 more|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies6 citations

TL;DR

This paper proposes a novel experimental probe for factorization-breaking effects in QCD by comparing azimuthal correlations between Z bosons and jets with those in multijet events at high transverse momentum (pT ≈ 100–1000 GeV). Using the Parton Branching (PB) TMD method matched to NLO calculations via MCatNLO, it shows that Z+jet azimuthal correlations are steeper than in dijet production at pT ~ 100 GeV but converge at pT ~ 1000 GeV, highlighting differences in initial- and final-state radiation that could reveal non-perturbative dynamics.

ABSTRACT

Azimuthal correlations in Z+jet production at large transverse momenta are computed by matching Parton - Branching (PB) TMD parton distributions and showers with NLO calculations via MCatNLO. The predictions are compared with those for dijet production in the same kinematic range. The azimuthal correlations $Δϕ$ between the Z boson and the leading jet are steeper compared to those in dijet production at transverse momenta ${\cal O}(100)$ GeV, while they become similar for very high transverse momenta ${\cal O}(1000)$ GeV. The different patterns of Z+jet and dijet azimuthal correlations can be used to search for potential {\it factorization - breaking} effects in the back-to-back region, which depend on the different color and spin structure of the final states and their interferences with the initial states. In order to investigate these effects experimentally, we propose to measure the ratio of the distributions in $Δϕ$ for Z+jet - and multijet production at low and at high transverse momenta, and compare the results to predictions obtained assuming factorization. We examine the role of theoretical uncertainties by performing variations of the factorization scale, renormalization scale and matching scale. In particular, we present a comparative study of matching scale uncertainties in the cases of PB-TMD and collinear parton showers.

Motivation & Objective

• To investigate differences in azimuthal correlations between Z+jet and dijet production at high transverse momentum.
• To probe potential factorization-breaking effects in QCD arising from color and spin interference in initial- and final-state radiation.
• To propose a measurable observable—ratio of ∆φ distributions in Z+jet vs. multijet events—for testing factorization assumptions.
• To assess theoretical uncertainties from scale variations and matching scale dependence in PB-TMD vs. collinear parton showers.
• To enable new experimental access to the back-to-back region (Δφ ≈ π) at high pT, previously unexplored with fine angular resolution.

Proposed method

• Uses the Parton Branching (PB) approach to model transverse momentum dependent (TMD) parton distribution functions (PDFs) and parton showers.
• Matches PB-TMD evolved PDFs with NLO calculations of Z+jet production using the MCatNLO framework.
• Employs MADGRAPH5_AMC@NLO for NLO matrix elements and combines them with the CAS3 and H6 parton shower algorithms for validation.
• Performs systematic scale variations of factorization, renormalization, and matching scales to quantify theoretical uncertainties.
• Compares predictions from PB-TMD with those from collinear parton showers (H6) to assess matching scale sensitivity.
• Uses identical LHE files for Z+jet and dijet events to ensure consistent comparison of ∆φ distributions.

Experimental results

Research questions

• RQ1 How do azimuthal correlations between Z bosons and jets differ from those in dijet production at high transverse momentum?
• RQ2 What observable can be used to experimentally probe factorization-breaking effects in QCD at high pT?
• RQ3 How do theoretical uncertainties from scale variations affect PB-TMD predictions compared to collinear showers?
• RQ4 To what extent do TMD dynamics and soft gluon resummation modify the ∆φ distribution in Z+jet events at pT ~ 1000 GeV?
• RQ5 Can the ratio of ∆φ distributions in Z+jet and multijet events serve as a clean probe of non-perturbative effects?

Key findings

• At pT ≈ 100 GeV, Z+jet azimuthal correlations are steeper than in dijet production due to stronger initial-state radiation and color interference effects.
• At pT ≈ 1000 GeV, the ∆φ distributions in Z+jet and dijet production become similar, indicating a convergence of perturbative dynamics.
• Theoretical uncertainties from scale variations are well-controlled and consistent across PB-TMD and collinear shower frameworks.
• The PB-TMD method successfully reproduces NLO+PS predictions when matched with MCatNLO, validating its use in high-pT studies.
• The comparison of ∆φ ratios between Z+jet and multijet events provides a robust, scale-invariant observable to test factorization.
• Discrepancies in pseudorapidity distributions at forward/backward regions are sensitive to parton shower parameters, especially VQCUT and VGCUT.