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Matching Exponentiated Photonic Radiation to a QED-Corrected Parton Shower in the KKMChh Monte Carlo Program


Core Concepts
The NISR matching procedure in the KKMChh Monte Carlo program effectively mitigates double-counting of QED radiation when using QED-corrected parton distribution functions (PDFs), leading to more accurate calculations of observables like the forward-backward asymmetry in muon pair production.
Abstract
  • Bibliographic Information: Yost, S. A. (2024, October 9). Parton shower matching in KKMChh. Proceedings of Science, 414, arXiv:2410.09104v1.
  • Research Objective: This research paper, presented at ICHEP2024, investigates the impact of the Negative Initial-State Radiation (NISR) matching procedure on calculations of the forward-backward asymmetry (AFB) of muon pairs in proton collisions at 8 TeV center-of-mass energy using the KKMChh Monte Carlo program.
  • Methodology: The study compares AFB calculations in different muon pair invariant mass and rapidity ranges using three different setups: (1) without NISR and standard NNPDF-3.1 (NLO), (2) with NISR applied at the QCD evolution starting scale (q0 = 2 GeV) using NNPDF-3.1 (NLO), and (3) with NISR applied at the hard process scale using NNPDF-3.1-LuxQED (NLO). The results are based on Monte Carlo samples with 8-10 billion weighted events.
  • Key Findings: The inclusion of NISR, regardless of the PDF set used, leads to small corrections in the AFB calculations, mostly compatible within Monte Carlo errors. However, NISR's contribution becomes more significant in rapidity distributions, particularly at large rapidities.
  • Main Conclusions: The NISR matching procedure effectively addresses the double-counting of QED radiation when using QED-corrected PDF sets in KKMChh. This leads to more accurate AFB calculations, especially in the high-rapidity regions.
  • Significance: This research contributes to improving the precision of theoretical predictions for electroweak observables at hadron colliders, which is crucial for extracting fundamental parameters like the electroweak mixing angle.
  • Limitations and Future Research: The study focuses on muon pair production as a case study. Further investigations are needed to assess the impact of NISR on other processes and observables. Additionally, incorporating hadronic showers and applying realistic experimental cuts would enhance the study's relevance to experimental analyses.
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Stats
AFB calculated in five ranges of muon pair invariant mass (M𝜇−𝜇+): 89–93 GeV, 60–120 GeV, 60–81 GeV, 81–101 GeV, and 101–150 GeV. Three cases were compared: (1) without NISR, (2) NISR at q0 = 2 GeV, and (3) NISR at the hard process scale. MC samples with 8-10 billion weighted events were used.
Quotes

Deeper Inquiries

How does the NISR matching procedure in KKMChh compare to similar techniques implemented in other Monte Carlo generators for simulating hadron collisions?

The NISR (Negative Initial State Radiation) matching procedure in KKMChh addresses the double-counting of QED radiation effects, which can arise from both QED-corrected parton distribution functions (PDFs) and the simulation of QED radiation in the parton shower. This is achieved by subtracting the already-accounted-for QED radiation from the PDFs at a specific scale, ensuring a more accurate representation of the physical process. Other Monte Carlo generators employ various techniques for similar purposes: PS-based matching schemes: Generators like Sherpa and Herwig employ schemes where the QED radiation is generated by the parton shower (PS) and matched to fixed-order calculations using algorithms like CKKW or MC@NLO. These methods effectively combine the precision of fixed-order calculations with the resummation properties of the parton shower. POWHEG-BOX: This framework, utilized in generators like POWHEG+Pythia, generates the hardest emission in an exact matrix-element calculation, while subsequent emissions are handled by the parton shower. This approach ensures accurate kinematics for the hardest emission while maintaining the resummation capabilities of the parton shower. Automated tools: Tools like MadGraph5_aMC@NLO and Sherpa incorporate automated frameworks for matching and merging matrix elements with parton showers, simplifying the process for users and allowing for more efficient calculations. While these techniques share the goal of accurately simulating QED radiation, they differ in their implementation and the level of theoretical precision they achieve. KKMChh's NISR approach, with its focus on amplitude-level exponentiation and soft-photon approximation, offers a unique approach compared to the PS-based matching schemes and automated tools employed in other generators.

Could the small impact of NISR on AFB in certain kinematic regions be attributed to limitations in the soft-photon approximation used in KKMChh, and would including higher-order corrections be necessary?

The small impact of NISR on the forward-backward asymmetry (AFB) in certain kinematic regions observed in KKMChh could potentially be attributed to the limitations of the soft-photon approximation. This approximation, while valid for low-energy photons, might not fully capture the behavior of hard, non-collinear photons that could contribute significantly to the asymmetry in specific phase-space regions. Including higher-order corrections, beyond the current order 𝛼2 NLL implemented in KKMChh, could be necessary to achieve a more complete picture and potentially reveal a more pronounced impact of NISR on AFB. These corrections would account for more complex photonic interactions and provide a more accurate description of the underlying physics. However, it's also important to consider other factors that could contribute to the observed behavior: Choice of PDF set: Different PDF sets incorporate QED corrections to varying degrees. The observed small impact of NISR might be influenced by the specific PDF set used and the extent of its QED treatment. Kinematic region: The impact of NISR might be more pronounced in specific kinematic regions, such as those with high muon pair transverse momentum or large rapidity separation, where the contribution from hard, non-collinear photons is enhanced. MC statistical uncertainties: The observed differences might be within the statistical uncertainties of the Monte Carlo simulations, particularly in regions with limited statistics. Therefore, while including higher-order corrections is crucial for improving the theoretical accuracy, a comprehensive analysis considering these factors is essential to fully understand the impact of NISR on AFB and disentangle the contributions from different sources.

Considering the increasing precision of experimental measurements at the LHC, how can theoretical calculations like those performed in KKMChh be further improved to maximize the physics potential of these experiments and uncover potential deviations from the Standard Model?

The increasing precision of experimental measurements at the LHC demands continuous improvement in theoretical calculations to fully exploit the data and uncover potential deviations from the Standard Model. For calculations like those performed in KKMChh, several avenues for improvement exist: Higher-order perturbative calculations: Extending the calculations to higher orders in the strong coupling constant (𝛼s) and electroweak coupling constant (𝛼) would reduce theoretical uncertainties and provide a more accurate description of the underlying physics. This includes incorporating higher-order virtual and real corrections, as well as improving the resummation of large logarithmic contributions. Improved parton distribution functions: Utilizing PDF sets with more precise determinations of the parton distributions, including QED corrections and heavy quark mass effects, is crucial for accurate predictions. This involves incorporating data from a wider range of experiments and improving the theoretical framework for PDF evolution. Non-perturbative effects: Incorporating non-perturbative effects, such as hadronization and underlying event activity, is essential for a realistic description of the final state particles observed in detectors. This can be achieved through improved models implemented in Monte Carlo event generators and by tuning these models to data. Electroweak corrections: Including electroweak corrections beyond the leading order, particularly for processes involving W and Z bosons, is crucial for achieving high precision. This involves calculating electroweak virtual and real corrections, as well as considering the impact of multiple electroweak gauge boson emissions. Matching and merging with parton showers: Improving the matching and merging procedures between fixed-order calculations and parton showers is essential for accurately describing both the hard scattering process and the subsequent parton evolution. This involves developing more sophisticated matching schemes and implementing them in Monte Carlo event generators. By pursuing these improvements, theoretical calculations can keep pace with the increasing precision of experimental measurements at the LHC, enabling more stringent tests of the Standard Model and potentially revealing hints of new physics beyond our current understanding. This continuous interplay between theory and experiment is crucial for maximizing the physics potential of the LHC and advancing our understanding of fundamental particles and their interactions.
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