Core Concepts

The authors demonstrate a novel method for calculating single isolated photon production at high energies using the NLO* approximation of the Parton Reggeization Approach, achieving good agreement with experimental data and offering predictions for future experiments.

Abstract

Chernyshev, A.A., & Saleev, V.A. (2024). Single isolated photon production in the NLO${}^\star$ approximation of the Parton Reggeization Approach. arXiv:2410.06644v1 [hep-ph].

This research paper investigates the production of single isolated photons in high-energy collisions, aiming to improve theoretical predictions by incorporating next-to-leading order (NLO*) corrections within the framework of the Parton Reggeization Approach (PRA).

The authors employ the NLO* approximation of the PRA, which combines the resummation of large logarithmic corrections with fixed-order calculations. They introduce a modified Multi-Regge Kinematics (mMRK) subtraction scheme to address double-counting issues between real corrections and unintegrated parton distribution functions (uPDFs). Numerical calculations are performed using the Suave Monte-Carlo algorithm and compared with experimental data from various collaborations.

The NLO* PRA calculations, including the mMRK subtraction scheme, demonstrate good agreement with experimental data on isolated photon transverse momentum spectra across a wide energy range (√S = 2 - 13 TeV) up to pγT/√S ≃ 0.2 - 0.3. The authors also find that the NLO* corrections, specifically the Q ¯Q →γg subprocess, have a small impact after the subtraction procedure, highlighting the self-consistency of the PRA.

The study confirms the effectiveness of the NLO* approximation of the PRA in describing single isolated photon production at high energies. The proposed mMRK subtraction scheme successfully addresses double-counting issues, leading to improved accuracy in theoretical predictions. The findings have implications for understanding photon production mechanisms and for making reliable predictions for future high-energy experiments.

This research contributes to the field of high-energy physics by refining theoretical tools for calculating photon production, a crucial process for probing the structure of matter and fundamental forces. The improved accuracy of the NLO* PRA calculations enhances our understanding of QCD dynamics at high energies and provides valuable input for future experimental analyses.

The study primarily focuses on the transverse momentum spectra of isolated photons. Further investigations could explore other observables, such as angular correlations, to provide a more comprehensive picture of photon production. Additionally, extending the calculations to higher orders in perturbation theory would further improve the precision of the predictions.

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The NLO* PRA calculations agree with experimental data up to pγT/√S ≃ 0.2 - 0.3.
The energy range considered for comparison with experimental data is √S = 2 - 13 TeV.
The isolation cone radius (R(iso)) used in the calculations varies depending on the experimental setup.
The central value of the hard scale (µ) is set to pγT/2.
The variation in cross-section due to hard scale variation is estimated by a factor of ξ = 2±1.

Quotes

Key Insights Distilled From

by Alexey Chern... at **arxiv.org** 10-10-2024

Deeper Inquiries

Inclusion of higher-order corrections beyond NLO* is expected to improve the accuracy of predictions for isolated photon production, particularly in the regions where the current NLO* predictions show discrepancies with experimental data. Here's how:
Reduced Uncertainties: Higher-order calculations typically involve the inclusion of more Feynman diagrams, representing a wider range of physical processes. This leads to a reduction in the theoretical uncertainties associated with the predictions. The scale dependence of the cross-section, estimated in the paper by varying the hard scale μ, is expected to decrease as more orders of perturbation theory are included.
Improved Kinematic Reach: The paper identifies that the Multi-Regge Kinematics (MRK) condition (µ ≪ √S) becomes questionable at high pγT. Higher-order calculations could potentially extend the kinematic reach of the Parton Reggeization Approach (PRA), allowing for more reliable predictions in the high-pT region.
Better Description of Data: The NLO* PRA currently overestimates data at high pγT in specific rapidity regions. Higher-order corrections might account for missing contributions or kinematic effects that are not fully captured at the NLO* level, leading to a better agreement with experimental observations.
However, it's important to note that calculating higher-order corrections is computationally demanding. The complexity of the calculations increases significantly with each additional order, requiring sophisticated techniques and significant computing resources.

Yes, alternative theoretical frameworks can indeed provide complementary insights into photon production mechanisms at high energies. While the Parton Reggeization Approach (PRA) offers a valuable framework for understanding high-energy scattering processes, exploring other approaches can offer a more comprehensive understanding. Here's how Soft-Collinear Effective Theory (SCET) could be particularly insightful:
Resummation of Large Logarithms: SCET is specifically designed to systematically resum large logarithms that arise in collider physics processes, including those associated with soft and collinear gluon emissions. In the context of photon production, SCET could help resum logarithms that are not fully accounted for in the PRA, potentially improving the accuracy of predictions, especially in kinematic regions where such logarithms become significant.
Factorization at Different Scales: SCET provides a powerful framework for separating physics at different energy scales. This could be particularly useful for disentangling the contributions from various production mechanisms of photons, such as direct production, fragmentation, and those arising from electroweak processes.
Treatment of Hadronization: SCET offers tools for systematically incorporating hadronization effects, which are important for connecting theoretical predictions to experimental observables. This could be particularly relevant for studying isolated photon production in conjunction with jets or other hadronic activity.
By combining insights from PRA, SCET, and potentially other frameworks like those based on transverse-momentum-dependent parton distributions (TMDs), physicists can aim for a more complete and precise description of photon production at high energies.

Precisely understanding photon production has profound implications for unraveling the mysteries of the early universe and the fundamental building blocks of matter:
Probing the Quark-Gluon Plasma: Photons, due to their lack of color charge, interact very weakly with the quark-gluon plasma (QGP) – a state of matter believed to have existed in the very early universe. This makes them excellent probes of the QGP formed in heavy-ion collisions. Precise measurements of photon production in these collisions provide crucial information about the properties and evolution of the QGP, offering insights into the conditions of the early universe.
Constraining Parton Distribution Functions: Photons, particularly at high energies, are sensitive probes of the gluon content of protons and nuclei. Accurate measurements of photon production in proton-proton and proton-nucleus collisions allow for precise determination of the gluon parton distribution functions (PDFs). These PDFs are essential inputs for theoretical calculations of a wide range of processes at high-energy colliders, including searches for new particles and interactions beyond the Standard Model.
Testing the Standard Model and Beyond: Precise predictions and measurements of photon production serve as stringent tests of the Standard Model of particle physics. Any deviations from the Standard Model predictions could hint at new physics, such as new particles or interactions. For instance, the production rate and kinematic distributions of photons can be affected by the presence of hypothetical particles like supersymmetric particles or dark matter candidates.
In essence, photons act as messengers from the most extreme environments, both in the laboratory and in the cosmos. By meticulously studying their production mechanisms, we gain invaluable knowledge about the fundamental forces and particles that govern our universe, from the smallest scales to the grandest cosmological events.

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