Lhc Study Probes Nuclear Parton Densities Using Fully Coherent Radiation and Virtual Photons

Understanding the internal structure of atomic nuclei remains a fundamental challenge in particle physics, and recent research led by François Arleo, Djessy Bourgeais, and Maxime Guilbaud, all from SUBATECH UMR 6457, investigates how particles interact within these complex systems. The team explores fully coherent radiation, a phenomenon arising from multiple scattering of particles inside the nucleus, and its influence on particle production at the Large Hadron Collider. Their work demonstrates that analysing this radiation provides a new way to map nuclear parton densities, essentially creating a detailed picture of the nucleus’s internal structure, and importantly, the researchers show that the Drell-Yan process offers a particularly powerful method for probing these densities with unprecedented accuracy, even reweighting existing models using simulated data representative of the LHC’s current run. This achievement promises to refine our understanding of nuclear physics and improve the precision of particle collision experiments.

Photons provide a sensitive probe of nuclear parton distribution functions. This study presents recent results on fully coherent radiation induced by parton multiple scattering, a cold nuclear matter effect that may influence the nuclear dependence of prompt photon production. At backward rapidities, photons are sensitive to fully coherent energy loss, while at forward rapidities, fully coherent energy gain plays a crucial role due to the dominance of the quark-gluon scattering channel.

Prompt Photon and Drell-Yan Production in pA Collisions

This paper investigates the production of prompt photons and the Drell-Yan process in proton-nucleus collisions, aiming to understand the impact of the cold nuclear medium on these processes. Researchers focus on fully coherent energy loss and energy gain mechanisms, and seek to constrain nuclear parton distribution functions, which describe the probability distributions of quarks and gluons inside a nucleus. The study also aims to separate the effects of fully coherent energy loss from other mechanisms that can modify photon and Drell-Yan production. Prompt photons, produced directly in hard scattering processes, are sensitive probes of the initial state of the collision and the nuclear medium.

The Drell-Yan process, involving a quark and an antiquark annihilating to produce a virtual photon or Z boson, provides a clean probe of the nuclear structure and is less susceptible to certain medium effects than hadron production. Fully coherent energy loss is a specific type of energy loss where the radiated gluons remain coherent with the original parton. The authors use perturbative quantum chromodynamics calculations to model photon and Drell-Yan production, modifying them to include the effects of fully coherent energy loss and energy gain. They utilize existing nuclear parton distribution function sets as input for their calculations and employ PDF reweighting, a technique that adjusts these functions to better match expected experimental observations.

This allows researchers to assess the sensitivity of the nuclear parton distribution functions to new measurements and generate pseudo-data for Drell-Yan processes to simulate experimental observations. The authors find that fully coherent energy loss can have a measurable impact on prompt photon production, particularly at low transverse momenta and backward rapidities. The Drell-Yan process is found to be a robust probe of the nuclear structure, being less sensitive to fully coherent energy loss than prompt photon production. Their reweighting analysis demonstrates that measurements of the Drell-Yan process can significantly constrain the nuclear parton distribution functions, especially the gluon distribution at small values of x.

The study highlights the sensitivity of both prompt photon and Drell-Yan production to the choice of nuclear parton distribution functions, and the reweighting procedure effectively reduces uncertainties in these functions. The authors conclude that fully coherent energy loss is a non-negligible effect that should be considered in analyses of proton-nucleus collisions. The Drell-Yan process is a powerful tool for probing the nuclear structure and constraining nuclear parton distribution functions. Future experiments, particularly those measuring the Drell-Yan process in proton-nucleus collisions, will be crucial for refining our understanding of the nuclear medium and the nuclear parton distribution functions. Combining measurements of prompt photons and the Drell-Yan process will provide a more complete picture of the nuclear environment.

Coherent Radiation Reveals Nuclear Parton Distributions

Scientists are investigating how photons produced in proton-nucleus collisions can reveal details about the internal structure of atomic nuclei, specifically the distribution of partons. This work focuses on understanding cold nuclear matter effects, particularly fully coherent radiation, which arises from the multiple scattering of partons within the nucleus and can alter observed photon yields. Researchers computed the differential cross section for prompt photon production in proton-proton collisions, analyzing individual partonic sub-processes contributing to photon creation. The team modeled the impact of fully coherent radiation on photon production in proton-nucleus collisions, demonstrating how this phenomenon can lead to both energy loss and energy gain depending on the specific interaction.

Calculations reveal that the dominant process, involving a gluon and a quark, experiences fully coherent energy gain, while other processes exhibit energy loss. The results show that the nuclear modification factor is significantly affected by these coherent radiation effects at a transverse momentum of 5 GeV. Furthermore, scientists explored the potential of the Drell-Yan process to probe nuclear parton distribution functions, expecting it to be less sensitive to coherent radiation and thus a valuable tool for isolating the underlying nuclear structure. Using realistic pseudo-data, researchers demonstrated the power of future LHCb measurements of the Drell-Yan process to constrain nuclear parton distribution functions.

Coherent Effects Refine Nuclear Collision Understanding

This research investigated the influence of fully coherent energy loss and energy gain on prompt photon production in proton-nucleus collisions, alongside an assessment of the Drell-Yan process as a probe of nuclear parton distribution functions. The team demonstrated that while these coherent effects are generally small, they are not negligible, particularly at lower momentum transfer and backward rapidities. These findings refine the understanding of particle production in nuclear collisions and highlight the importance of considering subtle effects beyond simple energy loss models. Importantly, the study confirms the Drell-Yan process remains largely unaffected by medium-induced coherent gluon radiation, establishing it as a powerful tool for probing nuclear parton distribution functions. By reweighting existing nuclear parton distribution function sets with simulated data representative of future LHC runs, researchers showed that Drell-Yan measurements can significantly constrain both quark and gluon densities at low values of x. The authors acknowledge limitations in the precision of these constraints, but emphasize the potential for future measurements to further refine our knowledge of nuclear parton distribution functions.