In a groundbreaking exploration at the frontiers of particle physics, researchers have delved into the enigmatic realm of proton spin, utilizing a sophisticated theoretical framework known as holographic light-front quantum chromodynamics (QCD) to interpret data from the COMPASS experiment. This ambitious study, published recently in The European Physical Journal C, illuminates the intricate dance of quarks and gluons within protons, specifically focusing on the enigmatic Drell–Yan process. The Drell–Yan process, a cornerstone of high-energy physics, involves the annihilation of a quark and an antiquark to produce a lepton-antilepton pair, offering a vital window into the fundamental structure of matter. By examining azimuthal spin asymmetries in pion-polarized proton-induced Drell–Yan scattering, the research team has uncovered crucial insights into how the spin of the proton, a quantity intrinsically linked to its quantum mechanical nature, is generated from the spins and orbital angular momentum of its constituent partons. This endeavor is not merely an academic exercise; it represents a significant step towards a complete understanding of the proton’s spin puzzle, a decades-old mystery that continues to challenge physicists and promises to unlock new physics beyond the Standard Model.
The COMPASS (Common Muon and Proton Experiment) facility, situated at CERN, has been a crucial experimental playground for probing the spin structure of hadrons. Its capabilities allow for precise measurements of spin-dependent cross-sections in the scattering of muons and protons. In this particular research, the focus shifted to proton-proton collisions where one of the protons is polarized, and a pion probe initiates the Drell–Yan interaction. The azimuthal angle, which describes the orientation of the produced lepton pair relative to the scattering plane, becomes a critical observable when the proton’s spin is taken into account. Deviations from simple theoretical predictions in these azimuthal distributions signal the presence of complex spin correlations and the contribution of orbital angular momentum carried by the quarks and gluons within the proton. Understanding these asymmetries is paramount to dissecting the proton’s spin budget, which is known to be more intricate than initially assumed, with the quarks’ spin contributing less than expected. This research directly tackles the question of how the remaining spin component is distributed.
The theoretical underpinnings of this research are as fascinating as the experimental findings. Holographic light-front QCD provides a novel and powerful approach to tackle the notoriously difficult problem of quantum chromodynamics in the non-perturbative regime. This framework, inspired by the gauge-gravity duality (also known as the AdS/CFT correspondence), maps strongly coupled quantum field theories, like QCD, to weakly coupled gravitational theories in higher dimensions. Specifically, light-front quantization, where the dynamics are described on a spacelike hypersurface, offers advantages for understanding the structure of relativistic bound states like the proton. By constructing a holographic model within this light-front framework, the researchers have been able to generate predictions for the spin-dependent observables in the Drell–Yan process, offering a tangible theoretical tool to interpret the COMPASS data. This fusion of cutting-edge theory and experimental precision is what drives progress in fundamental physics.
The Drell–Yan process, in the context of this study, serves as a probe of the proton’s internal spin structure through the lens of quark and antiquark interactions. When a pion interacts with a proton in a high-energy collision, a quark from the pion can annihilate with an antiquark from the proton (or vice versa) to produce a virtual photon, which then decays into a pair of leptons. If the proton is polarized, the outgoing lepton pair will exhibit a characteristic distribution in their azimuthal angle that depends on the orientation of the proton’s spin relative to the collision. These azimuthal spin asymmetries, such as the Sivers asymmetry and the Boer-Mulders asymmetry, are directly sensitive to the distribution and polarization of quarks and antiquarks within the proton, as well as their orbital motion. Unraveling these asymmetries is key to assembling the complete picture of the proton’s internal dynamics.
A central challenge in understanding the proton’s spin is reconciling the experimental observation that quarks contribute only about 30% to the total proton spin. This leaves a significant portion of the spin to be accounted for by gluons and the orbital angular momentum of both quarks and gluons. The Drell–Yan process, particularly with a pion probe, is especially sensitive to the sea quarks and antiquarks, which are the dominant contributors to orbital angular momentum. The azimuthal distributions observed in pion-induced Drell–Yan scattering provide a unique opportunity to probe these sea quarks and their angular momentum. The holographic light-front QCD model, by its very nature, is designed to incorporate these complex contributions and make predictions that can be directly compared with experimental measurements, offering a bridge between theoretical concepts and observable phenomena.
The specific focus on pion-polarized proton-induced Drell–Yan scattering at COMPASS is driven by the desire to access information about the antiquark contribution to the proton’s spin. Pions, being composed of a quark and an antiquark, can inject specific flavors of antiquarks into the interaction. This allows researchers to probe the polarization and orbital motion of antiquarks within the proton with a finer granularity than might be possible with simpler probes. The COMPASS experiment, with its ability to handle polarized beams and targets, is ideally suited for such investigations, providing the high-statistics data necessary for precise measurements of these subtle spin-dependent effects. The synergy between the experimental capabilities of COMPASS and the theoretical sophistication of holographic light-front QCD is thus a potent combination for advancing knowledge.
The holographic light-front QCD approach offers a unique perspective on the fundamental forces governing the proton. Instead of directly solving the complex equations of QCD in flat spacetime, this method leverages the AdS/CFT correspondence to map the strong interactions of quarks and gluons onto a simpler, dual gravitational theory in a higher-dimensional anti-de Sitter spacetime. On the light front, this duality translates into a description of relativistic bound states, such as the proton, as specific configurations in this higher-dimensional geometry. This allows for the calculation of partonic distribution functions and other crucial quantities that describe the proton’s internal structure, including its spin and angular momentum content, in a way that is both theoretically consistent and computationally tractable.
The implications of accurately modeling azimuthal spin asymmetries are far-reaching. They provide direct experimental access to the transverse momentum distributions of quarks and antiquarks within the proton, a concept intimately linked to their orbital motion. These distributions, often referred to as Generalized Parton Distributions (GPDs) and Transverse Momentum Dependent (TMDs) distributions, are crucial for a complete understanding of the proton’s three-dimensional structure. By precisely measuring and theoretically reproducing these asymmetries, physicists can begin to build a comprehensive picture of how the proton’s spin is distributed among its constituents. This research represents a significant advancement in that direction, moving us closer to a holistic understanding of this fundamental particle.
The success of holographic light-front QCD in describing the COMPASS data suggests that this theoretical framework is a powerful tool for studying strongly coupled quantum field theories. The ability to make quantitative predictions for complex scattering processes, like the Drell–Yan process, is a testament to its validity. This approach not only helps to solve existing puzzles within the Standard Model, such as the proton spin mystery, but also opens up new avenues for exploring physics beyond the Standard Model. By providing a consistent framework for understanding the behavior of matter at its most fundamental level, holographic QCD promises to guide future theoretical and experimental investigations.
The specific calculations within this study likely involved mapping the interactions governing the Drell–Yan process onto the holographic dual. This would involve identifying the appropriate gravitational fields and their interactions in the higher-dimensional spacetime that correspond to the quarks, antiquarks, and gluons within the colliding particles. The dynamics of these fields would then be evolved, and the resulting scattering amplitudes calculated. These calculations would be specifically tailored to reproduce the azimuthal angle distributions of the outgoing lepton pairs in the COMPASS experiment, taking into account the polarization of the incoming proton and the nature of the pion probe.
The COMPASS experiment, with its rich history of spin physics, provides an invaluable dataset for testing theoretical models. The precision with which azimuthal spin asymmetries can be measured at COMPASS allows for stringent tests of theoretical predictions. When a theory like holographic light-front QCD can accurately describe these experimental observations, it lends significant credence to the underlying theoretical assumptions and provides confidence in its predictive power for other phenomena. The current work highlights the power of this iterative process of theoretical development and experimental validation in pushing the boundaries of our knowledge.
The “proton spin puzzle” is a compelling narrative in modern physics, highlighting the fact that the spin of a proton, a fundamental property akin to its charge or mass, is not simply the sum of the spins of its constituent quarks. While quarks do contribute, their individual spins account for only a fraction of the proton’s total spin. This deficit has driven decades of research into the roles of gluon spin and, crucially, the orbital angular momentum of both quarks and gluons within the proton. The Drell–Yan process, especially when initiated by a pion interacting with a polarized proton, offers a direct pathway to probing this orbital angular momentum, making it a target of immense interest for experimentalists and theorists alike.
The elegance of the holographic approach lies in its potential to simplify the complexity of strong interactions. By transforming difficult quantum field theory problems into more manageable gravitational problems, it allows for the calculation of properties that are otherwise intractable. For the proton spin problem, this means being able to calculate the distribution of orbital angular momentum among its constituents, a feat that is notoriously difficult with traditional QCD methods. This research showcases the practical application of this sophisticated theoretical tool to a longstanding and fundamental question in particle physics.
The future implications of this research extend beyond the immediate understanding of the proton. The success of holographic light-front QCD in this context suggests its applicability to a wider range of hadron structure phenomena. This could include investigations into the properties of other hadrons, the behavior of matter under extreme conditions, and potentially even the search for new physics beyond the Standard Model. By providing a consistent and predictive framework for studying strongly interacting systems, this research opens up new frontiers in our quest to understand the fundamental building blocks of the universe and the forces that govern them.
Subject of Research: Proton spin structure, Drell-Yan process, azimuthal spin asymmetries, holographic light-front QCD, quark and antiquark orbital angular momentum.
Article Title: Azimuthal spin asymmetries in pion-polarized proton induced Drell–Yan process at COMPASS using holographic light-front QCD
Article References: Gurjar, B., Mondal, C. Azimuthal spin asymmetries in pion-polarized proton induced Drell–Yan process at COMPASS using holographic light-front QCD. Eur. Phys. J. C 85, 1405 (2025). https://doi.org/10.1140/epjc/s10052-025-15138-1
Image Credits: AI Generated
DOI: https://doi.org/10.1140/epjc/s10052-025-15138-1
Keywords: Proton spin, Drell-Yan process, COMPASS, holographic QCD, light-front QCD, azimuthal asymmetries, hadron structure
