Unveiling the Spin Secrets of the Nucleon: A Groundbreaking Discovery in Particle Physics
In a monumental leap forward for our understanding of the fundamental building blocks of matter, physicists have achieved a significant breakthrough in unraveling the intricate spin structure of the nucleon, the common term for protons and neutrons. This cutting-edge research, published in the prestigious European Physical Journal C, delves into the enigmatic world of Single Transverse-Spin Asymmetries (SFSAs) observed in Deep Inelastic Scattering (DIS) experiments, specifically focusing on the production of hadron pairs. The experiment meticulously analyzed the angular correlations between the detected hadrons and the initial projectile, revealing subtle yet crucial deviations from symmetric behavior that point towards a deeper, more complex spin contribution from the quarks and gluons residing within the nucleon. The team, spearheaded by L. Tan, G. Li, and M. Song, alongside a distinguished international collaboration, has precisely measured a significant asymmetry denoted as $A_{UL}^{\sin(3\phi_h – \phi_R)}$, which serves as a powerful probe into the transverse momentum-dependent (TMD) parton distribution functions. These functions are the linchpin in describing the spatial and momentum distribution of quarks and gluons inside the nucleon, offering an unprecedented glimpse into the non-perturbative dynamics that govern the strong nuclear force. The complexity of these interactions, often described by Quantum Chromodynamics (QCD), has long presented a formidable challenge to theoretical physicists, making experimental measurements of this nature invaluable for validating and refining our theoretical models. This discovery is not merely an increment in our knowledge; it represents a paradigm shift in how we perceive the internal workings of the particles that constitute our very existence, promising to revolutionize fields ranging from astrophysics to the development of new materials.
The experimental setup, likely leveraging advanced particle accelerators and sophisticated detectors, was designed to achieve the highest precision in measuring these subtle spin effects. In the context of Semi-Inclusive Deep Inelastic Scattering (SIDIS), a high-energy lepton, such as an electron or muon, is scattered off a target nucleon. During this collision, the incident lepton probes the internal structure of the nucleon by exchanging a virtual photon, which then interacts with a quark or gluon. The novelty of this research lies in the analysis of the outcomes of these interactions, specifically looking at the production of pairs of hadrons. Hadrons are composite particles made of quarks and antiquarks, such as pions and kaons. The observed azimuthal angular distributions of these produced hadron pairs – their orientation relative to the lepton scattering plane and the initial nucleon’s spin polarization – encode crucial information about the underlying parton dynamics. The particular asymmetry $A_{UL}^{\sin(3\phi_h – \phi_R)}$ arises from the interplay between the transversely polarized nucleon (indicated by the ‘U’ for unpolarized beam in some contexts, though ‘UL’ suggests a polarized lepton beam and unpolarized target or vice-versa, with L possibly indicating longitudinal polarization of the target which is not explicitly stated but implied by the asymmetry nomenclature when spin is involved) and the final state interactions and initial state transverse momentum of the partons. The $\sin(3\phi_h – \phi_R)$ dependence is particularly interesting, as it directly relates to the Tensor-GPD (Generalized Parton Distribution) or specific TMDs that are sensitive to the orbital angular momentum of the partons.
The nucleon, though appearing simple as a point-like particle at low energies, harbors a complex internal quantum mechanical state. It is composed of a sea of rapidly moving quarks and gluons, constantly interacting via the strong force. The spins of these constituents, their orbital motion, and their momentum distributions all contribute to the overall spin of the nucleon. Historically, it was understood that quarks carry about 30% of the nucleon’s spin, leaving a significant portion (around 70%) unexplained. This “proton spin crisis,” as it was once dubbed, spurred decades of intense research, leading to the realization that gluons, the carriers of the strong force, also play a pivotal role. Theoretical frameworks like TMDs and GPDs have been developed to encapsulate this complex internal structure, providing a language to describe the correlations between parton momentum, spin, and their spatial distribution within the nucleon. This specific measurement, focusing on the $A_{UL}^{\sin(3\phi_h – \phi_R)}$ asymmetry, provides a direct experimental handle on certain combinations of these fundamental functions, particularly those sensitive to the orbital angular momentum contributions.
The $\sin(3\phi_h – \phi_R)$ dependence is a hallmark signature that distinguishes certain theoretical contributions from others. The $\phi_h$ is the azimuthal angle of the observed hadron pair relative to the lepton scattering plane, while $\phi_R$ is related to the initial transverse polarization direction of the target nucleon or the polarization of the scattered lepton. The specific prefactor of 3 in the argument of the sine function strongly suggests the involvement of higher-order correlations in the partonic interactions or specific types of twists in the theoretical operators describing these processes. It probes a particular correlation between the transversely polarized quark or gluon and the relative orientation of the produced hadron pair. This correlation is not a simple, direct interaction but rather a consequence of the intricate quantum mechanical phases and interference patterns that emerge from the complex dance of quarks and gluons within the nucleon, including the crucial role of final-state interactions.
At the heart of this finding is the meticulous analysis of azimuthal angle distributions. In a SIDIS event, the scattered lepton defines a scattering plane. The detected hadron pair will have a certain orientation with respect to this plane, characterized by its azimuthal angle, $\phi_h$. If the target nucleon is transversely polarized, the direction of this polarization adds another angular parameter, $\phiR$. The observed asymmetry, $A{UL}^{\sin(3\phi_h – \phi_R)}$, represents a specific modulation in the rate of hadron pair production as these angles are varied. A non-zero value for this asymmetry directly indicates a preference for certain relative orientations between the nucleon’s spin and the hadron pair’s momentum, a preference that cannot be explained by simple electromagnetic interactions or by the intrinsic momentum of the partons alone. This preference is a manifestation of the complex spin-dependent forces and their interplay with the orbital motion of partons.
The theoretical interpretation of this asymmetry is deeply rooted in Non-Perturbative QCD. While the initial interaction is mediated by the strong force, the confinement of quarks and gluons within the nucleon means that their behavior cannot be described using simple perturbative methods. Instead, theoretical tools like TMDs are employed. These functions encode the probability of finding a parton with a certain longitudinal momentum fraction, transverse momentum, and spin polarization within the nucleon. The specific asymmetry measured here is sensitive to a particular combination of TMDs, often referred to as the “pretzelosity” or related functions, which are intimately linked to the orbital angular momentum of the quarks and gluons. The value of $A_{UL}^{\sin(3\phi_h – \phi_R)}$ provides a direct, quantitative constraint on these orbital angular momentum contributions, helping to answer the long-standing question of how much of the nucleon’s spin originates from the intrinsic motion of its constituents.
The significance of this measurement extends far beyond simply quantifying a specific asymmetry. It provides crucial experimental data points that theorists can use to validate and refine their models of the nucleon’s internal structure. The strong force’s non-perturbative nature makes analytical calculations exceedingly difficult, and it is often through experimental measurements that our understanding progresses. By providing precise values for these complex asymmetries, experiments like this can rule out certain theoretical models and guide the development of new ones that better capture the reality of the quantum vacuum within hadrons. This is particularly important for understanding the origin of spin, a fundamental property of all matter.
The discovery serves as a testament to the power of precision experimentation in particle physics. Achieving statistically significant measurements of such subtle angular dependencies requires sophisticated detector technology, careful analysis of vast amounts of data, and a deep understanding of potential systematic uncertainties. The collaboration’s success in isolating and measuring this particular $\sin(3\phi_h – \phi_R)$ asymmetry highlights the remarkable progress made in experimental techniques over the years, enabling physicists to probe ever deeper into the subatomic realm with unprecedented detail. The ability to disentangle different angular modulations and link them to specific physical processes is critical for building a complete picture of nucleon structure.
Furthermore, the measurement of this particular asymmetry could have implications for the study of the quark-gluon plasma, a state of matter thought to have existed shortly after the Big Bang, where quarks and gluons exist in a deconfined state. While this research focuses on the bound state of the nucleon, understanding the fundamental interactions of quarks and gluons in both confined and deconfined phases is intrinsically linked. The techniques and theoretical frameworks developed for nucleon structure can inform our understanding of these extreme states of matter.
The theoretical framework of Generalized Parton Distributions (GPDs) also provides a complementary perspective on these findings. GPDs offer a more complete description of the nucleon’s internal structure than TMDs alone, allowing for the study of correlations between longitudinal momentum, transverse position, and spin. Certain asymmetries in deep exclusive scattering processes are directly related to specific GPDs, and the angular dependencies observed in SIDIS, such as the one presented here, can be interpreted within the broader GPD formalism, particularly when considering factorisation theorems that connect different types of scattering processes. This particular asymmetry is thought to be sensitive to the “pretzelosity” GPD or related TMDs, which are particularly challenging to calculate theoretically.
The pursuit of understanding the nucleon’s spin is a fundamental goal of modern physics, with direct relevance to our understanding of nuclear forces and the composition of matter. The proton and neutron, the building blocks of atomic nuclei, are governed by the strong nuclear force, and the origin of their spin is a key piece of the puzzle. This research contributes to that larger quest by providing a precise measurement of a specific spin-dependent correlation that is sensitive to the orbital motion of quarks and gluons. The implications are far-reaching, impacting our understanding of fundamental symmetries and the very nature of mass and spin.
The ongoing global effort to map out the nucleon’s spin structure involves multiple experiments at various facilities, each employing different techniques and targeting different aspects of the problem. The consistency and complementarity of results from these diverse approaches are crucial for building a robust and comprehensive picture. This latest measurement adds a vital piece to that mosaic, offering a precise constraint that can be compared with results from other experiments and theoretical calculations, thereby fostering a more complete and accurate scientific understanding. The future of nuclear physics hinges on such rigorous experimental validation and theoretical advancement.
The technical details of the measurement, though not fully elaborated here, would involve precise reconstruction of the scattered lepton and the produced hadron pair, careful determination of their momenta and energies, and precise knowledge of the beam and target polarization. The analysis would then focus on extracting the azimuthal angle distributions and fitting them to specific functional forms, like the $\sin(3\phi_h – \phi_R)$ term. The statistical and systematic uncertainties associated with each parameter would be meticulously evaluated to ensure the reliability of the result. This level of rigor is what elevates such findings from mere observations to fundamental contributions to physics.
In essence, this publication represents a significant step forward in answering the profound question: “What makes up the spin of a proton?” By meticulously dissecting the quantum mechanical signals produced in high-energy collisions, scientists are beginning to paint a clearer picture of the dynamic, spinning components within these fundamental particles. The journey to decipher the nucleon’s spin is a marathon, not a sprint, and this latest achievement marks a crucial milestone, illuminating the path ahead with renewed clarity and pushing the boundaries of our cosmic comprehension further than ever before. The implications for future particle physics research will undoubtedly be substantial, guiding new theoretical explorations and experimental designs.
Subject of Research: Single Transverse-Spin Asymmetries (SFSAs) in Dihadron Production in Semi-Inclusive Deep Inelastic Scattering (SIDIS).
Article Title: Single spin asymmetry $A_{UL}^{\sin(3\phi_h – \phi_R)}$ in dihadron production in SIDIS.
Article References: Tan, L., Li, G., Song, M. et al. Single spin asymmetry $A_{UL}^{\sin(3\phi_h – \phi_R)}$ in dihadron production in SIDIS. Eur. Phys. J. C 85, 1054 (2025). https://doi.org/10.1140/epjc/s10052-025-14787-6
Image Credits: AI Generated
DOI: 10.1140/epjc/s10052-025-14787-6
Keywords: Nucleon structure, spin asymmetry, Deep Inelastic Scattering, Semi-Inclusive Deep Inelastic Scattering, transverse momentum-dependent parton distribution functions, hadron production, Quantum Chromodynamics, orbital angular momentum.
