ePIC Science
- Exploring the Quark-Gluon Structure of Matter
- Imaging of Nucleons
- Exploring Nuclei as QCD Laboratories
Exploring the Quark-Gluon Structure of Matter
The quest to understand and image the structure of matter, which builds the visible world around us, has been a major focus of physics in recent decades. Nucleons—protons and neutrons, the building blocks of atomic nuclei—are made up of fractionally charged valence quarks and dynamically produced quark-antiquark pairs, all bound together by gluons, the carriers of the strong force. Modern nuclear physics aims to understand the structure of protons and neutrons through quark and gluon interactions, as described by the theory of the strong force—quantum chromodynamics (QCD)—and to understand how interactions between protons and neutrons in nuclei emerge from this dynamics.
Despite significant progress, many fundamental questions about the nature of matter remain unanswered. One of the most profound mysteries is how quarks and gluons, which carry a type of charge related to the strong interaction known as “color”, combine to form colorless protons and neutrons, how the properties of nucleons and nuclei emerge from the dynamics of quarks and gluons, and how the residual color forces hold these nucleons together to form atomic nuclei. The ePIC Detector at the Electron-Ion Collider (EIC) is designed to tackle these challenges, providing new insights and enhancing our understanding of the fundamental nature of matter.
Harnessing Electron Scattering
The EIC will use scattering of energetic electrons on protons and ions to examine their internal structures with unprecedented precision of electromagnetic interactions. The ePIC detector will analyze particles from thousands of collisions per second, collecting crucial data to uncover the intricacies of matter. By studying these energetic collisions of electrons with protons and heavier nuclei, ePIC will enable the exploration of new regions in matter dominated by gluons and transient quark-antiquark pairs, known as the quark sea, which originate from gluons.
Deep Inelastic Scattering
How do scientists probe the structure of matter at such a fundamental level? At ePIC, this will be achieved through deep inelastic scattering (DIS) processes. These processes are termed “deep” because they involve examining the internal structure of protons and neutrons at very small distance scales, much smaller than the size of the protons and neutrons themselves. In DIS, an incoming electron emits a high-energy virtual photon that interacts with the interior of the nucleon, probing its internal structure. What about gluons? While the virtual photon in DIS primarily interacts with quarks, gluons are probed indirectly through their influence on quark behavior and the dynamics of the nucleon’s internal structure.
DIS is an excellent tool for studying the nucleon’s interior because it involves a simple initial state (electron-proton/ion), with the electron having no internal structure, allowing for precise control over the interaction and clean final states. The electromagnetic interaction in DIS provides a clear probe of the nucleon’s quark structure, based on well-understood theoretical frameworks. In contrast, proton-proton collisions are more complex due to multiple interactions and higher particle multiplicity, making it harder to disentangle the underlying parton dynamics.
The energy transferred to the quarks is often sufficient to disrupt the binding forces that hold the nucleon together. This process is termed “inelastic” because the internal structure of the nucleon is changed, and it is broken apart rather than remaining intact. The electron that participated in the collision is scattered and detected. Its energy and angle of scattering provide information about the interaction, specifically the fraction of the nucleon’s momentum carried by the struck quark, denoted as x, and the resolution power at which the nucleon is probed, Q². Additionally, the quarks and gluons released from the nucleon form new particles through a process called hadronization, which can also be detected and analyzed to further understand the details of the interaction.
For example, in the process of semi-inclusive deep inelastic scattering (SIDIS), ePIC will measure not only the scattered electron but also one or more of the particles produced from the hadronization process. By analyzing the properties of these produced particles (such as their type, momentum, and angle), scientists can gain additional insights into the scattering process. In deep exclusive processes, the entire final state is measured, providing detailed insights into the dynamics of the interaction. In these processes, such as deeply virtual Compton scattering and exclusive meson production, the nucleon often remains intact after the interaction. Both SIDIS and deep exclusive processes can provide insight into the more complex 3D structure of nucleons, which you can learn more about in the 3D Imaging of Nucleons section.
Imaging of Nucleons
The EIC and its ePIC Detector will address fundamental questions about nucleons, including how their properties like mass and spin emerge from their partonic structure, and how sea quarks and gluons, along with their spins, are distributed in space and momentum within the nucleon. Through different types of electron scattering processes, this research will illuminate the complex quark-gluon interactions that form the foundation of visible matter.
Unraveling the Mystery of Proton Spin
Understanding the origin of the proton’s spin is a fundamental challenge in nuclear physics. It is more than the s=1/2 we know from textbooks. We now know that it is the result of the complex interplay between the intrinsic properties and interactions of quarks and gluons. While protons are composed of three valence quarks, the spin contribution of these quarks accounts for only about 30% of the proton’s spin. The remaining spin is believed to arise from gluon spins, sea quarks, and the orbital motion of quarks and gluons. How all these various contributions add up to exactly 1/2 is one of the amazing puzzles in nature.
The ePIC detector at EIC is designed to probe these contributions in unprecedented detail. By colliding longitudinally polarized electrons with nucleons and analyzing both inclusive and semi-inclusive deep inelastic scattering events, we will probe the spin contributions from gluons and various quark flavors. This comprehensive approach will allow researchers to precisely quantify the factors contributing to the proton’s spin, offering new insights into one of the most profound puzzles of nuclear physics.
How Does the Mass of the Nucleon Arise?
One of the most significant discoveries at CERN’s Large Hadron Collider is the experimental discovery of a field responsible for giving mass to fundamental particles. This field, known as the Higgs field, plays a crucial role in explaining the mass of particles like electrons. However, for the majority of visible mass in the universe, which is found in protons and neutrons, the story is more complex.
Nucleons derive their mass not just from the Higgs mechanism, but primarily from the intricate interplay of gluons, sea quarks, and the kinetic energy of their constituent quarks. Gluons, despite being electrically neutral, contribute significantly to nucleon mass through the energy stored in their fields, a property that remains challenging to measure directly.
The Electron-Ion Collider, equipped with the advanced ePIC Detector, will revolutionize our understanding of these fundamental processes. Using a technique called parton tomography, the EIC will map the distribution of gluons within nucleons with great precision in both their spatial arrangements and their momenta. Traditional methods like deep inelastic scattering reveal longitudinal momentum fractions of probed nucleon constituents. In contrast, tomography adds a new dimension by exploring the transverse distances and momenta, providing critical insights into the 3D distributions of quarks and gluons within nucleons.
By generating detailed tomographic images across a wide spectrum of longitudinal momenta—from the surface-level valence quarks to deeper internal states—the EIC will uncover essential details about how gluons are spatially arranged within nucleons. The EIC’s broad kinematic range and high luminosity, facilitated by the ePIC Detector, will allow researchers to conduct comprehensive studies on the energy density and pressure exerted by gluons within nucleons. This research will refine our understanding of where nucleon mass originates and enhance theoretical models concerning the behavior of gluons.
3D Imaging of Nucleons
The Electron-Ion Collider, enhanced by the ePIC Detector, will enable precise 3D imaging of gluons and sea quarks in both momentum and position spaces within nucleons. Momentum imaging will primarily utilize semi-inclusive deep inelastic scattering processes. Imaging in position space will be studied by exclusive processes, where the probed nucleon remains intact and either a real photon (in deeply virtual Compton scattering) or a bound quark-antiquark state (in deeply virtual meson production) is produced.
Imaging in momentum space
With semi-inclusive deep inelastic scattering, the ePIC detector will probe both the large momentum transfer from the electron beam for high resolution Q² and the transverse momentum of produced hadrons. This approach enables detailed investigations into the confined motion of partons within nucleons, essential for understanding their three-dimensional dynamics encoded in Transverse Momentum Distributions (TMDs). TMDs provide critical insights into correlations between parton motion, spin, and orbital contributions in polarized nucleons, shedding light on the complex interplay of sea quarks and gluons at different momentum scales. These correlations may arise from spin-orbit coupling among the partons, a phenomenon currently underexplored. With its advanced particle identification capabilities and ability to access data from polarized beam collisions, the ePIC detector will significantly advance our understanding of the complex 3D structure of nucleons, which current facilities cannot fully explore.
Imaging in position space
With its broad range of collision energies and high luminosity, the EIC, equipped with the nearly hermetic ePIC detector, will achieve unparalleled precision in imaging nucleons across varying transverse distances. The accessible kinematic range of the EIC extends from regions dominated by sea quarks and gluons to those where valence quarks become prominent, linking to experiments sensitive to the valence region. Tomographic images from the measurements of previously mentioned exclusive processes are encapsulated within Generalized Parton Distributions (GPDs). GPDs provide detailed insights into spin-orbit correlations and the angular momentum of partons, including both their spin and orbital motion. The comprehensive kinematic coverage offered by the EIC and ePIC detector will be instrumental in discerning the contributions of quark and gluon angular momentum to the proton’s overall spin.
Exploring Nuclei as QCD Laboratories
The Electron-Ion Collider, equipped with the state-of-the-art ePIC detector, opens a new frontier in scientific exploration by using atomic nuclei as Quantum Chromodynamics laboratories. This endeavor aims to unravel the mysteries of quarks, gluons, and their interactions within the nucleus with unparalleled precision.
Central to these studies is the investigation of gluons. The EIC will particularly focus on probing the regime of low x, where gluon densities in the nuclei are extraordinarily high. In this high-density regime, known as the gluon saturation regime, the density of gluons becomes so large that their interactions lead to significant recombination effects, effectively limiting their further growth in density. Scientists anticipate exploring new states of matter composed of dense gluonic material, which are described by the Color Glass Condensate theory.
In the gluon saturation regime, gluons multiply rapidly but also start to recombine more frequently. This balance between their production and recombination creates a unique state where the number of gluons stabilizes and no longer increases indefinitely. This is known as reaching the saturation point. The regime is characterized by the saturation scale, which increases with the energy and size of the nucleus. By studying these interactions, researchers aim to gain deeper insights into the fundamental theory of quantum chromodynamics and the behavior of matter under extreme conditions. With high-energy electron and various nuclei collisions, ePIC will precisely measure the gluon distributions and their behavior in a high-density state. This will help uncover the properties of dense gluonic matter and its impact on the internal structure of matter.
Additionally, ePIC will explore the modification of quark distributions within atomic nuclei. By scrutinizing these distributions, researchers seek to understand how the nuclear environment influences the behavior and properties of quarks, shedding light on fundamental aspects of nuclear structure.
Another unique EIC capability lies in its ability to study the evolution of jets—energetic sprays of particles produced when quarks and gluons fragment within the nucleus. This phenomenon provides crucial insights into QCD dynamics in a controlled environment, offering researchers a window into how quarks lose energy and transform into observable particles in the process of hadronization. The study of jet evolution within the nuclear medium, characterized by its known properties such as size and density, poses fundamental questions about the nature of particle formation and the underlying mechanisms governing these processes.
By utilizing the ePIC Detector’s advanced technologies and the EIC’s capabilities of colliding electrons with ions, scientists aim to deepen our knowledge of nuclear matter, advance theoretical frameworks, and pave the way for future discoveries in nuclear physics and beyond.
