Nucleon Structure from lattice QCD Using a Nearly Physical Pion Mass
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
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Disentangling the EMC effect: from free to bound nucleon structure
Deep inelastic scattering experiments using electron beams on fixed targets allow for the study of the structure of nucleons. While the structure of the free proton has been well measured, the free neutron structure and the modification of nucleon structure in the nuclear environment, the EMC effect, remain open questions. Studies suggest that the EMC effect may arise due to strongly-interacting nucleon-nucleon systems, referred to as short-range correlations. The BAND experiment at the Thomas Jefferson National Accelerator Facility seeks to probe the bound proton structure when in a strongly-interacting state, using spectator-tagged deep inelastic scattering. This thesis presents the results of the BAND experiment, demonstrating the proton is modified when strongly-interacting with the spectator neutron in deuterium. This thesis also explores efforts to describe inclusive and tagged deep inelastic scattering data within a uniform theoretical framework to explore nucleon structure modification effects and extract the free neutron structure.
Segarra, Efrain↗
Strong QCD Insights from Excited Nucleon Structure Studies with CLAS and CLAS12
Studies of the spectrum of hadrons and their structure in experiments with electromagnetic probes offer unique insight into many facets of the strong interaction in the regime of large quark-gluon running coupling, i.e. the regime of strong QCD. The experimental program within Hall B at Jefferson Laboratory based on data acquired with the CLAS spectrometer using electron and photon beams with energies up to 6 GeV has already considerably extended the scope of research in hadron physics in joint efforts between experiment and phenomenological data analysis. Impressive progress in relating the hadron structure observables inferred from the data to the strong QCD mechanisms underlying hadron mass generation has been achieved in the past decade. Furthermore, these results will be considerably extended with data from the experimental program with the new CLAS12 spectrometer that has begun data taking using electron beams with energies up to 11 GeV. With this extended kinematic reach the structure of nucleon resonances will be probed at the highest photon virtualities ever achieved in the studies of exclusive electroproduction, which will allow for the exploration of the distance scale where>98% of light hadron mass emerges from QCD in the transition of the strong interaction from the regime of quark-gluon confinement to perturbative QCD.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Studies of the nucleon structure in back-to-back SIDIS
The Deep Inelastic Scattering (DIS) proved to be a great tool in testing of the theory of strong interactions, which was a major focus in last decades. Semi-Inclusive DIS (SIDIS), with detection of an additional hadron allowed first studies of 3D structure of the nucleon, moving the main focus from testing the QCD to understanding of strong interactions and quark gluon dynamics to address a number of puzzles accumulated in recent years. Detection of two hadrons in SIDIS, which is even more complicated, provides access to details of quark gluon interactions inaccessible in single-hadron SIDIS, providing a new avenue to study the complex nucleon structure. Large acceptance of the Electron Ion Collider, allowing detection of two hadrons, produced back-to-back in the current and target fragmentation regions, combined with clear separation of two regions, would provide a unique possibility to study the nucleon structure in target fragmentation region, and correlations of target and current fragmentation regions.
Avakian, Harut↗
Nucleon structure studies: DVCS on polarised protons with the CLAS12 experiment, and development of Micromegas detectors for EIC
The quark and gluon structure of nucleons is crucial for understanding the origin of their mass and spin. This information is encoded in structure function such as Generalised Parton Distributions (GPDs), which provide a three-dimensional picture of the nucleon in terms of its constituents. Deeply Virtual Compton Scattering (DVCS) offers the most direct access to GPDs, but their extraction requires high-precision measurements of multiple observables over a wide kinematic range. In 2022 and 2023 the CLAS12 experiment at Jefferson Lab (JLab) collected data from polarised electron scattering on a longitudinally polarised proton target, enabling the first measurement of polarised DVCS asymmetries at JLab 12GeV kinematics. This thesis will present preliminary measurement of the beam, target and double spin DVCS asymmetries from the CLAS12 polarised proton target data. Nucleon structure studies will also constitute a major part of the physics program at the future Electron Ion Collider (EIC). The development of the first detector at the EIC is ongoing and it requires light Micro-Pattern Gaseous Detectors (MPGDs) for its tracking system. This thesis further reports initial tests of Micromegas MPGDs with a two-dimensional readout developed for EIC.
Polcher, Samy [Univ. Paris-Saclay, Gif-sur-Yvette ↗
Nucleon Structure Studies at Jefferson Lab and the Electron Ion Collider
The research programs of Thomas Jefferson Laboratory (JLab) and the future Electron- Ion Collider (EIC) focus on one of the main goals of strong interaction studies : understanding the structure of nucleons in terms of the quarks and gluons composing them. Their structure is encoded in functions such as Generalized Parton Distributions (GPDs), which describe how quarks and gluons’ transverse position and longitudinal momentum are distributed inside nucleons. GPDs allow to obtain three-dimensional pictures of nucleons and to understand some of their fundamental properties, such as their internal pressure or the emergence of their spin from the dynamics of the partons composing them. At JLab and the EIC, electron beams are used to probe nucleons. Measurement of reactions such as Deeply Virtual Compton Scattering (DVCS) allows access to GPDs. The first longitudinally polarized-target experiment of the CLAS12 program at Jlab took place in 2022-2023. Combining polarized electron beams and nucleon targets, this experiment offers unique access to observables that allow the measurement of different types of GPDs. In particular, the DVCS beam- and target-spin asymmetries for protons and neutrons in deuterium will be measured for the first time. They give access to kinds of GPDs that are still poorly known, and the comparison between proton and neutron data will allow the extraction of the flavor dependence of the structure of nucleons. Specific analysis methods have been implemented to work with a polarized nuclear target and are presented in this thesis. These methods allow to obtain preliminary results for the asymmetries, waiting for the complete statistics to be available. In the long term, the experimental program for the EIC has been established with a strong emphasis on the measurement of the structure of nucleons at high energy. Measurements of reactions such as DVCS impose strict requirements on the electromagnetic calorimeter that will allow to measure the energy of the scattered electrons and photons. This calorimeter, which is under development, will be based on scintillating crystals read by Silicon Photomultipliers (SiPMs). A new type of glass-based scintillating material was tested, evaluating the possibilities to meet the technical requirements concerning their light yield and resistance to radiation damage in particular. Several models of SiPMs have been characterized, demonstrating they can operate over the vast energy range necessary to address the physics case at the EIC and providing guidelines for developing their readout electronics.
Pilleux, Noemie↗
Exploring the 3D nucleon structure with CLAS and CLAS12 at JLAB
Exploring the 3 dimensional structure of the nucleon can help to understand several fundamental questions of nature, such as the origin of the nucleon spin and the charge and density distributions inside the nucleon. In QCD, the 3-dimensional structure of the nucleon is described by Wigner functions. However, experimentally momentum and coordinate space have to be assessed independently. The momentum distribution can be accessed by transverse momentum dependent distribution functions (TMDs) measured in semi-inclusive deep inelastic scattering (SIDIS) or Drell-Yan processes while the distribution in transverse coordinate and longitudinal momentum space is described by generalized parton distributions (GPDs) which can be accessed for example by deeply virtual Compton scattering (DVCS) and hard exclusive meson production (DVMP). Based on the high quality data of CLAS and the recently upgraded CLAS12 detector at Jefferson Laboratory (JLAB), a detailed study of these distribution functions is being performed. With the new CLAS12 data, multidimensional, high precision studies in an extended kinematic range become possible for the first time. The talk will present the results of recent SIDIS, DVCS and DVMP studies with CLAS and CLAS12 and their impact on the understanding of the 3D nucleon structure.
Diehl, Stefan↗
Theoretical and Phenomenological Studies of the Nucleon Structure in High Energy Reactions
The understanding of the internal structure of the proton and other strongly interacting particles is at the forefront of modern nuclear physics research. Generalized Parton Distribution Functions (GPDs) are a powerful tool to advance the understanding of hadron structure. In addition to the information about the one- dimensional collinear momentum distributions of partons (quarks, anti-quarks, and gluons) known from studies of high energy deep-inelastic reactions, GPDs also carry information on the distribution of partons in the transverse plane, and allow us in this way to access the three-dimensional structure of the nucleon. GPDs can be studied in hard-exclusive reactions and contain also information on the energy-momentum tensor form factors which will allow us to gain insights on quantities like pressure or angular momentum distribution inside the nucleon. The goal of this thesis is to deepen our understanding of the three-dimensional structure of the nucleon. We investigate energy-momentum tensor form factors and densities, and all leading-twist GPDs in the bag model. This quark model provides a consistent theoretical framework to investigate many general concepts that have recently attracted interest, and allows one to study insightful limits like the large-Nc limit, heavy-quark limit or the non-relativistic limit. Another important aspect of this thesis is the discussion of the monopole and quadrupole contributions to the angular momentum density. Finally, the description of pseudoscalar meson production in exclusive processes in terms of GPDs is discussed in the Goloskokov-Kroll model and implemented in the PARTONS framework, a software development project which will provide direct support for experiments at the Jefferson National Lab and the future Electron-Ion Collider.
Tezgin, Kemal↗
Three-dimensional nucleon structure: Spin, spatial imaging, orbital motion
Recent theoretical advances in Quantum Chromodynamics make it possible to visualize hadrons as extended systems in relativistic space-time and describe their spin structure, spatial shape, and internal motion in terms of the basic quark and gluon degrees of freedom. Mapping these structures will require a dedicated experimental effort over the next two decades. The Jefferson Lab 12 GeV Upgrade will probe the valence quark component of the nucleon with unprecedented accuracy. A future high-luminosity Electron-Ion Collider (EIC) could open up the region of sea quarks and gluons and definitively determine their role in nucleon structure. In this talk I summarize our theoretical understanding and illustrate the impact of the future facilities on nucleon structure in QCD.
Ji, Xiangdong↗
Nucleon Structure on the Light Front
The light-front (or partonic) view of relativistic dynamics enables a description of hadrons as composite many-body systems that shows many analogies with traditional few-body systems (atoms, nuclei). It defines the spatial structure of hadrons and allows one to study the space-time evolution of strong and electromagnetic processes. Light-front methods represent an essential tool in the theory and phenomenology of nucleon structure and are used both in QCD and in formulations based on effective degrees of freedom. In this talk we explain the physical picture and highlight several novel applications. This includes (a) the transverse charge and current densities measured in elastic eN scattering and their interpretation; (b) the study of peripheral spatial nucleon structure using chiral effective field theory; (c) resonance structure on the light-front; (d) the mapping of the spatial distributions of QCD quarks and gluons (Generalized Parton Distributions, or GPDs) in the nucleon using exclusive processes at multi-GeV momentum transfer.
Weiss, Christian↗
Beyond the Born Approximation: Two Photon Exchange in Nucleon Structure Studies
Precision measurements of the electric form factor of the proton at Jefferson Lab have revealed a striking difference between Rosenbluth separation and polarization transfer data. Much of this discrepancy has been attributed to two-photon exchange, previously ignored in nucleon structure studies due to lack of precision in the data. Recently, experiments were conducted at Jefferson Lab where unpolarized electrons were scattered from neutrons polarized normal to the electron scattering plane. The polarized neutrons were produced by polarizing 3He nuclei using spin-exchange optical pumping. The single spin asymmetry for inclusive scattering, formed by flipping the target spin direction, is predicted to be exactly zero in the Born approximation by the Christ-Lee theorem. A non-zero asymmetry is an indication of contributions from two-photon exchange. This asymmetry provides a new tool to study the structure of the nucleon and to directly test models of two-photon exchange contributions. Results will be presented from measurements in the quasi-elastic and deep-inelastic regions.
Averett, Todd↗
Exploring the Nucleon Structure via Deep Electroproduction Processes
Understanding the internal structure of the nucleon is a fundamental goal of modern physics, which aims at a comprehensive framework describing the internal dynamics of quarks and gluons. Among other structure functions, Generalized Parton Distributions (GPDs) offer a powerful framework for describing the nucleon dynamics by correlating the longitudinal momentum and the transverse position of its internal partons. Such a correlation provides a three-dimensional picture of the nucleon and enables access to fundamental properties, including the internal pressure distributions and the parton’s angular momentum contribution to the nucleon's total spin, thereby playing a central role in resolving the nucleon spin puzzle. At the Thomas Jefferson National Facility (JLab), polarized electron beam experiments allow for probing GPDs through the measurement of hard exclusive processes. Among the cleanest experimental channels, we find the electro-production of a real photon through the Deeply Virtual Compton Scattering (DVCS) mechanism. The first data-taking period of the CLAS12 program, taking place in 2018, allowed for unique DVCS Beam Spin Asymmetry (BSA) measurements in the phase space covered by a 10.6 GeV polarized electron beam impinging on an unpolarized liquid hydrogen target. Although detecting all final-state particles ensures exclusivity of the process, conservation laws indicate that it is not mandatory. I adopt an approach omitting the direct detection of the recoil proton, providing a simplified yet effective event selection strategy that boosts statistics and gives access to a larger phase space sensitive to the underlying GPD dynamics through BSA and cross section measurements. The Double DVCS (DDVCS) process promises a dedicated mapping of GPDs. Contributing to the electro-production of a lepton pair cross-section, the DDVCS reaction extends DVCS by allowing the final-state photon to be virtual, enriching the kinematic phase space and providing unique access to the internal correlations encoded by GPDs. A feasibility study is conducted to assess the potential of future DDVCS measurements at Jefferson Lab and the future Electron-Ion Collider (EIC). While Jefferson Lab will provide DDVCS measurements in the valence region through the SoLID$\mu$ and $\mu$CLAS12 experimental projects, in the long term, the EIC will provide complementary measurements in the sea region, both accessing unprecedented information about GPDs in a phase space region otherwise inaccessible. Taken together, these investigations demonstrate both the current capabilities and future opportunities for probing GPDs through exclusive processes. The experimental analysis of DVCS at CLAS12 provides precise measurements within an established framework, while the phenomenological study of DDVCS opens the door to richer and more comprehensive explorations with future detectors and facilities.
Alvarado, Juan [Université Paris-Saclay: Gif-sur-Y↗
Recent progresses in few-nucleon structure and dynamics in chiral effective field theory
The most recent progresses made within the framework of chiral effective field theory for few-nucleon structure and low-energy reactions are here presented. In particular, for the 𝐴 = 2 sector, the study of muon capture on deuteron is reviewed. Then, the results obtained using the Hyperspherical Harmonics ab-initiomethod for the 4He monopole form factor and the 3He(®𝑛, 𝑝) 3H parity-conserving asymmetry are described.
Marcucci, Laura [University of Pisa, Pisa, Italy; ↗
Physical implications of the extrapolation and statistical bootstrap of nucleon structure function ratios F 2 n F 2 p for mirror nuclei He 3 and H 3
A nuclear physics example of statistical bootstrap is used on the MARATHON nucleon structure function ratio data in the quark momentum fraction regions x B → 0 and x B → 1. The extrapolated F 2 ratio as quark momentum fraction x B → 1 is $\frac{F^n_2}{F^p_2}$ → 0.4 ± 0.05 and this value is compared to theoretical predictions. The extrapolated ratio when x B → 0 favors the simple model of isospin symmetry with the complete dominance of sea quarks at low momentum fraction. At high- x B , the proton quark distribution function ratio d/u is derived from the F 2 ratio and found to be d/u → 1/6. Our extrapolated values for both the $\frac{F^n_2}{F^p_2}$ ratio and the d/u parton distribution function ratio are within uncertainties of perturbative QCD values from quark counting, helicity conservation arguments, and a Dyson-Schwinger equation with a contact interaction model. In addition, it is possible to match the statistical bootstrap value to theoretical predictions by allowing two compatible models to act simultaneously in the nucleon wave function. Finally, one such example is nucleon wave functions composed of a linear combination of a quark-diquark state and a three-valence quark correlated state with coefficients that combine to give the extrapolated F 2 ratio at x B = 1.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Lattice QCD Calculation of Nucleon Structure
It is emphasized in the 2015 NSAC Long Range Plan that "understanding the structure of hadrons in terms of QCD's quarks and gluons is one of the central goals of modern nuclear physics." Over the last three decades, lattice QCD has developed into a powerful tool for ab initio calculations of strong-interaction physics. Up until now, it is the only theoretical approach to solving QCD with controlled statistical and systematic errors. Since 1985, we have proposed and carried out first-principles calculations of nucleon structure and hadron spectroscopy using lattice QCD which entails both algorithmic development and large-scale computer simulation. We started out by calculating the nucleon form factors -- electromagnetic, axial-vector, πNN, and scalar form factors, the quark spin contribution to the proton spin, the strangeness magnetic moment, the quark orbital angular momentum, the quark momentum fraction, and the quark and glue decomposition of the proton momentum and angular momentum. The first round of calculations were done with Wilson fermions in the `quenched' approximation where the dynamical effects of the quarks in the sea are not taken into account in the Monte Carlo simulation to generate the background gauge configurations. Beginning in 2000, we have started implementing the overlap fermion formulation into the spectroscopy and structure calculations. This is mainly because the overlap fermion honors chiral symmetry as in the continuum. It is going to be more and more important to take the symmetry into account as the simulations move closer to the physical point where the u and d quark masses are as light as a few MeV only. We began with lattices which have quark masses in the sea corresponding to a pion mass at ~ 300 MeV and obtained the strange form factors, charm and strange quark masses, the charmonium spectrum and the D s meson decay constant f D s , the strangeness and charmness, the meson mass decomposition and the strange quark spin from the anomalous Ward identity. Recently, we have started to include multiple lattices with different lattice spacings and different volumes including large lattices at the physical pion mass point. We are getting quite close to being able to calculate the hadron structure at the physical point and to do the continuum and large volume extrapolations, which is our ultimate aim. We have now finished several projects which have included these systematic corrections. They include the leptonic decay width of the ρ, the πN sigma and strange sigma terms, and the strange quark magnetic moment. Over the years, we have also studied hadron spectroscopy with lattice calculations and in phenomenology. These include Roper resonance, pentaquark state, charmonium spectrum, glueballs, scalar mesons a 0 (1450) and σ(600) and other scalar mesons, and the 1 -+ meson. In addition, we have employed the canonical approach to explore the first-order phase transition and the critical point at finite density and finite temperature. We have also discovered a new parton degree of freedom -- the connected sea partons, from the path-integral formulation of the hadronic tensor, which explains the experimentally observed Gottfried sum rule violation. Combining experimental result on the strange parton distribution, the CT10 global fitting results of the total u and d anti-partons and the lattice result of the ratio of the momentum fraction of the strange vs that of u or d in the disconnected insertion, we have shown that the connected sea partons can be isolated. In this final technical report, we shall present a few representative highlights that have been achieved in the project.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Deep inelastic scattering from A = 3 nuclei: Nucleon structure and the EMC effect
The MARATHON experiment ran at Jefferson Laboratory in 2018, measuring deep inelastic scattering of 10.6 GeV electrons from the proton (p), deuterium (D), tritium ( 3 H), and helium-3 ( 3 He). MARATHON's experimental observables are cross section ratios, most notably 3 H/ 3 He. This ratio of mirror nuclei is used for a novel extraction of the nucleon structure function ratio $F^n_2$ /$F^p_2$ with minimal nuclear corrections. Determining leading-order $F^n_2$ /$F^p_2$ at high x provides important constrains on the ratio of nucleon valence quark distributions d/u. The ratios 3 H/D and 3 He/D allow a determination of the EMC effect in the lightest nuclei sensitive to possible isospin dependence of nuclear effects. Lastly, the D/p ratio allows a comparison of the MARATHON experiment to SLAC measurements, and provides data to which the A = 3 ratios can be renormalized. The MARATHON results provide new constraints on d/u. Additionally, the results emphasize the importance of higher-order effects in extracting $F^n_2$ /$F^p_2$ from DIS measurements, and can help constrain said effects. In the renormalization scheme applied in the MARATHON analysis, no significant isospin dependence of the EMC effect is observed.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Nucleon Structure from Lattice QCD Calculations
Parton distribution and correlation functions describe the relation between a hadron and the quarks and gluons (or collectively, the partons) within it, and carry rich information on hadron’s partonic structure that cannot be calculated by QCD perturbation theory. In this talk, I will review what lattice QCD can and cannot do for calculating the parton distribution and correlation functions, and the new ideas and efforts around the world to explore nucleon structure from lattice QCD calculations by combining the strength of both lattice QCD and perturbative QCD in such a way that is complementary to our on-going effort to extract these fundamental functions from experimental data.
Qiu, Jian-Wei↗
EIC in the US: Status of the Project and Outlook of the 3D Nucleon Structure Program
The interior landscape of nucleons includes a strong-force driven sea of quarks, antiquarks and gluons, with a net surplus of a few ever-present valence quarks. In order to understand how the properties and structure of all forms of nuclear matter emerge from the dynamics of QCD, it is essential to precisely image the gluons and quarks, and to understand the role they and their interactions play in nucleons and nuclei. For this, a new accelerator facility is required, the Electron-Ion Collider, to match the valence quark studies of the upgraded Jefferson Lab. Such a future facility would be the world's first polarized electron-proton collider, and the world's first e-A collider. The science foreseen at and the status of such a future US-based polarized Electron-Ion Collider will be presented.
Ent, Rolf↗