Marie Boer

This White Paper presents an overview of the current status and future perspective of QCD research, based on the community inputs and scientific conclusions from the 2022 Hot and Cold QCD Town Meeting. We present the progress made in the last decade toward a deep understanding of both the fundamental structure of the sub-atomic matter of nucleon and nucleus in cold QCD, and the hot QCD matter in heavy ion collisions. We identify key questions of QCD research and plausible paths to obtaining answers to those questions in the near future, hence defining priorities of our research over the coming decades.

Generalized parton Distributions (GPDs) are important functions to understand the three dimensional structure of the nucleon. Deeply Virtual Compton Scattering is one of the reaction accessing GPDs, and has been measured for the past 20 years. However, to move forward, we need to look for other reactions, such as Timelike Compton Scattering (TCS), its "time-reversal" equivalent. Indeed, accessing GPDs from both DVCS and TCS independently will allow us, for instance, to study their universality. Any assesment on GPD's universality would be a milestone in our field. In this article we discuss our preliminary studies on the feasibility of measuring unpolarized and beam polarized cross sections and beam spin asymmetry for TCS in the dilepton photoproduction reaction. For that purpose, we use a polarized photon beam and an unpolarized target at JLab Hall C. We will discuss our Geant4 simulations, with a dedicated detector setup along with the use of the SBS magnet for separating outgoing , pairs.

This paper presents our project and perspectives to measure for the first time beam spin asymmetries from Double Deeply Virtual Compton Scattering in the reaction at Jefferson Lab. Our goal is to constrain the so-called Generalized Parton Distribution (GPDs) in a kinematic region that isn't accessible from other reactions, such as Deeply Virtual Compton Scattering, to allow for their extrapolation to "zero skewness", i.e. at a specific kinematic point enabling for tomographic interpretations of the nucleon's partonic structure. We are discussing DDVCS phenomenology and our approach, as well as our experimental project aimed at complementing the SoLID experiment at JLab Hall A with a new muon detector.

PHENIX presents a simultaneous measurement of the production of direct and in Au collisions at GeV over a range of 7.5 to 18 GeV/ for different event samples selected by event activity, i.e. charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor is , consistent with unity for minimum-bias (MB) Au events. For event classes with moderate event activity, is consistent with the MB value within 5\% uncertainty. These results confirm that the previously observed enhancement of high- production found in small-system collisions with low event activity is a result of a bias in interpreting event activity within the Glauber framework. In contrast, for the top 5\% of events with the highest event activity, is suppressed by 20\% relative to the MB value with a significance of , which may be due to final-state effects.

It is currently understood that there are four fundamental forces in nature: gravitational, electromagnetic, weak and strong forces. The strong force governs the interactions between quarks and gluons, elementary particles whose interactions give rise to the vast majority of visible mass in the universe. The mathematical description of the strong force is provided by the non-Abelian gauge theory Quantum Chromodynamics (QCD). While QCD is an exquisite theory, constructing the nucleons and nuclei from quarks, and furthermore explaining the behavior of quarks and gluons at all energies, remain to be complex and challenging problems. Such challenges, along with the desire to understand all visible matter at the most fundamental level, position the study of QCD as a central thrust of research in nuclear science. Experimental insight into the strong force can be gained using large particle accelerator facilities, which are necessary to probe the very short distance scales over which quarks and gluons interact. The Long Range Plans (LRPs) exercise of 1989 and 1996 led directly to the construction of two world-class facilities: the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLab) that is focused on studying how the structure of hadrons emerges from QCD (cold QCD research), and the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab (BNL) that aims at the discovery and study of a new state of matter, the quark-gluon plasma (QGP), at extremely high temperatures (hot QCD research). The different collision systems used to access the incredibly rich field of hot and cold QCD in the laboratory are illustrated …

It is currently understood that there are four fundamental forces in nature: gravitational, electromagnetic, weak and strong forces. The strong force governs the interactions between quarks and gluons, elementary particles whose interactions give rise to the vast majority of visible mass in the universe. The mathematical description of the strong force is provided by the non-Abelian gauge theory Quantum Chromodynamics (QCD). While QCD is an exquisite theory, constructing the nucleons and nuclei from quarks, and furthermore explaining the behavior of quarks and gluons at all energies, remain to be complex and challenging problems. Such challenges, along with the desire to understand all visible matter at the most fundamental level, position the study of QCD as a central thrust of research in nuclear science. Experimental insight into the strong force can be gained using large particle accelerator facilities, which are necessary to probe the very short distance scales over which quarks and gluons interact. The Long Range Plans (LRPs) exercise of 1989 and 1996 led directly to the construction of two world-class facilities: the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLab) that is focused on studying how the structure of hadrons emerges from QCD (cold QCD research), and the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab (BNL) that aims at the discovery and study of a new state of matter, the quark-gluon plasma (QGP), at extremely high temperatures (hot QCD research). These past investments have produced major advances. Nucleons and nuclei are being studied with increasing precision …

The proposed energy upgrade to the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility would enable the only facility worldwide, planned or foreseen, that can address the complexity at the scientific frontier of emergent hadron structure with its high luminosity and probing precision at the hadronic scale. While high-energy facilities will illuminate the perturbative dynamics and discover the fundamental role of gluons in nucleons and nuclei, a medium energy electron accelerator at the luminosity frontier will be critical to understand the rich and extraordinary variety of non-perturbative effects manifested in hadronic structure.

Recently, the PHENIX Collaboration has published second-and third-harmonic Fourier coefficients v 2 and v 3 for midrapidity (| η|< 0.35) charged hadrons in 0%–5% central p+ Au, d+ Au, and He 3+ Au collisions at s N N= 200 GeV, utilizing three sets of two-particle correlations for two detector combinations with different pseudorapidity acceptance [Acharya et al., Phys. Rev. C 105, 024901 (2022)]. This paper extends these measurements of v 2 to all centralities in p+ Au, d+ Au, and He 3+ Au collisions, as well as p+ p collisions, as a function of transverse momentum (p T) and event multiplicity. The kinematic dependence of v 2 is quantified as the ratio R of v 2 between the two detector combinations as a function of event multiplicity for 0.5< p T< 1 and 2< p T< 2.5 GeV/c. A multiphase-transport (AMPT) model can reproduce the observed v 2 in most-central to midcentral d+ Au and He 3+ Au collisions. However, the …

This document presents the initial scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLab) to 22 GeV. It is the result of a community effort, incorporating insights from a series of workshops conducted between March 2022 and April 2023. With a track record of over 25 years in delivering the world's most intense and precise multi-GeV electron beams, CEBAF's potential for a higher energy upgrade presents a unique opportunity for an innovative nuclear physics program, which seamlessly integrates a rich historical background with a promising future. The proposed physics program encompass a diverse range of investigations centered around the nonperturbative dynamics inherent in hadron structure and the exploration of strongly interacting systems. It builds upon the exceptional capabilities of CEBAF in high-luminosity operations, the availability of existing or planned Hall equipment, and recent advancements in accelerator technology. The proposed program cover various scientific topics, including Hadron Spectroscopy, Partonic Structure and Spin, Hadronization and Transverse Momentum, Spatial Structure, Mechanical Properties, Form Factors and Emergent Hadron Mass, Hadron-Quark Transition, and Nuclear Dynamics at Extreme Conditions, as well as QCD Confinement and Fundamental Symmetries. Each topic highlights the key measurements achievable at a 22 GeV CEBAF accelerator. Furthermore, this document outlines the significant physics outcomes and unique aspects of these programs that distinguish them from other existing or planned facilities. In summary, this …

The proton is one of the main building blocks of all visible matter in the universe. Among its intrinsic properties are its electric charge, mass, and spin. These emerge from the complex dynamics of its fundamental constituents, quarks and gluons, described by the theory of quantum chromodynamics (QCD). Using electron scattering its electric charge and spin, shared among the quark constituents, have been the topic of active investigation until today. An example is the novel precision measurement of the proton's electric charge radius. In contrast, little is known about the proton's inner mass density, dominated by the energy carried by the gluons, which are hard to access through electron scattering since gluons carry no electromagnetic charge. In the present work we chose to probe this gluonic gravitational density using a small color dipole, the particle, through its threshold photoproduction. From our data we determined, for the first time, the proton's gluonic gravitational form factors, which encode its mass density. We used a variety of methods and determined in all cases a mass radius that is notably smaller than the electric charge radius. In some cases, the determined radius is in excellent agreement with first-principle predictions from lattice QCD. This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter.

The proton is one of the main building blocks of all visible matter in the Universe. Among its intrinsic properties are its electric charge, mass and spin. These properties emerge from the complex dynamics of its fundamental constituents—quarks and gluons—described by the theory of quantum chromodynamics, –. The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering. An example is the highly precise measurement of the electric charge radius of the proton. By contrast, little is known about the inner mass density of the proton, which is dominated by the energy carried by gluons. Gluons are hard to access using electron scattering because they do not carry an electromagnetic charge. Here we investigated the gravitational density of gluons using a small colour dipole, through the threshold photoproduction of the J/ψ particle. We …

Generalized Parton Distributions, functions correlating the transverse distribution of partons with their longitudinal momenta, can be interpreted in terms of''3D images''of the nucleon. They can be accessed in hard exclusive processes, such as Timelike Compton Scattering (TCS). This process is measured in the reaction γ (P)-> γ*(p)-> e+ e-(P)(photoproduction of a lepton pair at high invariant mass), where the virtual photon is scattered off a quark, and interferes with another process called Bethe-Heitler (BH). The spacelike-equivalent of TCS,(spacelike) Deeply Virtual Compton Scattering has been widely studied over the past two decades and provided data to parametrize GPD models. However, we still lack experimental data on some GPDs such as the GPD E, which contains information about the nucleon polarization and is needed to study parton's angular momenta from GPDs. We analyzed data in the context of …

The visible world is founded on the proton, the only composite building block of matter that is stable in nature. Consequently, understanding the formation of matter relies on explaining the dynamics and the properties of the proton’s bound state. A fundamental property of the proton involves the response of the system to an external electromagnetic field. It is characterized by the electromagnetic polarizabilities that describe how easily the charge and magnetization distributions inside the system are distorted by the electromagnetic field. Moreover, the generalized polarizabilities map out the resulting deformation of the densities in a proton subject to an electromagnetic field. They disclose essential information about the underlying system dynamics and provide a key for decoding the proton structure in terms of the theory of the strong interaction that binds its elementary quark and gluon constituents. Of particular …

The so-called Generalized Parton Distributions have been introduced more than 30 years ago now, and their interpretation allows for a better understanding of the multidimensionnal" position" versus" momentum" partonic distributions in the nucleon. Yet, very few measurement exist beyond DVCS and light vector mesons in worldwide experiments and data. Recent theory progresses however emphasize the importance of new measurements to constrain GPD models and allow for interpretations.In this talk, I will discuss why it is fundamental to complement the existing measurements with novel reactions that have not been measured yet, or are poorly studied. I will present a brief summary of the outcome of a dedicated workshop we are hosting in July 2022 about this topic, with theorist and experimental specialists working at different facilities. I will also provide projections and physics interest for some reactions that …

Deeply Virtual Compton Scattering (DVCS) refers to the reaction γ∗(q) P (p)→ P (p′) γ (q′) in the Bjorken limit of Deep Inelastic Scattering (DIS). Experimentally, we can access DVCS through electroproduction of real photons e (k) P (p)→ e (k′) P (p′) γ (q′), where the DVCS amplitude interferes with the so-called Bethe-Heitler (BH) process (Fig. 1). The BH contribution is calculable in QED since it corresponds to the emission of the photon by the incoming or the outgoing electron.

PHENIX has performed an extensive study on the evolution of medium effects from small to large systems. PHENIX has continued searching for Quark-Gluon Plasma (QGP) in small systems by measuring collectivity, modification of light hadron and quarkonia production, and jet substructure. In large systems, detailed studies on the property of the QGP have been done using direct photon, -hadron correlation, heavy-flavor electron, and flow with a large statistics of data collected in 2014. This report covers new results from the PHENIX experiment in various collision systems.

There is strong evidence for the formation of small droplets of quark-gluon plasma in p/d/He 3+ Au collisions at the Relativistic Heavy Ion Collider (RHIC) and in p+ p/Pb collisions at the Large Hadron Collider. In particular, the analysis of data at RHIC for different geometries obtained by varying the projectile size and shape has proved insightful. In the present analysis, we find excellent agreement with the previously published PHENIX at RHIC results on elliptical and triangular flow with an independent analysis via the two-particle correlation method, which has quite different systematic uncertainties and an independent code base. In addition, the results are extended to other detector combinations with different kinematic (pseudorapidity) coverage. These results provide additional constraints on contributions from nonflow and longitudinal decorrelations.

Hard exclusive Compton-like process, involving the exchange of at least one high virtuality photon off a quark of the nucleon, or hard exclusive vector meson production (where a hard scale is provided by the exchange of a virtual photon or by the meson mass), enable access to the partonic structure of the nucleon, in particular it the transverse distribution versus the longitudinal momentum of the partons. The latter can be parametrized by the so-called Generalized Parton Distributions (GPDs). We propose a simulation framework to study these reactions and will present the interest of such studies for accessing GPDs, our methods and simulations, and our projections for potential future experiments.

The so-called Generalized Parton Distributions (GPDs), containing information about the parton's longitudinal momenta and their transverse position, can be accessed through hard exclusive processes. Double Deeply Virtual Compton Scattering (DDVCS), corresponding to the scattering of a virtual photon off a quark followed by the emission of a photon of different virtuality, is a''golden channel''to study GPDs. Indeed, as for DVCS or TCS (Spacelike and Timelike Deeply Virtual Compton Scattering) it only involves one non-pertubative QCD part, corresponding to GPDs, while other parts can be calculated. In addition, the relative virtuality of the 2 photons provides a lever arm to vary the kinematic points at which we access GPDs, bringing new constrains for tomographic interpretations of the GPDs. We will discuss the interest of measuring DDVCS and some new opportunities at Jefferson Lab.

We propose to use the High Momentum Spectrometer of Hall C combined with the Neutral Particle Spectrometer (NPS) to perform high precision measurements of the Deeply Virtual Compton Scattering (DVCS) cross section using a beam of positrons. The combination of measurements with oppositely charged incident beams is the only unambiguous way to disentangle the contribution of the DVCS term in the photon electroproduction cross section from its interference with the Bethe-Heitler amplitude. This provides a stronger way to constrain the Generalized Parton Distributions of the nucleon. A wide range of kinematics accessible with an 11 GeV beam off an unpolarized proton target will be covered. The dependence of each contribution will be measured independently.