Garth M. Huber

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.

There is increasing interest in deep exclusive meson production (DEMP) reactions, as they provide access to Generalized Parton Distributions over a broad kinematic range, and are the only means of measuring pion and kaon charged electric form factors at high . Such investigations are a particularly useful tool in the study of hadronic structure in QCD's transition regime from long-distance interactions described in terms of meson-nucleon degrees of freedom, to short-dist ance interactions governed by hard quark-gluon degrees of freedom. To assist the planning of future experimental investigations of DEMP reactions in this transition regime, such as at Jefferson Lab and the Electron-Ion Collider (EIC), we have written a special purpose event generator, DEMPgen. Several types of DEMP reactions can be generated: -channel , , and from a polarized He target. DEMPgen is modular in form, so that additional reactions can be added over time. The generator produces kinematically-complete reaction events which are absolutely-normalized, so that projected event rates can be predicted, and detector resolution requirements studied. The event normalization is based on parameterizations of theoretical models, appropriate to the kinematic regime under study. Both fixed target modes and collider beam modes are supported. This paper presents the structure of the generator, the model parameterizations used for absolute event weighting, the kinematic distributions of the generated particles, some initial results using the generator, and instructions for its use.

We report the measurement of the helicity asymmetry for the and final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g.~) and circularly polarized photons (e.g.~) and a longitudinally polarized target. The new data cover a photon energy range 270 - 1400 MeV for the final state (230 - 842 MeV for the final state) and the full range of pion polar angles, , providing the most precise measurement of the observable . A moment analysis gives a clear observation of the cusp in the final state.

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 …

The nuclear dependence of the inclusive inelastic electron scattering cross section (the EMC effect) has been measured for the first time in B 10 and B 11. Previous measurements of the EMC effect in A≤ 12 nuclei showed an unexpected nuclear dependence; B 10 and B 11 were measured to explore the EMC effect in this region in more detail. Results are presented for Be 9, B 10, B 11, and C 12 at an incident beam energy of 10.6 GeV. The EMC effect in the boron isotopes was found to be similar to that for Be 9 and C 12, yielding almost no nuclear dependence in the EMC effect in the range A= 4–12. This represents important new data supporting the hypothesis that the EMC effect depends primarily on the local nuclear environment due to the cluster structure of these nuclei.

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 …

We report a first measurement of the double-polarisation observable, C x′, in π+ photoproduction off the proton. The C x′ double-polarisation observable represents the transfer of polarisation from a circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Crystal Ball calorimeter. Ancillary apparatus surrounding the target provided tracking, particle identification and determination of recoil nucleon polarisation. The C x′ observable is determined for photon energies 800-1400 MeV, providing new constraints on models aiming to elucidate the spectrum and properties of nucleon resonances. This is the first determination of any polarisation observable from the beam-recoil group of observables for this reaction. Inclusion of the new data in the database of the SAID partial wave analysis …

We describe the design and performance the calorimeter systems used in the ECCE detector to achieve the overall performance specifications cost-effectively with careful consideration of appropriate technical and schedule risks. The calorimeter systems consist of three electromagnetic calorimeters, covering the combined pseudorapdity range from− 3.7 to 3.8 and two hadronic calorimeters covering a combined range of− 1. 1< η< 3. 8. Key calorimeter performances which include energy and position resolutions, reconstruction efficiency, and particle identification will be presented.

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 …

Measurement of the Charged Pion Form Factor to High Q2 at JLab Page 1 Garth Huber Measurement of the Charged Pion Form Factor to High Q2 at JLab Precision tests of fundamental physics with light mesons ECT* Workshop, Trento, Italy June 14, 2023 Supported by: SAPIN-2021-00026 (on behalf of the PionLT Collaboration) Page 2 G a rth H u b e r, h u b e rg @ u re g in a .c a 2 Simple valence structure of mesons presents the ideal testing ground for our understanding of bound quark systems. Charged Meson Form Factors The meson wave function can be separated into φ π soft with only low momentum contributions (k<k 0 ) and a hard tail φ π hard. While φ π hard can be treated in pQCD, φ π soft cannot. From a theoretical standpoint, the study of the Q2–dependence of the form factor focuses on finding a description for the hard and soft contributions of the meson wave-function. qq In quantum field theory, the …

The GlueX experiment at Jefferson Lab studies photoproduction of mesons using linearly polarized 8.5 GeV photons impinging on a hydrogen target which is contained within a detector with near-complete coverage for charged and neutral particles. We present measurements of spin-density matrix elements for the photoproduction of the vector meson ρ (770). The statistical precision achieved exceeds that of previous experiments for polarized photoproduction in this energy range by orders of magnitude. We confirm a high degree of s-channel helicity conservation at small squared four-momentum transfer t and are able to extract the t dependence of natural-and unnatural-parity exchange contributions to the production process in detail. We confirm the dominance of natural-parity exchange over the full t range. We also find that helicity amplitudes in which the helicity of the incident photon and the photoproduced ρ …

We propose to conduct a measurement of the Virtual Compton Scattering reaction in Hall C that will allow the precise extraction of the two scalar Generalized Polarizabilities (GPs) of the proton in the region of to . The Generalized Polarizabilities are fundamental properties of the proton, that characterize the system's response to an external electromagnetic (EM) field. They describe how easily the charge and magnetization distributions inside the system are distorted by the EM field, mapping out the resulting deformation of the densities in the proton. As such, they reveal unique information regarding 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 together. Recent measurements of the proton GPs have challenged the theoretical predictions, particularly in regard to the electric polarizability. The magnetic GP, on the other hand, can provide valuable insight to the competing paramagnetic and diamagnetic contributions in the proton, but it is poorly known within the region where the interplay of these processes is very dynamic and rapidly changing.The unique capabilities of Hall C, namely the high resolution of the spectrometers combined with the ability to place the spectrometers in small angles, will allow to pin down the dynamic signature of the GPs through high precision measurements combined with a fine mapping as a function of . The experimental setup utilizes standard Hall C equipment, as was previously employed in the VCS-I (E12-15-001) experiment, namely the HMS …

We performed feasibility studies for various measurements that are related to unpolarized TMD distribution and fragmentation functions for the ECCE detector proposal. The processes studied include semi-inclusive Deep inelastic scattering (SIDIS) where single hadrons (pions and kaons) were detected in addition to the scattered DIS lepton. The single hadron cross sections and multiplicities were extracted as a function of the DIS variables x and Q 2, as well as the semi-inclusive variables z, which corresponds to the momentum fraction the detected hadron carries relative to the struck parton and P T, which corresponds to the transverse momentum of the detected hadron relative to the virtual photon. The expected statistical precision of such measurements is extrapolated to accumulated luminosities of 10 fb− 1 and potential systematic uncertainties are approximated given the deviations between true and …

Abstract The recently approved Electron-Ion Collider (EIC) will provide a unique new opportunity for searches of charged lepton flavor violation (CLFV) and other new physics scenarios. In contrast to the e↔ μ CLFV transition for which very stringent limits exist, there is still a relatively large discovery space for the e→ τ CLFV transition, potentially to be explored by the EIC. With the latest detector design of ECCE (EIC Comprehensive Chromodynamics Experiment) and projected integral luminosity of the EIC, we find the τ-leptons created in the DIS process e p→ τ X are expected to be identified with high efficiency. A first ECCE simulation study, restricted to the 3-prong τ-decay mode and with limited statistics for the Standard Model backgrounds, estimates that the EIC will be able to improve the current exclusion limit on e→ τ CLFV by an order of magnitude. The very high vertex resolution of the ECCE detector …

Abstract The EIC Comprehensive Chromodynamics Experiment (ECCE) detector has been recommended as a reference design for the proposed Electron-Ion Collider (EIC) program. This paper presents simulation studies of exclusive J/ψ detection and selected physics impact results in EIC using the projected ECCE detector concept. Exclusive quarkonium photoproduction is one of the most popular processes in EIC, which has a large cross section and a simple final state. Due to the gluonic nature of the exchange Pomeron, this process can be related to the gluon distributions in the nucleus. Preliminary results estimate the excellent statistics benefited from the large cross section of J/ψ photoproduction and superior performance of ECCE detector concept. The precise measurement of exclusive J/ψ photoproduction at EIC will help us to more deeply understand nuclear gluon distributions, near threshold production …

Photoinduced reactions on proton and neutron targets have played a key role in progressing our knowledge of the excited nucleon spectrum in the past decade [1], catalysed by quality nucleon photoproduction data from MAMI, JLab, ELSA, SPring-8, ELPH and other facilities [2, 3]. These have provided a step change in the number of measured observables, statistical accuracy, and kinematic coverage. Pion photoproduction is the simplest photoinduced reaction on the nucleon. The reaction can be described theoretically with four complex amplitudes, which can be fully constrained, up to an overall phase, by kinematically complete measurements of a chosen set of eight observables taken from the cross section, single polarisation observables (where the polarisation of either photon beam, target or recoiling nucleon is determined) and double-polarisation observables formed from simultaneous determination of two of the above polarisation quantities. Recent work has indicated that the properties of the different partial waves in the reaction may converge with fewer measurements than the mathematically complete eight, as discussed in Ref.[4]. Previous double-polarisation measurements for ????????+ photoproduction are limited to the beam-target group of observables. Measurements of the ???? observable (linearly polarised beam and transversely polarised target)[5], the ???? observable (circularly polarised beam and longitudinally polarised target [6]) and more limited data sets for ???? (circularly polarised beam and transversely polarised target) have recently been obtained [7, 8]. These double-polarisation data, combined with the cross section and single …

Quasielastic scattering on C 12 (e, e′ p) was measured in Hall C at Jefferson Lab for spacelike four-momentum transfer squared Q 2 in the range of 8–14.2 (GeV/c) 2 with proton momenta up to 8.3 GeV/c. The experiment was carried out in the upgraded Hall C at Jefferson Lab. It used the existing high-momentum spectrometer and the new super-high-momentum spectrometer to detect the scattered electrons and protons in coincidence. The nuclear transparency was extracted as the ratio of the measured yield to the yield calculated in the plane wave impulse approximation. Additionally, the transparency of the 1 s 1/2 and 1 p 3/2 shell protons in C 12 was extracted, and the asymmetry of the missing momentum distribution was examined for hints of the quantum chromodynamics prediction of color transparency. All of these results were found to be consistent with traditional nuclear physics and inconsistent with the …

This White Paper aims at highlighting the important benefits in the science reach of the EIC. High luminosity operation is generally desirable, as it enables producing and harvesting scientific results in a shorter time period. It becomes crucial for programs that would require many months or even years of operation at lower luminosity.

This article presents a collection of simulation studies using the ECCE detector concept in the context of the EIC’s exclusive, diffractive, and tagging physics program, which aims to further explore the rich quark-gluon structure of nucleons and nuclei. To successfully execute the program, ECCE proposed to utilize the detector system close to the beamline to ensure exclusivity and tag ion beam/fragments for a particular reaction of interest. Preliminary studies confirm the proposed technology and design satisfy the requirements. The projected physics impact results are based on the projected detector performance from the simulation at 10 or 100 fb−1 of integrated luminosity. Additionally, insights related to a potential second EIC detector are documented, which could serve as a guidepost for future development.

The Electron-Ion Collider (EIC) is a cutting-edge accelerator facility that will study the nature of the “glue” that binds the building blocks of the visible matter in the universe. The proposed experiment will be realized at Brookhaven National Laboratory in approximately 10 years from now, with detector design and R&D currently ongoing. Notably, EIC is one of the first large-scale facilities to leverage Artificial Intelligence (AI) already starting from the design and R&D phases. The EIC Comprehensive Chromodynamics Experiment (ECCE) is a consortium that proposed a detector design based on a 1.5 T solenoid. The EIC detector proposal review concluded that the ECCE design will serve as the reference design for an EIC detector. Herein we describe a comprehensive optimization of the ECCE tracker using AI. The work required a complex parametrization of the simulated detector system. Our approach dealt with an …