Quantum Chromodynamics Resolution of the ATOMKI Anomaly in 4He{\rm {^4He}} Nuclear Transitions

Recent observations of the angular correlation spectra in the decays ${\rm ^4He}^*\to {\rm ^4He}+ e^+e^-$ and ${\rm ^8Be}^*\to {\rm ^8Be}+ e^+e^-$ have been suggested as due to the creation and subsequent decay to an electron-positron pair of a new light particle with a mass of $\sim 17$ MeV. In this work, we present a calculation of the invariant $m_{e^+e^-}$ mass spectrum of the electromagnetic transition of an excited state of helium and estimate the differential and total width of the decay. We investigate the possibility that the source of the signal is an $e^+ e^-$ pair created by a new electromagnetic decay of $\rm ^4He$ caused by a proposed 12-quark hidden-color Fock state in the ${\rm {^4He}}$ nuclear wavefunction, the "hexadiquark.'' We find that we can fit the shape of the signal with the QCD Fock state at excitation energy ${\rm E^*}\simeq 17.9$ MeV and a Gaussian form factor for the electromagnetic decay. We address the physical issues with the fit parameters using properties of the hexadiquark state. In light of this work, we emphasize the need for independent experimental confirmation or refutation of the ATOMKI results as well as further experiments to detect the proposed new excitation of ${\rm ^4He}$.Comment: Condensed version, 8 pages, 3 figures, comments welcom

Diffractive Dissociation of Alpha Particles as a Test of Isophobic Short-Range Correlations inside Nuclei

The CLAS collaboration at Jefferson Laboratory has compared nuclear parton distributions for a range of nuclear targets and found that the EMC effect measured in deep inelastic lepton-nucleus scattering has a strongly "isophobic" nature. This surprising observation suggests short-range correlations between neighboring $n$ and $p$ nucleons in nuclear wavefunctions that are much stronger compared to $p-p$ or $n-n$ correlations. In this paper we propose a definitive experimental test of the nucleon-nucleon explanation of the isophobic nature of the EMC effect: the diffractive dissociation on a nuclear target $A$ of high energy $\rm ^4He$ nuclei to pairs of nucleons $n$ and $p$ with high relative transverse momentum, $\alpha + A \to n + p + A' + X$. The comparison of $n-p$ events with $p-p$ and $n-n$ events directly tests the postulated breaking of isospin symmetry. The experiment also tests alternative QCD-level explanations for the isophobic EMC effect. In particular it will test a proposal for hidden-color degrees of freedom in nuclear wavefunctions based on isospin-zero $[ud]$ diquarks.Comment: 5 pages, references added, clarifications due to helpful referee comments (latexdiff for all changes). Accepted for publication in Physics Letters

A new direct detection electron scattering experiment to search for the X17 particle

A new electron scattering experiment (E12-21-003) to verify and understand the nature of hidden sector particles, with particular emphasis on the so-called X17 particle, has been approved at Jefferson Lab. The search for these particles is motivated by new hidden sector models introduced to account for a variety of experimental and observational puzzles: excess in $e^+e^-$ pairs observed in multiple nuclear transitions, the 4.2$\sigma$ disagreement between experiments and the standard model prediction for the muon anomalous magnetic moment, and the small-scale structure puzzle in cosmological simulations. The aforementioned X17 particle has been hypothesized to account for the excess in $e^+e^-$ pairs observed from the $^8$Be M1, $^4$He M0, and, most recently, $^{12}$C E1 nuclear transitions to their ground states observed by the ATOMKI group. This experiment will use a high resolution electromagnetic calorimeter to search for or set new limits on the production rate of the X17 and other hidden sector particles in the $3 - 60$ MeV mass range via their $e^+e^-$ decay (or $\gamma\gamma$ decay with limited tracking). In these models, the $1 - 100$ MeV mass range is particularly well-motivated and the lower part of this range still remains unexplored. Our proposed direct detection experiment will use a magnetic-spectrometer-free setup (the PRad apparatus) to detect all three final state particles in the visible decay of a hidden sector particle for an effective control of the background and will cover the proposed mass range in a single setting. The use of the well-demonstrated PRad setup allows for an essentially ready-to-run and uniquely cost-effective search for hidden sector particles in the $3 - 60$ MeV mass range with a sensitivity of 8.9$\times$10$^{-8}$ - 5.8$\times$10$^{-9}$ to $\epsilon^2$, the square of the kinetic mixing interaction constant between hidden and visible sectors.Comment: 6 pages, 7 figures. arXiv admin note: substantial text overlap with arXiv:2108.1327

The present and future of QCD

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

Strong Interaction Physics at the Luminosity Frontier with 22 GeV Electrons at Jefferson Lab

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 document provides an exciting rationale for the energy upgrade of CEBAF to 22 GeV, outlining the transformative scientific potential that lies within reach, and the remarkable opportunities it offers for advancing our understanding of hadron physics and related fundamental phenomena.Comment: Updates to the list of authors; Preprint number changed from theory to experiment; Updates to sections 4 and 6, including additional figure

The present and future of QCD

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

Neutron spin structure from e-3He scattering with double spectator tagging at the electron-ion collider

The spin structure function of the neutron is traditionally determined by measuring the spin asymmetry of inclusive electron deep inelastic scattering (DIS) off polarized3He nuclei. In such experiments, nuclear effects can lead to large model dependencies in the interpretation of experimental data. Here we study the feasibility of suppressing such model dependencies by tagging both spectator protons in the process of DIS off neutrons in3He at the forthcoming Electron-Ion Collider (EIC). This allows reconstructing the momentum of the struck neutron to ensure it was nearly at rest in the initial state, thereby reducing sensitivity to nuclear corrections, and suppress contributions from electron DIS off protonsin3He. Using realistic accelerator and detector configurations, we find that the EIC can probe the neutron spin structure from xB of 0.003 to 0.651. We further find that the double spectator tagging method results in reduced uncertainties bya factor of 4 on the extracted neutron spin asymmetries over all kinematics, and by a factor of 10 in the low-xB region,thereby providing valuable insight to the spin and flavor structure of nucleonsComment: 8 pages, 5 figure