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. 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. Here, we chose to probe this gluonic gravitational density using a small color dipole, the \(J/\psi\) particle, through its threshold photoproduction. From our data, we determined, for the first time, the proton's gluonic gravitational form factors. We used a variety of models and determined, in all cases, a mass radius that is notably smaller than the electric charge radius. In some cases, the determined radius, although model dependent, 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 Electron-Ion Collider (EIC), a state-of-the-art facility for studying the strong force, is expected to begin commissioning its first experiments in 2028. This is an opportune time for artificial ...intelligence (AI) to be included from the start at this facility and in all phases that lead up to the experiments. The second annual workshop organized by the AI4EIC working group, which recently took place, centered on exploring all current and prospective application areas of AI for the EIC. This workshop is not only beneficial for the EIC, but also provides valuable insights for the newly established ePIC collaboration at EIC. This paper summarizes the different activities and R&D projects covered across the sessions of the workshop and provides an overview of the goals, approaches and strategies regarding AI/ML in the EIC community, as well as cutting-edge techniques currently studied in other experiments.
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 interest is a puzzle in the electric generalized polarizability of the proton that remains unresolved for two decades. Here we report measurements of the proton's electromagnetic generalized polarizabilities at low four-momentum transfer squared. We show evidence of an anomaly to the behaviour of the proton's electric generalized polarizability that contradicts the predictions of nuclear theory and derive its signature in the spatial distribution of the induced polarization in the proton. The reported measurements suggest the presence of a new, not-yet-understood dynamical mechanism in the proton and present notable challenges to the nuclear theory.
Evidence for a flavor asymmetry between the $\bar u$ and $\bar d$ quark
distributions in the proton has been found in deep-inelastic scattering and
Drell-Yan experiments. The pronounced dependence of ...this flavor asymmetry on
$x$ (fraction of nucleon momentum carried by partons) observed in the Fermilab
E866 Drell-Yan experiment suggested a drop of the $\bar d\left(x\right) / \bar
u\left(x\right)$ ratio in the $x > 0.15$ region. We report results from the
SeaQuest Fermilab E906 experiment with improved statistical precision for $\bar
d\left(x\right) / \bar u\left(x\right)$ in the large $x$ region up to $x=0.45$
using the 120 GeV proton beam. Two different methods for extracting the
Drell-Yan cross section ratios, $\sigma^{pd} /2 \sigma^{pp}$, from the SeaQuest
data give consistent results. The $\bar{d}\left(x\right) /
\bar{u}\left(x\right)$ ratios and the $\bar d\left(x\right) - \bar
u\left(x\right)$ differences are deduced from these cross section ratios for
$0.13 < x < 0.45$. The SeaQuest and E866/NuSea $\bar{d}\left(x\right) /
\bar{u}\left(x\right)$ ratios are in good agreement for the $x\lesssim 0.25$
region. The new SeaQuest data, however, show that $\bar d\left(x\right)$
continues to be greater than $\bar u\left(x\right)$ up to the highest $x$ value
($x = 0.45$). The new results on $\bar{d}\left(x\right) /
\bar{u}\left(x\right)$ and $\bar{d}\left(x\right) - \bar{u}\left(x\right)$ are
compared with various parton distribution functions and theoretical
calculations.
Quasi-elastic scattering on $^{12}$C$(e,e'p)$ was measured in Hall C at
Jefferson Lab for space-like 4-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 $1s_{1/2}$ and $1p_{3/2}$ shell protons in $^{12}$C 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 onset of Color Transparency.
The proposed measurement is a dedicated study of the exclusive electroproduction process,1H(e,e'p)pi0, in the backward-angle regime (u-channel process) above the resonance region. The produced pi0 is ...emitted 180 degrees opposite to the virtual-photon momentum. This study also aims to apply the well-known Rosenbluth separation technique that provides the model-independent differential cross-sections at the never explored u-channel kinematics region. Currently, the "soft-hard transition" in u-channel meson production remains an interesting and unexplored subject. The available theoretical frameworks offer competing interpretations for the observed backward-angle cross section peaks. In a "soft" hadronic Regge exchange description, the backward meson production comes from the interference between nucleon exchange and the meson produced via re-scattering within the nucleon. Whereas in the "hard" GPD-like backward collinear factorization regime, the scattering amplitude factorizes into a hard subprocess amplitude and baryon to meson transition distribution amplitudes (TDAs), otherwise known as super skewed parton distributions (SuperSPDs). Both TDAs and SPDs are universal non-perturbative objects of nucleon structure accessible only through backward-angle kinematics. The separated cross sections:sigma_T,sigma_L and T/L ratio at Q2=2-6 GeV2, provide a direct test of two predictions from the TDA model. The magnitude and u-dependence of the separated cross sections also provide a direct connection to the re-scattering Regge picture. The extracted interaction radius (from u-dependence) at different Q2 can be used to study the soft-hard transition in the u-channel kinematics. The acquisition of these data will be an important step forward in validating the existence of a backward factorization scheme of the nucleon structure function and establishing its applicable kinematic range.
Advanced detector R&D requires performing computationally intensive and detailed simulations as part of the detector-design optimization process. We propose a general approach to this process based ...on Bayesian optimization and machine learning that encodes detector requirements. As a case study, we focus on the design of the dual-radiator Ring Imaging Cherenkov (dRICH) detector under development as part of the particle-identification system at the future Electron-Ion Collider (EIC). The EIC is a US-led frontier accelerator project for nuclear physics, which has been proposed to further explore the structure and interactions of nuclear matter at the scale of sea quarks and gluons. We show that the detector design obtained with our automated and highly parallelized framework outperforms the baseline dRICH design within the assumptions of the current model. Our approach can be applied to any detector R&D, provided that realistic simulations are available.
The fundamental building blocks of the proton, quarks and gluons, have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles ...and their dynamics give rise to the quantum bound state of the proton and its physical properties, such as for example its spin. The two up and the single down quarks that comprise the proton in the simplest picture account only for a few percent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force. An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter-antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions where a matter-antimatter quark pair annihilates. In this picture of quark-antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, since their masses are quite similar and small compared to the mass of the proton. In the present manuscript, we show evidence from muon pair production measurements that these distributions are significantly different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momentum. These results revive interest in several proposed mechanisms as the origin of this antimatter asymmetry in the proton that had been disfavored by the previous results and point to the future measurements that can distinguish between these mechanisms.
Phys. Rev. Lett. 126, 082301 (2021) Quasielastic $^{12}$C$(e,e'p)$ scattering was measured at space-like
4-momentum transfer squared $Q^2$~=~8, 9.4, 11.4, and 14.2 (GeV/c)$^2$, the
highest ever ...achieved to date. Nuclear transparency for this reaction was
extracted by comparing the measured yield to that expected from a plane-wave
impulse approximation calculation without any final state interactions. The
measured transparency was consistent with no $Q^2$ dependence, up to proton
momenta of 8.5~GeV/c, ruling out the quantum chromodynamics effect of color
transparency at the measured $Q^2$ scales in exclusive $(e,e'p)$ reactions.
These results impose strict constraints on models of color transparency for
protons.
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