Vector meson photoproduction is an important probe of nuclear structure. Light vector mesons are most sensitive to <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>low</mi><mtext>−</mtext><mi>x</mi></mrow></math> structure, as long as they are not too light for perturbative QCD calculations. The <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>ρ</mi><mo>′</mo></msup></math> is of interest as an intermediate mass state (between the <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>ρ</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow></math>) that is easier to detect than the <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>ϕ</mi></math>. Using HERA data on proton targets, we make projections for lead/gold targets in ultraperipheral collisions (UPCs) at the CERN Large Hadron Collider and the BNL Relativistic Heavy Ion Collider, and for <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mi>p</mi></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mi>A</mi></mrow></math> collisions at a future electron-ion collider (EIC). We compare the UPC projections with ALICE data, and constrain the branching ratio divided by the square of the photon-<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>ρ</mi><mo>′</mo></msup></math> coupling. The data prefer large couplings and small branching ratio, probably less than 25%. The photon-meson coupling predicted by generalized vector meson dominance does not fit the data. The HERA <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mi>p</mi></mrow></math> and ALICE UPC <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mi>Pb</mi></mrow></math> data exhibit very similar <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>4</mn><mi>π</mi></mrow></math> mass spectra, indicating that, if the system is composed of two resonances, the products of their photon couplings with their four-pion branching ratios are similar. The predicted rates are high for both UPCs and the EIC. The <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>ρ</mi><mo>′</mo></msup><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></mrow></math> decay can be observed at the EIC with high efficiency. In <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mi>p</mi></mrow></math> collisions at the highest energy, the forward B0 detector is needed to observe this channel down to the lowest achievable <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>Bjorken</mi><mtext>−</mtext><mi>x</mi></mrow></math> values.

United States

Neha Devi

Janet Seger

Minjung Kim

Spencer R. Klein

Photoproduction and detection of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>ρ</mi><mo>′</mo></msup><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></mrow></math> decays in ultraperipheral collisions and at an electron-ion collider

It has been assumed that nuclear modification does not exist in the longitudinal-transverse structure-function ratio <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>R</mi><mi>N</mi></msub><mo>=</mo><msubsup><mi>F</mi><mi>L</mi><mi>N</mi></msubsup><mo>/</mo><mrow><mo>(</mo><mn>2</mn><mi>x</mi><msubsup><mi>F</mi><mn>1</mn><mi>N</mi></msubsup><mo>)</mo></mrow></mrow></math> in lepton deep inelastic scattering. This assumption is widely used in obtaining structure functions of the “nucleon” from nuclear data such as the deuteron ones. However, nuclear modifications do exist theoretically at least in medium- and large-<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi></math> regions because nucleons in a nucleus move in any direction, which is not necessarily the longitudinal direction of the virtual-photon or weak-boson momentum in lepton scattering. Because of this transverse motion, the nucleon's transverse and longitudinal structure functions should mix with each other in nuclei with the mixture probability proportional to the nucleon's transverse momentum squared <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msubsup><mover accent="true"><mi>p</mi><mo>⃗</mo></mover><mi>T</mi><mrow><mspace width="0.16em"></mspace><mspace width="0.16em"></mspace><mn>2</mn></mrow></msubsup><mo>/</mo><msup><mi>Q</mi><mn>2</mn></msup></mrow></math>. In this work, numerical results are explicitly shown regarding such nuclear modifications in the deuteron. These nuclear modifications are important for determining precise structure functions of the nucleon. Furthermore, modifications of <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>R</mi><mi>N</mi></msub></math> should be investigated also at small <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi></math> by the future electron-ion collider to find interesting gluon dynamics in nuclei. Hopefully, this nuclear effect of <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>R</mi><mi>N</mi></msub></math> could be found by future experimental measurements at lepton accelerator facilities.

Japan

S. Kumano

Existence of nuclear modifications of the nucleon longitudinal-transverse structure-function ratio

EPOS4 is based on a sophisticated (recently significantly improved) parallel-primary-scattering scenario followed by a hydrodynamic expansion, for all collision systems, from small ones such as proton-proton <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>(</mo><mi>p</mi><mi>p</mi><mo>)</mo></mrow></math> to large ones such as lead-lead (PbPb). Having already reported on identified particle spectra in recent publications (providing information about radial flow), I discuss here the multiplicity dependence of multiparticle cumulants and flow harmonics, to better understand collectivity in small systems. The model is not particularly tuned for flow results, but it is a “general purpose” approach, trying to accommodate various types of observables with the same model.

France

K. Werner

Flow in small systems in the EPOS4 approach for high-energy scattering

Hadronic resonances, with lifetimes of a few <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>fm</mi><mo>/</mo><mi>c</mi></mrow></math> are key tools for studying the hadronic phase in high-energy collisions. This work investigates resonance production in pp collisions at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13.6</mn><mspace width="0.28em"></mspace><mi>TeV</mi></mrow></math> and in Pb-Pb collisions at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>5.36</mn><mspace width="0.28em"></mspace><mi>TeV</mi></mrow></math> using the EPOS4 model. The EPOS4 model provides the capability of switching the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) ON or OFF, allowing the study of final-state hadronic interactions in the hadronic phase. Although this study is primarily focused on understanding the production of hadronic resonances, we also investigate the production of strange and nonstrange hadrons in relation to rescattering, regeneration, baryon-to-meson production, and strangeness enhancement, using transverse momentum (<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi mathvariant="normal">T</mi></msub></math>) spectra and particle ratios. Rescattering effects and strangeness enhancement play an important role in the low-<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi mathvariant="normal">T</mi></msub></math> region, where soft, nonperturbative processes dominate particle production. At intermediate <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi mathvariant="normal">T</mi></msub></math>, the baryon-to-meson yield ratios show an enhancement, and the strong mass-dependent radial flow observed for baryons in the most central Pb-Pb collisions reflects the interplay between hadronization dynamics and collective effects. The average <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi mathvariant="normal">T</mi></msub></math> scaled with the reduced hadron mass (i.e., the mass divided by the number of valence quarks) is examined, revealing a deviation from a linear trend for short-lived resonances, attributed to hadronic phase effects. By analyzing the yield ratios of short-lived resonances to stable hadrons in pp and Pb-Pb collisions, the time duration (<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>τ</mi></math>) of the hadronic phase as a function of average charged multiplicity is estimated. The results show that <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>τ</mi></math> increases with increasing multiplicity and system size, also exhibiting a nonzero value for high-multiplicity pp collisions. It is found that the production of baryons such as nonstrange (<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>p</mi></math>), strange (<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">Λ</mi></math>), and multistrange (<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">Ξ</mi></math>, <math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">Ω</mi></math>) in the most central Pb-Pb collisions is influenced by competing processes of strangeness enhancement and baryon-antibaryon annihilation. Comparing these findings with measurements at energies available at the CERN Large Hadron Collider offers valuable insights into the underlying dynamics of the hadronic phase and their production dynamics.

India

Vikash Sumberia

Dukhishyam Mallick

Sanjeev Singh Sambyal

Nasir Mehdi Malik

Examining hadronic resonance dynamics at energies available at the CERN Large Hadron Collider: Insights from EPOS4

In this study we examine the impact of the initial stage and dynamical evolution on final-state observables in heavy-ion collisions. For this goal we develop a novel approach, <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>EPOSir</mi><mo>+</mo><mi>PHSDe</mi></mrow></math>, which employs energy-conserving multiple parallel scattering approach, accommodating factorization and saturation (EPOS) initial conditions as the starting point for parton and hadron evolution within the parton-hadron-string dynamics (PHSD) microscopic transport approach. By examining the space-time evolution of matter in this model and comparing to EPOS (which starts with an <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>S</mi></mrow></math>-matrix approach for parallel scatterings for the initial conditions and uses a hydrodynamic evolution for the quark-gluon plasma stage with the UrQMD as afterburner) and PHSD (which starts with primary high-energy <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>N</mi><mi>N</mi></mrow></math> scattering realized via the LUND string model and continues with fully microscopic transport dynamics for strongly interacting partonic and hadronic matter), we identify the key differences in the final particle distributions among the three approaches. Our analysis focuses on rapidity, transverse momentum spectra, and flow harmonics <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>v</mi><mn>2</mn></msub></math> for <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>Au</mi><mo>+</mo><mi>Au</mi></mrow></math> collisions at the invariant energy of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>200</mn></mrow></math> GeV. We find a dominant influence of dynamical evolution over the initial conditions on the final observables.

Mahbobeh Jafarpour

JINR

Vadym Voronyuk

Klaus Werner

Germany

Elena Bratkovskaya

Damien Vintache

Disentangling initial-state and evolution effects in heavy-ion collisions using EPOS and PHSD

The <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>π</mi><mo>+</mo></msup><mi>p</mi></mrow></math> elastic scattering process is studied, using an effective Lagrangian approach that incorporates the <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>s</mi></math>-, <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>u</mi></math>-, and <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>t</mi></math>-channel amplitudes, including <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="normal">Δ</mi><mrow><mo>+</mo><mo>+</mo></mrow></msup><mrow><mo>(</mo><mn>1232</mn><mo>)</mo></mrow></mrow><mo>,</mo><mo> </mo><mrow><msup><mi mathvariant="normal">Δ</mi><mn>0</mn></msup><mrow><mo>(</mo><mn>1232</mn><mo>)</mo></mrow></mrow></math>, neutron, and <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>ρ</mi><mn>0</mn></msup></math> contributions. By constructing the spin-density matrices from the scattering amplitudes, the von Neumann (entanglement) entropy is derived and associated with the spin degrees of freedom of the initial and final state particles. The entropy is computed by performing a partial trace over the spin subsystems, and its behavior is analyzed as a function of the center-of-mass energy <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>W</mi></math> and scattering angle <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>θ</mi></math>. It is found that entropy exhibits nontrivial angular and energy dependences, reflecting the interplay among resonance contributions and background amplitudes. In particular, the <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi mathvariant="normal">Δ</mi><mrow><mo>+</mo><mo>+</mo></mrow></msup></math> region shows relatively low entanglement, suggesting spin coherence dominated by the specific resonant contribution, while the background contributions increase entropy due to mixed spin configurations. These results indicate that entanglement entropy can serve as a novel probe into the quantum structure of hadronic scattering amplitudes, providing complementary insights beyond conventional cross-section analyses.

South Korea

Seung-il Nam

Entanglement entropy for <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>π</mi><mo>+</mo></msup><mi>p</mi></mrow></math> elastic scattering using a spin-density matrix

The early stage of a heavy-ion collision is marked by rapid entropy production and the transition from a gluon saturated initial condition to a plasma of quarks and gluons that evolves hydrodynamically. However, during the early times of the hydrodynamic evolution, the chemical composition of the QCD medium is still largely unknown. We present a study of quark chemical equilibration in the (Q)GP using a novel model of viscous hydrodynamic evolution in partial chemical equilibrium. Motivated by the success of gluon saturated initial condition models, we initialize the QCD medium as a completely gluon dominated state. Local quark production during the hydrodynamic phase is then simulated through the evolution of time-dependent fugacities for each independent quark flavor, with the timescales set as free parameters to compare different rates of equilibration. We present the results of complete heavy-ion collision simulations using this partial chemical equilibrium model, and show the effects on hadronic and electromagnetic observables. In particular, we show that the development of flow is sensitive to the equilibration timescale, providing an empirical way to probe the chemical equilibration of the QCD medium.

Andrew Gordeev

Steffen A. Bass

Berndt Müller

Jean-François Paquet

Quark flavor equilibration of the quark-gluon plasma

Using the Monte Carlo <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>HYDJET</mi><mo>+</mo><mo>+</mo></mrow></math> model, we investigate the transverse momentum (<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math>) spectra of heavy-flavor hadrons, namely <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>D</mi><mn>0</mn></msup><mo>,</mo><mo> </mo><msup><mover><mi>D</mi><mo>¯</mo></mover><mn>0</mn></msup><mo>,</mo><mo> </mo><msup><mi>D</mi><mo>+</mo></msup><mo>,</mo><mo> </mo><msup><mi>D</mi><mo>−</mo></msup></math>, and <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi mathvariant="normal">Λ</mi><mi>c</mi></msub></math>, along with the nuclear modification factors of <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>D</mi><mn>0</mn></msup></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>D</mi><mo>±</mo></msup></math> in Au <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>+</mo></math> Au collisions at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>200</mn></mrow></math> GeV RHIC energy across various centrality bins. This work extends the earlier study by relaxing the assumption of identical chemical and thermal freezeout and by exploring the effect of a higher chemical freezeout temperature relative to thermal freezeout. The study also explores the centrality and mass dependence of the charm enhancement factor (<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>γ</mi><mi>c</mi></msub></math>). Furthermore, we investigate medium-induced energy loss, particle-antiparticle asymmetry, and the role of fragmentation in the hadronization process. The <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>HYDJET</mi><mo>+</mo><mo>+</mo></mrow></math> model, with its default parameters, is unable to reproduce heavy hadron yields due to their early thermal freezeout and initial state production of heavy quarks. Therefore, a systematic tuning of model parameters is performed to improve the overall agreement with experimental data. Additionally, to assess the performance of the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>HYDJET</mi><mo>+</mo><mo>+</mo></mrow></math> model relative to other theoretical approaches, its results are compared with the TAMU, SUBATECH, Torino, and Duke models, for which corresponding simulation data are available. This study shows a decreasing trend in <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>γ</mi><mi>c</mi></msub></math> from central to peripheral collisions. Moreover, <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>γ</mi><mi>c</mi></msub></math> increases with increasing charm-hadron mass. The model approximately reproduces the experimental data of <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math> spectra at low and intermediate <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math>. However, it overestimates the data at high <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math>, indicating a need for improvements in modeling the energy loss of heavy quarks. The nuclear modification factors (<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>R</mi><mrow><mi>A</mi><mi>A</mi></mrow></msub></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>R</mi><mrow><mi>C</mi><mi>P</mi></mrow></msub></math>) for <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>D</mi><mn>0</mn></msup></math> mesons exhibit significant suppression at high <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math>. At low <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub><mo>,</mo><mo> </mo><msub><mi>R</mi><mrow><mi>A</mi><mi>A</mi></mrow></msub></math> results show deviations from the data, reflecting limitations in the model where coalescence becomes important. The antiparticle-to-particle ratios are consistent with experimental data, whereas mixed-particle ratios are underestimated, revealing the model's limitation in estimating the baryon-to-meson enhancement.

Sunidhi Saxena

Rajiv Gupta

Gauri Devi

Ajay Kumar

Heavy hadron production in <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Au</mi><mo>+</mo><mi>Au</mi></math> collisions at a center-of-mass energy of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>200</mn></mrow></math> GeV

We propose a new method to investigate the existence and location of the conjectured high-temperature critical point of strongly interacting matter via contours of constant entropy density. By approximating these lines as a power series in the baryon chemical potential <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>μ</mi><mi>B</mi></msub></math>, one can extrapolate them from first-principle results at zero net-baryon density, and use them to locate the quantum chromodynamics (QCD) critical point, including the associated first-order and spinodal lines. As a proof of principle, we employ currently available continuum-extrapolated lattice data from the Wuppertal-Budapest collaboration to find a critical point at a temperature and a baryon chemical potential of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>T</mi><mi>c</mi></msub><mo>=</mo><mn>114.3</mn><mo>±</mo><mn>6.9</mn></mrow></math> MeV and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>μ</mi><mrow><mi>B</mi><mo>,</mo><mi>c</mi></mrow></msub><mo>=</mo><mn>602.1</mn><mo>±</mo><mn>62.1</mn></mrow></math> MeV, respectively, at expansion order <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi mathvariant="script">O</mi><mo>(</mo><msubsup><mi>μ</mi><mi>B</mi><mn>2</mn></msubsup><mo>)</mo></mrow></math>. We advocate for a more precise determination of the required expansion coefficients via lattice QCD simulations as a means of pinpointing the location of the critical endpoint in the phase diagram of strongly interacting matter.

Hitansh Shah

Brazil

Mauricio Hippert

Jorge Noronha

Claudia Ratti

Volodymyr Vovchenko

Locating the QCD critical point through contours of constant entropy density

The evolution of nuclear excitation in electron-gold (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mspace width="0.16em"></mspace><mo>+</mo><mspace width="0.16em"></mspace><mi>Au</mi></mrow></math>) collisions at the forthcoming Electron-Ion Collider (EIC) is investigated using the beagle event generator. The EIC's collider kinematics and detector-motivated rapidity selections enable a clear separation, in the laboratory frame, of the dominant reaction stages: hard scattering, intranuclear cascade (INC), and nuclear deexcitation. A systematic study of the event-by-event mean transverse momentum fluctuation observable <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>κ</mi></math> reveals strong sensitivity to the intranuclear-cascade formation time and to nuclear geometry, with only weak dependence on the electron-beam energy. These findings indicate that <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>κ</mi></math> can serve as an exploratory diagnostic for disentangling the cascade and deexcitation stages of nuclear excitation at the EIC. The pronounced sensitivity observed within the beagle framework underscores the model dependence of the present results while at the same time motivating systematic cross-model investigations to establish the broader applicability of <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>κ</mi></math>. The results yield stage-resolved, testable predictions for early EIC measurements.

Niseem Magdy

Tracing the evolution of nuclear excitation at the future Electron-Ion Collider