Nuclear Experiment

We studied the effects of centrality fluctuation and deuteron formation on the cumulants ( C n ) and correlation functions ( κ n ) of protons up to sixth order in most central ( b<3 fm) Au+Au collisions at s NN − − − √ = 3 GeV from a microscopic transport model (JAM). The results are presented as a function of rapidity acceptance within transverse momentum 0.4< p T <2 GeV/ c . We compared the results obtained by centrality bin width correction (CBWC) using charged reference particle multiplicity with CBWC done using impact parameter bins. It was found that at low energies the centrality resolution for determining the collision centrality using charged particle multiplicities is not good enough to reduce the initial volume fluctuations effect for higher-order cumulant analysis. New methods need to be developed to classify events with high centrality resolution for heavy-ion collisions at low energies. Finally, we observed that the formation of deuteron will suppress the higher-order cumulants and correlation functions of protons and is found to be similar to the efficiency effect. This work can serve as a noncritical baseline for the QCD critical point search at the high baryon density region.

Systematic differences in the the proton's charge radius, as determined by ordinary atoms and muonic atoms, have caused a resurgence of interest in elastic lepton scattering measurements. The proton's charge radius, defined as the slope of the charge form factor at Q 2 =0, does not depend on the probe. Any difference in the apparent size of the proton, when determined from ordinary versus muonic hydrogen, could point to new physics or need for the higher order corrections. While recent measurements seem to now be in agreement, there is to date no high precision elastic scattering data with both electrons and positrons. A high precision proton radius measurement could be performed in Hall B at Jefferson Lab with a positron beam and the calorimeter based setup of the PRad experiment. This measurement could also be extended to deuterons where a similar discrepancy has been observed between the muonic and electronic determination of deuteron charge radius. A new, high precision measurement with positrons, when viewed alongside electron scattering measurements and the forthcoming MUSE muon scattering measurement, could help provide new insights into the origins of the proton radius puzzle, and also provide new experimental constraints on radiative correction calculations.

Inelastic proton scattering experiments were performed at the Research Center for Nuclear Physics, Osaka, with a 295 MeV beam covering laboratory angles 0°-6° and excitation energies 6-22 MeV. Cross sections due to E1 and M1 excitations were extracted with a multipole decomposition analysis and then converted to reduced transition probabilities with the "virtual photon method" for E1 and the "unit cross section method" for M1 excitations, respectively. Including a theory-aided correction for the high excitation energy region not covered experimentally, the electric dipole polarizability was determined from the E1 strength distributions. Total photoabsorption cross sections derived from the E1 and M1 strength distributions show significant differences compared to those from previous ( γ ,xn) experiments in the energy region of the isocvector giant dipole resonance (IVGDR). The widths of the IVGDR deduced from the present data with a Lorentz parameterization show an approximately constant value of about 4.5 MeV in contrast to the large variations between isotopes observed in previous work. The IVGDR centroid energies are in good correspondence to expectations from systematics of their mass dependence. Furthermore, a study of the dependence of the IVGDR energies on bulk matter properties is presented. The E1 strengths below neutron threshold show fair agreement with results from ( γ , γ ') experiments on 112,116,120,124Sn in the energy region between 6 and 7 MeV. At higher excitation energies large differences are observed pointing to a different nature of the excited states with small ground state branching ratios. The isovector spin-M1 strengths exhibit a broad distribution between 6 and 12 MeV in all studied nuclei.

Electromagnetic and electroweak probes are the most versatile probes in the study of heavy-ion collisions. Produced at every stage in the evolution of QCD matter, its messengers are practically inert to the strongly interacting medium they travel through. In this contribution, I will discuss a selection of new results from experiments at the LHC, RHIC, and SIS facilities, spanning almost four orders of magnitude in beam energy. I will conclude with a brief overview of the experimental landscape in the near future.

Second-order processes in physics is a research topic focusing attention from several fields worldwide including, for example, non-linear quantum electrodynamics with high-power lasers, neutrinoless double- β decay, and stimulated atomic two-photon transitions. For the electromagnetic nuclear interaction, the observation of the competitive double- γ decay from 137m Ba has opened up the nuclear structure field for detailed investigation of second-order processes through the manifestation of off-diagonal nuclear polarizability. Here we confirm this observation with an 8.7σ significance, and an improved value on the double-photon versus single-photon branching ratio as 2.62× 10 −6 (30) . Our results, however, contradict the conclusions from the original experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small quadrupole-quadrupole element in the decay, hindered due to an evolution of the internal nuclear structure. The implied strongly hindered double-photon branching in 137m Ba opens up the possibility of the double-photon branching as a feasible tool for nuclear-structure studies on off-diagonal polarizability in nuclei where this hindrance is not present.

Lepton scattering is an established ideal tool for studying inner structure of small particles such as nucleons as well as nuclei. As a future high energy nuclear physics project, an Electron-ion collider in China (EicC) has been proposed. It will be constructed based on an upgraded heavy-ion accelerator, High Intensity heavy-ion Accelerator Facility (HIAF) which is currently under construction, together with a new electron ring. The proposed collider will provide highly polarized electrons (with a polarization of ??80%) and protons (with a polarization of ??70%) with variable center of mass energies from 15 to 20 GeV and the luminosity of (2-3) ? 10 33 cm ?? s ?? . Polarized deuterons and Helium-3, as well as unpolarized ion beams from Carbon to Uranium, will be also available at the EicC. The main foci of the EicC will be precision measurements of the structure of the nucleon in the sea quark region, including 3D tomography of nucleon; the partonic structure of nuclei and the parton interaction with the nuclear environment; the exotic states, especially those with heavy flavor quark contents. In addition, issues fundamental to understanding the origin of mass could be addressed by measurements of heavy quarkonia near-threshold production at the EicC. In order to achieve the above-mentioned physics goals, a hermetical detector system will be constructed with cutting-edge technologies. This document is the result of collective contributions and valuable inputs from experts across the globe. The EicC physics program complements the ongoing scientific programs at the Jefferson Laboratory and the future EIC project in the United States. The success of this project will also advance both nuclear and particle physics as well as accelerator and detector technology in China.

Electroweak bosons are sensitive probes of the initial state of heavy-ion collisions, of which a precise knowledge is required in order to disentangle initial state effects from phenomena induced by the presence of the quark--gluon plasma (QGP). The production rate of the Z and W ± bosons is especially sensitive to the nuclear modification of the Parton Distribution Functions (PDF), and the muonic decays (Z → μ + μ − and W ± → μ ± ν ) offer medium-blind processes carrying this information to the detector where it can be directly collected. In this contribution, new measurements of electroweak bosons in p--Pb collisions at s NN − − − √ = 8.16 TeV and Pb--Pb collisions at s NN − − − √ = 5.02 TeV measured by the ALICE Collaboration are reported. The data are collected at forward rapidity with the ALICE muon spectrometer and are compared to theoretical predictions with and without including nuclear modifications.

The measurements of the (anti)deuterons elliptic flow ( v 2 ) and the first measurements of triangular flow ( v 3 ) in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon collisions s NN − − − √ = 5.02 TeV are presented. A mass ordering at low transverse momentum ( p T ) is observed when comparing these measurements with those of other identified hadrons, as expected from relativistic hydrodynamics. The measured (anti)deuterons v 2 lies between the predictions from the simple coalescence and blast-wave models, which provide a good description of the data only for more peripheral and for more central collisions, respectively. The mass number scaling, which is violated for v 2 , is approximately valid for the (anti)deuterons v 3 . The measured v 2 and v 3 are also compared with the predictions from a coalescence approach with phase-space distributions of nucleons generated by iEBE-VISHNU with AMPT initial conditions coupled with UrQMD, and from a dynamical model based on relativistic hydrodynamics coupled to the hadronic afterburner SMASH. The model predictions are consistent with the data within the uncertainties in mid-central collisions, while a deviation is observed in central centrality intervals.

The elliptic flow of electrons from beauty hadron decays at midrapidity ( |y| < 0.8) is measured in Pb-Pb collisions at s NN − − − √ = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parameterized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, v 2 . The v 2 coefficient of electrons from beauty-hadron decays is measured for the first time in the transverse momentum ( p T ) range 1.3-6 GeV/ c in the centrality class 30-50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length cτ≈ 500 μ m compared to that of charm hadrons and most of the other background sources. The v 2 of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 σ . The results provide insights on the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavoured by the measurement at high p T , but is in agreement with the results at low p T . Transport models including substantial interactions of beauty quarks with an expanding strongly-interacting medium describe the measurement.

The discovery of QGP phenomena in small collision systems like pp and p-Pb collisions have challenged the basic paradigms of heavy-ion and high-energy physics. These proceedings give a brief overview of the key findings and their implications, as well as today's experimental and theoretical situation. An outlook of future measurement is made.