BESIII Results: Oscillating Features in the Neutron Electromagnetic Structure

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The nucleon constitutes most of the visible matter in the universe, however its nature has not been completely understood yet. Precise measurements of nucleon electromagnetic form factors are of crucial importance to reveal its complicated structure, which is not well calculated yet from the first principle of QCD theory. So far, experimental data for the neutron is rare in the time-like regime. In this talk we present new results for the Born cross section and the effective form factor of the neutron at the center-of-mass energies between 2.0 and 3.08 GeV, using 18 data sets corresponding to an integrated luminosity of 647.9 pb^-1 from electron-positron annihilation reactions collected at the BESIII experiment. Our results significantly improve the statistical precision on the neutron form factor over previous measurements, distinctly clarifying the photon-nucleon interaction puzzle persistent for over 20 years. In addition, the electromagnetic form factor of the neutron shows an oscillating behavior similar to that of the proton.

Higher-Order Cumulants of Proton Multiplicity Distributions in Au+Au Collisions at sqrt{s} = 3.0 GeV from STAR Fixed-Target Experiment

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In the search of conjectured QCD critical point and first-order phase boundary, higher-order cumulants of conserved quantities are proposed as sensitive observables. Recently the measurement of $\mathrm{C_{4}/C_{2}}$ from RHIC Beam Energy Scan (BES) phase I Au+Au collisions data at $\mathrm{\sqrt{s_{NN}}}$ = 7.7 - 200 GeV shows a non-monotonic energy dependence trend with 3.1$\sigma$ significance which is consistent with model predictions with a critical point. While there is still large statistical uncertainty below 20 GeV, the higher-order cumulant analysis from RHIC BES phase II will help shrink statistical uncertainty and confirm the trend.

In this talk, we report our measurement of higher-order cumulants of proton multiplicity distributions in Au+Au collisions at $\mathrm{\sqrt{s_{NN}}}$= 3 GeV collected by STAR at RHIC from the year 2018. Corresponding analysis techniques, like efficiency correction, pileup correction, and volume fluctuation correction will be discussed. A comparison with calculation from transport UrQMD model as well as implications of QCD critical point will be discussed.

Neutron Spin study using Double Spectator Tagging from 3He at the EIC

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The spin structure function of the neutron is traditionally determined by measuring the spin asymmetry of inclusive electron deep inelastic scattering (DIS) off polarized 3He nuclei. In such experiments, nuclear corrections are significant and must be treated carefully in the interpretation of experimental data. I will present our study of the feasibility of suppressing model dependencies by tagging both spectator protons in the process of DIS off neutrons in 3He at the forthcoming Electron-Ion Collider (EIC). This allows for a reconstruction of the momentum of the struck neutron to ensure it was nearly at rest in the initial state, thereby reducing sensitivity to nuclear corrections and suppressing contributions from electron DIS off protons in $^3$He. Using realistic accelerator and detector configurations, we demonstrate that the EIC can probe the neutron spin structure from xB of 0.003 to 0.651. Our study finds that the double spectator tagging method results in reduced uncertainties by a factor of 2 on the extracted neutron spin asymmetries over all kinematics and by a factor of 10 in the low-xB region, thereby providing valuable insight into the spin and flavor structure of nucleons.

The quark-gluon plasma: multi-messenger nuclear physics, microscopic relativistic fluids and big data

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The quark-gluon plasma is a new phase of matter that can be produced by colliding large nuclei at velocities close to the speed of light. This plasma is both the smallest and hottest liquid ever produced, extending the size of a few proton radii but reaching temperatures higher than those found in the most extreme astrophysical events. The explosive expansion of this plasma can be described with relativistic fluid dynamics, and provides a window into the equation of state and transport coefficients of strongly coupled nuclear matter. I present the latest constraints on the viscosities of the quark-gluon plasma, obtained through a large-scale Bayesian analysis combining measurements from two collider experiments. I discuss electromagnetic radiation from the quark-gluon plasma and their potential to provide additional constraints on the viscosities.

The Onset of Color Transparency from Holographic Light-Front QCD

The color transparency of a hadron, propagating without absorption in a nucleus, is a fundamental property of QCD, reflecting the hadron's internal structure and the effective size of its color distribution when it is produced at high transverse momentum Q. By using the framework of holographic light-front QCD, one can predict the Q^2 behavior of the effective transverse size of the hadronic cross section, its dependence on the hadron's twist \tau -- the number of constituent quarks of its valence state -- and the quark current which triggers the initial formation of a small color-singlet configuration which can propagate without interaction in a nucleus. One finds a significant delay in Q^2 for the onset of color transparency for hadrons with twist \tau \ge 3; this can explain the absence of color transparency for the electroproduction of a proton in the kinematic range of existing experimental tests. Remarkably, the onset in Q^2 of color transparency for baryons is predicted to strongly differ for electroproduction events corresponding to the spin-conserving (twist-3) Dirac form factor vs. the spin-flip (twist-4) Pauli form factor.

I will also review recent developments in intrinsic heavy quark phenomena, including predictions for intrinsic charm-anticharm asymmetry in the nucleon.

Near Threshold Heavy Quarkonium Photoproduction

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There has been a claim in the literature that the near threshold photo-production of heavy quarkonium can measure the gravitational form factors and hence to directly measure the proton mass distribution. In this talk, we will present our recent analysis based on perturbative QCD calculations for this process at large momentum transfer. We take into account the contributions from the leading three-quark Fock states of the nucleon. The dominant contribution comes from the three-quark Fock state with one unit quark orbital angular momentum (OAM) whereas that from zero quark OAM is suppressed at the threshold. From our analysis, we also show that there is no direct connection between the near threshold heavy quarkonium photoproduction and the gluonic gravitational form factors of the nucleon. Based on the comparison between our result and recent GlueX data of J/\psi photoproduction, we make predictions for \psi' and \Upsilon (1S,2S) states which can be tested in future experiments.

The puzzling behavior of tetraquarks and pentaquarks.

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Quarks that combine in much more complex ways than scientists expected can have repercussions in many fields, not only in particle physics. The recent observation by LHCb of a meson made of four charm quarks (T_cccc) and of new strange hidden charm tetraquark and pentaquark states has consecrated the subject of exotics as a hot topic. New experiments at LHC and Belle plan to shed light on this unexpected behavior and try to understand the strong force at long distances.

In this talk, some of the main experimental findings and theoretical predictions given before the experimental discoveries, regarding fully charm tetraquarks and pentaquarks, will be presented and discussed. Finally, the Electron Ion Collider (EIC) has the potential to produce such states in photoproduction reactions, which would confirm the observations in previous experiments and provide complementary insight into their composition.

Probing hadronization with flavor correlations of leading particles in jets

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In this talk I will discuss the study of nonperturbative flavor correlations between pairs of leading and next-to-leading charged hadrons within jets at the Electron-Ion Collider (EIC). We introduce a charge correlation ratio observable rc that distinguishes same- and opposite-sign charged pairs. Using Monte Carlo simulations with different event generators, rc is examined as a function of various kinematic variables for different combinations of hadron species, and the feasibility of such measurements at the EIC is demonstrated. The precision hadronization study we propose will provide new tests of hadronization models and hopefully lead to improved quantitative, and perhaps eventually analytic, understanding of nonperturbative QCD dynamics.

Gearing up for the EIC era: precision computations in the CGC

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Understanding the high energy limit of Quantum Chromodynamics (QCD) is one of the outstanding goals in nuclear and particle physics. At very high energies, it is conjectured that hadrons and nuclei transform into a universal form of matter known as the Color Glass Condensate (CGC). The CGC is an effective field theory for high-density saturated small-x gluons. This framework has been confronted with experimental data from HERA, RHIC, and the LHC, where hints of gluon saturation have been observed. In this talk, I will review the state of the art of the CGC with an eye towards the Electron-Ion Collider (EIC) era. I will emphasize recent next-to-leading-order computations for a variety of processes, and the potential for the discovery of gluon saturation at the EIC.

A new EIC pathfinder program with HERA data: Jet-based TMD measurements with machine-learning unfolding

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Recently, jet measurements in deep-inelastic scattering (DIS) events close to Born kinematics have been proposed as a new probe to study transverse-momentum-dependent (TMD) PDFs, TMD fragmentation functions, and TMD evolution. In this talk, I will report measurements of lepton-jet momentum imbalance in high-Q2 DIS events collected with the H1 detector at HERA (https://arxiv.org/abs/2108.12376). These data bridge DIS measurements from fixed target experiments and Drell-Yan measurements at colliders, thus providing a stringent test of TMD factorization, evolution and universality. This measurement also represents the first example of unfolding assisted with machine learning.

Experimental status of jet measurements in heavy-ion collisions

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Jet quenching refers to interaction of the jet shower with the QCD medium generated in relativistic heavy-ion collisions. Jet quenching has multiple phenomenological consequences - jet energy loss, modification of jet (sub)structure, and medium-induced acoplanarity, and it has been actively investigated in both theory and experiment. Due to the specific detector characteristics, kinematic coverage, and analysis methodologies of different experiments, different aspects of jet quenching are explored even with nominally the same jet observable. In this talk I will discuss the experimental status of jet measurements at RHIC and the LHC, focusing on selected jet observables.

Parton energy loss, momentum broadening, and eHIJING for the EIC

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Deep inelastic collisions of heavy nuclei are ideal for measuring energy loss and transverse momentum broadening of partons/jets in the nuclear medium. These measurements help us understand the gluon degree of freedom in large nuclei as probed by energetic parton. The jet-medium interactions are often quantified by the jet transport parameter defined in the higher-twist framework. On a microscopic level, one can also model these interactions by the exchange of transverse-momentum-dependent (TMD) gluon, probing the TMD gluon distribution of heavy nuclei. To study medium effects on hadron/jet production and correlations, we take an event generator approach. The resulting model, "electron-Heavy-Ion Jet INteraction Generator" (eHIJING), includes the parton momentum broadening based on the TMD gluon model, medium-modified parton evolution, and a model for medium modified hadronization process. With eHIJING, we present a detailed analysis of the SIDIS data, transverse-momentum broadening, and double-hadron correlation measured by the HERMES experiment and how they are sensitive to the jet transport parameter. Finally, we make projections for kinematics relevant for future electron-ion colliders (EIC). Precision measurements at EIC will extensively test our understanding of in-medium parton dynamics and nuclear structure at increasingly large Q^2 and small x regions.

Jet substructure to classify in-medium jet quenching

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Jet observables are crucial to deepen our understanding of the theory of the strong interactions. Being multi-scale objects, they are a probe of excellence to resolve the Quark-Gluon Plasma structure produced in ultra-relativistic heavy-ion collisions at RHIC and the LHC. While propagating through the created medium, jets will interact strongly with the created matter. As a result, jets lose energy, and their substructure is modified with respect to a proton-proton reference. The analysis of the collective in-medium modifications, generically known as jet quenching, allowed us to determine QGP transport properties and improve the current description of in-medium QCD propagation. However, these studies rely on an ensemble of jets that contain different magnitudes of in-medium modifications, thus endangering an accurate determination of in-medium QCD processes. In this talk, I will present novel jet substructure tools that can select jet populations by their quenching magnitude, thus helping to distinguish specific features of jet-QGP interaction.

Anomalous diffusion in QCD matter

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Transverse momentum broadening of energetic partons in QCD matter plays a central role in a variety of processes studied at colliders to probe QCD ranging from jet suppression in heavy ion collisions due to strong final state interactions that cause jets to lose energy to the QGP, to TMD gluon distributions that encode 3D information on the structure of the proton and nuclei in electron-proton or proton-proton collisions in particular at small Bjorken x where gluon saturation is expected to take place. I will discuss in this seminar the effects of quantum corrections on transverse momentum broadening of a fast parton passing through dense QCD matter. I will show that, at leading logarithmic accuracy the broadening distribution tends at late times to a universal distribution analogous to geometric scaling of gluon distributions in the saturation regime at high energy and traveling waves in reaction-diffusion processes. This super-diffusive process is reflected at large transverse momentum by the emergence of a heavy tail akin to Lévy random walks.

Isobar results from STAR in searching for the chiral magnetic effect

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The chiral magnetic effect (CME) is predicted to occur in relativistic heavy-ion collisions because of the chiral anomaly in quantum chromodynamics. To better control the background as well as the magnetic field contributions to CME-sensitive measurements, isobar collisions of ₄₄⁹⁶Ru+₄₄⁹⁶Ru and ₄₀⁹⁶Zr+₄₀⁹⁶Zr at √s_NN=200 GeV were conducted at BNL’s Relativistic Heavy Ion Collider in 2018. A large data sample of approximately 3.8 billion minimum-bias isobar collisions were acquired by the STAR experiment, on which a blind analysis has been performed. The findings from this blind analysis will be announced in a press release seminar on August 31 next week. This is the culmination of many years of work by the STAR collaboration. In this seminar, I will discuss these findings in the broad context of the CME search in heavy ion collisions.

Quantum simulation of non-equilibrium dynamics and thermalization in the Schwinger model

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Abstract: We present simulations of non-equilibrium dynamics of quantum field theories on digital quantum computers. As a representative example, we consider the Schwinger model, a 1+1 dimensional U(1) gauge theory, coupled through a Yukawa-type interaction to a thermal environment described by a scalar field theory. We use the Hamiltonian formulation of the Schwinger model discretized on a spatial lattice. With the thermal scalar fields traced out, the Schwinger model can be treated as an open quantum system and its real-time dynamics are governed by a Lindblad equation in the Markovian limit. The interaction with the environment ultimately drives the system to thermal equilibrium. In the quantum Brownian motion limit, the Lindblad equation is related to a field theoretical Caldeira-Leggett equation. By using the Stinespring dilation theorem with ancillary qubits, we perform studies of both the non-equilibrium dynamics and the preparation of a thermal state in the Schwinger model using IBM's simulator and quantum devices. The real-time dynamics of field theories as open quantum systems and the thermal state preparation studied here are relevant for a variety of applications in nuclear and particle physics, quantum information and cosmology.

Mapping the Electromagnetic Fields of Heavy-Ion Collisions with the Breit-Wheeler Process

Abstract: Ultra-relativistic heavy-ion collisions are expected to produce the strongest electromagnetic fields ($10^{13}-10^{16}$ Tesla) in the known Universe. These highly-Lorentz contracted fields can manifest themselves as linearly polarized quasi-real photons that interact via the Breit-Wheeler process. In this talk I will discuss recent experimental measurements that have spurred significant theoretical progress in our understanding of dilepton production in ultra-peripheral heavy ion collisions. Specifically, we now know that the energy and momentum distribution of the produced dileptons carry information about the strength and spatial distribution of the colliding fields. Moreover, the recent observation of quantum correlations between the interacting photon's spin (polarization) and momentum provides a clear connection to the semi-classical electromagnetic field distribution, thus making it possible to measure the magnetic field produced in heavy ion collisions for the first time. I'll end the talk with a look at how current and future measurements can be used to constraining the magnetic field and explore how they may provide novel input for the discussion of emergent magnetohydrodynamical phenomena driven by event-by-event fluctuations of the magnetic field.

Jet substructure: from proton-proton to heavy-ion collisions

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Jet substructure, defined by observables constructed from the distribution of constituents within a jet, provides the versatility to tailor observables to specific regions of QCD radiation phase space. This flexibility allows us to test not only our understanding of perturbative QCD but also the nature of nonperturbative effects including hadronization — and has resulted in jet substructure becoming an essential tool to study rare event topologies in searches for new physics. In this talk, I will highlight recent jet substructure measurements at the LHC and RHIC. I will discuss measurements in proton-proton collisions, which enable differential tests of our understanding of perturbative QCD, and measurements in heavy-ion collisions, which provide exciting new opportunities to reveal the nature of the quark-gluon plasma.

Rethinking Jets with Energy Correlators: Spin Correlations and Charged Energy Flow

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Abstract: Jets and their substructure have emerged over the last few years as a promising new method for probing the nature of the strong interactions. In this talk I will discuss progress in understanding energy flow in jets using recent developments in Conformal Field Theory. These developments allow one to rephrase the study of jet substructure in terms of correlation functions of particular lightray operators which can be studied using symmetries and the lightray operator product expansion. Rethinking jets in this manner has lead to a number of practical spinoffs of interest to the EIC physics program in particular an understanding of perturbative spin correlations in jets and higher order calculations of jet observables on subsets of hadrons (e.g. charged, strange, ...) using generalized fragmentation functions.

Study QCD Phase Structure with High-Energy Nuclear Collisions

- Selected results from RHIC beam energy scan program

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Abstract: Since 2010, STAR Collaboration has been investigating the QCD phase structure, especially the search for QCD critical point, with the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven National Laboratory. Over a wide energy range from 3 GeV to 200 GeV Au+Au collisions, the RHIC BES program has recorded the world’s largest dataset. In this talk, I will present selected results from the first phase of the Beam Energy Scan with focus on collective dynamics and fluctuations in high-energy nuclear collisions. Physics implications of these results and future prospects, especially the QCD phase structure at the high baryon density region, will be discussed

What can stop the flow? Azimuthal anisotropies at large mass, high pT, and in exotic systems with ATLAS

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Abstract: Experimental signatures of multi-particle collectivity persist for a broad variety of system sizes and interacting probes, such as in low-multiplicity proton-proton collisions and for charm quarks in proton-lead collisions. This behavior seems to be so ubiquitous, that it is almost as interesting to observe when a probe does not participate in the collective motion in a given system, as when it does. In this seminar, I will highlight some of our work in trying to delineate the boundaries of where collective phenomena start and stop using the ATLAS experiment at the LHC. Finding these boundaries can help better reveal the underlying dynamics of the many-body QCD systems produced in these collisions. I will discuss results on measurements of collectivity for charm- and bottom-separated heavy quarks in pp collisions, for very high-pT particles in p+Pb collisions, and in ultra-peripheral photo-nuclear collisions selected during Pb+Pb data-taking.

Non-perturbative Heavy Quark dynamics in ultrarelativistic collision: impact of vorticity, electromagnetic fields

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Abstract: The evidences collected in the last decades suggests that a new state of matter of deconfined quark and gluons plasma (QGP) has been created. Heavy quarks due to their large masses are excellent probes for its characterisation. The theoretical efforts made to describe D meson observables, have led in the last dcade to estimate a large non-perturbative D_s space diffusion of charm quarks in agreement with lattice QCD. Also a new insight on the heavy flavour hadronization process is emerging and could play a key role even in pp collisions. What remains to be more deeply explored is the very early stage dynamics where HQs can be further excellent probes of the initial glasma dynamics, and moreover of the huge bulk vorticity and electromagnetic field. I will discuss how the bulk vorticity can lead to a large directed flow v1 of neutral particles/anti-particles D0 and anti-D0 of few percent with a splitting generated by the strong initial e.m. field, both much larger compared to the observed light charged particles v1. We will see that the large v1 is a further sign of the large non-perturbative HQ drag (small D_s(T)) while the D0/anti-D0 v1 splitting can be considered as a possible probe of the formation of the quark-gluon plasma phase. However currently appears quite challenging to interpret coherently the first measurements at RHIC and LHC. We propose that especially at LHC the measurement of v1 of letpons from Z0 decay can provide further enlightening insight for understanding the origin of the observed splitting of D0/anti-D0 along with a measurement of v1 of B mesons.

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Diffractive vector meson photoproduction: a Swiss army knife for QCD

Abstract: Diffractive vector meson production is a deceptively simple process which allows us to ask several different questions about QCD in a relatively clean enviroment. In this talk I will present an overview of recent experimental results and discuss some of their implications,mainly regarding the search for stauration effects.

Neutron Stars with a Crossover Equation of State

Abstract: The question of whether quark matter exists in neutron stars is a long standing one. Generally one finds that a first order phase transition from baryons to quarks softens the equation of state so much that the star would collapse into a black hole. We consider a crossover equation of state, similar to the crossover that is found in lattice QCD studies at finite temperature and zero or small baryon chemical potentials. We find that with reasonable parameters it may be possible to support neutron stars up to about 2.2 solar masses. In that case 1 to 10% of the pressure would be contributed by quark matter in the central core of the highest mass stars.

Jet production in e+A collisions at the EIC

Jet production and jet substructure in reactions with nuclei at future electron ion colliders will play a preeminent role in the exploration of nuclear structure and the evolution of parton showers in strongly-interacting matter. In the framework of soft-collinear effective theory, generalized to include in-medium interactions, we present the first theoretical study of inclusive jet cross sections and the jet charge at the EIC. Predictions for the modification of these observables in electron-gold relative to electron-proton collisions reveal how the flexible center-of-mass energies and kinematic coverage at this new facility can be used to enhance the signal and maximize the impact of the electron-nucleus program. Importantly, we demonstrate theoretically how to disentangle the effects from nuclear parton distribution functions and the ones that arise from strong final-state interactions between the jet and the nuclear medium. A comparison to the modification of inclusive hadron production is also shown.

QCD Critical Point and Net-Proton Number Fluctuations

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Understanding the properties of quark matter and its phase structure can enhance our knowledge of universe evolution and the structure of visible matters. In the last two decades, many experimental evidences for the strongly interacting quark-gluon plasma (sQGP) have been observed in high energy heavy-ion collisions. Therefore, exploring the QCD phase structure at high baryon density, such as mapping the 1st order phase boundary and finding the QCD critical point, becomes one of the most important goals of the heavy-ion collisions. During 2010-2017, RHIC has finished the first phase of Beam Energy Scan program (BES-I), and STAR experiment has collected the data of Au+Au collisions at various collision energies from 200 to 7.7 GeV. To confirm the intriguing observations at BES-I, RHIC has started the second phase of beam energy scan program (BES-II) since 2018, focusing on the energies below 27 GeV. From 2018 to 2020, STAR experiment has taken the data of high statistics Au+Au collision at 9.2, 11.5, 14.6, 19.6 and 27 GeV (collider mode) and 3.0 - 7.7 GeV (fixed target mode). In this talk, I will discuss the recent experimental progress for exploring the QCD phase structure at RHIC-STAR experiment, especially focusing on the QCD critical point search. New facilities aiming for high baryon density region and future plan will be also discussed.

Toward seeing the chiral magnetic effect at long last

Quantum anomaly is a fundamental feature of chiral fermions. In chiral materials, the microscopic anomaly leads to nontrivial macroscopic transport processes such as the chiral magnetic effect (CME), which has been in the spotlight lately across disciplines of physics. The quark-gluon plasma (QGP) created in relativistic nuclear collisions provides the unique example of a chiral material consisting of intrinsically relativistic chiral fermions. The potential discovery of CME in QGP is of utmost significance, with extensive experimental searches carried out over the past decade. The path toward discovering it in heavy-ion collision experiments, however, has been rocky, due to relatively weak signal and strong background contamination. About 12 years after the first hint of possible CME signal reported at QM2009, excitement is mounting amid the expected release of analysis results later this year from a decisive isobar collision experiment. In this talk, I will discuss the early developments of this “hunt”, the painstaking process of recognizing and facing the background challenges and the key progress in remediating them. I will also discuss the status and prospect of relevant theoretical developments, highlighting the quantitative predictions for signatures of CME in the collisions of isobars.

Mass radius of the proton

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The mass radius is a fundamental property of the proton that so far has not been determined from experiment. Basing on my recent paper arXiv:2102:00110, I will show that the mass radius of the proton can be rigorously defined through the formfactor of the trace of the energy-momentum tensor (EMT) of QCD in the weak gravitational field approximation, as appropriate for this problem. I will then demonstrate that the scale anomaly of QCD enables the extraction of the formfactor of the trace of the EMT from the data on threshold photoproduction of J/ψ and Υ quarkonia, and use the recent GlueX Collaboration data to extract the r.m.s. mass radius of the proton R_m = 0.55 ± 0.03 fm. The extracted mass radius is significantly smaller than the r.m.s. charge radius of the proton R_C = 0.8409 ± 0.0004 fm. I will discuss the possible origin of this difference, and outline future measurements needed to determine the mass radius more precisely.

Asymmetry of antimatter in the proton from SeaQuest and SpinQuest perspectives

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Naively, the amounts of dbar and ubar in the proton were expected to be the same based

on the flavor-independence of the strong coupling. However, the muon deep inelastic

scattering experiment NMC at CERN found dbar>ubar in the proton. Drell--Yan experiments also obtained the results consistent with it. The Drell--Yan experiment

E866 at Fermilab showed that dbar(x)/ubar(x)>1.0 for 0.015 < x < 0.20. It also showed dbar(x)/ubar(x)<1.0 at large x (x~0.3), although it is consistent with 1.0 within statistical uncertainty. SeaQuest is a Drell--Yan experiment at Fermilab that measured the antiquark flavor asymmetry dbar/ubar precisely for a wide x range (0.13 < x < 0.45) including the intriguing region from E866. It uses a 120 GeV proton beam extracted from Fermilab Main Injector colliding with liquid hydrogen and deuterium targets. The antiquark flavor asymmetry dbar(x)/ubar(x) is derived from the cross section ratio of proton-deuterium to proton-proton Drell--Yan processes. SeaQuest results were published on Nature in February 2021. In this talk, the contents of Nature paper will be mainly presented. In addition to that, the perspective of the SpinQuest experiment will be also discussed.

Explore hadronization through heavy flavor probes at the Electron-Ion Collider

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How fundamental elementary blocks: quarks and gluons form into visible matter remains as a long-standing unresolved question. Such process known as the hadronization, can not be directly calculated by perturbative Quantum Chromodynamics (QCD). Existing e+e−, e + p, p + p, p + A and A + A experimental results provide limited constraints on the hadronization process either due to the limited kinematic reach or the complexity of hadron and heavy ion collision systems. The proposed high luminosity high energy Electron-Ion Collider (EIC) will provide a clean environment to explore the hadronization processes in vacuum and a heavy nucleus within a wide kinematic phase space. Heavy flavor products such as a D-meon or a charm jet at the EIC provide enhanced sensitivities to the nuclear transport properties in medium. Moreover, they will provide strong discriminating power to separate the hadronization happened inside or outside the nuclear medium. In this talk, I will discuss about studies of heavy flavor hadron and jet reconstruction in simulation and the corresponding physics studies associated with the hadronization process, such as the flavor dependent hadron nuclear modification factor in electron+nucleus collisions. Initial design and performance of a proposed forward (proton/nuclei going direction) silicon tracking detector, which is essential to carry out these measurements at the EIC will be shown as well.

Flow and jet quenching in small systems

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As a function of system size and center-of-mass energy, signatures of collectivity are known to arise smoothly and without any sharp threshold in ultra-relativistic proton-proton (pp), proton-nucleus and nucleus-nucleus collisions (AA). Yet, the physics concepts invoked in phenomenological descriptions of the smallest (pp) and the largest (central AA) hadronic collision systems are maximally different. Our baseline picture of pp collisions is a free-streaming but fragmenting partonic system in which final state interactions may be treat as perturbations. Our baseline picture of central heavy ion collisions is a perfect fluid, deviations of which are parametrized in terms of dissipative fluid dynamic properties. Given that the size of the smallest and the largest collision systems differ by only one order of magnitude, and given that the entire size-dependence is experimentally accessible, there are strong motivations for developing a phenomenological description that smoothly interpolates between almost free-streaming and almost perfect fluidity as a function of system size. In this talk, I discuss some of the resulting theoretical challenges and open experimental issues. I emphasize that such a phenomenological program can give access to some fundamental properties of hot QCD matter that escape a purely fluid-dynamic analysis of heavy ion data.

Dynamical modeling of the initial energy-momentum and baryon charge distributions for heavy-ion collisions at RHIC and LHC

Heavy-ion collisions at different beam energies offer us a unique opportunity to study the QGP properties in the $T$-$\mu_B$ QCD phase diagram. Building upon Ref. [1], we present an improved three-dimensional dynamical initialization model for heavy-ion collisions by implementing local energy-momentum conservation and baryon charge fluctuations at string junctions [2]. These improvements lead to an excellent description of the charged hadron and net proton rapidity distributions in Au+Au collisions from 7.7 to 200 GeV. By keeping all model parameters fixed, we extrapolate our model to asymmetric (p, d, 3He)+Au collisions at RHIC BES energies as well as p+Pb, O+O, and Pb+Pb at LHC energies. We achieve a good description of the measured particle rapidity distributions for those collision systems, demonstrating our model’s predictive power.

[1] C. Shen and B. Schenke, “Dynamical initial state model for relativistic heavy-ion collisions,” Phys.Rev. C97 (2018) no.2, 024907

[2] D. Kharzeev, “Can gluons trace baryon number?,” Phys. Lett. B 378, 238 (1996)

Correlated Dirac eigenvalues and axial anomaly in chiral symmetric QCD

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The phenomenon of anomaly plays an important role in quantum field theory. In QCD how axial anomaly manifests itself in the two-point correlation functions of iso-triplet scalar and pseudo-scalar mesons affects the nature of chiral phase transition. In this talk I first review current studies of the fate of UA (1) anomaly in the finite temperature lattice QCD, and then propose novel relations between the quark mass derivatives of Dirac eigenvalue spectrum and correlation functions among eigenvalues in order to study the microscopic origin of the axial anomaly. We finally show our results in the chiral and continuum limit in (2+1)-flavor lattice QCD at 1.6Tc. Our results suggest that the axial anomaly is driven by the weakly interacting (quasi-)instanton gas motived eigenvalue spectrum above 1.6Tc and the chiral phase transition is of 2nd order and belongs to 3-d O(4) universality class (talk is based on https://arxiv.org/abs/2010.14836).

Non-equilibrium attractor in high-temperature QCD plasmas

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I will discuss a recent paper in which my collaborators and I established the existence of a far-from-equilibrium attractor in weakly-coupled gauge theory that goes beyond the usual hydrodynamic attractor picture. We demonstrated that the resulting far-from-equilibrium evolution is insensitive to certain features of the initial condition, including both the initial momentum-space anisotropy and initial occupancy. We found that this insensitivity extends beyond the energy-momentum tensor to the detailed form of the one-particle distribution function. Based on our results, I will assess different procedures for reconstructing the full one-particle distribution function from the energy-momentum tensor along the attractor and discuss implications for the freeze-out procedure used in the phenomenological analysis of ultra-relativistic nuclear collisions.

References:

1. Dekrayat Almaalol, Aleksi Kurkela, and Michael Strickland, Phys. Rev. Lett. 125, 122302 (2020)

2. M. Strickland, Journal High Energy Physics 2018, 128 (2018).

Global polarization of \Xi and \Omega hyperons in heavy-ion collisions

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Global polarization in heavy-ion collisions arises from the partial conversion of the orbital angular momentum of colliding nuclei into the spin angular momentum of particles produced in the collisions. The STAR Collaboration observed \Lambda global polarization in Au+Au collisions at \sqrt{s_{NN}} = 7.7—200 GeV, indicating a thermal vorticity of the system. Based on thermal models, particles with the same spin are expected to have the same polarization, while other effects might contribute to the polarization and change the picture. In this talk, the first measurements of the global polarization of spin s=1/2 \Xi and spin s=3/2 \Omega hyperons in heavy-ion collisions will be presented and the physics implications will be discussed.

Mapping Nucleon Parton Distributions with Lattice QCD

The strong force which binds hadrons is described by the theory of quantum chromodynamics (QCD). Determining the character and manifestations of QCD is one of the most important and challenging outstanding issues necessary for a comprehensive understanding of the structure of hadrons. Within the context of the QCD parton picture, the parton distribution functions (PDFs) have been remarkably successful in describing a wide variety of processes. However, these PDFs have generally been confined to the description of collinear partons within the hadron. New experiments and facilities provide the opportunity to additionally explore the three-dimensional structure of hadrons, which can be described by generalized parton distributions (GPDs) for example.

In recent years, a breakthrough was made in calculating the Bjorken-x dependence of PDFs in lattice QCD by using large-momentum effective theory (LaMET) and other similar frameworks. The breakthrough has led to the emergence and rapid development of direct calculations of Bjorken-x dependent structure.

Constraining the jet transport coefficient q-hat in heavy-ion collisions with Bayesian analysis

The jet transport coefficient qhat is a characterization of the properties of the interaction between high energy parton and the quark-gluon plasma, created during relativistic heavy-ion collisions. I will present a new analysis by the JETSCAPE Collaboration of the dependence of qhat on jet energy, jet virtuality, and medium temperature. The analysis utilizes Bayesian parameter estimation techniques to compare theoretical calculation and experimental data of inclusive hadron suppression spanning a wide range of transverse momentum, at both RHIC and LHC center-of-mass energies. The correlation of the experimental uncertainties is an important piece of the analysis and is accounted for. Simulation is performed using the JETSCAPE framework, with a multi-stage approach based on the MATTER and LBT jet quenching models We will also discuss prospects of additional analyses to better constrain additional properties of the quark-gluon plasma.

Search for the elusive jet-induced diffusion wake in Z-jets with 2D jet tomography

Diffusion wake is an unambiguous part of the jet-induced medium response in high-energy heavy-ion collisions that leads to a depletion of soft hadrons in the opposite direction of the jet propagation. New experimental data on $Z$-hadron correlation in Pb+Pb collisions at the Large Hadron Collider show, however, an enhancement of soft hadrons in the direction of both the $Z$ and the jet. Using a coupled linear Boltzmann transport and hydro model, we demonstrate that medium modification of partons from the initial multiple parton interaction (MPI) gives rise to a soft hadron enhancement that is uniform in azimuthal angle while jet-induced medium response and soft gluon radiation dominate the enhancement in the jet direction. After subtraction of the contributions from MPI with a mixed-event procedure, the diffusion wake becomes visible in the near-side $Z$-hadron correlation. We further employ the longitudinal and transverse gradient jet tomography for the first time to localize the initial jet production positions in $Z/\gamma$-jet events in which the effect of the diffusion wake is apparent in $Z/\gamma$-hadron correlation even without the subtraction of MPI.

What can we learn from heavy neutron stars?

The observation of gravitational waves from a blackhole-mystery object binary opens the possibility for heavy neutron stars of 2.5 solar masses (potentially seen in GW190814). If this mystery object is a neutron star of 2.5 solar masses, it poses direct challenges to models of the equation of state. Interestingly, introducing non-trivial structure in the speed of sound sourced by changes in the degrees of freedom (possibly quarks) of ultra-dense matter can resolve this conflict, which may have large ramifications in nuclear and astrophysics. However, for a clear smoking gun signature of the mystery object being a neutron star, one requires a measurement of the tidal deformability that is non-zero. Because the predicted values are very small, a tenfold increase in sensitivity may be needed to test this possibility with gravitational waves, which is feasible with third generation detectors. Finally, I will comment on opportunities in heavy-ion collisions that exist for further constraining the equation of state relevant to neutron stars.