# Hadron Ion Tea (HIT) Seminar

[formerly Heavy Ion Tea Seminar]

Nuclear Science Division

Lawrence Berkeley National Laboratory

# HIT seminars are typically on Tuesdays at 3:30pm Pacific Time

Current Organizers: Alberto Collu, Sooraj Radhakrishnan, André Walker-Loud, Xin-Nian Wang

### Tues. 2021.03.09, 1:30 PM, Virtual Zoom HIT

**Dr. Urs Wiedemann** (CERN)

**Flow and jet quenching in small systems**

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.

# Upcoming seminars

09 Mar, 2021 -Urs Widemann (CERN) (1:30 pm)

16 Mar, 2021 - Xuan Li (LANL)

23 Mar, 2021 - Open

30 Mar, 2021 - Dima Kharzeev (SUNY@Stony Brook & BNL)

06 April, 2021 - JingFeng Liao (Indiana Univ)

13 April, 2021 - Xiaofeng Luo (Central China Normal University)

1 June, 2021 - Nu Xu (LBNL)

### Tues. 2021.03.30, 3:30 PM, Virtual Zoom HIT

**Prof. ****Dima Kharzeev** (Stony Brook Univ)

**Mass radius of the proton**

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.

### Tues. 2021.04.06, 3:30 PM, Virtual Zoom HIT

**Prof. ****Jingfeng Liao** (Indiana Univ)

https://physics.indiana.edu/about/directory/all-faculty-scientists/liao-jinfeng.html

**Toward ****s****eeing the ****c****hiral ****m****agnetic ****e****ffect ****a****t ****l****ong ****l****ast**** **

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.

### Tues. 2021.04.13, 4:30 PM, Virtual Zoom HIT

**Prof. ****Xiaofeng Luo** (Central China Normal Univ)

# Previous Seminars

### Tues. 2021.01.12, 3:30 PM, Virtual Zoom HIT

**Prof. Jaki Noronha-Hostler**, University of Illinois at Urbana-Champaign

**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.

**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.

**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.

### Tues. 2021.02.2, 3:30 PM, Virtual Zoom HIT

**Prof. ****Huey-Wen Lin** (Michigan State University)

**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.

### Tues. 2021.02.9, 3:30 PM, Virtual Zoom HIT

**Prof. ****Takafumi Niida** (University of Tsukuba)

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

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.

### Tues. 2021.02.16, 3:30 PM, Virtual Zoom HIT

**Prof. ****Michael Strickland** (Kent State Univ)

**Non-equilibrium attractor in high-temperature QCD plasmas**

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:

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

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

### Tues. 2021.02.23, 3:30 PM, Virtual Zoom HIT

**Prof. ****Hengtong Ding** (Central China Normal Univ)

https://inspirehep.net/authors/1259382?ui-citation-summary=true

**Correlated Dirac eigenvalues and axial anomaly in chiral symmetric QCD**

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).

### Tues. 2021.03.02, 3:30 PM, Virtual Zoom HIT

**Prof. ****Chun Shen** (Wayne State Univ)

**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)