## from Cosmos to Quarks

### The research of the Nuclear Theory Program at LBNL addresses many of the urgent scientific challenges in Nuclear Physics, including (1) properties of hot and dense matter, (2) the QCD structure of nucleons and nuclei, (3) nuclear and neutrino astrophysics, (4) many body physics, and (5) fundamental symmetries.

## Recent highlights

### Dijet Anisotropy at Colliders

Yoshitaka Hatta, Bo-Wen Xiao, Feng Yuan, and Jian Zhou, *Anisotropy in Dijet Production in Exclusive and Inclusive Processes*,** ****Phys. Rev. Lett. 126, 142001 (2021)**.

We investigate the effect of soft gluon radiations on the azimuthal angle correlation between the total and relative momenta of two jets in inclusive and exclusive dijet processes. We show that the final state effect induces a sizable cos(2ϕ) anisotropy due to gluon emissions near the jet cones. The phenomenological consequences of this observation are discussed for various collider experiments, including diffractive processes in ultraperipheral pA and AA collisions, inclusive and diffractive dijet production at the EIC, and inclusive dijet in pp and AA collisions at the LHC.

*figure:* Dijet in transverse plane perpendicular to the beam direction at hadron colliders. Their total transverse momentum q_{T}=k_{1T}+k_{2T} is much smaller than the individual jet momentum P_{T}=(k_{1T}−k_{2T})/2. Angular distribution between q_{T} and P_{T} has an anisotropy due to soft gluon radiation associated with the final state jet with a nonzero ⟨cos(2ϕ)⟩.

### Theory advance toward the sophistic lattice computations of parton distribution functions

Long-Bin Chen, Wei Wang, and Ruilin Zhu, *Next-to-Next-to-Leading Order Calculation of Quasiparton Distribution Functions*, **Phys. Rev. Lett. 126, 072002, (2021)**

In the last few years, there have been significant progresses toward a first principle computation of nucleon parton distribution functions, based on the so-called large momentum effective theory (LaMET), see, a recent review. In this formalism, a quasidistribution is constructed from the lattice calculable matrix element of the hadron state and the relevant light-cone distributions can be derived through a perturbative matching. This provides a powerful tool to calculate all parton observables from the first principle of QCD which can be directly confronted with the experimental measurements. The fixed-order calculation plays an important role in the development of LaMET. It provides not only the explicit expression of the matching coefficients needed for the lattice computation, but also the detailed instances showing how the factorization works. In this paper, we carry out, for the first time, the next-to-next-to-leading order (NNLO) calculation of quark quasiparton distribution functions (PDFs) in the large momentum effective theory. The nontrivial factorization at this order is established explicitly and the full analytic matching coefficients between the quasidistribution and the light-cone distribution are derived. We demonstrate that the NNLO numerical contributions can improve the behavior of the extracted PDFs sizably. With the unprecedented precision study of nucleon tomography at the planned electron-ion collider, high precision lattice QCD simulations with the NNLO results implemented will enable to test the QCD theory and more precise results on the PDFs of nucleons will be obtained.

*Figure:* Next-to-next-to-leading order improvement for the extraction of nucleon parton distirbution functions (up quark minus down quark) from the lattice computations. Also shown are results from the NNPDF global analysis (R. D. Ball et al., Eur. Phys. J. C 77, 663 (2017)).

### Nature Review Physics (cover of January Issue of 2021): Proton Spin

Xiangdong Ji, Feng Yuan, Yong Zhao, *What we know and what we don’t know about the proton spin after 30 years*, **Nature Reviews Physics volume 3, 27–38 (2021)**

More than three decades ago, the European Muon Collaboration published a surprising result on the spin structure of the proton: the spins of its three quark components account for only a small part of the spin of the proton. Ever since, theoretical and experimental progress has been made in understanding the origins of the proton spin. In this Review, we discuss what has been learned so far, what is still missing and what could be learned from the upcoming experiments, including the Jefferson Lab 12 GeV upgrade and the proposed Electron-Ion Collider. In particular, we focus on first-principles calculations and experimental measurements of the total gluon helicity ΔG, and the quark and gluon orbital angular momenta.

See an editorial and a comment on EIC project in the same issue of Nature Review Physics.

### Correlation studies of fission-fragment neutron multiplicities

M. Albertsson, B. G. Carlsson, T. Døssing, P. Möller, J. Randrup, and S. Åberg, Correlation studies of fission-fragment neutron multiplicities, **Phys. Rev. C 103, 014609**

We calculate neutron multiplicities from fission fragments with specified mass numbers for events having a specified total fragment kinetic energy. The shape evolution from the initial compound nucleus to the scission configurations is obtained with the metropolis walk method on the five-dimensional potential-energy landscape, calculated with the macroscopic-microscopic method for the three-quadratic-surface shape family. Shape-dependent microscopic level densities are used to guide the random walk, to partition the intrinsic excitation energy between the two proto-fragments at scission, and to determine the number of neutrons evaporated from the fragments. The contribution to the total excitation energy of the resulting fragments from statistical excitation and shape distortion at scission is studied. Good agreement is obtained with available experimental data on neutron multiplicities in correlation with fission fragments from 235U(n_{th},f). With increasing neutron energy a superlong fission mode grows increasingly prominent, which affects the dependence of the observables on the total fragment kinetic energy.

### Pion Condensation in the Early Universe

Volodymyr Vovchenko, Bastian B. Brandt, Francesca Cuteri, Gergely Endrődi, Fazlollah Hajkarim, Jürgen Schaffner-Bielich, *Pion Condensation in the Early Universe at Nonvanishing Lepton Flavor Asymmetry and Its Gravitational Wave Signatures*, **Phys. Rev. Lett. 126, 012701 (2021)**

The conditions for the formation of a Bose-Einstein condensed phase of pions in the early Universe are determined. Utilizing a hadron resonance gas model with pion interactions constrained to first-principle lattice QCD simulations at nonzero isospin density, we evaluate cosmic trajectories at various values of electron, muon, and tau lepton asymmetries that satisfy the available constraints on the total lepton asymmetry. The cosmic trajectory passes through the pion condensed phase if the combined electron and muon asymmetry is sufficiently large: |le + lμ| > 0.1, with little sensitivity to the difference le − lμ between the individual flavor asymmetries. Future constraints on the values of the individual lepton flavor asymmetries will thus be able to either confirm or rule out the condensation of pions during the cosmic QCD epoch. We demonstrate that the pion condensed phase leaves an imprint both on the spectrum of primordial gravitational waves and on the mass distribution of primordial black holes at the QCD scale, for example, the black hole binary of recent LIGO event GW190521 can be formed in that phase.

*Figure*: Cosmic trajectories in the plane of temperature and electric chemical potential evaluated for different values of the combined electric and muon asymmetry, le + lμ. For le + lμ > 0.1, the cosmic trajectories pass through the pion-condensed phase, depicted in the figure by the shaded grey area.

### Anisotropy of leptons from two photon process in heavy ion collisions

Bo-Wen Xiao, Feng Yuan, and Jian Zhou, *Momentum Anisotropy of Leptons from Two-Photon Processes in Heavy-Ion Collisions*, **Phys. Rev. Lett. 125, 232301 (2020)**

Di-lepton production in heavy ion collisions has attracted great attention in recent years. It may provide a unique window to explore the electro-magnetic property of the quark-gluon plasma created in these collisions. Experiments from RHIC and LHC have reported a number of interesting phenomena for these di-lepton in various collision configurations, including ultra-peripheral, peripheral, and in some occasions, even the central collisions. Significant Pt-broadening of the total transverse momentum of the lepton pair was observed from ultra-peripheral to central collisions. This can be explained as a final state interaction effects of the lepton pair when they traverse through the medium, see, our previous paper in Phys. Rev. Lett. 122, 132301 (2019). However, this could come from the initial state effects, as indicated by recent studies (*Phys.Lett.B* 800 (2020) 135089; *Phys.Rev.D* 102 (2020) 9, 094013). Therefore, the key is to identify/isolate the production mechanism of the lepton pair in the two photon processes.

In this paper, we carry out an analysis that can help to identify the production mechanism of the lepton pair in heavy ion collisions. In particular, we investigate the azimuthal angular correlation between the lepton transverse momentum P⊥. and the impact parameter b⊥ in noncentral heavy-ion collisions, where the leptons are produced through two-photon scattering. Among the Fourier harmonic coefficients, a significant v4 asymmetry is found for the typical kinematics at RHIC and LHC with a mild dependence on the P⊥, whereas v2 is power suppressed by the lepton mass over P⊥. This unique prediction, if confirmed from the experiments, shall provide crucial information on the production mechanism for the dilepton in two-photon processes.

*upper figure:* Illustration of the polarized photon flux associated with a relativistic heavy nucleus moving to the right. The physical polarization of the photon propagating to the right is along the direction of b1⊥ with respect to the center of the nucleus in the transverse plane. Because of this peculiar polarization states for both incoming photons in heavy ion collisions, the resulting lepton pair will have a cos(4ϕ) anisotropy.

l*ower figure*: Estimates of the cos(4ϕ) asymmetry as the function of b⊥ in Au-Au collisions at √s=200 GeV as an example to show the momentum anisotropy of leptons from two photon processes in heavy ion collision. Similar results exist for the LHC experiments as well.

*Bayesian Approach to Understand N**eu**tron Star Equation of State*

*Bayesian Approach to Understand N*

*eu*

*tron Star Equation of State*

C. Drischler, R. J. Furnstahl, J. A. Melendez, and D. R. Phillips, *How Well Do We Know the Neutron-Matter Equation of State at the Densities Inside Neutron Stars? A Bayesian Approach with Correlated Uncertainties**,* **Phys. Rev. Lett. 125, 202702 (2020)**

A new framework for quantifying correlated uncertainties of the infinite-matter equation of state derived from chiral effective field theory (χEFT) was introduced. Bayesian machine learning via Gaussian processes with physics-based hyperparameters allows us to efficiently quantify and propagate theoretical uncertainties of the equation of state, such as χEFT truncation errors, to derived quantities. We apply this framework to state-of-the-art many-body perturbation theory calculations with nucleon-nucleon and three-nucleon interactions up to fourth order in the χEFT expansion. This produces the first statistically robust uncertainty estimates for key quantities of neutron stars. We give results up to twice nuclear saturation density for the energy per particle, pressure, and speed of sound of neutron matter, as well as for the nuclear symmetry energy and its derivative. At nuclear saturation density, the predicted symmetry energy and its slope are consistent with experimental constraints.

*Figure*: Constraints on the Sv–L correlation. Our results (“GP–B”) are given at the 68% (dark-yellow ellipse) and 95% level (light-yellow ellipse). Experimental constraints are derived from heavy-ion collisions (HIC) [72], neutron-skin thicknesses of Sn isotopes [73], giant dipole resonances (GDR) [74], the dipole polarizability of 208 Pb [75, 76], and nuclear masses [77]. The intersection is depicted by the white area, which only barely overlaps with constraints from isobaric analog states and isovector skins (IAS+ΔR) [78]. In addition, theoretical constraints derived from microscopic neutron-matter calculations by Hebeler *et al.* (H) [79] and Gandolfi *et al.* (G) [80] as well as from the unitary gas (UG) limit by Tews *et al.* [69]. The figure has been adapted from Refs. [70, 71]. A Jupyter notebook that generates it is provided in Ref. [42].

*Equation of State of **Symmetric Nuclear Matter *

*Equation of State of*

*Symmetric Nuclear Matter*

M. Leonhardt, M. Pospiech, B. Schallmo, J. Braun, C. Drischler, K. Hebeler, and A. Schwenk, *Symmetric Nuclear Matter from the Strong Interaction*, **Phys. Rev. Lett. 125, 142502 ****(2020)**

We study the equation of state of symmetric nuclear matter at zero temperature over a wide range of densities using two complementary theoretical approaches. At low densities, up to twice nuclear saturation density, we compute the energy per particle based on modern nucleon-nucleon and three-nucleon interactions derived within chiral effective field theory. For higher densities, we derive for the first time constraints in a Fierz-complete setting directly based on quantum chromodynamics using functional renormalization group techniques. We find remarkable consistency of the results obtained from both approaches as they come together in density and the natural emergence of a maximum in the speed of sound at supranuclear densities. The presence of this maximum appears tightly connected to the formation of a diquark gap. Notably, this maximum is observed to exceed the asymptotic value 1/3 while its exact position in terms of the density cannot yet be determined conclusively.

*Left figure:* Pressure of symmetric nuclear matter as obtained from chiral effective field theory (EFT), functional renormalization group (FRG), and perturbative QCD (pQCD), in comparison with different models.

*Jet **Quenching **Tomography*

*Jet*

*Quenching*

*Tomography*

Yayun He, Long-Gang Pang, and Xin-Nian Wang, *Gradient Tomography of Jet Quenching in Heavy-Ion Collisions*, **Phys. Rev. Lett. 125, 122301 (2020)**

Transverse momentum broadening and energy loss of a propagating parton are dictated by the space-time profile of the jet transport coefficient in a dense QCD medium. The spatial gradient of the jet transport coefficient in the direction perpendicular to the parton propagation can lead to a drift and asymmetry in parton transverse momentum distribution. Such an asymmetry depends on both the spatial position along the transverse gradient and path length of a propagating parton as shown by numerical solutions of the Boltzmann transport in the simplified form of a drift-diffusion equation. In high-energy heavy-ion collisions, this asymmetry with respect to a plane defined by the beam and trigger particle (photon, hadron, or jet) with a given orientation relative to the event plane is shown to be closely related to the transverse position of the initial jet production in full event-by-event simulations within the linear Boltzmann transport model. Such a gradient tomography can be used to localize the initial jet production position for more detailed study of jet quenching and properties of the quark-gluon plasma.

*Left: *Illustration of the transverse geometry and a photon-triggered parton propagation in heavy-ion collisions.

### Two- and Three-Pion Finite-Volume Spectra at Maximal Isospin from Lattice QCD

Ben Hörz and Andrew Hanlon, **Phys. Rev. Lett. 123, 142002 (2019)**** **

The rich multiparticle dynamics of QCD, which includes resonances and bound states, leaves an imprint on the spectrum of the theory confined to a finite volume. Turning this relationship around, scattering amplitudes can be constrained through precise measurements of the small energy shifts due to the interaction, which are accessible in first-principle lattice QCD simulations. For the first time we have computed the spectrum of three positively charged pions in several symmetry sectors of the finite volume, enabling a peek at three-particle dynamics directly from QCD.

*Figure: *Three-pion spectrum in various symmetry sectors of the finite volume. The energy shifts between the measured values (white markers) and the noninteracting energies (dashed lines) encode the interaction between two and three pions.

### Microcanonical Particlization with Local Conservation Laws

Dmytro Oliinychenko and Volker Koch, **Phys. Rev. Lett. 123, 182302 (2019)**** **

For the first time we are able to construct a sampling method for the transition from relativistic hydrodynamics to particle transport, which, in every sample, preserves the local conservation of energy, momentum, baryon number, strangeness, and electric charge microcanonically. The proposed method is essential for studying fluctuations and correlations by means of stochastic hydrodynamics. It is also useful for studying small systems. The method is based on Metropolis sampling applied to particles within distinct patches of the switching space-time surface, where hydrodynamic and kinetic evolutions are matched.

*Figure: *Demonstration of the sampling with conservation laws over the patch, where total baryon number, strangeness, and charge are enforced to be 0, while total energy and momentum are fixed. The patch consists of 3 cells with arbitrarily selected normals dσ_{μ1}=(500.0,50.0,20.0,30.0), dσ_{μ2}=(500.0,40.0,80.0,30.0), and dσ_{μ3}=(500.0,20.0,20.0,20.0) fm^{3}; collective velocities v_{1}=(0.2,0.3,0.4), v_{2}=(0.1,0.5,0.5), v_{3}=(0.3,0.4,0.2); and temperatures T_{1}=0.155, T_{2}=0.165, and T_{3}=0.175 GeV. Mean multiplicities of selected hadrons in the cells are shown in panel (a): they are unchanged compared to standard grand-canonical Cooper-Frye sampling. However, the scaled variances of multiplicities in the whole patch, shown in panel (b), differ from the standard Cooper-Frye result and coincide within 0.5% with the microcanonical expectation in the thermodynamic limit, computed using analytic formulas from EPJC58, 83. In panel (c), the nontrivial correlations, generated by conservation laws, are shown in contrast to no correlations in the standard Cooper-Frye sampling. Correlations are defined as (A,B)≡⟨(A−⟨A⟩)(B−⟨B⟩)⟩, where ⟨⟩ denotes the average over samples; σ_{2A}≡(A,A).

### Quantum annealing for polynomial systems of equations

Chia Cheng Chang, Arjun Gambhir, Travis S. Humble, Shigetoshi Sota, *Quantum annealing for polynomial systems of equations,* **Nature ***Scientific Reports*** 9, 10258 (2019)**** **

The advent of Noisy Intermediate-Scale Quantum (NISQ) quantum computers has galvanized efforts towards discovering near-term applications. An algorithm for solving polynomial systems of equations was proposed and a linear solver on a D-Wave quantum annealer was implemented. While the problems are currently limited to sizes that are easily solved by classical computers, the team showed that the quantum algorithm exhibits constant scaling with increasing condition number, in direct contrast with classical methods. Additionally, the quantum algorithm may also be applied iteratively to exponentially decrease the relative residual, allowing for the classical solution to be reproduced by the quantum computer to single precision. However, the scaling with problem size is unfortunately exponentially bad, reflecting limitations of current quantum computers. Fortunately, there is a great amount of interest and effort put fourth by the greater quantum annealing community geared towards tackling this problem, including using inhomogeneous driving fields, reverse annealing, and even hardware developments towards universal quantum annealers.

*Figure: *Example QUBO for a system of second-order polynomial equations. The QUBO can be organized into four quadrants as indicated by the solid black lines, corresponding to bilinear (2nd quadrant), tri-linear (1st and 3rd quadrants) and quadra-linear (4th quadrant) contributions. Within the quadrants, the elements are colored to reflect the effective one (red), two (green), three (blue), and four (yellow) qubit interactions after accounting for repeated indices. The entries in 𝒬𝑠𝑝𝑎𝑟𝑠𝑒Qsparse are distributed to entries in the QUBO corresponding to the order of interaction: *Q*(1) to red, *Q*(2) to green, *Q*(3) to blue, *Q*(4) to yellow. The coefficients of the constraint equations contributes to entries in red text.

Further readings: LBNL News Release, Phys.Org Ariticle

### Bayesian Analysis of Jet Energy Loss in Heavy-Ion Collisions

Yayun He, Long-Gang Pang, and Xin-Nian Wang, *Bayesian Extraction of Jet Energy Loss Distributions in Heavy-Ion Collisions,*** ****Phys. Rev. Lett. 122, 252302****(2019)**

High energy heavy ion collisions produce jets of energetic partons that traverse the strongly coupled quark-gluon plasma (sQGP). The interactions between jet-shower and medium partons lead to suppression of jets and large transverse momentum hadrons, in heavy-ion collisions as compared to proton-proton collisions. The suppression known as jet quenching can be used to probe multiple properties of the QGP, such as the temperature, the gluon density and jet transport coefficient.

In this study, the researchers formulate jet cross section in heavy-ion collisions as the convolution of jet cross section in proton-proton collisions and the in-medium jet energy loss distribution. The trial jet energy loss distribution has three parameters whose functional space covers exponential distribution, Gamma distribution and gaussian distribution. The parameters of this statistical model for jet quenching is estimated through Bayesian analysis of experimental data from the Large Hadron Collider (LHC) with Markov Chain Monte Carlo (MCMC) method.

Results from Linear Boltzmann Transport (LBT) model simulations are consistent with the data-driven extraction and indicate that the observed jet quenching prefers a small number of out-of-cone scatterings. Reducing experimental uncertainties in finer transverse momentum bins should improve the precision of the Bayesian extraction. Such systematic extraction of jet energy loss distributions can help to shed light on jet-medium interaction and constrain jet transport coefficients in high-energy heavy-ion collisions.

*Figure:* Bayesian fits to R_{AA} for single inclusive jets from ATLAS collaboration at LHC, (middle) the extracted average jet energy loss ⟨Δp_{T}⟩as a function of the initial jet energy and (bottom) energy loss distributions W_{AA}(x=Δp_{T}/⟨Δp_{T}⟩) in Pb+Pb collisions at two LHC energies with different centralities. Blue lines with solid circles are mean averages from MCMC Bayesian fits and light blue lines are results with one sigma deviation from the average fits of R_{AA}. Red lines are from LBT simulations.

### Delineating properties of the Quark Gluon Plasma using jets with a minimized model bias

Jian-Wei Qiu, Felix Ringer, Nobuo Sato, and Pia Zurita, *Factorization of Jet Cross Sections in Heavy-Ion Collisions, ***Phys. Rev. Lett. 122, 252301 (2019).**

Nowadays powerful accelerators collide heavy-ions at high energies in order to recreate the Quark Gluon Plasma (QGP) which is a hot and dense state of matter that is believed to have filled our universe shortly after the Big Bang. Highly energetic jets which are produced in the same collisions are often used as hard probes of the produced QCD medium. Measuring the properties of the QGP such as the temperature or transport coefficients relies on whether it is possible to reliably separate the production of the probe and the formation of the medium. This concept is known as QCD factorization. Starting from an established factorization formalism in proton-proton collisions, we introduce medium modified jet functions to capture the interaction of jets with the QGP. Within a global analysis using a Monte Carlo sampling technique we find that it is indeed possible to describe the data obtained at the LHC. Our results thus support the validity of QCD factorization in the complex heavy-ion environment, and open up a new door to analyze heavy-ion jet data. In addition, our results may serve as guidance for constructing microscopic models of the QGP.

*Upper Figure: * Comparison of our results and LHC data of the nuclear modification factor.

*Lower Figure: *Ratio of the extracted medium jet functions relative to the vacuum for quarks and gluons.

### New applications of jet physics at the Electron-Ion Collider

Xiaohui Liu, Felix Ringer, Werner Vogelsang, and Feng Yuan, *Lepton-Jet Correlations in Deep Inelastic Scattering at the Electron-Ion Collider, ***Phys. Rev. Lett. 122, 192003 (2019)**

The high energy and high luminosity Electron-Ion Collider (EIC) is regarded as the next generation QCD machine which is aimed at exploring the structure of nucleons and nuclei. In this work we proposed the measurement of jets (collimated sprays of particles) at the EIC which constitute unique tools to study various interesting physics aspects. We focused specifically on electron-jet correlations studies in deep inelastic scattering processes. Jets can be used to explore the (transverse) spin structure of the proton and they constitute sensitive probes of cold nuclear matter effects such as transverse momentum broadening. In general, we expect that jet physics will set a new direction of the EIC science program. Our studies are presented in a timely manner such that they can be taken into account in the planning phase of the EIC.

*Upper Figure: *The schematic kinematics of lepton-jet correlation at the electron-ion collider.

*Lower Figure:* P_{T}-broadening effects for the lepton jet azimuthal correlation due to the interaction with cold nuclear matter as a function of Δϕ=|ϕ_{J}−ϕ_{ℓ}−π| for two typical values of ^qL.

### Snowballs in Hell: Explaining Light Ion Production in High-Energy Collisions

Dmytro Oliinychenko, Long-Gang Pang, Hannah Elfner, and Volker Koch, *Microscopic study of deuteron production in PbPb collisions at √s=2.76TeV via hydrodynamics and a hadronic afterburner*, **Phys. Rev. C 99, 044907 (2019)**

The deuteron yield in Pb+Pb collisions at √s_{NN}=2.76 TeV is consistent with thermal production at a freeze out temperature of T=155 MeV. The existence of deuterons with binding energy of 2.2 MeV at this temperature was described as “snowballs in hell” [P. Braun-Münzinger, B. Dönigus, and N. Löher, CERN Courier, August 2015]. We provide a microscopic explanation of this phenomenon, utilizing relativistic hydrodynamics and switching to a hadronic afterburner at the above-mentioned temperature of T=155 MeV. The measured deuteron p_{T} spectra and coalescence parameter B_{2}(p_{T}) are reproduced without free parameters, only by implementing experimentally known cross sections of deuteron reactions with hadrons, most importantly πd↔πnp.

*Figure: *deuteron yield (both in hydrodynamics and afterburner) versus time for the scenarios, described in the text. Bottom: relative amount of energy in the afterburner. This is to indicate, how much of the system is already treated by the afterburner.

Further readings: APS Physics, DOE NP Highlights

### Femtoscopy of stopped protons

Andrzej Bialas, Adam Bzdak, and Volker Koch, *Femtoscopy of stopped protonsicro*, **Phys. Rev. C 99, 034906**** (2019).**

The longitudinal proton-proton femtoscopy (Hanbury Brown–Twiss) correlation function, based on the idea that in a heavy-ion collision at √s≲20GeV stopped protons are likely to be separated in configuration space, is evaluated. It shows a characteristic oscillation which appears sufficiently pronounced to be accessible in experiment. The proposed measurement is essential for estimating the baryon density in the central rapidity region and can be also viewed as an (almost) direct verification of the Lorentz contraction of the fast-moving nucleus.

*Figure: *Time integrated source function for stopped protons as a function of z for (a) √s=20GeVand (b) √s=14GeV. The black dashed lines represent the result of our model calculation while the blue solid lines are obtained by doubling the value of width of the collision point distribution, Γ_{c}. The source functions shown are normalized to unity.

### Probing the Electromagnetic Property of Quark Matter

Spencer Klein, A.H. Mueller, Bo-Wen Xiao, Feng Yuan, *Acoplanarity of Lepton Pair to Probe the Electromagnetic Property of Quark Matter*, **Phys. Rev. Lett. 122, 132301 (2019)**

We investigate the PT-broadening effects in dilepton production through photon-photon scattering in heavy ion collisions. The QED multiple interaction effects with the medium is found to be consistent with a recent observation of low transverse momentum lepton pair from ATLAS collaboration at the LHC. We further comment on the magnetic effects and point out a number of ways to disentangle these two mechanisms. In particular, the rapidity dependence of the PT-broadening effects provide a unique probe to the magnetic effects.

Jet quenching is considered one of the major discoveries in relativistic heavy ion experiments from RHIC at Brookhaven National Laboratory and the LHC at CERN. These phenomena have been well formulated in QCD, where the energy loss and PT -broadening effects are closely related. The parameter qhat has been extracted from various experimental data. Meanwhile, qhat*L describes the typical transverse momentum squared that a parton acquires in the medium of length L. In the last few years, there have been significant progress in understanding the PT -broadening effects in dijet, photon-jet, and hadron-jet productions in heavy ion collisions.

We extend our previous studies on the dijet azimuthal correlation (Phys.Lett. B763 (2016) 208-212) to the di-lepton correlation and focus on two main areas. One is the QED Sudakov effect, where we show that the theory prediction for the UPC events agree very well with data from ATLAS. Second, we investigate the medium effects, including the QED multiple interaction effects similar to the PT -broadening of the QCD jet and the magnetic effects. We also discuss how to disentangle these two mechanisms.

The comparison of the PT -broadening effects in QCD and QED is of crucial importance to understand the medium property in heavy ion collisions. The lepton’s PT -broadening effects is sensitive to the electromagnetic property of the quark-gluon plasma, whereas the jet PT -broadening effects depends on the strong interaction property. The experimental and theoretical investigations of both phenomena will deepen our understanding of the hot medium created in these collisions. The clear measurements of lepton PT -broadening effects from ATLAS and STAR should stimulate further study on dijet azimuthal correlations in heavy ion collisions.

*Upper Figure: *Acoplanarity distribution for lepton pair production at mid-rapidity in UPC events at the LHC with a typical kinematics: lepton transverse momentum P⊥ > 4 GeV and pair invariant mass from 10 to 100 GeV. The detailed explanation of different curves is provided in the main text. The total contribution with resummation (solid curve) agrees well with the ATLAS measureme*nt.*

*Lower Figure:* Medium modifications to the acoplanarity distribution, with different values of the effective qL.

### How to Escape a Black Hole: Simulations Provide New Clues to What’s Driving Powerful Plasma Jets

Kyle Parfrey (Einstein Fellow), Alexander Philippov, Benoît Cerutti (IPAG), *First-Principles Plasma Simulations of Black-Hole Jet Launching*, **Phys.Rev.Lett. 122 (2019) no.3, 035101**

Black holes are known for their voracious appetites, binging on matter with such ferocity that not even light can escape once it’s swallowed up. Less understood, though, is how black holes purge energy locked up in their rotation, jetting near-light-speed plasmas into space to opposite sides in one of the most powerful displays in the universe. These jets can extend outward for millions of light years.

New simulations combine decades-old theories to provide new insight about the driving mechanisms in the plasma jets that allow them to steal energy from black holes’ powerful gravitational fields and propel it far from their gaping mouths. The simulations could provide a useful comparison for high-resolution observations from the Event Horizon Telescope, an array that is designed to provide the first direct images of the regions where the plasma jets form.

The simulations, for the first time, unite a theory that explains how electric currents around a black hole twist magnetic fields into forming jets, with a separate theory explaining how particles crossing through a black hole’s point of no return – the event horizon – can appear to a distant observer to carry in negative energy and lower the black hole’s overall rotational energy. It’s like eating a snack that causes you to lose calories rather than gaining them. The black hole actually loses mass as a result of slurping in these “negative-energy” particles. Computer simulations have difficulty in modeling all of the complex physics involved in plasma-jet launching, which must account for the creation of pairs of electrons and positrons, the acceleration mechanism for particles, and the emission of light in the jets. These simulations incorporate new numerical techniques that provide the first model of a collisionless plasma – in which collisions between charged particles do not play a major role – in the presence of a strong gravitational field associated with a black hole. The simulations naturally produce effects known as the Blandford-Znajek mechanism, which describes the twisting magnetic fields that form jets, and a separate Penrose process that describes what happens when negative-energy particles are gulped down by the black hole.

*Figure: *This visualization of a general-relativistic collisionless plasma simulation shows the density of positrons near the event horizon of a rotating black hole. Plasma instabilities produce island-like structures in the region of intense electric current.

Further readings: LBNL News, Viewpoint: Feeding a Black Hole Jet (APS Physics, Jan. 23, 2019)

### When Stars Collide: 3D Computer Simulation Captures Cosmic Event

Rodrigo Fernández, Alexander Tchekhovskoy, Eliot Quataert, Francois Foucart, Daniel Kasen, *Long-term GRMHD Simulations of Neutron Star Merger Accretion Disks: Implications for Electromagnetic Counterparts, * **Mon.Not.Roy.Astron.Soc. 482 (2019) 3373–3393**

The aftermath of the collision of two neutron stars has been fully captured in a 3D computer model for the first time. The achievement has led to a better understanding of the cosmic collision, showing how heavy elements like lead and gold are created and accounting for a phenomenon missing in other models. Neutron stars are the smallest and densest stars, mostly made of elementary particles called neutrons. In August 2017, scientists detected the collision of two neutron stars for the first time by using the **Laser Interferometer Gravitational-Wave Observatory**. When two of these stars collide, they merge in a flash of light and debris known as a kilonova, as material explodes outward. One of the important elements of studying the collision is the accretion disk—a collection of leftover debris that orbits the combined hyper-massive star—a cosmic footprint of the collision event. The material launched by the accretion disk should match up with the amount of matter that plays a part in the kilonova, helping scientists better understand the event. By modeling the aftermath of the collision in such detail, they have been able to account for both the accretion disk and another way matter is ejected from the collision: carried by an astrophysical jet, a narrow plume of particles and radiation shot out at nearly the speed of light as the stars collide, which is also thought to be the source of the gamma ray burst.

*Left Figure: *A cross section of the model of colliding neutron stars shows the dense regions of the accretion disk in red around the black hole at the very center. The astrophysical jet is the funnel above and below the black hole, a less dense region of blue, which gave rise to a gamma-ray burst. (Credit: Rodrigo Fernández)

### Lattice QCD is used to determine prospective short distance contributions to Neutrinoless Double Beta Decay

A. Nicholson et al., *Heavy Physics Contributions to Neutrinoless Double Beta Decay from QCD,* **Phys. Rev. Lett. 121 (2018) no.17, 172501**

The observation of neutrinoless double beta decay would be a definitive sign of new physics and would illuminate a mechanism for generating the observed excess of matter over anti-matter in the universe. Searches for this very rare decay place some of the strongest constraints on the search for new physics, and are therefore some of the most fascinating (and highest priority) experiments. In order to improve these constraints, and interpret a potential signal, we must understand how this new physics manifests inside large nuclei. To do this we must be able to make predictions regarding the strength of the prospective interactions directly from the Standard Model of particle physics.

With a computing allocation at LLNL and another through the DOE INCITE Program at ORNL, we have performed the first calculation of the dominant contribution to neutrinoless double beta decay arising from prospective short distance contributions, improving our understanding of this mechanism. As the theories used to describe nuclei improve, our result can be used to compute the full nuclear decay rate with our calculation as an input.

The calculations performed here are the first in an increasingly complex series of calculations that will provide a complete understanding of how prospective sources of new physics that drive neutrinoless double beta decay will manifest in nuclei. The foundational level calculations, like the one summarized here, utilize lattice QCD to compute the processes directly in terms of the quarks and gluons which bind into nucleons. These lattice QCD calculations are sufficiently challenging that they may only be performed in systems of two or perhaps up to four nucleons. The results will be matched onto theoretical calculations performed with neutrons and protons that can then be coupled onto calculations of nuclei. In this way, we will build a quantitative understanding of prospective neutrinoless double beta decay mechanisms rooted in the Standard Model.

### Calculating the axial coupling g_{A} with lattice QCD

_{A}with lattice QCD

C.C. Chang, *et al.,* *A percent-level determination of the nucleon axial coupling from Quantum Chromodynamics*, **Nature 558 (2018) 91-94 **

The neutron lifetime has profound consequences on the composition of the universe. This lifetime can be predicted from the Standard Model, and is dependent on the nucleon axial coupling g_{A}. Determining this fundamental parameter is very challenging as the underlying theory of quantum chromodynamics (QCD) is non-perturbative, precluding the use of known analytic methods. A team led LBNL staffers has recently performed the first ever percent-level determination of g_{A} using lattice QCD, a rigorous numerical implementation of QCD. Critical to their success was the use of an innovative method to perform the calculations, as well as access to powerful supercomputers. The results are statistics limited, signifying a straightforward path to improving the precision to a level commensurate with the experimental precision. This may eventually lead to the resolution of the discrepancies in different neutron lifetime measurements.

*Left Figure:* A comparison of previous lattice QCD determinations of g_{A }with the present work and the experimental determination, from the Particle Data Group, 2018.

### Machine Learning of EoS in Heavy-ion Collisions

LG Pang, K. Zhou, N. Su, H Petersen, H. Stocker &X.-N. Wang, *An equation-of-state-meter of quantum chromodynamics transition from deep learning*, **Nature Communications 9, 210 (2018)**

The researchers programmed powerful arrays known as neural networks to serve as a sort of hivelike digital brain in analyzing and interpreting the images of the simulated particle debris left over from the collisions. During this test run the researchers found that the neural networks had up to a 95 percent success rate in recognizing important features in a sampling of about 18,000 images.

The next step will be to apply the same machine learning process to actual experimental data. Powerful machine learning algorithms allow these networks to improve in their analysis as they process more images. The underlying technology is used in facial recognition and other types of image-based object recognition applications. The images used in this study – relevant to particle-collider nuclear physics experiments at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider and CERN’s Large Hadron Collider – recreate the conditions of a subatomic particle “soup,” which is a superhot fluid state known as the quark-gluon plasma believed to exist just millionths of a second after the birth of the universe.

*Left Figure: *The convolution neural network architecture. The architecture is designed to identify the quantum chromodynamics transition by using particle spectra with 15 transverse momentum *p*T bins and 48 azimuthal angle *ϕ* bins

**Top 50 Nature Communications physics articles published in 2018**

### Confirmed: Neutron star mergers create heavy elements

D. Kasen *et al.,* *Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event,* **Nature 551, 80-84 (2017)**

GW170817 was the first gravitational wave detection involving the merger of two neutron stars to form a black hole. Observations revealed an optical signal that faded after a few days, along with an infrared signal that persisted for nearly two weeks. These signals are consistent with computer model predictions for a kilonova which produces significant quantities of heavy elements via the r-process (rapid neutron capture). The shorter-lived, spectrally featureless optical emission is compatible with an initial ejecta component composed of lighter elements, while the long-lived infrared signal is from a secondary component which is powered by the radioactive decay of heavy elements which heat the plasma. Heavy elements with 58 < Z < 90 scatter the light strongly, leading to a long-lived emission.

*Left Figure*: Theoretical calculation of the evolution of the spectrum of light from a kilonova such as that associated with GW170817. The right panel shows an illustration of the expanding radioactive debris cloud ejected from a neutron star merger that gives rise to the light.