Nuclear Theory·nucl-th

Tuesday·December 7, 2021

18 papers—5 primary·13 cross-listed

06
Directed flow of charged particles within idealized viscous hydrodynamics at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider
Ze-Fang Jiang🇨🇳 · C. B. Yang🇨🇳 · Qi Peng🇨🇳
Following the Boz $e ˙$ k-Wyskiel parametrization tilted initial condition, an alternative way to construct a longitudinal tilted fireball based on the Glauber collision geometry is presented. This longitudinal tilted initial condition combined with the Ideal-CLVisc (3 + 1)D hydrodynamic model, a nonvanishing directed flow coefficient $v_{1}$ in a wide rapidity range is observed. After comparing the model's results with experimentally observed data of directed flow coefficient $v_{1} (η)$ from $s_{N N}$ = 200 GeV Cu + Cu, Au + Au collisions at RHIC energy to $s_{N N}$ = 2.76 TeV and $s_{N N}$ = 5.02 TeV Pb + Pb collisions at the LHC energy. We find that the directed flow measurements in heavy-ion collisions can set strong constraints on the imbalance of forward and backward incoming nuclei and on the magnitude asymmetry of pressure gradients along the $x$ direction.
↳ hep-phnucl-exnucl-thPRC(2021)·20 citations
07
Longitudinal distribution of initial energy density and directed flow of charged particles in relativistic heavy-ion collisions
Ze-Fang Jiang🇨🇳 · Shanshan Cao🇨🇳 · Xiang-Yu Wu🇨🇳 · C. B. Yang🇨🇳 · Ben-Wei Zhang🇨🇳
We study the origin of the directed flow of charged particles produced in relativistic heavy-ion collisions. Three different initial conditions, Boz $e ˙$ k-Wyskiel, CCNU and Shen-Alzhrani, of energy density distributions are coupled to the (3+1)-dimensional viscous hydrodynamic model CLVisc, and their effects on the development of the anisotropic medium geometry, pressure gradient and radial flow are systematically compared. By comparing to experimental data at both RHIC and LHC, we find that the directed flow provides a unique constraint on the tilt of the initial medium profile in the plane spanned by the impact parameter and space-time rapidity. Within mid-rapidity, the counter-clockwise tilt is shown to be a crucial source of the positive/negative force by the pressure gradient along the impact parameter ( $x$ ) direction at backward/forward rapidity, which drives a negative slope of the $x$ component of the medium flow velocity with respect to rapidity, and in the end the same feature of the charged particle directed flow.
↳ hep-phhep-thnucl-exnucl-thPRC(2022)·28 citations
08
Probing Invisible Vector Meson Decays with NA64 and LDMX
Philip Schuster🇺🇸 · Natalia Toro🇺🇸 · Kevin Zhou🇺🇸
Electron beam fixed target experiments such as NA64 and LDMX use missing energy-momentum to detect the production of dark matter and other long-lived states. The most studied production mechanism is dark Bremsstrahlung through a vector mediator. In this work, we explore a complementary source of missing energy-momentum signals: Bremsstrahlung photons can convert to hard vector mesons in exclusive photoproduction processes, which then decay to dark matter or other invisible particles, such as neutrinos. We find that existing NA64 data can improve the leading constraints on invisible light vector meson decays, while a future run of LDMX could improve them by up to $5$ orders of magnitude. For the examples of a dark photon and a $U (1)_{B}$ gauge boson mediator, accounting for meson decays substantially enhances these experiments' sensitivity, especially to thermal relic dark matter of mass $m_{χ} ≳ 0.1 GeV$ .
↳ hep-phhep-exnucl-thPRD(2022)·50 citations
09
Reconstruction of bottomonium spectral functions in thermal QCD using Kernel Ridge Regression
Sam Offler🇬🇧 · Gert Aarts🇬🇧 · Chris Allton🇬🇧 · Benjamin Jäger🇩🇰 · Seyong Kim🇰🇷 · Maria-Paola Lombardo🇮🇹 · Benjamin Page🇬🇧 · Sinead M. Ryan🇮🇪 · Jon-Ivar Skullerud🇩🇰 · Thomas Spriggs🇬🇧
We discuss results for bottomonium at nonzero temperature obtained using NRQCD on Fastsum Generation 2L ensembles, as part of the Fastsum collaboration's programme to determine the spectrum of the bottomonium system as a function of temperature using a variety of approaches. Here we give an update on results for spectral functions obtained using Kernel Ridge Regression. We pay in particular attention to the generation of training data and introduce the notion of using lattice QCD ensembles to learn how to improve the generation of training data. A practical implementation is given.
↳ hep-latnucl-thPoS(2022)·12 citations
10
New approach to lattice QCD at finite density: reweighting without an overlap problem
Attila Pasztor🇩🇪 · Szabolcs Borsanyi🇭🇺 · Zoltan Fodor🇭🇺 · Kornel Kapas🇭🇺 · Sandor D. Katz🇭🇺 · Matteo Giordano🇭🇺 · Daniel Nogradi🇭🇺 · Chik Him Wong🇩🇪
Approaches to finite baryon density lattice QCD usually suffer from uncontrolled systematic uncertainties in addition to the well-known sign problem. We test a method - sign reweighting - that works directly at finite chemical potential and is yet free from any such uncontrolled systematics: with this approach the only problem is the sign problem itself. In practice the approach involves the generation of configurations with the positive fermionic weights given by the absolute value of the real part of the quark determinant, and a reweighting by a sign. There are only two sectors, +1 and -1 and as long as the average $⟨ \pm ⟩ \neq = 0$ (with respect to the positive weight) this discrete reweighting has no overlap problem - unlike reweighting from $μ = 0$ - and the results are reliable. We also present results based on this algorithm on the phase diagram of lattice QCD with two different actions: as a first test, we apply the method to calculate the position of the critical endpoint with unimproved staggered fermions at $N_{τ} = 4$ ; as a second application, we study the phase diagram with 2stout improved staggered fermions at $N_{τ} = 6$ . This second one is already a reasonably fine lattice - relevant for phenomenology. We demonstrate that the method penetrates the region of the phase diagram where the Taylor and imaginary chemical potential methods lose predictive power.
↳ hep-lathep-phnucl-thPoS(2022)·4 citations
11
Lattice QCD Determination of the Bjorken- $x$ Dependence of Parton Distribution Functions at Next-to-next-to-leading Order
Xiang Gao🇨🇳 · Andrew D. Hanlon🇺🇸 · Swagato Mukherjee🇺🇸 · Peter Petreczky🇺🇸 · Philipp Scior🇺🇸 · Sergey Syritsyn🇺🇸 · Yong Zhao🇺🇸
We report the first lattice QCD calculation of pion valence quark distribution with next-to-next-to-leading order perturbative matching correction, which is done using two fine lattices with spacings $a = 0.04$ fm and $0.06$ fm and valence pion mass $m_{π} = 300$ MeV, at boost momentum as large as $2.42$ GeV. As a crucial step to control the systematics, we renormalize the pion valence quasi distribution in the recently proposed hybrid scheme, which features a Wilson-line mass subtraction at large distances in coordinate space, and develop a procedure to match it to the $MS$ scheme. We demonstrate that the renormalization and the perturbative matching in Bjorken- $x$ space yield a reliable determination of the valence quark distribution for $0.03 ≲ x ≲ 0.80$ with 5-20\% uncertainties.
↳ hep-lathep-exhep-phnucl-ex+1PRL(2022)·96 citations
12
Inhomogeneous Phases in the Chiral Gross-Neveu Model on the Lattice
Keita Horie🇯🇵 · Chiho Nonaka🇯🇵
We discuss possible existence of inhomogeneous phase in low temperature and high density region in the 1+1 dimensional chiral Gross-Neveu ( $χ$ GN $_{2}$ ) model on the lattice. First we investigate the phase structure of the $χ$ GN $_{2}$ model at vanishing chemical potential, changing temperature. From behavior of $Δ^{2} = Σ^{2} + Π^{2}$ as a function of Monte Carlo time, a candidate of an order parameter for chiral symmetry in the $χ$ GN $_{2}$ model is $Δ$ . At vanishing chemical potential, we observe restoration of chiral symmetry at high $T$ . In low temperature and high chemical potential region, we find existence of inhomogeneous phase in spatial correlation functions of $σ$ and $π$ . The signal of inhomogeneous phase in correlation function of $Δ$ is more clearly than that of $σ$ or $π$ .
↳ hep-latnucl-thPoS(2022)·12 citations
13
Lattice QCD noise reduction for bosonic correlators through blocking
Luis Altenkort🇩🇪 · Alexander M. Eller🇩🇪 · O. Kaczmarek🇩🇪 · Lukas Mazur🇩🇪 · Guy D. Moore🇩🇪 · H.-T. Shu🇩🇪
We propose a method to substantially improve the signal-to-noise ratio of lattice correlation functions for bosonic operators or other operator combinations with disconnected contributions. The technique is applicable for correlations between operators on two planes (zero momentum correlators) when the dimension of the plane is larger than the separation between the two planes which are correlated. In this case, the correlation arises primarily from points whose in-plane coordinates are close, but noise arises from all pairs of points. By breaking each plane into bins and computing bin-bin correlations, it is possible to capture these short-distance correlators exactly while replacing (small) correlators at large spatial extent with a fit, with smaller uncertainty than the data. The cost is only marginally larger than averaging each plane before correlating, but the improvement in signal-to-noise can be substantial. We test the method on correlators of the gradient-flowed topological charge density and squared field strength, finding noise reductions by a factor of $\sim$ 3 $-$ 7 compared to the conventional approach on the same ensemble of configurations.
↳ hep-latcond-mat.stat-mechnucl-thphysics.comp-phPRD(2022)·7 citations
14
Laser-coolable AcOH $^{+}$ ion for $C P$ -violation searches
Alexander V. Oleynichenko🇷🇺 · Leonid V. Skripnikov🇷🇺 · Andréi V. Zaitsevskii🇷🇺 · Victor V. Flambaum🇦🇺
The AcOH $^{+}$ molecular ion is identified as a prospective system to search for $C P$ -violation effects. According to our study AcOH $^{+}$ belongs to the class of laser-coolable polyatomic molecular cations implying the large coherence time in the experiments to study symmetry violating effects of fundamental interactions. We perform both nuclear and high level relativistic coupled cluster electronic structure calculations to express experimentally measurable $T$ , $P$ -violating energy shift in terms of fundamental quantities such as the nuclear magnetic quadrupole moment (MQM), electron electric dipole moment ( $e$ EDM) and dimensionless scalar-pseudoscalar nuclear-electron interaction constant. We further express nuclear MQM in terms of the strength constants of $C P$ -violating nuclear forces: quantum chromodynamics vacuum angle $θ ˉ$ and quark chromo-EDMs. The equilibrium geometry of AcOH $^{+}$ in the ground and the four lowest excited electronic states was found to be linear. The calculated Franck-Condon factors and transition dipole moments indicate that the laser cooling using optical cycle involving the first excited state is possible for the trapped AcOH $^{+}$ ions with the Doppler limit estimated to be~ $\sim 4$ ~nK. The lifetime of the (0,1 $^{1}$ ,0) excited vibrational state considered as a working one for MQM and $e$ EDM search experiments is estimated to be $\sim 0.4$ sec.
↳ physics.atom-phhep-phnucl-thPRA(2022)·13 citations
15
Production of $Ω N N$ and $ΩΩ N$ in ultra-relativistic heavy ion collisions
Liang Zhang🇨🇳 · Song Zhang🇨🇳 · Yu-Gang Ma🇨🇳
Even though lots of $Λ$ -hypernuclei have been found and measured, multi-strangeness hypernuclei consisting of $Ω$ are not yet discovered. The studies of multi-strangeness hypernuclei help us further understand the interaction between hyperons and nucleons. Recently the $Ω N$ and $ΩΩ$ interactions as well as binding energies were calculated by the HAL-QCD's lattice Quantum Chromo-Dynamics (LQCD) simulations and production rates of $Ω$ -dibaryon in Au + Au collisions at RHIC and Pb + Pb collisions at LHC energies were estimated by a coalescence model. The present work discusses the production of more exotic triple-baryons including $Ω$ , namely $Ω N N$ and $ΩΩ N$ as well as their decay channels. A variation method is used in calculations of bound states and binding energy of $Ω N N$ and $ΩΩ N$ with the potentials from the HAL-QCD's results. The productions of $Ω N N$ and $ΩΩ N$ are predicted by using a blast-wave model plus coalescence model in ultra-relativistic heavy-ion collisions at $s_{N N} = 200$ GeV and $2.76$ TeV. Furthermore, plots for baryon number dependent yields of different baryons ( $N$ and $Ω$ ), their dibaryons and hypernuclei are made and the production rate of a more exotic tetra-baryon ( $ΩΩ N N$ ) is extrapolated.
↳ hep-phnucl-exnucl-thEPJC(2022)·25 citations
16
Leptophobic dark photon interpretation of the $η^{(')} \to π^{0} (η) γ γ$ puzzle
Yaroslav Balytskyi🇺🇸
The decays of $η$ and $η^{'}$ mesons provide unique opportunities for testing the properties of low energy Quantum Chromodynamics and for the search of new physics beyond the Standard Model. However, recent experimental results on the rare decays of $η^{(')} \to π^{0} (η) γ γ$ cannot be self-consistently described by the combination of the Vector Meson Dominance and Linear Sigma Model employing the same set of parameters. We show that this tension can be attributed to the presence of a leptophobic dark photon $B$ , and find representative values of the parameters which provide consistent description of these three decays, simultaneously. Unlike existing strategies of Dalitz analysis searching for the bump at $m_{B}$ , we propose the usage of mismatch between these decays to constrain the parameters of the hypothetical dark photon.
↳ hep-phnucl-thPLB(2023)·7 citations
17
Effect of thermal shear on longitudinal spin polarization in a thermal model
Wojciech Florkowski🇵🇱 · Avdhesh Kumar🇮🇳 · Aleksas Mazeliauskas🇨🇭 · Radoslaw Ryblewski🇵🇱
By including the recently introduced thermal shear term that contributes to the spin polarization vector at local equilibrium, we determine longitudinal polarization of $Λ$ hyperons emitted from a hot and rotating hadronic medium using the thermal model with single freeze-out. In our analysis, we consider the RHIC top energies and use the model parameters which were determined in the earlier analyses of particle spectra and elliptic flow. We confirm that, unlike the previous calculations done by using only the thermal vorticity, the thermal shear term alone leads to the correct sign of the quadrupole structure of the longitudinal component of the polarization three-vector measured in experiments. However, we find almost complete cancellation between thermal shear and vorticity terms, which eventually leads to disagreement with the data. To clarify the role played by velocity and temperature gradient terms, we present a systematic analysis of different contributions to the longitudinal polarization.
↳ hep-phnucl-thPRC(2022)·61 citations
18
First Glimpse into the Kaon Gluon Parton Distribution Using Lattice QCD
Alejandro Salas-Chavira🇺🇸 · Zhouyou Fan🇺🇸 · Huey-Wen Lin🇺🇸
In this work, we present the first results on the gluon parton distribution of the kaon from lattice quantum chromodynamics. We carry out the lattice calculation at pion mass around 310~MeV and two lattice spacings, 0.15 and 0.12~fm, using $2 + 1 + 1$ -flavor HISQ ensembles generated by MILC Collaboration. The kaon correlators are calculated using clover fermions and momentum-smearing sources with maximum boost momentum around 2~GeV and high statistics (up to 324,000 measurements). We study the dependence of the resulting reduced Ioffe-time pseudo-distributions at multiple boost momenta and lattice spacings. We then extract the kaon gluon distribution function in the $MS$ scheme at $μ = 2$ ~GeV, neglecting the mixing between the gluon and singlet-quark sectors. Our results at the smaller lattice spacing are consistent with phenomenological determinations.
↳ hep-lathep-phnucl-thPRD(2022)·49 citations

Affiliations

first authorsco-authorsvia INSPIRE