Nuclear Theory·nucl-th

Monday·January 1, 2018

13 papers—8 primary·5 cross-listed

01
Nuclear modification of leading hadrons and jets within a virtuality ordered parton shower
Shanshan Cao🇺🇸 · Abhijit Majumder🇺🇸
The event generator based on the higher-twist energy loss formalism -- Modular All Twist Transverse-scattering Elastic-drag and Radiation (MATTER) -- is further developed and coupled to a hydrodynamic model for studying jet modification in relativistic nuclear collisions. The probability of parton splitting is calculated using the Sudakov form factor that is constructed by a combination of vacuum and medium-induced splitting functions; and the full parton showers are simulated, including both energy-momentum and space-time evolutions of all jet partons. With the assumption that partons below a virtual scale of 1 GeV is absorbed by the medium, this framework is able to provide a reasonable description of the nuclear modification of both leading hadrons and jets at high transverse momentum at the BNL Relativistic Heavy Ion Collider and the CERN Large Hadron Collider.
nucl-thhep-phnucl-exPRC(2020)·84 citations
02
Relativistic chiral description of the $^{1} S_{0}$ nucleon-nucleon scattering
Xiu-Lei Ren🇨🇳 · Chun-Xuan Wang🇨🇳 · Kai-Wen Li🇨🇳 · Li-Sheng Geng🇨🇳 · Jie Meng🇨🇳
Recently, a relativistic chiral nucleon-nucleon interaction is formulated up to leading order which provides a good description of the phase shifts of $J \leq 1$ partial waves [Chin. Phys. C 42 (2018) 014103]. Nevertheless, a separable regulator function that is not manifestily covariant was used in solving the relativistic scattering equation. In the present work, we first propose a covariant and separable form factor to regularize the kernel potential and then apply it to study the simplest but most challenging $^{1} S_{0}$ channel which features several low-energy scales. In addition to being self-consistent, we show that the resulting relativistic potential can describe quite well the unique features of the $^{1} S_{0}$ channel at leading order, in particular the pole position of the virtual bound state and the zero amplitude at the scattering momentum $\sim 340$ MeV, indicating that the relativistic formulation might be more natural from the point of view of effective field theories.
nucl-thhep-phChin.Phys.Lett.(2021)·28 citations
03
Reexamination of microscopic optical potentials based on multiple scattering theory
Kosho Minomo · Kouhei Washiyama · Kazuyuki Ogata
Microscopic optical potentials have been successful in describing nucleon-nucleus and nucleus-nucleus scattering. Some essential ingredients of the framework, however, have not been examined in detail. Applicability of the microscopic folding model is systematically investigated. Effect of an antisymmetrization factor (ASF) appearing in multiple scattering theory, theoretical uncertainty regarding the local density approximation (LDA), and the validity of a prescription for nonlocality, the Brieva-Rook (BR) localization, of the microscopic potential, are quantitatively estimated for nucleon-nucleus scattering; investigation on the ASF is carried out for also deuteron-nucleus scattering. A single folding model with the Melbourne g-matrix interaction and the SLy4 Skyrme-type Hartree-Fock-Bogoliubiv (SLy4-HFB) density is employed for evaluating a nucleon-nucleus microscopic optical potential. Deuteron-nucleus scattering is described by the continuum-discretized coupled-channels method incorporating the microscopic proton-nucleus and neutron-nucleus potentials. The ASF is found to affect proton total reaction cross sections for a 12C target below 200 MeV by about 10%. Effect of the ASF on total reaction cross sections is negligibly small if a target nucleus is heavy or scattering energy is above 200 MeV; elastic cross sections are hardly affected by the ASF for all the reaction systems considered. Below 65 MeV, still the BR localization works quite well. However, at energies below about 50 MeV, the LDA becomes less accurate for evaluating elastic cross sections at backward angles. This is the case also for the total reaction cross sections of p-12C below about 200 MeV. The microscopic model is applicable to nucleon-nucleus scattering above 25 MeV for target nuclei in a wide range of mass numbers. Deviation of calculated results from experimental data is less than about 10%.
nucl-th14 citations
04
Neutrino emission from Cooper pairs at finite temperatures
Lev B. Leinson🇷🇺
A brief review is given of the current state of the problem of neutrino pair emission through neutral weak currents caused by the Cooper pairs breaking and formation (PBF) in superfluid baryon matter at thermal equilibrium. The cases of singlet-state pairing with isotropic superfluid gap and spin-triplet pairing with anisotropic gap are analyzed with allowance for the anomalous weak interactions caused by superfluidity. It is shown that taking into account the anomalous weak interactions in both the vector and axial channels is very important for a correct description of neutrino energy losses through the PBF processes. The anomalous contributions lead to an almost complete suppression of the PBF neutrino emission in spin-singlet superfluids and strong reduction of the PBF neutrino losses in the spin-triplet superfluid neutron matter, which considerably slows down the cooling rate of neutron stars with superfluid cores.
nucl-thastro-ph.SRhep-phAdv.High Energy Phys.(2018)·8 citations
05
Small Bits of Cold Dense Matter
Stefano Gandolfi🇺🇸 · Joseph Carlson🇺🇸 · Alessandro Roggero🇺🇸 · Joel Lynn🇩🇪 · Sanjay Reddy🇺🇸
The behavior of QCD at high baryon density and low temperature is crucial to understanding the properties of neutron stars and gravitational waves emitted during their mergers. In this paper we study small systems of baryons in periodic boundary conditions to probe the properties of QCD at high baryon density. By comparing calculations based on nucleon degrees of freedom to simple quark models we show that specific features of the nuclear spectrum, including shell structure and nucleon pairing, emerge if nucleons are the primary degrees of freedom. Very small systems should also be amenable to studies in lattice QCD, unlike larger systems where the fermion sign problem is much more severe. Through comparisons of lattice QCD and nuclear calculations it should be possible to gain, at at least a semi-quantitative level, more understanding of the cold dense equation of state as probed in neutron stars.
nucl-thhep-latPLB(2018)·7 citations
06
Pion-less effective field theory for atomic nuclei and lattice nuclei
A. Bansal🇺🇸 · S. Binder🇺🇸 · A. Ekström🇸🇪 · G. Hagen🇺🇸 · G. R. Jansen🇺🇸 · T. Papenbrock🇺🇸
We compute the medium-mass nuclei $^{16}$ O and $^{40}$ Ca using pionless effective field theory (EFT) at next-to-leading order (NLO). The low-energy coefficients of the EFT Hamiltonian are adjusted to experimantal data for nuclei with mass numbers $A = 2$ and $3$ , or alternatively to results from lattice quantum chromodynamics (QCD) at an unphysical pion mass of 806 MeV. The EFT is implemented through a discrete variable representation in the harmonic oscillator basis. This approach ensures rapid convergence with respect to the size of the model space and facilitates the computation of medium-mass nuclei. At NLO the nuclei $^{16}$ O and $^{40}$ Ca are bound with respect to decay into alpha particles. Binding energies per nucleon are 9-10 MeV and 30-40 MeV at pion masses of 140 MeV and 806 MeV, respectively.
nucl-thhep-latPRC(2018)·79 citations
07
The link between two-body model space and many-body model space
C.-J. Yang
An exact relation which links the ideal model space to be used in A-body calculations when the two-body interaction is given in a truncated model space is derived. Its implications on the effective field theory (EFT) approach to no-core-shell-model (NCSM) is analyzed. Some insights regarding whether details of two-body interaction becomes less important in the calculations of many-body system are given. The result suggests that there might be a way to establish an EFT expansion for heavy nuclei and nuclear matter with an effective intereaction which has a much simpler form than the nucleon-nucleon (NN) interaction in the vacuum.
nucl-th0 citations
08
Hydro+: hydrodynamics with parametric slowing down and fluctuations near the critical point
M. Stephanov🇺🇸 · Y. Yin🇺🇸
The search for the QCD critical point in heavy-ion collision experiments requires dynamical simulations of the bulk evolution of QCD matter as well as of fluctuations. We consider two essential ingredients of such a simulation: a generic extension of hydrodynamics by a_parametrically_ slow mode or modes ("Hydro+") and a description of fluctuations out of equilibrium. By combining the two ingredients we are able to describe the bulk evolution and the fluctutations within the same framework. Critical slowing down means that equilibration of fluctuations could be as slow as hydrodynamic evolution and thus fluctuations could significantly deviate from equilibrium near the critical point. We generalize hydrodynamics to partial-equilibrium conditions where the state of the system is characterized by the off-equilibrium magnitude of fluctuations in addition to the usual hydrodynamic variables -- conserved densities. We find that the key element of the new formalism -- the extended entropy taking into account the off-equilibrium fluctuations -- is remarkably similar to the 2PI action in quantum field theory. We show how the new Hydro+ formalism reproduces two major effects of critical fluctuations on the bulk evolution: the strong frequency dependence of the anomalously large bulk viscosity as well as the stiffening of the equation of state with increasing frequency or wave-number. While the agreement with known results confirms its validity, the fact that Hydro+ achieves this within a local and deterministic framework gives it significant advantages for dynamical simulations.
nucl-thhep-thPRD(2018)·236 citations
09
Finite Temperature Phase Diagrams of a Two-band Model of Superconductivity
Heron Caldas · A. Celes · David Nozadze
We explore the temperature effects in the superconducting phases of a hybridized two-band system. We show that for zero hybridization between the bands, there are two different critical temperatures. However, for any finite hybridization there are only one critical temperature at which the two gaps vanish simultaneously. We construct the phase diagrams of the critical temperature versus hybridization parameter $α$ and critical temperature versus critical chemical potential asymmetry $δ μ$ between the bands, identifying the superconductor and normal phases in the system. We find an interesting reentrant behavior in the superconducting phase as the parameters $α$ or $δ μ$ , which drive the phase transitions, increase. We also find that for optimal values of both $α$ and $δ μ$ there is a significant enhancement of the critical temperature of the model.
↳ cond-mat.supr-concond-mat.quant-gasnucl-thAnnals Phys.(2018)·1 citation
10
Prospects for $γ γ \to$ Higgs observation in ultraperipheral ion collisions at the Future Circular Collider
David d'Enterria🇨🇭 · Daniel E. Martins🇧🇷 · Patricia Rebello Teles🇧🇷
We study the two-photon production of the Higgs boson, $γ γ \to H$ , at the Future Circular Collider (FCC) in ultraperipheral PbPb and pPb collisions at $s_{N N} = 39$ and 63 TeV. Signal and background events are generated with MADGRAPH 5, including $γ$ fluxes from the proton and lead ions in the equivalent photon approximation, yielding $σ (γ γ \to H)$ = 1.75 nb and 1.5 pb in PbPb and pPb collisions respectively. We analyse the H $\to b b ˉ$ decay channel including realistic reconstruction efficiencies for the final-state $b$ -jets, showered and hadronized with PYTHIA 8, as well as appropriate selection criteria to reduce the dominant exclusive $γ γ \to b b ˉ$ continuum background. Observation of $PbPb γ γ (Pb) H (Pb)$ is achievable in the first year with the expected PbPb integrated luminosities.
↳ hep-phhep-exnucl-exnucl-thCERN Proc.(2018)·4 citations
11
Directed flow in Au+Au collisions from the RHIC Beam Energy Scan at the STAR experiment
Subhash Singha (for the STAR Collaboration)🇺🇸
We report results of $v_{1} (y)$ and $d v_{1} / d y$ near mid-rapidity for $π^{\pm}$ , $K^{\pm}$ , $K_{s}^{0}$ , $p$ , $p$ , $Λ$ , $Λ$ and $ϕ$ from Beam Energy Scan Au+Au collisions at $s_{N N} =$ 7.7 - 200 GeV using the STAR detector at RHIC. The $d v_{1} / d y$ of $π^{\pm}$ , $K^{\pm}$ and $K_{s}^{0}$ mesons remains negative over all beam energies. The $d v_{1} / d y$ of $p$ and $Λ$ baryons shows a sign change around 10 - 15 GeV, while net baryons (net p and net $Λ$ ) indicate a double sign change. The $d v_{1} / d y$ of $p$ , $Λ$ and $ϕ$ show a similar trend for $s_{N N} >$ 14.5 GeV. For the first time, $v_{1}$ measurements are used to test a quark coalescence hypothesis. Many measurements are found to be consistent with the particles being formed via coalescence of constituent quarks. The observed deviations from that consistency offer a new approach for probing the collision process at the quark level.
↳ nucl-exhep-exnucl-thPoS(2018)·0 citations
12
Traps in hadron spectroscopy: Thresholds, triangle singularities, etc
Feng-Kun Guo🇨🇳
The knowledge of hadron spectrum is based on experimental observations of hadronic resonances. The resonances are normally observed as peaks in certain invariant mass distributions. However, neither is a peak necessarily due to the presence of a resonance, nor does a resonance necessarily lead to a peak. Kinematic singularities can also produce peaks. Here, we discuss such possibilities and methods distinguishing genuine resonances from kinematic effects.
↳ hep-phhep-exnucl-thPoS(2018)·9 citations
13
Exclusive photoproduction of $π^{0}$ up to large values of Mandelstam variables $s, t$ and $u$ with CLAS
M.C. Kunkel · 32 · 18 M.J. Amaryan · 32 · I.I. Strakovsky · 16 J. Ritman · 3 · 18 G.R. Goldstein · 43 K.P. Adhikari · 28 S Adhikari · 13 H. Avakian · 39 J. Ball and 160 other authors
Exclusive photoproduction cross sections have been measured for the process $γ p \to p π^{0} (e^{+} e^{-} (γ))$ with the Dalitz decay final state using tagged photon energies in the range of $E_{γ} = 1.275 - 5.425$ GeV. The complete angular distribution of the final state $π^{0}$ , for the entire photon energy range up to large values of $t$ and $u$ , has been measured for the first time. The data obtained show that the cross section $d σ / d t$ , at mid to large angles, decreases with energy as $s^{- 6.89 \pm 0.26}$ . This is in agreement with the perturbative QCD quark counting rule prediction of $s^{- 7}$ . Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD based Generalized Parton Distribution mechanism at highest available invariant energy $s = 11$ GeV $^{2}$ . At the same time, the Regge exchange based models for $π^{0}$ photoproduction are more consistent with experimental data.
↳ hep-exhep-phnucl-exnucl-thPRC(2018)·27 citations

Affiliations

first authorsco-authorsvia INSPIRE