Nuclear Theory·nucl-th

Friday·April 21, 2017

12 papers—6 primary·6 cross-listed

01
Evidence for chiral symmetry restoration in heavy-ion collisions
Pierre Moreau🇩🇪 · Alessia Palmese🇩🇪 · Wolfgang Cassing🇩🇪 · Eduard Seifert🇩🇪 · Thorsten Steinert🇩🇪 · Elena Bratkovskaya🇩🇪
We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range $s_{N N}$ = 3-20 GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for particle production. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at $s_{N N}$ = 3-20 GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. Our results provide a microscopic explanation for the "horn" structure in the excitation function of the $K^{+} / π^{+}$ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to $s_{N N} \approx$ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the "horn" structure is investigated as a function of the system size. We additionally present an analysis of strangeness production in the ( $T, μ_{B}$ )-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.
nucl-thNPA(2017)·6 citations
02
Existence problem of proton semi-bubble structure in the $2_{1}^{+}$ state of $^{34}$ Si
Feng Wu · C.L. Bai · J.M. Yao · H.Q. Zhang · X.Z. Zhang
The fully self-consistent Hartree-Fock (HF) plus random phase approximation (RPA) based on Skyrme-type interaction is used to study the existence problem of proton semi-bubble structure in the $2_{1}^{+}$ state of $^{34}$ Si. The experimental excitation energy and the B(E2) strength of the $2_{1}^{+}$ state in $^{34}$ Si can be reproduced quite well. The tensor effect is also studied. It is shown that the tensor interaction has a notable impact on the excitation energy of the $2_{1}^{+}$ state and a small effect on the B(E2) value. Besides, its effect on the density distributions in the ground and $2_{1}^{+}$ state of $^{34}$ Si is negligible. Our present results with T36 and T44 show that the $2_{1}^{+}$ state of $^{34}$ Si is mainly caused by proton transiton from $π 1 d_{5/2}$ orbit to $π 2 s_{1/2}$ orbit, and the existence of a proton semi-bubble structure in this state is very unlikely.
nucl-thEPJA(2017)·1 citation
03
Nuclear final-state interactions in deep inelastic scattering off the lightest nuclei
W. Cosyn🇧🇪 · M. Sargsian🇺🇸
We review recent progress in studies of nuclear final-state interactions in deep inelastic scattering (DIS) off the lightest nuclei tagged by a recoil nucleon. These processes hold a lot of potential for resolving the outstanding issues related to the dynamics of hadronization in QCD. Within the minimal Fock component framework, valid at large Bjorken $x$ , the main features of the theoretical approach based on the virtual nucleon approximation are elaborated. In this approach, the strong final-state interaction of the DIS products with the nuclear fragments is described by an effective eikonal amplitude, whose parameters can be extracted from the analysis of semi-inclusive DIS off the deuteron target. The extraction of the $Q^{2}$ and $W$ mass dependences of these parameters gives a new observable in studying the QCD structure of DIS final states. Another important feature of tagged DIS off the lightest nuclei is the possibility of performing pole extrapolation with a high degree of accuracy. Such extrapolation allows an extraction of the neutron structure function in a model independent way due to suppression of the final-state interaction in the on-shell limit of the struck nucleon propagator. We review the first application of the pole extrapolation to recent experimental data. Finally, we outline the extension of the framework to inclusive DIS, including a polarized deuteron target as well as its application to the tagged DIS reactions for future experiments at fixed target and collider energies.
nucl-thhep-phnucl-exIJMPE(2017)·18 citations
04
On the possibility of revealing the transition of a baryon pair state to a six-quark confinement state
V.I. Komarov🇷🇺
Proton-proton collisions are considered to find favourable conditions for searching for the transition of a baryon pair state to a hexa-quark confinement state $(3 q) + (3 q) \to (6 q)_{cnf}$ . It is admitted that central $pp$ collisions in a definite range of the initial energy can lead to creation of an intermediate compound system where the hexa-quark dibaryon can be formed. Criteria for selection of central collision events and for manifestation of the quark-structure dibaryon production are proposed.
nucl-thnucl-exPhys.Part.Nucl.Lett.(2018)·5 citations
05
A derivation of the entropy-based relativistic smoothed particle hydrodynamics by variational principle
Philipe Mota · Weixian Chen · Wei-Liang Qian
In this work, a second order smoothed particle hydrodynamics is derived for the study of relativistic heavy ion collisions. The hydrodynamical equation of motion is formulated in terms of the variational principle. In order to describe the fluid of high energy density but of low baryon density, the entropy is taken as the base quantity for the interpolation. The smoothed particle hydrodynamics algorithm employed in this study is of the second order, which guarantees better particle consistency. Furthermore, it is shown that the variational principle preserves the translational invariance of the system, and therefore improves the accuracy of the method. A brief discussion on the potential implications of the model in heavy ion physics is also presented.
nucl-thCommun.Theor.Phys.(2017)·3 citations
06
Final state interactions and the extraction of neutron single spin asymmetries from SIDIS by a transversely polarized $^{3}$ He target
A. Del Dotto🇮🇹 · L.P. Kaptari🇷🇺 · E. Pace🇮🇹 · G. Salmè🇮🇹 · S. Scopetta🇮🇹
The semi-inclusive deep inelastic electron scattering off transversely polarized $^{3}$ He, i.e. the process, $e +^{3} He \to e^{'} + h + X$ , with $h$ a detected fast hadron, is studied beyond the plane wave impulse approximation. To this end, a distorted spin-dependent spectral function of a nucleon inside an A=3 nucleus is actually evaluated through a generalized eikonal approximation, in order to take into account the final state interactions between the hadronizing system and the (A-1) nucleon spectator one. Our realistic description of both nuclear target and final state is a substantial step forward for achieving a reliable extraction of the Sivers and Collins single spin asymmetries of the free neutron. To illustrate how and to what extent the model dependence due to the treatment of the nuclear effects is under control, we apply our approach to the extraction procedure of the neutron single spin asymmetries from those measured for $^{3}$ He for values of the kinematical variables relevant both for forthcoming experiments at Jefferson Lab and, with an exploratory purpose, for the future Electron Ion Collider.
nucl-thhep-phPRC(2017)·8 citations
07
Simulating chiral magnetic effect and anomalous transport phenomena in the pre-equilibrium stages of heavy-ion collisions
Mark Mace🇺🇸 · Niklas Mueller🇩🇪 · Soeren Schlichting🇺🇸 · Sayantan Sharma🇺🇸
We present a first principles approach to study the Chiral Magnetic Effect during the pre-equilibrium stage of a heavy-ion collision. We discuss the dynamics of the Chiral Magnetic Effect and Chiral Magnetic Wave based on real-time lattice simulations with dynamical (Wilson and Overlap) fermions simultaneously coupled to color and electromagnetic fields. While for light quarks we observe a dissipation-less transport of charges as in anomalous hydrodynamics, we demonstrate that for heavier quarks the effects of explicit chiral symmetry breaking lead to a significant reduction of the associated currents.
↳ hep-lathep-phnucl-thNPA(2017)·6 citations
08
Physics with ions at the Future Circular Collider
David d'Enterria🇨🇭 · L. Apolinario🇨🇭 · N. Armesto🇨🇭 · A. Dainese🇨🇭 · J. Jowett🇨🇭 · J.P. Lansberg🇨🇭 · S. Masciocchi🇨🇭 · G. Milhano🇨🇭 · C. Roland🇨🇭 · C.A. Salgado🇨🇭 · M. Schaumann🇨🇭 · M. van Leeuwen🇨🇭 · U.A. Wiedemann🇨🇭
The unique physics opportunities accessible with nuclear collisions at the CERN Future Circular Collider (FCC) are summarized. Lead-lead (PbPb) and proton-lead (pPb) collisions at $s_{N N}$ = 39 and 63 TeV respectively with $L_{in t}$ = 33 nb $^{- 1}$ and 8 pb $^{- 1}$ monthly integrated luminosities, will provide unprecedented experimental conditions to study quark-gluon matter at temperatures $O$ (1 GeV). The following topics are succinctly discussed: (i) charm-quark densities thrice larger than at the LHC, leading to direct heavy-quark impact in the bulk QGP properties, (ii) quarkonia, including $Υ (1 S)$ , melting at temperatures up to five times above the QCD critical temperature, (iii) access to initial-state nuclear parton distributions (nPDF) at fractional momenta as low as $x \approx 1 0^{- 7}$ , (iv) availability of $5 \cdot 1 0^{5}$ top-quark pairs per run to study the high- $x$ gluon nPDF and the energy loss properties of boosted colour-antennas, (v) study of possible Higgs boson suppression in the QGP, and (vi) high-luminosity $γ γ$ (ultraperipheral) collisions at c.m. energies up to 1 TeV.
↳ hep-exhep-phnucl-exnucl-thNPA(2017)·10 citations
09
The QCD Equation of state and critical end-point estimates at $O (μ_{B}^{6})$
Sayantan Sharma · for Bielefeld-BNL-CCNU collaboration
We present results for the QCD Equation of State at non-zero chemical potentials corresponding to the conserved charges in QCD using Taylor expansion upto sixth order in the baryon number, electric charge and strangeness chemical potentials. The latter two are constrained by the strangeness neutrality and a fixed electric charge to baryon number ratio. In our calculations, we use the Highly Improved Staggered Quarks (HISQ) discretization scheme at physical quark masses and at different values of the lattice spacings to control lattice cut-off effects. Furthermore we calculate the pressure along lines of constant energy density, which serve as proxies for the freeze-out conditions and discuss their dependence on $μ_{B}$ , which is necessary for hydrodynamic modelling near freezeout. We also provide an estimate of the radius of convergence of the Taylor series from the 6th order coefficients which provides a new constraint on the location of the critical end-point in the T- $μ_{B}$ plane of the QCD phase diagram.
↳ hep-lathep-phnucl-exnucl-thNPA(2017)·9 citations
10
Electromagnetic decays of the neutral pion
Esther Weil🇩🇪 · Gernot Eichmann🇩🇪 · Christian S. Fischer🇩🇪 · Richard Williams🇩🇪
We complement studies of the neutral pion transition form factor pi^0 --> gamma^(*) gamma^(*) with calculations for the electromagnetic decay widths of the processes pi^0 --> e^+ e^-, pi^0 --> e^+ e^- gamma and pi^0 --> e^+ e^- e^+ e^-. Their common feature is that the singly- or doubly-virtual transition form factor serves as a vital input that is tested in the non-perturbative low-momentum region of QCD. We determine this form factor from a well-established and symmetry-preserving truncation of the Dyson-Schwinger equations. Our results for the three- and four-body decays match results of previous theoretical calculations and experimental measurements. For the rare decay we employ a numerical method to calculate the process directly by deforming integration contours, which in principle can be generalized to arbitrary integrals as long as the analytic structure of the integrands are known. Our result for the rare decay is in agreement with dispersive calculations but still leaves a 2 sigma discrepancy between theory and experiment.
↳ hep-phhep-latnucl-thPRD(2017)·65 citations
11
Scale Invariance in Heavy Hadron Molecules
Lisheng Geng🇨🇳 · Junxu Lu🇨🇳 · Manuel Pavon Valderrama🇨🇳
We discuss a scenario in which the $P_{c} (4450)^{+}$ heavy pentaquark is a $Σ_{c} D ˉ^{*}$ - $Λ_{c} (2595) D ˉ$ molecule. The $Λ_{c 1} D ˉ \to Σ_{c} D ˉ^{*}$ transition is mediated by the exchange of a pion almost on the mass shell that generates a long-range $1/ r^{2}$ potential. This is analogous to the effective force that is responsible for the Efimov spectrum in three-boson systems interacting through short-range forces. The equations describing this molecule exhibit approximate scale invariance, which is anomalous and broken by the solutions. If the $1/ r^{2}$ potential is strong enough this symmetry survives in the form of discrete scale invariance, opening the prospect of an Efimov-like geometrical spectrum in two-hadron systems. For a molecular pentaquark with quantum numbers $\frac{3}{2}^{-}$ the attraction is not enough to exhibit discrete scale invariance, but this prospect might very well be realized in a $\frac{1}{2}^{+}$ pentaquark or in other hadron molecules involving transitions between particle channels with opposite intrinsic parity and a pion near the mass shell. A very good candidate is the $Λ_{c} (2595) Ξ ˉ_{b} - Σ_{c} Ξ ˉ_{b}^{'}$ molecule. Independently of this, the $1/ r^{2}$ force is expected to play a very important role in the formation of this type of hadron molecule, which points to the existence of $\frac{1}{2}^{+}$ $Σ_{c} D^{*}$ - $Λ_{c} (2595) D$ and $1^{+}$ $Λ_{c} (2595) Ξ_{b} - Σ_{c} Ξ_{b}^{'}$ molecules and $0^{+}$ / $1^{-}$ $Λ_{c} (2595) Ξ ˉ_{b} - Σ_{c} Ξ ˉ_{b}^{'}$ baryonia.
↳ hep-phnucl-thPRD(2018)·39 citations
12
Chiral Extrapolations of the $ρ (770)$ Meson in $N_{f} = 2 + 1$ Lattice QCD Simulations
B. Hu🇺🇸 · R. Molina🇺🇸 · M. Döring🇺🇸 · M. Mai🇺🇸 · A. Alexandru🇺🇸
Recent $N_{f} = 2 + 1$ lattice data for meson-meson scattering in $p$ -wave and isospin $I = 1$ are analyzed using a unitarized model inspired by Chiral Perturbation Theory in the inverse-amplitude formulation for two and three flavors. Chiral extrapolations are performed that postdict phase shifts extracted from experiment quite well. In addition, the low-energy constants are compared to the ones from a recent analysis of $N_{f} = 2$ lattice QCD simulations to check for the consistency of the hadronic model used here. Some inconsistencies are detected in the fits to $N_{f} = 2 + 1$ data, in contrast to the previous analysis of $N_{f} = 2$ data.
↳ hep-latnucl-thPRD(2017)·42 citations

Affiliations

first authorsco-authorsvia INSPIRE