Nuclear Theory·nucl-th

Friday·December 4, 2015

10 papers—4 primary·6 cross-listed

01
An analytic hydrodynamical model of rotating 3D expansion in heavy-ion collisions
M. I. Nagy🇭🇺 · T. Csörgő🇭🇺
A new exact and analytic solution of non-relativistic fireball hydrodynamics is presented. It describes an expanding triaxial ellipsoid that rotates around one of its principal axes. The observables are calculated using simple analytic formulas. Azimuthal oscillation of the off-diagonal Bertsch-Pratt radii of Bose-Einstein correlations as well as rapidity dependent directed and third flow measurements provide means to determine the magnitude of the rotation of the fireball. Observing this rotation and its dependence on collision energy may lead to new information on the equation of state of the strongly interacting quark gluon plasma produced in high energy heavy ion collisions.
nucl-thInt.J.Mod.Phys.A(2016)·5 citations
02
Charm production in Pb+Pb collisions at the Large Hadron Collider energy
Taesoo Song🇩🇪 · Hamza Berrehrah🇩🇪 · Daniel Cabrera🇩🇪 · Wolfgang Cassing🇩🇪 · Elena Bratkovskaya🇩🇪
We study charm production in Pb+Pb collisions at $s_{NN} =$ 2.76 TeV in the Parton-Hadron-String-Dynamics transport approach and the charm dynamics in the partonic and hadronic medium. The charm quarks are produced through initial binary nucleon-nucleon collisions by using the PYTHIA event generator taking into account the (anti-)shadowing incorporated in the EPS09 package. The produced charm quarks interact with off-shell massive partons in the quark-gluon plasma and are hadronized into $D$ mesons through coalescence or fragmentation close to the critical energy density, and then interact with hadrons in the final hadronic stage with scattering cross sections calculated in an effective Lagrangian approach with heavy-quark spin symmetry. The PHSD results show a reasonable $R_{AA}$ and elliptic flow of $D$ mesons in comparison to the experimental data for Pb+Pb collisions at $s_{N N}$ = 2.76 TeV from the ALICE Collaboration. We also study the effect of temperature-dependent off-shell charm quarks in relativistic heavy-ion collisions. We find that the scattering cross sections are only moderately affected by off-shell charm degrees of freedom. However, the position of the peak of $R_{AA}$ for $D$ mesons depends on the strength of the scalar partonic forces which also have an impact on the $D$ meson elliptic flow. The comparison with experimental data on the $R_{AA}$ suggests that the repulsive force is weaker for off-shell charm quarks as compared to that for light quarks. Furthermore, the effects from radiative charm energy loss appear to be low compared to the collisional energy loss up to transverse momenta of $\sim$ 15 GeV/c.
nucl-thnucl-exPRC(2016)·189 citations
03
Systematic study of complete fusion suppression in reactions involving weakly bound nuclei at energies above the Coulomb barrier
Bing Wang🇨🇳 · Wei-Juan Zhao🇨🇳 · Alexis Diaz-Torres🇮🇹 · En-Guang Zhao🇨🇳 · Shan-Gui Zhou🇨🇳
Complete fusion excitation functions of reactions involving breakup are studied by using the empirical coupled-channel (ECC) model with breakup effects considered. An exponential function with two parameters is adopted to describe the prompt-breakup probability in the ECC model. These two parameters are fixed by fitting the measured prompt-breakup probability or the complete fusion cross sections. The suppression of complete fusion at energies above the Coulomb barrier is studied by comparing the data with the predictions from the ECC model without the breakup channel considered. The results show that the suppression of complete fusion are roughly independent of the target for the reactions involving the same projectile.
nucl-thnucl-exPRC(2016)·30 citations
04
Ab-initio calculations of charge symmetry breaking in the A=4 hypernuclei
Daniel Gazda🇸🇪 · Avraham Gal🇮🇱
We report on ab-initio NCSM calculations of the A=4 mirror Lambda hypernuclei Lambda-4H and Lambda-4He, using the Bonn-Juelich LO chiral EFT YN potentials plus a CSB Lambda0--Sigma0 mixing vertex. In addition to reproducing rather well the 0+ (g.s.) and 1+ (exc.) binding energies, these four-body calculations demonstrate for the first time that the observed CSB splitting of mirror levels, reaching hundreds of keV for 0+ (g.s.), can be reproduced using realistic theoretical interaction models, although with a non-negligible momentum cutoff dependence. Our results are discussed in relation to recent measurements of the Lambda-4H (0+ g.s.) binding energy [MAMI A1 Collaboration, Phys. Rev. Lett. 114, 232501 (2015)] and the Lambda-4He (1+ exc.) excitation energy [J-PARC E13 Collaboration, Phys. Rev. Lett. 115, 222501 (2015)].
nucl-thhep-phnucl-exPRL(2016)·72 citations
05
Calculating Soft Radiation at One Loop
Tomas Kasemets🇳🇱 · Wouter J. Waalewijn🇳🇱 · Lisa Zeune🇳🇱
We present an efficient way to calculate the effect of soft QCD radiation at one loop, which is needed for predictions at next-to-next-to-leading logarithmic accuracy. We use rapidity coordinates and isolate the divergences in the integrand. By performing manipulations with cumulative variables, we avoid complications from plus distributions. We address rapidity divergences, divergences with an azimuthal dependence, complicated jet boundaries and multi-differential measurements. The process and measurements can be easily adjusted, as we demonstrate by reproducing many existing soft functions. The results for a general LHC process with multiple Wilson lines are obtained by treating Wilson lines that are not back-to-back using a boost. We also obtain, for the first time, the N-jettiness soft function for generic jet angularities, and the collinear-soft function for the measurement of two angularities.
↳ hep-phnucl-thJHEP(2016)·20 citations
06
Model-independent determination of the shear viscosity of a trapped unitary Fermi gas: Application to high temperature data
Marcus Bluhm (North Carolina State University) · Thomas Schaefer (North Carolina State University)
Determinations of the shear viscosity of trapped ultracold gases suffer from systematic, uncontrolled uncertainties related to the treatment of the dilute part of the gas cloud. In this work we present an analysis of expansion experiments based on a new method, anisotropic fluid dynamics, that interpolates between Navier-Stokes fluid dynamics at the center of the cloud and ballistic behavior in the dilute corona. We validate the method using a comparison between anisotropic fluid dynamics and numerical solutions of the Boltzmann equation. We then apply anisotropic fluid dynamics to the expansion data reported by Cao et al. In the high temperature limit we find $η = 0.282 (m T)^{3/2}$ , which agrees within about 5\% with the theoretical prediction $η = 0.269 (m T)^{3/2}$ .
↳ cond-mat.quant-gasnucl-thPRL(2016)·38 citations
07
Real and virtual $N N ˉ$ pair production near the threshold
V. F. Dmitriev🇷🇺 · A. I. Milstein🇷🇺 · S. G. Salnikov🇷🇺
Nucleon-antinucleon optical potential, which explains the experimental data for the processes $e^{+} e^{-} \to p p ˉ$ and $e^{+} e^{-} \to \mbox p i o n s$ near the threshold of $p p ˉ$ pair production, is suggested. To obtain this potential we have used the available experimental data for $p p ˉ$ scattering, $p p ˉ$ pair production in $e^{+} e^{-}$ annihilation, and the ratio of electromagnetic form factors of a proton in the timelike region. It turns out that final-state interaction via the optical potential allows one to reproduce the available experimental data with good accuracy. Our results for the cross sections of $e^{+} e^{-} \to 6 π$ process near the threshold of $p p ˉ$ pair production are in agreement with the recent experiments.
↳ hep-phnucl-thPRD(2016)·27 citations
08
Pion Production via Proton Synchrotron Radiation in Strong Magnetic Fields in Relativistic Field Theory: Scaling Relations and Angular Distributions
Tomoyuki Maruyama🇯🇵 · Myung-Ki Cheoun🇯🇵 · Toshitaka Kajino🇯🇵 · Grant J. Mathews🇺🇸
We study pion production by proton synchrotron radiation in the presence of a strong magnetic field when the Landau numbers of the initial and final protons are $n_{i, f} \sim 1 0^{4} - 1 0^{5}$ . We find in our relativistic field theory calculations that the pion decay width depends only on the field strength parameter which previously was only conjectured based upon semi-classical arguments. Moreover, we also find new results that the decay width satisfies a robust scaling relation, and that the polar angular distribution of emitted pion momenta is very narrow and can be easily obtained. This scaling implies that one can infer the decay width in more realistic magnetic fields of $1 0^{15}$ G, where $n_{i, f} \sim 1 0^{12} - 1 0^{13}$ , from the results for $n_{i, f} \sim 1 0^{4} - 1 0^{5}$ . The resultant pion intensity and angular distributions for realistic magnetic field strengths are presented and their physical implications discussed.
↳ astro-ph.HEhep-phnucl-thPLB(2016)·8 citations
09
Pion-induced production of the $Z_{c} (3900)$ off a nuclear target
Yin Huang🇨🇳 · Jun He🇨🇳 · Xiang Liu🇨🇳 · Hong Fei Zhang🇨🇳 · Ju Jun Xie🇨🇳 · Xu Rong Chen🇨🇳
We investigate the possibility to study the charmoniumlike state $Z_{c} (3900)$ through the pion-induced production off a nuclear target. By using a high-energy pion beam, the $Z_{c} (3900)$ can be produced off a proton or nucleus though the Primakoff effect. The production amplitude is calculated in an effective Lagrangian approach combined with the vector dominance model. The total cross sections of the $p (π^{-}, Z_{c}^{-} (3900))$ and $p (π^{-}, Z_{c}^{-} (3900) \to J / ψ π^{-})$ reactions are calculated, and their order of magnitude is about 0.1 and 0.01nb, respectively, with an assumption of branch ratio 10\% for the $Z_{c} (3900)$ decay in $J / ψ π$ channel. If the proton target is replaced by a nuclear target, the production of the $Z_{c} (3900)$ enhances obviously. The predicted total cross sections for the $A (π^{-}, Z_{c}^{-} (3900))$ and $A (π^{-}, Z_{c}^{-} (3900) \to J / ψ π^{-})$ reactions with $A =^{12}$ C or $^{208}$ Pb are on the order of magnitude of 100 and 10 nb, respectively, which is about one thousand times larger than the cross sections off a proton target. Based on these the results, we suggest the experimental study of the $Z_{c} (3900)$ by using high-energy pion beams with a nuclear target at facilities such as COMPASS and J-PARC.
↳ hep-phhep-exnucl-exnucl-thPRD(2016)·8 citations
10
Axial form factors of the octet baryons in a covariant quark model
G. Ramalho🇧🇷 · K. Tsushima🇧🇷
We study the weak interaction axial form factors of the octet baryons, within the covariant spectator quark model, focusing on the dependence of four-momentum transfer squared, Q^2. In our model the axial form factors G_A(Q^2) (axial-vector form factor) and G_P(Q^2) (induced pseudoscalar form factor), are calculated based on the constituent quark axial form factors and the octet baryon wave functions. The quark axial current is parametrized by the two constituent quark form factors, the axial-vector form factor g_A^q(Q^2), and the induced pseudoscalar form factor g_P^q(Q^2). The baryon wave functions are composed of a dominant S-state and a P-state mixture for the relative angular momentum of the quarks. First, we study in detail the nucleon case. We assume that the quark axial-vector form factor g_A^q(Q^2) has the same function form as that of the quark electromagnetic isovector form factor. The remaining parameters of the model, the P-state mixture and the Q^2-dependence of g_P^q(Q^2), are determined by a fit to the nucleon axial form factor data obtained by lattice QCD simulations with large pion masses. In this lattice QCD regime the meson cloud effects are small, and the physics associated with the valence quarks can be better calibrated. Once the valence quark model is calibrated, we extend the model to the physical regime, and use the low Q^2 experimental data to estimate the meson cloud contributions for G_A(Q^2) and G_P(Q^2). Using the calibrated quark axial form factors, and the generalization of the nucleon wave function for the other octet baryon members, we make predictions for all the possible weak interaction axial form factors G_A(Q^2) and G_P(Q^2) of the octet baryons. The results are compared with the corresponding experimental data for G_A(0), and with the estimates of baryon-meson models based on SU(6) symmetry.
↳ hep-phhep-exhep-latnucl-ex+1PRD(2016)·32 citations

Affiliations

first authorsco-authorsvia INSPIRE