Nuclear Theory·nucl-th

Tuesday·December 23, 2014

17 papers—4 primary·13 cross-listed

05
Dirac semimetals $A_{3} B i$ ( $A$ =Na,K,Rb) as $Z_{2}$ Weyl semimetals
E. V. Gorbar🇺🇦 · V. A. Miransky🇨🇦 · I. A. Shovkovy🇺🇸 · P. O. Sukhachov🇺🇦
We demonstrate that the physical reason for the nontrivial topological properties of Dirac semimetals $A_{3} B i$ (A=Na,K,Rb) is connected with a discrete symmetry of the low-energy effective Hamiltonian. By making use of this discrete symmetry, we argue that all electron states can be split into two separate sectors of the theory. Each sector describes a Weyl semimetal with a pair of Weyl nodes and broken time-reversal symmetry. The latter symmetry is not broken in the complete theory because the time-reversal transformation interchanges states from different sectors. Our findings are supported by explicit calculations of the Berry curvature. In each sector, the field lines of the curvature reveal a pair of monopoles of the Berry flux at the positions of Weyl nodes. The $Z_{2}$ Weyl semimetal nature is also confirmed by the existence of pairs of surface Fermi arcs, which originate from different sectors of the theory.
↳ cond-mat.str-elhep-thnucl-thPRB(2015)·22 citations
06
Observation of the chiral magnetic effect in ZrTe5
Qiang Li🇺🇸 · Dmitri E. Kharzeev🇺🇸 · Cheng Zhang🇺🇸 · Yuan Huang🇺🇸 · I. Pletikosic🇺🇸 · A. V. Fedorov🇺🇸 · R. D. Zhong🇺🇸 · J. A. Schneeloch🇺🇸 · G. D. Gu🇺🇸 · T. Valla🇺🇸
The chiral magnetic effect is the generation of electric current induced by chirality imbalance in the presence of magnetic field. It is a macroscopic manifestation of the quantum anomaly in relativistic field theory of chiral fermions (massless spin $1/2$ particles with a definite projection of spin on momentum) -- a dramatic phenomenon arising from a collective motion of particles and antiparticles in the Dirac sea. The recent discovery of Dirac semimetals with chiral quasi-particles opens a fascinating possibility to study this phenomenon in condensed matter experiments. Here we report on the first observation of chiral magnetic effect through the measurement of magneto-transport in zirconium pentatelluride, ZrTe_5. Our angle-resolved photoemission spectroscopy experiments show that this material's electronic structure is consistent with a 3D Dirac semimetal. We observe a large negative magnetoresistance when magnetic field is parallel with the current. The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect. The observed phenomenon stems from the effective transmutation of Dirac semimetal into a Weyl semimetal induced by the parallel electric and magnetic fields that represent a topologically nontrivial gauge field background.
↳ cond-mat.str-elhep-phnucl-exnucl-thNat.Phys.(2016)·599 citations
07
Recent Results of the Hadron Resonance Gas Model and the Chemical Freeze-out of Strange Hadrons
K. A. Bugaev🇺🇦 · A. I. Ivanytskyi🇺🇦 · D. R. Oliinychenko🇺🇦 · E. G. Nikonov🇷🇺 · V. V. Sagun🇺🇦 · G. M. Zinovjev🇺🇦
A detailed discussion of recent results obtained within the hadron resonance gas model with the multi-component hard core repulsion is presented. Among them there are the adiabatic chemical freeze-out criterion, the concept of separate chemical freeze-out of strange particles and the effects of enhancement and sharpening of wide resonances and quark gluon bags occurring in a thermal medium. These findings are discussed in order to strengthen the planned heavy-ion collision experimental programs at low collision energies. We argue, that due to found effects, at the center of mass collision energy 4-8 GeV the quark gluon bags may appear directly or in decays as new heavy resonances with the narrow width of about 50-150 MeV and with the mass above 2.5 GeV.
↳ hep-phnucl-th5 citations
08
The equation of state in two-, three-, and four-color QCD at non-zero temperature and density
Tyler Gorda🇺🇸 · Paul Romatschke🇺🇸
We calculate the equation of state at non-zero temperature and density from first principles in two-, three- and four-color QCD with two fermion flavors in the fundamental and two-index, antisymmetric representation. By matching low-energy results (from a `hadron resonance gas') to high-energy results from (resummed) perturbative QCD, we obtain results for the pressure and trace anomaly that are in quantitative agreement with full lattice-QCD studies for three colors at zero chemical potential. Our results for non-zero chemical potential at zero temperature constitute predictions for the equation of state in QCD-like theories that can be tested by traditional lattice studies for two-color QCD with two fundamental fermions and four-color QCD with two two-index, antisymmetric fermions. We find that the speed of sound squared at zero temperature can exceed one third, which may be relevant for the phenomenology of high-mass neutron stars.
↳ hep-phhep-latnucl-thPRD(2015)·9 citations
09
The QCD equation of state to $O (μ_{B}^{4})$
Prasad Hegde (for the BNL-Bielefeld-CCNU collaboration)🇨🇳
We present results from an ongoing calculation of the QCD equation of state at finite baryon chemical potential $μ_{B}$ . We use the method of Taylor expansions to circumvent the sign problem and calculate the expansion coefficients to sixth order using HISQ fermions. We work at two lattice spacings, namely $N_{τ} = 6$ and 8 and, though we do not take the continuum limit, demonstrate that cutoff effects remain under control. We also use our results to construct an equation of state along the freeze-out curve. Using our sixth-order results as a cross-check, we demonstrate that our fourth-order equation of state is suitable for the modeling of dense matter created in heavy ion collisions with center-of-mass energies down to $s_{N N}^{1/2} \sim 20$ GeV.
↳ hep-lathep-phnucl-thPoS(2017)·10 citations
10
Hadron Masses in Strong Magnetic Fields
Hidetoshi Taya🇯🇵
Hadron masses under strong magnetic fields are studied. In the presence of strong magnetic fields exceeding the QCD energy scale $e B ≫ Λ_{QCD}^{2}$ , $SU (3)_{flavor} \otimes SU (2)_{spin}$ symmetry of hadrons is explicitly broken so that the quark components of hadrons differ from those with zero or weak magnetic fields $e B ≲ Λ_{QCD}^{2}$ . Also, squeezing of hadrons by strong magnetic fields affects the hadron mass spectrum. We develop a quark model which appropriately incorporates these features and analytically calculate various hadron masses including mesons, baryons and those with strangeness.
↳ hep-phnucl-thPRD(2015)·46 citations
11
Nonlinear susceptibilities under the framework of Dyson-Schwinger equations
A-Meng Zhao🇨🇳 · Zhu-Fang Cui🇨🇳 · Yu Jiang🇨🇳 · Hong-Shi Zong🇨🇳
Since the baryon-number susceptibilities are correlated with the cumulant of baryon-number fluctuations in experiments, we do calculations of the susceptibilities and compare them with the experimental fluctuation data under the framework of the Dyson-Schwinger equations (DSEs) approach. We compare our results with lattice QCD and experimental data at RHIC. The fitness of the results indicates that under the framework of DSEs, we can deal with the problems of heavy ion collisions properly.
↳ hep-phnucl-thPRD(2014)·30 citations
12
Non-Extensive Statistics, New Solution to the Cosmological Lithium Problem
J.J. He · S.Q. Hou · A. Parikh · D. Kahl · C.A. Bertulani · other collaborators
In the primordial Big Bang nucleosynthesis (BBN), only the lightest nuclides (D, $^{3}$ He, $^{4}$ He, and $^{7}$ Li) were synthesized in appreciable quantities, and these relics provide us a unique window on the early universe. Currently, BBN simulations give acceptable agreement between theoretical and observed abundances of D and $^{4}$ He, but it is still difficult to reconcile the predicted $^{7}$ Li abundance with the observation for the Galactic halo stars. The BBN model overestimates the primordial $^{7}$ Li abundance by about a factor of three, so called the cosmological lithium problem, a long-lasting pending issue in BBN. Great efforts have been paid in the past decades, however, the conventional nuclear physics seems unable to resolve such problem. It is well-known that the classical Maxwell-Boltzmann (MB) velocity distribution has been usually assumed for nuclei in the Big-Bang plasma. In this work, we have thoroughly investigated the impact of non-extensive Tsallis statistics (deviating from the MB) on thermonuclear reaction rates involved in standard models of BBN. It shows that the predicted primordial abundances of D, $^{4}$ He, and $^{7}$ Li agree very well with those observed ones by introducing a non-extensive parameter $q$ . It is discovered that the velocities of nuclei in a hot Big-Bang plasma indeed violate the classical Maxwell-Boltzmann (MB) distribution in a very small deviation of about 6.3--8.2%. Thus, we have for the first time found a new solution to the cosmological lithium problem without introducing any mysterious theories. Furthermore, the implications of non-extensive statistics in other exotic high-temperature and density astrophysical environments should be explored, which might offer new insight into the nucleosynthesis of heavy elements.
↳ astro-ph.COnucl-th0 citations
13
Possibility of ferromagnetic neutron matter
Koji Hashimoto🇯🇵
We study ferromagnetism at high density of neutrons in the QCD hadron phase, by using the simplest chiral effective model incorporating magnetic fields and the chiral anomaly. Under the assumption of spatial homogeneity, we calculate the energy density as a function of neutron density, with a magnetization and a neutral pion condensation a la Dautry and Neyman. We find that at a high density the energy of the ferromagnetic order is lower than that of the ordinary neutron matter, and the reduction effect is enhanced by the anomaly. Compared to the inhomogeneous phase with the alternating layer structure, our ferromagnetic phase turns out to be unfavored. However, once an axial vector meson condensation is taken into account in our simplest model, the ferromagnetic energy density is lowered significantly, which still leaves some room for a possible realization of a QCD ferromagnetic phase and ferromagnetic magnetars.
↳ hep-phnucl-thPRD(2015)·15 citations
14
Production of 92Nb, 92Mo, and 146Sm in the gamma-process in SNIa
T. Rauscher (1,2,3) · C. Travaglio (4) · R. Gallino (5) · N. Nishimura (3,6) · R. Hirschi (3,6,7) ((1) U Hertfordshire, UK, (2) U Basel, Switzerland, (3) UK Network for Bridging Disciplines of Galactic Chemical Evolution (BRIDGCE), (4) INAF-Astrophysical Observatory Turin, Italy, (5) U Turin, Italy, (6) Keele University, UK, (7) U Tokyo, Japan)
The knowledge of the production of extinct radioactivities like 92Nb and 146Sm by photodisintegration processes in ccSN and SNIa models is essential for interpreting abundances in meteoritic material and for Galactic Chemical Evolution (GCE). The 92Mo/92Nb and 146Sm/144Sm ratios provide constraints for GCE and production sites. We present results for SNIa with emphasis on nuclear uncertainties.
↳ astro-ph.HEastro-ph.SRnucl-thPoS(2015)·1 citation
15
Quantification of nuclear uncertainties in nucleosynthesis of elements beyond Iron
T. Rauscher
Nucleosynthesis beyond Fe poses additional challenges not encountered when studying astrophysical processes involving light nuclei. Generally higher temperatures and nuclear level densities lead to stronger contributions of transitions on excited target states. This may prevent cross section measurements to determine stellar reaction rates and theory contributions remain important. Furthermore, measurements often are not feasible in the astrophysically relevant energy range. Sensitivity analysis allows not only to determine the contributing nuclear properties but also is a handy tool for experimentalists to interpret the impact of their data on predicted cross sections and rates. It can also speed up future input variation studies of nucleosynthesis by simplifying an intermediate step in the full calculation sequence. Large-scale predictions of sensitivities and ground-state contributions to the stellar rates are presented, allowing an estimate of how well rates can be directly constrained by experiment. The reactions 185W(n,gamma) and 186W(gamma,n) are discussed as application examples. Studies of uncertainties in abundances predicted in nucleosynthesis simulations rely on the knowledge of reaction rate errors. An improved treatment of uncertainty analysis is presented as well as a recipe for combining experimental data and theory to arrive at a new reaction rate and its uncertainty. As an example, it is applied to neutron capture rates for the s-process, leading to larger uncertainties than previously assumed.
↳ nucl-exastro-ph.HEastro-ph.SRnucl-thPoS(2015)·0 citations
16
Status of the Thermal Model and Chemical Freeze-Out
J. Cleymans🇿🇦
A brief review is presented of the status of the thermal model and chemical freeze-out in relativistic heavy-ion collisions. Some interewsting aspects at lower energies are emphasized.
↳ hep-phnucl-thEPJ Web Conf.(2015)·10 citations
17
Flavor instabilities in the neutrino line model
Huaiyu Duan🇺🇸 · Shashank Shalgar🇺🇸
A dense neutrino medium can experience collective flavor oscillations through nonlinear neutrino-neutrino refraction. To make this multi-dimensional flavor transport problem more tractable, all existing studies have assumed certain symmetries (e.g., the spatial homogeneity and directional isotropy in the early universe) to reduce the dimensionality of the problem. In this work we show that, if both the directional and spatial symmetries are not enforced in the neutrino line model, collective oscillations can develop in the physical regimes where the symmetry-preserving oscillation modes are stable. Our results suggest that collective neutrino oscillations in real astrophysical environments (such as core-collapse supernovae and black-hole accretion discs) can be qualitatively different from the predictions based on existing models in which spatial and directional symmetries are artificially imposed.
↳ hep-phastro-ph.HEnucl-thPLB(2015)·90 citations

Affiliations

first authorsco-authorsvia INSPIRE