arXiv subjects

Feng Yuan

Publications and source records attributed to Feng Yuan.

At least 379 records · Page 21Linked to original sources

arXiv2003

Why only half of the fermionic atoms were converted to molecules by a Feshbach resonance?

In some recent experiments an ultracold gas of $^{40}$K$_2$ (or $^{6}$Li$_2$) molecules has been produced from a degenerate two-component Fermi gas of $^{40}$K (or $^{6}$Li) atoms by adiabatic passage through a Feshbach resonance. The maximum atom-molecule transfer efficiency is reported to be about 50%. We propose a simple microscopic model to characterize the ground state of the gas in the vicinity of the resonance, and show that the term describing the atom-molecule coupling is responsible for the observed efficiency 50%. Our result also suggests that the experiments have produced a molecular condensate.

cond-mat.stat-mech↗

arXiv2003

Sivers Function in the MIT Bag Model

The Sivers function, an asymmetric transverse-momentum distribution of the quarks in a transversely polarized nucleon, is calculated in the MIT bag model. The bag quark wave functions contain both $S$-wave and $P$-wave components, and their interference leads to nonvanishing Sivers function in the presence of the final state interactions. We approximate these interactions through one-gluon exchange. An estimate of another transverse momentum dependent distribution $h_1^\perp$ is also performed in the same model.

hep-ph↗

arXiv2003

Quark Imaging in the Proton Via Quantum Phase-Space Distributions

We develop the concept of quantum phase-space (Wigner) distributions for quarks and gluons in the proton. To appreciate their physical content, we analyze the contraints from special relativity on the interpretation of elastic form factors, and examine the physics of the Feynman parton distributions in the proton's rest frame. We relate the quark Wigner functions to the transverse-momentum dependent parton distributions and generalized parton distributions, emphasizing the physical role of the skewness parameter. We show that the Wigner functions allow to visualize quantum quarks and gluons using the language of the classical phase space. We present two examples of the quark Wigner distributions and point out some model-independent features.

hep-ph↗

arXiv2003

Luminous hot accretion flows: thermal equilibrium curve and thermal stability

In a previous paper, we presented the global solution of a new accretion flow model, namely luminous hot accretion flows (LHAFs). In this {\em Letter}, we first show the corresponding thermal equilibrium curve of LHAFs in the mass accretion rate vs. surface density diagram. Then we examine its thermal stability again local perturbations. We find that LHAFs are thermally unstable when thermal conduction is neglected. However, when the accretion rate is not very large, the timescale of the growth of perturbations is longer than the accretion timescale, therefore the instability has no dynamical effect on the accretion flow. When the accretion rate is large, the perturbations can grow very fast at a certain radius. As a result, some cold clumps may form and the accretion flow will become multi-phase.

astro-ph↗

arXiv2003

Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A*

We investigate radiatively inefficient accretion flow models for Sgr A*, the supermassive black hole in our Galactic Center, in light of new observational constraints. Confirmation of linear polarization in the submm emission argues for accretion rates much less than the canonical Bondi rate. We consider models with low accretion rates and calculate the spectra produced by a hybrid electron population, consisting of both thermal and nonthermal particles. The thermal electrons produce the submm emission and can account for its linear polarization properties. As noted in previous work, the observed low-frequency radio spectrum can be explained if a small fraction ($\approx 1.5$%) of the electron thermal energy resides in a soft power-law tail. In the innermost region of the accretion flow, turbulence and/or magnetic reconnection events may occasionally accelerate a fraction of the electrons into a harder power-law tail. We show that the synchrotron emission from these electrons, or the Compton up-scattering of synchrotron photons by the same electrons, may account for the X-ray flares observed by {\it Chandra}.

astro-ph↗

arXiv2003

Perturbative QCD Analysis of the Nucleon's Pauli Form Factor F_2(Q^2)

We perform a perturbative QCD analysis of the nucleon's Pauli form factor $F_2(Q^2)$ in the asymptotically large $Q^2$ limit. We find that the leading contribution to $F_2(Q^2)$ has a $1/Q^6$ power behavior, consistent with the well-known result in the literature. Its coefficient depends on the leading- and subleading-twist light-cone wave functions of the nucleon, the latter describing the quarks with one unit of orbital angular momentum. We also derive at the logarithmic accurary the asymptotic scaling $F_2(Q^2)/F_1(Q^2) \sim (\log^2 Q^2/Λ^2)/Q^2$ which describes recent Jefferson Lab data well.

hep-ph↗

arXiv2003

Charge transport in underdoped bilayer cuprates

Within the t-J model, we study the charge transport in underdoped bilayer cuprates by considering the bilayer interaction. Although the bilayer interaction leads to the band splitting in the electronic structure, the qualitative behavior of the charge transport is the same as in the case of single layer cuprates. The conductivity spectrum shows a low-energy peak and the unusual midinfrared band. This midinfrared band is suppressed severely with increasing temperatures, while the resistivity in the heavily underdoped regime is characterized by a crossover from the high temperature metallic-like to the low temperature insulating-like behaviors, which are consistent with the experiments.

cond-mat.str-el↗

arXiv2003

Classification and Asymptotic Scaling of Hadron Light-Cone Wave-Function Amplitudes

We classify the hadron light-cone wave-function amplitudes in terms of parton helicity, orbital angular momentum, and quark flavor and color symmetries. We show in detail how this is done for the pion, $ρ$ meson, nucleon, and delta resonance up to and including three partons. For the pion and nucleon, we also consider four-parton amplitudes. Using the scaling law derived recently, we show how these amplitudes scale in the limit that all parton transverse momenta become large.

hep-ph↗

arXiv2003

Counting Rule for Hadronic Light-Cone Wave Functions

We introduce a systematic way to write down the Fock components of a hadronic light-cone wave function with $n$ partons and orbital angular momentum projection $l_z$. We show that the wave function amplitude $ψ_n(x_i,k_{i\perp},l_{zi})$ has a leading behavior $1/(k^2_\perp)^{[n+|l_z|+{\rm min}(n'+|l_z'|)]/2-1}$ when all parton transverse momenta are uniformly large, where $n'$ and $l_z'$ are the number of partons and orbital angular momentum projection, respectively, of an amplitude that mixes under renormalization. The result can be used as a constraint in modeling the hadronic light-cone wave functions. We also derive a generalized counting rule for hard exclusive processes involving parton orbital angular momentum and hadron helicity flip.

hep-ph↗

arXiv2002

Three-Quark Light-Cone Amplitudes of The Proton And Quark-Orbital-Motion Dependent Observables

We study the three-quark light-cone amplitudes of the proton including quarks' transverse momenta. We classify these amplitudes using a newly-developed method in which light-cone wave functions are constructed from a class of light-cone matrix elements. We derive the constraints on the amplitudes from parity and time-reversal symmetries. We use the amplitudes to calculate the physical observables which vanish when the quark orbital angular momentum is absent. These include transverse-momentum dependent parton distributions $Δq_T(x, k_\perp)$, $q_T(x, k_\perp)$, $δq(x, k_\perp)$, and $δq_L(x,k_\perp)$, twist-three parton distributions $g_T(x)$ and $h_L(x)$, helicity-flip generalized parton distributions $E(x, ξ=0, Q^2)$ and its associates, and the Pauli form factor $F_2(Q^2)$.

hep-ph↗

arXiv2002

Azimuthal asymmetry of J/Psi suppression in non-central heavy-ion collisions

The azimuthal asymmetry of $J/ψ$ suppression in non-central heavy-ion collisions is studied within a dynamic model of $J/ψ$ suppression in a deconfined partonic medium. Within this model, $J/ψ$ suppression in heavy-ion collisions is caused mainly by the initial state nuclear absorption and dissociation via gluon-$J/ψ$ scattering in deconfined partonic medium. Only the second mechanism gives arise to azimuthal asymmetry of the final $J/ψ$ production. We demonstrate that if there is an onset of suppression by quark-gluon plasma (QGP) in the NA50 data, it must be accompanied by the non-vanishing azimuthal asymmetry. Using the same critical density above which the QGP effect enters, we predict the azimuthal asymmetric coefficient $v_2$ as well as the survival probability for $J/ψ$ at the RHIC energy.

nucl-th↗

arXiv2002

Parton Distributions in Light-Cone Gauge: Where Are the Final-State Interactions?

We show that the final-state interaction effects in the single target spin asymmetry discovered by Brodsky et al. can be reproduced by either a standard light-cone gauge definition of the parton distributions with a prescription of the light-cone singularities consistent with the light-cone gauge link, or a modified light-cone gauge definition with a gauge link involving the gauge potential at the spatial infinity.

hep-ph↗

arXiv2002

Convergence of energy-dependent incommensurate antiferromagnetic neutron scattering peaks to commensurate resonance in underdoped bilayer cuprates

The recently discovered coexistence of incommensurate antiferromagnetic neutron scattering peaks and commensurate resonance in underdoped YBa$_2$Cu$_3$O$_{6+x}$ is calling for an explanation. Within the t-J model, the doping and energy dependence of the spin dynamics of the underdoped bilayer cuprates in the normal state is studied based on the fermion-spin theory by considering the bilayer interactions. Incommensurate peaks are found at $[(1\pmδ)π,π] $ and $[π,(1\pmδ)π]$ at low energies with $δ$ initially increasing with doping at low dopings and then saturating at higher dopings. These incommensurate peaks are suppressed, and the parameter $δ$ is reduced with increasing energy. Eventually it converges to the $[π,π]$ resonance peak. Thus the recently observed coexistence is interpreted in terms of bilayer interactions.

cond-mat.str-el↗

arXiv2002

NGC4258: a jet-dominated low-luminosity AGN?

Low-luminosity AGNs (LLAGNs) are a very important class of sources since they occupy a significant fraction of local galaxies. Their spectra differ significantly from the canonical luminous AGNs, most notably by the absence of the ``big blue bump''. In the present paper, taking a typical LLAGN--NGC4258--as an example, we investigate the origin of their spectral emission. The observational data of NGC4258 is extremely abundant, including water maser emission, putting very strict constraints to its theoretical models. The infrared (IR) spectrum is well described by a steep power-law form $f_ν\propto ν^{-1.4}$, and may extend to the optical/UV band. Up until now there is no model which can explain such a steep spectrum, and we here propose a coupled jet plus accretion disk model for NGC4258. The accretion disk is composed of an inner ADAF (or radiatively inefficient accretion flow) and an outer standard thin disk. A shock occurs when the accretion flow is ejected out of the ADAF to form the jet near the black hole, accelerating the electrons into a power-law energy distribution. The synchrotron and self-Comptonized emission from these electrons greatly dominates over the underlying accretion disk and can well explain the spectrum ranging from IR to X-ray bands. The further propagation of the shocked gas in the jet can explain the flat radio spectrum of NGC4258. Several predictions of our model are presented for testing against future observations, and we briefly discuss the application of the model to other LLAGNs.

astro-ph↗

arXiv2002

Scale Dependence of Hadronic Wave Functions and Parton Densities

We study how the components of hadronic wave functions in light-cone quantization depend on the ultraviolet cut-off by relating them in a systematic way to the matrix elements of a class of quark-gluon operators between the QCD vacuum and the hadrons. From this, we derive an infinite set of scale-evolution equations for the individual contributions to parton distributions from the Fock expansion. When summed over all the contributions, we recover the well-known DGLAP equation.

hep-ph↗

arXiv2002

Pseudogap effects on the charge dynamics in the underdoped copper oxide materials

Within the t-J model, the charge dynamics of copper oxide materials in the underdoped regime is studied based on the fermion-spin theory. It is shown that both in-plane charge dynamics and c-axis charge dynamics are mainly governed by the scattering from the in-plane fluctuation, which would be suppressed when the holon pseudogap opens at low temperatures, leading to the temperature linear to the nonlinear range in the in-plane resistivity and crossovers to the semiconducting-like range in the c-axis resistivity.

cond-mat.str-el↗

arXiv2002

Critical Correlations of Wilson Lines in SU(3) and the High Energy $γ^*p$ Cross Section

We discuss deep inelastic scattering at high energies as a critical phenomenon in 2+1 space - time dimensions. In the limit of Bjorken $x \to 0$, $QCD$ SU(3) with quark fields becomes a critical theory with a diverging correlation length $ξ(x) \propto x^{-\frac{1}{2 λ_2}}$ where the exponent $λ_2=2.52$ is obtained from the center group Z(3) of SU(3). We conjecture that the dipole wave function of the virtual photon for transverse sizes $1/Q<x_{\bot}<ξ$ obeys correlation scaling $Ψ\propto (x_{\bot})^{-(1+n)}$ before exponentially decaying for distances larger than the correlation length. Using this behavior combined with different $x$ -independent dipole proton cross sections we calculate the proton structure function and compare with the experimental data. We take the good agreement with the measured proton structure function F$_2(x,Q^2)$ as an indication that at high energies dimensional reduction to an effective three dimensional theory with a critical point occurs.

hep-ph↗

arXiv2002

Effect of the additional second neighbor hopping on the charge dynamics in the t-J model

The effect of the additional second neighbor hopping t' on the charge dynamics of the t-J model in the underdoped regime is studied within the fermion-spin theory. The conductivity spectrum of the t-t'-J model shows the low-energy peak and unusual midinfrared band, while the resistivity exhibits a nearly temperature linear dependence with deviation at low temperature in the underdoped regime. Although the qualitative feature of the charge dynamics in the t-t'-J model is the same as in the case of the t-J model, the additional second neighbor hopping t' leads to a clear shift of the position of the midinfrared band to the higher energies in the conductivity spectrum, and suppress the range of the deviation from the temperature linear dependence in the resistivity.

cond-mat.str-el↗