arXiv subjects

Lang Yu

Publications and source records attributed to Lang Yu.

32 records · Page 2Linked to original sources

arXiv2020

Ellipticity of photon emission from strongly magnetized hot QCD plasma

By making use of an explicit representation for the imaginary part of the photon polarization tensor in terms of transitions between the Landau levels of light quarks, we study the angular dependence of direct photon emission from a strongly magnetized quark-gluon plasma. Because of the magnetic field, the leading order photon rate comes from the three processes of the zeroth order in the coupling constant $α_s$: (i) the quark splitting ($q\rightarrow q+γ$), (ii) the antiquark splitting ($\bar{q} \rightarrow \bar{q}+γ$), and (iii) the quark-antiquark annihilation ($q + \bar{q}\rightarrow γ$). In a wide range of moderately high temperatures, $T\gtrsim m_π$, and strong magnetic fields, $|eB|\gtrsim m_π^2$, the direct photon production is dominated by the two splitting processes. We show that the Landau-level quantization of quark states plays an important role in the energy and angular dependence of the photon emission. Among other things, it leads to a nontrivial momentum dependence of the photon ellipticity coefficient $v_2$, which takes negative values at small transverse momenta and positive values at large transverse momenta. The crossover between the two regimes occurs around $k_T\simeq \sqrt{|eB|}$. In application to heavy-ion collisions, this suggests that a large value of $v_2$ for the direct photons could be explained in part by the magnetic field in the quark-gluon plasma.

hep-ph↗

arXiv2020

Assessing Phrasal Representation and Composition in Transformers

Deep transformer models have pushed performance on NLP tasks to new limits, suggesting sophisticated treatment of complex linguistic inputs, such as phrases. However, we have limited understanding of how these models handle representation of phrases, and whether this reflects sophisticated composition of phrase meaning like that done by humans. In this paper, we present systematic analysis of phrasal representations in state-of-the-art pre-trained transformers. We use tests leveraging human judgments of phrase similarity and meaning shift, and compare results before and after control of word overlap, to tease apart lexical effects versus composition effects. We find that phrase representation in these models relies heavily on word content, with little evidence of nuanced composition. We also identify variations in phrase representation quality across models, layers, and representation types, and make corresponding recommendations for usage of representations from these models.

cs.CL↗

arXiv2020

The pole and screening masses of neutral pion in hot and magnetized medium: a comprehensive study in the Nambu--Jona-Lasinio model

In this work, we investigate not only the pole masses but also the screening masses of neutral pions at finite temperature and magnetic field by utilizing the random phase approximation (RPA) approach in the framework of the two-flavor Nambu--Jona-Lasinio (NJL) model. And two equivalent formalisms in the presence of a magnetic field, i.e. the Landau level representation (LLR) and the proper-time representation (PTR), are applied to obtain the corresponding analytical expressions of the polarization functions (except the expressions for the pole masses in the PTR). In order to evaluate the applicable region of the low-momentum expansion (LME), we compare the numerical results within the full RPA (FRPA) with those within the reduced RPA (RRPA), i.e. the RPA in the LME. It is confirmed that the pole masses of π0in the FRPA suffer a sudden mass jump at the Mott transition temperature when in the presence of external magnetic field, and the Mott transition temperature is catalyzed by the magnetic field. And by analyzing the behaviors of the directional sound velocities of π0, which are associated with the breaking of the Lorentz invariance by the heat bath and the magnetic field, we clarify the two problems existing in previous literatures: one is that the transverse sound velocities in the medium are always larger than unity and thus violate the law of causality on account of the non-covariant regularization scheme, the other is that the longitudinal sound velocities are identically equal unity at finite temperature on account of the limitation of the derivative expansion method used.

hep-ph↗

arXiv2018

Neutral and charged (pseduo)scalar mesons and diquarks under magnetic fields

We investigate both (pseudo)scalar mesons and diquarks in the presence of external magnetic field in the framework of the two-flavored Nambu--Jona-Lasinio (NJL) model, where mesons and diquarks are constructed by infinite sum of quark-loop chains by using random phase approximation. The polarization function of the quark-loop is calculated to the leading order of $1/N_c$ expansion by taking the quark propagator in the Landau level representation. We systematically investigate the masses behaviors of scalar $σ$ meson, neutral and charged pions as well as the scalar diquarks, with respect to the magnetic field strength at finite temperature and chemical potential. It is shown that the numerical results of both neutral and charged pions are consistent with the lattice QCD simulations. The mass of the charge neutral pion keeps almost a constant under the magnetic field, which is preserved by the remnant symmetry of QCD$\times$QED in the vacuum. The mass of the charge neutral scalar $σ$ is around two times quark mass and increases with the magnetic field due to the magnetic catalysis effect, which is an typical example showing that the polarized internal quark structure cannot be neglected when we consider the meson properties under magnetic field. For the charged particles, the one quark-antiquark loop contribution to the charged $π^{\pm}$ increases essentially with the increase of magnetic fields due to the magnetic catalysis of the polarized quarks. However, the one quark-quark loop contribution to the scalar diquark mass is negative comparing with the point-particle result and the loop effect is small.

hep-ph↗

arXiv2016

Possible formation of high temperature superconductor at early stage of heavy-ion collisions

We investigate the effect of the inverse magnetic catalysis (IMC) on the charged $ρ$ meson condensation at finite temperature in the framework of the Nambu--Jona-Lasinio model, where mesons are calculated to the leading order of $1/N_c$ expansion by summing up infinity quark-loops. IMC for chiral condensate has been considered in three different ways, i.e. fitting Lattice data, using the running coupling constant and introducing the chiral chemical potential, respectively. It is observed that, with no IMC effect included, the critical magnetic field $eB_c$ for charged $ρ$ condensation increases monotonically with the temperature. However, including IMC substantially affects the polarized charged $ρ$ condensation around the critical temperature $T_c$ of chiral phase transition, the critical magnetic field $eB_c$ for charged $ρ$ condensation decreases with the temperature firstly, reaches to a minimum value around $T_c$, then increases with the temperature. Our calculation indicates that the charged $ρ$ condensation can exist in the temperature region of $1-1.5 T_c$ with critical magnetic field $eB_c\sim 0.15-0.3 {\rm GeV}^2$, which suggests that high temperature superconductor might be created through non-central heavy ion collisions at LHC energies. We also show that a growing electric conductivity in early stage of non-central heavy-ion collisions substantially delays the decay of strong magnetic field, which is helpful for the formation of the high temperature superconductor.

hep-ph↗

arXiv2015

The effect of the chiral chemical potential on the chiral phase transition in the NJL model with different regularization schemes

We study the chiral phase transition in the presence of the chiral chemical potential $μ_5$ using the two-flavor Nambu--Jona-Lasinio model. In particular, we analyze the reason why one can obtain two opposite behaviors of the chiral critical temperature as a function of $μ_5$ in the framework of different regularization schemes. We compare the modifications of the chiral condensate and the critical temperature due to $μ_5$ in different regularization schemes, analytically and numerically. Finally, we find that, for the conventional hard-cutoff regularization scheme, the increasing dependence of the critical temperature on the chiral chemical potential is an artifact, which is caused by the fact that it does not include complete contribution from the thermal fluctuations. When the thermal contribution is fully taken into account, the chiral critical temperature should decrease with $μ_5$.

hep-ph↗

arXiv2015

Survival of charged rho condensation at high temperature and density

The charged vector $ρ$ mesons in the presence of external magnetic fields at finite temperature $T$ and chemical potential $μ$ have been investigated in the framework of the Nambu--Jona-Lasinio model. We compute the masses of charged $ρ$ mesons numerically as a function of the magnetic field for different values of temperature and chemical potential. The self-energy of the $ρ$ meson contains the quark-loop contribution, i.e. the leading order contribution in $1/N_c$ expansion. The charged $ρ$ meson mass decreases with the magnetic field and drops to zero at a critical magnetic field $eB_c$, which means that the charged vector meson condensation, i.e. the electromagnetic superconductor can be induced above the critical magnetic field. Surprisingly, it is found that the charged $ρ$ condensation can even survive at high temperature and density. At zero temperature, the critical magnetic field just increases slightly with the chemical potential, which indicates that the charged $ρ$ condensation might occur inside compact stars. At zero density, in the temperature range $0.2-0.5~ {\rm GeV}$, the critical magnetic field for charged $ρ$ condensation is in the range of $0.2-0.6~ {\rm GeV}^2$, which indicates that the high temperature electromagnetic superconductor could be created at LHC.

hep-ph↗

arXiv2014

Inverse Magnetic Catalysis in the three-flavor NJL model with axial-vector interaction

In this paper we explore the chiral phase transition in QCD within the three-flavor Nambu-Jona-Lasinio (NJL) model with a negative coupling constant in the isoscalar axial-vector channel, which is associated with a polarized instanton--anti-instanton molecule background. The QCD phase diagram described in this scenario shows a new first order phase transition around the transition temperature $T_c$ toward a phase without chiral condensates, but with nontrivial dynamic chiral chemical potentials for the light quarks, spontaneously giving rise to local $\mathcal {CP}$ violation and local chirality imbalance. The corresponding critical temperature $T_{5c}$ for this phase transition decreases with the magnetic field and it gives a natural explanation to the inverse magnetic catalysis effect for light quarks when incorporating a reasonable value of the coupling constant in the isoscalar axial-vector channel. Furthermore, when the isoscalar axial-vector interaction is dominant in light quark sector and suppressed in strange quark sector, it is found that there is no inverse magnetic catalysis for strange quark condensate, which agrees with lattice results.

hep-ph↗

arXiv2014

Spontaneous generation of local CP violation and inverse magnetic catalysis

In the chiral symmetric phase, the polarized instanton--anti-instanton molecule pairing induces a nontrivial repulsive interaction in the iso-scalar axial-vector channel. As a consequence, one unusual property is observed that in the chiral restoration phase, there is a first order phase transition for the spontaneous generation of local CP violation and chiral imbalance. Furthermore, it is found that external magnetic fields will lower the critical temperature for the local CP-odd phase transition and catalyze the chiral imbalance, which destroys the chiral condensate with pairing quarks between different chiralities. A reasonable strength of the repulsive interaction in the iso-scalar axial-vector channel can naturally explain the inverse magnetic catalysis around the critical temperature under external magnetic fields.

hep-ph↗

arXiv2014

Charged and neutral vector meson under magnetic field

The vector meson $ρ$ in the presence of external magnetic field has been investigated in the framework of the Nambu--Jona-Lasinio model, where mesons are constructed by infinite sum of quark-loop chains by using random phase approximation. The $ρ$ meson polarization function is calculated to the leading order of $1/N_c$ expansion. It is found that the constituent quark mass increases with magnetic field, the masses of the neutral vector meson $ρ^{0}$ with spin component $s_z=0,\,\pm1$ and the charged vector meson $ρ^{\pm}$ with $s_z=0$ also increases with magnetic field. However, the mass square of the charged vector meson $ρ^{+}$ ($ρ^{-}$) with $s_z=+1$ ($s_z=-1$) decreases linearly with magnetic field and drops to zero at the critical magnetic field $e B_c \simeq 0.2 {\rm GeV}^2$, which indicates the possible condensation of charged vector meson in the vacuum. This critical magnetic field is much lower than the value $eB_c=0.6 {\rm GeV}^2$ predicted by a point-like vector meson. We also show that if we use lowest Landau level approximation, the mass of the charged vector meson $ρ^{\pm}$ for $s_z=\pm1$ cannot drop to zero at high magnetic fields.

hep-ph↗

arXiv2014

Zeta Function Regularization of Photon Polarization Tensor for a Magnetized Vacuum

In this paper, we have developed a systematic technique to regularize double summations of Landau levels and analytically evaluated the photon vacuum polarization at an external magnetic field. The final results are described by Lerch transcendent $Φ(z,s,v)$ or its $z$-derivation. We have found that the tensor of vacuum polarization is split into not only longitudinal and transverse parts but also another mixture component. We have obtained a complete expression of the magnetized photon vacuum polarization at any kinematic regime and any strength of magnetic field for the first time. In the weak $B$-fields, after canceling out a logarithmic counter term, all three scalar functions are limited to the usual photon polarization tensor without turning on magnetic field. In the strong $B$-fields, the calculations under Lowest Landau Level approximation are only valid at the region $M^2\gg q_{\shortparallel}^2$, but not correct while $q_{\shortparallel}^2\gg M^2$, where, an imaginary part has been missed. It reminds us, a recalculation of the gap equation under a full consideration of all Landau Levels is necessary in the next future.

hep-th↗

arXiv2012

Directional dependence of color superconducting gap in two-flavor QCD in a magnetic field

We study the effect of a magnetic field on the pairing dynamics in two-flavor color superconducting dense quark matter. The study is performed in the weakly coupled regime of QCD at asymptotically high density, using the framework of the Schwinger-Dyson equation in the improved rainbow approximation. We show that the superconducting gap function develops a directional dependence in momentum space. Quasiparticles with momenta perpendicular to the direction of the magnetic field have the largest gaps, while quasiparticles with momenta parallel to the field have the smallest gaps. We argue that the directional dependence is a consequence of a long-range interaction in QCD. The quantitative measure of the ellipticity of the gap function is determined by a dimensionless ratio, proportional to the square of the magnetic field and inversely proportional to the fourth power of the quark chemical potential. For magnetic fields in stars, $B\lesssim 10^{18} G$, the corresponding ratio is estimated to be less than about $10^{-2}$, justifying the use of the weak magnetic field limit in all stellar applications.

hep-ph↗

arXiv2009

Pion Electroproduction Amplitude Relations in the 1/N_c Expansion

We derive expressions for pion electroproduction amplitudes in the 1/N_c expansion of QCD, and obtain from them linear relations between the electromagnetic multipole amplitudes that hold at all energies. The leading-order relations in 1/N_c compare favorably with available data (especially away from resonances), but the next-to-leading-order relations tend to provide only small or no improvement.

hep-ph↗

arXiv2006

Radiative Decays of Decuplet Baryons, $Λ(1405)$ and $Λ(1520)$ Hyperons

The chiral quark model gives a reasonably good description of many low-energy observables by incorporating the effective degrees carried by the constituent quarks and Goldstone bosons. We calculate the decuplet to octet transition magnetic moments and the decay widths of several excited hyperons using this model. The various radiative decay widths from the chiral quark roughly agree with experimental data including the recent JLAB measurement.

hep-ph↗