Resonant amplification of quantum fluctuations in a spinor gas

Bose-Einstein condensates of atoms with non-zero spin are known to constitute an ideal system to investigate fundamental properties of magnetic superfluids. More recently it was realized that they also provide the fascinating opportunity to investigate the macroscopic amplification of quantum and classical fluctuations. This is strikingly manifested in a sample initially prepared in the m _F = 0 state, where spin-changing collisions triggered by quantum fluctuations may lead to the creation of correlated pairs in m _F = ±1. We show that the pair creation efficiency is strongly influenced by the interplay between the external trapping potential and the Zeeman effect. It thus reflects the confinement-induced magnetic field dependence of elementary spin excitations of the condensate. Remarkably, pair production in our experiments is therefore characterized by a multi-resonant dependence on the magnetic field. Pair creation at these resonances acts as strong parametric matter-wave amplifier. Depending on the resonance condition, this amplification can be extremely sensitive or insensitive to the presence of seed atoms. We show that pair creation at a resonance which is insensitive to the presence of seed atoms is triggered purely by quantum fluctuations and thus the system acts as a matter-wave amplifier for the vacuum state.     

Quark pair creation in color electric fields and effects of magnetic fields

The time evolution of a system where a uniform and classical SU(3) color electric field and quantum fields of quarks interact with each other is studied focusing on non-perturbative pair creation and its back reaction. We characterize a color direction of an electric field in a gauge invariant way, and investigate its dependence. Momentum distributions of created quarks show plasma oscillation as well as quantum effects such as the Pauli blocking and interference. Pressure of the system is also calculated, and we show that pair creation moderates degree of anisotropy of pressure. Furthermore, enhancement of pair creation and induction of chiral charge under a color magnetic field which is parallel to an electric field are discussed.     

Dynamically assisted Schwinger mechanism

We study electron-positron pair creation from the Dirac vacuum induced by a strong and slowly varying electric field (Schwinger effect) which is superimposed by a weak and rapidly changing electromagnetic field (dynamical pair creation). In the subcritical regime where both mechanisms separately are strongly suppressed, their combined impact yields a pair creation rate which is dramatically enhanced. Intuitively speaking, the strong electric field lowers the threshold for dynamical particle creation--or, alternatively, the fast electromagnetic field generates additional seeds for the Schwinger mechanism. These findings could be relevant for planned ultrahigh intensity lasers.     

推广的Glauber理论及其在晕核散射中的应用

介绍了推广到晕核散射的Glauber理论,并用其研究晕核¹⁴Be的散射问题.弹核的密度分布分别采用谐振子密度分布和相对论平均场理论计算得到具有两个晕中子结构的密度分布,对晕核模型的多重积分采用蒙特卡洛数值积分方法.计算了不同能量下¹⁴Be,¹²Be与靶核¹²C散射的反应截面,并与实验结果进行比较,¹⁴Be的两个中子采用具有晕中子密度分布的理论计算与实验符合较好,而采用不具有晕中子密度分布的结果与实验值相差较大.     

Holographic Schwinger effect

We study tunneling pair creation of W bosons by an external electric field on the Coulomb branch of N=4 supersymmetric Yang-Mills theory. We use AdS/CFT holography to find a generalization of Schwinger's formula for the pair production rate to the strong coupling, planar limit which includes the exchange of virtual massless particles to all orders. We find that the pair creation formula has an upper critical electric field beyond which the process is no longer exponentially suppressed. The value of the critical field is identical to that which occurs in the Born-Infeld action of probe D3-branes in the AdS(5)×S(5) background, where AdS(5) and S(5) are 5-dimensional anti-de Sitter space and the 5-sphere, respectively.     

Electron-positron pair creation in the electric fields generated by micro-bubble implosions

We show that electron-positron pair production from the vacuum is possible via the strong Coulomb fields generated by micro-bubble implosions induced by ultra-high intensity lasers. Even in the case where the Coulomb fields are lower than the pair creation threshold, externally injected high energy electrons or photons could be used to generate pairs.     

Reaction mechanisms for light halo nuclei

Current nuclear physics focuses on exploring nucleon matter under extreme conditions, such as those that can be created in modern accelerator laboratories. On the neutron-rich side of stability, radioactive beams have already led to the discovery of halos in nuclei with neutron distributions extending to large distances. Halo nuclei are composite systems with prominent features of few-body correlations, which reveal themselves in various reactions involving these systems. We will discuss experiments that probe a halo structure through studying various reactions involving halo nuclei, with special emphasis on how, from the theoretical point of view, such reactions contribute to our knowledge of the structure and dynamics of the nuclear halo.     

Artifact Halo Reduction in Phase Contrast Microscopy Using Apodization

A new phase contrast microscopy technique for halo reduction is proposed. This technique is based on an apodization method combined with the Zernike phase contrast method. Although it has been a difficult theoretical problem, the proposed technique achieves halo reduction by considering angles of diffraction and phase differences. The technique utilizes an apodized phase plate consisting of a quarter wave phase shift ring with a 25% transmittance, and a pair of adjacent rings, which have 50% transmittance. This element is placed at the back focal plane of the objective. The result is startling, halo reduced images of phase objects providing enhanced inner details.     

Electron-positron pair creation and Oleinik resonances

We consider the process of electron-positron pair creation by an impact of ultra-intense laser beam on a highly relativistic nucleus, with an exact account for its finite mass. It is shown that the probability rates of pair production increase tremendously due to the nuclear recoil. Numerical calculations also show the existence of very narrow resonances in differential probability rates of pair production, which are recognized to be Oleinik resonances, as they originate from poles of the photon propagator. Using our exact treatment of the colliding nucleus, we analyze also the effect of dressing it by the laser field on the process of pair creation.     

Nonlinear bound-free pair creation in the strong electromagnetic fields of a heavy nucleus and an intense x-ray laser

We study e(-)e(+) pair creation in the collision of a heavy nucleus with an intense x-ray laser, where the produced e(-) is simultaneously captured into the K shell of the projectile nucleus. The parameters of the colliding system are chosen such that the absorption of at least two photons from the laser wave is required in order to surmount the energy threshold of the reaction. Considering this fundamental nonlinear strong-field process for the first time, we estimate the total production rate as well as the angular and energetic distributions of the emitted positrons. According to our results, the process of nonlinear bound-free pair creation could become observable by the aid of the upcoming x-ray laser facilities.     

Extended Glauber Theory and Its Application in Halo Nucleus Scattering

We introduced an extended Glauber theory for a halo nucleus scattering, where the halo nucleons and the nuclear core were treated separately. Expressions of reaction and interaction cross sections of the halo nucleus scattering were derived. We took the halo structure of the projectile nucleus into account and adopted an improved optical limit approximation. In the framework of the extended Glauber theory, we studied the reaction cross section for the halo nucleus ¹⁴Be scattering on a target ¹²C. For comparison, the reaction cross sections of ¹²Be+¹²C were calculated as well. The density distribution of target ¹²C is taken from experiments, and those of the projectiles ¹²Be and ¹⁴Be were obtained by two methods. One is that the harmonic oscillator wave functions for ¹²Be and ¹⁴Be are used. The length of harmonic oscillator is adjusted to reproduce the reaction cross section of ¹²Be+¹²C at the high energy E=790MeV/u . The density distribution of ¹⁴Be was also calculated self-consistently in the relativistic mean field (RMF) theory, with a long tail wave functions for the two neutrons in ¹⁴Be. It was found that the calculated reaction cross sections for ¹²Be+¹²C at E=790MeV/u and E=56.5MeV/u were in good agreement with the experimental data no matter harmonic oscillator or RMF wave functions were used. In contrast, the experiments of the reaction cross sections for ¹⁴Be+ ¹²C could only be reproduced when the wave functions of two 2s_1/2 neutrons spreaded over with a long tail. It comes to a conclusion that two outside neutrons in ¹⁴Be form a halo structure.