X-ray Imaging through Machine LearningI 本周物理学讲座

1 X-ray Imaging through Machine Learning

报告人：Ge Wang，Rensselaer Polytechnic Institute

时间：8月9日（周四）15:00

单位：中科院高能所

地点：化学楼C305

Computer vision and image analysis are great examples of machine learning, especially deep learning. While computer vision and image analysis deal with existing images and produce features of these images (images to features), tomographic image reconstruction produces images of internal structures from measurement data, which are various features (attenuated/non-attenuated line integrals, Fourier/harmonic components, echoed/scattered/transmitted ultrasound signatures, diffused/excited/interfered light signals, and so on) of the underlying images (features to images). Recently, machine learning, especially deep learning, techniques are being actively developed worldwide for tomographic image reconstruction, which is a new area of research as shown by the perspective https://ieeexplore.ieee.org/document/7733110, the 20 high-quality papers included in the latest TMI special issue https://ieee-tmi.org/fast-facts/featured-article.asp?id=30&title=Special-Issue-on-Machine-Learning-for-Image-Reconstruction,-June-2018, as well as similar publications in other journals and conferences. In addition to well-known analytic and iterative methods for tomographic image reconstruction, machine learning is an emerging approach for image reconstruction, and likewise image reconstruction is a new frontier of machine learning. There are exciting research and application opportunities ahead for smart imaging and precision medicine. 

2Characterisation of gravity and Rossby modes in hundreds of Kepler γ Doradus stars

报告人：Gang LI

时间：8月7日（周二）12:00

单位：北京大学

地点：1st floor meeting room, KIAA

We present our analysis of ∼450 Kepler γ Doradus stars. These stars are pulsating mainly in gravity modes, but the periods are no longer equally-spaced due to core rotation and molecular weight gradients. So far we have discovered ∼600 period spacing patterns from ∼450 stars. These results constitute the largest γ Dor sample thus it is possible to reveal some new statistical relations. For dozens of slowly-rotating stars, we show how rotation distorts the asymptotic relation. In rapid rotators, we find that g-mode period spacings and their gradient (slope) are both positively correlated with period. Additionally, Rossby modes are seen in tens of stars. They have a negative correlation between the slope and mean period, which is twice the mean period of prograde gravity modes.  These stars are very useful for understanding the physics of rotation and angular momentum transport in main sequence stars, and can place much needed constraints on diffusive mixing and chemical gradients, whilst also providing stellar ages. As this is first sample with significant Rossby modes, we have the unprecedented opportunity to study these unfamiliar modes, like their instability strip, excitation mechanism and surface rotation.

3Quantum Dynamics ?Space-time Crystal and Bethe String States

报告人：CongjunWu

时间：8月8日（周三）10:00

单位：北京大学

地点：Room W563, Physics Building

We present recent works on two different aspects of quantum dynamics ? symmetry and strong correlations. For the symmetry aspect, we propose a new concept of ?space-time? crystal as a general framework for studying intertwined space-time periodic structures, which include both the static crystal and the Floquet lattice as special cases. A new mathematic structure of ?space-time? group is constructed to describe the symmetries of a space-time crystal, which augments the space group with non-symmorphic operations involving fractional translations along the time domain: ?time-screw rotation? and ?time-glide reflection?. Classifications for the 1+1 D and 2+1D space-time crystals (groups) are completed, and their consequences on dynamic band structures will be discussed. For the strong correlation aspect, we have studied the real frequency response at high energy which is a hard core problem of condensed matter physics. We studied the role of Bethe-string states in the quantum spin dynamics in antiferromagnetic spin chains in high magnetic fields based on algebraic Bethe ansatz via the form-factor formulae. Close to quantum criticality, the string excitations govern the quantum spin dynamics, whereas the fractional excitations, which are dominant at low energies, reflect the antiferromagnetic quantum fluctuations. These states have been recently observed in the electron-spin-resonance spectroscopy measurement on SrCo2V2O8. This work is helpful for experimental studies on spin dynamics in both condensed matter and cold atom systems beyond the low energy effective Luttinger liquid theory.

4 We don’t know how supernovae explode

报告人：Boaz Katz

时间：8月9日（周四）16:00

单位：北京大学

地点：KIAA-PKU Auditorium

How do supernovae explode? The simple truth is that we don't know.  While decades of observational and theoretical research have lead to many important breakthroughs in our understanding of these powerful explosions, for all we know, they need not occur. In this talk I will focus on type Ia supernovae and describe our recent ongoing efforts to find an answer to this long standing question.

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