Branching Ratio and Polarization of B→ρ(ω)ρ(ω)B\to\rho(\omega)\rho(\omega) Decays in Perturbative QCD Approach

In this work, we calculated the branching ratios, polarization fractions and CP asymmetry of decay modes $B\to\rho(\omega)\rho(\omega)$ in the Perturbative QCD approach, which is based on $\mathbf{k}_T$ factorization. After calculation, we find the the branching ratios of $B^0 \to \rho^+ \rho^-$, $B^+ \to \rho^+ \rho^0$ and $B^+ \to \rho^+ \omega$ are at the order of $10^{-5}$, and their longitudinal polarization fractions are more than 90%. The above results agree with BarBar's measurements. We also predict the branching ratios and polarizations of $B^0 \to \rho^0 \rho^0$, $B^0 \to \rho^0 \omega$ and $B^0 \to \omega \omega$, which will be measured in future. We predicted the CP asymmetry of $B^0 \to \rho^+ \rho^-$ and $B^+ \to \rho^+ \rho^0$, which will play important role in determining angle $\alpha$.Comment: 13 pages, 4 figure

Performance Analysis of Millimeter Wave Massive MIMO Systems in Centralized and Distributed Schemes

This paper considers downlink multi-user millimeter-wave massive multiple-input multiple-output (MIMO) systems in both centralized and distributed configurations, referred to as C-MIMO and D-MIMO, respectively. Assuming the fading channel is composite and comprised of both large-scale fading and small-scale fading, a hybrid precoding algorithm leveraging antenna array response vectors is applied into both the C-MIMO system with fully connected structure and the D-MIMO system with partially connected structure. First, the asymptotic spectral efficiency (SE) of an arbitrary user and the asymptotic average SE of the cell for the C-MIMO system are analyzed. Then, two radio access unit (RAU) selection algorithms are proposed for the D-MIMO system, based on minimal distance (D-based) and maximal signal-to-interference-plus-noise-ratio (SINR) (SINR-based), respectively. For the D-MIMO system with circular layout and D-based RAU selection algorithm, the upper bounds on the asymptotic SE of an arbitrary user and the asymptotic average SE of the cell are also investigated. Finally, numerical results are provided to assess the analytical results and evaluate the effects of the numbers of total transmit antennas and users on system performance. It is shown that, from the perspective of the cell, the D-MIMO system with D-based scheme outperforms the C-MIMO system and achieves almost alike performance compared with the SINR-based solution while requiring less complexity.Peer reviewe

The Investigation of Hierarchical α Microstructure in the Metastable β Titanium Alloys Using Advanced Electron Microscopy

Metastable β-titanium alloys are increasingly popular due to their excellent mechanical properties and biocompatibilities. They play a paramount role in aerospace and biomedical industries due to being light-weight, high-strength, and corrosion-resistant. The α microstructure in the β grains is crucial to determining the mechanical properties, such as strength and ductility. The morphology, distribution, texture, and volume fraction of α precipitates in metastable β titanium alloys can be tuned to achieve strength-ductility trade-off. Quite a few factors, including grain boundary, twinning boundary, dislocations, and metastable phases are able to assist the nucleation of α through different phase transformation mechanisms and thus can affect the α microstructure. Recently, a novel highly-indexed {10 9 3}β type twinning was found in the metastable β Ti-5Al-5Mo-5V-3Cr (Ti-5553, wt.%) commercial alloy, widely used in aerospace and biomedical fields, such as landing gears of Boeing 787 Dreamliner airplane and artificial joints. With the pre-formed of highly-indexed twinning in Ti-5553 alloy, hierarchical α microstructure can be generated via isothermal aging. This kind of α microstructure is related to achieving the strength-ductility trade-off. The hierarchal α microstructure combines coarse α layers, alpha sub-layers, and fine-scaled α precipitates. This study utilizes advanced electron microscopy techniques such as scanning electron microscopy and transmission electron microscopy, combined with machine-learning-based microstructure quantifications, to characterize the hierarchical α microstructure in Ti-5553 alloy. There are three aspects to be discussed: the highly-indexed {10 9 3}β type twinning in Ti-5553 alloys, which possesses hierarchical substructure with different metastable phases; the α microstructure without the influence of pre-formed highly-indexed twinning, where heating rates plays essential roles in α phase transformation mechanisms; the hierarchical α microstructure with three types of α precipitates influenced by the pre-formed highly-indexed twinning. Overall, this study presents the hierarchical α microstructure in Ti-5553 alloy, and explores the influence of highly-indexed {10 9 3}β type in the α phase transformation pathways. It can help understanding α microstructural evolutions and tuning α microstructures with different pre-formed interfaces/metastable phases in metastable β titanium alloys