The new interaction suggested by the anomalous 8^8Be transition sets a rigorous constraint on the mass range of dark matter

The WIMPs are considered one of the favorable dark matter (DM) candidates, but as the upper bounds on the interactions between DM and standard model (SM) particles obtained by the upgraded facilities of DM direct detections get lower and lower. Researchers turn their attentions to search for less massive DM candidates, i.e. light dark matter of MeV scale. The recently measured anomalous transition in $^8$Be suggests that there exists a vectorial boson which may mediate the interaction between DM and SM particles. Based on this scenario, we combine the relevant cosmological data to constrain the mass range of DM, and have found that there exists a model parameter space where the requirements are satisfied, a range of $10.4 \lesssim m_{\phi} \lesssim$ 16.7 MeV for scalar DM, and $13.6 \lesssim m_{V} \lesssim$ 16.7 MeV for vectorial DM is demanded. Then a possibility of directly detecting such light DM particles via the DM-electron scattering is briefly studied in this framework.Comment: 13 Pages, 7 figures, references added, version accepted by journa

Development of a pneumatic GM cryocooler with dual-displacer

As an alternative to conventional low temperature superconducting (LTS) materials, high temperature superconducting (HTS) materials have the potential to significantly reduce the cooling cost due to their higher critical temperature. Usually proportional to the physical size of the cooling object, cost reduction is especially conspicuous in large scale applications including power transmission cable and power generator. On the other hand, relatively high cooling capacity, roughly around 102 W to 104 W, remains indispensable for most cryocooler consumers in HTS area. In order to fulfill this requirement, a single-stage, dual-displacer, pneumatic type GM cryocooler has been developed at Sumitomo Heavy Industries, Ltd.(SHI). GM type cryocoolers can be more compact and efficient compared with the currently available large-scale Turbo-Brayton cryocoolers. The main difficulty, however, is that the force needed to drive the displacer increases with the cooling capacity at the same time, which leads to the scale-up of the housing and consequently impairs the reliability of the whole system. To overcome this problem, a pneumatic type design is proposed in which the driving force is substituted by the pressure difference of helium gas. Additionally, a novel concept, called dual-displacer structure, is introduced to further increase the cooling efficiency. Since supply and discharge timing is reversed in the two expansion spaces, the pressure oscillation amplitude at the compressor side is reduced and thus the overall efficiency of the cryocooler is improved. A prototype unit was designed, built and tested in 2016. As preliminary results, a cooling capacity of 550W at 80K was achieved with an input power of about 13 kW. Detailed design concept and performance results will be presented in this report

Phase-field simulations for dripping-to-jetting transitions: Effects of low interfacial tension and bulk diffusion

The dripping-to-jetting transitions in coaxial flows have been experimentally well studied for systems of high interfacial tension, where the capillary number of the outer fluid and the Weber number of the inner fluid are in control. Recent experiments have shown that in systems of low interfacial tension, the transitions driven by the inner flow are no longer dominated by the inertial force alone, and the viscous drag force due to the inner flow is also quantitatively important. In the present work, we carry out numerical simulations based on the Cahn-Hilliard-Navier-Stokes model, aiming for a more complete and quantitative study that is needed for understanding the effects of interfacial tension when it becomes sufficiently low. The Cahn-Hilliard-Navier-Stokes model is solved by using an accurate and efficient spectral method in a cylindrical domain with axisymmetry, and numerical results obtained for jet and drop radii demonstrate the accuracy of our computation. Plenty of numerical examples are systematically presented to show the dripping-to-jetting transitions driven by the outer flow and inner flow respectively. In particular, for transitions dominated by inner flow, detailed results reveal how the magnitude of interfacial tension quantitatively determines the relative importance of the inertial and viscous forces due to the inner flow at the transition point. Our numerical results are found to be consistent with the experimental observation. Finally, the degree of bulk diffusion is varied to investigate its quantitative effect on the condition for the occurrence of transition. Such effect is expected for systems of ultralow interfacial tension where interfacial motion is more likely to be driven by bulk diffusion.Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Phys. Fluids 35, 074105 (2023

Optimal three-state field-free molecular orientation with terahertz pulses

We present a combined analytical and numerical investigation to show how an optimal control field can be designed to generate maximum field-free orientation of molecules for three populated rotational states. Based on a model involving pure rotational ladder-climbing excitation between rotational states, a set of optimal amplitude and phase conditions are analytically derived for the applied control fields. The maximum degree of orientation can be achieved when the field satisfies amplitude and phase conditions at the two transition frequencies. Multiple optimal solutions exist and to examine these conditions, we devise a quantum coherent control scheme using two terahertz pulses and successfully apply it to the linear polar molecule HCN at ultracold temperature. The sensitivity of both populations and phases of rotational states to control field parameters, i.e., the detuning, bandwidth, and time delay, is analyzed for understanding the optimal orientation mechanism. This work thus examines the frequency domain landscape belonging to optimal pulses.Comment: 21 pages and 10 figure