Quasi-elastic polarization-transfer measurements on the deuteron in anti-parallel kinematics

We present measurements of the polarization-transfer components in the $^2$H$(\vec e,e'\vec p)$ reaction, covering a previously unexplored kinematic region with large positive (anti-parallel) missing momentum, $p_{\rm miss}$, up to 220 MeV$/c$, and $Q^2=0.65$ $({\rm GeV}/c)^2$. These measurements, performed at the Mainz Microtron (MAMI), were motivated by theoretical calculations which predict small final-state interaction (FSI) effects in these kinematics, making them favorable for searching for medium modifications of bound nucleons in nuclei. We find in this kinematic region that the measured polarization-transfer components $P_x$ and $P_z$ and their ratio agree with the theoretical calculations, which use free-proton form factors. Using this, we establish upper limits on possible medium effects that modify the bound proton's form factor ratio $G_E/G_M$ at the level of a few percent. We also compare the measured polarization-transfer components and their ratio for $^2$H to those of a free (moving) proton. We find that the universal behavior of $^2$H, $^4$He and $^{12}$C in the double ratio $\frac{(P_x/P_z)^A}{(P_x/P_z)^{^1\rm H}}$ is maintained in the positive missing-momentum region

Direct TPE Measurement via e+p/e−pe^+p/e^-p Scattering at low ε\varepsilon in Hall A

The proton elastic form factor ratio can be measured either via Rosenbluth separation in an experiment with unpolarized beam and target, or via the use of polarization degrees of freedom. However, data produced by these two approaches show a discrepancy, increasing with $Q^2$. The proposed explanation of this discrepancy - two-photon exchange - has been tested recently by three experiments. The results support the existence of a small two-photon exchange effect but cannot establish that theoretical treatments at the measured momentum transfers are valid. At larger momentum transfers, theory remains untested, and without further data, it is impossible to resolve the discrepancy. A positron beam at Jefferson Lab allows us to directly measure two-photon exchange over an extended $Q^2$ and $\epsilon$ range with high precision. With this, we can validate whether the effect reconciles the form factor ratio measurements, and test several theoretical approaches, valid in different parts of the tested $Q^2$ range. In this proposal, we describe a measurement program in Hall A that combines the Super BigBite, BigBite, and High Resolution Spectrometers to directly measure the two-photon effect. Though the limited beam current of the positron beam will restrict the kinematic reach, this measurement will have very small systematic uncertainties, making it a clean probe of two photon exchange.Comment: 8 pages, 6 figures, 1 tabl

Modeling and Simulation of Compliant Mechanisms in MATLAB Simscape

Compliant mechanisms are the mechanisms that transfer the input force, displacement, or torque from one point to another through the deformation of its compliant members and flexible body rather than the joints such as ball bearings. They have superiorities over the traditionally designed mechanisms such that since they can be designed and manufactured as a single piece using injection molding or additive manufacturing, they are light weighted, and no need for assembly and have no friction loss. Thus, the compliant mechanisms have better performance and accuracy and find application areas in the design of locomotive robots, grippers, medical robots, and microelectromechanical (MEMs) devices. Despite all the advantages, deriving the mathematical model of compliant mechanisms is much more challenging compared to traditional rigid body mechanisms as the complexity of the design increases. Although there are several methods available to find the load-deflection curves of flexible members such as pseudo rigid body modeling (PRBM) and the first and second of the Elliptica theory, they are limited to the simple geometries including fixed-free, fixed-guided, and fixed-fixed buckling beams. In this study, we present the design, modeling, and simulation of several compliant mechanisms in MATLAB Simscape. We adopted two approaches: the model can be created using the Simulink library blocks or by importing the cad model and then introducing the flexibility using discrete beam elements. We created the models of a fully compliant five-bar mechanism including 4 rigid bars connected by large deflecting flexure hinges, a compliant dwell mechanism incorporating buckling beams, a slider, and a rail, and a compliant bistable mechanism consisting of 6 rigid bars, a slider, and fixed-free flexible beams. The Simscape models not only provide kinematic insight but also visualizes the displacement and motion of each mechanism in the mechanics explorer

Determination of two-photon exchange via

The proton elastic form factor ratio shows a discrepancy between measurements using the Rosenbluth technique in unpolarized beam and target experiments and measurements using polarization degrees of freedom. The proposed explanation of this discrepancy is uncorrected hard two-photon exchange (TPE), a type of radiative correction that is conventionally neglected. The effect size and agreement with theoretical predictions has been tested recently by three experiments. While the results support the existence of a small two-photon exchange effect, they cannot establish that theoretical treatments are valid. At larger momentum transfers, theory remains untested. This proposal aims to measure two-photon exchange over an extended and so far largely untested $$Q^2$$ and $$\varepsilon$$ range with high precision using the CLAS12 experiment. Such data are crucial to clearly confirm or rule out TPE as the driver for the discrepancy as well as test several theoretical approaches, believed valid in different parts of the tested $$Q^2$$ range