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

## TL;DR

This study measures polarization transfer in deuteron electrodisintegration at high missing momentum, finding results consistent with free proton behavior and setting limits on medium modifications of nucleon form factors.

## Contribution

It provides new polarization transfer data at large missing momentum in deuteron, supporting the absence of significant medium modifications and validating theoretical models.

## Key findings

- Measured polarization transfer components agree with free-proton calculations.
- Established upper limits on medium modifications of proton form factors.
- Observed universal behavior across different nuclei in polarization ratios.

## Abstract

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.