Measurement of polarization-transfer to bound protons in carbon and its virtuality dependence

Abstract

We measured the ratio P x /P z of the transverse to longitudinal components of polarization transferred from electrons to bound protons in 12 C by the 12 C( e, e p) process at the Mainz Microtron (MAMI). We observed consistent deviations from unity of this ratio normalized to the free-proton ratio, (P x /P z )12 C /(P x /P z )1 H , for both s-and p-shell knocked out protons, even though they are embedded in averaged local densities that differ by about a factor of two. The dependence of the double ratio on proton virtuality is similar to the one for knocked out protons from 2 H and 4 He, suggesting a universal behavior. It further implies no dependence on average local nuclear density.

The influence of Fermi motion on the comparison of the polarization transfer to a proton in elastic e→p and quasi-elastic ePaul1, Brecelj2, Arenhövel3 et al. 2019Physics Letters BSelf Cite61170View full textAdd to dashboardCite

“…In these experiments, the comparison to the free proton was done either by measurements of the polarization transfer ratio to 1 H at similar kinematics [17,18], or by calculations using Eq. 1 and a fit to the world data of proton FFs [11,22], which were used in [13,14,16].…”

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

“…Following the convention used in [13][14][15]27] we use the struck-proton virtuality ("off-shellness"), ν, as our parameter of choice. The virtuality is defined as [13]:…”

“…7, we find that the double ratio in the new 2 H data agrees very well with the 4 He and 12 C in the positive p miss region, just as it was shown for the earlier 2 H data in Refs. [15,27] [13,14] (green, open symbols), s-shell knockout in 12 C data sets [15] (black filled symbols), and 4 He data [18] (yellow filled squares). In each of these ratios, the denominator is calculated using the "movingproton" prescription [27,28].…”

“…Quasi-free polarization-transfer experiments have been carried out on 2 H and 12 C at the Mainz Microtron (MAMI) [13][14][15], as well as on 2 H, 4 He and 16 O at Jefferson Lab (JLab) [16][17][18][19], in search of medium modification in the proton's internal structure. These include measurements on far off-shell nucleons, characterized by high missing momentum, which is equivalent (neglecting final-state interactions (FSI)) to the struck protons having high initial momentum.…”

“…See the supplementary material 6. Polarization transfer in p 3/2 -shell knockout was also measured in[15]; however, sinceFig. 7compares the carbon data set with 2 H and4 He, which only contain s-shell protons, only the s-shell knockout from carbon is shown in the figure.…”