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 pro… Show more
“…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].…”
A comparison between polarization-transfer to a bound proton in quasi-free kinematics by the A( e, e p) knockout reaction and that in elastic scattering off a free proton can provide information on the characteristics of the bound proton. In the past the reported measurements have been compared to those of a free proton with zero initial momentum. We introduce, for the first time, expressions for the polarization-transfer components when the proton is initially in motion and compare them to the 2 H data measured at the Mainz Microtron (MAMI). We show the ratios of the transverse (P x ) and longitudinal (P z ) components of the polarization transfer in 2 H( e, e p)n, to those of elastic scattering off a "moving proton", assuming the proton's initial (Fermi-motion) momentum equals the negative missing momentum in the measured reaction. We found that the correction due to the proton motion is up to 20% at high missing momentum. However the effect on the double ratio (Px/Pz) A (Px/Pz) 1 H is largely canceled out, as shown for both 2 H and 12 C data. This implies that the difference between the restingand the moving-proton kinematics is not the primary cause for the deviations between quasi-elastic and elastic scattering reported previously.
“…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].…”
A comparison between polarization-transfer to a bound proton in quasi-free kinematics by the A( e, e p) knockout reaction and that in elastic scattering off a free proton can provide information on the characteristics of the bound proton. In the past the reported measurements have been compared to those of a free proton with zero initial momentum. We introduce, for the first time, expressions for the polarization-transfer components when the proton is initially in motion and compare them to the 2 H data measured at the Mainz Microtron (MAMI). We show the ratios of the transverse (P x ) and longitudinal (P z ) components of the polarization transfer in 2 H( e, e p)n, to those of elastic scattering off a "moving proton", assuming the proton's initial (Fermi-motion) momentum equals the negative missing momentum in the measured reaction. We found that the correction due to the proton motion is up to 20% at high missing momentum. However the effect on the double ratio (Px/Pz) A (Px/Pz) 1 H is largely canceled out, as shown for both 2 H and 12 C data. This implies that the difference between the restingand the moving-proton kinematics is not the primary cause for the deviations between quasi-elastic and elastic scattering reported previously.
“…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]:…”
Section: Measured Polarization-transfermentioning
confidence: 99%
“…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].…”
Section: (Px/pz) 1 Hmentioning
confidence: 99%
“…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.…”
Section: Introductionmentioning
confidence: 99%
“…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.…”
We present measurements of the polarization-transfer components in the 2 H( e, e p) reaction, covering a previously unexplored kinematic region with large positive (anti-parallel) missing momentum, p miss , up to 220 MeV/c, and Q 2 = 0.65 (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 (Px/Pz) A (Px/Pz) 1 H is maintained in the positive missing-momentum region.
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