No evidence for medium effects in the12C(e, e'p)11Bg.s. reaction

Abstract

Abstract. The 12C(e, e'p)llB~ reaction has been measured in parallel and anti-parallel kinematics over a p.~ range of -120<pm<120MeV/c at a central Tp of 90MeV. In contrast to previous measurements at NIKHEF, both kinematics can be described without enhancement of the transverse form factor of the bound proton (~/=0.98(5)). The spectroscopic factor for this transition as determined with the new Mainz data (S~= 1.54(5)) is 22% smaller than the NIKHEF value.PACS: 21.10.Pc; 25.30.Dh; 24.50. + g; 27.20. + n As part of the commisioning of the large-acceptance and high-resolution three-spectrometer setup [1] we have measured the 12C(e,e'p) reaction. Since a Carbon target is easy to handle and to manufacture, this reaction is a good choice for calibration purposes, though not such a large set of accurate 12C(e,e'p) data exists as is the case for the 12C(e,e') reaction [2]. However, there is one set of high quality data measured at NIKHEF [3]. For comparison and in order to have a good indication of the size of systematic errors it is important to measure the same reaction in similar kinematics at different experimental facilities.Using the 12C(e,e'p) reaction for calibration purposes also offers the opportunity to study the discrepancy between the cross sections measured at NIKHEF in parallel (p > q) and anti-parallel (p < q) kinematics, an effect observed only for proton knock-out from 1~C. In reference [3] van der Steenhoven et al. interpreted this discrepancy as a possible signature of the enhancement of the transverse form factor of a proton bound in the nuclear medium. Because of the great importance of the question of nuclear medium effects, it is of interest to measure again this reaction at the facility in Mainz with its 100%-duty-cycle accelerator MAMI.The relevant parameters of the experimental setup in Mainz have been discussed in reference [4], describing an 160(e,e'p) experiment. There we concluded that the systematic uncertainty in the (e,e'p) cross section was 8% (adding up linearly the different contributions) with * E-mail address! offerman @kph.uni-mainz.de the main contribution coming from the target thickness (4%). Now we estimate a target thickness uncertainty of 3%, thereby arriving at a total systematic uncertainty of 7%. The target thickness was determined by measuring its weight and area This value was checked through elastic electron scattering from 12C. Data acquisition and analysis proceeded along the same lines as in the 160 experiment. In table I the central kinematical settings are given. Note that settings 1 and 2 were performed in parMlel kinematics while in setting 3 data was accumulated in anti-parallel kinematics. In settings 1 and 3 maximum solid-angle acceptances were used in both electron (5.6 msr) and proton (28 msr) spectrometer. Setting 2 has a large kinematical overlap with setting 1. In order to check the luminosity and the large proton acceptance we changed the target thickness and reduced the proton solid angle to 15 msr (see table I).In Figure 1 the measured distorted momen...

Transparency of12Cfor protons

“…In order to further corroborate this finding three new 12 C(e, e ′ p) measurements were performed at the AmPS facility of NIKHEF in kinematics that were chosen, as close as possible, to resemble the kinematics used in Refs. [7] and [11]. The new data are not in agreement with the Mainz results, but are in good agreement with all other data sets available.…”

“…For these reasons we have re-analyzed all existing 12 C(e, e ′ p) data for knockout from the 1p-and 1s-shell that were taken in the quasi-elastic domain at Q 2 < 0.4 (GeV/c) 2 in one consistent approach. The results of this analysis indicate that the normalization of the Mainz data set [11] deviates with respect to all other existing data. In order to further corroborate this finding three new 12 C(e, e ′ p) measurements were performed at the AmPS facility of NIKHEF in kinematics that were chosen, as close as possible, to resemble the kinematics used in Refs.…”

“…* on leave of absence from PSU An early comparison of part of the world's 12 C(e, e ′ p) data for knockout from the 1p-shell in the quasi-elastic domain [12] demonstrated the mutual consistency of these data. On the other hand, recent 12 C(e, e ′ p) data collected in Mainz [11] suggest that the normalization of previous data was off by 22%. It is important to resolve this discrepancy for the following reasons.…”

“…The data set for such (e, e ′ p) measurements on 12 C in particular is quite sizable [3][4][5][6][7][8], possibly because the energy-resolution requirements are modest (≤ 1 MeV) and the target handling is easy. Therefore, it is no surprise that 12 C(e, e ′ p) measurements are often among the first calibration experiments to be carried out at new high-duty factor electron accelerators in the intermediate energy domain, such as AmPS [9], TJNAF [10] and Mainz [11]. * on leave of absence from PSU An early comparison of part of the world's 12 C(e, e ′ p) data for knockout from the 1p-shell in the quasi-elastic domain [12] demonstrated the mutual consistency of these data.…”

Importance of the direct knockout mechanism in relativistic calculations for (γ,p) reactions

“…This problem was considered further by van der Steenhoven [23] who found no justification for this enhancement factor. The newer data from Mainz on this nucleus, reported by Blomqvist et al [24] have a higher final proton kinetic energy of T p ≈ 90 MeV. In this case, with the increase in normalization of the data by a factor of 1.19 [25], both nonrelativistic and our relativistic calculations can describe the shape of the measured missing momentum distribution using the spectroscopic factor as obtained from matching to the positive missing momentum peak of the NIKHEF data.…”