Polarizations for 12C(p, 2p) Reactions at 1 GeV
H. P. Yoshida∗ , T. Noro† , O. V. Miklukho∗∗ , V. A. Andreev∗∗ ,
M. N. Andronenko∗∗ , G. M. Amalsky∗∗ , S. L. Belostotski∗∗ ,
O. A. Domchenkov∗∗ , O. Ya. Fedorov∗∗ , K. Hatanaka∗, A. A. Izotov∗∗ ,
A. A. Jgoun∗∗ , J. Kamiya∗ , A. Yu. Kisselev∗∗ , M. A. Kopytin∗∗ ,
E. Obayashi∗ , A. N. Prokofiev∗∗ , D. A. Prokofiev∗∗ , H. Sakaguchi‡ ,
V. V. Sulimov∗∗ , A. V. Shvedchikov∗∗ , H. Takeda‡, S. I. Trush∗∗ ,
V. V. Vikhrov∗∗ , T. Wakasa∗, Y. Yasuda‡ and A. A. Zhdanov∗∗
∗
Research Center for Nuclear Physics, Osaka University, Ibaraki 567-0047, Japan
†
Department of Physics, Kyushu University, Fukuoka 812-8581, Japan
∗∗
Petersburg Nuclear Physics Institute, , Gatchina 188350, Russia
‡
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
Abstract.
We have measured polarizations P of outgoing protons in (p, 2p) reactions at an incident energy
of 1 GeV for three kinds of targets. The experimental result shows a distinct reduction from IA
calculation values using NN interaction in free space and the reduction is found to be monotonic
to the effective mean density estimated with DWIA. This is consistent with the previous result
obtained at 392 MeV, though the incident energy is quite different. We have also measured an
angular distribution of the polarization for 12 C target. All of the data, including both for forward nd
backward outgoing protons, show similar reduction from the IA calculation, though, again, outgoing
energies are quite different from 130 MeV to 890 MeV. From these results, it is concluded that these
reductions are nuclear structure or interaction originated and not caused by the reaction mechanism
such as multi-step processes or distortions.
INTRODUCTION
It is a long standing problem that the analyzing power A y for proton quasifree scattering
is reduced from values predicted with NN interactions in free space. Since theoretical calculations based on the Schrödinger equation have failed to reproduce this phenomenon, it has been taken interest in as a phenomenon which shows appearance of a
relativistic effect or a medium-modification effect in a hadron level.
The (p, 2p) reaction, where both of two outgoing nucleons are detected, this reaction
is regarded as an NN scattering in nuclear field. Thus we expect to extract information
on modification of the NN interaction in nuclear field from direct comparison of this
reaction with the NN scattering in free space.
By a TRIUMF group, it is found that the analyzing power A y for this reaction leading
to a 1s1/2 -hole state shows a distinct reduction from the free p–p scattering values.[1, 2]
At RCNP, Ay of this reaction was measured for several target nuclei at 392 MeV and it
was pointed out that this reduction is monotonically depends on the averaged density,
which was estimated by using the DWIA and the local density approximation.[3]
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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C-block
PC2
PC1
S1
D
Beam dump
M1-M3
C-block S2 S1
D
Q2
Q1 Col
PC1
PC2
PC3
MAP spectrometer
PC4
PC4
S2 PC3
NES spectrometer
Q2
Q1
Col
Scattering chamber
1GeV beam
FIGURE 1. Schematic view of the two arm spectrometer system at PNPI. Four sets of MWPC’s (PC1–
PC4), two trigger scintillators (S1,S2) and a carbon analyzer block form a focal-plane polarimeter system
on each spectrometer. Each of PC1–PC4 consists of two MWPC’s for measurements of horizontal and
vertical positions. Collimeters are used in front of two spectrometers and luminosity is monitored by
using the beam-monitor which consists of three scintillators (M1–M3).
In this article, we show our new measurement of polarizations P for the (p, 2p)
reactions at 1 GeV, which is a significantly different energy from previous measurements
at TRIUMF and RCNP. In addition, we have also measured an angular distribution of
polarizations for 12 C target, where outgoing energies changes significantly, from 740
MeV to 890 MeV for forward outgoing protons and from 130 MeV to 265 MeV for
backward outgoing protons.
A discussion is given on the reaction mechanism of this reaction, which is helpful in
investigating by measuring polarizations at this higher energy and in such a wide energy
range.
EXPERIMENT AND RESULT
Experimental detail
The experiment has been performed by using a 1 GeV unpolarized proton beam from
the synchrocyclotron at Petersburg Nuclear Physics Institute(PNPI) in Gatchina. The
beam line we used equipped with the two-arm spectrometer system which consists of
two QQD-type magnetic spectrometers, called MAP and NES. Each spectrometer has a
polarimeter system using a carbon block analyzer on the focal plane and polarizations
of both the forward and backward outgoing protons were measured. MAP analyses The
forward scattered protons with higher energies were analyzed with MAP, while the
backward ones with lower energies were analyzed with NES. The schematic view of
the detection system is shown in Fig. 1.
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Yields (a.u.)
p-p
6Li(p,2p)5 He
3/2-
4.3MeV
3/2+
0
20
40
60
12 C(p,2p)11 B
3/2-
1/2+
Yields (a.u.)
0
20
0
20
40
60
80
Separation Energy (MeV)
FIGURE 2. Separation energy spectra for p–p scattering and (p, 2p) reactions. Each vertical axis is
linear scale in arbitrary unit. The states with arrows are used for data analysis.
Throughout the (p, 2p) measurement, setting angles and field strengths of spectrometers are kept to those corresponding to the zero-recoil condition for the s-shell knockout
from each target nucleus. This is the condition where the cross section for the s-shell
knockout gives a maximum and the reaction mechanism is expected to be the simplest.
Figure 2 shows typical spectra. The overall energy-resolution for the p–p measurement
is 4.3 MeV, as shown in the figure.
Experimental Result
The experimental result is shown in Fig. 3. The polarizations of forward and backward
outgoing protons are denoted as P1 and P2 , respectively.
The data in the left figure are plotted as a function of the averaged density, which
is defined in [3]. The solid line is a PWIA calculation by using the free NN interaction.
The calculation increases with the averaged density, caused by changes of the kinematics
in fact, while the experimental data, both of P1 and P2 , decrease. And the reduction, the
difference between the theoretical and experimental values, is monotonic to the averaged
density. This result is consistent with results obtained at 392MeV.[4]
The data in the right figure shows the angular distribution for the 12 C(p, 2p) reaction.
The data corresponding to the NES angle of 64.0, 59.7, and 53.3 degrees are plotted as
a function of transferred momentum q. The solid and dash lines are PWIA and DWIA
calculations by using the free NN interaction. It shows that the distortion effect, the
difference between DWIA and PWIA, is negligibly small for this kinematics. Though the
outgoing energies is quite different from 130 MeV to 890 MeV, the data show essentially
the same amount of reduction from IA calcurations, independent of energies.
732
0.6
IA
40
Ca
(2s 1/2 )
0.1
6
Li
(1s 1/2 )
P1
12
C
(1s 1/2 )
Polarization
0.2
0.4
0.2
12
P2
0
0
0.1
0.2
P1
P2
DWIA
PWIA
0.3
p+p
Polarization
0.4
0
0.3
Averaged Density ( ρ/ρ0)
C
(1s 1/2)
2
3
4
Momentum transfer q (fm-1)
FIGURE 3. The left panel: Target dependence of polarization data P for p–p scattering and (p, 2p)
reactions. The data are plotted as a funcion of the averaged density. The right panel: Angular dependence
of P for 12 C(p, 2p) reaction. The data are plotted as a funcion of momentum transfer values. In both
panels, open circles are polarizations P 1 for forward outgoing protons and black circles are polarizations
P2 for backward ones.
Reaction Mechanism
In the case of nuclear reactions by using hadron projectile, contribution of multi-step
processes should be considered carefully. In this measurement at 1 GeV, the incident
energies are quite different from previous experiment at 392 MeV and the energies of
two emitted protons are also quite different as well. But the result of present data at
1GeV is consistent with that at 392 MeV. This results show that contributions by multistep processes are not the main reason of the reaction.
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Hatanaka,K., et al., Phys. Lev. Lett. 78, 1014–1017 (1997).
Noro, T. et al., Nucl. Phys. A629, 324c–333c (1998).
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