Quasielastic
À
-nucleus scattering at 950 MeVÕ c
Y. Fujii,
1,
* O. Hashimoto,
1
T. Nakagawa,
1
Y. Sato,
1
T. Takahashi,
1
J. T. Brack,
2
C. J. Gelderloos,
2
M. V. Keilman,
2
R. J. Peterson,
2
M. Itoh,
3
H. Sakaguchi,
3
H. Takeda,
3
K. Aoki,
4
H. Hotchi,
4
H. Noumi,
4
Y. Ohta,
4
H. Outa,
4
M. Sekimoto,
4
M. Youn,
4
S. Ajimura,
5
T. Kishimoto,
5
H. Bhang,
6
H. Park,
6
and R. Sawafta
7
1
Department of Physics, Tohoku University, Sendai 980-8578, Japan
2
Nuclear Physics Laboratory, University of Colorado, Boulder, Colorado 80309-0446
3
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
4
High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
5
Department of Physics, Osaka University, Osaka 560-0043, Japan
6
Department of Physics, Seoul National University, Seoul 151-742, Korea
7
Physics Department, North Carolina A & T State University, Greensboro, North Carolina27411
Received 14 May 1999; revised manuscript received 31 May 2000; published 22 August 2001
Quasielastic scattering cross sections have been measured with a 950 MeV/ c
-
beam on targets of
2
H,
6
Li, C, Ca, Zr, and
208
Pb, over a range of three-momentum transfers from 350 through 650 MeV/ c . Results for
carbon are compared to a finite-nucleus continuum random-phase approximation calculation including distor-
tions. The pion spectra at our lowest range of momentum transfers show less scalar/isoscalar correlation than
predicted.
DOI: 10.1103/PhysRevC.64.034608 PACS numbers: 25.80.Ls
I. INTRODUCTION
Quasielastic scattering is a process in which an incident
particle elastically and incoherently interacts with only one
nucleon inside a nucleus, with all the other nucleons being
spectators. At intermediate energies this process dominates
the nuclear response, showing two characteristic features:
one is that the position of the peak corresponds to that of
elastic scattering by a free nucleon, and the other is that the
width of the peak reflects internal motion of individual
nucleons in the target nucleus. Nuclear correlations can be
studied with this process, by measuring the distribution of
the quasielastic strength 1. Quasielastic scattering of elec-
trons has been extensively studied using their short wave-
lengths, deep penetration, and well-known couplings to
nucleons. However, the coupling of electrons to nucleons is
almost entirely electromagnetic, and hadronic probes are
needed to explore the full set of spin and isospin couplings.
A recent example is the study of isovector spin-transverse
and spin-longitudinal quasielastic scattering with the polar-
ized ( p , n ) reaction 2,3.
Mesonic probes can be used to study other couplings than
reached by lepton and baryon beams. Quasielastic noncharge
exchange NCX scattering by pion and K
+
beams occurs
largely through scalar-isoscalar couplings, while pion single-
charge exchange SCX acts through an isovector, largely
scalar, coupling. Both ( , ' ) NCX4 and (
,
0
)
SCX 5 reactions were measured using pions of
624 MeV/ c at LAMPF. The quasielastic peak positions for
the (
,
0
) reaction showed shifts toward higher energy
loss than for free
-
p scattering below about q
=400 MeV/ c , while no shifts were observed for the ( , ' )
reaction. This difference was tentatively explained as arising
from the difference in effective particle-hole interactions for
different channels. These studies demonstrated that pion
quasielastic scattering can be used to study nuclear responses
in the scalar-isoscalar and scalar-isovector channels. At large
momentum transfers, however, the outgoing pions from
those studies have such low energies as to be likely to inter-
act again to form the prominent resonance. A higher pion
beam energy is needed to avoid this complication and to
maintain a long mean free path within the nuclear medium.
The ( K
+
, K
+
' ) quasielastic reaction at 705 MeV/ c 6 was
measured at the Alternating Gradient Synchrotron AGS at
Brookhaven National Laboratory, taking advantage of the
small K
+
N cross sections to give a long mean free path
within nuclei to reach high densities of nucleons. Theoretical
calculations compared to these data were performed in a
finite-nucleus continuum random-phase approximation
RPA framework 7. The calculation described the experi-
mental results well and provided a constraint on the strength
of the effective particle-hole interaction in the scalar-
isoscalar channel. The model used in that calculation is also
applied below to the present data.
The present experiment is a study of quasielastic
-
-nucleus scattering at 950 MeV/ c , principally to investi-
gate the nuclear response using a scalar-isoscalar dominant
probe. An advantage of the present experiment at
950 MeV/ c compared to the previous one at 624 MeV/ c is
the greater predominance of the scalar-isoscalar channel. The
momentum transfer dependence of the spin/isospin content
of pion-nucleon scattering cross section at the beam momen-
tum of 950 MeV/ c is shown in Fig. 1. The
-
N cross sec-
tions in the four allowed spin/isospin channels are taken
from the SM95 solution to SAID 8, assuming charge sym-
metry. The fraction of the scalar-isoscalar channel is more
than 55% and up to 75% over a momentum transfer range
from 350 to 550 MeV/ c , which our spectra will emphasize.
II. EXPERIMENT AND DATAANALYSIS
The experiment was performed at the 12 GeV proton syn-
chrotron of the High Energy Accelerator Research Organiza- *Electronic address: fujii@lambda.phys.tohoku.ac.jp
PHYSICAL REVIEW C, VOLUME 64, 034608
0556-2813/2001/643/0346088/$20.00 ©2001 The American Physical Society 64 034608-1