Quasi-free photoproduction of η-mesons off 3He nuclei
Crystal Ball/TAPS experiment at MAMI
the A
9
Collaboration
L. Witthauer
10
D. Werthmuller
10
I. Keshelashvili
10
P. Aguar-Bartolome
9
J. Ahrens
9
J.R.M. Annand
8
H.J. Arends
9
K. Bantawa
15
R. Beck
9
14
V. Bekrenev
13
A. Braghieri
12
D. Branford
17
W.J. Briscoe
16
J. Brudvik
11
S. Cherepnya
6
B. Demissie
16
M. Dieterle
10
E.J. Downie
8
9
16
P. Drexler
7
L.V. Fil'kov
6
A. Fix
4
D.I. Glazier
17
D. Hamilton
8
E. Heid
9
D. Hornidge
5
D. Howdle
8
G.M. Huber
2
I. Jaegle
10
O. Jahn
9
T.C. Jude
17
A. Kaser
10
V.L. Kashevarov
6
9
R. Kondratiev
3
M. Korolija
0
S.P. Kruglov
13
B. Krusche
10
A. Kulbardis
13
V. Lisin
3
K. Livingston
8
I.J.D. MacGregor
8
Y. Maghrbi
10
J. Mancell
8
D.M. Manley
15
Z. Marinides
16
M. Martinez
9
J.C. McGeorge
8
E. McNicoll
8
V. Metag
7
D.G. Middleton
5
A. Mushkarenkov
12
B.M.K. Nefkens
11
A. Nikolaev
9
14
R. Novotny
7
M. Oberle
10
M. Ostrick
9
B. Oussena
9
16
P. Pedroni
12
F. Pheron
10
A. Polonski
3
S. Prakhov
9
11
16
J. Robinson
8
G. Rosner
8
M. Rost
9
T. Rostomyan
10
S. Schumann
9
14
M.H. Sikora
17
D. Sober
1
A. Starostin
11
I. Supek
0
M. Thiel
7
9
A. Thomas
9
M. Unverzagt
9
14
D.P. Watts
17
0
Rudjer Boskovic Institute
, HR-10000 Zagreb,
Croatia
1
The Catholic University of America
,
Washington, DC 20064, USA
2
University of Regina
, Regina, SK S4S-0A2 Canada
3
Institute for Nuclear Research
, RU-125047 Moscow,
Russia
4
Laboratory of Mathematical Physics, Tomsk Polytechnic University
, Tomsk,
Russia
5
Mount Allison University
, Sackville, New Brunswick E4L 1E6,
Canada
6
Lebedev Physical Institute
, RU-119991 Moscow,
Russia
7
II. Physikalisches Institut, University of Giessen
, D-35392 Giessen,
Germany
8
SUPA,
School of Physics and Astronomy, University of Glasgow
, Glasgow G12 8QQ,
UK
9
Institut fur Kernphysik, University of Mainz
, D-55099 Mainz,
Germany
10
Department of Physics, University of Basel
, CH-4056 Basel,
Switzerland
11
University of California Los Angeles
,
Los Angeles, California 90095-1547, USA
12
INFN, Sezione di Pavia, I-27100 Pavia, Pavia,
Italy
13
Petersburg Nuclear Physics Institute
, RU-188300 Gatchina,
Russia
14
Helmholtz-Institut fur Strahlen- und Kernphysik, University of Bonn
, D-53115 Bonn,
Germany
15
Kent State University
, Kent,
Ohio 44242, USA
16
Institute for Nuclear Studies, The George Washington University
,
Washington, DC 20052, USA
17
SUPA,
School of Physics, University of Edinburgh
, Edinburgh EH9 3JZ,
UK
Quasi-free photoproduction of -mesons has been measured off nucleons bound in 3He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost 4 covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The -mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the S11(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final-state interaction effects.
1 Introduction
Currently the only practical method to investigate the
electromagnetic excitation spectrum of the neutron is
photoproduction of mesons off nucleons bound in light
nuclei. Photoproduction of mesons off the free proton
has been (and still is) intensively investigated with the
measurement of differential cross sections and single- and
double-polarization observables for many different final
states (pseudoscalar mesons, vector mesons, meson pairs).
The aim is to establish a reliable electromagnetic
excitation scheme of the nucleon; however, since the
electromagnetic interaction is isospin dependent, measurements with
neutron targets are mandatory for a complete picture.
Programs to measure such reactions off neutrons bound in
the deuteron are currently under way at several
laboratories (see [1] for an overview). Such measurements are
complicated in comparison to those with a free proton target
by three factors. The first is of technical nature, the
indispensable detection of recoil nucleons in coincidence with
the produced mesons complicates the experiments. The
detection of recoil neutrons lowers significantly the overall
detection efficiency and the determination of the latter
introduces an additional systematic uncertainty. Fermi
motion of the bound nucleons smears out all structures in the
measured reaction cross section. It can significantly
modify angul (...truncated)