46 Complete Photoproduction Experiments - AIP Conference Proceedings, October 2011

Complete Photoproduction Experiments
A. D’Angelo, K. Ardashev, C. Bade, O. Bartalini, V. Bellini et al.
Citation: AIP Conf. Proc. 1374, 17 (2011); doi: 10.1063/1.3647092
View online: http://dx.doi.org/10.1063/1.3647092
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Complete Photo-production Experiments
A. D’Angelo∗,†
, K. Ardashev∗∗,‡
, C. Bade§
, O. Bartalini∗,†
, V. Bellini¶,
,
M. Blecher††
, J. -P. Bocquet‡‡
, M. Capogni∗,§§
, A. Caracappa¶¶
, L. E. Casano†
,
M. Castoldi∗∗∗
, R. Di Salvo†
, A. Fantini∗,†
, D. Franco∗,†††
, G. Gervino‡‡‡
, F. Ghio§§§
,
G. Giardinaa,¶¶¶,
, C. Gibson‡
, B. Girolami§§§
, A. Giusaa,¶,
, H. Glüklerb, K. Hicks§
,
S. Hoblit∗∗,¶¶
, A. Honigc, T. Kageyad,¶¶,
, M. Khandakerd,e,
, O. C. Kistner¶¶
,
S. Kizilgul§
, S. Kucuker∗∗
, A. Lapikf , A. Lehmann‡
, P. Levi Sandrig, A. Lleres‡‡
,
M. Lowryd,¶¶,
, M. Lucas§
, J. Mahon§
, F. Mammolitia,¶,
, G. Mandaglioa,¶¶¶,
,
M. Manganaroa,¶¶¶,
, L. Miceli¶¶
, D. Moricciani†
, A. Mushkarenkovf ,
V. Nedorezovf , B. Norum∗∗
, M. Papb, B. Preedom‡
, H. Seyfarthb, C. Randieria,¶,
,
D. Rebreyend‡‡
, N. Rudnevf , G. Russoa,¶,
, A. Sandorfid,¶¶,
, C. Schaerf∗,†
,
M. -L. Sperdutoa,¶,
, H. Stro¨herb,, M. -C. Suteraa,¶,
, C. E. Thorn¶¶
, A. Turingef ,
V. Vegna∗,†
, C. S. Whisnanth, K. Wang∗∗
and X. Weid,¶¶,
∗
Dipartimento di Fisica, Universitá di Roma "Tor Vergata", Via della Ricerca Scientifica 1, I-00133 Roma, Italy
†
INFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Roma, Italy
∗∗
Department of Physics, University of Virginia, Charlottesville, Virginia 22901, USA
‡
Department of Physics, University of South Carolina, Columbia, South Carolina 29208, USA
§
Department of Physics, Ohio University, Athens Ohio 45701, USA
¶
Dipartimento di Fisica, Universitá di Catania , Via Santa Sofia 44, I-95123 Catania, Italy
INFN Laboratori Nazionali del Sud, Via Santa Sofia 44, I-95123 Catania, Italy
††
Physics Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
‡‡
IN2P3, Laboratoire de Physique Subatomique et de Cosmologie, 38026 Grenoble, France
§§
Present affiliation: ENEA - C.R. Casaccia, via Anguillarese 301, I-00060 Roma, Italy
¶¶
Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA
∗∗∗
Dipartimento di Fisica, Università degli Studi di Genova, via Dodecaneso 33, I-16146 Genova, Italy
†††
Present affiliation: Albert Einstein Center for Fundamental Physics - LHEP - University of Bern, Switzerland
‡‡‡
Dipartimento di Fisica Sperimentale, Università degli Studi di Torino and INFN Sezione di Torino, via Pietro
Giuria 1, I-10125 Torino, Italy
§§§
Istituto Superiore di Sanità, viale Regina Elena 299, I-00161 Roma, Italy and INFN Sezione di Roma,
piazzale Aldo Moro 2, I-00185 Roma, Italy
¶¶¶
Dipartimento di Fisica, Università di Messina, salita Sperone 31, I-98166 Messina, Italy
aINFN Sezione di Catania, via Santa Sofia 64, I-95123 Catania, Italy
bForschungszentrum Jülich GmbH, D-52425 Jülich, Germany
cDepartment of Physics, Syracuse University, Syracuse, New York 13210, USA
dPresent affiliation:Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
eNorfolk State University, Norfolk, Virginia 23606, USA
f Institute for Nuclear Research, 60-letiya Oktyabrya prospekt 7a, 117312 Moscow, Russia
gINFN Laboratori Nazionali di Frascati, via E. Fermi 40, I-00044 Frascati, Italy
hJames Madison University, Harrisonburg, Virginia 22807, USA
Abstract. The extraction of resonance parameters from meson photo-reaction data is a challenging effort, that would greatly
benefit from the availability of several polarization observables, measured for each reaction channel on both proton and
neutron targets. In the aim of obtaining such complete experiments, polarized photon beams and targets have been developed
at facilities, worldwide. We report on the latest results from the LEGS and GRAAL collaborations, providing single and
double polarization measurements on pseudo-scalar meson photo-production from the nucleon.
Keywords: Photoreactions, polarized photon beams, polarized targets.
12th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU 2010)
AIP Conf. Proc. 1374, 17-22 (2011); doi: 10.1063/1.3647092
© 2011 American Institute of Physics 978-0-7354-0934-7/$30.00
17
2014.05.1
4 12:47:01
+01'00'

PACS: 25.20-x,24.70+s
1. INTRODUCTION
One of the most significant challenges of hadron physics is the quantitative description of the properties of hadrons
from QCD. Recently, both numerical simulations of unquenched lattice QCD (LQCD)[1] and solutions of the Dyson-
Swinger equation (DSE) for the quark propagator[2] have found that the current-quarks of perturbative QCD evolve
into constituent quarks at low momenta; that is to say that the constituent quark masses arise from low momentum
gluons attaching themselves to current quarks.
This finding provides a theoretical justification for QCD -inspired Constituent Quark Models (CQM) that have been
used in the last decades to predict the resonant structure of the nucleon[3, 4, 5]. In these models the asymmetry of the
baryon wave function is guaranteed by color, but color degrees of freedom are integrated out and play no dynamical
role. Excited baryon states (N∗) are classified by isospin, parity and spin within each oscillator band. Comparison
with experimental data shows, however, that only the lowest few resonances in each band have been seen in πN
reactions. Moreover the g(πN) couplings are predicted to decrease rapidly with increasing mass in each oscillator
band and higher levels are expected to have larger couplings to KΛ, KΣ,ππN,ηN and ωN channels. Therefore,
unseen or missing resonances may show their presence in other reaction channels such as K,η,ω and multi-pion
photo-production on the nucleons.
An alternative explanation comes from the di-quark model in which two quarks in nucleon are assumed to be quasi-
bound in a color isotriplet; the baryon states are made of diquark-quark combinations that are net color iso-singlet. If
all possible internal di-quark excitations are considered the full spectrum of CQM is obtained. But if internal di-quark
excitations are frozen out (having both spin and and isospin equal to zero) a large reduction in the number of baryon
degrees of freedom is predicted, providing about the same number of N∗ states, seen in πN reactions.
Unravelling the N∗ spectrum is still a challenge. Photo-nuclear reactions on the proton, in the resonance energy
region, have been deeply investigated in recent years and detailed knowledge of differential and total cross sections is
presently available for most of the possible reaction channels. Extraction of resonance parameters from experiments
is not straightforward. The idealized path to search for N∗ and Δ∗ states via meson photo-production would be to
determine the production amplitude from experiments and to search for resonant structure in a model independent
way (using Argand circles, phase motion, speed plots, etc.). Then models would be needed to separate resonance from
background components and determine resonant γN∗ and decay couplings, which could be compared with LQCD,
DSE and other hadron models predictions.
After fifty years of photo-nuclear experiments, however, amplitude extraction from data has never been accom-
plished and models have conjectured resonances and adjusted couplings to compare with limited data. The next exper-
imental challenge is to provide measurements for the highest number of spin and isospin dependent observables.
In the case of pseudo-scalar meson photo-production on the nucleon, there are eight possible spin states, corre-
sponding to four independent complex amplitudes, which describe the transition matrix.
From four complex amplitudes it is possible to construct fifteen polarization observables, in addition to the unpolar-
ized differential cross section. While seven carefully chosen polarization observables could be enough to completely
determine the amplitudes from the data, it has been found that an higher number of observables are needed to com-
pensate for limited experimental precision[6]. Moreover each reaction amplitude has two components resulting from
coupling of the I = 1/2 nucleon isospin with the isoscalar and the isovector components of the photon. These two
isospin-dependent contributions appear in linear combinations in the final reaction amplitude and they may be disen-
tangled only using measurements on both the neutron and the proton targets.
An experiment capable of providing enough single and double polarization observable on both proton and neutron
targets, so that a model independent extraction of all spin- and isospin- dependent amplitudes may be performed
from the data, is called complete experiment[7, 6]. It requires linearly and circularly polarized photon beams and
longitudinally and transversely polarized targets. A large effort is going on in worldwide facilities to realize this goal.
The most recent results from the LEGS and the GRAAL experiments are here shown, providing new single and double
polarization measurements on single pion and kaon photo-production channels.
18

FIGURE 1. Reference frames used to define the beam, target and recoil polarization directions.
2. THE LEGS AND GRAAL EXPERIMENTS
Both LEGS and GRAAL experiments used Compton backscattered photon beams, produced by the interaction of an
Ion-Argon Laser with the electrons circulating inside a storage ring: the National Synchrotron Radiation Facility at
the Brookhaven National Laboratory (USA), in the case of LEGS, and the European Synchrotron Radiation Facility
in Grenoble (France), in the case of GRAAL. The two tagged photon beams had complementary energy ranges: (220-
480) MeV, covering the first nucleon resonance region, in the case of LEGS; (550-1480)MeV, corresponding to the
second and third nucleon resonance region, in the case of GRAAL. Since relativistic electrons conserve helicity in
their scattering process with the incoming Laser photon beam, also backscattered photons retain their initial degree of
polarization. Therefore if the Laser light is linearly or circularly polarized, the Compton backscattered photon beam
is also linearly or circularly polarized to a high degree. The polarization degree slowly decreases as a function of the
electron scattering angle and scattered photon energy, but it may be higher than 90% at LEGS and 60% at GRAAL,
over the entire tagged energy range, by changing the Ion-Argon Laser line. Both experiments were equipped with
a large solid angle detector. In the case of the LEGS experiment a NaI crystal box was used to detect photons and
charged particles for polar angles between 20◦ and 160◦. It was alternatively equipped with an internal neutron barrel,
made of plastic scintillators, optimized for neutron detection or with a Time Projection Chamber (TPC), for charged
particle tracking. At forward angles two walls, made of plastic scintillators and of Lead Glass crystals, respectively,
allowed to detect both charged and neutral particles. A polarized frozen-spin HD target completed the experimental
set-up. Both proton and deuterons could be independently polarized, with polarization degrees up to 60% and 35% for
protons and deuterons, respectively.
In the case of the GRAAL set-up the LAGRANγE detector was made of a BGO ball covering polar angles between
25◦ and 155◦, internally equipped with a plastic scintillator barrel and a double layer of cylindrical Multi Wire
Proportional Chambers (MWPC). The detector was optimized for photon detection up to 1.5 GeV and for proton
detection up to 300 MeV. At forward angles a double wall of planar MWPC was backed by a double wall of plastic
scintillators for charged particles detection and a shower wall made of sandwiches of plastic scintillators and lead, for
neutral particles detection.
Both experiments were characterized by very low levels of backgrounds, due to unpolarized or untagged photons
impinging the target. Since only H or D targets were used, very low backgrounds coming from heavier nuclear target
components (such as Carbon in buthanol targets or Nitrogen in ammonia targets) contaminated the data acquisition,
allowing for excellent signal/background separation.
3. POLARIZED OBSERVABLES
For a polarized photon beam impinging on a polarized target and assuming the spins of the outgoing baryons are
measured, the differential cross section for a pseudo-scalar meson photo-production reaction on a nucleon, may be
expressed in terms of polarization observables, to leading order in polarization, as follows:
dσ =
1
2
{dσ0 + ˆΣ[−P
γ
L cos(2φγ)]+ ˆT[PT
y ]+ ˆP[PR
y ]
+ Ê[−Pγ
c PT
z ]+ ˆG[P
γ
L PT
z sin(2φγ)]+ ˆF[Pγ
c PT
x ]+ ˆH[P
γ
L PT
x sin(2φγ)]
+ ˆCx [P
γ
CPR
x ]+ ˆCz [P
γ
CPR
z ]+ Ôx [P
γ
L PR
x sin(2φγ)]+ Ôz [P
γ
L PR
z sin(2φγ)]
+ ˆLx [PT
z PR
x ]+ ˆLz [PT
z PR
z ]+ ˆTx [PT
x PR
x ]+ ˆTz [PT
x PR
z ]}
19

TABLE 1. Polarization observables for pseu-
doscalar meson photoproduction[6].
Simbol Required Polarization
Beam,Target,Recoil
Type
dσ
dt 0
-;-;- Single
ˆΣ = Σdσ
dt 0
L( π
2 ,0);-;-
ˆT = T dσ
dt 0
-;y;-
ˆP = T dσ
dt 0
-;-;y
ˆG = Gdσ
dt 0
L(π
4 ,0);z;- Beam
ˆH = H dσ
dt 0
L(π
4 ,0);x;- and
Ê = E dσ
dt 0
C;z;- Target
ˆF = F dσ
dt 0
C;x;-
Ôx = Ox
dσ
dt 0
L( π
4 ,0);-;x’ Beam
Ôz = Oz
dσ
dt 0
L( π
4 ,0);-;z’ and
ˆCx = Cx
dσ
dt 0
C;-;x’ Recoil
ˆCz = Cz
dσ
dt 0
C;-;z’
ˆTx = Tx
dσ
dt 0
-;x;x’ Target
ˆTz = Tz
dσ
dt 0
-;x;z’ and
ˆLx = Lx
dσ
dt 0
-;z;x’ Recoil
ˆLz = Lz
dσ
dt 0
-;z;z’
where dσ0 is the unpolarized differential cross section, P
γ
L (P
γ
C) is the degree of linear (circular) beam polarization, PT
x
(PT
y or PT
z ) is the degree of target polarization along the x (y or z) direction (see Fig. 1), PR
x (PR
y or PR
z ) is the degree
of recoil polarization along the x’ (y’ or z’) directions (see Fig. 1) and φγ is the relative angle between the linear beam
polarization direction and the scattering plane. The single (ˆΣ, ˆT, ˆP) and double ( ˆG, ˆH, Ê, ˆF, Ôx, Ôz, ˆCx, ˆCz, ˆTx, ˆTz, ˆLx, ˆLz)
polarization observables are shown schematically in Table 1 and defined explicitly in [6]. The sixteen observables
are not independent and are related by several relations, called Fierz identities. These relations may be used to check
consistency among different measured observables.
4. RESULTS FROM THE LEGS AND GRAAL EXPERIMENTS
Circularly polarized photon beams have been used on a longitudinally polarized HD target to measure the Ê observable
for both γ p → π+n and γ p → π0 p reactions, in the energy range between 220 and 480 MeV, by the LEGS experiment.
The Ê observable may be directly related to the difference of cross sections for parallel and anti-parallel polarizations
of beam and target, as it appears in the GDH spin sum rule. The results are shown in Fig. 2 for a few energy bins,
together with predictions of the SAID[FA07k] solution of partial wave analysis[8]. The curves slightly overestimate
the experimental results and LEGS measurements allowed for a new evaluation of the GDH sum rule[9], closer to the
theoretical prediction.
Using linearly polarized photons on a longitudinally polarized HD target it was also possible to measure the ˆG
observable for the same pion photo-production channels on the proton. Some preliminary results are shown in Fig. 3
together with previous data from [10] and predictions form the SAID[FA07k] solution. The new data show that while
a reasonable agreement exists with both existing results and predictions for the charged pion production, opposite sign
and higher strength than predicted are found for the neutral pion channel. This effect is still under investigation and
it may indicate that a larger D-wave component of the Δ(1232) than currently established is necessary to describe the
data.
Σ beam asymmetries of pseudo-scalar meson photo-production reactions on the nucleons have been systematically
measured by the GRAAL collaboration[11]. Some of latest results from π− photo-production on the neutron[12] are
shown in Fig.4 together with existing data and partial wave analysis predictions from SAID and MAID[13]. The new
precise GRAAL data required a revision of the SAID solution leading to several changes from the previous [SP09]
20

FIGURE 2. Measurements of the ˆE polarization observable for the γ p → π+n (left two panels), and γ p → π0 p (right two panels)
at LEGS.
FIGURE 3. Measurements of the ˆG polarization observable for the γ p → π+n (left panel), and γ p → π0 p (right panel) at LEGS.
Results are compared with previous measures at Mainz and with SAID[SP09k] partial wave analysis predictions.
version to the updated [MA09].
The study of the γ + p → K+ +Λ reaction is particularly interesting because the weak decay of the Λ recoil allows
for the measurement of the P recoil asymmetry, from the Λ → π− + p decay distribution. If a linearly polarized photon
beam is used, the Ox and the Oz double polarization asymmetries may be measured. A sample of the results obtained
by the GRAAL collaboration on this observables[14] are shown in Fig.5, together with Ghent Isobar (Regge plus
resonance) model (dashed curve)[15] and the Bonn-Gatchina model (soulid curves)[16]. A general good agreement is
found between data and the Bonn-Gatchina Model, which require the introduction of new or poorly known resonances
in the 1900 MeV mass region (P and/or D) to correctly reproduce the data.
5. CONCLUSIONS
The latest results from the LEGS and GRAAL collaboration have been reported in the aim of enlarging the experi-
mental data-set on single and double polarization observables for meson photo-production reactions, in the nucleon
resonance energy region. Similar efforts are currently made by other collaborations such as CLAS at the Thomas Jef-
ferson National Accelerator Facility, the CB-ELSA collaboration at the Bonn University, Mami at Mainz and LEPS
at Spring8 in Japan. A very attractive reaction candidate is the γN → K +Λ reaction, because of the "self-analyzing"
properties of the Λ recoil. These reactions are being measured by the CLAS collaboration on both buthanol and HD
polarized targets with polarized photon beams. The goal of performing the ﬁrst complete experiment is very close to
being achieved.
21

FIGURE 4. Results on the Σ beam asymmetry of the π− photo-production on a quasi-free neutron in a deuteron target from the
GRAAL collaboration. Results are compared with existing data and partial wave analysis predictions.
FIGURE 5. Results on the Ox and Oz double polarization observables for the γ + p → K+Λ reaction from the GRAAL
collaboration. Results are compared with Ghent Isobar (Regge plus resonance) model (dashed curve) and the Bonn-Gatchina model
(soulid curves).
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22

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