31 Recent Results from the Graal and LEGS Beams - Sixth International Conference on Perspectives in Hadronic Physics, ICTP, October 2008

Recent Results from the Graal and LEGS Beams
Carlo Schaerf
Citation: AIP Conf. Proc. 1056, 404 (2008); doi: 10.1063/1.3013072
View online: http://dx.doi.org/10.1063/1.3013072
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Recent Results from the Graal and LEGS Beams
Presented by Carlo Schaerf for the Graal ^ and LSC collaborations ^
Universita degli Studi di Roma "Tor Vergata"
INFN - Sezione di Roma "Tor Vergata"
Abstract. The polarized and tagged Graal gamma-ray beam is obtained by backward Compton
scattering of laser light on the high-energy electrons circulating in the ESRF storage ring. This
technique, first developed for the Ladon beam on the storage ring Adone at LNF [1], provides
gamma-ray beams with linear or circular polarizations close to one and well known. The Graal beam
covers the energy region between 600 and 1500 MeV thus allowing the study of baryon resonances
up to an energy of 1916 MeV, in large part by precision measurements of beam polarization
asymmetries in meson photoproduction on the nucleons.
Keywords: Polarized photons, meson photoproduction, polarization observables
PACS: 13.60.Le
The differential cross section for the photoproduction of mesons with linearly polar-
ized y-rays on unpolarized nucleons is:
^ ^ ^ = ^ [ l + P . A ( M ) - ( 2 ^ ) ] (1)
where —"J'^ is the cross section for unpolarized y-ray beam (function of k, the y-ray
energy, and 6 the polar angle of the meson in the CMS), (j) the azimuthal angle between
the polarization and reaction planes, Pyik) the polarization of the y-ray beam as function
of k, and A{k, 6) the beam polarization asymmetry (function of k and 6).
The Graal detector [2] has been designed to have cylindrical symmetry around the
beam axis and we have collected data with two orthogonal orientations of the polariza-
tion plane exploiting the feature of Ladon beams that the polarization of the y-ray beam
is in the same direction of that of the laser and therefore can be easily rotated with optical
half-wave plates. Graal has produced high quality asymmetries for the reactions:
in Hydrogen
Y + p ^ Jt^ + p , Y + p ^ rj + p , y - | - / 5 — > ; r + - | - «
^ V. Bellini, J. P. Bocquet, L. Casano, M. Castoldi, A. D'Angelo, J. P Didelez, R. Di Salvo, A. Fantini,
D. Franco, G. Gervino, F Ghio, G. Giardina, B. Girolami, A. Giusa, M. Guidal, E. Hourany, R. Kunne,
A. Lapik, P. Levi Sandri, A. Lleres, F Mammoliti, G. Mandaglio, M. Manganaro, D. Moricciani, A.
Mushkarenkov, V. Nedorezov, L. Nicoletti, C. Randieri, D. Rebreyend, N. Rudnev, G. Russo, C. Schaerf,
M.-L. Sperduto, M.-C. Sutera, and A. Turinge, V. Vegna.
^ K. Ardashev, C. Bade, M. Blecher, A. Caracappa, A. D'Angelo, R. Di Salvo, A. Fantini, C. Gibson,
H. Gluckler, K. Hicks, S. Hoblit, T. Kageya, M. Khandaker, O.C. Kistner, S. Kizigul, S. Kucuker, A.
Lehmann, F Lincoln, R. Lindgren, M. Lowry, M. Lucas, J. Mahon, L. Miceli, D. Moricciani, M. Pap, B.
Preedom, A.M. Sandorfi, H. Seyfarth, C. Schaerf, H. Stroher, C.E. Thornl, C.S. Whisnant, K. Wang, X.
Wei.
CP1056, Sixth International Conference on Perspectives in Hadronic Physics,
edited by S. Boffi, C. Ciofi degli Atti, M. Giannini, and D. Treleani
© 2008 American Institute of Pliysics 978-0-7354-0586-8/08/$23.00
404
2014.05.1
4 12:36:56
+01'00'

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FIGURE 1. Beam polarization asymmetries 2 for jr" photoproduction on free (open circles) and quasi-
free proton (triangles). The results are almost identical with some smoothing of the quasi-free curves near
maxima or minima probably due to Fermi motion of the target nucleon.
Y + P
Y + P
in Deuterium
^ K+ + A , V -
Y + p -i- Jt -
p ^ K+ + lP (2)
Y + d ^ n + p
Y + p{n) -^ 7] + p(n) .
Y + p{n) -> JT*^ + p{n)
n(p) -^ 7] + n{p)
- n(p) —^ JT" + n{p) (3)
A Deuterium target has been used to study the reactions on the neutron. To com-
pare the experimental results obtained by us on a neutron bound in the deuteron with
the available theoretical results for a free neutron, we have compared the asymmetries
obtained for the free proton in Hydrogen with those obtained for the bound proton in
Deuterium. This comparison is indicated in figures 1 and 2 for JT" and r photoproduc-
tion, respectively. Figures 3 and 4 present a comparison of the results for the quasi-free
neutron with those for the quasi-free proton for JT" and r, respectively.
In Fig. 6 we have indicated the tantalizing enhancement of the 77-n yield compared to
that for 77-p above the peak of the S11 resonance. This anomalous peak at an invariant
mass around 1.68 GeV, first discovered at Graal [5] and successively confirmed by the
Bonn cross section measurements [6], has been interpreted in various ways [7]. As
405

FIGURE 2. Beam polarization asymmetries 2 for 77 photoproduction on free (full dots) and quasi-free
proton (open squares). The different results between 900 and 1100 MeV coincide with the minimum
of the cross section and have been explained with the Fermi motion of the target nucleon. The energy
values outside and inside parenthesis indicate the mean value of the bin for quasi-free and free protons
respectively. Dotted and solid lines are the predictions of Maid2001 [3] for the free and quasi-free protons
respectively while the dashed lines are those for the reggeized model of ref. [4].
function of y-ray energy its width can be explained completely as due to the Fermi
motion of the neutron in the deuteron while as function of the 17-n invariant mass
its width (~ 40 MeV FWHM) is due entirely to the experimental resolution of our
apparatus. No lower limit of the width can be derived from these experiments.
The production of strange particles in the reactions
y + ;? ^ /<:+ + A , and y + p ^ K^ + iP (4)
allows the measurement of double-polarization beam-recoil observables by the asym-
metries in the weak decay of the strange baryons. Fig. 7 compares our preliminary re-
sults [8] for the angular distributions of the beam-recoil observables Ox and O^ with the
theoretical Coupled Channel calculations of Bonn [9] and the Regge Plus Resonance
calculations of Ghent [10]. Beam-recoil double-polarization asymmetries for the same
reaction have been measured at CLAS with a circularly polarized y-ray beam producing
the Cx and Q observables. The consistency of the Graal and CLAS results can be tested
comparing the angular distributions of a combination of the observables measured by
each experiment. We must have:
(1 + r^ - I? - ol - o)1/2 (f -rCl^Cfl^< (5)
406

1
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A < E ^ = 1,2B1 GeV
Q <E,> = 1,2611 GeV
0,5 -
O A I
T
100
1
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0.5 ^
0.25 '-
0 -
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O <E,>=0.89547 GeV
AAdSo
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^ O < g > ^ 1 . 1 7 4 S G e V
- a
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8
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-0.5 -
o}
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FIGURE 3. Sample of beam polarization asymmetries 2 for jr" photoproduction on quasi-free neutron
(open circles) and quasi-free proton (empty triangles). The striking differences between the two nucleons
is due to isospin T=l of the jr".
I
075
0.5
0.26
0
1
E
07=
0.6
0,25
0
1
E
0.75
0-6
0.25
(H >^0-746C;tV
i * tT*t = j
(I'J^O.SfMOcV
i '
•*?T ,
(R Msci-'sri^v
V ' ___i i
• • • i f
I 1
' T
(E^M.99 GeV <E,)=1J (E,)=l.lGcy
1 * oSi;
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t ? . j j - ..t .•.'*.:
..°..i
(B,)=l <U,)=
t t h««t *•;
<R|.>=1.435 GeV
...i
FIGURE 4. Beam polarization asymmetries 2 for rj photoproduction on quasi-free neutron (full trian-
gles) and quasi-free proton (open squares). The main differences between the two nucleons are at forward
angles and at energies above 1 GeV where the proton has a peak while the neutron exhibits a more
symmetric behavior around 90°.
407

FIGURE 5. The same data of Fig. 4 for rj on quasi-free neutron (full triangles). The mean energies for
each bin are indicated. The solid and dashed lines illustrate the predictions for neutrons of Maid2001 and
the reggeized model of ref. [4].
where the first quantity has been obtained with the Graal results and the last with those
of CLAS. As indicated in Fig. 8 both conditions are reasonably satisfied.
The LEGS Spin Collaboration (LSC) has successfully operated a frozen-spin polar-
ized HD target on the polarized and tagged Ladon beam installed on the NSLS storage
ring at BNL. The HD molecule allows the independent polarization of its p and d nuclei.
Figure 9 indicates the p and d polarizations during its 2004 and 2005 runs.
The LEGS tagged beam provides y-ray with an energy between 180 and 421 MeV
in the region of the A33 pion-nucleon resonance. Therefore the LSC collaboration has
focused on the measurement of the E double polarization observable using a mainly
circularly polarized beam on a longitudinally polarized target to verify the Drell-Hearn-
Gerassimov sum rule on the nucleons. Fig. 10 provides a sample of the results obtained
for the Jt^ and Jt^ yields as a function of the missing energy in the reconstructed
kinematics when the spins of the target and the beam are oriented parallel or antiparallel.
The angular distribution of the difference between the parallel and antiparallel cross
sections are shown in Fig. 11. The LSC are compared with those of Mainz.
REFERENCES
1. L. Federici, et al., // Nuovo Cimento 59B, 247 (1980).
2. O. Bartalmi et al., Eur. Phys. J. A 26, 399 (2005).
3. W.-T. Chiang et al., Nucl. Phys. A 700, 429 (2002).
4. W.-T. Chiang et al., Phys. Rev. C 68, 045202 (2003).
408

" • profon-
A neutron
f i
J ^
Egamma(GeV) Lgamma(GeV)
^ ttli • ^
f *u'rt 1 120.<l5,r<150
f lift ::rn
w r% ^
; • - . . ' »
nl"
j
- 1
''
liIf«.
"T"5D".""<"-iy,^"'<:"T8a.""
11 • -proton •••
^
' • ^ ' %
FIGURE 6. The yield for rj photoproduction on the neutron (triangles) and the proton (squares) at
different angles. The four picture on the left are plotted as a function of the y-ray energy assuming a
neutron at rest while those on the right are plotted as function of the invariant mass of the rj -n system
calculated from the measured kinematical variables of the two particles.
L5
O 1.0
60 no ISO 60 120 ISO
60 no ISO 60 no iso 60 no ISO 60 no iso
9™
FIGURE 7. The beam-recoil double-polarization observables Ox and O^ are compared with the theo-
retical calculations of Bonn (solid lines) and Ghent (dotted lines) as indicated in the text.
409

GRAAL vs CLAS data
+
+
9
1^
2.0
1.5
1.0
0.5
0.0
7.5 P
7.0 p
0.5 -
0.0
1.5
1.0
0.5
0.0
7.5 P
1
0.5 h
980 MeV
1122 MeV
(1132 MeV)
Oh
+^Vj%4'-4-+-i-i-r + ^ 4
1272 MeV
+-t-H;t
1421 MeV
(1433 MeV)
•4-*4U'''-^-+-^-H-r^-h-J
1027 MeV
(1032 MeV)
md-t-
1171 MeV
-ft
1321 MeV
(1332 MeV)
1466MeV
1074 MeV
>-f- f
1222 MeV
(1232 MeV)
-h ,*
1372 MeV
^-i^-f-f
60 120 180
• GRAAL data
(l + f-I.^-0^2.02f
D CLAS data
(I^+C2 + C2f^
60 120 180 60 120 180
FIGURE 8. Angular distribution of the quantities: (1 + T ^ - 1 : ^ - O^ - O^) ^/^ = (p2 ^ ^2 ^ ^2^ 1/2 < ^
For a comparison between the Graal and CLAS results. The combination of Graal data is indicated by full
points while that of CLAS by empty squares. The Graal energies are indicated above and those of CLAS
below in parenthesis.
Fall'2004: Sprmg'2005 _
10 15 20 25 30
Time (days)
10 15 20 25 30 35
Time (days)
FIGURE 9. Polarizations of H (full dots) and D (gray diamonds-vector and full squares-tensor) nuclei
in HD during the two data collection periods. Mid-way through each, the H polarization was flipped using
a RF transition.
410

Figure 10. Differences between 2-
body kinematics and the measured
energy for jr^ (top panels) and jr^
(bottom panels), in the cases of par-
allel (left panels) and anti-parallel
(right panels) beam and target spin
alignments. The simulated energy
differences are shown as the solid
curves.
-100 -50 0 50 100 -100 -50 0 50 100
E missing (MeV) E missing (MeV)
0 45 90 135 0 45 90 135 180
I • ' I • ' I • ' I ' • I ' • I • ' I ' •
^
•g-40
^
i I
X^•^
EY=304MeV f Ey= 320 MeV
Figure 11. Angular depen-
dence of the [da{P) - da{A)]
spin-difference cross section for
polarized H at beam energies near
the A peak. The full data from
Fair04 and Spring'05 are shown
as solid circles. Unpolarized limits
(solid squares) at 0^ and 180^ are
the mean of SAID [8] and MAID
[9]. Open diamonds are results
from Mainz [11] at 310 MeV (left)
and at 330 MeV (right). Predictions
from SAID and MAID are shown
as dotted and dashed curves, re-
spectively. The solid curves in the
top panels are a Legendre fit to the
new data.
0 46 90 135 0 45 90 135 180
5. C. Schaerf et al., Proceedings ofNSTAR2005, World Scientific, Tallahasse (Fl), 2005, page 176; V.
Kuznetsov et al., ibidem page 336.
6. I. Jaegle et al., Phys. Rev. Lett. 100,252002 (2008).
7. For example see: M. V. Polyakov and A. Rathke, Eur. Phys. J. A 18,691 (2003).
8. A. Lleres etal. to be published.
9. A. V. Anisovich et al., Eur. Phys. J. A 25, 427 (2005); A. V. Sarantsev et al., Eur. Phys. J. A 25,441
(2005).
10. T. Corthals, J. Ryckerbush and T. Van Cauteren, Phys. Rev. C 73, 045207 (2006).
11. J. Ahrens et al.,Eur. Phys. J. All,323 (2004).
411

31 Recent Results from the Graal and LEGS Beams - Sixth International Conference on Perspectives in Hadronic Physics, ICTP, October 2008

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31 Recent Results from the Graal and LEGS Beams - Sixth International Conference on Perspectives in Hadronic Physics, ICTP, October 2008