First Photo-Pion Double Polarization
Experiments Using Polarized HD at LEGS
S. Hoblit , K. Ardashev† , C. Bade† , M. Blecher, C. Cacace,
A. Caracappa, A. Cichocki‡ , C. Commeaux§ , A. d’Angelo¶ ,
R. Deininger† , J.-P. Didelez§ , C. Gibson , K. Hicks† , A. Honig†† ,
T. Kageya , M. Khandaker‡‡, O. Kistner , A. Lehmann , F. Lincoln ,
M. Lowry , M. Lucas† , J. Mahon† , H. Meyer , L. Miceli ,
D. Morizzianni¶ , B. Norum‡ , B.M. Preedom, T. Saitoh , A.M. Sandorfi,
R. di Salvo¶, C. Schaerf¶, C. Thorn , K. Wang‡, X. Wei and
C.S. Whisnant§§
Brookhaven National Laboratory, Upton, NY 11973, USA
†
Ohio University, Athens, OH 45701, USA
Virginia Polytechnic Inst. & State Univ., Blacksburg, VA 24061, USA
‡
University of Virginia, Charlottesville, VA 22904, USA
§
Université de Paris-sud / IN2P3, Orsay, France
¶
Università di Roma-“Tor Vergata” and INFN-Sezione di Roma2, Rome, Italy
University of South Carolina, Columbia, SC 29208, USA
††
Syracuse University, Syracuse, NY 13244, USA
‡‡
Norfolk State University, Norfolk, VA 23504, USA
§§
James Madison University, Harrisonburg, VA 22807, USA
Abstract. We report preliminary results of π photo-production using polarized γ beams and a
polarized HD target. Four observables can be extracted simultaneously from the data, the cross
section, the beam asymmetry Σ, and the double-polarization observables G and E. The latter
determines the GDH sum rule integral.
In this paper we described the first photo pion double polarization measurements using
polarized HD at the Laser-Electron-Gamma-Source (LEGS) facility at Brookhaven National Laboratory. The LEGS facilities produces polarized gamma beams ranging from
π threshhold to 470 MeV with either linear or circular polarization, by backscattering
laser light from relativistic electrons circulating in a 2.8 GeV storage ring. The main focus of our current experimental program is the measurement of polarized asymmetries
and cross sections with the aim of constraining π -production amplitudes, particularly
from the neutron for which there is relatively little available data. One of the observables measured at LEGS, with circularly polarized photons on longitudinally polarized
nucleons, is the asymmetry that enters the Gerasimov-Drell-Hearn (GDH) sum rule integral. The LEGS energy range covers the first 65% of the sum rule integral.
Measurements of the GDH sum rule require integrals of hadron cross sections and for
this we have assembled a large solid angular detector. In the central angular range, from
50Æ 130Æ lab, 432 sodium iodide detectors surround the target. Forward angles,
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
651
from 10Æ 35Æ lab, are covered by walls of segmented plastic scintilators and lead
glass arrays. This system has nearly complete coverage for π Æ detection and about 3π
coverage for charged pions.
The most novel feature of this experiment is the use of a new class of polarized target
consisting of molecular HD in the solid phase. At the low temperatures required for
target polarization the lowest state of the two identical protons in the H 2 molecule is the
para configuration with the proton spins opposed, which cannot be polarized. The HD
molecule, consisting of non- identical particles, does not have this symmetry restriction
and in fact its lowest state has H and D spins parallel with relative orbital angular
momentum L 0. There are no phonons that couple an S-wave HD molecule to its
crystal lattice and so, once polarized, HD has an extremely long relaxation time. While
this is of course very desirable another mechanism must be found to polarize it. For this
we introduced a small (a few parts in 104 ) concentration of H2 which initially has 3/4 of
its molecules in the orbital configuration with S 1 and L 1 (ortho-). These readily
polarize in high magnetic fields and spin-spin coupling between an H in H 2 and an H
in HD transfers polarization to HD. This process occurs very rapidly (on the order of
hours). Once cooled, the ortho-H2 decays to the magnetically inert para-H2 state with a
half life of about six days. Thus, maintaining the target at high field and low temperature
for several ortho para time constants results in a frozen spin target.
The targets we have used so far are two and a half centimeters diameter by five
centimeters long. They were made in a dilution refrigerator and the spins were frozen
out at 18 mK and 15 tesla over a period of three months. They were then extracted from
the dilution refrigerator and placed into a in-beam cryostat at 1.25 K and 0.7 tesla. Apart
from the frozen HD the only other material seen by the beam are the cell walls made of
CTFE (Chlorotrifluoroethylene) and 2050 µ m aluminum wires which represent 20% of
the target by weight. These are needed to take away the heat from ortho-H2 to para-H2
conversion. Nonetheless, the yield from this background is easily measured by pumping
out the HD material and collecting data from an empty cell.
The target used in these measurements was condensed July 27, 2001 and taken to high
fields and low temperatures. On September 17 the field was lowered and polarization
measured to be 70% for H and 17% for D. The targets then went through a long period of
tests, including five transfers between the dilution refrigerator and the in-beam cryostat.
The experimental run did not start until mid November, at which point the polarization
had decreased to 30% for H and 6% for D. The measured in-beam relaxation times
were 13 days for H and 36 days for D. Unfortunately, an accelerator shutdown occurred
3 1/2 days after the start of the data taking, but the results from this short period are
nonetheless impressive.
Missing energy spectra for γ p π n and γ n π Æ n are shown in Fig. 1 for
the two helicity states h 12 (parallel beam+target spins), h 32 (antiparallel spins),
and their difference. The spectra in both helicity states for π n has two components, a
narrow spike on top of a broad peak coming from bound protons in deuterium. The latter
is almost absent in the helicity difference, since the deuterium polarization was low.
The π Æ n spectra are considerably broader since they only come from bound neutrons in
deuterium.
In these measurements we flipped between six gamma ray polarization states with randomly chosen intervals: left circular, right circular, 0 Æ linear, 45Æ linear, 45Æ linear,
652
r r r
HD( a , / + n )
A
+
A
a + p A/ n
r r
D( a , /°n )
A
helicity 1/2
k
A
a + n A/ n
helicity 1/2
3000
2000
1500
2000
1000
1000
500
0
0
@
+
A
a + p A/ n
@
helicity 3/2
k
A
a + n A/ n
helicity 3/2
3000
1500
2000
1000
1000
500
0
A
@
A
a + p
A
@
0
A
a + n
h(1/2 ) - h(3/2)
h(1/2 ) - h(3/2)
600
400
300
400
200
100
200
0
0
-200
2000
-150
-100 -50
0
50
100 150
E Missing Energy (MeV)
200
-200
-150
-100 -50
0
50
100 150
E Missing Energy (MeV)
200
/k
/+
FIGURE 1. Missing energy spectra for π n from polarized HD (left panels), and for π Æ n from polarized
D (right panels), for the indicated initial helicity configurations.
and 90Æ linear. The small unpolarized component in the beam due to bremsstrahlung
of the electron in the residual gas of the storage ring vacuum chamber ( 1%) was also
monitored regularly by blocking the laser from entering the straight section. Since the
γ -ray production process is simply Klein-Nishina scattering of light from free electrons
in the vacuum, the γ -ray polarization is very precisely determined by measuring the laser
Stokes vector (Q, U , V ). Here Q is the intensity of linear polarization at 0 Æ , U is the linear
intensity at 45Æ and V is the intensity of right-circular polarization. With a longitudinally
polarized target of polarization Pz , the yield at a given angle and energy is determined
by four independent quantities: the cross section, the beam asymmetry Σ (independent
of target polarization) and two double-polarization quantities, E and G, shown schematically in Fig. 2. The G asymmetry is obtained in reactions on longitudinally polarized
nucleons by flipping the beam polarization between linear states oriented at 45 Æ and
45Æ to the reaction plane. E is the asymmetry entering GDH sum rule, as measured
with longitudinal target polarization and circular beam polarization. The dependence of
the cross sections on these observables is shown in Eq. (1), where the subscript i refers
to the incident polarization state characterized by polarization (Q i , Ui , Vi ).
d σi
dσ
θ φ Eγ θ Eγ 1 Qi Eγ Σθ Eγ Pz Ui Eγ Gθ Eγ cos 2φ
dΩ
dΩ
Pz QiEγ Gθ Eγ UiEγ Σθ Eγ sin2φ
Pz Vi Eγ E θ Eγ (1)
From the six polarization states we form three asymmetries, the left/right circular,
45Æ 45Æ linear and 0Æ 90Æ linear asymmetry. These three asymmetries are then
simultaneously fit using the measured stokes parameters for each state to yield Σ, E, and
653
mŒ
m ||
m <z / / 4
m +z / / 4
m par
manti
Y
G
E
FIGURE 2. Polarization states entering the three measured asymmetries.
0.6
0.6
0.4
0.4
0.2
0.2
G 0.0
0.0
-0.2
-0.2
-0.4
G
Ea = 285 MeV
Ea = 331 MeV
30 60 90 120 150
e (deg)
30 60 90 120 150
e (deg)
cm
cm
Ea = 364 MeV -0.4
30 60 90 120 150
e (deg)
cm
FIGURE 3. Preliminary asymmetries for the γ p
π n (solid circles), compared with data from
Ref. [1], open symbols. The curves are the SM02k multipole solution from SAID.
G. The resulting G asymmetries for π n are shown in Fig. 3. Also shown are three of
the four points from Khar’kov which comprises the world’s published data.
The E asymmetry is a ratio formed from the cross sections with photon and targetnucleon spins parallel (helicity = S PP 12) and anti-parallel (helicity = 3/2).
In terms of the helicity cross sections, E σ 32 σ12 σ32 σ12 , or Ê σ unp
where σ unp is the unpolarized cross section and we denote the numerator by Ê σ32 σ122. Examples of these asymmetries are shown for two energy bins in Fig. 4
Dγ π n, left panels, and H
Dγ π Æ p , right panels. (At
for the exclusive reactions H
present, only events with pions detected in the central NaI calorimeter are included in the
plots of Fig. 4. The analysis of forward going pions is underway.) The π n asymmetries
are very close to multipole predictions from either SAID or MAID. In contrast, the
π Æ p E-asymmetries near 90Æ are always larger. This arises from the bound protons in
deuterium. Although the D polarization is small and the free H dominates the numerator
Ê, the contribution of bound protons in D does not cancel in σ unp , the denominator
of E. The exclusive cross sections for Dγ π Æ p have been measured at LEGS and in
the central angular range they are about half of the corresponding cross sections on the
free proton.[2] It is this large reduction in σ unp that produces the larger asymmetries in
654
1.0
1.0
Ea = 317 MeV
0.5
E
asy
E
0.0
asy
-0.5
-1.0
0
LEGS'01
30
SAID
MAID
1.0
90
A A A
+
120
HD(a, 150
/ n)
1.5
asy
0
LEGS'01
30
SAID
MAID
1.0
Ea = 363 MeV
0.5
E
0.0
-0.5
-1.0
1.5
Ea = 317 MeV
0.5
90
A A
A
k
/ p)
HD(a, 150
120
Ea = 363 MeV
0.5
E
0.0
asy
0.0
-0.5
-0.5
-1.0
-1.0
0
30
60
e
cm
90
120
(deg)
150
180
0
30
60
e
cm
90
120
(deg)
150
180
FIGURE 4. Preliminary E asymmetries for exclusive π n (left panels) and π Æ n (right panels) at
energies near the ∆, compared with predictions from recent multipole solutions.
Fig. 4.
Charged π production from the neutron requires separating the Dγ π p and
Dγ π n channels. The recoil proton can serve to tag the reaction of interest, provided
that it has sufficient energy to emerge from the target cryostat. Unfortunately, below
about 280 MeV this is not the case for more than half of the angular distribution. The
only direct method of isolating charged π reactions on the neutron is to track π ’s in
a magnetic field and measure the pion charge. For this we are constructing a Time
Projection Chamber (TPC) which will be housed in a large-bore 1.8 Tesla field. We
expect this to yield the most accurate determination of the neutron multipoles at low
energy.
ACKNOWLEDGMENTS
This work is supported by the U. S. Dept. of Energy under contract No. DE- AC0298CH10886, the U.S. National Science Foundation, and the Istituto Nazionale di Fisica
Nucleare - Italy.
REFERENCES
1.
2.
A.A. Belyaev et al., Sov. J. Nucl. Phys. 40, 83 (1984).
LEGS collaboration: K.H. Hicks et al., Proc. Int. Sym. on Electromagnetic Interactions in Nucl.
and Hadron Phy., Osaka (ed. M. Fujiwara and T. Shima) page 144, World Scientific River Edge, NJ
(2002).
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