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CLAS Hadron Spectroscopy Group Meeting, JLab, September 26, 2009
Steffen StrauchUniversity of South Carolina
π+ Photoproduction from FROSTAnalysis Update
Polarized cross section (beam/target polarization)g9 Experiment
PT : transverse beam polarizationP⨀ : degree of circular beam polarizationPX, PY : transverse target polarizationPZ : longitudinal target polarization
I.S. Barker, A. Donnachie, and J.K. Storrow, Nucl. Phys. B75, 347 (1975)
Single- and Double Polarization Observables
dσdΩ
⎛⎝⎜
⎞⎠⎟
dσdΩ
⎛⎝⎜
⎞⎠⎟ unpol
= 1− PTΣ cos(2φ)
+PX[−PTH sin(2φ) + P°F]−PY [−T + PTP cos(2φ)]−PZ [−PTG sin(2φ) + P°E]
π+
p
xy
z
2
Polarization Observable E
dσdΩ
⎛⎝⎜
⎞⎠⎟=
dσdΩ
⎛⎝⎜
⎞⎠⎟ unpol
1− PZP°E( )
E = −xi∑
hPZP° x∑ i
2
σ 2 (E) ≥ 1− h2PZ
2P°2E2
h2PZ2P°
2N
Circularly polarized beam / longitudinally polarized target
3
xi = +1xi = −1
beamspin
targetspin
The sum is running over all events from the butanol targetBound-nucleon background is accounted for by the dilution factor h.
Estimator
π+ Photoproduction - Event Selection
One and only one positively charged particle detected with:
hit in ST, SC, and DCβ > 0, p > 0, and l > 0crude pion ID
No negatively charged particles detectedPhoton with good tagger status, and vertex time closest to that of the pionNo energy-loss correction, yet
4
γ p→π +n
Vertex-time difference (ns)-4 -2 0 2 4
Coun
ts
0.05
0.10
0.15
610×
(ns)!t"-4 -2 0 2 4
Coun
ts
0.02
0.04
0.06
610×
(ns)!t"-4 -2 0 2 4
(ns)
#t"
-4
-2
0
2
4
Particle Identification and Photon Selection
Vertex-time difference: |Δtγ| < 1 ns Particle ID: |Δtπ| < 1 ns
5
Δtπ =lc1β−
mπ2
p2+1
⎛
⎝⎜
⎞
⎠⎟
Vertex-z Position (cm)0 10 20
Coun
ts
1
10
210
310ButanolCarbon
2CH
Transverse Vertex Position (cm)-10 -5 0 5 10
Coun
ts
210
310
410
MVRT-XMVRT-Y
Reaction Vertex
6
Butanol events from Z ∈ [-3, 3] cm, r < 2 cmCarbon events from Z ∈ [5, 7.5] cm, r < 2 cmButanol background under carbon ≈ 1.3% of total
Missing Mass (GeV)0.5 1.0 1.5 2.0
Coun
ts
0
1
2
610×
ButanolC (unscaled)12
W = 1.200 - 2.250 GeV
γ(p,π+)X - Missing Mass Distribution
7
Identification of reaction channel;mX ≈ mN
Background from reactions off bound (unpolarized) nucleons ⇒
quenching of the asymmetry signalπ+ production off free and bound nucleons
Dilution factor
For the butanol target (C4H9OH) thesimple estimate is ≈ 10/74 ≈ 0.14Statistical uncertainty in E scales as
Figure of merit
Summer project, UG student Garry Hollis (USC)
Dilution Factor
8
h =SB= 1− C
B
F = h2B
Carbon
Butanol
free-proton events
Signal
mX
σ E ∝1F
Missing Mass Bin (GeV)0.6 0.8 1.0
C) ]
12No
rmal
izat
ion
[ N(B
utan
ol) /
N(
4
5
6
7
5%
Butanol/Carbon Normalization
Missing mass distributions for butanol and carbon data are of the same shape up to 0.8 GeV.Relatively more butanol events at ≈ mN (expected)Depletion at 0.8 - 0.9 GeV?
9
W (GeV)1.0 1.5 2.0 2.5
C) ]
12No
rmal
izat
ion
[ N(B
utan
ol) /
N(
4
5
6
7
DataNormalization s (fit)
5%±s
Energy Dependence of Normalization
Normalization factor is energy independent within ±5%Fit Χ2/d.o.f = 35/20Angular dependence not studied, yet
10
Missing Mass (GeV)0.5 1.0 1.5 2.0
Coun
ts
0.0
0.2
610×
Butanol
C12
Missing Mass (GeV)0.5 1.0 1.5 2.0
Diffe
renc
e
0.0
0.1
0.2
610×
W = 1.450 - 1.500 GeV
Missing Mass (GeV)0.5 1.0 1.5 2.0
Coun
ts
0
10
310×
Butanol
C12
Missing Mass (GeV)0.5 1.0 1.5 2.0
Diffe
renc
e
0
1
2
3310×
W = 1.950 - 2.000 GeV
Free-Proton Missing Momentum Distribution
Carbon data normalized to butanol data for mX < 0.8 GeVThe difference distribution is for illustration only and not used in the further analysis.
11
W (GeV)1.0 1.5 2.0 2.5
Dilu
tion
Fact
or
0.4
0.6
0.8
Dilution factor h (fit) 5%±h
= 0.93 to 0.97 GeV Xm
Dilution Factor
h ≈ 0.51; i.e. factor four improvement in this reactionDilution factor decreases for larger WModulation of dilution factor not yet understoodAngular dependence of the dilution factor to be tested
12
# Run Numbers E0 (GeV) Pe (?) HWP Holding Field
Target Polarization
1 55521 - 55536 1.645 - + + -
2 55537 - 55555 1.645 - - + -
3 55556 - 55595 1.645 - + + +
4 55604 - 55625 2.478 + + - +
5 55630 - 55678 2.478 + + - -
6 56164 - 56193 2.478 + + + +
7 56196 - 56233 2.478 + + + -
g9a Runs with Circularly Polarized Beam
13
Runs0 50 100 150
Asym
met
ry0
0.5
Butanol DataC Data12
55512 - 55595 55604 - 55676 56164 - 56193
Confirm Polarization Orientations
14
Status of half-wave plate, helicity signal, sign of electron-beam polarization, proton-target spin orientation understood on a per run base (up to an even number of sign changes).Pion data may help study the time dependence of electron / target polarization
A =Pi∑ xiP∑ i
2
Run Nunmber55600 55800 56000 56200
Elec
tron
Beam
Pol
ariz
atio
n
0.7
0.8
0.9
1.0
Beam and Target Polarizations
15
Photon beam polarization
Target polarization
) e / E! Photon Beam Energy (E
0.0 0.5 1.0
) e
/ P
! Ph
oton
Bea
m P
olar
izat
ion
(P
0.0
0.5
1.0
Pe ≈ 85%
Pγ = Pe4x − x2
4 − 4x + 3x2
PZ ≈ 80%
Beam-Charge Asymmetry
Helicity-gated beam-charge signals from the synchrotron light monitor in the HLS bankAll runs have beam-charge asymmetries within ± 0.5%.
16Summer project, UG student Theo DuBose (USC)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0E
-1
0
1W = 1.450 - 1.500 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.400 - 1.450 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.350 - 1.400 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.300 - 1.350 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0E
-1
0
1W = 1.250 - 1.300 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.200 - 1.250 GeV
SAID (SP09)MAID (2007)SAID (SM95)
Preliminary Results (I)
17
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0E
-1
0
1W = 1.750 - 1.800 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.700 - 1.750 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.650 - 1.700 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.600 - 1.650 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0E
-1
0
1W = 1.550 - 1.600 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.500 - 1.550 GeV
SAID (SP09)MAID (2007)SAID (SM95)
Preliminary Results (II)
18
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0E
-1
0
1W = 2.050 - 2.100 GeV
SAID (SP09)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 2.000 - 2.050 GeV
SAID (SP09)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.950 - 2.000 GeV
SAID (SP09)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.900 - 1.950 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0E
-1
0
1W = 1.850 - 1.900 GeV
SAID (SP09)MAID (2007)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 1.800 - 1.850 GeV
SAID (SP09)MAID (2007)SAID (SM95)
Preliminary Results (III)
19
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 2.200 - 2.250 GeV
SAID (SP09)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0E
-1
0
1W = 2.150 - 2.200 GeV
SAID (SP09)SAID (SM95)
)! cm"cos(
-1.0 -0.5 0.0 0.5 1.0
E
-1
0
1W = 2.100 - 2.150 GeV
SAID (SP09)SAID (SM95)
Preliminary Results (IV)
20
Very preliminary results for E in π+ photoproduction covering a wide energy and angular range.-0.9 < cos(θπcm) < +0.91.20 GeV < W < 2.25 GeVPresently more than 730 data points.
Systematic Uncertainties
21
Quantity UncertaintyElectron-beam polarization 5%Beam-charge asymmetry < 0.5%Photon-beam polarization from Pe ?Target polarization ?Dilution factor 5%Kinematics (W, cos(θ)) ?Total < 10%
present best guesses
Summary
Very preliminary results for E in π+ photoproduction
Issues:Obtain improved values for electron beam and target polarizations on a per run-period baseStudy of systematic effects in dilution factor (carbon/butanol normalization, butanol contamination of carbon data, energy and angular dependence)Implement energy-loss corrections and study of the reconstruction of event kinematics
22