Transcript Polarization measurements of proton in π° photo-production --on behalf of the Jefferson Lab Hall C GEp-III collaboration Hall C user meeting, W.

Polarization measurements of
proton in π° photo-production
--on behalf of the Jefferson Lab Hall C
GEp-III collaboration
Hall C user meeting,
W. Luo
Lanzhou University, China
January 22, 2010
Outline
• Introduction
 π° photo-production in GEp-III & 2 γ experiments
 Physics motivation
• π° events identification with BigCal

2 γ detected

1 γ detected
• Preliminary results of polarization components
• Conclusion
π° photo-production in GEp-III & 2 γ experiments
GEp reaction:
e p e  p
'
To get clean ep elastic events, at hardware level:
BigCal – HMS coincidence trigger
BigCal threshold ≈ 1/2 E’ elastic
The EM calorimeter is sensitive to e, e+ and γ, while the other particles have smaller
amplitude. The BigCal threshold at 1/2 of elastic electron energy will further narrow
down the reactions that could pass the trigger(10cm of absorber in front of BigCal
also eliminate part of low energy particles).
20cm(15cm) LH2 long target is equivalent to a 2.3% radiator.
Two reactions have the possibilities to pass the trigger settings:
Real Compton Scattering:
π° production:
the cross section is much larger than ep elastic at high Cross section is much smaller than the π°
Q2, most of the background comes from this reaction. production at high Q2.
  p  0  p
 
  p   p
'
Kinematics of π° production
π° will decay into 2γ(fraction 98.798%) right after its
production
if one of the two γs with energy > E’/2 hits BigCal , a
trigger will form and be recorded.
The minimum open angle between two γs is
determined by the energy of π°, in some of kinematics
we can measure both γs.
121.8cm
E1
Open distance
γ
γ
217.6cm
P
Target chamber
Beam line
249.4cm
E2
Physics motivation
Polarization transfer components at target:
Px, Pz (Pt, Pl)
Induced polarization:
Py (Pn)
Helicity independent variable.
H
1. Evidence of baryon resonances:
In low photon energy range(Eγ <2GeV),
the π° production is dominated by the
baryon resonances. This has been
largely tested by the measurement of
2. Hadron Helicity Conservation(HHC) Rule:
the cross section and the induced
PQCD predicts the HHC. The HHC predicts
polarization observable Py.
that transferred polarization Px and induced
One would expect the polarization
polarization Py both vanish; Pz should
components to behave smoothly
becomes independent of beam energy in
above the baryon resonance regime.
high Eγ regime.
π° event identification with BigCal
HMS:
Acceptance cut
TDC cut
Ep
θp
BigCal:
Acceptance cut
TDC cut
Eγ
π° photoproduction
Eπ°
kinematics
θπ°
Cluster:
θ1
E1’
θ1
π° decay
kinematics
θ2’
E2’
E1
θ2
E2
1
2
π° event identification with two clusters found in BigCal
Event distribution: Q2 = 8.5 GeV2
Φ is the angle between measured cluster position and HMS predicted π°
position in the following coordinate system.
BigCal
γ2
φ2
φ1
Target
π°
γ1
Two clusters found in BigCal: after cuts
Fitting π° events with
gauss + landau function
Background with pol4
function
Simulation of π° decay as
a function of open
distance between two
clusters
π° event identification with one clusters found in BigCal
In case we only find one cluster in BigCal, the only information we can use to identify
the π° decay is the energy correlation between HMS predicted energy and BigCal
measrued energy.
The E1/E1_hms has similar energy resolution as to ep elastic data. Fit the E1/E1_hms
distribution as a gauss function for π° , and a landau function for background.
Preliminary result s of polarization components
FPP analyzing power calibrated by ep elastic events , applied all the detector acceptance
cut, correlation cut and anti-elastic cut.
Q2=8.5 GeV2 One γ identified at BigCal
Two γs identified at BigCal
Two γs identified in BigCal
Q2=8.5 GeV2
One γ identified in BigCal
Preliminary result s of polarization components
Background correction for Q2 = 8.5 GeV2
Px
Py
Pz
No correction
After correction
No correction
After correction
No correction
After correction
1 cluster
-0.192 +/- 0.007
-0.197 +/- 0.007
-0.292 +/- 0.015
-0.280 +/- 0.016
0.723 +/- 0.008
0.788 +/- 0.009
2 clusters
-0.095 +/- 0.060
-0.101 +/- 0.065
0.01 +/- 0.16
0.02+/-0.16
0.815+/- 0.062
0.814+/-0.065
Polarization components for all kinematics
Ee, GeV
(GeV)
Eγ
range
(GeV)
θπ°c.m.
(deg)
R1
(%)
Px
Py
Pz
1.867
1.60 - 1.86
135.0149.0
6.1
-0.263
+/- 0.002
-0.585
+/- 0.004
0.082
+/- 0.002
2.839
2.35 - 2.73
89.0-104.0
8.5
-0.574
+/- 0.003
-0.015
+/- 0.007
0.212
+/- 0.003
3.549
2.62 - 3.38
72.091.0
29.5
-0.022
+/- 0.003
0.200
+/- 0.008
0.332
+/- 0.004
4.053
3.4 0- 3.98
115.0129.0
6.4
-0.107
+/- 0.015
-0.0237
+/- 0.013
0.905
+/- 0.044
5.714
5.00 - 5.70
100.0119.0
26.4
-0.223
+/- 0.018
0.538
+/- 0.019
5.714
5.0 0- 5.70
129.0143.0
20.4
-0.197
+/- 0.007
-0.280
+/- 0.016
R2
(%)
Px
Py
Pz
-0.076
+/- 0.030
14.7
-0.159
+/- 0.084
0.570
+/- 0.090
-0.32
+/- 0.12
0.788
+/- 0.009
17.6
-0.101
+/- 0.065
0.02
+/- 0.16
0.814
+/- 0.065
Px
Px
Eγ (GeV)
Eγ (GeV)
Py
Py
Eγ (GeV)
Eγ (GeV)
Pz
Pz
Eγ (GeV)
Eγ (GeV)
Conclusion
 Measured π° production polarization components agrees
with the Hall A dedicated measurement data in GEp-2γ
kinematics.
 First measurement of π° production polarization
components at Eγ=5.0-5.7GeV .
To do:
• Analysis of FPP2 will be done and that will increase the statistic;
• False asymmetry correction to Py
• Systematic error