Proton polarization measurements in π° photo-production
--on behalf of the Jefferson Lab Hall C GEp-III and GEp-2γ collaboration
2010 Annual Fall Meeting of the APS Division of Nuclear Physics
Santa Fe, New Mexico
Wei Luo, Lanzhou University, ChinaPresented by Mark Jones, Jefferson Lab
• Experiments were run in Hall C at Jefferson Lab
acce
lera
tor
Hall A Hall CHall B
Overview of Gep3 and Gep2g experiments
Overview of Gep3 and Gep2g experiments
Elastic
Hall A Hall CHall B
BigCal
Polarized electron beam
P
θp
Target chamber
HMSElectron
pepe '
FPP
217.6cm
Calorimeter (BigCal)121.8cm
Elastic
Hall A Hall CHall B
BigCal
Polarized electron beam
P
θp
Target chamber
HMSElectron
Focal Plane Polarimeter (FPP) inHigh Momentum Spectrometer
pepe '
FPP
Overview of Gep3 and Gep2g experiments
• Measure proton GE/GM from ratio of PT/PL
• Gep3 to Q2 = 8.5 GeV2
• Gep2g covered 3e at Q2 = 2.5GeV2
• Talks Sat ND.0002 & ND.0003 by Andrew Puckett.
π° photo-production in GEp-III & 2g experiments
pp 0
π° photo-production:the cross section is much larger than ep elastic at high Q2, most of the background comes from this reaction.
Real Compton Scattering: Cross section is much smaller than the π° production at high Q2.
Two reactions have the possibilities to pass the trigger settings:
pp '
•BigCal sensitive to electrons, positrons and photons• Trigger is coincidence between proton and cluster in BigCal with more than ½ the elastic electron energy
• 20cm LH2 target, 2.3% radiator
HMS Spectra with no BIGCAL cuts
LargeBackground
Elastics
1¡ pp(µp)=ppC
ount
s
Physics motivationPolarization transfer components at target: Px, Pz (PT, PL)Induced polarization: Py (PN)Helicity independent variable.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 the cross section* and the induced polarization observable Py.One would expect the polarization components to behave smoothly above the baryon resonance regime.
2. Hadron Helicity Conservation(HHC) Rule**:As a consequence of pQCD, the HHC predicts that transferred polarization Px and induced polarization Py both vanish and Pz should become independent of beam energy at high beam energy.
** S.J. Brodsky et. al., Phy. Rev. D 24, 2848(1981)
* D. Menze, et. al., Compilation of Pion Photoproduction Data (Physikalisches Institut der Universitat, bonn, 1977)
In some kinematics settings, BigCal can detect both γs(depends on θ12 and distance from target to BigCal).Polarization of proton are measured by Focal Plane Polarimeter inside HMS hu in the ideal dipole approximation:
BigCal
Polarized electron beam
P
γ
θp
θ12γ
Target chamber
EM Calorimeter: BigCal
High Momentum Spectrometer (HMS)
Kinematics of π° photo-production
Experiment
Ee
(GeV)PHMS
(GeV/c) ΘHMS
(°)RBigCal (cm)
Χ (°)
Eγ
(GeV) θC.M,
π°
(°)
GEp-2γ 1.867 2.068 14.49 493.2 109.1 1.60 - 1.86 135.-149.
GEp-III4.053 3.589 17.94 605.0 176.5 3.40 - 3.98 115.-129.
5.714 4.464 19.10 608.2 219.2 5.00 - 5.70 100.-119.
5.714 5.407 11.59 430.4 262.8 5.00- 5.70 129.-143.
GEp-III & 2γ kinematics table*:
*only the kinematics which can be used for π° analysis are shown.
P f pt = PxP f pn = Py cosÂ+Pz sinÂ
MeasureProton
Energy & angle
Calculate • Initial photon energy
• Outgoing pion angle
and energy
Use both and assume
π° photo-productionkinematics
Measuredecay
photonEnergy &
angle
Use photon angle and pion energy & angle
π° decay kinematics
Calculatedecay photon energy
HMS:Acceptance cut
BigCal: Acceptance cutCoin TDC cut
Identification of reaction One detected from 0 decay:
• high statistics• high background contamination
°p! ¼±p
π° event identification: one cluster found in BigCal In case only one photon hit BigCal, to select π° event one can use the
energy correlation between the energy measured by BigCal and photon energy predicted by proton measured in HMS.
The E1/E1’ resolution is similar to energy resolution of ep elastic data. Fitting the E1/E1’ distribution as a gauss function for π °, and Landau function for background. The total background contamination is about 7.7%.
Cut: Ethreshold <E1 < Eelastic
Ethreshold <E1’ < Eelastic
π° event identification: 1 photon detected
E1/E1’E1’
E 1
MeasureProton
Energy & angle
Calculate • Initial photon energy
• Outgoing pion angle
and energy
Use both and assume
π° photo-productionkinematics
MeasureBoth decay
photonsEnergy &
angle
Use one photon’s angle
and pion energy & angle
π° decay kinematics
Calculate photon energyand 2nd photon’s energy
and angle
HMS:Acceptance cut
BigCal: Acceptance cutCoin TDC cut
Identification of reaction Two photons detected from 0 decay:
• low statistics• low background contamination
°p! ¼±p
π° γ1
φ1
φ2
Target
BigCal 0
γ2
GEp-III: Beam energy 5.71 GeV, BigCal at closest position to target, two photons decayed from π° could be detected.
π° event identification: 2 photons detected
Δφ cut
Simulation Data
Estimated background contamination: < 4.2%
Asymmetry at Focal Plane Polarimeter
cos'fptAyPsin'fp
nAyP1)0N
)(N
0N
)(N(21
Helicity-independent distribution:
Helicity-dependent distribution:
Θπ°c.m. = 122°, χ =176.5° (1γ
detected)
Θπ°c.m. = 137°, χ =
262.8° (2γ detected)
)cosfptAyPsinfp
nAyP(h)0N)(N
0N)(N(
21
In the ideal dipole approximation:
cos2psin1p0pfit cos1psin0pfit
'fpnP
'fptP , Helicity-independent components at
focal planefpnP
fptP , Helicity-dependent components at
focal plane Ay Analyzing power of CH2h Beam helicity Total number of helicity(+/-)
events/number of bins0N
yzfp
n
xfpt
PPP
PP
cossin
Preliminary results for Px
Eγ (GeV)
Px
PRELIMINARY
FPP analyzing power and false asymmetry are calibrated by ep elastic events; Using COSY spin transport matrix
and maximum likelihood method Estimated maximum systematic
error < 3% for each bin
Preliminary results for Pz
Eγ (GeV)
Pz
PRELIMINARY
FPP analyzing power and false asymmetry are calibrated by ep elastic events; Using COSY spin transport matrix
and maximum likelihood method Estimated maximum systematic
error < 3% for each bin
Preliminary results for Py
Eγ (GeV)
Py
PRELIMINARY
FPP analyzing power and false asymmetry are calibrated by ep elastic events; Using COSY spin transport matrix
and maximum likelihood method Estimated maximum systematic
error < 3% for each bin
Conclusion Identifying π° with two different methods give
consistent results.First measurement of π° photo-production
polarization components in range of 5.0 GeV< Eγ < 5.7 GeV.
Preliminary results show:•Consistency with the results of previous
experiment at low Eγ
• Non-zero induced polarization Py at large Eγ
• Transferred Polarization component Pz changes dramatically as a function of θπ°
cm when Eγ > 5.0 GeV
Backup slides
GEp-III: Q2=8.5 GeV2 : ep elastic data Analyzing power:
312
0
ptPpp
t ePpAy
Polarization at Focal Plane Polarimeter: ep elastic
Two γs identified in BigCal One γ identified in BigCalQ2 = 8.5 GeV2 background correction
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