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