Hall C Summer WorkshopAugust 6, 2009

W. LuoLanzhou University, China

Analysis of GEp-III&2γ Inelastic Data

--on behalf of the Jefferson Lab Hall C GEp-III collaboration

OutlineGEp-III & 2γ: π°

•Physics motivation•Kinematics of π° production

SIMC simulation of π° photo-productionPreliminary result of π° production analysis

• Polarization transfer vs. Eγ

• Induced polarization Py vs. Eγ

• Induced polarization Py vs. θπ°

Conclusion

GEp-III & 2γ: π°

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).

pp 0

π° 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.

GEp reaction: pepe '

To get clean ep elastic events, at hardware level: BigCal – HMS coincidence trigger BigCal threshold ≈ 1/2 E’ elastic

20cm(15cm) LH2 long target is equivalent to a 2.3% radiator.Two reactions may pass the trigger setting:

pp '

Physics motivation

H

Polarization transfer components: 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: PQCD predicts the HHC. The HHC predicts the transferred polarization Px and induced polarization Py both vanish; Pz becomes independent of beam energy in high Eγ regime.

121.8cm121.8cm

217.6cm217.6cm

249.4cm

Open distance

HMS

BigCa

l

Beam line

P

γ

θp

θ12γ

Target chamber

Kinematics of π° productionπ° decay into 2γ(fraction 98.798%)Only if one gamma energy > 1/2 E’ hits BigCal will form a trigger signal.In 2 kinematics with BigCal close to the target we can measure both γs, and reconstruct the π° mass:

E1

E2

))cos(-(1E2Em 1221

o

Ee, GeV pp Ee’ p, deg BigCal γrange π° θC.M,

1.867 2.068 0.527 14.49 105 1.6 - 1.86 135.-149.

2.839 2.068 1.507 30.98 45.3 2.35 - 2.73 89.-104.

3.549 2.068 2.207 35.39 32.9 2.615 - 3.38 72.-91.

Kinematics of π° production GEp-2gamma: dependence of R at 2.5 GeV2

GEp-3: high Q2 measurementsEe, GeV pp Ee’ p, deg BigCal γrange π° θC.M

4.053 3.589 1.274 17.94 60.3 3.4 - 3.98 115-129

5.714 4.464 2.090 19.10 44.2 5.0 - 5.6 100-119

5.714 5.407 1.164 11.6 69.0 5.0 - 5.6 129-143

The polarization component of π° production has not been measured in this energy region.

The polarization components of π° production was measured by Hall A experiment in range of 0.8GeV<Eγ <4.0GeV. (K. Wijesooriya et al, Phys.Rev.C66: 034614(2002))Note: polarization of π° photo- and electro-production was expected and measured to be the same in the Hall A experiment.

3σ Cut

SIMC simulation of π° photo-production

20cm(15cm) LH2 long target is equivalent to a 2.3% radiator. Assuming the photon comes from the bremsstrahlung reaction. An example of the data and the simulation comparison at Ebeam=2.839GeV , Q2=2.5GeV2

Note: SIMC spectrum was normalized to the elastic peak of the data.The normalization parameters applied to elastic and π° are different.

Goals of elastic and π° simulation: ep elastic simulation: radiation tail under inelastic spectrum ep elastic + π° simulation: π° events under elastic peak.

Target wall c

ut

perform

ed

3σ Cut

SIMC simulation of π° photo-production

Ee, GeV

(GeV)

γrange

(GeV)

π° θC.M, R1

(%)

R2

(%)

1.867 1.6 - 1.86 135.-149. 7.26 1.69

2.839 2.35 - 2.73 89.-104. 10.2 0.77

3.549 2.615 - 3.38 72.-91. 12.3 0.44

4.053 3.4 - 3.98 115-129 4.94 1.27

5.714 5.0 - 5.6 100-119 2.1 0.9

5.714 5.0 - 5.6 129-143 0.2 9.4

Applied all the other elastic cuts(3σ) except pmiss_p , then evaluate the number of events left under the elastic peak.

simcepsimc

simc

NN

NR

o

o

__

_2

ineldata

simcep

N

NR

_

_1Estimated ratio of number of elastic tail events under π° events.

Estimated ratio of number of π° events under elastic peak.

Preliminary result

Note:Target wall cut and anti-elastic cut applied to the analysis.Only FPP1 data was analyzed. With the high statistic of π° data, we can split the data into several bins for each kinematics .Px must be reversed to be compared with the Hall A data.

),(),(

),(),(),(

NN

NNf

)cos()sin()(2 21 ppf

Same method applied to π° data as the ep elastic analysis.

0

),(),(),(1

N

NNf

Helicity sum:

)cos()sin(1)(1 21 ppf

Helicity diff:

The asymmetry is related to the induced polarization. In Born approximation, Py = 0 for ep elastic ;

PRELIMINARY

Eγ (GeV)Eγ (GeV)

PRELIMINARY

Eγ (GeV)Eγ (GeV)

PRELIMINARY

Eγ (GeV)Eγ (GeV)

PRELIMINARY

θC.M.θC.M.

PRELIMINARY

θC.M.θC.M.

Conclusion

To do:• Analysis of FPP2 will be done and that will increase the statistic;• Subset of events at Q2=8.5GeV2 and Q2=2.5GeV2 lowest є which can be used to do π° identification;• ep elastic correction • False asymmetry correction to Py• Systematic error

π° photo-production simulation agrees well with the data.

Measured π° production polarization components agrees with the Hall A data in GEp-2γ kinematics.

First measurement of π° production polarization components at Eγ=5.0-5.6GeV .