11

Primakoff Experiments with EIC

A. GasparianNC A&T State University, Greensboro, NC

For the PrimEx Collaboration

Outline

Physics motivation: The first experiment at JLab: 0 lifetime

Development of precision technique Results for 0 lifetime

Experiments with EIC Summary

2

chiral limit: is the limit of vanishing quark masses mq→ 0.

QCD Lagrangian with quark masses set to zero:

qq

s

d

u

q

GgD

GGqiDqqiDqL

LR

s

RRLLoQCD

)1(2

1

2/

4

1

5,

)(

Large global symmetry group:Large global symmetry group:

)1()1()3()3( BARL UUSUSU

The QCD LagrangianThe QCD Lagrangian

3

Fate of QCD SymmetriesFate of QCD Symmetries

4

• Chiral SUL(3)XSUR(3) spontaneously broken Goldstone mesons π0, η8

• Chiral anomalies Mass of η0 P→γγ ( P: π0, η, η׳)

• Quark flavor SU(3) breaking

The mixing of π0, η and η׳

The The 00, , ηη and and ηη’ system provides a rich ’ system provides a rich laboratory to study the symmetry structure of laboratory to study the symmetry structure of

QCD at low energyQCD at low energy..

Lightest Pseudoscalar MesomsLightest Pseudoscalar Mesoms

55

The PrimEx Experimental ProjectThe PrimEx Experimental Project

Experimental program Precision measurements of:

Two-Photon Decay Widths: Γ(0→), Γ(→), Γ(’→)

Transition Form Factors at low Q2 (0.001-0.5 GeV2/c2): F(*→ 0), F(* →), F(* →)

Test of Chiral Symmetry and Anomalies via the Primakoff Effect

66

Physics Outcome

Fundamental input to Physics:

precision test of chiral anomaly predictions determination of quark mass ratio -’ mixing angle 0, and ’ interaction electromagnetic radii is the ’ an approximate Goldstone boson?

7

First experiment: 0 decay width

eVF

mNc 725.7576 23

3220

0→ decay proceeds primarily via the chiral anomaly in QCD. The chiral anomaly prediction is exact for massless quarks:

Corrections to the chiral anomaly prediction: (u-d quark masses and mass differences)

Calculations in NLO ChPT:(J. Goity, at al. Phys. Rev. D66:076014, 2002)Γ(0) = 8.10eV ± 1.0%

~4% higher than LO, uncertainty: less than 1%

Precision measurements of (0→) at the percent level will provide a stringent test of a fundamental prediction of QCD.

0→

Recent calculations in QCD sum rule: (B.L. Ioffe, et al. Phys. Lett. B647, p. 389, 2007)

Γ() is only input parameter 0- mixing includedΓ(0) = 7.93eV ± 1.5%

8

Decay Length Measurements (Direct Method)

1x10-16 sec too small to measure

solution: Create energetic 0 ‘s,

L = vE/m

But, for E= 1000 GeV, Lmean 100 μm very challenging experiment

Measure 0 decay length

1984 CERN experiment: P=450 GeV proton beamTwo variable separation (5-250m) foilsResult:(0) = 7.34eV3.1% (total)

Major limitations of method unknown P0 spectrum needs higher energies for improvement

0→

9

e+e- Collider Experiment

e+e-e+e-**e+e-0e+e-

e+, e- scattered at small angles (not detected)

only detected

DORIS II @ DESY

Results: Γ(0) = 7.7 ± 0.5 ± 0.5 eV ( ± 10.0%)

Not included in PDG average

Major limitations of method knowledge of luminosity unknown q2 for **

0→

1010

Primakoff Method

22

..4

43

3

2Pr

3

sin)(8

QFQ

E

m

Z

d

dme

ρ,ω

Challenge: Extract the Primakoff amplitude

from the experimental cross section

12C target

Primakoff Nucl. Coherent

Interference Nucl. Incoh.

)log(

2

2Pr

4Pr

2

2

Pr

EZd

Ed

dE

m

peak

peak

11

Previous Primakoff Experiments

DESY (1970) bremsstrahlung beam,

E=1.5 and 2.5 GeVTargets C, Zn, Al, Pb Result: (0)=(11.71.2) eV

10.%

Cornell (1974) bremsstrahlung beam

E=4 and 6 GeV targets: Be, Al, Cu, Ag, U Result: (0)=(7.920.42) eV

5.3%

All previous experiments used: Untagged bremsstrahlung beam Conventional Pb-glass calorimetry

12

PrimEx Experiment at Hall B JLab

JLab Hall B high resolution, high intensity photon tagging facility

New pair spectrometer for photon flux control at high intensities New high resolution hybrid multi-channel calorimeter (HYCAL)

Requirements of Setup: high angular resolution (~0.5 mrad)

high resolutions in calorimeter small beam spot size (‹1mm)

Background: tagging system needed

Particle ID for (-charged part.) veto detectors needed

1313

Fit to Extract Γ(0) Decay Width Theoretical angular distributions smeared with experimental

resolutions are fit to the data

12

C 208Pb

14L. Gan APS, April 15, 2008 14

Estimated Systematic Errors

Contributions Errors

Photon flux 1.0%

Target number 0.1%

Background subtraction 0.9%

Event selection 0.5%

HYCAL response function 0.5%

Beam parameters 0.4%

Acceptance 0.3%

Model errors (theory) 0.25%

Physics background 0.24%

Branching ratio (PDG) 0.03%

Total 1.6%

1515

Current PrimEx Result

() = 7.93eV2.3%1.6%

16

Next Run

16

1717

PrimEx @ High Energies with EICPrimEx @ High Energies with EIC

Experimental program

Precision measurements of:

Transition Form Factors at low Q2 (0.001-0.5 GeV2/c2):

F(*→ 0), F(* →), F(* →)

1818

Primakoff Method

22

..4

43

3

2Pr

3

sin)(8

QFQ

E

m

Z

d

dme

ρ,ω

Challenge: Extract the Primakoff amplitude

12C target

Primakoff

Nucl. Coherent

Interference

Nucl. Incoh.

19

)log( 2

Pr4Pr EZdE

d

d

peak

Increase Primakoff cross section:

Better separation of Primakoff reaction from nuclear processes:

Momentum transfer to the nuclei becomes less reduce the incoherent background

3/12

2

Pr

2

2 AEE

mNCpeak

Why do we need high energy?

20

Direct measurements of slopes:

F(*→ 0), F(* →), F(* →)

Interaction radii:

Fγγ*P(Q2) ≈ 1 - 1/6▪<r2>PQ2

ChPT for large Nc predicts relation between the slopes.

Extraction of Ο(p6) low-energy constant in the chiral Lagrangian

Extraction of decay widths:

Γ(0→), Γ(→), Γ(’→)

Precision test of chiral anomaly predictions

Transition Form Factors at Law Q2

21

Experimental Status for Experimental Status for F(*→ 0)

F(*→ 0) ≈ 1 – a Q2/m2

22

Experimental Status for Experimental Status for F(* →)

2323

PrimEx @ High Energies with EIC Precision Measurement of → decay width

All decay widths are calculated from decay width and experimental Branching Ratios (B.R.):

ΓΓ((η→η→ decay) = decay) = ΓΓ((→→) × B.R.) × B.R.

Any improvement in ΓΓ((→→))

will change the whole will change the whole - sector in PDB- sector in PDB

24

)(2

1ˆ ,

22

222

duud

s mmmmm

mmQ

..)()3( RB

There are two ways to determine the quark mass ratio:

•Γ(η→3π) is the best observable for determining the quark mass ratio, which is obtained from Γ(η→γγ) and known branching ratios:

•The quark mass ratio can also be given by a ratio of The quark mass ratio can also be given by a ratio of meson masses: meson masses:

)(1)(

222

22

2

22 m

mm

mm

m

mQ

QCDKK

kk

o

Determination of quark mass ratioDetermination of quark mass ratio

2525

Corr. )( ..0 meKKmm

)(2

1ˆ re whe,

22

222

duud

s mmmmm

mmQ

ΓΓ((ηη→→33)=)=ΓΓ((→→))××B.B.R.R.

Determination of quark mass ratioDetermination of quark mass ratio

26

• Mixing corrections:

)(cos)(sin

)(sin)(cos00

0

008

008

• DecayDecay constant corrections:

000

888

000

888

cos ,sin

sin ,cos

ffff

ffff

Γ(η/η´→γγ) widths are crucial inputs for obtaining fundamental mixing parameters.

Mixing Angles Mixing Angles

2727

Summary

Extrapolation to Q2=0 will define the radiative decay widths: Γ(0→), Γ(→), Γ(’→)

It looks possible to perform high precision transition form factor measurements of light pseudoscalar mesons at low Q2 with EIC at high energies

Fundamental input to Physics:

precision test of chiral anomaly predictions 0, and ’ interaction electromagnetic radii

extraction of Ο(p6) low-energy constant in the chiral Lagrangian

determination of quark mass ratio -’ mixing angle is the ’ an approximate Goldstone boson?

28A. Gasparian Hall D, March 7, 2008 28

The End

2929

The Primakoff Effect

22

..4

43

3

2Pr

3

sin)(8

QFQ

E

m

Z

d

dme

ρ, ω

Challenge: Extract the Primakoff amplitude

3030

(0→) World Data

0 is lightest quark-antiquark hadron

The lifetime:

= B.R.( 0 →γγ)/(0 →γγ) 0.8 x 10-16 second

Branching ratio: B.R. ( 0→γγ)= (98.8±0.032)% 0

→

±1%

3131

Estimated Systematic Errors

Contributions Errors

Photon flux 1.0%

Target number 0.1%

Background subtraction 0.9%

Event selection 0.5%

HYCAL response function 0.5%

Beam parameters 0.4%

Acceptance 0.3%

Model errors (theory) 0.25%

Physics background 0.24%

Branching ratio (PDG) 0.03%

Total 1.6%

32

Electromagnetic Calorimeter: HYCAL Energy resolution Position resolution Good photon detection efficiency @ 0.1 – 5 GeV; Large geometrical acceptance

PbWO4 crystals resolutionPb-glass budget

HYCALonly

Kinematicalconstraint

33

15 Days

Beam Time and Statistics

Target: L=20 cm, LHe4 NHe = 4x1023 atoms/cm2 Nγ = 1x107 photon/sec (10-11.5 GeV part)<Δσ(prim.)> = 1.6x10-5 mb

N() = NHexNγx<Δσ>xεx(BR)

= 4x1023x 1x107x 1.6x10-32x0.7x0.4 = 64 events/hour = 1500 events/day = 45,000 events/30 days

Will provide sub-percent systematic error