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A Precision Measurement of the η Radiative Decay Width via the Primakoff Effect

Explore PrimEx program at 12 GeV, study previous experiments, propose future experiment, control systematic errors, and improve chiral symmetry understanding in QCD. Investigate decay widths and transition form factors.

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A Precision Measurement of the η Radiative Decay Width via the Primakoff Effect

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  1. A Precision Measurement of the η Radiative Decay Width via the Primakoff Effect Outline • PrimEx program @12 GeV • Previous Γ(→) experiments • The proposed experiment • Control of systematic errors • Summary A. Gasparian NC A&T State University, Greensboro, NC for the PrimEx Collaboration

  2. 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(* →) PrimEx Project @ 12 GeV Input to Physics: • precision tests of Chiral symmetry and anomalies; • determination of quark mass ratio • -’ mixing angle • 0, and ’ interaction electromagnetic radii • is the ’ an approximate Goldstone boson? PAC34, Jan 27, 2009

  3. Symmetries in QCD • Classical QCD Lagrangian in Chiral limit is invariant under: • Chiral SUL(3)xSUR(3) spontaneously broken: • 8 Goldstone Bosons (π,K,η,η/) • UA(1) is explicitly broken: (Chiral anomalies) • Γ(0→), Γ(→), Γ(’→) • Mass of ’ • Massive quarks, SU(3) broken: • GB are massive • Mixing of π0ηη/ The π0, η, η’ system provides a rich laboratory to study the symmetry structure of QCD at low energy. PAC34, Jan 27, 2009

  4. PrimEx 12 GeV Project History • This program had been reviewed by 3 special high energy PACs: PAC18 (2000) PAC23 (2003) PAC27 (2005) • It is included in the 12 GeV CDR under: “Test of the Standard Model of electro-weak interactions and the determination of fundamental Parameters of this model” • With the following statement in the CDR Abstract: “… Precision measurements of the two-photon decay widths and transition form factors of the three neutral pseudoscalar mesons via the Primakoff effect will lead to a significant improvement on our knowledge of chiral symmetry in QCD, in particular on the ratios of quark masses and on chiral anomalies.” PAC34, Jan 27, 2009

  5. PrimEx-I Experiment: Γ(0) Decay Width • Nuclear targets: 12C and 208Pb; • 6 GeV Hall B tagged beam; • experiment performed in 2004 12C 208Pb PAC34, Jan 27, 2009

  6. PrimEx-I Result () = 7.93eV2.3%1.6% PAC34, Jan 27, 2009

  7. PrimEx-II Planned Run • Approved by PAC33 for 20 days to get high statistics 1.4% PAC34, Jan 27, 2009

  8. → Decay Width:e+e- Collider Experiments • e+e-  e+e-** e+e- η  e+e-  • e+,e- scattered at small angles (not detected); • Only  detected; • Error in individual experiments: from 7.6% to 25% η→ • PDG average for collider experiments: Γ(η) = 0.510 ± 0.026 keV ( ± 5.1%) • Major limitations of method • unknown q2 for ** • knowledge of luminosity PAC34, Jan 27, 2009

  9. ρ,ω Primakoff Method η • Difficulties of →experiment: • cross section is smaller • larger overlap between Primakoff and hadronic processes Challenge: Separate Primakoff amplitude from hadronic processes. • larger momentum transfer: We propose to use hydrogen target to address all tose issues. PAC34, Jan 27, 2009

  10. Cornell Primakoff Experiment • Cornell (PRL, 1974) • brems.  beam, E=5.8, 9.0, 11.45 GeV • targets: Be, Al, Cu, Ag, U • Result: (η)=(0.3240.046) keV (14.2%) PAC34, Jan 27, 2009

  11. Comments on Cornell Experiment • Comments on Cornell experiment: • Untagged bremsstrahlung  beam • Conventional Pb-glass calorimetor: (size: 4.5x4.5x49. cm3 target to calorimeter dist.: 4.7m) • As a result: • insufficient resolutions in experimental parameters; • hard to resolve Primakoff from hadronic contribution; • model dependent calculation for hadronic contributions. PAC34, Jan 27, 2009

  12. Precision Measurement of → decay width • We propose to measure Γ(→) with 2.2% total error • all-decay widths are normalized to  decay width and experimental Branching Ratios (B.R.): Γ(η→all-decays) =Γ(→)/B.R. • Improvement in Γ(→) will change the whole -sector in PDG PAC34, Jan 27, 2009

  13. Physics Outcome from  Experiment • light quark mass ratio • ( - ’) mixing angle: Γ(η→3)=Γ(→)×B.R. PAC34, Jan 27, 2009

  14.  Experiment with GlueX Setup • Advantages of Hall D/GlueX setup: • High energy tagged photon beam Eγ=10 – 11.7 GeV • High acceptance Pb-glass calorimeter (FCAL) • LH2 and LHeTargets (30 cm) • Pair Spectrometer Counting House • Disadvantages of standard Hall D/GlueX setup for Γ(→) precision measurement: • Beam line: 3.4 mm collimator on 75 m distance: • very low tagging efficiency (~15%); • Radiation damage effects of diamond crystal on endpoint spectrum? • Compton cross section measurement to control overall systematic errors: • FCAL with its central hole unable to detect double arm Compton (need more than 10 m distance) 75 m PAC34, Jan 27, 2009 14

  15. Propositions for Existing Setup • 5.0 mm collimator • double the tagging efficiency from 15% to ~30% • help to control photon flux stability within 1% 5.0mm coll. Design limit • Amorphous radiator (~10-4 r.l. Au) • better control of photon flux at endpoint • HyCal Calorimeter • enables Compton detection • factor of 2 better experimental resolutions: • better event selection; • extraction of Primakoff process from hadronic background. PAC34, Jan 27, 2009 15

  16. Proposed Experiment with GlueX Setup • General characteristics of proposed experiment: • Incoherent bremsstrahlung photon beam Eγ =10.5 – 11.7 GeV (~10-4 r.l. Au radiator, 5.0 mm beam collimator) • High resolution, high segmentation HyCal Calorimetor • 30 cm LH2 target (~3.6 r.l.) • Advantages of Hydrogen target: • no inelastic hadronic contribution; • no nuclear final state interactions; • proton form factor is well known; • better separation between Primakoff and nuclear processes; • new theoretical developments of Regge description of hadronic processes. PAC34, Jan 27, 2009 16

  17. Experimental Resolutions: Prod. Angle • Precision Primakoff measurement requires high resolutions in: • Production angle (fit); • Invariant mass (background) • Energy (elasticity) PAC34, Jan 27, 2009 17

  18. Experimental Resolutions (contd.) • γγinvariant mass • Energy conservation (elasticity) • High resolution, high granularity calorimeter is critical in: • event selection; • extraction of Primakoff from hadronic processes (fitting stage). PAC34, Jan 27, 2009 18

  19. Statistics and Beam Time Request • Target: 30 cm (3.46% r.l.) LH2, Np=1.28x1024 p/cm2 • Photon intensity: 7.6x106γ/sec in Eγ= 10.5–11.7 GeV • Total cross section on P for θη=0 - 3.50, Δσ = 1.1x10-5 mb (10% is Primakoff). • N(evts) = Np x Nγ x Δσ x ε(eff.)x(Br. Ratio) = 1.28x1024x7.6x106x1.1x10-32x0.6x0.4 = 2.6 x 10-2events/sec = 2200 events/day = 220 Primakoff events/day • Beam time request: • Statistics: 45 days of run on LH2: 1% stat. error PAC34, Jan 27, 2009

  20. Estimated Error Budget • Systematical errors: • Total estimated error: PAC34, Jan 27, 2009

  21. Control of Overall Systematics: Compton Cross Section PrimEx-I Result Δσ/ΔΩ (mb/6.9 msrad) Data with radiative corrections • Total error: 1.3% reached in PrimEx-I run PrimEx-I Result PAC34, Jan 27, 2009

  22. Responses to Theory and TAC Comments • Theory comments: • “… How do hadronic contribution effect the decay width?” • The suggested hydrogen target will minimize the hadronic contributions: (a) no nuclear incoherent processes; (b) no final state interaction: better control of the measured nuclear coherent amplitude at relatively large angles; • “… new article with refitting the Cornell data …” • (a) the data from the Cornel experiment are not available for reanalysis; (b) this article fitted the data without the angular resolutions in; (c) it demonstrates the need for a new optimized Primakoff experiment to address the long standing discrepancy between the two data sets. • TAC comments: • About the “… DAQ readout electronics and trigger…” • It was discussed with Hall D leadership how to best organize the replacement of FCAL with HyCal for the proposed experiment. We agreed that building an “interface module” to connect the HyCal output cables into the FCAL output cables is the most cost effective and straightforward solution. It may cost ~$10K and will solve all questions for signal processing, trigger and DAQ. PAC34, Jan 27, 2009

  23. Summary • PrimEx-12 experimental program has been developed to perform precision tests of chiral symmetry and anomaly effects in the light pseudoscalar meson sector. • This program has been a part of the “Science Driving the 12 GeV Upgrade”. It is included in the “Abstract” of the CDR as a part of the physics program. • Hall D/GlueX experimental facility with its high resolution and high intensity 12 GeV photon tagger is uniquely suited for the Г() Primakoff measurement with high accuracy. • The first experiment, the 0 lifetime measurement, has been performed in fall, 2004 in Hall B with 6 GeV beam. (0) has been extracted with high precision at 3% level. A new PrimEx-II run is approved to reach the projected 1.4% precision. • New Primakoff precision experiment for Г(→) will: • Potentially solve the discrepancy between collider and Primakoff results; • Improve all  decay widths in PDG by more than a factor of 2; • Determine the light quark mass ratio in model independent way; • Significantly improve the (-’) mixing angle determination. PAC34, Jan 27, 2009

  24. Summary (contd.) • To reach the 2.2% accuracy in Γ(→) measurement, we propose to: • Use hydrogen target to minimize the effects of hadronic processes: • No incoherent contribution; • No final state interactions: better control of nuclear coherent contribution; • Use high resolution HyCalcalorimeter to significantly improve the systematic errors in this experiment. • Use larger (5 mm) beam collimator to control the photon flux at 1% level with PS. • Simultaneous measurement of Compton cross section to control the overall error in the experiment. • Requesting 60 days of beam time to measure the Γ(→) with 2.2% accuracy. PAC34, Jan 27, 2009

  25. The End PAC34, Jan 27, 2009

  26. Electromagnetic Background PAC34, Jan 27, 2009

  27. Geometrical Acceptance • Coverage of γγevents in HyCal vs. distance • Coverage of γγevents in HyCal vs. production angle • HyCal with its moderate size will provide a good (~60%) at 6.5 m for these energies. PAC34, Jan 27, 2009 27

  28. Comments on Cornell’s Result on η(I will redo this slide) Major concerns: • Large overlap between the Primakoff production and the nuclear background. • Uncertainty of the theoretical calculations on the charge form factors and hadronic distributions. • A conventional lead glass calorimeter • a poor 2γ invariant mass resolution for the η events selection, and a poor angular resolution to discern the characteristic shapes of the different production mechanisms. • Untagged photon beam. M2γγ(GeV2)

  29. Compton as Stability Control(maybe to question section) σ (mb) PAC34, Jan 27, 2009

  30. Physics Motivation Fundamental input to Physics: • precision tests of chiral anomaly • determination of quark mass ratio • -’ mixing angle • 0, and ’ interaction electromagnetic radii • is the ’ an approximate Goldstone boson? PAC34, Jan 27, 2009

  31. The Need for a Dedicated Run • Instability of the photon flux at the endpoint of the bremsstrahlung spectrum due to radiation damage of the diamond radiator. • Propose to use an amorphous metallic radiator (~10-4 R. L. Au) • Low tagging efficiency (~15%) due to a 3.4 mm diameter collimator. • Propose to use 5.0 mm diameter collimator to double the tagging efficiency and control of the photon flux at 1% level. • Unable to detect the atomic Compton scattering simultaneously with the η production to control the overall systematic errors. • The new HyCal calorimeter with its small beam hole size (6x6 cm2) will access the Compton electrons at small angles (0.450 at 6.5 m distance) PAC34, Jan 27, 2009

  32. Advantages of the proton Target(Will work on this one) • The incoherent hadronic contribution vanishes within our kinematical range. • The proton form factor is well known, and is free of corrections from the nuclear final state interactions. • The coherent hadronic photoproduction on proton is peaked at much larger angles than the Promakoff peak. • New theoretical development of a Regge description of meson photoproduction in the proton strong field. PAC34, Jan 27, 2009

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