Test of QCD Symmetries via Measurements on Light Pseudoscalar Mesons Liping Gan University of North Carolina Wilmington China, 2009 Liping Gan, UNCW

Contents Physics Motivation PrimEx experimental program at Jlab 12 GeV Symmetries of QCD Properties of π0, η and η’ PrimEx experimental program at Jlab 12 GeV Primakoff experiments Rare decays of η and η’ China, 2009 Liping Gan, UNCW

Symmetry Conservation Law What is symmetry? Symmetry is an invariance of a physical system to a set of changes . Noether’s theorem Symmetry Conservation Law Symmetry and symmetry breaking are fundamental in the laws of physics China, 2009 Liping Gan, UNCW

Continuous Symmetrys of QCD in the Chiral Limit chiral limit: is the limit of vanishing quark masses mq→ 0. QCD Lagrangian with quark masses set to zero: Large global symmetry group: China, 2009 Liping Gan, UNCW

Fate of Symmetrys China, 2009 Liping Gan, UNCW

Discrete Symmetries of QCD Charge: C Parity: P Time-Reversal: T Combinations: CP, CT, PT, CPT China, 2009 Liping Gan, UNCW

Lightest pseudoscalar mesons Chiral SUL(3)XSUR(3) spontaneously broken Goldstone mesons π0, η8 Chiral anomalies Mass of η0 P→ ( P: π0, η, η׳) Quark flavor SU(3) breaking The mixing of π0, η and η׳ China, 2009 Liping Gan, UNCW

Some Interesting η Rare Decay Channels Mode Branching Ratio Physics Highlight π0 π0 <3.5 × 10 − 4 CP, P π0 2γ ( 2.7 ± 0.5 ) × 10 − 4 χPTh, Ο(p6) π+ π− <1.3 × 10 − 5 π0 π0 γ <5 × 10 − 4 C 3γ <1.6 × 10 − 5 π0 π0 π0 γ <6 × 10 − 5 π0 e+ e− <4 × 10 − 5 4π0 <6.9 × 10 − 7 The π0, η and η’ system provides a rich laboratory to study the symmetry structure of QCD. China, 2009 Liping Gan, UNCW

PrimEx Program at Jlab (1) Primakoff experiments to measure: Two-Photon Decay Widths: Γ(π0 →) @ 6 GeV Γ(η →), Γ(η׳ →) Transition Form Factor Fγγ*P of π0, η and η׳ at low Q2 (0.001--0.5 GeV2/c2) (2) Measure the branching ratios of η and η’ rare decays * China, 2009 Liping Gan, UNCW

Transition Form Factor Experimental Status Decay width Transition Form Factor China, 2009 Liping Gan, UNCW

Determine the quark masse ratio Γ(η→3)=Γ(→)×B.R. China, 2009 Liping Gan, UNCW

Mixing angles of η-η׳ Mixing angles: Decay constants: Γ(η/η’ →) widths are crucial inputs for obtaining fundamental mixing parameters. China, 2009 Liping Gan, UNCW

Two-Photon Decay Widths Test chiral anomaly predictions: Features of anomaly: Unique property of the quantum theory. Calculable exactly to all orders in the chiral limit China, 2009 Liping Gan, UNCW

Number of colors in QCD In the past 30 years, many textbooks stated that Γ(π0 →γγ) was the best probe to determine the number of quark colors at low energy Recent calculations pointed out that Γ(π0→γγ) is less sensitive to Nc due to partial cancellations of the WZW term with a Goldstone-Wilczek term The decay amplitude of the single field (η0) depends strongly on Nc and yield under the inclusion of mixing also a strong Nc dependence for the η decay Both the Γ(η→γγ) and Γ(η’→γγ) decays are suited to confirm the number of colors China, 2009 Liping Gan, UNCW

Transition Form Factors at Low Q2 Direct measurement of slopes Interaction radii: Fγγ*P(Q2)≈1-1/6▪<r2>PQ2 ChPT for large Nc predicts relation between the three slopes. Extraction of Ο(p6) low-energy constant in the chiral Lagrangian Input for light-by-light scattering for muon (g-2) calculation Test of future lattice calculations China, 2009 Liping Gan, UNCW

Primakoff Process ρ,ω Challenge: Extract the Primakoff amplitude with unprecedented accuracy China, 2009 Liping Gan, UNCW

Features of Primakoff cross section: Beam energy sensitive Peaked at very small forward angle Coherent process China, 2009 Liping Gan, UNCW

PrimEx Experiment on 0 at 6 GeV 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 China, 2009 Liping Gan, UNCW

PrimEx-I Experiment: Γ(0) Decay Width Nuclear targets: 12C and 208Pb; 6 GeV Hall B tagged beam; experiment performed in 2004 12C 208Pb China, 2009 Liping Gan, UNCW

0 = 7.82eV ± 2.2%stat. ± 2.1%syst. (± 3.0% total) PrimEx-I Result 0 = 7.82eV ± 2.2%stat. ± 2.1%syst. (± 3.0% total) PrimEx China, 2009 Liping Gan, UNCW

PrimEx-II run @ 6 GeV Projected PrimEx-II China, 2009 Liping Gan, UNCW

12 GeV Experimental Setup New high energy photon tagger Improved PrimEx calorimeter HYCAL with all PbWO4 Choose the light targets 4He and 1H China, 2009 Liping Gan, UNCW

Proposed Experiment on Γ(η→) with GlueX Setup Counting House 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.) China, 2009 Liping Gan, UNCW 23

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. China, 2009 Liping Gan, UNCW

Statistics and Beam Time Request Target: 30 cm (3.46% r.l.) LH2, Np=1.28x1024 p/cm2 Photon intensity: 7.6x106 γ/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 10-2 events/sec = 2200 events/day = 220 Primakoff events/day Beam time request: LH2 target run 45 days Empty target run 5 days Tagger efficiency, TAC 3 days Setup calibration and checkout 7 days Total 60 days Statistics: 45 days of run on LH2: 1% stat. error China, 2009 Liping Gan, UNCW

Estimated Error Budget on Γ(η →) Systematical errors: Contributions Estimated Error Photon flux 1.0% Target number 0.5% Background subtraction Event selection 0.8% Acceptance, misalign. Beam energy 0.2% Branching ratio (PDG) 0.66% Total Systematic 1.9% Total estimated error: Statistical error 1.0% Systematic error 1.9% Total Error 2.2% China, 2009 Liping Gan, UNCW

Some η Rare Decay Channels Mode Branching Ratio Physics Highlight π0 π0 <3.5 × 10 − 4 CP, P π0 2γ ( 2.7 ± 0.5 ) × 10 − 4 χPTh, Ο(p6) π+ π− <1.3 × 10 − 5 π0 π0 γ <5 × 10 − 4 C 3γ <1.6 × 10 − 5 π0 π0 π0 γ <6 × 10 − 5 π0 e+ e− <4 × 10 − 5 4π0 <6.9 × 10 − 7 China, 2009 Liping Gan, UNCW

Study of η→0 0 Reaction The Origin of CP violation is still a mystery CP violation is described in SM by the phase in the Cabibbo-Kobayashi-Maskawa quark mixing matrix. A recent SM calculation predicts BR(η→0 0)<3x10-17 The η→0 0 is one of a few available flavor-conserving reactions listed in PDG to test CP violation. Unique test of P and PC symmetries, and search for new physics beyond SM China, 2009 Liping Gan, UNCW

History of the η→0 Measurements After 1980 A long standing “η” puzzle is still un-settled. China, 2009 Liping Gan, UNCW

High Energy η Production (GAMS Experiment on η→0 at Serpukhov) Experimental result was first published in 1981 The η’s were produced with 30 GeV/c - beam in the -p→ηn reaction Decay ’s were detected by lead-glass calorimeter Final result (D. Alde et al.) ~40 of η→0 events BR(η→0γγ)=(7.1±1.4)x10-4 (η→0γγ)=0.84±0.17 eV Major Background -p→ 00n η →000 China, 2009 Liping Gan, UNCW

Low energy η production (CB experiment on η→0 at AGS, by S Low energy η production (CB experiment on η→0 at AGS, by S. Prakhov et al. ) η →000 -p→ 00n The η’s were produced with 720 MeV/c - beam through the -p→ηn reaction Decay ’s energy range: 50-500 MeV Final result 1600 of η→0 events (η→0γγ)=0.45±0.12 eV China, 2009 Liping Gan, UNCW

What can be improved at 12 GeV Jlab? High energy tagged photon beam to reduce the background from η→ 30 Lower relative threshold for -ray detection Improve calorimeter resolution Tag η by measuring recoiled particles to reduce non-resonance 00 background High resolution PWO Calorimeter Higher energy resolution → improve 0γγ invariance mass Higher granularity→ better position resolution and less overlap clusters Large statistics to provide a precision measurement of Dalitz plot July, 2009 Liping Gan, UNCW

Suggested Experiment in Hall D at Jlab Counting House Photon Tagger GlueX FCAL Simultaneously measure the η→0, η →00: η produced on LH2 target with 11 GeV tagged photon beam γ+p → η+p Tag η by measuring recoil p with GlueX detector Forward calorimeter with PWO insertion to detect multi-photons from the η decay China, 2009 Liping Gan, UNCW

S/N Ratio vs. Calorimeter Granularity PWO dmin=4cm S/N=1.4 Pb Glass dmin=8cm S/N=0.024 July, 2009 Liping Gan, UNCW

Summary Fundamental input to Physics: (1) Primakoff experiments to measure: Two-Photon Decay Widths: Γ(0 →), Γ(η→), Γ(η׳ →) Transition Form Factor Fγγ*P of π0, η and η׳ at low Q2 (0.001-0.5 GeV2/c2) (2) Measure the branching ratios for η and η’ rare decays Fundamental input to Physics: Determination of quark mass ratio Mixing parameters ⇒ decay constants and mixing angles of η―η׳ Test chiral anomaly predictions Confirm number of colors Nc Ο(p6) low-energy-constant in Chiral Lagrangian Test P, CP and C symmetries, and search for new physics beyond Standard Model China, 2009 Liping Gan, UNCW

Invariant Mass Resolution σ=6.9 MeV σ=3.2 MeV PWO M0 M σ=6.6 MeV σ=15 MeV Pb glass M M0 Oct 31, 2008 Liping Gan, UNCW

Why do we need 12 GeV beam? Increase Primakoff cross section: Better separation of Primakoff reaction from nuclear processes: Momentum transfer to the nuclei becomes less reduce the incoherent background Unique CEBAF beam quality China, 2009 Liping Gan, UNCW

Challenges in Modern Physics What are the building blocks of matter? Strong force Strong force obeys the rules of Quantum Chromodynamics (QCD) What happens if one tries to separate two quarks? Color confinement: the potential energy between (in this case) a quark and an antiquark increases while increasing the distance between them. The properties of strong force at large distance represents one of the biggest intellectual challenges in physics. Solution: Understanding symmetries of nature China, 2009 Liping Gan, UNCW

Experimental Resolutions: Prod. Angle Precision Primakoff measurement requires high resolutions in: Production angle (fit); Invariant mass (background) Energy (elasticity) China, 2009 Liping Gan, UNCW 39 A. Gasparian, Hall D, May 9, 2008 39

Experimental Resolutions (contd.) γγ invariant mass Energy conservation (elasticity) High resolution, high granularity calorimeter is critical in: event selection; extraction of Primakoff from hadronic processes China, 2009 Liping Gan, UNCW 40 A. Gasparian, Hall D, May 9, 2008 40

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 CEBAF 12 GeV CDR With the following statement in the 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.” China, 2009 Liping Gan, UNCW

Low Energy η Production Continue (KLOE, by B. Micco et al. , Acta Phys Low Energy η Production Continue (KLOE, by B. Micco et al., Acta Phys. Slov. 56 (2006) 403) Produce Φ through e+e- collision at √s~1020 MeV The decay η→0γγproceeds through: Φ→η, η→0γγ, 0→γγ Final result 68±23 of η→0 events BR(η→0γγ)=(8.4±2.7±1.4)x10-5 (η→0γγ)=0.109±0.035±0.018 eV China, 2009 Liping Gan, UNCW

Determine the quark masse ratio 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 meson masses: China, 2009 Liping Gan, UNCW