The “New” Charge Symmetry precision experiments in few nucleon systems from meson-exchange to effective field theory Ed Stephenson Indiana University Cyclotron Facility SPIN 2006 Kyoto, Japan How do we understand charge symmetry breaking? examples from meson exchange scattering lengths n-p analyzing power differences examples from effective field theory n+p→d+π 0 fore-aft asymmetry d+d→ 4 He+π 0 Keys to experimental precision

Traditional view of charge symmetry: Charge symmetry requires that any process/property be invariant under neutron-proton swap. Electromagnetic effects completely violate charge symmetry. We may still ask whether the strong interaction obeys this symmetry. Things which may violate charge symmetry: mass differences (n–p, π ± –π 0, …) meson isospin mixing (ρ 0 –ω, π 0 –η, …) (residual EM effects) Not considered here: isospin multiplets of baryons/mesons mirror nuclei (Not charge conjugation)

Charge Symmetry Breaking in the N-N Interaction m N – m P = MeV Different NN scattering lengths fm a pp (corrected) a nn a np CSB Charge Independence Breaking How are these effects be understood within a meson exchange picture? ρ 0 – ω mixing contribution of the π ± - π 0 mass difference in OPEP and TPEP EM contributions in π+γ exchange

n-p analyzing power difference. n n p p CS. p p n n Layout of TRIUMF experiment beam and target polarized, same geometry for both cases look near zero crossing to avoid calibration issues Keys:

Results from IUCF and TRIUMF experiments residual EM effect comes from neutron magnetic moment in magnetic field of moving proton most of effect is attributed to n-p and pion mass differences additional effect result of ρ 0 –ω mixing

Shift in approach to sources of CSB: emphasize quark level sources rather than nucleon level ud d uu d swap neutronproton Charge symmetry requires that no process/property depend on the swap of the down and up quarks (in strong force). Use effective field theory to define contribution to CSB. New experiments involve pion production.

n+p→d+π 0 Charge symmetry requires no change when n and p are swapped, so cross section is symmetric about 90°. all angles recorded in SASP at once detector efficiency independently calibrated data compared to Monte Carlo simulation with A fb variable Keys: [PRL 91, (’03)] SASP spectrometer particles move through system target point focal plane detectors

The data forward deuterons backward deuterons This difference is an artifact of the SASP. So asymmetry requires a detailed model of the experiment. All model properties determined independently, except: beam energy central SASP momentum target thickness A 1 /A 0 = 2A fb where Results from Monte Carlo study: A fb = ± ± % (stat) (sys)

d+d→ 4 He+π 0 forbidden by: isospin conservation π 0 is T = 1 charge symmetry π 0 is odd under CS major physics background: d+d→ 4 He+γ+γ clean selection of candidate events particle ID on 4 He [scintillators] Pb-glass selection of energetic photons good missing mass (π 0 ) reconstruction scattering angle [WC1] TOF in channel [ΔE 2 –ΔE 1 ] (include channel energy loss, compensate for PMT time drift) NOTE: cross section normalized to d+p elastic Keys: [PRL 91, (’03)]

Particle Identification (using scintillator signals) E ΔE2ΔE2 ΔE2ΔE2 Windows select 4 He events but rate is 10 3 too high due to d-induced reactions on residual gas and beam pipe ΔE1ΔE1 Select 2-photon events with left and right Pb-glass all events that pass particle identification events inside window final cut Final cut leaves no background, only π 0 and γ+γ

Results at two energies near threshold σ TOT = 12.7 ± 2.2 pb 15.1 ± 3.1 pb π 0 peak γ+γ continuum (scaled for channel acceptance) peak positions correct to 60 keV systematic errors about 7% (excluding normalization) upper 2 MeV of γ+γ continuum: σ = 6.9 ± 0.9 pb and 9.5 ± 1.4 pb (about twice prediction) average η = p π /m π σ TOT /η results consistent with S-wave

Cross section normalized to d+p elastic scattering online monitor is d+d elastic at 90° cm To calibrate online monitor, use HD gas and observe d+p elastic scattering at 25°(d) – 44°(p). [see K. Ermisch, PRC 71, ] 108 MeV 120 MeV 135 MeV interpolate to 116 MeV

but the KVI measurements disagree with Japanese data [Sekiguchi, PRL 95, ] Energy dependence We need the cross section here. KVI data Japanese data compare Ermisch data Other measurements on graph: 93.6 MeV: Chamberlain/Stern, PR 94, 666 (’54) 146 MeV: Postma/Wilson, PR 121, 1229 (’61) 155 MeV: Kuroda et al., NP 88, 33 (’66) 198 MeV: Adelberger/Brown, PR 5, 2139 (’72) More work is needed! 116 MeV new RCNP data

Charge Symmetry Breaking contributions from Effective Field Theory van Kolck, Niskanen, and Miller, PL B 493 (2000) 65 Leading order contributions: Down-up quark mass difference Electro- magnetic Nucleons and pions are components of model. Scale parameters determined from experiment. nucleon-only contributionnucleon-pion scattering (free pion-nucleon system limited to π + or π – with protons; large EM corrections limit view of CSB) In π 0 production, consider: π0π0 x CSB here Cottingham sum rule: so: Estimate size:

Theory status (work in progress): Pion rescattering large for n+p→d+π 0, but vanishes (except recoil and ISI) for d+d→ 4 He+π 0. Other terms matter! Re-introduce meson- exchange and meson mixing to stand in for missing EFT terms. Add π 0 –η mixing % n+p→d+π 0 π 0 –η mixing (Niskanen) EFT pion rescattering van Kolck prediction:re-evaluation: n-p mass difference d+d→ 4 He+π 50 0 pb data ρ 0 –ω mixing π 0 –η mixing EFT EM EFT quark mass difference wavefunction isospin mixing π 0 –η mixing downscaled [contributions to amplitude] [S, P-wave interference]

Further comments: Pion production experiments and EFT theory happened together. After several attempts, experiments have succeeded. New approaches achieved required sensitivity. Interpretation is still in progress: EFT has focused us on quark origins of CSB, including meson mixing. Even at threshold, pion production required high momentum transfer. This does not fit easily into EFT expansion scheme. Next order EFT large, make quantitative by bringing meson exchange back. Input still not well controlled (strength of eta-nucleon coupling). Theory sensitive to all ingredients: wavefunctions (high p), isospin mixing, ISI. There may be more: excite deuteron to T=1 state at beginning. Four-body calculations just beginning. Experiment in 2002 on d+d elastic (cross section and analyzing power) at IUCF. Further extension to reaction channels at KVI.

New experimental efforts at COSY (with WASA 4π detector): (further work on d+d→ 4 He+π 0 ) Get P-wave from higher energies (new number for EFT). But new CS allowed channels (p+t+π 0, n+ 3 He+π 0 ) open. Examine region of a 0 –f 0 to explore mixing.

Theory A. Gårdestig C.J. Horowitz A. Nogga A.C. Fonseca C. Hanhart G.A. Miller J.A. Niskanen U. van Kolck TRIUMF group A.K. Opper E. Korkmaz D.A. Hutcheon R. Abegg C.A. Davis R.W. Finlay P.W. Green L.G. Greeniaus D.V. Jordan J.A. Niskanen G.V. O’Reilly T.A. Percelli S.D. Reitzner P.L. Walden S. Yen IUCF group E.J. Stephenson A.D. Bacher C.E. Allgower A. Gårdestig C.M. Lavelle G.A. Miller H. Nann J. Olmsted P.V. Pancella M.A. Pickar J. Rapaport T. Rinckel A. Smith H.M. Spinka U. van Kolck elastic analysis A. Micherdzinska Many thanks to… Teams working on the “new” charge symmetry: