The “New” Charge Symmetryprecision experiments in few nucleon systemsfrom meson-exchange to effective field theory

Ed StephensonIndiana University Cyclotron Facility

SPIN 2006Kyoto, Japan

How do we understand charge symmetry breaking?examples from meson exchange scattering lengths n-p analyzing power differencesexamples from effective field theory n+p→d+π0 fore-aft asymmetry d+d→4He+π0

Keys toexperimental

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±–π0, …)meson isospin mixing (ρ0–ω, π0–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

mN – mP = 1.293 MeV

Different NN scattering lengths

-18

-20

-22

-24

fm app (corrected)

ann

anp

CSB

ChargeIndependenceBreaking

How are these effectsbe understood within ameson exchange picture?

ρ0 – ω mixing

contribution of the π± - π0 mass differencein OPEP and TPEP

EM contributions in π+γ exchange

n-p analyzing power difference

. nn

pp

CS.

pp

nn

Layout of TRIUMFexperiment

beam and target polarized, same geometry for both caseslook near zero crossing to avoid calibration issues

Keys:

Results from IUCF and TRIUMF experiments

residual EMeffect comesfrom neutronmagneticmoment inmagneticfield of movingproton

most of effect is attributed to n-p and pionmass differences

additional effectresult of ρ0–ω mixing

Shift in approach to sources of CSB: emphasize quark level sources rather than nucleon level

u d

d

u u

d

swap

neutron proton

Charge symmetry requires that no process/property dependon 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+π0

Charge symmetryrequires no change when nand p are swapped, so crosssection is symmetric about 90°.

all angles recorded in SASP at oncedetector efficiency independently calibrateddata compared to Monte Carlo simulation with Afb variable

Keys:[PRL 91, 212302 (’03)]

SASP spectrometer

particles movethrough system

targetpoint

focal planedetectors

The data

forwarddeuterons

backwarddeuterons

This differenceis an artifactof the SASP.So asymmetryrequires adetailed modelof the experiment.

All model properties determinedindependently, except: beam energy central SASP momentum target thickness A1/A0 = 2Afb

)(cos)(cos)( 22110 PAPAA where

Results from Monte Carlo study:

Afb = 0.172 ± 0.080 ± 0.055 % (stat) (sys)

d+d→4He+π0

forbidden by: isospin conservation π0 is T = 1 charge symmetry π0 is odd under CS

major physics background: d+d→4He+γ+γ

clean selection of candidate events particle ID on 4He [scintillators] Pb-glass selection of energetic photonsgood missing mass (π0) reconstruction scattering angle [WC1] TOF in channel [ΔE2–ΔE1] (include channel energy loss, compensate for PMT time drift)NOTE: cross section normalized to d+p elastic

Keys:[PRL 91, 142302 (’03)]

Particle Identification (using scintillator signals)

E

ΔE2

ΔE2 Windows select4He events

but rate is 103

too high due tod-induced reactions

on residual gasand beam pipe

ΔE1

Select 2-photonevents with left

and right Pb-glass

all events that passparticle identification

eventsinside

window

finalcut

Final cut leavesno background,only π0 and γ+γ

Results at two energies near threshold

σTOT =

12.7 ± 2.2 pb

15.1 ± 3.1 pb

π0 peak

γ+γ continuum

(scaled forchannel

acceptance)

peak positionscorrect to 60 keV

systematicerrors about 7%

(excludingnormalization)

upper 2 MeVof γ+γ continuum:σ = 6.9 ± 0.9 pband 9.5 ± 1.4 pb

(about twiceprediction)

average

0 0.1 0.20

50

100

η = pπ/mπ

σTOT/η

results consistentwith S-wave

Cross section normalized to d+p elastic scattering

onlinemonitoris d+delastic

at 90°cm

To calibrate online monitor, use HDgas and observe d+p elasticscattering at 25°(d) – 44°(p).[see K. Ermisch, PRC 71, 064004]

108 MeV

120 MeV

135 MeV

interpolateto 116 MeV

but the KVI measurements disagree with Japanese data[Sekiguchi, PRL 95, 162301]

Energy dependenceWe need the cross section here.

KVIdataJapanese data

compare

Ermischdata

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!

116MeV

newRCNPdata

Charge Symmetry Breakingcontributions from Effective Field Theory

van Kolck, Niskanen, and Miller, PL B 493 (2000) 65

Leading order contributions:

Down-upquark massdifference

Electro-magnetic

Nucleons and pions arecomponents of model.

Scale parameters determinedfrom experiment.

nucleon-only contributionMeVmmmm pnNN 29.1

nucleon-pion scattering

(free pion-nucleon system limitedto π+ or π– with protons; large EMcorrections limit view of CSB)

In π0 production,consider:

π0

xCSB here

Cottingham sum rule:

MeVmN 30.076.0

so: MeVmN 3.01.2~

Estimate size:

Theory status (work in progress):Pion rescattering large for n+p→d+π0,but vanishes (except recoil and ISI) for d+d→4He+π0.Other terms matter!

Re-introduce meson-exchange and mesonmixing to stand in formissing EFT terms.Add π0–η mixing.

0.5

0

%

n+p→d+π0

π0–η mixing(Niskanen)

EFT pion rescattering

van Kolck prediction: re-evaluation:

n-p massdifference

d+d→4He+π

50

0

pb

datadata

ρ0–ω mixing

π0–η mixing

EFT EM

EFT quark mass difference

wavefunction isospin mixingπ0–η mixingdownscaled

[contributions to amplitude][S, P-wave interference]

Further comments:

Pion production experiments and EFT theory happened together.

After several attempts, experiments have succeeded. Newapproaches 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) atIUCF. Further extension to reaction channels at KVI.

New experimental efforts at COSY (with WASA 4π detector):(further work on d+d→4He+π0)

Get P-wave from higher energies (new number for EFT).

But new CS allowed channels (p+t+π0, n+3He+π0) open.

Examine region of a0–f0 to explore mixing.

Theory

A. GårdestigC.J. HorowitzA. NoggaA.C. FonsecaC. HanhartG.A. MillerJ.A. NiskanenU. van Kolck

TRIUMF group

A.K. OpperE. KorkmazD.A. HutcheonR. AbeggC.A. DavisR.W. FinlayP.W. GreenL.G. GreeniausD.V. JordanJ.A. NiskanenG.V. O’ReillyT.A. PercelliS.D. ReitznerP.L. WaldenS. Yen

IUCF group

E.J. StephensonA.D. BacherC.E. AllgowerA. GårdestigC.M. LavelleG.A. MillerH. NannJ. OlmstedP.V. PancellaM.A. PickarJ. RapaportT. RinckelA. SmithH.M. SpinkaU. van Kolck

elastic analysisA. Micherdzinska

Many thanks to…

Teams working on the “new” charge symmetry: