Abelian Anomaly & Neutral Pion Production

Abelian Anomaly & Neutral Pion Production

Craig Roberts

Physics Division

Craig Roberts, Physics Division, APS April Meeting 2011: Abelian Anomaly & Neutral Pion Production

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Why γ* γ → π0 ?

The process γ* γ → π0 is fascinating– To explain this transition form factor within the standard model on the full

domain of momentum transfer, one must combine• an explanation of the essentially nonperturbative Abelian anomaly • with the features of perturbative QCD.

– Using a single internally-consistent framework! The case for attempting this has received a significant boost with the

publication of data from the BaBar Collaboration (Phys.Rev. D80 (2009) 052002) because:– They agree with earlier experiments on their common domain of squared-

momentum transfer (CELLO: Z.Phys. C49 (1991) 401-410; CLEO: Phys.Rev. D57 (1998) 33-54)

– But the BaBar data are unexpectedly far above the prediction of perturbative QCD at larger values of Q2.

http://inspirebeta.net/record/821653?ln=en









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Why γ* γ → π0 ?

The process γ* γ → π0 is fascinating– To explain this transition form factor within the standard model on the full

domain of momentum transfer, one must combine• an explanation of the essentially nonperturbative Abelian anomaly • with the features of perturbative QCD.

– Using a single internally-consistent framework! The case for attempting this has received a significant boost with the

publication of data from the BaBar Collaboration (Phys.Rev. D80 (2009) 052002) because:– They agree with earlier experiments on their common domain of squared-

momentum transfer (CELLO: Z.Phys. C49 (1991) 401-410; CLEO: Phys.Rev. D57 (1998) 33-54)

– But the BaBar data are unexpectedly far above the prediction of perturbative QCD at larger values of Q2.

pQCD










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Transition Form Factor γ*(k1)γ(k2) → π0

S(l1) – dressed-quark propagator

Γπ(l1,l2) – pion

Bethe-Salpeter amplitude γν

*(k1)

γμ(k1)

Γν (l12,l2) – dressed quark-photon vertex

S(l2)

S(l12)

Γμ (l1,l12)

π0

• All computable quantities• UV behaviour fixed by pQCD• IR Behaviour informed by DSE- and lattice-QCD


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S(p) … Dressed-quark propagator- nominally, a 1-body problem

Gap equation

Dμν(k) – dressed-gluon propagator Γν(q,p) – dressed-quark-gluon vertex


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S(p):Dressed-quark propagator- nominally, a 1-body problem

Dμν(k) – dressed-gluon propagator

~ 1/(k2 + m(k2)2)

Γν(q,p) – dressed-quark-gluon vertex

~ numerous tensor structures

DSE- and Lattice-QCD results

Frontiers of Nuclear Science:Theoretical Advances


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In QCD a quark's mass depends on its momentum. The function describing this can be calculated and is depicted here. Numerical simulations of lattice QCD (data, at two different bare masses) have confirmed model predictions (solid curves) that the vast bulk of the constituent mass of a light quark comes from a cloud of gluons that are dragged along by the quark as it propagates. In this way, a quark that appears to be absolutely massless at high energies (m =0, red curve) acquires a large constituent mass at low energies.


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Dynamical Chiral Symmetry Breaking = Mass from Nothing Critical for understanding pionIn QCD a quark's mass depends on its

momentum. The function describing this can be calculated and is depicted here. Numerical simulations of lattice QCD (data, at two different bare masses) have confirmed model predictions (solid curves) that the vast bulk of the constituent mass of a light quark comes from a cloud of gluons that are dragged along by the quark as it propagates. In this way, a quark that appears to be absolutely massless at high energies (m =0, red curve) acquires a large constituent mass at low energies. Jlab 12GeV: Scanned by 2<Q2<9 GeV2

elastic & transition form factors.

Frontiers of Nuclear Science:Theoretical Advances


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*(k1)

γμ(k1)


S(l2)

S(l12)

Γμ (l1,l12)

π0



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π0: Goldstone Mode & bound-state of strongly-dressed

quarks Pion’s Bethe-Salpeter amplitude

Dressed-quark propagator

Axial-vector Ward-Takahashi identity entails

Maris, Roberts and Tandynucl-th/9707003

Exact inChiral QCD

Critically!Pseudovector components

are necessarily nonzero. Cannot be ignored!

Goldstones’ theorem: Solution of one-body problem solves the two-body problem




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*(k1)

γμ(k1)


S(l2)

S(l12)

Γμ (l1,l12)

π0



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Dressed-quark-photon vertex

Linear integral equation– Eight independent amplitudes

Readily solved Leading

amplitude

ρ-meson polegenerated dynamically- Foundation for VMD

Asymptotic freedomDressed-vertex → bare at large spacelike Q2

Ward-Takahashi identity

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γν*(k1)

γμ(k2)

π0

S(l1)

S(l12)S(l2)

• Calculation now straightforward• However, before proceeding, consider slight modification

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Transition Form Factor γ*(k1)γ*(k2) → π0


γν*(k1)

γμ*(k2)

π0

S(l1)

S(l12)S(l2)

Only changes cf. γ*(k1)γ(k2) → π0

• Calculation now straightforward• However, before proceeding, consider slight modification


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Anomalous Ward-Takahashi Identity

chiral-limit: G(0,0,0) = ½ Inviolable prediction

– No computation believable if it fails this test– No computation believable if it doesn’t confront this

test. DSE prediction, model-independent:

Q2=0, G(0,0,0)=1/2Corrections from mπ

2 ≠ 0, just 0.4%


Maris & Tandy, Phys.Rev. C65 (2002) 045211

Maris & Roberts, Phys.Rev. C58 (1998) 3659








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pQCD prediction

Obtained if, and only if, asymptotically, Γπ(k2) ~ 1/k2 Moreover, absolutely

no sensitivity to φπ(x); viz., pion distribution amplitude

Q2=1GeV2: VMD broken Q2=10GeV2:

GDSE(Q2)/GpQCD(Q2)=0.8 pQCD approached

from below


pQCD





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Pion Form Factor Fπ(Q2)

γ(Q)

π(P)

π(P+Q)


DSE computationappeared before data; viz., a prediction

pQCD-scale Q2Fπ(Q2)→16πα(Q2)fπ

2

VMD-scale: mρ2

Q2=10GeV2

pQCD-scale/VMD-scale= 0.08

Internally consistent calculationCAN & DOES overshoot pQCD limit,and approach it from above; viz, at ≈ 12 GeV2




Single-parameter, Internally-consistent

Framework Dyson-Schwinger Equations – applied extensively to spectrum &

interactions of mesons with masses less than 1 GeV; & nucleon & Δ. On this domain the rainbow-ladder approximation

– leading-order in systematic, symmetry-preserving truncation scheme, nucl-th/9602012 – is accurate, well-understood tool: e.g., Prediction of elastic pion and kaon form factors: nucl-th/0005015 Pion and kaon valence-quark distribution functions: 1102.2448 [nucl-th] Unification of these and other observables – ππ scattering: hep-ph/0112015 Nucleon form factors: arXiv:0810.1222 [nucl-th]

Readily extended to explain properties of the light neutral pseudoscalar mesons (η cf. ή): 0708.1118 [nucl-th]

One parameter: gluon mass-scale = mG = 0.8 GeV


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http://arxiv.org/abs/1102.2448











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γν*(k1)

γμ(k2)

π0

S(l1)

S(l12)S(l2)


DSE result no parameters varied; exhibits ρ-pole; perfect agreement with

CELLO & CLEO





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Three , internally-consistent calculations– Maris & Tandy

• Dash-dot: γ*(k1)γ(k2) → π0

• Dashed: γ*(k1)γ*(k2) → π0

– H.L.L Roberts et al.• Solid: γ*(k1)γ(k2) → π0

contact-interaction, omitting pion’s pseudovector component

All approach UV limitfrom below

Hallmark of internally-consistent computations

γν*(k1)

S(l12)

γμ(k2)

S(l2)

S(l1)

π0

H.L.L. Roberts et al., Phys.Rev. C82 (2010) 065202





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All approach UV limitfrom below

UV scale in this case is 10-times larger than for Fπ(Q2):– 8 π2fπ

2 = ( 0.82 GeV )2

– cf. mρ2 = ( 0.78 GeV )2

Hence, internally-consistent computations can and do approach the UV-limit from below.

Hallmark of internally-consistent computations

γν*(k1)

S(l12)

γμ(k2)

S(l2)

S(l1)

π0






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UV-behaviour: light-cone OPE

Integrand sensitive to endpoint: x=1– Perhaps φπ(x) ≠ 6x(1-x) ?– Instead, φπ(x) ≈ constant?

γν*(k1)

S(l12)

γμ(k2)

S(l2)

S(l1)

π0


There is one-to-one correspondence between behaviour of φπ(x) and short-range interaction between quarks

φπ(x) = constant is achieved if, and only if, the interaction between quarks is momentum-independent; namely, of the Nambu – Jona-Lasinio form




Pion’s GT relationContact interaction


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Guttiérez, Bashir, Cloët, RobertsarXiv:1002.1968 [nucl-th]

Pion’s Bethe-Salpeter amplitude

Dressed-quark propagator

Bethe-Salpeter amplitude can’t depend on relative momentum;

propagator can’t be momentum-dependent Solved gap and Bethe-Salpeter equations

P2=0: MQ=0.4GeV, Eπ=0.098, Fπ=0.5MQ

1 MQ

Nonzero and significant

Remains!





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Comparison between Internally-consistent calculations:

φπ(x) ≈ constant, in conflict with large-Q2 data here, as it is in all cases– Contact interaction cannot

describe scattering of quarks at large-Q2

φπ(x) = 6x(1-x) yields pQCD limit, approaches from below

γν*(k1)

S(l12)

γμ(k2)

S(l2)

S(l1)

π0






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2σ shift of any one of the last three high-points – one has quite a different

picture η production η' production Both η & η’ production in

perfect agreement with pQCD

γν*(k1)

S(l12)

γμ(k2)

S(l2)

S(l1)

π0


CLEO

BaBar





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Epilogue

In fully-self-consistent treatments of pion: static properties; and elastic and transition form factors, the asymptotic limit of the product

Q2G(Q2)which is determined a priori by the interaction employed, is not exceeded at any finite value of spacelike momentum transfer: – The product is a monotonically-increasing concave function.

A consistent approach is one in which: – a given quark-quark scattering kernel is specified and solved in

a well-defined, symmetry-preserving truncation scheme; – the interaction’s parameter(s) are fixed by requiring a

uniformly good description of the pion’s static properties; – and relationships between computed quantities are faithfully

maintained.


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Epilogue

The large-Q2 BaBar data is inconsistent with – pQCD – All extant, fully-self-consistent studies

Conclusion: the large-Q2 data reported by BaBar is not a true representation of the γ ∗ γ → π0 transition form factor

Explanation? – Possible erroneous way to extract pion transition form factor from the data is

problem of π0 π0 subtraction. – This channel – γ ∗ γ → π0 π0

• scales in the same way (Diehl et al., Phys.Rev. D62 (2000) 073014)• Misinterpretation of some events, where 2nd π0 is not seen, may be larger

at large-Q2.

π0(p)

π0(p’)






Abelian Anomaly & Neutral Pion Production

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