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1. Introduction 2. g g  p 0 p 0 3. g p  g p + n

Review on Experimental and Theoretical Results on the Pion Polarizabilities L.V. Fil’kov Lebedev Physical Institute. 1. Introduction 2. g g  p 0 p 0 3. g p  g p + n 4. g g  p + p - 5. p - A  g p - A 6. Discussion

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1. Introduction 2. g g  p 0 p 0 3. g p  g p + n

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  1. Review on Experimental and Theoretical Results on the Pion PolarizabilitiesL.V. Fil’kovLebedev Physical Institute 1. Introduction 2. g g  p0 p0 3.g p  g p+n 4. g g  p+ p- 5. p-A  g p- A 6. Discussion 7. Summary Bosen Workshop 2007

  2. Pion polarizabilities characterize the behavior of the pion in an external electromagnetic field.

  3. The dipole (a1, b1) and quadrupole (a2, b2) pion polarizabilities are defined through the expansion of the non-Born helicity amplitudes of the Compton scattering on the pion over t at s=m2 s=(q1+k1)2, u=(q1–k2)2, t=(k2–k1)2 M++(s=μ2,t)=pm[ 2(α1 - β1) + 1/6(α2 - β2)t ] + O(t2) M+-(s=μ2,t)=p/m[ 2(α1 + β1) + 1/6(α2+β2)t] + O(t2) (α1, β1 and α2, β2 in units 10-4 fm3 and 10-4 fm5, respectively)

  4. g g→p0p0 L. Fil’kov, V. Kashevarov, Eur. Phys. J. A5, 285 (1999); Phys. Rev. C72, 035211 (2005)

  5. s-channel: ρ(770), ω(782), φ(1020); t-channel: σ, f0(980), f0(1370), f2(1270), f2(1525) Free parameters: mσ, Γσ, Γσ→ gg, (α1-β1), (α1+β1), (α2-β2), (α2+β2) The σ-meson parameters were determined from the fit to the experimental data on the total cross section in the energy region 270 - 825 MeV. As a result we have found: mσ=(547± 45) MeV, Γσ=(1204±362) MeV, Γσ→ gg=(0.62±0.19) keV p0 meson polarizabilities have been determined in the energy region 270 - 2250 MeV. A repeated iteration procedure was used to obtain stable results.

  6. The total cross section of the reaction gg→p0p0 H.Marsiske et al., Phys.Rev.D 41, 3324 (1990) J.K.Bienlein, 9-th Intern. Workshop on Photon-Photon Collisions, La Jolla (1992) our best fit

  7. The total cross section of the reaction gg→p0p0at E g<850 MeV our fit

  8. The sensitivity of the cross section calculations to different values of the quadrupole polarizabilities (α2 –β2)p0 bigger by 5% (α2 –β2)p0 less by 5% (α2 +β2)p0 bigger by 5% (α2 +β2)p0 less by 5%

  9. p0 meson polarizabilities [1] L .Fil’kov, V. Kashevarov, Eur.Phys.J. A 5, 285 (1999) [2] L. Fil’kov, V. Kashevarov, Phys.Rev. C 72, 035211 (2005) [3] J. Gasser et al., Nucl.Phys. B728, 31 (2005) [4] A. Kaloshin et al., Z.Phys. C 64, 689 (1994) [5] A. Kaloshin et al., Phys.Atom.Nucl. 57, 2207 (1994) Two-loop ChPT calculations predict a positive value of (α2+β2)p0, in contrast to experimental result. One expects substantial correction to it from three-loop calculations.

  10. g + p →g + p+ + n (MAMI)

  11. where t = (pp –pn )2 = -2mp Tn

  12. The pion polarizabilities can extracted from the experimental data on radiative pion photoproduction either by extrapolating these data to the pion pole or by comparing the experimental cross section with prediction of different theoretical models. Extrapolation: 1. Data with small errors in a wide region of t, in particular, very close to t=0. 2. The pion pole amplitude alone is not gauge invariant. The sum of the pion andnucleon pole amplitudes does not vanish at t=0. The cross section of g p→ gp+ n has been calculated in the framework of two different models:

  13. Model-1: Contribution of all the pion and nucleon diagrams

  14. Model-2: Contribution of the pion and nucleon pole diagrams andD(1232), P11(1440), D13(1520), S11(1535) resonances, ands meson

  15. To decrease the model dependence we limited ourselves to kinematical regions where the difference between model-1 and model-2 does not exceed 3% when (α1 – β1)p+ =0. I. The kinematical region where the contribution of (α1 – β1)p+ is small: 1.5 m2 < s1 < 5 m2 Model-1 Model-2 Fit of the experimental data The small difference between the theoretical curves and the experimental data was used for a normalization of the experimental data.

  16. II. The kinematical region where the (α1 – β1)p+ contribution is substantial: 5m2 < s1 < 15m2, -12m2 < t < -2m2 (a1- b1)=0 (a1- b1)=0 model-2 model-1 (a1- b1)=14 (a1- b1)=14

  17. (a1-b1)p+ = 11.6  1.5st  3.0syst  0.5mod ChPT (Gasser et al. (2006)): (a1-b1)p+= 5.7  1.0

  18. gg→p+p- L.V. Fil’kov, V.L. Kashevarov, Phys. Rev. C 73, 035210 (2006) Old analyses: energy region 280 - 700 MeV (α1-β1)p±= 4.4 - 52.6 Our analysis: energy region 280 - 2500 MeV, DRs at fixed t with one subtraction at s=m2, DRs with two subtraction for the subtraction functions, subtraction constants were defined through the pion polarizabilities. s-channel: ρ(770), b1(1235), a1(1260), a2(1320) t-channel: σ, f0(980), f0(1370), f2(1270), f2(1525) Free parameters: (α1-β1)p±, (α1+β1)p±, (α2-β2)p±, (α2+β2)p±

  19. Charged pion polarizabilities [1] L. Fil’kov, V. Kashevarov, Phys. Rev. C 72, 035211 ( 2005). [2] J. Gasser et all., Nucl. Phys. B 745, 84 (2006).

  20. Total cross section of the process gg→p+p- our best fit calculations with α1 and β1 from ChPT Born contribution fit with α1 and β1 from ChPT

  21. Angular distributions of the differential cross sections Mark II – 90 CELLO - 92 VENUS - 95 ╬ ds/d(|cosQ*|<0.6) (nb) Calculations using our fit a1, b1: Bürgi-97, a2, b2 : our fit a1, b1, a2, b2: Gasser-06 |cosQ*|

  22. Fits of the experimental data on total cross section of the different collaborations separately

  23. p- A→ p- g A (alab.s.) a1+b1 - sgp, qcm 90o

  24. t  10-4(GeV/c)2 dominance of Coulomb bremsstrahlung t  10-3 Coulomb and nuclear contributions are of similar size t  102  dominance of nuclear bremsstrahlung (a1 + b1 )=0: (a1- b1 )=5.6 (a1- b1 )=0 Born + (a 1- b1 )=5.6

  25. Serpukhov (1983): Yu.M. Antipov et al., Phys. Lett. B121, 445 (1983) E1=40 GeV, Be, C, Al, Fe, Cu, Pb tmin=6x108 Maximum at t=2 tmin t < 6x104(GeV/c)2 t=(2 – 4)x103  estimation of the strong interactions t x 103

  26. 1. (a1 + b1)=0 w=w2/E1 (lab. syst.) b1= -6.8  1.4  1.2 2. (a1 + b1)0 (Yu.M. Antipov et al., Z. Phys. C 26, 495 (1985) ) a1 + b1 =1.4  3.1  2.5 b1 = -7.1  2.8  1.8

  27. Charged pion dipole polarizabilities

  28. Dispersion sum rules for the pion polarizabilities

  29. The DSR predictions for the charged pions polarizabilities in units 10-4 fm3 for dipole and 10-4 fm5 quadrupole polarizabilities. The DSR predictions for the p0 meson polarizabilities

  30. Contribution of vector mesons DSR ChPT

  31. Discussion • (α1 - β1)p± The σ meson gives a big contribution to DSR for (α1 –β1). However, it was not taken into account in the ChPT calculations. Different contributions of vector mesons to DSR and ChPT. 2. one-looptwo-loopsexperiment (α2-β2)p± = 11.9 16.2 [21.6] 25 +0.8-0.3 The LECs at order p6are not well known. The two-loop contribution is very big (~100%). • (α1,2+β1,2)p± Calculations at order p6 determine only the leading order term in the chiral expansion. Contributions at order p8could be essential.

  32. Summary • The values of the dipole and quadrupole polarizabilities of p0 have been found from the analysis of the data on the process gg→p0p0. • The values of (α1± β1)p0 and (α2 –β2)p0 do not conflict within the errors with the ChPT prediction. 3.Two-loop ChPT calculations have given opposite sign for (α2+β2)p0. 4. The value of (α1 –β1)p± =13.0+2.6-1.9 found from the analysis of the data on the process gg→ p+p - is consisted with results obtained at MAMI (2005) (g p→ g p+ n), Serpukhov (1983) (p-Z → g p-Z), and Lebedev Phys. Inst. (1984) (g p→ g p+ n). 5. However, all these results are at variance with the ChPT predictions. One of the reasons of such a deviation could be neglect of the σ- meson contribution in the ChPT calculations. 6. The values of the quadrupole polarizabilities (α2 ±β2 )p± disagree with the present two-loop ChPT calculations. 7. All values of the polarizabilities found agree with the DSR predictions.

  33. pp and rr contributions to (a1– b1) D(a1b1)p± - 1.88

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