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Lecture III

Lecture III. 5. The Balitsky-Kovchegov equation. Properties of the BK equation. The basic equation of the Color Glass Condensate

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Lecture III

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  1. Lecture III

  2. 5. The Balitsky-Kovchegov equation

  3. Properties of the BK equation The basic equation of the ColorGlassCondensate -Rapid growth of the gluon density when the field is weak, and saturation of the scattering amplitude N(x,y) 1 as s  infinity also unitarity is restored -Energy dependence of saturation scale computable from linear + saturation. Known up to NLO BFKL Qs  large as s  large - Absence of infrared diffusion problem (cf BFKL) - Geometric scaling exists in a wide region Q2 < Qs4/L2 - Phenomenological success The CGC fit for F2 at HERA

  4. Saturation scale from linear regime Matching the linear solution to saturated regime The BK equation is known only at the LO level, but one can compute Qs(x) up to (resummed) NLO level by using this technique.

  5. Absence of Infrared diffusion • BFKL equation has the infrared diffusion problem: even if one starts from the initial condition well localized around hard scale, eventually after the evolution, the solution enters the nonperturbative regime. Thus, BFKL evolution is not consistent with the perturbative treatment. • However, there is no infrared diffusion problem in the BK eq. • Most of the gluons are around Qs(x).  Justifies perturbative treatment aS(Q=Qs(x))

  6. Geometric Scaling from the BK eq. • Numerical solution to the BK equation shows the geometric scaling and its violation ~ F.T. of N(x) scaling variable

  7. Geometric Scaling above Qs

  8. BFKL BFKL, BK Energy (low high) Parton gas DGLAP Transverse resolution (low high) “Phase diagram” as a summary

  9. Froissart bound from gluon saturation BK equation gives unitarization of the scattering amplitude at fixed impact parameter b. However, the physical cross section is obtained after the integration over the impact parameter b. The Froissart bound is a limitation for the physical cross section, and it is highly nontrivial if this is indeed satisfied or not.

  10. Froissart bound from gluon saturation

  11. Froissart bound from gluon saturation

  12. Froissart bound from gluon saturation

  13. Coefficient in front of ln2 s s ~ B ln2s -- Froissart Martin bound B = p/mp2 = 62 mb -- Experimental data (COMPETE) B = 0.3152 mb -- CGC + confinement initial condition LO BFKL B = 2.09 ~ 8.68 mb (aS=0.1 ~ 0.2) rNLO BFKL B = 0.446 mb (aS=0.1 )

  14. 6. Recent progress in phenomenologyHERA (Lecture III)RHIC AuAu (Lecture IV) RHIC dAu (Lecture IV)

  15. Attempts with saturation (I)Golec-Biernat, Wusthoff model

  16. Attempts with saturation (II)Improvements of the GBW model

  17. Attempts with saturation (III)our approach

  18. Geometric scaling and its violation Total g* p cross section (Stasto,Kwiecinski,Golec-Biernat) in log-log scale deviation from the pure scaling in linear scale Figure by S.Munier

  19. Attempts with saturation (III)our approach

  20. The CGC fit

  21. The CGC fit DGLAP regime

  22. Effects of charm (not shown in the paper) • Performed the fit with charm included (for example ) • Still have a good fit (c2=0.78), but saturation scale becomes smaller

  23. Other observables (I) Vector meson production, F2Diff Forshaw, et al. PRD69(04)094013 hep-ph/0404192

  24. Other observables (II) • FL Goncalves and Machado, hep-ph/0406230

  25. Summary for lecture III The Balitsky-Kovchegov equation is the evolution equation for the change of scattering energy when it is large enough. It is a nonlinear equation, and leads to -- saturation (unitarization) of the scattering amplitude -- geometric scaling and its violation also free from the infrared diffusion problem. One can compute the cross section and its increase as increasing energy. Froissart bound is satisfied if one adds the information of the confinement. HERA data at small x is well described by the CGC fit.

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