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Gluon Evolution at small-x : Extending the PT Domain of QCD

Gluon Evolution at small-x : Extending the PT Domain of QCD. Dimitri Colferai University of Firenze M. Ciafaloni G.P. Salam A.M. Stasto. {. In collaboration with:. DIS 03 St. Petersburg, 23-27 April 2003. Outline. vs picture. BFKL. DGLAP. ’ ’ ’.

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Gluon Evolution at small-x : Extending the PT Domain of QCD

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  1. Gluon Evolution at small-x:Extending the PT Domain of QCD Dimitri Colferai University of Firenze M. Ciafaloni G.P. Salam A.M. Stasto { In collaboration with: DIS 03St. Petersburg, 23-27 April 2003

  2. Outline

  3. vs picture BFKL DGLAP ’ ’ ’ ’ ’ ’

  4. h’ hG’ Gh’ h Scale change: BFKL vs DGLAP kernel in the collinear limit k >> k’ ; Simplified case:

  5. What have we learnt? • Position of collinear g-poles of evolution kernels depends on s0 • Origin of cubic poles in c1(g ): expansion around w= 0 of shifted poles ~1/(g+ w/2) • Non-singular part A(w) of anomalous dimension ggg(w) = 1/w + A(w) can be taken into account by a subleading kernel in 2 ways: (remember )

  6. Definition of RG Improved Approach Adapting the procedure for running asand with s0= Q1Q2 : resums scale dependent terms of a pure BFKL approach includes left-over exact NL BFKL infos resums exact 1-loop gluon anomalous dimension (note w-dependence of A) This procedure implements kinematical constraints

  7. Diffusion corrections: Scheme A Scheme B fixed running numeric Y-evolution of Green’s function Hard Pomeron

  8. Domain of applicability of perturbative QCD

  9. Gluon density Evolution eqn. Resummed splitting function is independent of : important check of RG factorization High-energy exponent wc(as)

  10. Conclusions • 2 main problems of high-energy perturbative QCD: • occurrence of large leading log s contributions and of subleading ones of comparable size and opposite sign • increasing importance of wee partons, whence a strong coupling non-perturbative Pomeron regime • RGI approach tames both problems: • through an understanding and consequent resummation of the most important subleading contributions • Subleading corrections and running coupling effects lower high-energy exponents and diffusion/tunneling into NP region • We expect a large domain of applicability of PT QCD • We have the tools to make reliable physical predictions: g *-g *, forward jets, Mueller-Navelet jets, …

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