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Theory summary: what I would like to understand

Theory summary: what I would like to understand. Ji ří Chýla, Institute of Ph ysics, Prague. Often difficult to distinguish between theory/phenome- and experiment. A lot of comparison of older calcula- tions with new data, as well as new calculations.

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Theory summary: what I would like to understand

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  1. Theory summary: what I would like to understand Jiří Chýla, Institute of Physics, Prague Often difficult to distinguish between theory/phenome- and experiment. A lot of comparison of older calcula- tions with new data, as well as new calculations. Overview on direct photons: Heinrich Total cross sections: Pancheri, GPD’s & related: Friot,Landsberg, Pire, Wallon, Szymanowski Unintegrated PDF: Szczurek, Hautman Standard PDF: JC, Hejbal, Sasaki Power corrections: Hautman Scales, schemes: Geiser, Grindhammer QED processes: Serbo, Da Silva Photon 2007, Paris, July 9-13, 2007

  2. I will not attempt to summarize what the speakers actually said, but will attach to each of the topics comments/questions, the latter mostly reflecting my ignorance of the respective subjects, but which, nevertheless, might be interesting to answer. Photon 2007, Paris, July 9-13, 2007

  3. Photon 2007, Paris, July 9-13, 2007

  4. What about dijets? Photon 2007, Paris, July 9-13, 2007

  5. Photon 2007, Paris, July 9-13, 2007

  6. significant achievement! Photon 2007, Paris, July 9-13, 2007

  7. Total cross sections G. Pancheri: Photon 2007, Paris, July 9-13, 2007

  8. What drives the rise of the total cross sections? G. Pancheri has the answer: minijet production driven by the rise of PDF at low x Photon 2007, Paris, July 9-13, 2007

  9. Photon 2007, Paris, July 9-13, 2007

  10. To get the rise she needs very small lower cut off on minijet, or better, produced parton, tranverse momentum • But as such low transverse momenta: • lowest order partonic cross sections are unreliable and • highly ambiguous (scales!). • These XS grow rapidly at low pT and thus the rise of the total XS is very sensitive to the choice of pTmin • I do not understand how one can use in minijet models PDF extracted from genuine hard processes. Photon 2007, Paris, July 9-13, 2007

  11. Generalized PDF and similar quantities From Wallon’s talk: Factorization on the level of partonic cross sections Factorization on the level of partonic amplitudes Photon 2007, Paris, July 9-13, 2007

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  14. Choice of C(1) here is compensated by change of C(1) here Great, but factorization implies ambiguities, scales and schemes, which at least for inclusive processes, play very important phenomenological role. I would expect them to play analogous role in GPD’s and related quantities as well. Is that true? If yes, there must a mechanism to guarantee independence of physical quantities of these ambiguities, similarly as in inclusive processes: compensation Mechanism guaranteeing factorization scale and scheme invariance of proton structure function Photon 2007, Paris, July 9-13, 2007

  15. Photon 2007, Paris, July 9-13, 2007

  16. DA: TDA: Photon 2007, Paris, July 9-13, 2007

  17. No unknown nonperturbative input needed ? No factorization scale introduced? How to choose ? Looks like the LO purely perturbative quantity ? + contain information on the structure of hadrons Photon 2007, Paris, July 9-13, 2007

  18. Unintegrated PDF Used by Szczurek to describe data on inclusive direct photon production in hadronic collisions meaning of this scale? Photon 2007, Paris, July 9-13, 2007

  19. Photon 2007, Paris, July 9-13, 2007

  20. but where is ? Photon 2007, Paris, July 9-13, 2007

  21. I miss the mechanism by which the dependence of UPDF’s on the scale is cancelled. In the standard integrated PDF’s this cancellation is provided is by the explicit depen- dence of hard scattering cross sections on . So the off-shell cross sections should depend on it as well, and one would probably need NLO QCD calculation of these cross sections to get the cancellation mechanism. At the present time the scale dependence of calculations involving UPDF’s is therefore large. But perhaps, I am wrong.... Photon 2007, Paris, July 9-13, 2007

  22. Standard, integrated PDF and structure of the photon Real photon: alternative organization of finite order QCD approximations to photon structure function Plea: apply the terms “LO”, “NLO” etc to QCD contribu- tions only Basic message: parton distribution functions of the photon do not behave like Together with the plea above this implies significantly different definitions of LO and NLO approximations to as well as other photon induced processes. Photon 2007, Paris, July 9-13, 2007

  23. Numerical differences between standard and alternative approaches and their phenomenological relevance shown by J. Hejbal: Photon 2007, Paris, July 9-13, 2007

  24. Virtual photon: new NNLO (in the standard definition) calculations of the “Order” defined in terms of high Q2 behavior of the structure function, as understood in the pioneering papers of Witten, De Witt et al. and Llewelyn-Smith. Photon 2007, Paris, July 9-13, 2007

  25. Photon 2007, Paris, July 9-13, 2007

  26. apart from the last term behaves as for Photon 2007, Paris, July 9-13, 2007

  27. Photon 2007, Paris, July 9-13, 2007

  28. Scales, schemes Boring, frustrating but inevitable perturbation theory ambiguities. We can ignore them, but to our own peril. Achim Geiser: How well do we understand choice of QCD scales? Theseambiguities are inevitable consequence of renor- malization and factorization procedures and truncation of PT to any fixed order. Contrary to conventional view, they do not go away when we go to higher orders, as at each order of PT new free parameters defining renormalized/factorized quantities do appear. There is no genuine “resolution” of these ambiguities, just a few recipes how to fix the scales and schemes. Photon 2007, Paris, July 9-13, 2007

  29. Achim Geiser’s Photon 2007, Paris, July 9-13, 2007

  30. Photon 2007, Paris, July 9-13, 2007

  31. Achim Geiser’s Photon 2007, Paris, July 9-13, 2007

  32. I generally agree with Achim’s strategy, but want to emphasize two points: • Renormalization schemesare equally important as renormalization scales. We can fix one and very the other or the other way round. The number “1/2” in Achim’s suggested new default scale corresponds to the standard MSbar RS. In other RS it will be different and could easily be 1. • The renormalization and factorization scales should not be identified (as usually done). Reply to Samuel Wallon:BLM scale setting method cannot be applied to quantities involving factorization scale. Photon 2007, Paris, July 9-13, 2007

  33. Example of different renormalization and factorization scale dependence: NLO calculation of bbbar production at the Tevatron Photon 2007, Paris, July 9-13, 2007

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  39. Power corrections and low x physics Photon 2007, Paris, July 9-13, 2007

  40. Photon 2007, Paris, July 9-13, 2007

  41. But: separation of perturbative part and power corrections is ambiguous and the numerical relevance of the latter thus depends on the choice of free parameters (scales, schemes) of perturbative calculations. Example: Klaus Hammacher at ICHEP 2002 in Amsterdam Photon 2007, Paris, July 9-13, 2007

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  47. QED processes are not boring V. Serbo: Large contributionof the Delbruckscattering into processof a photon emissionin collisions of relatis-tic nuclei. Photon 2007, Paris, July 9-13, 2007

  48. Photon 2007, Paris, July 9-13, 2007

  49. But these processes are of great interest for heavy quark production in collisions! Da Silva:Four fermion two pair production from gamma- gamma collisions: from PLC to LHC • Motivation • -Need for a reference • process forluminosity • measurement at a PLC • QED and QCD back- • groundsource to rare • processes • Only a realistic Monte- • Carlo cangive a correct • result Photon 2007, Paris, July 9-13, 2007

  50. Au revoir in 2009 at DESY Photon 2007, Paris, July 9-13, 2007

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