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Work done in collaboration with Stefano Frixione

INFN, Sezione di Genova. 1. Universit à di Genova, DiFi. 2. Jet and di-jet production at NLO in Photon – Photon collisions. Leonardo Bertora. 1,2. 1. Work done in collaboration with Stefano Frixione. Leonardo Bertora, 2003 April 8. Situation. Photon – Photon Hard Collisions.

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Work done in collaboration with Stefano Frixione

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  1. INFN, Sezione di Genova 1 Università di Genova, DiFi 2 Jet and di-jet production at NLO in Photon – Photon collisions Leonardo Bertora 1,2 1 Work done in collaboration with Stefano Frixione Leonardo Bertora, 2003 April 8

  2. Situation Photon – Photon Hard Collisions Motivations: • Hard production of jets is a complementary method (with respect to DIS) to study the “structure” of the photon • It is rich test of QCD (since the beginning of history…Han-Nambu integer charges model) Experimental Situation: • OPAL, see talk by T. Wengler • L3, see talk by M. Kienzle • DELPHI, see Photon2001 proceedings • Future Linear Collider … LEPII Theoretical results: • As of now only the theoretical prediction by M.Klasen, Kleinwort, G.Kramer (KKK) (hep-ph/9611450 and hep-ph/9712256) is available • It is based upon slicing method Leonardo Bertora, 2003 April 8

  3. Motivations Any prediction has to be checked ! Factorization theorem • We are not able to compute differential cross section analytically • We fight against large unphysical oscillations and computer precision • A general method should be adopted in order to cancel out divergences analytically, and finite reminders can be numerical (MC) integrated • We use subtraction method, KKK used slicing one • … double test: a different computation with a different method Leonardo Bertora, 2003 April 8

  4. About subtraction method: Real emission plus virtual correction Neglected O() terms Large-number cancellations without any approximation and large-number cancellation (beware of narrow bins) Leonardo Bertora, 2003 April 8

  5. The structure of the “home made” computation NLO terms + + … + • We have recalculated and checked (with respect to KKK’s) the matrix elements • We implemented a “partons generator” (hep-ph/9512328 and hep-ph/9706545) divided in three classes of Feynman diagrams: double resolved, single resolved, direct diagrams • We have to rely on non-perturbative input such as PDFs and …paper in preparation Leonardo Bertora, 2003 April 8

  6. The first check A first check: A comparison with OPAL 1997 (one inclusive jet) KKK Inclusive Jet Production in Photon-Photon Collision at sqrt(s)=130 and 136 GeV, Opal coll., Zeit. Fur Physik(1997) Ok, at least we are on the right path… Leonardo Bertora, 2003 April 8

  7. Scheme DI-MS But we need self consistence tests!!! Unfortunately they are very boring! Let’s try to see their physical meaning: what about the true photon contribution to the cross section? Each class of diagrams is separately divergent: We have removed analytically the infinities modulo finite, scheme dependent, terms Thus colored histos do not have a real physical meaning Changing the factorization scheme Leonardo Bertora, 2003 April 8

  8. Try another way… Changing the factorization scale Changing the PDF Leonardo Bertora, 2003 April 8

  9. Dangerous regions: Recall our simple model to deal with the final state direct contribution in the dangerous region Dangerous regions: PYTHIA HERWIG 350% Zoom OPAL Failure of the perturbative computation Let’s try to understand where our troubles come from Leonardo Bertora, 2003 April 8

  10. Phi angle Cuts on the phase space for di-jet observables Dangerous regions II: • Symmetric cuts on Et imply large log corrections • Opal coll. have chosen to impose: NLO Limit Opal Phase Space Observation: • In NLO computation • In the limit we run into IR divergences! Et min … thus even if in principle Opal’s choice could be dangerous, in practice it is far from the singularity sources! Leonardo Bertora, 2003 April 8

  11. Results:opal Results:Theo.vs.Opal KKK Data from Di-Jet Production in photon-photon collisions at sqrt(s) from 189 to 209 GeV, OPAL coll., hep-ex/0301013 Leonardo Bertora, 2003 April 8

  12. Results:opal2 Results:Theo.vs.Opal II Data from Di-Jet Production in photon-photon collisions at sqrt(s) from 189 to 209 GeV, OPAL coll., hep-ex/0301013

  13. Results:l3 Results:Theo.vs.L3 Good shape, but not really good normalization Wrong shape and normalization: Preliminary data seem to confirm L3-hadron production discrepancy with NLO prediction L3 Preliminary Leonardo Bertora, 2003 April 8

  14. Conclusions Conclusions: • We implemented a computer code to compute jet and di-jet observables using the subtraction method; our results are in agreement with the predictions by KKK. • The comparisons of our results with OPAL data are really satisfactory for inclusive observables. The comparison with L3 data is troublesome! • Finally, we analyzed some theoretical QCD (NLO) difficulties to describe well particular exclusive observables measured by OPAL. Theoretical uncertainties for inclusive observables are under control. Leonardo Bertora, 2003 April 8

  15. Results:opalB PHYTIA KKK NLO/(1+dhadr) KKK KKK Data from Di-Jet Production in photon-photon collisions at sqrt(s) from 189 to 209 GeV, OPAL coll., preprint L. Bertora

  16. Results:opalc

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