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Lev Khein (Moscow State University) on behalf of the ZEUS collaboration

Lev Khein (Moscow State University) on behalf of the ZEUS collaboration DIS 2007 Munich. Forward jet production in DIS. Parton dynamics at low x. QCD Calculations. MC Models. Forward Jets. Forward Jet + Dijet. Summary & conclusions.

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Lev Khein (Moscow State University) on behalf of the ZEUS collaboration

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  1. Lev Khein (Moscow State University) on behalf of the ZEUS collaboration DIS 2007 Munich Forward jet production in DIS Parton dynamics at low x QCD Calculations MC Models Forward Jets Forward Jet + Dijet Summary & conclusions Lev Khein Forward jets in DIS

  2. Parton dynamics at low x e´ e *(Q2) q Xn,kT,n ln Q2 q Xn-1,kT,n-1 . . . High Density Region CCFM X2,kT,2 DGLAP X1,kT,1 Strong ordering in x,no kTordering BFKL Non-perturbative region x1 >> x2 >> ... >> xn-1 >> xn = xBj ln x p θn >> θn-1 >> ... >> θ2 >> θ1 Perturbative expansion of parton evolution equations Cannot be explicitly calculated to all orders Approximations summing subsets of terms • DGLAP:∑ (s ln Q2 )n Ordering in x, strong ordering in kT x1 > x2 > ... > xn-1 > xn = xBj - kT,12 << kT,22 << ... kT,n-12 << kT,n2 ≈ Q 2 • BFKL :∑ (s ln (1/x))n • CCFM :ln Q2 andln (1/x) Angular ordering • DGLAP successful at high Q2 but expected to break down at low Q2 and low x • BFKL should be applicable at very low x • CCFM expected to be valid in whole x, Q2 range Lev Khein Forward jets in DIS

  3. QCD Calculations DISENT/NLOJET++:Fixed order QCD partonic cross section, on mass shell ME + DGLAP , (collinear factorization) DISENT NLOJET++ + Virtual Corr. to LO Scales R2 = F2 =Q2 Lev Khein Forward jets in DIS

  4. MCModels ARIADNE: implementation of Color Dipole Model (CDM) Independently radiating dipoles formed by emitted gluons Random walk in kT LEPTO: LO ME on mass shell + PS in DGLAP Strong ordering in kT CASCADE:LO off mass shell ME + PS based on kT factorized CCFM evolution transverse momentum of emitted gluon k> kcut uPDF set1 : kcut = 1.33 GeV uPDF set2 : kcut = 1.18 GeV Non-singular term in splitting function At small xBj no ordering in kT Two versions, with defalt tuning (default) and retuned by H1 (tuned) Lev Khein Forward jets in DIS

  5. Forward Jets e’ e 20 < Q2 < 100 GeV2 0.0004 < xBj < 0.005 0.04 < y < 0.7 Event & Jet selection Kinematicrange 98-00 Data, L  82 pb-1 Forward Jet selection Inclusive KT algorithm p Etjet > 5 GeV 2< ηjet<4.3 1.4 unit more forward than before (Etjet)2 0.5 < (Etjet)2/Q2 < 2 (Etjet)2Q2 suppresses DGLAP evolution xjet = Ejet/Eproton >> xBj enhances BFKL evolution xjet > 0.036 Lev Khein Forward jets in DIS

  6. Forward Jets DISENT vs data NLO below data, especially at small xBj but theoretical uncertainty is large Lev Khein Forward jets in DIS

  7. Forward Jets • Lepto doesn’t suffice • Ariadne default • overestimates high Etjet, • overestimates high ηjet • (proton remnant) • Ariadne tuned is good Lev Khein Forward jets in DIS

  8. Forward Jets Non singular term (in set 2) reduces cross section, but not improves agreement Shape of all distributions disagrees with data Shape is a problem Lev Khein Forward jets in DIS

  9. Forward Jets Triple differential cross sections NLO is below data Large theoretical uncertainty prevents decisive conclusion Lev Khein Forward jets in DIS

  10. Forward Jets Triple differential cross sections Pronounced excess of high Etjet in Ariadne default Ariadne tuned is fine Lev Khein Forward jets in DIS

  11. Forward Jet + Dijet Event & Jet selection Kinematicrange the same as for inclusive forward jets Forward jet the same, 0.5 < (Etjet)2/Q2 < 2 constraint excluded Two additional jets with Etjet > 5 GeV el < jet 1 < jet-2 < forward-jet Lev Khein Forward jets in DIS

  12. Forward Jet + Dijet NLOJET++ vs data Small and jets are most forward. At small xBj space is left for additional partons closer to the photon. NLOJET++ underpredicts many partons below data 1 2 Lev Khein Forward jets in DIS

  13. Forward Jet + Dijet Lepto below data Ariadne default pronouncedly above data –> too high multigluon emission rate Ariadne tuned is fine Lev Khein Forward jets in DIS

  14. Summary & Conclusions • ZEUS measured jets at small xBj in highly extended forward region • NLO is significantly below the data for inclusive forward jets • NLO for forward jet + dijet undershoots data in the region of pseudorapidities where multigluon emission is favoured • LO DGLAP-based MC, Lepto, is clearly below the data • CDM (Ariadne) is capable of successfully describing the whole volume of the data • Cascade MC fails to successfully describe the measurements, other sets of uPDF are to be tried. The data obtained could be highly useful for further uPDF adjusting Lev Khein Forward jets in DIS

  15. Forward Jets Neither set accommodates for all data features Lev Khein Forward jets in DIS

  16. Forward Jet + Dijet • Set 1 is above data • Set 2 is below data Lev Khein Forward jets in DIS

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