S mall-x effects in forward-jet production at HERA

1. Small-x effectsin forward-jet production at HERA Cyrille Marquet SPhT, Saclay DIS 2005, Madison

2. Contents • Introductionfixed-scale BFKL evolution and saturationforward-jet emission in terms of dipoles • Parametrizations of the forward-jet cross-section BFKL and GBW parametrizations • Fits to the HERA forward-jet data for d/dx • Comparison with the ZEUS new data for d/dx • Predictions for the H1 preliminary data for d/dxdQ²dk² • Conclusion and outlook

3. Introduction

4. example: *-* total cross-section**  X suitable to test fixed-scale evolution BFKLprediction: Saturation: restores unitarity • High-energy behavior:determined by dipole-dipole scattering • Tîmneanu, Kwiecinski and Motyka (2002) Kozlovand Levin (2003) BFKL evolution and saturation

5. p+*  jet+XQ, kT » QCD exp()» 1 Same kind of process than *-* but more statisticsdata from H1 and ZEUS Is the BFKL resummation seen at HERA? Are saturation effects sizable at HERA? Forward-jet production The high-energy behavior is also determined by dipole-dipole scattering

6. Gluon emission by a hadron C. M., Nucl. Phys. B 705 (2005) 319 It can be formulated in terms of the gluon dipole cross-section (gg)t : • xJ is the longidudinal momentum fraction of the gluon-jet • kis the transverse momentum of the gluon-jet • gh is the gluon density inside the incident hadron •  is the factorization scale • This formula is valid in the leading-logarithmic approximation • One recovers the kT-factorization formula by restricting (gg)t to a two-gluon exchange (BFKL)

7. with the hard cross-section given by The forward-jet cross-section • x, y, Q2 : usual kinematic variables of DIS • xJ , k: longitudinal and transverse momentum of the jet •  = log(xJ /x) : rapidity interval

8. Parametrizations of (gg)d

9. BFKL parametrization • The BFKL dipole-dipole cross-section:with the BFKL kernel • We fit the parameter =4 ln(2)and a normalization

10. The saturation radius iswe fit the parameters  , 0and a normalization Q0  1 GeV Saturation parametrization • An extension of the GBW model Tîmneanu, Kwiecinski and Motyka (2002)with

11. Fitting the HERA data J. Contreras, R. Peschanski and C. Royon, Phys. Rev. D 62 (2000) 034006C.M., R. Peschanski and C. Royon, Phys. Lett. B 599 (2004) 236

12. Results of the fits • The BFKL fit is a measure of the hard Pomeron intercept • We obtain two ² minima for the saturation fit: The « sat. » solution corresponds to significant saturation effects The « weak sat. » solution corresponds to weak saturation effects The intercept l is in both cases higher than what was found for F2 (lGBW = 0.288)

13. The BFKL fit

14. The saturation fit

15. The saturation scales • The saturation scale is QS 1/R0(Dh) • The plot represents • The weak saturation solution is compatible with the F2 parametrization • The other solution shows a harder saturation scale

16. Comparisons with new data and predictions C. M. and C. Royon, in preparation

17. Comparisons with ZEUS data

18. Predictions for H1 preliminary data

19. Predictions for d/dxdQ²dk²

20. Conclusion and outlook • Studies of small-xeffects in forward jets at HERA using BFKL and GBW parametrizationsusing the parameters fit to the old data for d/dx: we describe well the new data for d/dxwe make predictions for d/dxdQ²dk² (H1) • Study to be completed with d/dQ², d/dJ, d/dk² (ZEUS) • At small-xNLO QCD is below the data BFKL LL agrees wellThe saturation model is compatible with the data, (as it is for F2) • But two different saturation scales workwe need other observables to distinguish themOther forward-jet cross-sections? Mueller-Navelet at LHC?