# Presentation: 747853

## Testing QCD at small-x with jet production

- Cyrille Marquet
- SPhT, Saclay

- Low-x meeting 2005, Sinaia, Romania

## Contents

- Introductionhigh-energy scattering in perturbative QCDobservables at small-x and the dipole picture
- Forward-jet production in DISBFKL resummation and saturation consistent with the data
- Diffractive photon dissociation with jet productionstrongly sensitive to unitarity effects and saturation
- Conclusion and outlook

## Introduction

## QCD at high energies

- 2

- The Regge limit: s >> t >> QCD
- The degrees of freedom for quarks and gluons propagating through a target are Wilson lines:

## High-energy scattering

- r: transverse size of the dipole
- b: impact parameter
- z: longitudinal momentum fraction of the quark

## Observables at high energies

- In DDIS:DVCS, vector mesons, ...

- Y: total rapidity

## J. Contreras, R. Peschanski and C. Royon, Phys. Rev. D 62 (2000) 034006C.M., R. Peschanski and C. Royon, Phys. Lett. B 599 (2004) 236

- C. M. and C. Royon, in preparation

- Forward-jet production at HERA

- In collaboration with Christophe Royon and Robi Peschanski

## The forward-jet cross-section

- p+*  jet+X	     Q, *k* » QCD         exp()» 1
*x*, *y*, Q*2** *: usual kinematic variables of DIS
*x**J *, *k** *: longitudinal and transverse momentum of the jet
-  = log(*x**J */*x*) : rapidity interval

## The BFKL dipole-dipole cross-section:with the BFKL kernel

we fit the parameter =4 ln(2)  and a normalization

- we fit the parameter =4 ln(2)  and a normalization

- BFKL parametrization

- Parametrizations of (gg)d

## Results of the fits to the old data

- The BFKL fit

- The saturation fit

## Comparisons with ZEUS new data

## Predictions for H1 preliminary data

## Diffractive photon dissociation with jet production

- In collaboration with Krzysztof Golec-Biernat

- C. M., Nucl. Phys. B 705 (2005) 319

- K. Golec-Biernat and C. M., Phys. Rev. D 71 (2005) 114005

## Diffractive photon dissociation

- y = log(1/) = log(MX2/Q2)        <<1

- This is the dominant contribution to the diffractive cross-section ***diff* at large MX in DIS:***elas*: involves the qq dipole fluctuation, dominant for small-mass final states ***dissoc*: involves higher Fock state fluctuations: qqg, ...dominant for large-mass final states

-  = log(1/xpom)       xpom<<1

- k: transverse momentum of the gluon

## Diffractive gluon production 

- The cross-section is derived for an arbitrary target and for an incident dipole of size r0= x0-x1
- one obtains

**x**0: transverse position of the quark
**x**1: transverse position of the antiquark
- y: rapidity of the gluon
- k: transverse momentum of the gluon

- b = (x0+x1)/2

## Analytical insight

- Independently of the precise form of the S-matrices

- as k goes to zero

- as k goes to infinity

## Behaviour of the cross-section as a function of k

- 1/*k*0: typical size at which the S-matrices are cut off
-  observable strongly sensitive to unitarity effects

- we studied this
- cross-section in the
- framework of
- saturation theory

## GBW parametrization of the S-matrices

*R**p*: proton radius

- 1/QS: size at which the S-matrices start decreasing to zero

- QS: saturation scale

- Scales of the problem: QS, Q2, k

## Plots of			

- marked bump for k = kmax

- Can we experimentally test this? extract QS?
- Major limitation: at HERA QS < 1 Gev and k > 3 Gev
- One cannot reach the whole bump

## Predictions of the model withand the parameters , x0 and 0 taken from the F2 fits:

- In the HERA energy range

-  = 0.288, *x*0 = 3.10-4 and 0  23 mb
- for full lines (no charm)

-  = 0.277, *x*0 = 4.10-5 and 0  29 mb
- for dashed lines (charm included)

## Jet-production cross-sections are sensitive to the small-x regime of QCD contain complementary information w.r.t. DIS (Tgg, Tqqg, ...)

Studies of small-x effects in forward jets at HERA using BFKL and GBW parametrizationsAt small-x, NLO QCD is below the data while BFKL LL agrees wellThe saturation model is compatible with the data, (as it is for F2) BFKL resummation seen at HERA?  Encouraging for Mueller-Navelet jets studies at LHC 

In diffractive DIS at large mass (MX2 >> Q2 ), can one measure	      ?
As a function of the gluon transverse momentum, the cross-section is resonant with the scale at which unitarity effects become important observable with a great potential to study high-energy QCD
Study using a saturation model for the dipole amplitudes: strong potential for extracting QS and testing models

- Studies of small-x effects in forward jets at HERA using BFKL and GBW parametrizationsAt small-x, NLO QCD is below the data while BFKL LL agrees wellThe saturation model is compatible with the data, (as it is for *F**2*) BFKL resummation seen at HERA?  Encouraging for Mueller-Navelet jets studies at LHC
- In diffractive DIS at large mass (MX2 >> Q2 ), can one measure	      ?
- As a function of the gluon transverse momentum, the cross-section is resonant with the scale at which unitarity effects become important observable with a great potential to study high-energy QCD
- Study using a saturation model for the dipole amplitudes: strong potential for extracting QS and testing models

- Conclusion and outlook