Testing QCD at smallx with jet production

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Testing QCD at small-x with jet production

• Cyrille Marquet
• SPhT, Saclay

Low-x meeting 2005, Sinaia, Romania
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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

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Introduction
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QCD at high energies
²

• The Regge limit s gtgt t gtgt ?QCD
• The degrees of freedom for quarks and gluons
  propagating through a target are Wilson lines

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High-energy scattering
r transverse size of the dipole b impact
parameter z longitudinal momentum fraction of
the quark
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Observables at high energies

• In DIS
• In DDISDVCS, vector mesons,

Y total rapidity
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Forward-jet production at HERA
In collaboration with Christophe Royon and Robi
Peschanski

• 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

C. M. and C. Royon, in preparation
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The forward-jet cross-section

• p? ? jetX Q, k  ?QCD exp(??) 1
• x, y, Q2 usual kinematic variables of DIS
• xJ , k longitudinal and transverse momentum of
  the jet
• ?? log(xJ /x) rapidity interval

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BFKL parametrization
Parametrizations of ?(gg)d

• The BFKL dipole-dipole cross-sectionwith the
  BFKL kernel
• we fit the parameter ?4? ln(2) and a
  normalization

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Results of the fits to the old data
The BFKL fit
The saturation fit
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Comparisons with ZEUS new data
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Predictions for H1 preliminary data
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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
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Diffractive photon dissociation

• 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

k transverse momentum of the gluon
?? log(1/xpom) xpomltlt1
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Diffractive gluon production

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

x0 transverse position of the quark x1
transverse position of the antiquark y rapidity
of the gluon k transverse momentum of the gluon
b (x0x1)/2
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Analytical insight

• Independently of the precise form of the
  S-matrices

as k goes to zero
as k goes to infinity
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Behaviour of the cross-section as a function of k

• 1/k0 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
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GBW parametrization of the S-matrices
Rp proton radius
QS saturation scale
1/QS size at which the S-matrices start
decreasing to zero

Scales of the problem QS, Q², k
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Plots of
marked bump for k kmax
Can we experimentally test this? extract
QS? Major limitation at HERA QS lt 1 Gev and k gt
3 Gev One cannot reach the whole bump
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In the HERA energy range

• Predictions of the model withand the
  parameters ?, x0 and ?0 taken from the F2 fits

? 0.288, x0 3.10-4 and ?0 ? 23 mb for full
lines (no charm)
? 0.277, x0 4.10-5 and ?0 ? 29 mb for dashed
lines (charm included)
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Conclusion and outlook

• 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 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 F2)? BFKL
  resummation seen at HERA? ? Encouraging for
  Mueller-Navelet jets studies at LHC
• In diffractive DIS at large mass (MX2 gtgt 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