Inclusive diffraction at HERA

1
A Deeply Virtual Compton Scattering Amplitude
(hep-ph/0605319 submitted to Phys. Lett. B)

• The model
• Fits to the HERA data

e?p?e?p
Marcella Capua Calabria University and INFN
Cosenza (Italy)
2
DVCS at HERA
Inclusive DIS measurements
Allow the extraction of proton PDFs which are a
crucial input to pQCD calculations. But PDFs do
not provide a complete picture of the partonic
structure of nucleons (correlations between
partons, their trasverse motion)
DVCS processes
The study of processes (like DVCS,VM,) provide
important additional information on the proton
structure via GPDs etc.
3
Kinematics of the DVCS process
At HERA the ep scattering is dominated by single
photon exchange What we are studing is then the
diffractive ?p??p scattering the simplest
process sensitive to GPD
t
In analogy with electroproduction of VM we
introduce a new variable z -Q2t
4
A model for DVCS
Q2 and t dependences determined by the V1 vertex
generalizing the new z variable used on the VM or
virtual photon electroproduction only in the
upper vertex
DVCS amplitude
the t dependence of the pIPp vertex is introduced
via the ?p? vertex is introduced by the
trajectory The amplitude can be rewritten as
5
Fits to the HERA data

• A two parameters fit has been performed to the
  HERA 1996-2000 data
• H1 Coll. Eur. Phys. J. C. 44 (2005)
• ZEUS Coll. Phys. Lett B 573 (2003)
• Assuming a(0)1.25 a11 and a0.38 GeV-2 and t
  0.17 GeV2 according with the H1 measurements
  with slope b6.02 GeV-2

A0, b
6
Results of the fit
7
Results of the fit
ZEUS measurements have been rescaled to the W and
Q2 H1 values
8
The B slope
for t0 became A shrinkage in s and
anti-shrinkage in Q2 is expected
9
B slope vs Q2 and W
10
Conclusions

• A factorized Regge-pole model has been applied
  to the HERA DVCS data
• The model gives a reasonable description of the
  HERA data on DVCS
• The model can be used to study various extreme
  regimes of the scattering amplitude in all three
  (W,Q2,t) variables
• With the new HERA data in progress the fit can be
  improved by accounting for the Pomeron and other
  Reggeon term