Exclusive (elastic) vector meson photoproduction

1
Exclusive (elastic) vector meson photoproduction
(JPC1--) r, f, J/y,U,...
g
IP
Pomeron (vacuum quantum numbers)

• Pomeron goes back to the 60s Regge trajectory,
  ie a moving pole
• in complex angular momentum plane
• We understand it now in terms of quarks, gluons
  and QCD (need a hard process)

2
Look at photon in the proton rest frame
Virtual photon fluctuates to qq states (colour
dipoles)
g

• Lifetime of dipoles very long because of large g
  boost (Eg ? 50TeV at HERA) ? it is the dipole
  that interacts with the proton
• Transverse size proportional to 1/ ? (Q2 Mqq2)
• This is why can do diffraction in ep collisions !

3
Vector Meson production
(JPC1--) r, f, J/y,U,...
IP
2-gluon exchange LO realisation of vacuum
quantum numbers in QCD
!
Cross section proportional to probability of
finding 2 gluons in the proton
Gluon density in the proton
4
Vector Meson production
(JPC1--) r, f, J/y,U,...
IP
Growth of cross section with decreasing x, hence
with increasing W, at large Q2 MV2, reflecting
large gluon density at low x
Ryskin (1993), Nikolaev et al (1994), Brodsky et
al (1994),...
Wphoton-proton centre of mass energy
5
Sensitivity to gluons in proton
? W0.2
xg(x)
? W0.8
? W1.7
x
6
Sensitivity to gluons in proton

• At small MV (MV2 ? 1 GeV2)
• Incoming dipole behaves like a normal-size
  hadron the two exchanged gluons are soft
• cf stot(gp)
• Flat s vs W reflects flat gluon distribution for
  Q2 ? 0
• At large MV
• Fast growth of s with W reflects growth of gluon
  distribution with decreasing x

7
Sensitivity to gluons in proton
V
p
p
At large MV, data well reproduced by pQCD
8
Generalised PDFs
In general, x1 ? x2
x1
x2
t
Generalised PDFs (also non-diagonal, skewed
PDF) sensitive to parton-parton correlations in
the proton !
In the kinematic regions probed so far, GPDs are
approximated by standard PDFs, modulo a
normalisation factor (2-3 in ? case)
9
t-dependence
t
As in optical diffraction, the size of the
diffractive cone is related to the size of the
interacting objects t is the Fourier conjugate
of the impact parameter b b ? ?(Rqq2 Rp2)
with Rp ? 5 GeV-2
10
t-dependence
11
Summary

• Elastic production of heavy vector mesons
  sensitive to
• proton structure gluon density and GPDs
• Cross section can be calculated using pQCD
• Important to measure dependence of cross section
  vs W
• and t
• Not discussed decay angular distributions,
  which give
• information on helicity structure of cross
  section (but
• difficult to measure at LHC)

12
Diffraction in optics
o) Forward peak for q0 (diffraction peak) o)
Diffraction pattern related to size of target and
wavelength of beam
13
Diffraction in hadron scattering
Diffraction is a feature of hadron-hadron
interactions (30 of stot)
a
a
a
X
b
b
b
Y
Elastic
Double Dissociation
o) Beam particles emerge intact or dissociated
into low-mass states. Energy ? beam energy
(within a few )
o) Final-state particles separated by large polar
angle (or pseudorapidity, ln tan(q/2))
Large Rapidity Gap (LRG)
o) Interaction mediated by t-channel exchange of
object with vacuum quantum numbers (no
colour) the Pomeron (IP)
14
Generalised PDFs (GPDs)

• Evidence of GPDs (so far) in DVCS and ?
  production
• In the kinematic regions
• probed so far, GPDs are approximated by standard
• PDFs, modulo a normalisation factor (2-3 in ?
  case)
• A field in its infancy. Holds the promise of
  mapping parton-parton correlations in the proton
• Sensitivity to GPD modest at
• HERA largest at large x (JLab, Hermes,
  Compass)

GPD-based calculations