HEP2005 Europhysics Conference in Lisboa, Portugal
Inclusive NC and CC Diffraction
Laurent Schoeffel
CEA/Saclay (DAPNIA/SPP)
On behalf of H1 and ZEUS collaborations
1. Results from H1 and ZEUS experiments : Q²<120 GeV²
2. Interpretation in perturbative QCD
3. Large Q² Rapidity Gap Events (Q²>200 GeV²) : NC and CC
Inclusive diffraction
Diffractive event in the H1 detector :
Kinematic variables :
Y
xIP fractional loss of the proton longitudinal momentum
MX invariant mass of the diffractive final state
 = xBj/xIP fraction of the + momentum carried by the interacting parton / Diff. Exchange
Diffractive cross section :
with rD ~ F2D
F2D => diffractive parton densities of the Diffractive Exchange
New measurement by ZEUS
With the Forward Plug Calorimeter
MY>2.3 GeV => EFPC>1 GeV
recognised and rejected
Mx selection method :
d*pdiff /dlnMX² ~ constant
for a diffractive event
New measurement by ZEUS
With the Forward Plug Calorimeter
d*pdiff /dMX² ~ 1/MX²
for a diffractive event…
Mx=1120 GeV
We observe :
d*pdiff /dMX ~ Wa
rising with W (a>0)
From Regge theory :
a=4(IP(t)-1)
For soft IP (IP~1.08)
=> a ~ 0.2/0.3 (Mx<2 GeV)
Note :
d*pdiff /dMx²~4²/[Q²(Q²+Mx²)] xIPF2D
Perturbative QCD and hard diffraction
ddiff /dMX ~ Wa with a = 2.(2IP-2)
MX < 2 GeV => soft pomeron
MX > 2 GeV => transition from
soft to hard process(Q²)
=> large Q² + large MX :
perturbative regime
Mx > 2 GeV
Q² > 15 GeV²
Scaling violations
Large scaling violations even at large  => large Gluon
~xIP F2D(3)
Low xIP
Note : H1 measures in the range MY<1.6 GeV
97 data with 10 pb-1 for Q²>13.5 GeV²
Combination of both
data sets H1/ZEUS
=> Compatible
After scaling the
measurements in the
same MY range
Plot from the HERA/LHC workshop
J. Bartels at al., Eur. Phys. J. C7, 443 (1999).
Phenomenology of the 2-gluon exchange model
LO realisation of the Singlet Exchange
L or T
=Q²/(Q²+Mx²)
~1/Q²
at large 
=> HigherTwist
behaviour
Fraction of Diffraction vs the total
cross-section
Behaviour well explained in the
dipole saturation model
Mx>8 GeV bins :
=> Diffraction is dominated by
qq and qqg
For example at very low , one
gluon carries the momentum xBjp+
=> We expect the same W dep. // F2
Rapidity Gap Event at Large Q² NC/CC
Example of event in the H1 detector
X
Rapidity Gap
Missing transverse
energy
Results on CCDiff
H1 : CCDiff/CCIncl = 2.50.80.6%
NC ratio decreases with Q² :
It was ~15/20% at medium Q² and it
reaches ~2/3% for Q²>200 GeV²
CC and NC fractions are compatible
Conclusions
New results from ZEUS with a Forward Plug Calorimeter to measure the forward
activity : Ratio Diff/Incl ~ 15/20% at medium Q²
Well described by the 2-gluon exchange in the perturbative domain
A large gluon content of diffractive exchange is found by H1 by scaling
violations (QCD fit)
Rapidity Gap Events at large Q² : interesting as it gives access to CC events
Fractions NCDiff/NCIncl and CCDiff/CCIncl are compatible and decreasing with Q² :
CCDiff/CCIncl ~ 3% in the range of measurement (Q²>200 GeV²) : H1 and ZEUS
give compatible results