Laurent Schoeffel CEA/Saclay (DAPNIA/SPP)

Laurent SchoeffelCEA/Saclay (DAPNIA/SPP)

On behalf of H1 and ZEUS collaborations

1. Results from H1 and ZEUS experiments : Q²<120 GeV²2. Interpretation in perturbative QCD3. Large Q² Rapidity Gap Events (Q²>200 GeV²) : NC and CC

Inclusive NC and CC Diffraction

HEP2005 Europhysics Conference in Lisboa, Portugal

Inclusive diffraction

Diffractive event in the H1 detector :Kinematic variables :

Diffractive cross section :

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

with rD ~ F2

D

F2D => diffractive parton densities of the Diffractive Exchange

Y

New measurement by ZEUSWith the Forward Plug Calorimeter

MY>2.3 GeV => EFPC>1 GeV recognised and rejected

Mx selection method :d*p

diff /dlnMX² ~ constant for a diffractive event

d*pdiff /dMX² ~ 1/MX²

for a diffractive event…Mx=1120 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)

New measurement by ZEUSWith the Forward Plug Calorimeter

Note :d*p

diff /dMx²~4²/[Q²(Q²+Mx²)] xIPF2D

ddiff /dMX ~ Wa with a = 2.(2IP-2) MX < 2 GeV => soft pomeron MX > 2 GeV => transition from soft to hard process(Q²)

Perturbative QCD and hard diffraction

=> large Q² + large MX : perturbative regime Mx > 2 GeV Q² > 15 GeV²

Scaling violations

~xIP F2D(3)

Large scaling violations even at large => large Gluon

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 bothdata sets H1/ZEUS=> Compatible

After scaling the measurements in thesame MY range

Plot from the HERA/LHC workshop

Phenomenology of the 2-gluon exchange model

L or T

~1/Q²at large

=Q²/(Q²+Mx²)

=> HigherTwist behaviour

LO realisation of the Singlet Exchange

J. Bartels at al., Eur. Phys. J. C7, 443 (1999).

Fraction of Diffraction vs the totalcross-section

Behaviour well explained in thedipole saturation model

Mx>8 GeV bins :=> Diffraction is dominated by qq and qqgFor example at very low , onegluon 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 transverseenergy

H1 : CCDiff/CC

Incl = 2.50.80.6%

Results on CCDiff

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 NC

Diff/NCIncl and CC

Diff/CCIncl are compatible and decreasing with Q² :

CCDiff/CC

Incl ~ 3% in the range of measurement (Q²>200 GeV²) : H1 and ZEUSgive compatible results