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Marta Ruspa

Univ. of Eastern Piedmont-Novara and INFN-Torino (Italy)

XII International Workshop on Deep Inelastic Scattering

Strbske Pleso, High Tatras, Slovakia

April 14-18, 2004

on behalf of

- Diffractive cross section and diffractive structure function
- Comparison with colour dipole models
- NLO QCD fit

Inclusive diffraction γ*p Xp

Q2 = virtuality of photon =

= (4-momentum exchanged at e vertex)2

t = (4-momentum exchanged at p vertex)2

typically: |t|<1 GeV2

W = invariant mass of photon-proton system

MX = invariant mass of photon-Pomeron system

xIP = fraction of proton’s momentum

taken by Pomeron

ß = Bjorken’s variable for the Pomeron

= fraction of Pomeron’s momentum

carried by struck quark

= x/xIP

e’

Q2

e

g*

W

MX

xIP

IP

p

p’

t

Exchange of an object with the vacuum q. n.

Proton almost intact after the collision

Diffractive DIS in the Breit frame

(Breit frame)

- DIS of a pointlike virtual photon off the exchanged object
- PDFs

HARD SCATTERING FACTORISATION

fi/pD(z,Q2,xIP,t): probability to find in a proton, with a probe of resolution

Q2, parton i with momentum fraction z, under the condition that the proton remains intact and emerges with small energy loss, xIP, and momentum transfer,t

Diffractive Deep Inelastic Scattering probes the diffractive PDFs of

the proton relevant when the vacuum quantum numbers are exchanged

- Lifetime of dipoles very long due to large γ boost (E γ ~ W2 ~ 1/x 50TeV ! )it is thedipole that interactswith theproton !

- Transverse size of dipoles proportional to can be so small
- that the strong interaction with proton can be treatedperturbatively !

Diffractive DIS in the colour dipole picture

We can learn more about the structure of the proton by studying DDIS in a frame in which the virtual photon is faster than the proton

(γ* much faster than p)

2 gluon exchange: LO QCD realisation of vacuum q.n.

Diffractive DIS in the colour dipole picture

We can learn more about the structure of the proton by studying DDIS in a frame in which the virtual photon is faster than the proton

(γ* much faster than p)

2 gluon exchange: LO QCD realisation of vacuum q.n.

- BEKW model : at medium β; at small β
- saturation model : (colour transparency)
- as Q2 0, growth tamed by saturating

Inclusive diffraction γ*p Xp

Exchange of

color singlet

producing a GAP

in the particle

flow

e

p

- No activity in the forward direction

MX method

- Proton suffers only a small energy loss

Selection of events γ*p Xp with Mx method

Properties of Mx distribution:

- exponentially falling for decreasing Mx for non-diffractive events

- flat vs ln Mx2 for diffractive events

Diffr.

Non-diffr.

Non-diffr.

Diffr.

- Forward Plug Calorimeter (FPC):
- CAL acceptance extended by 1 unit in pseudorapidity from η=4 to η=5
- higher Mx and lower W
- if MN > 2.3 GeV deposits EFPC > 1 GeV recognized and rejected!

c, bfrom fit n.d. events subtracted

contamination fromreaction epeXN

Inclusive diffraction γ*p Xp

Exchange of

color singlet

producing a GAP

in the particle

flow

e

p

- No activity in the forward direction

MX method

- Proton suffers only a small energy loss

LPS method

Selection of events γ*p Xp with LPS

Diffractive peak

- Free of p-diss background
- Low acceptance
- low statistics

0.03 < Q2 < 100 GeV2

25 < W < 280 GeV

1.5 < Mx < 70 GeV

xIP< 0.1

Higher xIP region

99-00 FPC sample

(Mx method)

22 < Q2 < 80 GeV2

37 < W < 245 GeV

Mx < 35 GeV

MN < 2.3 GeV

Higher β region

Data samples

Cross section and structure function

- xIP dependence of F2D(3) and
W dependence of dσ/dMX

- - extraction of αIP
- - Regge factorisation
- Q2 dependence of F2D(3) and dσ/dMX
- -sensitivity to diffractive
- PDFs
- comparison to BEKW model
and to saturation model

diffractiveγ*p cross section

- diffractive structure function
(assumes )

F2D(3)xIPdependence

(LPS)

Regge fit (xIP<0.01):

with

Data agree with Regge factorisation assumptionin the

region of the fit

xIPdep. of F2D(3) equivalent to W dep. of dσ/dMx

(Mx method)

p-dissociation events with MN<2.3 GeVincluded

MX< 2 GeV: weak W dep.

MX> 2 GeV: d/dMX rises

rapidly with W

power-like fit

αIP from diffractive and total γ*p scattering

(Mx method)

fit to diffractive cross section data:

- IPdiff higher than soft Pomeron

- Evidence of a rise of IPdiff with Q2 mild Regge factorisation violation.

fit to total cross section data:

- Similar W dep. of diffractive and total cross section

diff(MX<35 GeV)/tot

~ 20 %Q2= 2.7 GeV

10 %Q2= 27 GeV

- at W=220 GeV:

σdiff/ σtot W dependence

(Mx method)

Regge expectation:

- BUT
- low MX :strong decrease of
diff/tot with increasing Q2

- high MX :no Q2dependence !
ratio ~ flat in W

Explained by saturation model

Cross section Q2 dependence

(LPS)

Transition to a constant

cross section as Q20

(similar to total cross section )

Main features of the

data described by BEKW

parametrization (xIP<0.01)

(Bartels, Ellis, Kowalski and Wüsthoff)

medium β

small β

qqg fluctuations dominant

at low Q2

F2D(3) Q2 dependence

(LPS)

Data well described by BGK saturation model (xIP<0.01)

Positive scaling violation at all values of β

QCD fit

- xIP <0.01
- QCDNUM
- Regge factorisation assumption possible for this small data set
- DL flux
- initial scale Q2=2 GeV2
- zf(z)=ΣPi(1-x)aat initial scale
- other PDFs parametrisation tried
- Thorne-Robert variable-flavour-number-scheme

(LPS)

QCD fit describes data

fractional gluon momentum

is

at initial scale

[F2D(3)cc from DESY-03-094, see N. Vlasov talk]

LPS QCD fit compared to Mxdata

ZEUS (MX method)

NB: fits scaled by 0.69

to account for p-diss

background in Mx data

Mx method data described by the fit in the region of overlap LPS-Mxmethod

Main discrepancies at

high β, where no LPS data

available

- Recent data from ZEUS with improved precision and extended kinematic range
- Data described by colour dipole models (BEKW, saturation)
- Data described by a NLO QCD fit (+model)
- Possible indication that αIP increases with Q2 in diffraction
- W dep. of diffractive and total cross section similar at high Q2

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