Marta Ruspa, "Inclusive diffraction", DIS 2004 1

Inclusive diffraction

Diffractive cross section and diffractive structure function

Comparison with colour dipole models

NLO QCD fit

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

Marta Ruspa, "Inclusive diffraction", DIS 2004 2

IP

Q2

W MX

e’

p’

*e

p

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

xIP

t

Inclusive diffraction γ*p Xp

Exchange of an object with the vacuum q. n. Proton almost intact after the collision

Marta Ruspa, "Inclusive diffraction", DIS 2004 3

(Breit frame)

Diffractive DIS in the Breit frame

Diffractive Deep Inelastic Scattering probes the diffractive PDFs of the proton relevant when the vacuum quantum numbers are exchanged

)ˆ 2iγIP

2pi

* Q(z,σt),x,Q(z,f~Xp)pσ(γ *

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

HARD SCATTERING FACTORISATION

DIS of a pointlike virtual photon off the exchanged object PDFs

Marta Ruspa, "Inclusive diffraction", DIS 2004 4

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)

• Lifetime of dipoles very long due to large γ boost (E γ ~ W2 ~ 1/x 50TeV ! ) it is the dipole that interacts with the proton !

• Transverse size of dipoles proportional to can be so small that the strong interaction with proton can be treated perturbatively !

)M(Q1/ _qq22

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

Marta Ruspa, "Inclusive diffraction", DIS 2004 5

Diffractive DIS in the colour dipole picture

BEKW model : at medium β; at small β

saturation model : (colour transparency)

as Q2 0, growth tamed by saturating

22qq

1/Qrσ _

_qq

σ

β)β(1~FTqq γT

gqqβ)(1~F _

_qq

σ

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.

Marta Ruspa, "Inclusive diffraction", DIS 2004 6

e pExchange ofcolor singletproducing a

GAPin the

particle flow

Inclusive diffraction γ*p Xp

No activity in the forward direction

Proton suffers only a small energy loss

MX method

Marta Ruspa, "Inclusive diffraction", DIS 2004 7

Diffr. Non-diffr.

c, b from fit n.d. events subtracted

contamination from reaction epeXN

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 Mx

2 for diffractive events

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!

Diffr. Non-diffr.

Marta Ruspa, "Inclusive diffraction", DIS 2004 8

e pExchange ofcolor singletproducing a

GAPin the

particle flow

Inclusive diffraction γ*p Xp

No activity in the forward direction

Proton suffers only a small energy loss

LPS method

MX method

Marta Ruspa, "Inclusive diffraction", DIS 2004 9

Free of p-diss background Low acceptance

low statistics

z

zIP p

p'x1

Selection of events γ*p Xp with LPS

Diffractive peak

IPx1

Marta Ruspa, "Inclusive diffraction", DIS 2004 10

97 LPS sample

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

Marta Ruspa, "Inclusive diffraction", DIS 2004 11

diffractive γ*p cross section

dWdMdQ

σd

)y)(α(WπQ

dM

dσ

X

De'Xp'ep

X

Dpγ*

2

3

2

2

11

diffractive structure function (assumes ) 0)3( D

LF

IP

XpeepD

IPD

dxdQdd

yyQ

xQF2

''22

42)3(

2 )2/1(4),,(

Cross section and structure function

xIP dependence of F2D(3)

andW 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

Marta Ruspa, "Inclusive diffraction", DIS 2004 12

xIP dep. of F2

D(3) equivalent to W dep. of dσ/dMx

F2D(3) xIP

dependence

Data agree with Regge factorisation assumption in the region of the fit

)(02.0)(02.016.1)0( sysstatIP

(LPS)

Regge fit (xIP<0.01):

),()( 22

)3(2 QFxfF IP

IPIPD

dtx

exftIP

t

IP

tb

IPIP

1)(2)(

with

tt IPIPIP ')0()(

Marta Ruspa, "Inclusive diffraction", DIS 2004 13

p-dissociation events with MN<2.3 GeV included

MX< 2 GeV: weak W dep.

MX> 2 GeV: d/dMX rises rapidly with W

Cross section W dependence (Mx method)

power-like fit

Marta Ruspa, "Inclusive diffraction", DIS 2004 14

fit to total cross section data:

fit to diffractive cross section data:

Evidence of a rise of IPdiff with

Q2 mild Regge factorisation violation .

αIP from diffractive and total γ*p scattering

IPdiff higher than soft Pomeron

Similar W dep. of diffractive and total cross section

(Mx method)

(0)αtotIP

(0)αdiffIP

Marta Ruspa, "Inclusive diffraction", DIS 2004 15

BUT

low MX : strong decrease of

diff/tot with increasing Q2 high MX : no Q2 dependence !

ratio ~ flat in W

Regge expectation:

19.01)0(2

222

*

*

)()(/

WWWdMd

IP

IP

totp

XD

p

at W=220 GeV: diff(MX<35 GeV)/tot

~ 20 % Q2= 2.7 GeV 10 % Q2= 27 GeV

σdiff/ σtot W dependence(Mx method)

Explained by saturation model

Marta Ruspa, "Inclusive diffraction", DIS 2004 16

Main features of the data described by BEKW parametrization (xIP<0.01)

Cross section Q2 dependence

Transition to a constant cross section as Q20(similar to total cross section )

qqg fluctuations dominant at low Q2

(Bartels, Ellis, Kowalski and Wüsthoff)

medium β

small β

)1(~ TqqF

)1(~ TqgqF

totp*

(LPS)

Marta Ruspa, "Inclusive diffraction", DIS 2004 17

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

Marta Ruspa, "Inclusive diffraction", DIS 2004 18

QCD fit describes data

fractional gluon momentum is at initial scale

NLO QCD fit on LPS+charm data

))%(9)(882( sysstat

)36/9.37/( 2 ndf

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

talk]

• 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)a at initial scale• other PDFs parametrisation tried• Thorne-Robert variable-flavour- number-scheme

(LPS)

Marta Ruspa, "Inclusive diffraction", DIS 2004 19

LPS QCD fit compared to Mx data

Main discrepancies at high β, where no LPS data available

NB: fits scaled by 0.69to account for p-dissbackground in Mx data

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

ZEUS (MX method)

Marta Ruspa, "Inclusive diffraction", DIS 2004 20

xIP.F2D(3)/F2

Q2 and xBJ dependences(LPS) (LPS)

Marta Ruspa, "Inclusive diffraction", DIS 2004 21

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

Summary