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Inclusive Diffraction at HERAMarcella Capua – INFN and Calabria University
Small X and DiffractionFNAL Chicago (USA) 17 – 20 September 2003
on behalf of
• experimental methods and diffractive *p cross section
• Regge phenomenology measurement of t and xIP dependence comparison with inclusive ep cross section • QCD analysis of diffraction - diffractive pdf
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
2
Diffractive γ*p interactions at HERA
IPz
zL x
pp
x 1'
L
t
x
pppt
22)'(
22
22
)'(2 XMQ
Q
ppq
Q
22
22)'(WQ
MQpq
ppqx X
IP
squared 4-momentum transfer to p:
fraction of p momentum:
fraction of the p momentum carried by the IP:
fraction of the IP momentum carried by the struck parton:
- large kinematic range and cross section: the order 10 % of DIS is diffractive- ~4 acceptance (important for * dissociative system)- point-like couplings to probe the Pomeron structure- wide Q2 range: to access the transition region from soft to hard processes
Q2
W
MY
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
3
Large Rapidiy Gap methodX system and e’ measured
System Y not measured, some theoretical and experimental uncertanties
Integrate over t<1GeV2 and MY<1.6 GeV
High acceptance
p tag method o Measurement of t o Free of p-diss backgroundo Lower acceptance
MX methodHigh acceptance t measurement not possible systematics from p-diss
Diffractive peak
z
zL p
px
'
NB: if scattered proton not detected, background from proton dissociative events
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
4
Factorization
Regge factorization - “resolved IP model” (IP with partonic structure):
1)(2/ ),( tIP
Bt
IPpIP xe
txf
(Breit frame)
QCD Hard Scattering factorization (by Collins; Trentadue, Veneziano; Berera, Soper…:)
),(ˆ),,,()( 22** QxQxtxpXpp
pIPpq
),(ˆ),(),()( 22** QQptxfXpp
ppqIPpIP
Regge motivated pomeron flux
At fixed xIP and t diffractive Parton Densities evolve according to DGLAP
Shape of diffractive pdfs independent of xIP and t
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
5
What we know about t-distribution (pTag method)
Exponential fit to t distribution:
Regge phenomenology predicts “shrinkage” of the diffractive peak (b rise as xIP → 0):
no Q2 dependence
For xIP < 10 –2, data
prevent any firm conclusion
We are waiting new data from H1 and ZEUS proton tagger
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
6
• MX < 2 GeV (resonance range) weak W dependance• MX > 2 GeV d/dMX rises with Q2 adiff typically higher then soft
Cross section energy dependence (MX method)
As expected
for soft IP
diffIP aα
XDγ*p Wh)(Wh/dMdσ 222Fit:
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
7
Energy dependence (MX method)
Data (4<MX<8 GeV) consistent with the same W-dependence for the diffractive and the total cross section:
222 IPαX
Dγ*p )(W/dMdσ
total cross section:
diffractive cross section:
102*
IPWtot
p
value of IP diff higher than soft Pomeron
indication of a rise of IPdiff with Q2.
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
8
Ratio diff/tot (MX method)
In DIS diff/tot independent of W !
• low MX: strong decrease of diff/tot
with increasing Q2
• high MX: no Q2 dependence
Regge th: 19.01)0(2
222
*
*
)(
)(/W
W
WdMdIP
IP
totp
XD
p
• at W=220 GeV: diff(MX<35 GeV)/tot
~ 20 % Q2= 2.7 GeV 10 % Q2= 27 GeV
Same results from H1 and ZEUS-LPS
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
9
H1 Measurement of σrD(3)
(LRG method)
FL unknown, FL = 0 or FL = F2 few % error
New low and very high Q2 data!!!
Agreement between measurements
Red sample used for DGLAP QCD interpretation and IP(0) calculation well described by the fit
3-fold differential diffr. struct. func.
)3(
2
2)3(
22)3(
)1(1),,( D
LD
IPDr F
y
yFxQ
Diffractive reduced cross section rD
2)3(2
4
2
2
3
,,2
14
Qxy
yQdQddx
dIP
Dr
IP
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
10
and Q2 dependences
large scaling violation except at high → gluon dominatedsimilar at all xIP → support for Regge factorization
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
11
NLO DGLAP QCD fit
Gluon momentum fraction 75 ±15 % at Q2 = 10 GeV2
and remains large up to high Q2
-Regge factorization
-Singlet Σ and gluon g with Q20=3GeV2
-NLO DGLAP evolution
-Fit medium Q2 (6 < Q2 < 120 GeV2)
-Experimental and theor incertainties
Diff PDF’s:
Extended to large z
Gluon dominate
Σ well constrained
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
12
Test of QCD factorization
Good agreement with prediction with NLO fit at medium Q2 NLO Comparisons with Diff DIS Jets
Marcella Capua, INFN and Calabria University
Small X and Diffraction - FNAL September 2003
13
Summary
• recent data from H1 and ZEUS with improved precision and extended kinematic range
• data prevent any firm conclusion about shrinkage (waiting for new proton tagged data)
• value of diffractive IP higher than soft Pomeron
• W-dependence of diffractive and total cross section similar on DIS
• similar Q2 and dependencies to the inclusive DIS at medium
• NLO QCD fit of inclusive diffractive gives a good description of the data:
- diffractive pdf dominated by gluons
- diffractive pdf predictions well confirmed by tests