Kein Folientitel

1
DIFFRACTION AT HERA
The HERA Collider and the Experiments Luminosity
and its Measurement Kinematics of Deep-Inelastic
(DIS) and Diffractive Reactions Diffraction in
High-Energy Particle Scattering Regge
Phenomenology versus pQCD Exclusive Vectormeson
Production and Deeply-Virtual Compton Scattering
(DVCS) Inclusive Diffraction and Diffractive
Structure Functions Exclusive Diffractive
Reactions Jets and Heavy Quarks

Page 1
Bernd Löhr, DESY
2
The HERA Collider
Electron - proton collisions
electrons
protons
This corresponds to an electron energy in a
fixed-target experiment of Ee 26952 GeV
Bernd Löhr, DESY
Page 2
3
The H1 Experiment
HERA I Configuration
Tracking
protons
backward
Liquid-Argon calorimeter
EM section
protons
HAC section
electrons
Bernd Löhr, DESY
Page 3
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A rear view of the H1 detector
Bernd Löhr, DESY
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The ZEUS Experiment
Tracking
HERA I Configuration
backward
Uranium-scintillator- calorimeter
protons
EM section
protons
electrons
HAC section
Bernd Löhr, DESY
Page 5
6
protons
The ZEUS detector seen from above
electrons
Bernd Löhr, DESY
Page 6
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The Concept of Luminosity
Proton and electron beams are not continuous but
they consist of bunches.
Typically 180 proton bunches and electron
bunches collided.
Individual bunches from each beam collide
What is the probability that a given process
happens ?
reaction rate
(instantaneous) luminosty
cm-2s-1
cross section
The luminosity can be calculated from HERA
parameters
where
-beam revolution frequency
-number of electrons/protons in the i-th bunch
- transverse size of interaction region
cm-2
L
integrated luminosity
Cross section
Bernd Löhr, DESY
Page 7
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Measurement of the Luminosity
Measure the rate of a process whose cross-section
is well known
initial state radiation
final state radiation
Bremsstrahlung in ep collisions or Bethe-Heitler p
rocess
proton
proton
Measure energy and rate of bremsstrahlung photons
in specialized photon detector under very small
angles w.r.t. the electron beam
ZEUS interaction point
p- beam
very well known cross-section
photon detector
e-- beam
(universal) integrated luminosity, for all
cross-section calculations
about 100m away from interaction point
Bernd Löhr, DESY
Page 8
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From Optical Diffraction to High Energy Particle
Scattering
High energetic elastic scattering
Optical diffraction
Generalization in high energy particle
interactions
Diffraction is the exchange of an object with
vacuum quantum numbers.
Hypothetical object Pomeron IP
double dissociation
double Pomeron exchange
single dissociation
elastic
Bernd Löhr, DESY
Page 9
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Kinematics of DIS and Diffraction
Inclusive nondiffr. DIS events
center of mass energy squared
virtuality, size of the probe
- proton cms energy squared
x fraction of the proton carried by the
struck parton
y inelasticity, fraction of the electron
momentum carried by the virtual photon
Diffractive DIS events
mass of the diffractive system x
For diffractive events in addition 2 variables
k
k
four-momentum transfer squared at the proton
vertex
q k - k
momentum fraction of the proton carried by the
Pomeron
fraction of the Pomeron momentum which enters the
hard scattering
Bernd Löhr, DESY
Page 10
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DIS- and Diffractive Events
Inclusive DIS events
Considerable energy flow in the very forward
direction
Diffractive DIS events
Large rapidity gap in forward direction
k
k
?? ?ln tan(??2)
Pseudo-Rapidity

• is the angle w.r.t.
• proton direction.

Bernd Löhr, DESY
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Regge Phenomenology vs. pQCD
Regge Phenomenology
Peripheral (soft) processes
e
e
X
R
I
Reggeon trajectory
I
P, R
I
I
P Pomeron trajectory
p
P
t
Diffraction
shrinkage
From fit to hadronic data
(Donnachie, Landshoff)
0.16
Bernd Löhr, DESY
Page 12
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Diffractive DIS in pQCD
In pQCD the simplest way to describe the exchange
of vacuum quantum numbers is the exchange of a
colour neutral system of two gluons.
This is realized in the colour dipole picture.


The wave function of the incoming virtual photon
can be calculated from QED. The outgoing wave
function might be known for some exclusive Final
states, like e.g. vector mesons.
higher orders
Bernd Löhr, DESY
Page 13
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Exclusive Vectormeson Production I
VDMRegge
?
VM
VM
Shrinkage
p
P
pQCD
r
r2 small if Q2 large or MV large
?
z
VM
1-z
Ryskin
p
P
and

fast rise with W
no or little shrinkage
Do pQCD models describe the data ? Which
variables can provide a hard scales ?
Bernd Löhr, DESY
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Exclusive Vectormeson Production II
DIS
Bernd Löhr, DESY
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Can the Vectormeson Mass be a Hard Scale ? I
Photoproduction Q20
The w-dependence of the light vector-meson (
r, w, f ) production is described by Regge
phenomenology
For higher mass vector mesons the rise of the
production cross section with W gets steeper.
This indicates the onset of hard diffractive
scattering
Bernd Löhr, DESY
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Can the Vectormeson Mass be a Hard Scale ? II
Photoproduction Q20
Shrinkage
W dependence
d0.74 from H1 alone
GeV
50 100
200 WGeV
pQCD models can describe photoproductionof J/Y
mesons.
Still some shrinkage seen in J/Y photoproduction
but compatible with pQCD models.
Bernd Löhr, DESY
Page 17
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Can Q2 be a Hard Scale ? I
r-electroproduction
-electroproduction
For r production the W-dependence gets steeper
with Q2.
ZEUS
.
J/Y
The rise with W is essentially independent of Q2
for production
pQCD models can describe exclusive
production
Bernd Löhr, DESY
Page 18
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Can Q2 be a Hard Scale ? II
Slopes of the t-distributions
The b-slope of r-production decreases with Q2.
At high Q2 it reaches the value of
-production.
The b-slope of -production is independent
of Q2 . It is a hard process already at Q2 0.
Q2 provides a hard scale
Bernd Löhr, DESY
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Can t be a Hard Scale ? I
Complication at high t, the vector-meson
production proceeds
predominantly through proton-dissociative
processes.
e
e
VM
Low mass MY particles of system Y disappear
undetected through the
beampipe hole in the detector. High mass MY a
fraction of the particles from system Y
are detected in the calorimeter.
Y
p
t
Within experimental uncertainties vertex
factorization holds .
We have to assume vertex factorization
The ratio should not depend on Q2.
Bernd Löhr, DESY
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Can t be a Hard Scale ? II
Ivanov et al.
pQCD models
Bartels et al.
not exponential at high t, predicted by pQCD
models.
t-dependence of p-dissociative vector-meson
production can be described by pQCD models.
Large t may provide a hard scale to apply pQCD.
Bernd Löhr, DESY
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The Pomeron Trajectory I
e
e
Measure W-dependence separately for different
t-bins Pomeron trajectory
VM
I
P
p
P
t
?IP(t) ?IP(0)?t
1.198 0.115.t
DIS r
1.14 0.04.t
r photoproduction
1.0960.125.t
F photoproduction
1.0810.158.t
Soft pomeron (DL)
1.08.25.t
Bernd Löhr, DESY
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The Pomeron Trajectory II
Slope of the Pomeron trajectory at higher t
depends on t
However
for high t , proton diffractive processes
dominate.
In Regge language
is not linear in t
Bernd Löhr, DESY
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Decay Matrix Density and Ratio of
Longitudinal to Transverse Cross-Section for ro
Production
The ro is a spin 1 particles. It decays into two
pions ro -gt pp-. The angular distribution
ofthe decay particles p and p- is described by
a function of 3 angles
In total 15 density matrix elements describe the
decay angular distribution, which depend on the
helicity amplitudes for spin non-flip, single
spin-flip, and double spin-flip
T00, T01, T10, T11, T1-1
Notation
S-channel helicity conservation
Only non-flip amplitudes T00 and T11 are
non-zero
Bernd Löhr, DESY
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Measured Decay Matrix Elements for ro
Production I
If S-channel helicity conservation (SCHC) is
valid only the following matrix elements should
be different from zero
Bernd Löhr, DESY
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Measured Decay Matrix Elements for ro
Production II
If S-channel helicity conservation is valid only
the following spin density matrix elements should
be zero r100 r111 r500 r511 0
At very small t SCHC holds, as t grows SCHS
is increasingly violated.
A pQCD based model (I.Royen and J.Cudell) is able
to describe the SCHC violation.
Bernd Löhr, DESY
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Ratio of longitudinal to transverse
cross-section
A virtual photon has two polarization
states transverse helicity 1 (normal
light) longitudinal helicity 0 (only for Q2gt0)
Ratio of longitudinal to transverse
cross-section is function of the photon
virtuality
If SCHS holds -gt D0.
R is qualitatively described by pQDC models
Bernd Löhr, DESY
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Deeply Virtual Compton Scattering I
DVCS
Bethe-Heitler
Bethe-Heitler processes lead to the same final
state
Difficult to disentangle experimentally
Interference between QCD and QED gives access to
DVCS-QCD amplitude
Interference term can be derived from
asymmetries, e.g. Beam-Spin-Asymmetry
x - skewedness
Probably cleanest test of pQCD , no hadrons in
the final state.
Generalized parton distributions GPD -gt
information on transverse distribution of partons
Bernd Löhr, DESY
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Deeply Virtual Compton Scattering II
s Wd
Q2 8 GeV2 d 1.0
DVCS is a hard process
NLO-QCD calculations can describe
DVCS qualitatively, magnitude of the
predicted cross-section depends on used
parton-density functions
Bernd Löhr, DESY
Page 29
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Deeply Virtual Compton Scattering III
t-dependence of DVCS measured at several Q2 and W
values
For Q2gt 5 GeV2 b 5 -6 GeV-2
Compatible with b values for hard production of
vector mesons
Bernd Löhr, DESY
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DIFFRACTION AT HERA
The HERA Collider and the Experiments Luminosity
and its Measurement Kinematics of Deep-Inelastic
(DIS) and Diffractive Reactions Diffraction in
High-Energy Particle Scattering Regge
Phenomenology versus pQCD Exclusive Vectormeson
Production and Deeply-Virtual Compton Scattering
(DVCS) Inclusive Diffraction and Diffractive
Structure Functions Exclusive Diffractive
Reactions Jets and Heavy Quarks

Page 1
Bernd Löhr, DESY
32
Summary of Exclusive Vectormeson Production

• A high vectormeson mass provides a hard scale.
• Photoproduction of r, w, f is described by
  soft Pomeron exchange,
• whereas J/Y photoproduction is a hard process,
  it can be described by pQCD models.
• Vectormeson production at high Q2 is a hard
  process, pQCD can be applied.
• The vertex factorization holds.
• Vectormeson production at high t is a hard
  process.
• The t-n dependence is in agreement with pQCD
  expectations.
• The Pomeron trajectory for r, f photoproduction
  is compatible with the
• soft Pomeron trajectory
  and shrinkage is observed.
• DIS r and J/Y trajectories are not compatible
  with a soft Pomeron,
• little shrinkage is observed.
• pQCD based models are able to describe the
  features of exclusive hard-production
• of vector mesons

Bernd Löhr, DESY
Page 31
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Summary of Deeply Virtual Compton Scattering

• At high Q2 the W-dependence of the DVCS
  cross-section
• and the slope of the t-distribution are
  compatible with a
• hard production process
• DVCS described in terms of generalized parton
  distributions (GPD)
• which contain information on transverse
  distributions of partons in
• the proton.
• Recently high statistics measurements of DVCS
  by H1 became available
• for electron-proton and positron-proton
  scattering.
• It is, however, still along way before one
  can extract GPDs.
• Apologies to HERMES ! The HERMES experiment
  have measured DVCS to
• a larger extent, also beam-spin asymmetries.
  These data are typically at
• Q2 of 1-2 GeV2. It is not really clear
  whether pQCD is already applicable
• at these low Q2.

Bernd Löhr, DESY
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Definition of Inclusive Diffraction
Model of a resolved pomeron
Colour dipole model
e
e
e
e
no colour exchange, vacuum quantum numbers
no colour exchange, vacuum quantum numbers
P
p
P
p
Definition of inclusive diffractive scattering

• The quantum numbers of the vacuum are exchanged
• between the proton and the virtual photon.
• (The proton stays intact.)

Attention different definitions of diffractive
cross-sections, proton
dissociative events are (partially) included or
not.
Bernd Löhr, DESY
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Inclusive Diffraction I
Diffractive/proton dissociative scattering in the
parton picture
Two additional variables
e
e
Momentum fraction of the proton carried by the
Pomeron
MX
ß
W
Large rapidity gap
xIP
Momentum fraction of the Pomeron carried by the
struck quark
MY
p
t
There is no method available to measure directly
inclusive diffractive cross-sections
Experimentally there are three different methods
used to select diffractive events
1. Detection of outgoing proton includes
reggeon exchange 2. Large rapidity gap
includes reggeon exchange and proton dissociation
into small mass 3. MX method includes
proton dissociation into small mass
Bernd Löhr, DESY
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The Proton Detection Method
1.) Detection of outgoing proton ( H1,ZEUS)
The ZEUS Leading Proton Spectrometer LPS
Z96 m
diffractive peak
Proton-beam direction
S4S5S6 vertical
spectrometer
Silicon strip detectors
xL 1-xIP
S1S2S3 horizontal
spectrometer
Fraction of initial proton momentum carried by
the diffractive proton
Z26 m
Detection of the diffractive proton is the only
method to measure the t-distribution
Advantage no proton dissociaton
Disadvantage small acceptance
possible Reggeon contributions
Bernd Löhr, DESY
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The Rapidity Gap Method
?min .
No tracks or energy deposits in calorimeter for
rapidities greater than ?max or at angles less
than ?min.
H1
LEPTO nondiffractice MC-simulation
Requiring a rapidity gap Dh (e.g. hmaxlt2) in
the events removes most of the nondiffractive
contribution.
mostly diffractive
This is equivalent to restrict measurements to
low xIP .
Advantage large acceptance -gt high statistics
Disadvantage contains contributions from
nondiffractive and
proton dissociative reactions and
Reggeon contributions
hmax
Bernd Löhr, DESY
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The MX-Method (I)
Non diffractive events
Rapidity
Uncorrelated particle emission between incoming
p-direction and scattered quark.
Property of a produced particle
Idealised rapidity distribution
Length of rapidity distribution is given by
cms-energy
Rapidity gap as a statistical fluctuation
Poisson distr. for Dy in nondiffractive events
Bernd Löhr, DESY
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The MX-Method (II)
Experimentally at high energies and not too low
MX
with
This follows also from a triple Regge model
Cortousy of K.Goulianos
t-averaged
Bernd Löhr, DESY
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The MX-Method (III)
Nondiffractive diffractive contributions
Two approaches for fit to the data
D is the diffractive contribution
1.) take Dconst. for a limited range in ln
M2X
Fit slope b, c and D
for
2.) take D from a BEKW-model (see later)
parametrization which describes the
measured data. This is an iterative
procedure.
Determine diffractive events by subtracting
nondiffractive events from measured data bin by
bin as calculated from fitted values b and c.
Advantage no nondiffractive contribution
large acceptance -gt high
statistics
Disadvantage contains contribution from
proton dissociation
Both approaches give the same results
Bernd Löhr, DESY
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Diffractive Cross-Section and Diffractive
Structure Functions
sizeable only at high y, can safely be neglected
in the range of HERA measurements.
If t is not measured, i.e. integrated over
and anlogously
H1 uses
, ZEUS neglects the longitudinal part and uses
Bernd Löhr, DESY
Page 40
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Diffractive DIS Factorization
In the same way as for DIS, diffractive deep
inelastic cross sections can be expressed as a
convolution of diffractive parton densities with
a deep inelastic cross section.
diffractive parton distribution function
(dpdf) number density of a parton i in the
proton under the condition that it undergoes a
diffractive reaction dpdfs at fixed t and xIP
evolve with Q2 according to DGLAP
hard universal DIS cross section
DDIS factorization rigorously proven Collins
Berera, Soper Trentadue, Veneziano
dpdfs are universal for all hard diffractive
processes
Bernd Löhr, DESY
Page 41
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Regge-Factorization
Regge theory originally developed for eleastich
2-body scattering Extension to inclusive
diffraction
Triple-Regge formalism
This expression allows the decomposition into
a flux factor and a cross section
Based on the triple-Regge formalism the
diffractive parton distribution functions are
factorized accordingly
with
This Regge- or vertex-factorization cannot be
proven in pQCD
Bernd Löhr, DESY
Page 42
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Separation of Pomeron from Reggeon Contributions
Results obtained with LPS/FPS method and the LRG
method may contain nondiffractive contributions
from Reggeon exchanges.
They are fitted by a sum of Pomeron and Reggeon
contributions. From the fit the Pomeron
contribution can be extracted.
is taken from a parametrization of the pion
structure function
The fluxes are normalized according to
with
Fitted values are
and
, the normalization of the Reggeon contribution
Other parameters, like
, are also fitted or taken from other
measurements.
Bernd Löhr, DESY
Page 43
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QCD DGLAP Fits
In a picture where the Pomeron has a partonic
structure (Ingelmann-Schlein Model)
can be interpreted as the Pomeron structure
function
Pomeron structure function expressed as a sum
of universal Pomeron parton distribution
functions
Pomeron pdfs obey DGLAP evolution
Parametrization at a starting value Q02 as
z is the longitudinal momentum fraction of the
parton entering the hard sub-process. z b
for lowest order quark-parton modell process, 0 lt
b lt z for higher order processes.
A DGLAP fit and the Regge fit are usually carried
out simultaneously
Bernd Löhr, DESY
Page 44
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Results from Proton Detection Method
H1 Forward Proton Spectrometer (FPS)
ZEUS Leading Proton Spectrometer (LPS)
combined Regge and DGLAP fit
Regge fit at two t-values
---
extrapolation to nonmeasured Q2
.....
Pomeron contribution only
Bernd Löhr, DESY
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Comparison FPS with LPS data
Fair agreement in shape between the FPS and
LPS results
Both datasets have systematic uncertainties from
the beam divergence H1 /-
10 ZEUS 12/-10 They are not
shown in the figures.
Bernd Löhr, DESY
Page 46
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H1 LRG Results
DGLAP fit to H1 data
Regge fit to ZEUS data
Both data show qualitatively the same features
Data contain contributions from proton
dissociation
Bernd Löhr, DESY
Page 47
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Comparison between H1 and ZEUS LRG Results
Good agreement in shape
The ZEUS data are normalized to the H1 data
Reason The H1 dataset and the ZEUS dataset
contain different amounts of
proton dissociation. The data are not corrected
for that.
Bernd Löhr, DESY
Page 48
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Comparison between H1 FPS and LRG Results
The LRG data contain contributions from proton
dissociation up to masses of 1.6 GeV.
The FPS data are multiplied by a factor 1.23 such
that they correspond to the LRG data
Bernd Löhr, DESY
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H1 DPDF Fits
Singlet Structure function
Gluon structure function
Differences between fit A and fit B
For both fits only data with Q2gt8.5 GeV2 have
been used.
Aq, Bq, Cq are always fitted
Qo2 1.75 GeV2
Fit A
Bg is set to zero
Fit B
Qo2 2.5 GeV2
Bg, Cg is set to zero
For fit A the fraction of exchanged momentum
carried by gluons in the range 0.0043ltzlt0.8 is
around 70 throughout the studied Q2 range. For
fit B it is a little less but compatible with
fit A.
The gluon density is only weakly constraint by
the data
Bernd Löhr, DESY
Page 50
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Results from the MX-Method dsdiff/dMX
ZEUS results FPC I published 2005 , FPC II
preliminary 2007
1.2 GeV lt MX lt 30 GeV
FPC I 2.7 GeV2 lt Q2 lt 55 GeV2
FPC II 2.7 GeV2 lt Q2 lt 320 GeV2
Data contain contributions from proton
dissociation with MNlt2.3 GeV
Bernd Löhr, DESY
Page 51
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Fit of W-dependence of inclusive DIS and
inclusive diffractive DIS cross sections
Inclusive DIS
For small x, F2 rises rapidly as x-gt 0
lt
lt
lt
Inclusive diffractive DIS

From W dependence, averaged over t
e.g.measured by ZEUS LPS
With
Inclusive DIS and inclusive diffractive DIS are
not described by the same Pomeron.
Bernd Löhr, DESY
Page 52
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Ratio of total diffractive cross-section to total
DIS cross-section
r sdiff(0.28ltMXlt35 GeV)/stot
Within the errors of the measurements r is
independent of W.
Mx 98-99
Ratio plotted at W220 GeV because only there
the full MX range is covered by measurments
Mx 99-00 (prel.)
At W220 GeV, r can be fitted by
r 0.22 0.034.ln(1Q2)
This logarithmic dependence of the ratio of total
diffractive cross-section to the total DIS cross
section indicates that diffraction is a leading
twist process for not too low Q2.
Bernd Löhr, DESY
Page 53
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ZEUS BEKW(mod) Fit
Dipole Model
The ZEUS data support taking nT(Q2)ng(Q2)nL(Q2
) n1ln(1Q2/Q20)
Taking x0 0.01 and Q20 0.4 GeV2 results in
the modified BEKW model with the 5 free
papameters
cT , cL , cg , n1T,L,g , g
Bernd Löhr, DESY
Page 54
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ZEUS-MX Diffractive Structure Functions with
BEKW(mod.) Fit
FPC I
FPC II
gt 400 points 5 parameters c2/nD 0.71
BEKW-fit
transverse qq contribution
sum of all contributions
transverse qqg contribution
longitudinal qq contribution
Bernd Löhr,
Page 55
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ZEUS-MX Diffractive Structure Functions
Relation between differential diffractive cross
section and diffractive structure function
F2D(3) specifies the probability to find a in
diffractive reaction a quark in a proton
carrying a fraction xbxIP of the proton
momentum.
rises approximately proportional to
.
This rise reflects the increase of the
diffractive cross section
with W.
At small xIP the development of the diffractive
cross section is governed by the increase of the
gluon density as W increases or xIP decreases.
Bernd Löhr,
Page 56
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ZEUS-MX Diffractive Structure Function and the
BEKW(mod) Fit
-
-
For fixed Q2 and fixed xIP the data show a broad
maximun around b0.5 which originates from the
b(1-b) dependence of the tansverse qqT
contribution.
Towards low b values the qqg contribution
rises strongly due to the increase in gluon
density. The longitudinal qqL contribution is
sizeable only at very high b and is responsible
for a finite value of the diffractive
structure function at b -gt 1.
Bernd Löhr, DESY
Page 57
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ZEUS MX QCD Scaling Violations
xIPF2D(3) in fixed (xIP,b)-bins
ZEUS BEKW(mod) fit
xIPF2D(3) shows considerable scaling
violations from positive scaling violations
over near constancy to negative scaling
violations.
Scaling violations well described by BEKW model
-gt effective DGLAP evolution present in this
model.
Bernd Löhr, DESY
Page 58
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H1 and ZEUS QCD Scaling Violations
ZEUS BEKW(mod) fit
H1 DPDF fit
Bernd Löhr, DESY
Page 59
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Summary of Inclusive Diffraction
1.) Three experimental methods to measure
inclusive diffraction at HERA PS/LPS,
LRG, and MX. All include contributions of
different magnitude from proton dissociation or
from Reggeon exchange or from both. 2.)
Inclusive diffraction is a leading twist
reaction. 2.) The diffractive cross-section
can be expressed in terms of diffractive
structure functions. 3.) Assuming Regge
factorization H1 separated Pomeron and Reggeon
contributions and extracted DPDFs from
the LRG data. 4.) QCD inspired dipole models
can describe inclusive diffraction as well.
The ZEUS MX data are described rather precisely
over the whole kinematic range by the
BEKW(mod) parametrization. 5.) The BEKW modell
explains the diffractive structure function
F2D(3) in terms of transverse and
longitudinal quark-antiquark and
quark-antiquark-gluon contributions. 6.) The
diffractive structure function F2D(3) shows large
scaling violations. This points to a
large gluon fraction in the colourless exchange.
Bernd Löhr, DESY
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Semi-Inclusive Diffraction And Tests of QCD
Factodization
Exclusive diffractive processes
?
exclusive vector meson production

transition from soft to hard

diffractive reactions
VM
z
1-z
p
P
Inclusive diffraction
diffractive structure functions

description by quark-dipole models

universal diffractive parton densities
Semi-inclusive diffractive processes
diffractive final states with

special properties
jets, heavy
quarks, ....
Semi-inclusive diffractive processes are a
testing ground for the universality of the
diffractive parton distributions derived from
inclusive reactions.
Bernd Löhr, DESY
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Semi-Inclusive Diffractive DIS D(2010) Production
Kinematical selection hmaxlt3, xIP lt 0.035, b lt
0.8
The pQCD fit using the DPDFs from inclusive
diffraction gives acceptable descriptions of
diffractive D(2010) Production.
ACTW fit (Alvero et al.) Combined fit of DPDFs
to H1 and ZEUS inclusive diffractive data and to
ZEUS diffractive di-jet photoproduction data, all
data up to 1998.
Confirmation of the universality of DPDFs from
inclusive diffraction.
Bernd Löhr, DESY
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Semi-Inclusive Diffractive DIS Di-Jet Production
Diffractive photon-gluon fusion
Fit A
Fit B
diffractive System X
zIP part of the Pomeron momentum that
goes into the hard interaction
Diffractive di-jet production is sensitive to
the gluon DPDF
H1 fits A and B used in NLO calculation (Z.Nagy)
Confirmation of the universality of DPDFs from
inclusive diffraction.
Fit B gives an acceptable description of the data
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Combined Fit to DIS Inclusive Diffractive and
Diffractive Di-Jet Production
make combined fit
Experimental validation of universality of DPDFs

The information from diffractive di-jet
production improves the fit of the gluon
density in particular at high zIP.
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HERA DPDFs and Predictions for the Tevatron
Do the DIS-DPDFs from HERA also describe
diffraction in hadron-hadron interactions ?
Note QCD factorization has not been
proven for hadron-hadron interactions
Diffracitve di-jet production at CDF
Convolution of 2 structure functions
Predictions from DIS diffractive structure
functions fail by factor 5-7.
Final state interaction between proton remnent
and antiproton possible gap
survival probability is not one.
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Semi-Inclusive Diffractive Photoproduction at
HERA
What is special about photoproduction (Q2gt0 GeV2)
?
Direct photoproduction
Resolved photoproduction
Photon fluctuates into a hadronic state of which
only a part enters the hard interaction
Photon takes part directly in hard interaction
Xg lt 1
Xg 1
Resolved photoproduction is similar to
hadron-hadron interactions
Do final state interactions play a role also in
photoproduction ?
operational definition
Xg lt 0.7 resolved
Is diffractive resolved photoproduction also
suppressed ?
Xg gt 0.7 direct
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Diffractive D(2010) Photoproduction at HERA
NLO calculations Frixione et al. FMNR-code and
H1 DPDFs
Qualitative good agreement with NLO QCD
calculations using inclusive DPDFs
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Diffractive Di-Jet Photoproduction at HERA
H1 Q2 d 1 GeV2 165 lt W lt 242 GeV
hmax 3.2 xIP lt 0.03 ET,jet1
gt 5 GeV ET,jet2 gt 4 GeV -1 lt
hjetlab lt 2
ZEUS Q2 lt 1 GeV2 hmax 2.8
xIP lt 0.028 ETjet1 gt 7.5 GeV
ETjet2 gt 6.5 GeV -1.5 lt hjetlab lt
1.5
Note H1 and ZEUS are quoting results for
different kinematical regions
Note NLO predictions scaled by 0.5
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Diffractive Di-Jet Photoproduction at HERA cont.
Results from H1 and ZEUS separately for direct
and resolved photoproduction
ZEUS diffractive di-jet photoproduction

• H1 data for all xg
• in NLO calculation
• xg,PL gt 0.9 no scaling factor
• xgPL lt 0.9 scale factor 0.44
• -gt unacceptable fit

direct enriched
Xgobs lt 0.75
resolved enriched
Xgobs gt 0.75
Agreement within errors between data and NLO
calculations with inclusive DPDFs
Fit with 2 free normalizations for xg,PL lt 0.9
and xg,PL gt 0.9 yielded 0.47 /-0.16 and
0.53/-0.14
Note, however, that kinematic regions for H1 and
ZEUS data are different -gt may
not be a contradiction
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Summary of Semi-Inclusive Diffraction and Tests
of QCD Factorization

• 1.) QCD inspired models using the DPDFs
  determined from inclusive diffractive DIS
• describe at least qualitatively
  semi-inclusive diffractive production of heavy
  quarks
• and di-jets.
• 2.) The QCD factorization scheme in DIS is
  experimentally verified in diffractive DIS.
• 3.) Although theoretically not expected, Regge
  factorization seems to hold in practice.
• 4.) These are experimental indications that
  DPDFs might be universal in DIS.
• 5.) QCD factorization does not hold for
  diffractive hadron-hadron interactions.
• Using HERA DPDFs leads to a gross
  overestimation of dijet production at the
• Tevatron. The survival probability of
  diffractive gaps is smaller than one.
• 6.) The situation in semi-inclusive diffractive
  photoproduction needs further
• clarification. For heavy quark production
  the QCD factorization seems to hold also
• for the resolved photon contribution.
  For di-jet production there are indications
• from H1 that QCD factorization might not
  be valid whereas ZEUS results confirm
• QCD factorization although in a slightly
  different kinematical regime.

Bernd Löhr, DESY
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