Regge trajectory determination in exclusive electroproduction
of rho^0
mesons
Exclusive production of rho^0 mesons has been studied in ep collisions
with the ZEUS detector at HERA in the kinematic range 2 < Q^2
< 30 GeV^2
and 32 < W < 150 GeV. The study of the W dependence of cross
sections
at fixed t allows to make a direct measurement of the exchanged
trajectory
at different values of Q^2. The ratio of the production cross sections,
R = sigma_L / sigma_T, for longitudinally (sigma_L) and transversely
(sigma_T)
polarized virtual photons is determined from the angular distribution
of the decay products of the rho^0, and the t and W dependence
of R
is presented.
Exclusive photoproduction of J/psi mesons
The exclusive photoproduction of J/psi mesons, gamma p -> J/psi
p,
has been studied in ep collisions with the ZEUS detector at HERA
using
two independent measurements with an integrated luminosity of 38
pb-1
of the photon-proton centre-of-mass energy, W, in the kinematic
range
20 < W < 290 GeV. The differential cross-section d\sig/dt,
where t is
the squared four-momentum transfer at the proton vertex, has been
measured
in the kinematic range 30 < W < 170 GeV and |t| < 1.8
GeV .
The Pomeron trajectory has been determined to be different from
the soft one.
Diffractive photoproduction of Vector Mesons
at large transverse momenta
Proton-dissociative diffractive photoproduction of
vector mesons at large -t
Photoproduction of vector mesons, gamma p -> VN , where N is a low-mass
dissociated state, has been measured in e+ p interactions with
the ZEUS
detector at HERA using an integrated luminosity of 24 pb-1.
The data were taken at a photon-proton center-of-mass energy W
~= 100 GeV
and extend up to -t = 12 GeV^2, where t is the sqare of the four-momentum
transferred to the vector meson. The differential cross section,
dsig/dt,
and the results of a decay-angle analysis as a function of t for
rho^0,
phi and J/psi production are compared to the expectation from the
phenomenology models. A two-dimensinal analysis in the decay-angle
analysis is performed for the rho mesons and the spin density
matrix elements r^04_1-1, r^04_00 and r^04_10 are determined.
Exclusive photoproduction of J/psi mesons
The exclusive photoproduction of J/psi mesons, gamma p -> J/psi
p,
has been studied in ep collisions with the ZEUS detector at HERA
using
two independent measurements with an integrated luminosity of 38
pb-1
of the photon-proton centre-of-mass energy, W, in the kinematic
range
20 < W < 290 GeV. The differential cross-section d\sig/dt,
where t is
the squared four-momentum transfer at the proton vertex, has been
measured
in the kinematic range 30 < W < 170 GeV and |t| < 1.8
GeV .
The Pomeron trajectory has been determined to be different from
the soft one.
Measurement of DME for Rho meson production at intermediate t
A sample of e^+ pi^+ pi^- events with Q^2 > 2.5 GeV^2, 40 < W < 120 GeV and a measured value of |t| in the range 0.5 < |t| < 3 GeV^2 is used to extract the density matrix elements r^04_00, which is related to the ratio of longitudinal to transverse photon cross section, and r^5_00, which is related to the dominant spin flip amplitude T_(lambda_rho=0, lambda_gamma=1). The |t| dependence of r^04_00 and r^5_00 are determined.
H1 Photoproduction of J/psi at high t.
A measurement of the diffractive production of J/Psi vector mesons with four-momentum transfer squared |t| at the proton vertex up to 20 GeV is presented in photoproduction at HERA. The J/Psi is identified via its decay into two unlikesign leptons. The differential cross section d sigma / d |t| is extracted and compared to theoretical predictions.
Electroproduction of rho, omega and phi mesons
at HERMES
HERMES has performed extensive measurements on diffractive production
of
Jan Olsson, H1
Proton dissociative Rho Electroproduction
The electroproduction of rho mesons is studied in the process e
p -> e rho Y,
Odderon in QCD
A review on how the Odderon idea does appear in QCD is given.
Possible Odderon discovery at HERA via charge asymmetry
in diffractive pi+pi- photoproduction
Search for contribution of Odderon-photon fusion to
exclusive pi0 production
A search for contributions of odderon-photon fusion to the exclusive
pi^0
John Collins
The relation between QCD and diffraction
Measurement of photoproduction cross section with a
leading proton
The total cross-section for the semi-inclusive photoproduction process
with
Measurement of the Diffractive Cross Section in ep
Interactions at low Q2
Diffractive dissociation of virtual photons, $\gamma^* p \rightarrow
X p$, has been studied in $ep$ interactions down to small photon virtualities
$Q^2$ of the order of 0.1~GeV$^2$. The data, taken with the ZEUS detector
at HERA between 1995 and 1997, correspond to a total integrated luminosity
of about 10~pb$^{-1}$. Diffractive events were selected with two methods:
one method is based on the distribution of the mass of the hadronic final-state
system $M_X$, while the other requires the detection of the scattered proton
in the ZEUS Leading Proton Spectrometer. The dependence of the diffractive
cross section on the photon virtuality $Q^2$ and on the virtual-photon
proton centre-of-mass energy $W$ is studied down to low $Q^2 \simeq$ 0.1~GeV$^2$
and compared to that of the total $\gamma^* p$ cross section. In the range
$0.1 \lsim Q^2 \lsim 0.7$~GeV$^2$, the $Q^2$ dependence is found to differ
significantly from what has previously been observed at higher values of
$Q^2$, analogously to what observed for the total $\gamma^* p$ cross section.
In the same $Q^2$ range, the energy dependence is found to be compatible
with Regge theory and used to determine the Pomeron intercept.
QCD Analysis of F2D(3)
It is now experimentally well established at HERA
The purpose of this presentqtion is to derive parton distributions
-the precise determination of the effective Pomeron and Reggeon
-the determination of the parton distributions at Next-to-Leading
Order
-the differences between parton distributions derived from ZEUS
and H1
-the extrapolation of the diffractive structure function to
ZEUS Diffractive results from the ZEUS Leading Proton
Spectrometer
Diffractive dissociation of virtual photons,
ZEUS Leading baryon production in ep scattering
Leading proton production in ep collisions at HERA
Events with a final state proton carrying a large fraction of
The production of energetic neutrons in e+ p collisions at HERA
is studied
Diffractive parton distributions from the saturation
model
On a unified approach to hard diffraction
S-matrix for dipole proton scattering in the compact
parameter representation
In this talk we present a method of extraction the S-matrix element
for
Diffraction in CDF: Run I results and plans for Run
II
Run I results in diffraction in D0 and prospects for
Run II
Scaling at low x in Gamma* proton interactions and
the generalized vector dominance/color-dipole picture
The GVD/CDP combines the diffraction ansatz from GVD at
Deeply Virtual Compton scattering, a NLO QCD analysis
Single Spin Azimuthal Asymmetries
DVCS from ZEUS, H1
A measurement is presented of Deeply Virtual Compton Scattering
at HERA.
Higgs prospects in double tagged diffractive events
at the Tevatron
Hadronic cross sections of untagged and double tagged
2 photon events at LEP
The production of hadrons by quasi-real
photons is studied
NLO corrections to the virtual photon impact factor
We report on progress in the calculation of NLO corrections to the
impact
Hard colour singlet exchange from the BFKL equation
with nonleading corrections
We study elastic parton-parton scattering by hard BFKL pomeron
Theoretical results on the parton level are obtained and compared
to
Properties of hadronic final states in diffractive
DIS
Characteristics of the hadronic final state of diffractive DIS events
Three-jet production in diffractive deep inelastic
scattering at HERA
Measurements are reported of three-jet production in diffractive
Diffractive jet production in DIS
A measurement is presented of dijet and 3-jet cross sections in
low-|t|
Study of D*+- Meson Production in Diffractive ep Scattering
at HERA
Non-linear evolution and parton distributions at low
x
We suggest a new procedure for extrapolating the parton
Black body limit of DIS at small x: diffractive funal
states
Skewed parton distributions and F2D(3) at beta ->1
light vector mesons (rho^0, omega, phi) in the intermediate
energy region (4.0
where the proton dissociates into the hadronic system Y. The skewing
of
the rho resonance mass shape in the pi^+ pi^- mass distribution
is measured
as a function of Q^2. The differential cross sections are presented
as a
function of Q^2, of the centre-of-mass energy W and of the four-momentum
transfer squared at the proton vertex, t.
An increase in the ratio of proton dissociative to elastic cross
sections
is observed. Violation of S-channel helicity conservation is confirmed
in the proton dissociative rho meson electroproduction process.
G.P.Vacca
In the last years it has been developed a non-perturbative QCD
approach
based on the stochastic vacuum model and a perturbative one based
on resummation tecniques in the small x QCD region.
Last developments on the perturbative analysis are shown in some
details.
I. Ivanov
Abstract:
We discuss how the evasive odderon signal can be enhanced
by the final state interaction. We suggest the charge asymmetry of pion
spectra in diffractive $\pi^{+}\pi^{-}$ photoproduction as a promising
signature of the odderon exchange. A dozen per cent charge asymmetry
signal of the odderon is feasible around the $f_{2}(1270)$ resonance.
T. Golling, H1
production in the process ep -> e pi^0 N^* is performed with a
leading neutron
from the N^* decay detected in a forward neutron calorimeter. With
no indication for the presence of odderon contributions, a 95% confidence
level upper
limit for the odderon cross-section is obtained.
I give a critical review of the relation between real QCD and the methods
used to estimate cross sections for diffraction and small x processes:
hard scattering factorization and the dipole model. The key to a correct
unification of all treatments is a correct understanding of the
unintegrated parton densities and of its quantitative relation to both the
dipole cross section and the ordinary MS-bar parton densities.
K.-H. Hiller, H1
a leading proton in the final state has been
measured at centre-of-mass energies between 91 and 231~GeV. The
measured
cross-sections refer to the kinematic range
with transverse momenta of the scattered proton restricted to
$p_T \leq 0.2 \gev$ and $0.66 \leq z \leq 0.90$, where $z = E_p'/E_p$
is the scattered proton energy normalized to the beam energy. That
is a region,
in which diffractive processes are
suppressed relative to processes with pion and reggeon exchange.
The cross-section is measured to be about $7.7 \mub$ per
unit $z$. It is found to be independent of the photon-proton centre-of-mass
energy and the scattered proton momentum. The
measured cross-sections are compared with semi-inclusive deep inelastic
scattering data with a leading proton in the final
state, which is subject to the same kinematic restrictions as in
the case
of photoproduction. This allows to study the transition
from finite photon virtuality $Q^2$ to $Q^2=0$ in a model in which
the
proton structure function at low $Q^2$ is split into a
nonperturbative vector meson dominated contribution and a partonic
contribution. The VDM contribution determined in the
framework of this model in case of the semi-inclusive process is
only $23\%$ of the amount found in the inclusive reaction.
Talk 7: A. Pellegrino, ZEUS
Laurant Schoeffel
that a substantial fraction of ep events is contributable to
diffraction, i.e. color singlet exchange, initiated by a highly
virtual photon.
Starting with the pioneering theoretical work of Schlein and Ingelman,
the idea
of a point-like structure of the Pomeron exchange opens the way
to the determination
of its parton (quark and gluon) distributions, where the Pomeron
point-like structure
can be treated in a similar way as (and compared to) the proton
one.
of the Pomeron from QCD fits of diffractive DIS cross-sections
determined at HERA. By comparison with previous
determinations
contained in the experimental papers, we introduce some new techniques
of
parametrizations and discuss various points which have appeared
in the
domain:
trajectories in Regge fits
(NLO) in
the Pomeron comparing two methods, either using cuts in order
to damp
the
expected higher twist contributions
or by a global fit including a higher twist contribution
taken
from
models
data
-the need to evaluate precisely
the gluon content of the Pomeron and its error
since it has strong impact on the discussion
of diffractive charm and longitudinal diffractive structure functions
single diffraction at Tevatron
could give a precise evaluation of the factorization breaking.
B. Smalska
gamma* p -> X p, with Q2>4GeV2 has been studied at HERA
in ep interactions using ZEUS Leading Proton Spectrometer.
The analysis is based on a luminosity of 13 pb-1
collected in 1997. The cross section is presented
as a function of four-momentum transfer squared t
as well as the azimuthal angle between the electron
scattering plane and the proton production plane.
A. Garfagnini
the proton beam energy have been studied in ep collisions at HERA,
using the ZEUS detector and its Leading Proton Spectrometer.
The study has been performed on two samples with different
photon virtualities: 0.1 < Q^2 < 0.74 GeV^2 and 3 < Q^2
< 254 GeV^2 .
In both cases, events were selected with a proton in the final
state
with at least 60% of the beam energy, xL > 0.6, and a transverse
momentum pT^2 < 0.5 GeV^2 . The cross section for leading proton
production is studied as a function of xL and pT . The dependence
of
the cross section on Q^2 and on the Bjorken scaling variable x
is
discussed in terms of the semi-inclusive structure function F_^LP
.
Leading neutron production in e+ p collisions at HERA
using the ZEUS detector. The semi-inclusive cross section ep ->
e'nX is
measured relative to the inclusive cross section ep -> e'X , considerably
reducing the systematic errors. The events are divided into three
samples
depending upon the virtuality of the photon: photoproduction,
Q^2 < 0.02 GeV^2 ; an intermediate region, 0.1 < Q^2 <
0.74 GeV^2 ;
and deep inelastic scattering, 4 < Q^2 < 10^4 GeV^2 .
The neutrons studied have small production angles, theta_n <
0.8 mrad,
and carry at least 20% of the proton beam energy, x_L > 0.2. Scaling
and factorization properties are studied as a function of Q^2 and
x,
the Bjorken scaling variable. A reduction in the relative yield
of neutrons is observed in photoproduction and at intermediate
Q^2
as compared to the deep inelastic scattering region. The diffractive
dissociation of the incident proton into an energetic neutron accounts
for only a small fraction of the events. The semi-inclusive structure
function F_2^{LN(3)}(x, Q^2, x_L) shows a rise at low values of
x similar
to that of the inclusive F_2(x, Q^2) of the proton. For x_L > 0.6,
pion
exchange dominates, allowing the determination of the total cross
section
and the structure function of the pion, F_2^pi(x, Q^2). At fixed
Q^2 , F_2^pi
has the same Bjorken x dependence as F_2 . Uncertainties in the
magnitude
of the pion flux and, therefore, in the normalization of F_2^pi
are discussed.
Krystof Golec-Biernat
I review inclusive diffractive DIS in the light of the
collinear factorization theorem, discussing
diffractive parton distributions in the Ingelman-Schlein approach
and the approach which starts from detailed pQCD description
of diffractive system. In the latter case their form
can be found using the saturation model. I discuss the relation between
the two approaches, pointing out the differences bewteen them.
R. Peschanski
Using a combination of S-Matrix and perturbative QCD properties in the small
$x_{Bj}$ regime, we propose a formulation of hard diffraction unifying
the partonic Pomeron, Soft Colour Interaction and QCD dipole descriptions.
A. Stasto
dipole-proton scattering using the data on diffractive electroproduction
of vector mesons at HERA. By considering the full t dependence
of this
process we are able to reliably unfold the profile of the S-matrix
for
impact parameter values b>0.3 fm. We show that the results depend
only
weakly on the choice of the form for the vector meson wave
function.
Using this result we can give an estimate of the magnitude
of the
saturation scale at HERA.
K. Goulianos, CDF
Results on diffraction obtained by CDF in Run I of the Tevatron
collider are presented and discussed. Published CDF results on
soft single diffraction, hard single diffraction (W, dijet and
b-quark production), hard double diffraction (jet-gap-jet) and
hard double Pomeron exchange (gap-dijet-gap) are briefly reviewed,
and new results are presented on soft double diffraction and
diffractive J/Psi production. The emphasis of the presentation
is on what has been learned from Run I data about the QCD nature
of diffraction, and in particular about the diffractive structure
function of the (anti)proton and its factorization properties.
The CDF plans for diffractive studies in Run II are also presented,
focusing on the relevant detector upgrades and physics goals.
Christophe Royon, D0
I will cover the main results on diffraction at run I in D0 based on
rapidity gap events. I will also discuss the new roman pot detectors which
are being installed now for Run II.
Dieter Schildknecht
low x with the generic two-gluon exchange sructure for
the (qq-bar)-proton interaction. It contains the empirically
established scaling of the virtual-photon -proton -interaction
cross section and implies hadronlike behaviour at any Q**2,
provided W**2 is sufficiently large.
Andreas Freund
Dieter Mueller
Jochen Vollmer
Laurant Favart, H1
The cross section is measured as a function of
$Q^2$ and $W$. The data are compared to LO QCD calculations based
on the two gluon exchange model.
C. Royon
I. Vorobiev
at LEP. Total cross-sections sigma(e+e- -> e+e- hadrons)
and sigma(gamma gamma -> hadrons) are measured for sqrt(s) from
161 GeV to 202 GeV and two-photon centre-of-mass energies
W_gammagamma between 5 GeV and 185 GeV.
New results on double tag events will also be presented for
sqrt(s) = 189 - 208 GeV and the gamma^* gamma^* cross-sections
will be discussed.
S. Gieseke
factor for virtual photons. The calculation of virual corrections
will be
briefly reviewed and the status of the calculation of real corrections
will be discussed.
Rikard Enberg
exchange at |t| >> 0 in p-pbar collisions at the Tevatron, leading
to
rapidity gaps between jets. Earlier studies based on the Mueller-Tang
prescription are extented to include a more complete BFKL treatment
with a running coupling and the consistency constraint accounting
for non-leading effects.
previous calculations. Furthermore, the parton level cross sections
are implemented in PYTHIA, facilitating detailed modelling of gap
survival probabilities and comparison with the data from D0 and
CDF.
T. Yamashita, ZEUS
Hadronic final states and 3-jet production
ep -> epX were studied with the ZEUS detector at HERA with an integrated
luminosity of 13.8 pb-1 collected in 1997. The events were tagged
by
identifying the diffractively scattered proton using the Leading
Proton
Spectrometer. The properties of the hadronic final state X were
studied
using thrust, thrust angle, sphericity, energy flow, transverse
energy and "seagull" distributions. Comparisons of the hadronic
final state
in terms of these distributions with predictions from various Monte
Carlo
Model generators suggest that the final state is dominated by q\bar{q}g
states at the parton level.
deep inelastic scattering with the ZEUS detector at HERA.
The data
were measured in the kinematic region $5 < Q^2 < 100$ GeV$^2$,
$200 < W < 250$ GeV, $23 < M_X < 40$ GeV and $x_{\Pma}
< 0.025$. The
diffractive signal is selected by requiring a large rapidity gap
in
the outgoing proton direction. Jets are reconstructed in the centre-of-mass
system of the observed hadronic final state with the exclusive-$k_T$
jet-algorithm. The three-jet final-state topology and the partonic
nature
of the jets are studied. The measurements are compared to models
for
diffraction in deep inelastic scattering.
Frank.-P. Schilling, H1
diffractive deep-inelastic scattering interactions of the type
ep -> eXY,
where the system X is separated by a large rapidity gap from a
low-mass
baryonic system Y. Data taken with the H1 detector at HERA,
corresponding to an integrated luminosity of 18.0 pb^(-1), are
used to
measure hadron level single and double differential cross sections
for 4
The measurements are consistent with a factorising diffractive
exchange with
trajectory intercept close to 1.2 and tightly constrain the dominating
diffractive
gluon distribution.
Viewed in terms of the diffractive scattering of partonic fluctuations
of
the photon, the data require the dominance of qqbarg over qqbar
states.
Soft colour neutralisation models in their present form cannot
simultaneously
reproduce the shapes and the normalisations of the differential
cross sections.
Models based on 2-gluon exchange are able to reproduce the shapes
of the
cross sections at low x_pom values.
Irina Korzhavina, ZEUS
Measurements of D^*+/- production in diffractive interactions at the HERA
collider are presented. The data, collected by the ZEUS experiment,
correspond to an integrated luminosity of 38 pb^-1 for photoproduction and
44 pb^-1 for DIS. Diffractive interactions were identified by a large gap
in rapidity distribution of final states. D^*+/- mesons were reconstructed
from decay channel D*+ -> D0pi_s^+ with K^-pi^+ (+c.c.). Integrated and
differential cross sections, measured for the first time in
photoproduction, were compared to theoretical expectations.
Michael Lublinsky
distributions from HERA to much higher energies. The procedure
suggested consists of two steps. First, we solve the non-linear
evolution
equation. Second, we introduce a correcting function for which
we write a
DGLAP-type linear evolution equation. The nonlinear equation is
solved numerically and estimates for the saturation scale, as well
as
for the gluon density at THERA and LHC energies are made.
V. Guzey
In the limit of complete absorption by the target of hadronic
fluctuations of the photon, we formulate predictions for inclusive and
diffractive structure functions as well as diffractive final states.
T . Teubner
We show that the diffractive structure function is perturbatively
calculable in the domain where the diffractive mass is small but still
outside the resonance region. In this domain, which can be characterized
by Lambda^2/Q^2 << 1-beta << (Lambda^2/Q^2)^1/2, the structure function
represents a new observable, which is highly sensitive to the small-x
skewed gluon distribution. Our leading order calculation and the
estimate of next-to-leading order corrections are consistent with
available data and demonstrate the potential of more precise data to put
further constraints on skewing effects."