19 research outputs found
Charm production in deep inelastic and diffractive scattering
We consider the production of charm by real and virtual photons. Special
attention is paid to diffractive charm production, which provides information
on the gluonic content of the Pomeron. Our calculations are based on the gluon
distributions of the CKMT-model, which is shown to lead to agreement with the
data on open charm production in deep inelastic scattering. We compare
predictions for diffractive charm production of different models for the
distribution of gluons in the Pomeron. Experiments at HERA should be able to
discriminate between them. Predictions for beauty production in diffractive and
non-diffractive interactions of photons are also given.Comment: 14 pages REVTEX and 24 figures include
The BFKL Pomeron with running coupling constant: how much of its hard nature survives?
We discuss the BFKL equation with a running gauge coupling and identify in
its solutions the contributions originating from different transverse momentum
scales. We show that for a running coupling constant the distribution of the
gluons making up the BFKL Pomeron shifts to smaller transverse momenta so that
the dominant part of Pomeron can have a nonperturbative origin. It is
demonstrated how this soft physics enters into the BFKL solution through the
boundary condition. We consider two kinematical regimes leading to different
behaviour of the rapidity and transverse momentum dependence of the gluon
distribution. In the diffusion approximation to the BFKL kernel with running
, we find a sequence of poles which replaces the cut for fixed
. The second regime corresponds to the singular part of the kernel,
which gives the dominant contribution in the limit of very large transverse
momenta. Finally, a simple more general picture is obtained for the QCD Pomeron
in hard processes: it is of soft, nonperturbative nature, but has hard ends of
DGLAP-type.Comment: 23 pages REVTEX and 4 figures include
Photoproduction of vector mesons in the Soft Dipole Pomeron model
Exclusive photoproduction of all vector mesons by real and virtual photons is
considered in the Soft Dipole Pomeron model. It is emphasized that being the
Pomeron in this model a double Regge pole with intercept equal to one, we are
led to rising cross-sections but the unitarity bounds are not violated. It is
shown that all available data for rho, omega, phi, J/psi and Upsilon in the
region of energies 1.7 <= W <= 250 GeV and photon virtualities 0 <= Q^2 <= 35
GeV^2, including the differential cross-sections in the region of transfer
momenta 0 <= |t| <= 1.6 GeV^2, are well described by the model.Comment: 17 pages, 19 figure
NLO BFKL Equation, Running Coupling and Renormalization Scales
I examine the solution of the BFKL equation with NLO corrections relevant for
deep inelastic scattering. Particular emphasis is placed on the part played by
the running of the coupling. It is shown that the solution factorizes into a
part describing the evolution in Q^2, and a constant part describing the input
distribution. The latter is infrared dominated, being described by a coupling
which grows as x decreases, and thus being contaminated by infrared
renormalons. Hence, for this part we agree with previous assertions that
predictive power breaks down for small enough x at any Q^2. However, the former
is ultraviolet dominated, being described by a coupling which falls like
1/(\ln(Q^2/\Lambda^2) + A(\bar\alpha_s(Q^2)\ln(1/x))^1/2)with decreasing x, and
thus is perturbatively calculable at all x. Therefore, although the BFKL
equation is unable to predict the input for a structure function for small x,
it is able to predict its evolution in Q^2, as we would expect from the
factorization theory. The evolution at small x has no true powerlike behaviour
due to the fall of the coupling, but does have significant differences from
that predicted from a standard NLO in alpha_s treatment. Application of the
resummed splitting functions with the appropriate coupling constant to an
analysis of data, i.e. a global fit, is very successful.Comment: Tex file, including a modification of Harvmac, 46 pages, 8 figures as
.ps files. Correction of typos, updating of references, very minor
corrections to text and fig.
Glueball spectrum and the Pomeron in the Wilson loop approach
Using a nonperturbative method based on asymptotic behaviour of Wilson loops
we calculate masses of glueballs and corresponding Regge-trajectories. The only
input is string tension fixed by meson Regge slope, while perturbative
contributions to spin splittings are defined by standard alpha_s values. The
masses of lowest glueball states are in a perfect agreement with lattice
results. The leading glueball trajectory which is associated with Pomeron is
discussed in details and its mixing with f and f' trajectories is taken into
account.Comment: LaTeX2e, 49 pages, 2 figure
Q^2 dependence of diffractive vector meson electroproduction
We give a general formula for the cross section for diffractive vector meson
electroproduction, gamma^* p -> Vp. We first calculate diffractive qqbar
production, and then use parton-hadron duality by projecting out the J^P = 1^-
state in the appropriate mass interval. We compare the Q^2 dependence of the
cross section for the diffractive production of rho and J/psi mesons with
recent HERA data. We include the characteristic Q^2 dependence associated with
the use of the skewed gluon distribution. We give predictions for
sigma_L/sigma_T for both rho and J/psi production.Comment: 15 pages, LaTeX, including five PostScript figure
Deep Inelastic Scattering Data and the Problem of Saturation in Small-x Physics
We investigate the role of unitarization effects in virtual photon-proton
(\gamma^*p) interactions at small x. The q\bar{q}-fluctuation of the initial
photon is separated into a small distance and a large distance component and a
model for the unitarization of each component is proposed. The Born
approximation for the small size component is calculated using QCD perturbation
theory. Reggeon diagram technique is used in order to obtain a self-consistent
scheme for both total \gamma^*p cross section and diffractive production. The
model gives a good description of HERA data in the small-x region, with a
single Pomeron of intercept 1.2.Comment: 27 pages, 8 postscript figure
Photoproduction off Nuclei and Point-like Photon Interactions Part I: Cross Sections and Nuclear Shadowing
High energy photoproduction off nuclear targets is studied within the
Glauber-Gribov approximation. The photon is assumed to interact as a
-system according to the Generalized Vector Dominance Model and as a
``bare photon'' in direct scattering processes with target nucleons. We
calculate total cross sections for interactions of photons with nuclei taking
into account coherence length effects and point-like interactions of the
photon. Results are compared to data on photon-nucleus cross sections, nuclear
shadowing, and quasi- elastic -production. Extrapolations of cross
sections and of the shadowing behaviour to high energies are given.Comment: 15 pages, 12 figure
Double-Spin Asymmetry in the Cross Section for Exclusive rho^0 Production in Lepton-Proton Scattering
Evidence for a positive longitudinal double-spin asymmetry = 0.24
+-0.11 (stat) +-0.02 (syst) in the cross section for exclusive diffractive
rho^0(770) vector meson production in polarised lepton-proton scattering was
observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA
positron beam was scattered off a longitudinally polarised pure hydrogen gas
target. The average invariant mass of the photon-proton system has a value of
= 4.9 GeV, while the average negative squared four-momentum of the virtual
photon is = 1.7 GeV^2. The ratio of the present result to the
corresponding spin asymmetry in inclusive deep-inelastic scattering is in
agreement with an early theoretical prediction based on the generalised vector
meson dominance model.Comment: 10 pages, 4 embedded figures, LaTe
asymmetry and semi-inclusive production of pions in DIS
We investigate unambiguities in the extraction of the
asymmetry from semi-inclusive production of pions in DIS. The role of several
effects beyond the quark-parton model (QPM) which lead to and and may therefore cloud such an
extraction is studied. The results are discussed in the context of the recent
HERMES data. We find that the interaction of the resolved photon with the
nucleon significantly modifies the observed asymmetry. The
exclusive elastic production of mesons plays a similar role for the
large- data sample. Our estimate shows a rather small effect of the
spectator mechanism. Nuclear effects in the deuteron also look potentially
important but are difficult to estimate quantitatively.
Throughout the paper we make in addition several general remarks concerning
semi-inclusive and exclusive production of mesons.Comment: 47 pages, LaTeX with 19 eps figure