59 research outputs found
Hadron Diffractive Processes: the Structure of Soft Pomeron and Colour Screening
On the basis of the experimental data on diffractive processes in ,
and collisions at intermediate, moderately high and high
energies, we restore the scattering amplitude related to the -channel
exchange by vacuum quantum numbers by taking account of the diffractive
-channel rescatterings. At intermediate and moderately high energies, the
-channel exchange amplitude turns, with a good accuracy, into an effective
pomeron which renders the results of the additive quark model. At superhigh
energies the scattering amplitude provides a Froissart-type behaviour, with an
asymptotic universality of cross sections such as at . The quark structure of hadrons
being taken into account at the level of constituent quarks, the cross sections
of pion and proton (antiproton) in the impact parameter space of quarks,
and , are found as functions of
. These cross sections implicate the phenomenon of colour screening: they
tend to zero at . The effective colour
screening radius for pion (proton) is found for different . The predictions
for the diffractive cross sections at superhigh energies are presented.Comment: LaTeX, epsfig.st
Phenomenological analysis of the double pion production in nucleon-nucleon collisions up to 2.2 GeV
With an effective Lagrangian approach, we analyze several NN \to NN\pi\pi
channels by including various resonances with mass up to 1.72 GeV. For the
channels with the pion pair of isospin zero, we confirm the dominance of
N*(1440)\to N\sigma in the near threshold region. At higher energies and for
channels with the final pion pair of isospin one, we find large contributions
from N*(1440)\to \Delta\pi, double-Delta, \Delta(1600) \to N*(1440)\pi,
\Delta(1600) \to \Delta\pi and \Delta(1620) \to \Delta\pi. There are also
sizeable contributions from \Delta \to \Delta\pi, \Delta \to N\pi, N \to
\Delta\pi and nucleon pole at energies close to the threshold. We well
reproduce the total cross sections up to beam energies of 2.2 GeV except for
the pp\to pp\pi^0\pi^0 channel at energies around 1.1 GeV and our results agree
with the existing data of differential cross sections of pp \to pp\pi^+\pi^-,
pp \to nn\pi^+\pi^+ and pp \to pp\pi^0\pi^0 which are measured at CELSIUS and
COSY.Comment: 36 pages, 18 figure
D^+_s -> pi^+ pi^+ pi^- decay: the 1^3P_0 s anti-s component in scalar-isoscalar mesons
On the basis of data on the decay D^+_s -> pi^+ pi^+ pi^-, which goes
dominantly via the transition D_s -> pi^+ s anti-s, we evaluate the 1^3P_0 s
anti-s components in the scalar-isoscalar resonances f0(980), f0(1300),
f0(1500) and broad state f0(1200-1600)$. The data point to a large s anti-s
component in the f0(980): 40% < s anti-s < 70%. Nearly 30% of the 1^3P_0 s
anti-s component flows to the mass region 1300-1500 MeV being shared by
f0(1300), f0(1500) and broad state f0(1200-1600): the interference of these
states results in a peak near 1400 MeV with the width around 200 MeV.Comment: 17 pages, 4 figures, epsfi
Pomeron in diffractive processes and at large Q^2: the onset of pQCD
We study the reactions and
at large Q^2 and and small
momentum transfer, , to the nucleon where the pomeron exchange
dominates. At large Q^2 the virtual photon selects a hard pair, thus
selecting the hard pomeron component (the BFKL pomeron). The amplitudes for
both transverse and longitudinal polarizations of the initial photon and
outgoing -meson (photon) are calculated in the framework of the BFKL
pomeron exchange. Our calculations show that one cannot expect the early onset
of the pure perturbative regime in the discussed diffractive processes: the
small interquark distances, fm, start to dominate not
earlier than at in
and in
.Comment: 20 pages, LaTeX, epsfig.st
Theoretical study of the N N --> N N pi pi reaction
We have developed a model for the N N --> N N pi pi reaction and evaluated
cross sections for the different charged channels. The low energy part of those
channels where the pions can be in an isospin zero state is dominated by N*
excitation, driven by an isoscalar source recently found experimentally,
followed by the decay N* --> N (pi pi, T=0, s-wave). At higher energies, and in
channels where the pions are not in T=0, Delta excitation mechanisms become
relevant. A rough agreement with the experimental data is obtained in most
channels. Repercussions of the present findings for the ABC effect and the p p
--> p p pi0 reaction close to threshold are also suggested.Comment: 22 pages, 12 postscripts figures, revised: a more general N* N pi pi
Lagrangian is considered, to be published in Nucl. Phys.
Luminosity determination for the pd reaction at 2.14 GeV with WASA-at-COSY
The luminosity for a WASA-at-COSY experiment involving the pd reaction at
2.14 GeV proton-beam energy is determined by the forward pd elastic scattering,
which yields an average beam-on-target value of
[5.2\pm0.3(stat)\pm0.3(syst)]*10^{30} s^{-1}cm^{-2}. In addition, the forward
pd elastic-scattering angular distribution is obtained with four-momentum
transfer squared -t between 0.16 (GeV/c)^{2} and 0.78 (GeV/c)^{2} at this beam
energy, which is compared with other experimental data and the pd double
scattering model.Comment: 5 pages, 6 figure
The study of the negative pion production in neutron-proton collisions at beam momenta below 1.8 GeV/c
A detailed investigation of the reaction np -> pp\pi^{-} has been carried out
using the data obtained with the continuous neutron beam produced by charge
exchange scattering of protons off a deuterium target. A partial wave
event-by-event based maximum likelihood analysis was applied to determine
contributions of different partial waves to the pion production process. The
combined analysis of the np -> pp\pi^{-} and pp -> pp\pi^{0} data measured in
the same energy region allows us to determine the contribution of isoscalar
partial waves (I=0) in the momentum range from 1.1 up to 1.8 GeV/c. The decay
of isoscalar partial waves into (^1S_0)_{pp}\pi$ channel provides a good tool
for a determination of the pp S-wave scalar scattering length in the final
state which was found to be a_{pp}=-7.5\pm 0.3 fm.Comment: 6 pages, 6 figure
Color coherent phenomena on nuclei and the QCD evolution equation
We review the phenomenon of color coherence in quantum chromodynamics (QCD),
its implications for hard and soft processes with nuclei, and its experimental
manifestations. The relation of factorization theorems in QCD with color
coherence phenomena in deep inelastic scattering (DIS) and color coherence
phenomena in hard exclusive processes is emphasized. Analyzing numerically the
QCD evolution equation for conventional and skewed parton densities in nuclei,
we study the onset of generalized color transparency and nuclear shadowing of
the sea quark and gluon distributions in nuclei as well as related phenomena.
Such novel results as the dependence of the effective coherence length on
and general trends of the QCD evolution are discussed. The limits of the
applicability of the QCD evolution equation at small Bjorken are estimated
by comparing the inelastic quark-antiquark- and two gluon-nucleon (nucleus)
cross sections, calculated within the DGLAP approximation, with the dynamical
boundaries, which follow from the unitarity of the matrix for purely QCD
interactions. We also demonstrate that principles of color coherence play an
important role in the processes of soft diffraction off nuclei.Comment: 58 pages, 19 figures, Revtex. Minor editor's changes, final version
published in J.Phys. G27 (2001) R23-6
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