277 research outputs found

### Eta and eta-prime meson production in the reaction pn -> dM near threshold

The two-step model with pi-, rho- and omega- exchanges taken into account is
applied to investigate the reactions pn -> d eta and pn -> d eta-prime. The
existing experimental data on the reaction pn -> d eta are analysed and
predictions for the cross section of the reaction pn -> d eta-prime are
presented. It is found that pi- as well as rho exchange yield significant
contributions in both reactions. The effect of the final state interaction is
also studied.Comment: 15 pages, 4 figure

### Suppression of nucleon resonances in the total photoabsorption on nuclei

Abstract We analyze the recent nuclear-photoabsorption data which have shown the disappearance on nuclei of the resonances higher than the Δ. We discuss how the Fermi motion, the collision broadening, and the Pauli blocking might distort the shape of nucleon resonances in the nuclear medium, and show that the Fermi motion and the collision broadening play the major role. From a fit to the photoabsorption data on uranium we have estimated the total cross sections for scattering of the resonances P 33 (1232), D 13 (1520), F 15 (1680), and D 33 (1700) on nucleons. The estimate for the P 33 -resonance cross section is in fair agreement with the value derived from the NN → Δ N cross sections, while those for the other resonances are high although do not violate the unitary limits

### In-medium broadening of nucleon resonances

We analyze the effects of an in-medium broadening of nucleon resonances on
the exclusive photoproduction of mesons on nuclei as well as on the total
photoabsorption cross sections in a transport calculation. We show that the
resonance widths observed in semi-inclusive photoproduction on nuclei are
insensitive to an in-medium broadening of nucleon resonances. This is due to a
simple effect: the sizeable width of the nuclear surface and Fermi motion.Comment: 4 pages, 3 figures, minor changes in the tex

### Medium effects in the production and decay of $\omega$- and $\rho$-resonances in pion-nucleus interactions

The $\omega$- and $\rho$-resonance production and their dileptonic decay in
$\pi^- A$ reactions at GSI energies are calculated within the intranuclear
cascade (INC) approach. The invariant mass distribution of the dilepton pair
for each resonance is found to have two components which correspond to the
decay of the resonances outside and inside the target nucleus. The latter
components are strongly distorted by the nuclear medium due to
resonance-nucleon scattering and a possible mass shift at finite baryon
density. These medium modifications are compared to background sources in the
dilepton spectrum from $\pi N$ bremsstrahlung and the Dalitz decays of
$\Delta$'s and $\eta$ mesons produced in the reaction.Comment: 10 pages, LaTeX, plus 4 postscript figures, submitted to Phys. Lett.

### Shadowing in photo-production : role of in-medium hadrons

We study the effects of in-medium hadronic properties on shadowing in
photon-nucleus interactions in Glauber model as well as in the multiple
scattering approach. A reasonable agreement with the experimental data is
obtained in a scenario of downward spectral shift of the hadrons. Shadowing is
found to be insensitive to the broadening of the spectral functions. An impact
parameter dependent analysis of shadowing might shed more light on the role of
in-medium properties of hadrons.Comment: Title modified; version to appear in PRC, Rapid Communication

### Superfluidity in a Model of Massless Fermions Coupled to Scalar Bosons

We study superfluidity in a model of massless fermions coupled to a massive
scalar field through a Yukawa interaction. Gap equations for a condensate with
total spin J=0 are solved in the mean-field approximation. For the Yukawa
interaction, the gaps for right- and left-handed fermions are equal in
magnitude and opposite in sign, so that condensation occurs in the J^P = 0^+
channel. At finite scalar mass, there are two different gaps for fermions of a
given chirality, corresponding to condensation of particle pairs or of
antiparticle pairs. These gaps become degenerate in the limit of infinite
scalar mass.Comment: 26 pages, 9 figures, RevTeX, epsf and psfig style files required.
Revised version, discussion of the excitation spectrum extended, Fig. 2 adde

### Open charm enhancement by secondary interactions in relativistic nucleus-nucleus collisions?

We calculate open charm production in $Pb+Pb$ reactions at SPS energies
within the HSD transport approach - which is based on string, quark, diquark
($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom - including
the production of open charm pairs from secondary 'meson'-'baryon' (or
quark-diquark and antiquark-diquark) collisions. It is argued that at collision
energies close to the $c\bar{c}$ pair threshold the dominant production
mechanism is related to the two body (or quasi two body) reactions $\pi N \to
\bar{D} (\bar{D^*}) \Lambda_c, (\Sigma_c)$. Estimates within the framework of
the Quark-Gluon String model suggest cross sections of a few $\mu b$ for $\pi N
\to \bar{D} \Lambda_c$ in the region of 1 GeV above threshold. The dynamical
transport calculations for $Pb + Pb$ at 160 A$\cdot$GeV indicate that the open
charm enhancement reported by the NA50 Collaboration might be due to such
secondary reaction mechanisms.Comment: 14 pages, LaTeX, including 5 postscript figures, submitted to Phys.
Lett.

### Relativistic Three-Dimensional Two- and Three-Body Equations on a Null Plane and Applications to Meson and Baryon Regge Trtajectories

We start from a field-theoretical model of zero range approximation to derive
three-dimensional relativistic two- and three-body equations on a null plane.
We generalize those equations to finite range interactions. We propose a
three-body null-plane equation whose form is different from the one presented
earlier in the framework of light-cone dynamics. We discuss the choices of the
kernels in two- and three-body cases and apply our model to the description of
meson and baryon Regge trajectories. Our approach overcomes some theoretical
and phenomenological difficulties met in preceding relativized treatments of
the three-body problem.Comment: 35 pages LaTex, 6 figs (available from [email protected]

### Critical Examination of the "Field-Theoretical Approach" to the Neutron-Antineutron Oscillations in Nuclei

We demonstrate that so called "infrared divergences" which have been
discussed in some publications during several years, do not appear within the
correct treatment of analytical properties of the transition amplitudes, in
particular, of the second order pole structure of the amplitudes describing the
$n - \bar n$ transition in nuclei. Explicit calculation with the help of the
Feynman diagram technique shows that the neutron-antineutron oscillations are
strongly suppressed in the deuteron, as well as in heavier nuclei, in
comparison with the oscillations in vacuum. General advantages and some
difficulties of the field theoretical methods applied in nuclear theory are
reminded for the particular example of the parity violating $np\to d\gamma$
capture amplitude.Comment: 15 pages, 4 figures; prepared for Eur.Phys.J.

### Spin Structure of the Pion in a Light-Cone Representation

The spin structure of the pion is discussed by transforming the wave function
for the pion in the naive quark model into a light-cone representation. It is
shown that there are higher helicity ($\lambda_{1}+\lambda_{2}=\pm1$) states in
the full light-cone wave function for the pion besides the ordinary helicity
($\lambda_{1}+\lambda_{2}=0$) component wave functions as a consequence from
the Melosh rotation relating spin states in light-front dynamics and those in
instant-form dynamics. Some low energy properties of the pion, such as the
electromagnetic form factor, the charged mean square radius, and the weak decay
constant, could be interrelated in this representation with reasonable
parameters.Comment: 15 Latex pages, 2 figures upon reques

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