2,024 research outputs found

### Forward antiproton-deuteron elastic scattering and total spin-dependent antiproton-deuteron cross sections at intermediate energies

Spin-dependent total pbar-d cross sections are considered using the optical
theorem. For this aim the full spin dependence of the forward pbar-d elastic
scattering amplitude is considered in a model independent way. The
single-scattering approximation is used to relate this amplitude to the
elementary amplitudes of pbar-p and pbar-n scattering and the deuteron form
factor. A formalism allowing to take into account Coulomb-nuclear interference
effects in polarized pbar-d cross sections is developed. Numerical calculations
for the polarized total pbar-d cross sections are performed at beam energies
20-300 MeV using the Nbar-N interaction models developed by the Julich group.
Double-scattering effects are estimated within the Glauber approach and found
to be in the order of 10-20%. Existing experimental data on differential pbar-d
cross sections are in good agreement with the performed Glauber calculations.
It is found that for the used Nbar-N models the total longitudinal and
transversal pbar-d cross sections are comparable in absolute value to those for
pbar-p scattering.Comment: 19 pages, 8 figures; typos in appendix removed, Section about
Coulomb-nuclear interference re-structured, figures improved; accepted for
publication in Phys. Rev.

### Antiproton scattering off $^3 He$ and $^4 He$ nuclei at low and intermediate energies

Antiproton scattering off ^3\He and ^4\He targets is considered at beam
energies below 300 MeV within the Glauber-Sitenko approach, utilizing the $\bar
N N$ amplitudes of the J\"ulich model as input. A good agreement with available
data on differential \bar p ^4\He cross sections and on \bar p ^3\He and
\pbar ^4\He reaction cross sections is obtained. Predictions for polarized
total $\bar p ^3$He cross sections are presented, calculated within the
single-scattering approximation and including Coulomb-nuclear interference
effects. The kinetics of the polarization buildup is discussed.Comment: 11 pages, 6 figure

### Density resummation of perturbation series in a pion gas to leading order in chiral perturbation theory

The mean field (MF) approximation for the pion matter, being equivalent to
the leading ChPT order, involves no dynamical loops and, if self-consistent,
produces finite renormalizations only. The weight factor of the Haar measure of
the pion fields, entering the path integral, generates an effective Lagrangian
$\delta \mathcal{L}_{H}$ which is generally singular in the continuum limit.
There exists one parameterization of the pion fields only, for which the weight
factor is equal to unity and $\delta \mathcal{L}_{H}=0$, respectively. This
unique parameterization ensures selfconsistency of the MF approximation. We use
it to calculate thermal Green functions of the pion gas in the MF approximation
as a power series over the temperature. The Borel transforms of thermal
averages of a function $\mathcal{J}(\chi ^{\alpha}\chi ^{\alpha})$ of the pion
fields $\chi ^{\alpha}$ with respect to the scalar pion density are found to be
$\frac{2}{\sqrt{\pi}}\mathcal{J}(4t)$. The perturbation series over the scalar
pion density for basic characteristics of the pion matter such as the pion
propagator, the pion optical potential, the scalar quark condensate
$$, the in-medium pion decay constant ${\tilde{F}}$, and the
equation of state of pion matter appear to be asymptotic ones. These series are
summed up using the contour-improved Borel resummation method. The quark scalar
condensate decreases smoothly until $T_{max}\simeq 310$ MeV. The temperature
$T_{max}$ is the maximum temperature admissible for thermalized non-linear
sigma model at zero pion chemical potentials. The estimate of $T_{max}$ is
above the chemical freeze-out temperature $T\simeq 170$ MeV at RHIC and above
the phase transition to two-flavor quark matter $T_{c} \simeq 175$ MeV,
predicted by lattice gauge theories.Comment: Replaced with revised and extended version. Results are compared to
lattice gauge theories. 16 pages REVTeX, 13 eps figure

### On the possibility of $f_0$ observation in low energy $pp$ collisions

Within the meson-exchange model we calculate $f_0$-meson production cross
section in \piN and $NN$ reactions and investigate the possibility for $f_0$
observation via the $K\bar{K}$ decay mode in $pp$ collisions. Our studies
indicate that an extraction of the $f_0$ signal is unlikely due to the large
background from other reaction channels.Comment: 6 pages, LaTeX, including 10 postscript figures, to appear in Eur.
Phys. J.

### Medium effects in the production and decay of vector mesons in pion-nucleus reactions

The $\omega$-, $\rho$- and $\phi$-resonance production and their dileptonic
decay in $\pi^- A$ reactions at 1.1 - 1.7 GeV/c are calculated within the
intranuclear cascade (INC) approach. The invariant mass distribution of the
dilepton pair for each resonance can be decomposed in two components which
correspond to their decay 'inside' the target nucleus and in the vacuum,
respectively. The first 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 as well as the Dalitz decays of
$\omega$ and $\eta$ mesons produced in the reaction. Detailed predictions for
$\pi^- Pb$ reactions at 1.3 and 1.7 GeV/c are made within several momentum bins
for the lepton pair.Comment: 29 pages, LaTeX, including 12 postscript figures, UGI-97-07, Nucl.
Phys. A., in pres

### Probing the interactions of charmed mesons with nuclei in $\bar p$ induced reactions

We study the perspectives of resonant and nonresonant charmed meson
production in $\bar{p} + A$ reactions within the Multiple Scattering Monte
Carlo (MSMC) approach. We calculate the production of the resonances
$\Psi(3770), \Psi(4040)$ and $\Psi(4160)$ on various nuclei, their propagation
and decay to $D, \bar{D}, D^*, \bar{D}^*, D_s, \bar{D}_s$ in the medium and
vacuum, respectively. The modifications of the open charm vector mesons in the
nuclear medium are found to be rather moderate or even small such that dilepton
spectroscopy will require an invariant mass resolution of a few MeV.
Furthermore, the elastic and inelastic interactions of the open charm mesons in
the medium are taken into account, which can be related to $(u,d)$-, $s$- or
$c$-quark exchange with nucleons. It is found that by studying the $D/\bar{D}$
ratio for low momenta in the laboratory ($\leq 2-2.2$ GeV/c) as a function of
target mass $A$ stringent constraints on the $c$-quark exchange cross section
can be obtained. On the other hand, the ratios
$D^-_s/D^+_s$ as well as $D/D^-_s$ and $D/D^+_s$ at low momenta as a function
of $A$ will permit to fix independently the strength of the $s$-quark exchange
reaction in $D^-_s N$ scattering.Comment: 9 pages, LaTeX, including 9 postscript figures, submitted to Eur.
Phys. J.

### Semileptonic Decays of Heavy Lambda Baryons in a Quark Model

The semileptonic decays of Lambda_c and Lambda_b are treated in the framework
of a constituent quark model. Both nonrelativistic and semirelativistic
Hamiltonians are used to obtain the baryon wave functions from a fit to the
spectra, and the wave functions are expanded in both the harmonic oscillator
and Sturmian bases. The latter basis leads to form factors in which the
kinematic dependence on q^2 is in the form of multipoles, and the resulting
form factors fall faster as a function of q^2 in the available kinematic
ranges. As a result, decay rates obtained in the two models using the Sturmian
basis are significantly smaller than those obtained using the harmonic
oscillator basis. In the case of the Lambda_c, decay rates calculated using the
Sturmian basis are closer to the experimentally reported rates. However, we
find a semileptonic branching fraction for the Lambda_c to decay to excited
Lambda* states of 11% to 19%, in contradiction with what is assumed in
available experimental analyses. Our prediction for the Lambda_b semileptonic
decays is that decays to the ground state Lambda_c provide a little less than
70% of the total semileptonic decay rate. For the decays Lambda_b to Lambda_c,
the analytic form factors we obtain satisfy the relations expected from
heavy-quark effective theory at the non-recoil point, at leading and
next-to-leading orders in the heavy-quark expansion. In addition, some features
of the heavy-quark limit are shown to naturally persist as the mass of the
heavy quark in the daughter baryon is decreased.Comment: 51 pages, 8 figures, submitted to Physical Review

### Near-threshold production of a0(980) mesons in the reaction pp -> d K^+ \bar{K}^0

Using an effective Lagrangian approach as well as the Quark-Gluon Strings
Model we analyze near-threshold production of a0(980)-mesons in the reaction NN
-> d K \bar{K} as well as the background of non-resonant K\bar{K}-pair
production. We argue that the reaction pp -> d K^+ \bar{K}^0 at an energy
release Q<=100 MeV is dominated by the intermediate production of the
a0(980)-resonance. At larger energies the non-resonant K^+\bar{K}^0-pair
production - where the kaons are produced in a relative P-wave - becomes
important. Then effects of final-state interactions are evaluated in a
unitarized scattering-length approach and found to be in the order of a 20%
suppression close to threshold. Thus in present experiments at the Cooler
Synchrotron COSY-J\"ulich for Q<=107 MeV the a_0^+ signal can reliably be
separated from the non-resonant K^+\bar{K^0} background.Comment: 14 pages, 12 figures, submitted to EPJ

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