Baryonic contributions to the dilepton spectrum of nucleon-nucleon collisions

We study the production of dileptons in relativistic nucleon-nucleon collisions. Additionally to the traditional dilepton production channels (vector meson decays, meson and Delta(1232) Dalitz decays) we included in our model as new dilepton sources the Dalitz decay of higher unflavored baryon resonances with spin<=5/2 and mass<=2.25 GeV/c^2. The contributions of these new channels are estimated using experimental information about the Ngamma decays of the resonances and have large uncertainties. The obtained dilepton spectra are compared to the experimental data by the DLS collaboration. Predictions for the HADES detector (SIS, GSI) are also discussed. In spite of the large uncertainties of the higher resonance Dalitz decay contributions we are able to draw the conclusion that these contributions are negligible compared to the other dilepton sources and do not influence the detectability of the phi and omega vector meson peaks.Comment: 9 pages, 4 figures, version accepted for publication in Phys. Rev.

Dilepton anisotropy from p+Bep + Be and Ca+CaCa + Ca collisions at BEVALAC energies

A full calculation of lepton-pair angular characteristics is carried out for $e^+e^-$ pairs created in $p + Be$ and $Ca + Ca$ collisions from 1.0 to 2.1 GeV/A. It is demonstrated that the dilepton decay anisotropy depends sensitively on the different sources and may be used for their disentangling. Due to the dominance of the $\eta$-and $\Delta$-Dalitz decays and only a small anisotropy coefficient for $\pi^+\pi^-$ annihilation, the expected anisotropy coefficients show a decrease with invariant mass of the dilepton pair and change only moderately when comparing $p + Be$ and $Ca + Ca$ reactions at the same bombarding energy.Comment: 10 pages, LaTeX, plus 4 postscript figures tarred, compressed and uuencoded in separate fil

Tunneling Conductance Between Parallel Two Dimensional Electron Systems

We derive and evaluate expressions for the low temperature {\it dc} equilibrium tunneling conductance between parallel two-dimensional electron systems. Our theory is based on a linear-response formalism and on impurity-averaged perturbation theory. The disorder broadening of features in the dependence of tunneling conductance on sheet densities and in-plane magnetic field strengths is influenced both by the finite lifetime of electrons within the wells and by non-momentum-conserving tunneling events. Disorder vertex corrections are important only for weak in-plane magnetic fields and strong interwell impurity-potential correlations. We comment on the basis of our results on the possibility of using tunneling measurements to determine the lifetime of electrons in the quantum wells.Comment: 14 pages, 5 Fig. not included, revtex, IUcm92-00

Tunneling Spectroscopy in Degenerate p-Type Silicon

Coordinated Science Laboratory was formerly known as Control Systems LaboratoryJoint Services Electronics Program / DAAB 07-67-C-0199Jet Propulsion Lab / 95238

Role of baryonic resonances in the dilepton emission in nucleon-nucleon collisions

Within an effective Lagrangian model, we present calculations for cross sections of the dilepton production in proton-proton and proton-neutron collisions at laboratory kinetic energies in 1-5 GeV range. Production amplitudes include contributions from the nucleon-nucleon bremsstrahlung as well as from the mechanism of excitation, propagation, and radiative decay of Delta(1232) and N*(1520) intermediate baryonic resonances. It is found that the delta isobar terms dominate the cross sections in the entire considered beam energy range. Our calculations are able to explain the data of the DLS collaboration on the dilepton production in proton-proton collisions for beam energies below 1.3 GeV. However, for incident energies higher than this the inclusion of contributions from other dilepton sources like Dalitz decay of pi0 and eta mesons, and direct decay of rho and omega mesons is necessary to describe the data.Comment: 22 pages, 7 figures, more details of the calculations added, version to appear in Phys. Rev

Dilepton Production from AGS to SPS Energies within a Relativistic Transport Approach

We present a nonperturbative dynamical study of $e^+e^-$ production in proton-nucleus and nucleus-nucleus collisions from AGS to SPS energies on the basis of the covariant transport approach HSD. For p + Be reactions the dilepton yield for invariant masses $M \leq 1.4$ GeV is found to be dominated by the decays of the $\eta, \rho, \omega$ and $\Phi$ mesons at all energies from 10 -- 450 GeV. For nucleus-nucleus collisions, however, the dilepton yield shows an additional large contribution from $\pi^+\pi^-$, $K^+K^-$ and $\pi \rho$ channels. Systematic studies are presented for the 'free' meson mass scenario in comparison to a 'dropping' meson mass scenario at finite baryon density. We find that for 'dropping' meson masses the invariant dilepton mass range 0.35 GeV $\leq M \leq$ 0.65 GeV is increased in comparison to the 'free' meson mass scenario and that the data of the CERES-collaboration for nucleus-nucleus collisions can be described much better within the 'dropping' mass scheme. We study in detail the contributions from the various dilepton channels as a function of the transverse momentum and rapidity of the lepton pair as well as a function of the charged particle multiplicity. Furthermore, various direct photon channels for S + Au at 200 GeV/u are computed and found to be well below the upper bounds measured by the WA80-collaboration.Comment: 38 pages, LaTeX, including 19 postscript figures, to appear in Nucl. Phys.

Magnetic-field dependence of electron spin relaxation in n-type semiconductors

We present a theoretical investigation of the magnetic field dependence of the longitudinal ($T_1$) and transverse ($T_2$) spin relaxation times of conduction band electrons in n-type III-V semiconductors. In particular, we find that the interplay between the Dyakonov-Perel process and an additional spin relaxation channel, which originates from the electron wave vector dependence of the electron $g$-factor, yields a maximal $T_2$ at a finite magnetic field. We compare our results with existing experimental data on n-type GaAs and make specific additional predictions for the magnetic field dependence of electron spin lifetimes.Comment: accepted for publication in PRB, minor changes to previous manuscrip

e+e−e^+e^- production in pAp A reactions at SIS energies

Detailed predictions for dilepton production from $p A$ reactions at SIS energies are presented within a semi-classical BUU transport model that includes the off-shell propagation of vector mesons nonperturbatively and calculates the width of the vector mesons dynamically. Different scenarios of in-medium modifications of vector mesons, such as collisional broadening and dropping vector meson masses, are investigated and the possibilities for an experimental observation of in-medium effects in $p A$ reactions at 1--4 GeV are discussed for a variety of nuclear targets.Comment: 38 pages, LaTeX, including 20 postscript figures, to be published in Nucl. Phys.

Electron Spin Relaxation in a Semiconductor Quantum Well

A fully microscopic theory of electron spin relaxation by the D'yakonov-Perel' type spin-orbit coupling is developed for a semiconductor quantum well with a magnetic field applied in the growth direction of the well. We derive the Bloch equations for an electron spin in the well and define microscopic expressions for the spin relaxation times. The dependencies of the electron spin relaxation rate on the lowest quantum well subband energy, magnetic field and temperature are analyzed.Comment: Revised version as will appear in Physical Review

Perspectives of e+e−e^+e^- production in pp, pd and p Be reactions at SIS energies

We study dilepton production from pp, pd and p Be collisions from 1 - 5 GeV including the $\pi^0$, $\eta$, $\omega$ and $\Delta$ Dalitz decays, direct decays of vector mesons ($\rho$, $\omega$) as well as subthreshold $\rho$ production via baryonic resonances (e.g. $D_{13}(1520), P_{11}(1710)$). Our calculations compare rather well with the pp and pd data from the DLS Collaboration, however, overestimate slightly the 'old' p Be data from that group. Futhermore, detailed predictions for differential dilepton spectra at SIS energies are made with a high mass resolution that can be controlled experimentally by the HADES Collaboration in near future.Comment: 27 pages, LaTeX, including 13 postscript figures, to be published in Nucl. Phys.