401 research outputs found
Searching a Dark Photon with HADES
The existence of a photon-like massive particle, the Îłâ or dark photon, is postulated in several extensions of the Standard Model to explain some recent puzzling astrophysical observations, as well as to solve the sofar unexplained deviation between the measured and calculated values of the muon anomaly. The dark photon, unlike the conventional photon, would have mass and would be detectable via its mixing with the latter. We present a search for the e+eâ decay of such a hypothetical particle, also named U vector boson, in inclusive dielectron spectra measured by HADES in the p (3.5 GeV) +p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared (Ï”2) at 90% CL has been obtained for the mass range M(U)= 0.02 - 0.55 GeV/c2 and is compared with the present world data set. Furthermore, an improved upper limit of 2.3 Ă 10â6 at 90% CL has been set on the branching ratio of the helicity-suppressed direct decay of the η meson η â e+eâ
Relativistic calculations of the x-ray emission following the Xe-Bi collision
We study the x-ray emission following the collision of a Bi ion with
a neutral Xe atom at the projectile energy 70 MeV/u. The collisional and
post-collisional processes are treated separately. The probabilities of various
many-electron processes at the collision are calculated within a relativistic
independent electron model using the coupled-channel approach with atomic-like
Dirac-Fock-Sturm orbitals. The analysis of the post-collisional processes
resulting in the x-ray emission is based on the fluorescence yields, the
radiation and Auger decay rates, and allows to derive intensities of the x-ray
emission and compare them with experimental data. A reasonable agreement
between the theoretical results and the recent experimental data is observed.
The role of the relativistic effects is investigated.Comment: 11 figures, 2 table
Electron gas polarization effect induced by heavy H-like ions of moderate velocities channeled in a silicon crystal
We report on the observation of a strong perturbation of the electron gas
induced by 20 MeV/u U ions and 13 MeV/u Pb ions channeled in
silicon crystals. This collective response (wake effect) in-duces a shift of
the continuum energy level by more than 100 eV, which is observed by means of
Radiative Electron Capture into the K and L-shells of the projectiles. We also
observe an increase of the REC probability by 20-50% relative to the
probability in a non-perturbed electron gas. The energy shift is in agreement
with calculations using the linear response theory, whereas the local electron
density enhancement is much smaller than predicted by the same model. This
shows that, for the small values of the adiabaticity parameter achieved in our
experiments, the density fluctuations are not strongly localized at the
vicinity of the heavy ions
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