220 research outputs found
Two-photon decay of pseudoscalar quarkonia
We report on our recent evaluation of the two-photon width of the
pseudoscalar quarkonia, eta_c(nS) and eta_b(nS) in an approach based on
Heavy-Quark Spin Symmetry (HQSS). To what concerns the 1S state eta_c, our
parameter-free computation agrees with experiments, as well as most of other
theoretical works. On the other hand, our computation for the 2S-state looks 2S
like a confirmation that there may exist an anomaly related to the decay of
eta_c(2S), especially in the light of the new preliminary result of the Belle
collaboration. We also point out that the essentially model-independent ratio
of eta_b two-photon width to the Upsilon leptonic width and the eta_b
two-photon width could be used to extract the strong coupling constant alpha_s.Comment: Presented by T.N. Pham at the Joint Meeting
Heidelberg-Liege-Paris-Wroclaw (HLPW08), Spa, Belgium, 6-8 March 2008, 9
pages, 2 figures, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty
(included
Spin configurations in circular and rectangular vertical quantum dots in a magnetic field: Three-dimensional self-consistent simulation
The magnetic field dependence of the electronic properties of \textit{real}
single vertical quantum dots in circular and rectangular mesas is investigated
within a full three-dimensional multiscale self-consistent approach without any
{\it \'a priori} assumptions about the shape and strength of the confinement
potential. The calculated zero field electron addition energies are in good
agreement with available experimental data for both mesa geometries. Charging
diagrams in a magnetic field for number of electrons up to five are also
computed. Consistent with the experimental data, we found that the charging
curves for the rectangular mesa dot in a magnetic field are flatter and exhibit
less features than for a circular mesa dot. Evolution of the singlet-triplet
energy separation in the two electron system for both dot geometries in
magnetic field was also investigated. In the limit of large field, beyond the
singlet-triplet transition, the singlet-triplet energy difference continues to
become more negative in a circular mesa dot without any saturation within the
range of considered magnetic fields whilst it is predicted to asymptotically
approach zero for the rectangular mesa dot. This different behavior is
attributed to the symmetry "breaking" that occurs in the singlet wave-functions
in the rectangular mesa dot but not in the circular one.Comment: 12 pages, 8 gifure
Astrophysical tests of mirror dark matter
Mirror matter is a self-collisional dark matter candidate. If exact mirror
parity is a conserved symmetry of the nature, there could exist a parallel
hidden (mirror) sector of the Universe which has the same kind of particles and
the same physical laws of our (visible) sector. The two sectors interact each
other only via gravity, therefore mirror matter is naturally "dark". The most
promising way to test this dark matter candidate is to look at its
astrophysical signatures, as Big Bang nucleosynthesis, primordial structure
formation and evolution, cosmic microwave background and large scale structure
power spectra.Comment: 9 pages, 2 figure
J/psi production at the Tevatron and RHIC from s-channel cut
We report on our recent evaluation of the s-channel cut contribution to J/psi
hadroproduction. We show that it is likely significantly larger than the usual
cut contribution of the colour-singlet model (CSM), which is known to
underestimate the experimental measurements. Here the s-channel cut develops
for configurations with off-shell quarks in the bound state. A correct
treatment of its contribution requires the introduction of a four point
function, partially constrained by gauge invariance and limiting behaviours at
small and large momenta. When the unconstrained degrees of freedom are fixed to
reproduce the Tevatron data, we show that RHIC data are remarkably well
reproduced down to very low transverse momenta P_T without need of resummation
of initial-state gluon effects. This unique feature might be typical of
s-channel cut contribution.Comment: Presented by J.P. Lansberg at the Joint Meeting
Heidelberg-Liege-Paris-Wroclaw (HLPW08), Spa, Belgium, 6-8 March 2008, 10
pages, 8 figures, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty
(included
Central Exclusive Dijet Production
Calculations of central exclusive production are affected by very large
perturbative and non-perturbative corrections. In this talk, we summarize the
results of a study of the uncertainties on these corrections in the case of
exclusive dijet production.Comment: To appear in the Proceedings of the 43rd Rencontres de Moriond, QCD
and High Energy Interactions, La Thuile, Italy, 1-8 March 200
Allowed and forbidden transitions in artificial hydrogen and helium atoms
The strength of radiative transitions in atoms is governed by selection
rules. Spectroscopic studies of allowed transitions in hydrogen and helium
provided crucial evidence for the Bohr's model of an atom. Forbidden
transitions, which are actually allowed by higher-order processes or other
mechanisms, indicate how well the quantum numbers describe the system. We apply
these tests to the quantum states in semiconductor quantum dots (QDs), which
are regarded as artificial atoms. Electrons in a QD occupy quantized states in
the same manner as electrons in real atoms. However, unlike real atoms, the
confinement potential of the QD is anisotropic, and the electrons can easily
couple with phonons of the material. Understanding the selection rules for such
QDs is an important issue for the manipulation of quantum states. Here we
investigate allowed and forbidden transitions for phonon emission in one- and
two-electron QDs (artificial hydrogen and helium atoms) by electrical
pump-and-probe experiments, and find that the total spin is an excellent
quantum number in artificial atoms. This is attractive for potential
applications to spin based information storage.Comment: slightly longer version of Nature 419, 278 (2002
Perspectives on heavy-quarkonium production at the LHC
We summarise the perspectives on heavy-quarkonium production at the LHC, both
for proton-proton and heavy-ion runs, as emanating from the round table held at
the HLPW 2008 Conference. The main topics are: present experimental and
theoretical knowledge, experimental capabilities, open questions, recent
theoretical advances and potentialities linked to some new observables.Comment: Summary of the round table on quarkonium production held at the Joint
Meeting Heidelberg-Liege-Paris-Wroclaw: Three Days of Strong Interactions and
Astrophysics (HLPW08), Spa, Liege, Belgium, 6-8 March 2008. 30 pages, 12
figures, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included
Gravitational Lensing, Dark Matter and the Optical Gravitational Lens Experiment
After briefly reviewing the history of gravitational lensing, we recall the basic principles of the theory. We then describe and use a simple optical gravitational lens experiment which has the virtue of accounting for all types of image configurations observed so far among the presently known gravitational lens systems. Finally, we briefly present the 4m International Liquid Mirror Telescope project in the context of a photometric monitoring of multiply imaged quasars
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