4,911 research outputs found
The Resummed Photon Spectrum in Radiative Upsilon Decays
We present a theoretical prediction for the photon spectrum in radiative
Upsilon decay including the effects of resumming the endpoint region, E_\gamma
-> M_\Upsilon/2. Our approach is based on NRQCD and the soft collinear
effective theory. We find that our results give much better agreement with data
than the leading order NRQCD prediction.Comment: 4 pages, 6 figure
Flavor-singlet light-cone amplitudes and radiative Upsilon decays in SCET
We study the evolution of flavor-singlet, light-cone amplitudes in the
soft-collinear effective theory (SCET), and reproduce results previously
obtained by a different approach. We apply our calculation to the color-singlet
contribution to the photon endpoint in radiative Upsilon decay. In a previous
paper, we studied the color-singlet contributions to the endpoint, but
neglected operator mixing, arguing that it should be a numerically small
effect. Nevertheless the mixing needs to be included in a consistent
calculation, and we do just that in this work. We find that the effects of
mixing are indeed numerically small. This result combined with previous work on
the color-octet contribution and the photon fragmentation contribution provides
a consistent theoretical treatment of the photon spectrum in radiative Upsilon
decay.Comment: 19 pages with 8 figure
Spectrum and Franck-Condon factors of interacting suspended single-wall carbon nanotubes
A low energy theory of suspended carbon nanotube quantum dots in weak
tunnelling coupling with metallic leads is presented. The focus is put on the
dependence of the spectrum and the Franck-Condon factors on the geometry of the
junction including several vibronic modes. The relative size and the relative
position of the dot and its associated vibrons strongly influence the
electromechanical properties of the system. A detailed analysis of the complete
parameters space reveals different regimes: in the short vibron regime the
tunnelling of an electron into the nanotube generates a plasmon-vibron
excitation while in the long vibron regime polaron excitations dominate the
scenario. The small, position dependent Franck-Condon couplings of the small
vibron regime convert into uniform, large couplings in the long vibron regime.
Selection rules for the excitations of the different plasmon-vibron modes via
electronic tunnelling events are also derived.Comment: 23 pages, 8 figures, new version according to the published on
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