869 research outputs found
Can the X(3872) be a 1^{++} four-quark state?
We use QCD spectral sum rules to test the nature of the meson X(3872),
assumed to be an exotic four-quark (c\bar{c}q\bar{q}) state with J^{PC}=1^{++}.
For definiteness, we work with the current proposed recently by Maiani et al
[1], at leading order in \alpha_s, consider the contributions of higher
dimension condensates and keep terms which are linear in the light quark mass
m_q. We find M_X=(3925+- 127) MeV which is compatible, within the errors, with
he experimental candidate X(3872), while the SU(3) breaking-terms lead to an
unusual mass-splitting M_{X^{s}}-M_X=- (61+-30) MeV. The mass-difference
between the neutral states due to isospin violation of about (2.6-3.9) MeV is
much smaller than the value (8+-3) MeV proposed in [1]. For the b-quark, we
predict M_{X_b}= (10144+-106) MeV for the X_b(b\bar{b}q \bar{q}), which is much
below the {\bar B}B* threshold in contrast to the {\bar B}B* molecule
prediction [2], and for the X_b^s(b\bar{b}s \bar{s}), a mass-splitting
M_{X^s_{b}}-M_{X_b}=-(121+-182) MeV. Our analysis also indicates that the
mass-splitting between the ground state and the radial excitation of about
(225~250) MeV is much smaller than in the case of ordinary mesons and is
(within the errors) flavour-independent. We also extract the decay constants,
analogous to f_\pi, of such mesons, which are useful for further studies of
their leptonic and hadronic decay widths. The uncertainties of our estimates
are mainly due to the ones from the c and b quark masses.Comment: 16 pages, 10 figures. Version to appear in Phys. Rev.
Mimicking Nanoribbon Behavior Using a Graphene Layer on SiC
We propose a natural way to create quantum-confined regions in graphene in a
system that allows large-scale device integration. We show, using
first-principles calculations, that a single graphene layer on a trenched
region of mimics i)the energy bands around the Fermi level
and ii) the magnetic properties of free-standing graphene nanoribbons.
Depending on the trench direction, either zigzag or armchair nanoribbons are
mimicked. This behavior occurs because a single graphene layer over a
surface loses the graphene-like properties, which are restored solely over the
trenches, providing in this way a confined strip region.Comment: 4 pages, 4 figure
Past production constrains current energy demands: persistent scaling in global energy consumption and implications for climate change mitigation
Climate change has become intertwined with the global economy. Here, we
describe the importance of inertia to continued growth in energy consumption.
Drawing from thermodynamic arguments, and using 38 years of available
statistics between 1980 to 2017, we find a persistent time-independent scaling
between the historical time integral of world inflation-adjusted economic
production , or , and
current rates of world primary energy consumption , such that
Gigawatts per trillion 2010 US dollars.
This empirical result implies that population expansion is a symptom rather
than a cause of the current exponential rise in and carbon dioxide
emissions , and that it is past innovation of economic production efficiency
that has been the primary driver of growth, at predicted rates
that agree well with data. Options for stabilizing are then limited to
rapid decarbonization of through sustained implementation of over
one Gigawatt of renewable or nuclear power capacity per day. Alternatively,
assuming continued reliance on fossil fuels, civilization could shift to a
steady-state economy that devotes economic production exclusively to
maintenance rather than expansion. If this were instituted immediately,
continual energy consumption would still be required, so atmospheric carbon
dioxide concentrations would not balance natural sinks until concentrations
exceeded 500 ppmv, and double pre-industrial levels if the steady-state was
attained by 2030
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