44,697 research outputs found
Tests for the asymptotic behaviour of the gamma^* --> gamma pi^0 form factor
The gamma^* --> gamma pi^0 transition measured at different photon
virtualities already provides us with a clean test for the behaviour of the
pi^0 gamma^* gamma off-shell axial anomaly at large time-like squared momenta.
It also allows reliable predictions for the branching ratio of heavy quarkonium
decays into omega pi^0.Comment: Latex, 7 pages + 1 .ps figure, comments on the Brodsky-Lepage limit
and 5 new references adde
Relativistic Coulomb scattering of spinless bosons
The relativistic scattering of spin-0 bosons by spherically symmetric Coulomb
fields is analyzed in detail with an arbitrary mixing of vector and scalar
couplings. It is shown that the partial wave series reduces the scattering
amplitude to the closed Rutherford formula exactly when the vector and scalar
potentials have the same magnitude, and as an approximation for weak fields.
The behavior of the scattering amplitude near the conditions that furnish its
closed form is also discussed. Strong suppressions of the scattering amplitude
when the vector and scalar potentials have the same magnitude are observed
either for particles or antiparticles with low incident momentum. We point out
that such strong suppressions might be relevant in the analysis of the
scattering of fermions near the conditions for the spin and pseudospin
symmetries. From the complex poles of the partial scattering amplitude the
exact closed form of bound-state solutions for both particles and antiparticles
with different scenarios for the coupling constants are obtained. Perturbative
breaking of the accidental degeneracy appearing in a pair of special cases is
related to the nonconservation of the Runge-Lenz vector
New solutions of the D-dimensional Klein-Gordon equation via mapping onto the nonrelativistic one-dimensional Morse potential
New exact analytical bound-state solutions of the D-dimensional Klein-Gordon
equation for a large set of couplings and potential functions are obtained via
mapping onto the nonrelativistic bound-state solutions of the one-dimensional
generalized Morse potential. The eigenfunctions are expressed in terms of
generalized Laguerre polynomials, and the eigenenergies are expressed in terms
of solutions of irrational equations at the worst. Several analytical results
found in the literature, including the so-called Klein-Gordon oscillator, are
obtained as particular cases of this unified approac
Dynamical charge density waves rule the phase diagram of cuprates
In the last few years charge density waves (CDWs) have been ubiquitously
observed in high-temperature superconducting cuprates and are now the most
investigated among the competing orders in the still hot debate on these
systems. A wealth of new experimental data raise several fundamental issues
that challenge the various theoretical proposals. Here, we account for the
complex experimental temperature vs. doping phase diagram and we provide a
coherent scenario explaining why different CDW onset curves are observed by
different experimental probes and seem to extrapolate at zero temperature into
seemingly different quantum critical points (QCPs) in the intermediate and
overdoped region. We also account for the pseudogap and its onset temperature
T*(p) on the basis of dynamically fluctuating CDWs. The nearly singular
anisotropic scattering mediated by these fluctuations also account for the
rapid changes of the Hall number seen in experiments and provides the first
necessary step for a possible Fermi surface reconstruction fully establishing
at lower doping. Finally we show that phase fluctuations of the CDWs, which are
enhanced in the presence of strong correlations near the Mott insulating phase,
naturally account for the disappearance of the CDWs at low doping with yet
another QCP.Comment: 13 pages, 7 figure
Tests of flavor symmetry in J/psi decays
We use SU(3) flavor symmetry to analyze the and baryon-antibaryon
decays of . Both, the SU(3)-invariant and -violating contributions are
considered. Particular attention is paid to the interference of the
electromagnetic and strong amplitudes.Comment: 8 pages, latex. Talk given at CAM-94 Physics Meetin
Phase Separation close to the density-driven Mott transition in the Hubbard-Holstein model
The density driven Mott transition is studied by means of Dynamical
Mean-Field Theory in the Hubbard-Holstein model, where the Hubbard term leading
to the Mott transition is supplemented by an electron-phonon (e-ph) term. We
show that an intermediate e-ph coupling leads to a first-order transition at
T=0, which is accompanied by phase separation between a metal and an insulator.
The compressibility in the metallic phase is substantially enhanced. At quite
larger values of the coupling a polaronic phase emerges coexisting with a
non-polaronic metal.Comment: 4 pages, 3 figures. Slightly revised text. More details in Fig.1 and
2. Smaller size version of Fig.
Doping-driven transition to a time-reversal breaking state in the phase diagram of the cuprates
Motivated by recent tunnelling and Andreev-reflection experiments, we study
the conditions for a quantum transition within the superconducting phase of the
cuprates,in which a bulk imaginary (time-reversal breaking) component
appears in addition to the order parameter.
We examine in detail the role of some important physical features of the
cuprates.In particular we show that a closed Fermi surface,a bilayer splitting,
an orthorhombic distortion,and the proximity to a quantum critical point around
optimal doping favor the appearance of the imaginary component. These findings
could explain why the mixed order parameter is
observed in YBCO and LSCO, and suggest that it could appear also in Bi2212. We
also predict that, in all cuprates, the mixed state should be stable only in a
limited doping region all contained beneath the dome. The
behavior of the specific heat at the secondary transition is discussed.Comment: 8 pages, 3 figures. Expanded text, 1 extra figur
Electronic polymers and soft-matter-like broken symmetries in underdoped cuprates
Empirical evidence in heavy fermion, pnictide, and other systems suggests
that unconventional superconductivity appears associated to some form of
real-space electronic order. For the cuprates, despite several proposals, the
emergence of order in the phase diagram between the commensurate
antiferromagnetic state and the superconducting state is not well understood.
Here we show that in this regime doped holes assemble in "electronic polymers."
Within a Monte Carlo study we find, that in clean systems by lowering the
temperature the polymer melt condenses first in a smectic state and then in a
Wigner crystal both with the addition of inversion symmetry breaking. Disorder
blurs the positional order leaving a robust inversion symmetry breaking and a
nematic order, accompanied by vector chiral spin order and with the persistence
of a thermodynamic transition. Such electronic phases, whose properties are
reminiscent of soft-matter physics, produce charge and spin responses in good
accord with experiments.Comment: 10 pages, 4 figures plus supplementary informatio
Proof of Luck: an Efficient Blockchain Consensus Protocol
In the paper, we present designs for multiple blockchain consensus primitives
and a novel blockchain system, all based on the use of trusted execution
environments (TEEs), such as Intel SGX-enabled CPUs. First, we show how using
TEEs for existing proof of work schemes can make mining equitably distributed
by preventing the use of ASICs. Next, we extend the design with proof of time
and proof of ownership consensus primitives to make mining energy- and
time-efficient. Further improving on these designs, we present a blockchain
using a proof of luck consensus protocol. Our proof of luck blockchain uses a
TEE platform's random number generation to choose a consensus leader, which
offers low-latency transaction validation, deterministic confirmation time,
negligible energy consumption, and equitably distributed mining. Lastly, we
discuss a potential protection against up to a constant number of compromised
TEEs.Comment: SysTEX '16, December 12-16, 2016, Trento, Ital
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