1,307 research outputs found
Factorization Approach for Inclusive Production of Doubly Heavy Baryon
We study inclusive production of doubly heavy baryon at a collider
and at hadron colliders through fragmentation. We study the production by
factorizing nonpertubative- and perturbative effects. In our approach the
production can be thought as a two-step process: A pair of heavy quarks can be
produced perturbatively and then the pair is transformed into the baryon. The
transformation is nonperturbative. Since a heavy quark moves with a small
velocity in the baryon in its rest frame, we can use NRQCD to describe the
transformation and perform a systematic expansion in the small velocity. At the
leading order we find that the baryon can be formed from two states of the
heavy-quark pair, one state is with the pair in state and in color
, another is with the pair in state and in color . Two matrix elements are defined for the transformation from the two
states, their perturbative coefficients in the contribution to the
cross-section at a collider and to the function of heavy quark
fragmentation are calculated. Our approach is different than previous
approaches where only the pair in state and in color is
taken into account. Numerical results for colliders at the two
-factories and for hadronic colliders LHC and Tevatron are given.Comment: Add results for large p_t, minor change
The J-triplet Cooper pairing with magnetic dipolar interactions
Recently, cold atomic Fermi gases with the large magnetic dipolar interaction
have been laser cooled down to quantum degeneracy. Different from
electric-dipoles which are classic vectors, atomic magnetic dipoles are
quantum-mechanical matrix operators proportional to the hyperfine-spin of
atoms, thus provide rich opportunities to investigate exotic many-body physics.
Furthermore, unlike anisotropic electric dipolar gases, unpolarized magnetic
dipolar systems are isotropic under simultaneous spin-orbit rotation. These
features give rise to a robust mechanism for a novel pairing symmetry: orbital
p-wave (L=1) spin triplet (S=1) pairing with total angular momentum of the
Cooper pair J=1. This pairing is markedly different from both the He-B
phase in which J=0 and the He- phase in which is not conserved. It
is also different from the p-wave pairing in the single-component electric
dipolar systems in which the spin degree of freedom is frozen
Photoacid behaviour in a fluorinated green fluorescent protein chromophore:Ultrafast formation of anion and zwitterion states
The photophysics of the chromophore of the green fluorescent protein in Aequorea victoria (avGFP) are dominated by an excited state proton transfer reaction. In contrast the photophysics of the same chromophore in solution are dominated by radiationless decay, and photoacid behaviour is not observed. Here we show that modification of the pKa of the chromophore by fluorination leads to an excited state proton transfer on an extremely fast (50 fs) time scale. Such a fast rate suggests a barrierless proton transfer and the existence of a pre-formed acceptor site in the aqueous solution, which is supported by solvent and deuterium isotope effects. In addition, at lower pH, photochemical formation of the elusive zwitterion of the GFP chromophore is observed by means of an equally fast excited state proton transfer from the cation. The significance of these results for understanding and modifying the properties of fluorescent proteins are discusse
Three-body interactions with cold polar molecules
We show that polar molecules driven by microwave fields give naturally rise
to strong three-body interactions, while the two-particle interaction can be
independently controlled and even switched off. The derivation of these
effective interaction potentials is based on a microscopic understanding of the
underlying molecular physics, and follows from a well controlled and systematic
expansion into many-body interaction terms. For molecules trapped in an optical
lattice, we show that these interaction potentials give rise to Hubbard models
with strong nearest-neighbor two-body and three-body interaction. As an
illustration, we study the one-dimensional Bose-Hubbard model with dominant
three-body interaction and derive its phase diagram.Comment: 8 pages, 4 figure
Heavy Flavour Production at Tevatron and Parton Shower Effects
We present hadron-level predictions from the Monte Carlo generator Cascade
and numerical calculations of charm and beauty production at the Fermilab
Tevatron within the framework of the -factorization QCD approach. Our
consideration is based on the CCFM-evolved unintegrated gluon densities in a
proton. The performed analysis covers the total and differential cross sections
of open charm and beauty quarks, and mesons (or rather muons from their
semileptonic decays) and the total and differential cross sections of di-jet hadroproduction. We study the theoretical uncertainties of our
calculations and investigate the effects coming from parton showers in initial
and final states. Our predictions are compared with the recent experimental
data taken by the D0 and CDF collaborations. Special attention is put on the
specific angular correlations between the final-state particles. We demonstrate
that the final state parton shower plays a crucial role in the description of
such observables. The decorrelated part of angular separations can be fully
described, if the process is included.Comment: Fig 8,9 10 replaced, small corrections in text A discussion of the
delta phi results is adde
Doubly Heavy Baryon Production at \gamma \gamma Collider
The inclusive production of doubly heavy baryons and at
collider is investigated. It is found that the contribution from
the heavy quark pair in color triplet and color sextet are important.Comment: errors/typos are correcte
The Superconducting Instabilities of the non half-filled Hubbard Model in Two Dimensions
The problem of weakly correlated electrons on a square lattice is formulated
in terms of one-loop renormalization group. Starting from the action for the
entire Brillouin zone (and not with a low-energy effective action) we reduce
successively the cutoff about the Fermi surface and follow the
renormalization of the coupling as a function of three energy-momenta. We
calculate the intrinsic scale where the renormalization group flow
crosses over from the regime () where the electron-electron
(e-e) and electron-hole (e-h) terms are equally important to the regime
() where only the e-e term plays a role. In the low energy
regime only the pairing interaction is marginally relevant, containing
contributions from all renormalization group steps of the regime . After diagonalization of , we identify its most
attractive eigenvalue . At low filling,
corresponds to the representation ( symmetry), while near half
filling the strongest attraction occurs in the representation
( symmetry). In the direction of the van Hove singularities, the
order parameter shows peaks with increasing strength as one approaches half
filling. Using the form of pairing and the structure of the renormalization
group equations in the low energy regime, we give our interpretation of ARPES
experiments trying to determine the symmetry of the order parameter in the
Bi2212 high- compound.Comment: 24 pages (RevTeX) + 11 figures (the tex file appeared incomplete
Instability of Anisotropic Fermi Surfaces in Two Dimensions
The effect of strong anisotropy on the Fermi line of a system of correlated
electrons is studied in two space dimensions, using renormalization group
techniques. Inflection points change the scaling exponents of the couplings,
enhancing the instabilities of the system. They increase the critical dimension
for non Fermi liquid behavior, from 1 to 3/2. Assuming that, in the absence of
nesting, the dominant instability is towards a superconducting ground state,
simple rules to discern between d-wave and extended s-wave symmetry of the
order parameter are given.Comment: 5 pages, revte
Direct J/psi and psi' hadroproduction via fragmentation in the collinear parton model and k_T-factorization approach
The p_T-spectra for direct J/psi and psi' in hadroproduction at Tevatron
energy have been calculated based on NRQCD formalism and fragmentation
approximation in the collinear parton model and k_T-factorization approach. We
have described the CDF data and obtained a good agreement between the
predictions obtained in the parton model and k_T-factorization approach. We
performed the calculations using the relevant leading order in alpha_s hard
amplitudes and the equal values of the color-octet long-distance matrix
elements for the both models.Comment: 10 pages, Latex, 4 eps figures, epsfig.sty, graphics.st
The water vapour continuum in near-infrared windows – current understanding and prospects for its inclusion in spectroscopic databases
Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300 cm−1) and include reference to the window centred on 2600 cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption
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