5,668 research outputs found
Parametric cooling of a degenerate Fermi gas in an optical trap
We demonstrate a novel technique for cooling a degenerate Fermi gas in a
crossed-beam optical dipole trap, where high-energy atoms can be selectively
removed from the trap by modulating the stiffness of the trapping potential
with anharmonic trapping frequencies. We measure the dependence of the cooling
effect on the frequency and amplitude of the parametric modulations. It is
found that the large anharmonicity along the axial trapping potential allows to
generate a degenerate Fermi gas with anisotropic energy distribution, in which
the cloud energy in the axial direction can be reduced to the ground state
value
Revealing treacherous points for successful light-front phenomenological applications
Light-front dynamics(LFD) plays an important role in hadron phenomenology as
evidenced from recent development of generalized parton distributions and other
quantities involving hadrons. For successful LFD applications to hadron
phenomenology, however, treacherous points such as zero-mode contributions
should be taken into account. For a concrete example of zero-mode contribution,
we present Standard Model analysis of vector anomaly in the CP-even form
factors of W^{\pm} gauge bosons. Main distinguished features of LFD is
discussed in comparison with other Hamiltonian dynamics. We also present a
power counting method to correctly pin down which hadron form factors receive
the zero-mode contribution and which ones do not. Indications from our analysis
to hadron phenomenology are discussed.Comment: 6 pages; requires espcrc2.sty; to appear in the proceedings of
Workshop on Light-Cone QCD and Nonperturbative Hadron Physics 2005 (LC 2005),
Cairns, Queensland, Australia, 7-15 Jul 200
Frame-Independence of Exclusive Amplitudes in the Light-Front Quantization
While the particle-number-conserving convolution formalism established in the
Drell-Yan-West reference frame is frequently used to compute exclusive
amplitudes in the light-front quantization, this formalism is limited to only
those frames where the light-front helicities are not changed and the good
(plus) component of the current remains unmixed. For an explicit demonstration
of such criteria, we present the relations between the current matrix elements
in the two typical reference frames used for calculations of the exclusive
amplitudes, i.e. the Drell-Yan-West and Breit frames and investigate both
pseudoscalar and vector electromagnetic currents in detail. We find that the
light-front helicities are unchanged and the good component of the current does
not mix with the other components of the current under the transformation
between these two frames. Thus, the pseudoscalar and vector form factors
obtained by the diagonal convolution formalism in both frames must indeed be
identical. However, such coincidence between the Drell-Yan-West and Breit
frames does not hold in general. We give an explicit example in which the
light-front helicities are changed and the plus component of the current is
mixed with other components under the change of reference frame. In such a
case, the relationship between the frames should be carefully analyzed before
the established convolution formalism in the Drell-Yan-West frame is used.Comment: 14 pages, 4 figure
Light-Front Model of Transition Form-Factors in Heavy Meson Decay
Electroweak transition form factors of heavy meson decays are important
ingredients in the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix
elements from experimental data. In this work, within a light-front framework,
we calculate electroweak transition form factor for the semileptonic decay of
mesons into a pion or a kaon. The model results underestimate in both cases
the new data of CLEO for the larger momentum transfers accessible in the
experiment. We discuss possible reasons for that in order to improve the model.Comment: Paper with 5 pages and 2 eps figures. To appear to Nuclear Physics B.
Talk at Light Cone 2009: Relativistic Hadronic and Particle Physics (LC
2009), Sao Jose dos Campos, S.P, Brazil, 8-13 Jul 2009
A covariant investigation of Neutral Vector Mesons: dynamical properties and electromagnetic decay widths
A simple, but fully-covariant model for describing neutral Vector Mesons, in
both light and heavy sectors, is briefly illustrated. The main ingredients of
our relativistic constituent model are i) an Ansatz for the Bethe-Salpeter
vertex for Vector Mesons, and ii) a Mandelstam-like formula for the
electromagnetic decay widths. The free parameters of our approach are fixed
through a comparison with the valence transverse-momentum distribution,
, obtained within phenomenological Light-Front Hamiltonian Dynamics
models reproducing the mass spectra. Preliminary results for the
transverse-momentum distributions, the parton distribution and the
electromagnetic decay constants are shown.Comment: 6 pages, 4 figures. Proceedings of LC2009, to appear in Nucl. Phys. B
supp
New Developments in Treacherous Points of Light-Front Dynamics
Light-front dynamics(LFD) plays an important role in hadron phenomenology.
Last few years, however, it has been emphasized that treacherous points such as
zero-mode contributions should be taken into account for successful LFD
applications to hadron phenomenology. We discuss examples of treacherous points
and present new progresses made last few years to handle them correctly.Comment: 5 pages, espcrc1.sty. proceedings of FB XVIII (August 2006, Brazil),
to be published in Nucl. Phys.
Electromagnetic form factor of the pion in the space- and time-like regions within the front-form dynamics
The pion electromagnetic form factor is calculated in the space- and
time-like regions from -10 up to 10 , within a
front-form model. The dressed photon vertex where a photon decays in a
quark-antiquark pair is depicted generalizing the vector meson dominance
ansatz, by means of the vector meson vertex functions. An important feature of
our model is the description of the on-mass-shell vertex functions in the
valence sector, for the pion and the vector mesons, through the front-form wave
functions obtained within a realistic quark model. The theoretical results show
an excellent agreement with the data in the space-like region, while in the
time-like region the description is quite encouraging.Comment: 9 pages + 4 figures. To appear in Phys. Lett.
Exploring the timelike region for the elastic form factor in the light-front quantization
Even though the Drell-Yan-West formulation is the most rigorous and
well-established framework to compute the exclusive processes, its utility has
been limited only to the spacelike region because of the intrinsic kinematic
constraint . We present an explicit example demonstrating how one may
obtain the necessary information (i.e. nonvalence or so called Z-graph
contribution) in the timelike region of exclusive process without encountering
a formidable task of direct calculation that has hindered so far the progress
in this area. In the analysis of bound state form factors using an
exactly solvable model of dimensional scalar field theory interacting
with gauge fields, the results analytically continued from the spacelike region
coincide exactly with the direct results in the timelike region. This example
verifies that the method of analytic continuation is capable of yielding the
effect of complicate nonvalence contributions. The meson peaks analogous to the
vector meson dominance(VMD) phenomena are also generated at the usual VMD
positions.Comment: 21 pages, 8 figures, we changed the title, added some references and
included some paragraphs in the introduction and conclusions; version to
appear in Nucl. Phys.
The Vector Meson Form Factor Analysis in Light-Front Dynamics
We study the form factors of vector mesons using a covariant fermion field
theory model in dimensions. Performing a light-front calculation in the
frame in parallel with a manifestly covariant calculation, we note the
existence of a nonvanishing zero-mode contribution to the light-front current
and find a way of avoiding the zero-mode in the form factor calculations.
Upon choosing the light-front gauge (\ep^+_{h=\pm}=0) with circular
polarization and with spin projection , only the
helicity zero to zero matrix element of the plus current receives zero-mode
contributions. Therefore, one can obtain the exact light-front solution of the
form factors using only the valence contribution if only the helicity
components, , and , are used. We also compare our
results obtained from the light-front gauge in the light-front helicity basis
(i.e. ) with those obtained from the non-LF gauge in the instant form
linear polarization basis (i.e. ) where the zero-mode contributions to
the form factors are unavoidable.Comment: 33 pages; typo in Eq.(15) is corrected; comment on Ref.[9] is
corrected; version to appear in Phys. Rev.
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