115 research outputs found
2d gravity stress tensor and the problem of the calculation of the multi-loop amplitudes in the string theory
It is shown that the vacuum value of the gravity stress tensor in the
free field theory is singular in the fundamental region on the complex plane
where the genus- Riemann surface are mapped. Because of the above
singularity, one can not construct modular invariant multi-loop amplitudes. The
discussed singularity is due to the singularity in the vacuum value of the
gravity field that turns out to be on the genus- Riemann surfaces.Comment: 13 pages, LATEX, November 199
BFKL Pomeron in string models
We consider scattering amplitudes in string models in the Regge limit of high
energies and fixed momentum transfers with the use of the unitarity in direct
channels. Intermediate states are taken in the multi-Regge kinematics
corresponding to the production of resonances with fixed invariant masses and
large relative rapidities. In QCD such kinematics leads to the BFKL equation
for the Pomeron wave function in the leading logarithmic approximation. We
derive a similar equation in the string theory and discuss its properties. The
purpose of this investigation is to find a generalization of the BFKL approach
to the region of small momentum transfers where non-perturbative corrections to
the gluon Regge trajectory and reggeon couplings are essential. The BFKL
equation in the string theory contains additional contributions coming from a
linear part of the Regge trajectory and from the soft Pomeron singularity
appearing already in the tree approximation. In higher dimensions in addition,
a non-multi-Regge kinematics corresponding to production of particles with
large masses is important. We solve the equation for the Pomeron wave function
in the string theory for D=4 and discuss integrability properties of analogous
equations for composite states of several reggeised gluons in the multi-colour
limit.Comment: 48 pages, 2 figure
Molecular regimes in ultracold Fermi gases
The use of Feshbach resonances for tuning the interparticle interaction in
ultracold Fermi gases has led to remarkable developments, in particular to the
creation and Bose-Einstein condensation of weakly bound diatomic molecules of
fermionic atoms. These are the largest diatomic molecules obtained so far, with
a size of the order of thousands of angstroms. They represent novel composite
bosons, which exhibit features of Fermi statistics at short intermolecular
distances. Being highly excited, these molecules are remarkably stable with
respect to collisional relaxation, which is a consequence of the Pauli
exclusion principle for identical fermionic atoms. The purpose of this review
is to introduce theoretical approaches and describe the physics of molecular
regimes in two-component Fermi gases and Fermi-Fermi mixtures, focusing
attention on quantum statistical effects.Comment: Chapter of the book: "Cold Molecules: Theory, Experiment,
Applications" edited by R. V. Krems, B. Friedrich and W. C. Stwalley
(publication expected in March 2009
Parity-violating neutron spin rotation in hydrogen and deuterium
We calculate the (parity-violating) spin rotation angle of a polarized
neutron beam through hydrogen and deuterium targets, using pionless effective
field theory up to next-to-leading order. Our result is part of a program to
obtain the five leading independent low-energy parameters that characterize
hadronic parity-violation from few-body observables in one systematic and
consistent framework. The two spin-rotation angles provide independent
constraints on these parameters. Using naive dimensional analysis to estimate
the typical size of the couplings, we expect the signal for standard target
densities to be 10^-7 to 10^-6 rad/m for both hydrogen and deuterium targets.
We find no indication that the nd observable is enhanced compared to the np
one. All results are properly renormalized. An estimate of the numerical and
systematic uncertainties of our calculations indicates excellent convergence.
An appendix contains the relevant partial-wave projectors of the three-nucleon
system.Comment: 44 pages, 17 figures; minor corrections; to be published in EPJ
Three-body problem in Fermi gases with short-range interparticle interaction
We discuss 3-body processes in ultracold two-component Fermi gases with
short-range intercomponent interaction characterized by a large and positive
scattering length . It is found that in most cases the probability of 3-body
recombination is a universal function of the mass ratio and , and is
independent of short-range physics. We also calculate the scattering length
corresponding to the atom-dimer interaction.Comment: 4 pages, 2 figure
Parity violation in nuclear systems
Parity violation in nuclear systems is reviewed. A few ingredients relevant
to the description of the parity-violating nucleon-nucleon force in terms of
meson exchanges are reminded. Effects in nuclear systems are then considered.
They involve pp scattering, some complex nuclei and the deuteron system.Comment: 4 pages, to be published in the proceedings of the worksho
Is the Luttinger liquid a new state of matter?
We are demonstrating that the Luttinger model with short range interaction
can be treated as a type of Fermi liquid. In line with the main dogma of
Landau's theory one can define a fermion excitation renormalized by interaction
and show that in terms of these fermions any excited state of the system is
described by free particles. The fermions are a mixture of renormalized right
and left electrons. The electric charge and chirality of the Landau
quasi-particle is discussed.Comment: paper 10 pages. This version of the paper will be published in
Foundations of Physic
Multiloop Amplitudes and Vanishing Theorems using the Pure Spinor Formalism for the Superstring
A ten-dimensional super-Poincare covariant formalism for the superstring was
recently developed which involves a BRST operator constructed from superspace
matter variables and a pure spinor ghost variable. A super-Poincare covariant
prescription was defined for computing tree amplitudes and was shown to
coincide with the standard RNS prescription.
In this paper, picture-changing operators are used to define functional
integration over the pure spinor ghosts and to construct a suitable ghost.
A super-Poincare covariant prescription is then given for the computation of
N-point multiloop amplitudes. One can easily prove that massless N-point
multiloop amplitudes vanish for N<4, confirming the perturbative finiteness of
superstring theory. One can also prove the Type IIB S-duality conjecture that
terms in the effective action receive no perturbative contributions above
one loop.Comment: 45 pages harvmac tex, added minor clarification
Parity nonconservation in deuteron photoreactions
We calculate the asymmetries in parity nonconserving deuteron
photodisintegration due to circularly polarized photons gamma+d to n+p with the
photon laboratory energy ranging from the threshold up to 10 MeV and the
radiative capture of thermal polarized neutrons by protons n+p to gamma+d. We
use the leading order electromagnetic Hamiltonian neglecting the smaller
nuclear exchange currents. Comparative calculations are done by using the
Reid93 and Argonne v18 potentials for the strong interaction and the DDH and
FCDH "best" values for the weak couplings in a weak one-meson exchange
potential. A weak NDelta transition potential is used to incorporate also the
Delta(1232)-isobar excitation in the coupled-channels formalism.Comment: 14 pages, 13 figures (18 eps files), LaTeX2
Parity nonconserving cold neutron-parahydrogen interactions
Three pion dominated observables of the parity nonconserving interactions
between the cold neutrons and parahydrogen are calculated. The transversely
polarized neutron spin rotation, unpolarized neutron longitudinal polarization,
and photon-asymmetry of the radiative polarized neutron capture are considered.
For the numerical evaluation of the observables, the strong interactions are
taken into account by the Reid93 potential and the parity nonconserving
interactions by the DDH model along with the two-pion exchange.Comment: 17 pages, 2 figure
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