66 research outputs found
Two-particle irreducible effective actions versus resummation: analytic properties and self-consistency
Approximations based on two-particle irreducible (2PI) effective actions
(also known as -derivable, Cornwall-Jackiw-Tomboulis or Luttinger-Ward
functionals depending on context) have been widely used in condensed matter and
non-equilibrium quantum/statistical field theory because this formalism gives a
robust, self-consistent, non-perturbative and systematically improvable
approach which avoids problems with secular time evolution. The strengths of
2PI approximations are often described in terms of a selective resummation of
Feynman diagrams to infinite order. However, the Feynman diagram series is
asymptotic and summation is at best a dangerous procedure. Here we show that,
at least in the context of a toy model where exact results are available, the
true strength of 2PI approximations derives from their self-consistency rather
than any resummation. This self-consistency allows truncated 2PI approximations
to capture the branch points of physical amplitudes where adjustments of
coupling constants can trigger an instability of the vacuum. This, in effect,
turns Dyson's argument for the failure of perturbation theory on its head. As a
result we find that 2PI approximations perform better than Pad\'e approximation
and are competitive with Borel-Pad\'e resummation. Finally, we introduce a
hybrid 2PI-Pad\'e method.Comment: Version accepted for publication in Nuclear Physics B. 31 pages, 16
figures. Uses feynm
Scattering of low energy neutrinos and antineutrinos by atomic electrons
Studies of neutrino mixing and oscillations, solar neutrinos as background in dark matter searches involving electron detection, detection of sterile neutrino warm dark matter, and of possible electromagnetic properties of neutrinos, have generated interest in the low energy O(10 keV) scattering of electron neutrinos and antineutrinos by atomic electrons where the binding of the atomic electron cannot be ignored. Of particular interest is the ionization of atoms by neutrinos and antineutrinos. Most existing calculations are based upon modifications of the free electron differential cross section which destroy the relationship between the neutrino helicities and the orbital and spin angular momenta of the atomic electrons. The present calculations maintain the full collision dynamics by formulating the scattering in configuration space using the Bound Interaction Picture, rather than the usual formulation in the Interaction Picture in momentum space as appropriate to scattering by free electrons. Energy spectra of ionization electrons produced by scattering of neutrinos and antineutrinos with energies of 5, 10, 20, and 30 keV by hydrogen, helium and neon have been calculated using Dirac central field eigenfunctions, and are presented as ratios to the spectra for scattering by free electrons. Binding effects increase strongly with atomic number, are largest for low neutrino energy and, for each neutrino energy, greatest at the high electron energy end of the spectrum. The most extreme effects of binding are for 5 keV scattering by Ne where the ratios are less than 0.1. The energy spectra have been calculated for both a Coulombic final electron state and a free final electron state. The results indicate that the binding effects from the continuum state of the final electron are significant and can be comparable to those arising from the bound initial electron state
Scattering of low energy neutrinos and antineutrinos by neon and argon
The theory of scattering of low energy neutrinos and antineutrinos by atomic electrons has recently been developed [I.B. Whittingham, Phys. Rev. D 105, 013008 (2022)] using the bound interaction picture in configuration space to fully implement the relationship between the neutrino helicities and the orbital and spin angular momenta of the atomic electrons. The energy spectra of ionization electrons produced by scattering of neutrinos and antineutrinos with energies of 5, 10, 20, and 30 keV by hydrogen, helium and neon were calculated using Dirac screened Coulombic eigenfunctions. This paper reports further applications of this theory, to a new calculation of the energy spectra for neon, as the original calculation used some screening constants which underestimated the effects of screening in the inner subshells, and to scattering by argon. The results are presented as ratios to the corresponding quantities for scattering by Z free electrons. The new spectra ratios for neon are larger than the original ratios by ≈0.03–≈0.14, with the greatest increases occurring for 10 keV neutrinos and antineutrinos. Integrated spectra ratios range from 0.16 to 0.59 for neon, and from 0.15 to 0.48 for argon, as the neutrino energy increases from 5 to 30 keV
Photoassociation spectra and the validity of the dipole approximation for weakly bound dimers
Photoassociation (PA) of ultracold metastable helium to the 2s2p manifold is
theoretically investigated using a non-perturbative close-coupled treatment in
which the laser coupling is evaluated without assuming the dipole
approximation. The results are compared with our previous study [Cocks and
Whittingham, Phys. Rev. A 80, 023417 (2009)] that makes use of the dipole
approximation. The approximation is found to strongly affect the PA spectra
because the photoassociated levels are weakly bound, and a similar impact is
predicted to occur in other systems of a weakly bound nature. The inclusion or
not of the approximation does not affect the resonance positions or widths,
however significant differences are observed in the background of the spectra
and the maximum laser intensity at which resonances are discernable. Couplings
not satisfying the dipole selection rule |J-1| <= J' <= |J+1| do not lead to
observable resonances.Comment: 5 pages, 2 figures; Minor textual revision
Dark matter relic density in Gauss-Bonnet braneworld cosmology
The relic density of symmetric and asymmetric dark matter in a Gauss-Bonnet (GB) modified Randall-Sundrum (RS) type II braneworld cosmology is investigated. The existing study of symmetric dark matter in a GB braneworld (Okada and Okada, 2009) found that the expansion rate was reduced compared to that in standard General Relativity (GR), thereby delaying particle freeze-out and resulting in relic abundances which are suppressed by up to O(10^−2). This is in direct contrast to the behaviour observed in RS braneworlds where the expansion rate is enhanced and the final relic abundance boosted. However, this finding that relic abundances are suppressed in a GB braneworld is based upon a highly contrived situation in which the GB era evolves directly into a standard GR era, rather than passing through a RS era as is the general situation. This collapse of the RS era requires equating the mass scale m(α) of the GB modification and the mass scale m(σ) of the brane tension. However, if the GB contribution is to be considered as the lowest order correction from string theory to the RS action, we would expect m(α) > m(σ). We investigate the effect upon the relic abundance of choosing more realistic values for the ratio Rm ≡ mα/mσ and find that the relic abundance can be either enhanced or suppressed by more than two orders of magnitude. However, suppression only occurs for a small range of parameter choices and, overwhelmingly, the predominant situation is that of enhancement as we recover the usual Randall-Sundrum type behaviour in the limit Rm >> 1. We use the latest observational bound Ω(DM)h^2 = 0.1187 ± 0.0017 to constrain the various model parameters and briefly discuss the implications for direct/indirect dark matter detection experiments as well as dark matter particle models
Symmetry improvement of 3PI effective actions for O(N) scalar field theory
[Abridged] n-Particle Irreducible Effective Actions (PIEA) are a powerful
tool for extracting non-perturbative and non-equilibrium physics from quantum
field theories. Unfortunately, practical truncations of PIEA can
unphysically violate symmetries. Pilaftsis and Teresi (PT) addressed this by
introducing a "symmetry improvement" scheme in the context of the 2PIEA for an
O(2) scalar theory, ensuring that the Goldstone boson is massless in the broken
symmetry phase [A. Pilaftsis and D. Teresi, Nuc.Phys. B 874, 2 (2013), pp.
594--619]. We extend this by introducing a symmetry improved 3PIEA for O(N)
theories, for which the basic variables are the 1-, 2- and 3-point correlation
functions. This requires the imposition of a Ward identity involving the
3-point function. The method leads to an infinity of physically distinct
schemes, though an analogue of d'Alembert's principle is used to single out a
unique scheme. The standard equivalence hierarchy of PIEA no longer holds
with symmetry improvement and we investigate the difference between the
symmetry improved 3PIEA and 2PIEA. We present renormalized equations of motion
and counter-terms for 2 and 3 loop truncations of the effective action, leaving
their numerical solution to future work. We solve the Hartree-Fock
approximation and find that our method achieves a middle ground between the
unimproved 2PIEA and PT methods. The phase transition predicted by our method
is weakly first order and the Goldstone theorem is satisfied. We also show
that, in contrast to PT, the symmetry improved 3PIEA at 2 loops does not
predict the correct Higgs decay rate, but does at 3 loops. These results
suggest that symmetry improvement should not be applied to PIEA truncated to
loops. We also show that symmetry improvement is compatible with the
Coleman-Mermin-Wagner theorem, a check on the consistency of the formalism.Comment: 27 pages, 15 figures, 2 supplemental Mathematica notebooks. REVTeX
4.1 with amsmath. Updated with minor corrections. Accepted for publication in
Phys. Rev.
Ultracold collisions in tight harmonic traps: Quantum defect model and application to metastable helium atoms
We analyze a system of two colliding ultracold atoms under strong harmonic
confinement from the viewpoint of quantum defect theory and formulate a
generalized self-consistent method for determining the allowed energies. We
also present two highly efficient computational methods for determining the
bound state energies and eigenfunctions of such systems. The perturbed harmonic
oscillator problem is characterized by a long asymptotic region beyond the
effective range of the interatomic potential. The first method, which is based
on quantum defect theory and is an adaptation of a technique developed by one
of the authors (GP) for highly excited states in a modified Coulomb potential,
is very efficient for integrating through this outer region. The second method
is a direct numerical solution of the radial Schr\"{o}dinger equation using a
discrete variable representation of the kinetic energy operator and a scaled
radial coordinate grid. The methods are applied to the case of trapped
spin-polarized metastable helium atoms. The calculated eigenvalues agree very
closely for the two methods, and with those computed self-consistently using
the generalized self-consistent method.Comment: 11 pages,REVTEX, text substantially revised, title modifie
Spin-dipole induced lifetime of the least-bound quintet sigma state of He(2S)+He(2S)
The properties of the least-bound vibrational level (v=14) of the quintet
sigma state formed during the ultracold collision of two spin-polarized
metastable helium atoms are crucial to studies of photoassociation spectroscopy
of metastable helium. We report a calculation of the autoionization lifetime of
this state induced by spin-dipole coupling of the quintet sigma state to the
singlet sigma state from which Penning and associative ionization processes are
highly probable. We find a lifetime of about 150 microseconds, significantly
larger than the recent experimental estimates of (4-5) microseconds.Comment: REVTEX4, four double-column page
Laser Intensity Dependence of Photoassociation in Ultracold Metastable Helium
Photoassociation of spin-polarized metastable helium to the three lowest
rovibrational levels of the J=1, state asymptoting to 2SP is studied using a second-order perturbative
treatment of the line shifts valid for low laser intensities, and two variants
of a non-perturbative close-coupled treatment, one based upon dressed states of
the matter plus laser system, and the other on a modified radiative coupling
which vanishes asymptotically, thus simulating experimental conditions. These
non-perturbative treatments are valid for arbitrary laser intensities and yield
the complete photoassociation resonance profile. Both variants give nearly
identical results for the line shifts and widths of the resonances and show
that their dependence upon laser intensity is very close to linear and
quadratic respectively for the two lowest levels. The resonance profiles are
superimposed upon a significant background loss, a feature for this metastable
helium system not present in studies of photoassociation in other systems,
which is due to the very shallow nature of the excited state potential.
The results for the line shifts from the close-coupled and perturbative
calculations agree very closely at low laser intensities.Comment: 14 pages, 7 figures, title altered, text reduce
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