750 research outputs found
Extrapolation of the Zalpha-Expansion and Two--Loop Bound--State Energy Shifts
Quantum electrodynamic (QED) effects that shift the binding energies of
hydrogenic energy levels have been expressed in terms of a semi-analytic
expansion in powers of Zalpha and ln[(Zalpha)^{-2}], where Z is the nuclear
charge number and alpha is the fine-structure constant. For many QED effects,
numerical data are available in the domain of high Z where the Zalpha expansion
fails. In this Letter, we demonstrate that it is possible, within certain
limits of accuracy, to extrapolate the Zalpha-expansion from the low-Z to the
high-Z domain. We also review two-loop self-energy effects and provide an
estimate for the problematic nonlogarithmic coefficient B_60.Comment: 10 pages, LaTeX, Phys. Lett. B, in pres
Calculation of the Characteristic Functions of Anharmonic Oscillators
The energy levels of quantum systems are determined by quantization
conditions. For one-dimensional anharmonic oscillators, one can transform the
Schrodinger equation into a Riccati form, i.e., in terms of the logarithmic
derivative of the wave function. A perturbative expansion of the logarithmic
derivative of the wave function can easily be obtained. The Bohr-Sommerfeld
quantization condition can be expressed in terms of a contour integral around
the poles of the logarithmic derivative. Its functional form is B_m(E,g) = n +
1/2, where B is a characteristic function of the anharmonic oscillator of
degree m, E is the resonance energy, and g is the coupling constant. A
recursive scheme can be devised which facilitates the evaluation of
higher-order Wentzel-Kramers-Brioullin (WKB) approximants. The WKB expansion of
the logarithmic derivative of the wave function has a cut in the tunneling
region. The contour integral about the tunneling region yields the instanton
action plus corrections, summarized in a second characteristic function
A_m(E,g). The evaluation of A_m(E,g) by the method of asymptotic matching is
discussed for the case of the cubic oscillator of degree m=3.Comment: 11 pages, LaTeX; three further typographical errors correcte
A Problematic Set of Two-Loop Self-Energy Corrections
We investigate a specific set of two-loop self-energy corrections involving
squared decay rates and point out that their interpretation is highly
problematic. The corrections cannot be interpreted as radiative energy shifts
in the usual sense. Some of the problematic corrections find a natural
interpretation as radiative nonresonant corrections to the natural line shape.
They cannot uniquely be associated with one and only one atomic level. While
the problematic corrections are rather tiny when expressed in units of
frequency (a few Hertz for hydrogenic P levels) and do not affect the
reliability of quantum electrodynamics at the current level of experimental
accuracy, they may be of importance for future experiments. The problems are
connected with the limitations of the so-called asymptotic-state approximation
which means that atomic in- and out-states in the S-matrix are assumed to have
an infinite lifetime.Comment: 12 pages, 3 figures (New J. Phys., in press, submitted 28th May
Non-uniform convergence of two-photon decay rates for excited atomic states
Two-photon decay rates in simple atoms such as hydrogenlike systems represent
rather interesting fundamental problems in atomic physics. The sum of the
energies of the two emitted photons has to fulfill an energy conservation
condition, the decay takes place via intermediate virtual states, and the total
decay rate is obtained after an integration over the energy of one of the
emitted photons. Here, we investigate cases with a virtual state having an
energy intermediate between the initial and the final state of the decay
process, and we show that due to non-uniform convergence, only a careful
treatment of the singularities infinitesimally displaced from the photon
integration contour leads to consistent and convergent results.Comment: 3 pages; LaTe
Light Sea Fermions in Electron-Proton and Muon-Proton Interactions
The proton radius conundrum [R. Pohl et al., Nature vol.466, p.213 (2010) and
A. Antognini et al., Science vol.339, p.417 (2013)] highlights the need to
revisit any conceivable sources of electron-muon nonuniversality in
lepton-proton interactions within the Standard Model. Superficially, a number
of perturbative processes could appear to lead to such a nonunversality. One of
these is a coupling of the scattered electron into an electronic as opposed to
a muonic vacuum polarization loop in the photon exchange of two valence quarks,
which is present only for electron projectiles as opposed to muon projectiles.
However, we can show that this effect actually is part of the radiative
correction to the proton's polarizability contribution to the Lamb shift,
equivalent to a radiative correction to double scattering. We conclude that any
conceivable genuine nonuniversality must be connected with a nonperturbative
feature of the proton's structure, e.g., with the possible presence of light
sea fermions as constituent components of the proton. If we assume an average
of roughly 0.7*10^(-7) light sea positrons per valence quark, then we can show
that virtual electron-positron annihiliation processes lead to an extra term in
the electron-proton versus muon-proton interaction, which has the right sign
and magnitude to explain the proton radius discrepancy.Comment: 6 pages; RevTeX; published in Physical Review A in 2013; as compare
to the journal version, we have added a note at the end of the paper which
pertains to the (new) Ref. [42]; otherwise unchange
Gravitational Correction to Vacuum Polarization
We consider the gravitational correction to (electronic) vacuum polarization
in the presence of a gravitational background field. The Dirac propagators for
the virtual fermions are modified to include the leading gravitational
correction (potential term) which corresponds to a coordinate-dependent fermion
mass. The mass term is assumed to be uniform over a length scale commensurate
with the virtual electron-positron pair. The on-mass shell renormalization
condition ensures that the gravitational correction vanishes on the mass shell
of the photon, i.e., the speed of light is unaffected by the quantum field
theoretical loop correction, in full agreement with the equivalence principle.
Nontrivial corrections are obtained for off-shell, virtual photons. We compare
our findings to other works on generalized Lorentz transformations and combined
quantum-electrodynamic gravitational corrections to the speed of light which
have recently appeared in the literature.Comment: 9 pages; RevTeX; typographical errors corrected and references adde
Techniques in Analytic Lamb Shift Calculations
Quantum electrodynamics has been the first theory to emerge from the ideas of
regularization and renormalization, and the coupling of the fermions to the
virtual excitations of the electromagnetic field. Today, bound-state quantum
electrodynamics provides us with accurate theoretical predictions for the
transition energies relevant to simple atomic systems, and steady theoretical
progress relies on advances in calculational techniques, as well as numerical
algorithms. In this brief review, we discuss one particular aspect connected
with the recent progress: the evaluation of relativistic corrections to the
one-loop bound-state self-energy in a hydrogenlike ion of low nuclear charge
number, for excited non-S states, up to the order of alpha (Zalpha)^6 in units
of the electron mass. A few details of calculations formerly reported in the
literature are discussed, and results for 6F, 7F, 6G and 7G states are given.Comment: 16 pages, LaTe
Separation of Transitions with Two Quantum Jumps from Cascades
We consider the general scenario of an excited level |i> of a quantum system
that can decay via two channels: (i) via a single-quantum jump to an
intermediate, resonant level |bar m>, followed by a second single-quantum jump
to a final level |f>, and (ii) via a two-quantum transition to a final level
|f>. Cascade processes |i> -> |bar m> -> | f> and two-quantum transitions |i>
-> |m> -> |f> compete (in the latter case, |m> can be both a nonresonant as
well as a resonant level). General expressions are derived within second-order
time-dependent perturbation theory, and the cascade contribution is identified.
When the one-quantum decay rates of the virtual states are included into the
complex resonance energies that enter the propagator denominator, it is found
that the second-order decay rate contains the one-quantum decay rate of the
initial state as a lower-order term. For atomic transitions, this implies that
the differential-in-energy two-photon transition rate with complex resonance
energies in the propagator denominators can be used to good accuracy even in
the vicinity of resonance poles.Comment: 9 pages; RevTe
Dirac Hamiltonian with Imaginary Mass and Induced Helicity-Dependence by Indefinite Metric
It is of general theoretical interest to investigate the properties of
superluminal matter wave equations for spin one-half particles. One can either
enforce superluminal propagation by an explicit substitution of the real mass
term for an imaginary mass, or one can use a matrix representation of the
imaginary unit that multiplies the mass term. The latter leads to the tachyonic
Dirac equation, while the equation obtained by the substitution m->i*m in the
Dirac equation is naturally referred to as the imaginary-mass Dirac equation.
Both the tachyonic as well as the imaginary-mass Dirac Hamiltonians commute
with the helicity operator. Both Hamiltonians are pseudo-Hermitian and also
possess additional modified pseudo-Hermitian properties, leading to constraints
on the resonance eigenvalues. Here, by an explicit calculation, we show that
specific sum rules over the spectrum hold for the wave functions corresponding
to the well-defined real energy eigenvalues and complex resonance and
anti-resonance energies. In the quantized imaginary-mass Dirac field,
one-particle states of right-handed helicity acquire a negative norm
("indefinite metric") and can be excluded from the physical spectrum by a
Gupta--Bleuler type condition.Comment: 8 pages; RevTeX; published in J.Mod.Phy
Muonic bound systems, virtual particles and proton radius
The proton radius puzzle questions the self-consistency of theory and
experiment in light muonic and electronic bound systems. Here, we summarize the
current status of virtual particle models as well as Lorentz-violating models
that have been proposed in order to explain the discrepancy. Highly charged
one-electron ions and muonic bound systems have been used as probes of the
strongest electromagnetic fields achievable in the laboratory. The average
electric field seen by a muon orbiting a proton is comparable to hydrogenlike
Uranium and, notably, larger than the electric field in the most advanced
strong-laser facilities. Effective interactions due to virtual annihilation
inside the proton (lepton pairs) and process-dependent corrections (nonresonant
effects) are discussed as possible explanations of the proton size puzzle. The
need for more experimental data on related transitions is emphasized.Comment: 11 pages; RevTe
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