262 research outputs found
Spontaneous Breaking of Lorentz Invariance
We describe how a stable effective theory in which particles of the same
fermion number attract may spontaneously break Lorentz invariance by giving
non-zero fermion number density to the vacuum (and therefore dynamically
generating a chemical potential term). This mecanism yields a finite vacuum
expectation value could relate to work on signals of Lorentz violation
in electrodynamics.Comment: revtex4, 11 pages, 5 figures; v2:references added; v3:more references
added, typos fixed, some points in sect. IV clarified; v4:even more
references added, discussion in sect. V extended; v5:replaced to match
published version (minor corrections of form
Effects of exposure of rat erythrocytes to a hypogeomagnetic field
The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.Background:Hypomagnetic fields can disrupts the normal functioning of living organisms by a mechanism thought to involve oxidative stress. In erythrocytes, oxidative stress can inter alia lead to changes to hemoglobin content and to hemolysis. Objective:To study the effects of hypomagnetism on the state of rat erythrocytes in vitro. Methods:Rat erythrocytes were exposed to an attenuated magnetic field (AMF) or Earth’s magnetic field (EMF), in the presence of tert-butyl hydroperoxide (TBHP) as inducer of oxidative stress. Determinations: total hemoglobin (and its three forms – oxyhemoglobin, methemoglobin, and hemichrome) released from erythrocytes, spectral data (500–700 nm); oxygen radical concentrations, electron paramagnetic resonance. Results:AMF and EMF exposed erythrocytes were compared. After 4 h incubation at high TBHP concentrations (>700 μM), AMF exposed erythrocytes released significantly more (p<0.05) hemoglobin (Hb), mostly as methemoglobin (metHb). Conversely, after 24 h incubation at low TBHP concentrations (⩽350 μM), EMF exposed erythrocytes released significantly more (p<0.001) hemoglobin, with metHb as a significant proportion of the total Hb. Erythrocytes exposed to AMF generated more radicals than those exposed to the EMF. Conclusion:Under particular conditions of oxidative stress, hypomagnetic fields can disrupt the functional state of erythrocytes and promote cell death; an additive effect is implicated
Spontaneous Lorentz Violation via QED with Non-Exact Gauge Invariance
We reconsider an alternative theory of the QED with the photon as a massless
vector Nambu-Goldstone boson and show that the underlying spontaneous Lorentz
violation caused by the vector field vacuum expectation value, while being
superficial in gauge invariant theory, becomes physically significant in the
QED with a tiny gauge non-invariance. This leads, through special dispersion
relations appearing for charged fermions, to a new class of phenomena which
could be of distinctive observational interest in particle physics and
astrophysics. They include a significant change in the GZK cutoff for UHE
cosmic-ray nucleons, stability of high-energy pions and W bosons, modification
of nucleon beta decays, and some others.Comment: 15 pages, to appear in Eur.Phys.J.
Aspects of Nucleon Compton Scattering
We consider the spin-averaged nucleon forward Compton scattering amplitude in
heavy baryon chiral perturbation theory including all terms to order . The chiral prediction for the spin-averaged forward Compton scattering
amplitude is in good agreement with the data for photon energies MeV. We also evaluate the nucleon electric and magnetic Compton
polarizabilities to this order and discuss the uncertainties of the various
counter terms entering the chiral expansion of these quantities.Comment: 17 pp, TeX, 7 figures available from the authors, preprint CRN-93/5
Modulation Instability of Ultrashort Pulses in Quadratic Nonlinear Media beyond the Slowly Varying Envelope Approximation
We report a modulational instability (MI) analysis of a mathematical model
appropriate for ultrashort pulses in cascaded quadratic-cubic nonlinear media
beyond the so-called slowly varying envelope approximation. Theoretically
predicted MI properties are found to be in good agreement with numerical
simulation. The study shows the possibility of controlling the generation of MI
and formation of solitons in a cascaded quadratic-cubic media in the few cycle
regimes. We also find that stable propagation of soliton-like few-cycle pulses
in the medium is subject to the fulfilment of the modulation instability
criteria
Using Heavy Quark Spin Symmetry in Semileptonic Decays
The form factors parameterizing the B_c semileptonic matrix elements can be
related to a few invariant functions if the decoupling of the spin of the heavy
quarks in B_c and in the mesons produced in the semileptonic decays is
exploited. We compute the form factors as overlap integral of the meson
wave-functions obtained using a QCD relativistic potential model, and give
predictions for semileptonic and non-leptonic B_c decay modes. We also discuss
possible experimental tests of the heavy quark spin symmetry in B_c decays.Comment: RevTex, 22 pages, 2 figure
Four-fermion heavy quark operators and light current amplitudes in heavy flavor hadrons
We introduce and study the properties of the "color-straight" four-quark
operators containing heavy and light quark fields. They are of the form (\bar
b\Gamma_b b)(\bar q\Gamma_q q) where both brackets are color singlets. Their
expectation values include the bulk of the nonfactorizable contributions to the
nonleptonic decay widths of heavy hadrons. The expectation values of the
color-straight operators in the heavy hadrons are related to the momentum
integrals of the elastic light-quark formfactors of the respective heavy
hadron. We calculate the asymptotic behavior of the light-current formfactors
of heavy hadrons and show that the actual decrease is 1/(q^2)^3/2 rather than
1/q^4. The two-loop hybrid anomalous dimensions of the four-quark operators and
their mixing (absent in the first loop) are obtained. Using plausible models
for the elastic formfactors, we estimate the expectation values of the
color-straight operators in the heavy mesons and baryons. Improved estimates
will be possible in the future with new data on the radiative decays of heavy
hadrons. We give the Wilson coefficients of the four-fermion operators in the
1/m_b expansion of the inclusive widths and discuss the numerical predictions.
Estimates of the nonfactorizable expectation values are given.Comment: 51 pages. The case of flavor-singlet operators is added for the
two-loop anomalous dimension
Proton-proton scattering above 3 GeV/c
A large set of data on proton-proton differential cross sections, analyzing
powers and the double polarization parameter A_NN is analyzed employing the
Regge formalism. We find that the data available at proton beam momenta from 3
GeV/c to 50 GeV/c exhibit features that are very well in line with the general
characteristics of Regge phenomenology and can be described with a model that
includes the rho, omega, f_2, and a_2 trajectories and single Pomeron exchange.
Additional data, specifically for spin-dependent observables at forward angles,
would be very helpful for testing and refining our Regge model.Comment: 16 pages, 19 figures; revised version accepted for publication in
EPJ
Nucleon axial and pseudoscalar form factors from the covariant Faddeev equation
We compute the axial and pseudoscalar form factors of the nucleon in the
Dyson-Schwinger approach. To this end, we solve a covariant three-body Faddeev
equation for the nucleon wave function and determine the matrix elements of the
axialvector and pseudoscalar isotriplet currents. Our only input is a
well-established and phenomenologically successful ansatz for the
nonperturbative quark-gluon interaction. As a consequence of the axial
Ward-Takahashi identity that is respected at the quark level, the
Goldberger-Treiman relation is reproduced for all current-quark masses. We
discuss the timelike pole structure of the quark-antiquark vertices that enters
the nucleon matrix elements and determines the momentum dependence of the form
factors. Our result for the axial charge underestimates the experimental value
by 20-25% which might be a signal of missing pion-cloud contributions. The
axial and pseudoscalar form factors agree with phenomenological and lattice
data in the momentum range above Q^2 ~ 1...2 GeV^2.Comment: 17 pages, 7 figures, 1 tabl
Standard Model with Partial Gauge Invariance
We argue that an exact gauge invariance may disable some generic features of
the Standard Model which could otherwise manifest themselves at high energies.
One of them might be related to the spontaneous Lorentz invariance violation
(SLIV) which could provide an alternative dynamical approach to QED and
Yang-Mills theories with photon and non-Abelian gauge fields appearing as
massless Nambu-Goldstone bosons. To see some key features of the new physics
expected we propose partial rather than exact gauge invariance in an extended
SM framework. This principle applied, in some minimal form, to the weak
hypercharge gauge field B_{mu} and its interactions leads to SLIV with B field
components appearing as the massless Nambu-Goldstone modes, and provides a
number of distinctive Lorentz beaking effects. Being naturally suppressed at
low energies they may become detectable in high energy physics and
astrophysics. Some of the most interesting SLIV processes are considered in
significant detail.Comment: 32 pages, extended version, to appear in Eur.Phys.J.
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