On the Size of Hadrons

The form factor and the mean-square radius of the pion are calculated analytically from a parametrized form of a $q\bar q$ wave function. The numerical wave function was obtained previously by solving numerically an eigenvalue equation for the pion in a particular model. The analytical formulas are of more general interest than just be valid for the pion and can be generalized to the case with unequal quark masses. Two different parametrizations are investigated. Because of the highly relativistic problem, noticable deviations from a non-relativistic formula are obtained.Comment: 14 pages, minor typos corrected, several points clarified, results unchange

Determination of the Kobayashi-Maskawa-Cabibbo matrix element V_{us} under various flavor-symmetry-breaking models in hyperon semileptonic decays

We study the success to describe hyperon semileptonic decays of four models that incorporate second-order SU(3) symmetry breaking corrections. The criteria to assess their success is by determining V_{us} in each of the three relevant hyperon semileptonic decays and comparing the values obtained with one another and also with the one that comes from K_{l3} decays. A strong dependence on the particular symmetry breaking model is observed. Values of V_{us} which do not agree with the one of K_{l3} are generally obtained. However, in the context of chiral perturbation theory, only the model whose corrections are O(m_s) and O(m_s^{3/2}) is successful. Using its predictions for the f_1 form factors one can quote a value of V_{us} from this model, namely, V_{us}=0.2176\pm 0.0026, which is in excellent agreement with the K_{l3} one.Comment: Final versio

Beta decay of hyperons in a relativistic quark model

A relativistic constituent quark model is used to calculate the semileptonic beta decay of nucleons and hyperons. The parameters of the model, namely, the constituent quark mass and the confinement scale, are fixed by a previous calculation of the magnetic moments of the baryon octet within the same model. We discuss the momentum dependence of the form factors, possible configuration mixing and SU(3) symmetry breaking. We conclude that the relativistic constituent quark model is a good framework to analyze electroweak properties of the baryons.Comment: RevTeX, 14 pages, SLAC-PUB-666

Baryon Current Matrix Elements in a Light-Front Framework

Current matrix elements and observables for electro- and photo-excitation of baryons from the nucleon are studied in a light-front framework. Relativistic effects are estimated by comparison to a nonrelativistic model, where we use simple basis states to represent the baryon wavefunctions. Sizeable relativistic effects are found for certain transitions, for example, to radial excitations such as that conventionally used to describe to the Roper resonance. A systematic study shows that the violation of rotational covariance of the baryon transition matrix elements stemming from the use of one-body currents is generally small.Comment: 32 pages, LaTeX, 10 postscript figures, uses epsf.sty; figures uuencoded with uufiles (or available by request in .ps or hardcopy form

Masses of the physical mesons from an effective QCD--Hamiltonian

The front form Hamiltonian for quantum chromodynamics, reduced to an effective Hamiltonian acting only in the $q\bar q$ space, is solved approximately. After coordinate transformation to usual momentum space and Fourier transformation to configuration space a second order differential equation is derived. This retarded Schr\"odinger equation is solved by variational methods and semi-analytical expressions for the masses of all 30 pseudoscalar and vector mesons are derived. In view of the direct relation to quantum chromdynamics without free parameter, the agreement with experiment is remarkable, but the approximation scheme is not adequate for the mesons with one up or down quark. The crucial point is the use of a running coupling constant $\alpha_s(Q^2)$, in a manner similar but not equal to the one of Richardson in the equal usual-time quantization. Its value is fixed at the Z mass and the 5 flavor quark masses are determined by a fit to the vector meson quarkonia.Comment: 18 pages, 4 Postscript figure

Precise Neutron Magnetic Form Factors

Precise data on the neutron magnetic form factor G_{mn} have been obtained with measurements of the ratio of cross sections of D(e,e'n) and D(e,e'p) up to momentum transfers of Q^2 = 0.9 (GeV/c)^2. Data with typical uncertainties of 1.5% are presented. These data allow for the first time to extract a precise value of the magnetic radius of the neutron.Comment: 10 pages, 2 figures, submitted to Physics Letters

Pion-photon and photon-pion transition form factors in light-cone formalism

We derive the minimal Fock-state expansions of the pion and the photon wave functions in light-cone formalism, then we calculate the pion-photon and the photon-pion transition form factors of $\gamma ^{\ast}\pi ^{0}\to \gamma$ and $\gamma ^{\ast}\gamma \to \pi ^{0}$ processes by employing these quark-antiquark wave functions of the pion and the photon. We find that our calculation for the $\gamma ^{\ast}\gamma \to \pi ^{0}$ transition form factor agrees with the experimental data at low and moderately high energy scale. Moreover, the physical differences and inherent connections between the transition form factors of $\gamma ^{\ast}\pi ^{0}\to \gamma$ and $\gamma ^{\ast}\gamma \to \pi ^{0}$ have been illustrated, which indicate that these two physical processes are intrinsically related. In addition, we also discuss the $\pi ^{0}\to \gamma \gamma$ form factor and the decay width $\mathit{\Gamma}(\pi \to \gamma \gamma)$ at $Q^{2}=0$.Comment: 20 pages, 2 figure

Electromagnetic Moments of the Baryon Decuplet

We compute the leading contributions to the magnetic dipole and electric quadrupole moments of the baryon decuplet in chiral perturbation theory. The measured value for the magnetic moment of the $\Omega^-$ is used to determine the local counterterm for the magnetic moments. We compare the chiral perturbation theory predictions for the magnetic moments of the decuplet with those of the baryon octet and find reasonable agreement with the predictions of the large--$N_c$ limit of QCD. The leading contribution to the quadrupole moment of the $\Delta$ and other members of the decuplet comes from one--loop graphs. The pionic contribution is shown to be proportional to $I_z$ (and so will not contribute to the quadrupole moment of $I=0$ nuclei), while the contribution from kaons has both isovector and isoscalar components. The chiral logarithmic enhancement of both pion and kaon loops has a coefficient that vanishes in the $SU(6)$ limit. The third allowed moment, the magnetic octupole, is shown to be dominated by a local counterterm with corrections arising at two loops. We briefly mention the strange counterparts of these moments.Comment: Uses harvmac.tex, 15 pages with 3 PostScript figures packed using uufiles. UCSD/PTH 93-22, QUSTH-93-05, Duke-TH-93-5

Nuclei in a chiral SU(3) model

Nuclei can be described satisfactorily in a nonlinear chiral SU(3)-framework, even with standard potentials of the linear $\sigma$-model. The condensate value of the strange scalar meson is found to be important for the properties of nuclei even without adding hyperons. By neglecting terms which couple the strange to the nonstrange condensate one can reduce the model to a Walecka model structure embedded in SU(3). We discuss inherent problems with chiral SU(3) models regarding hyperon optical potentials.Comment: 25 pages, RevTe

Sunscreens - Which and what for?

It is well established that sun exposure is the main cause for the development of skin cancer. Chronic continuous UV radiation is believed to induce malignant melanoma, whereas intermittent high-dose UV exposure contributes to the occurrence of actinic keratosis as precursor lesions of squamous cell carcinoma as well as basal cell carcinoma. Not only photocarcinogenesis but also the mechanisms of photoaging have recently become apparent. In this respect the use of sunscreens seemed to prove to be more and more important and popular within the last decades. However, there is still inconsistency about the usefulness of sunscreens. Several studies show that inadequate use and incomplete UV spectrum efficacy may compromise protection more than previously expected. The sunscreen market is crowded by numerous products. Inorganic sunscreens such as zinc oxide and titanium oxide have a wide spectral range of activity compared to most of the organic sunscreen products. It is not uncommon for organic sunscreens to cause photocontact allergy, but their cosmetic acceptability is still superior to the one given by inorganic sunscreens. Recently, modern galenic approaches such as micronization and encapsulation allow the development of high-quality inorganic sunscreens. The potential systemic toxicity of organic sunscreens has lately primarily been discussed controversially in public, and several studies show contradictory results. Although a matter of debate, at present the sun protection factor (SPF) is the most reliable information for the consumer as a measure of sunscreen filter efficacy. In this context additional tests have been introduced for the evaluation of not only the protective effect against erythema but also protection against UV-induced immunological and mutational effects. Recently, combinations of UV filters with agents active in DNA repair have been introduced in order to improve photoprotection. This article reviews the efficacy of sunscreens in the prevention of epithelial and nonepithelial skin cancer, the effect on immunosuppression and the value of the SPF as well as new developments on the sunscreen market. Copyright (C) 2005 S. Karger AG, Basel