arXiv subjects

Peter Lichard

Publications and source records attributed to Peter Lichard.

25 records · Page 2Linked to original sources

arXiv1999

Relations between the K_{l3} and tau --> K pi nu_tau decays

We investigate the relations between the K_{l3} and tau --> K pi nu_tau decays using the meson dominance approach. First, the experimental branching fractions (BF) for K^-_{e3} and K^0_{e3} are used to fix two normalization constants (isospin invariance is not assumed). Then, the BF of tau^- --> K^*(892)^- nu_tau is calculated in agreement with experiment. We further argue that the nonzero value of the slope parameter lambda_0 of the K^-_{mu3} and K^0_{mu3} form factors f_0(t) implies the existence of the tau^- --> K_0^*(1430)^- nu_tau decay. We calculate its BF, together with BF's of the K^-_{mu3}, K^0_{mu3}, tau- --> K^- pi^0 nu_tau, and tau- --> antiK^0 pi^0 nu_tau decays, as a function of the lambda_0 parameter. At some value of lambda_0, different for charged and neutral kaons, calculated BF's seem to match existing data and a prediction is obtained for the tau^- --> K pi nu decays going through the K_0^*(1430)^- resonance.

hep-ph↗

arXiv1999

Running mass of the rho0 meson's implication for the dilepton mass spectrum and the mu+mu-/e+e- branching ratio in the K+ --> pi+l+l- decays

We make an attempt to resolve the discrepancy of the observed e+e- mass spectrum in the K+ --> pi+e+e- decay with that predicted by meson dominance. To this end we investigate the properties of the rho0 propagator. We use dispersion relations to evaluate the running mass squared m_rho^2(t) of the rho0 resonance without adjustable parameters. To improve the convergence of the dispersion integral, the momentum dependence of strong vertices is taken from the flux-tube-breaking model of Kokoski and Isgur. The obtained behavior of m_rho^2(t) at small momentum squared t makes the K+ --> pi+e+e- form factor rise faster with increasing $t$ than in the original meson-dominance calculation and more in agreement with the published data. As a consequence, the meson-dominance prediction of the mu+mu-/e+e- branching ratio changes slightly, from 0.224 to 0.236. We do not see any possibility to accommodate into the meson-dominance approach an even steeper e+e- spectrum, indicated by the preliminary data of the E865 collaboration at BNL AGS.

hep-ph↗

arXiv1998

Consistency of data on soft photon production in hadronic interactions

The glob model of Lichard and Van Hove and the modified soft annihilation model (MSAM) of Lichard and Thompson are used as a phenomenological tool for relating results from various experiments on soft photon production in high energy collisions. The total phenomenological expectation is composed of contributions from classical bremsstrahlung, the soft annihilation model and the glob model. The empirical excess above the background from hadronic decays at very small longitudinal momenta of photons is well reproduced, as well as that for transverse momenta pT >~ 10 MeV/c. Some data do not require the glob model and MSAM components in the phenomenological mixture, but do not exclude them. On the basis of consistency of all data with the total theoretical expectation we argue that the results of all experiments are mutually consistent. The models are unable to describe the excess of ultrasoft photons (pT <~ 10 MeV/c), seen by some, but not all, experiments. This may indicate an as yet unknown projectile-mass-dependent production mechanism. Possible relations of soft photon production to other phenomena are discussed. A simple-to-use, but physically equivalent version of the glob model is developed, which enables an easy check of presented results.

hep-ph↗

arXiv1998

Formalism for dilepton production via virtual photon bremsstrahlung in hadronic reactions

We derive a set of new formulas for various distributions in dilepton production via virtual photon bremsstrahlung from pseudoscalar mesons and unpolarized spin-one-half fermions. These formulas correspond to the leading and sub-leading terms in the Low-Burnett-Kroll expansion for real photon bremsstrahlung. The relation of our leading-term formulas to previous works is also shown. Existing formulas are examined in the light of Lorentz covariance and gauge invariance. Numerical comparison is made in a simple example, where an "exact" formula and real photon data exist. The results reveal large discrepancies among different bremsstrahlung formulas. Of all the leading-term bremsstrahlung formulas, the one derived in this work agrees best with the exact formula. The issues of M_T-scaling and event generators are also addressed.

hep-ph↗

arXiv1998

Are the production and decay of a resonance always independent?

The widely accepted assumption that the decay of a resonance proceeds independently of its production, quantitatively expressed as a factorizing formula for the differential cross section in the invariant mass of unpolarized resonance debris, is put under scrutiny. It is shown that the factorization is always valid for scalar and pseudoscalar resonances, although the usual version of the formula is not entirely correct. For resonances with nonzero spins the factorization does not generally take place. We deal in more detail with the spin-one case, where we show a condition on the decay matrix element that ensures the validity of the same factorizing formula as in the spinless case. This condition is satisfied for rho --> pi pi but not, e.g., for K* --> pi K or a1 --> pi rho. The formalism is applied also to the case when the resonance is produced not in two-body collisions but in the decay of a heavier particle or resonance (chain decay). Application of our formulas to the e+e- production in one-photon approximation agrees with what is known from quantum electrodynamics and thus provides another test of their soundness.

hep-ph↗

arXiv1997

Some implications of meson dominance in weak interactions

The hypothesis is scrutinized that the weak interaction of hadronic systems at low energies is dominated by the coupling of the pseudoscalar, vector, and axial-vector mesons to the weak gauge bosons. The strength of the weak coupling of the rho(770) meson is uniquely determined by vector-meson dominance in electromagnetic interactions; flavor and chiral symmetry breaking effects modify the coupling of other vector mesons and axial-vector mesons. Many decay rates are calculated and compared to experimental data and partly to predictions of other models. A parameter-free description of the decay K^+ --> pi^+ e^+e^- is obtained. Predictions for several not yet observed decay rates and reaction cross sections are presented. The relation between the conserved vector current hypothesis and meson dominance is clarified. Phenomenological success of the meson dominance suggests that in some calculations based on the standard model the weak quark-antiquark annihilation and creation diagrams may be more important than anticipated so far. The processes are identified where the meson dominance fails, implying that they are governed, on the quark level, by some other standard model diagrams.

hep-ph↗

arXiv1993

Lepton pairs from thermal mesons

We study the net dielectron production rates from an ensemble of thermal mesons, using an effective Lagrangian to model their interaction. The coupling between the electromagnetic and the hadronic sectors is done through the vector meson dominance approach. For the first time, a complete set of light mesons is considered. We include contributions from decays of the type V~(PS)~$\rightarrow$~PS~(V)~+~$e^+~e^-$, where V is a vector meson and PS is a pseudoscalar, as well as those from binary reactions PS~+~PS, V~+~V, and V~+~PS~$\rightarrow~e^+e^-$. Direct decays of the type V~$\rightarrow~e^+ e^-$ are included and shown to be important. We find that the dielectron invariant mass spectrum naturally divides in distinct regions: in the low mass domain the decays from vector and pseudoscalar mesons form the dominant contribution. The pion--pion annihilation and direct decays then pick up and form the leading signal in an invariant mass region that includes the $ρ- ω$ complex and extends up to the $ϕ$. Above invariant mass $M\ \approx$~1~GeV other two-body reactions take over as the prominent mechanisms for lepton pair generation. These facts will have quantitative bearing on the eventual identification of the quark--gluon plasma.

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