Electric polarizability of pions in semirelativistic quark model

The generalized and static polarizabilities of charged pions, which are considered a relativistic system of two pointlike spinor quarks with a linear interaction potential, are calculated. The relationship between static electricity and generalized polarizabilities of pions is studied in the context of this approach.     

Photoproduction of pions in the constituent quark model

Threshold photoproduction of pions has been calculated for composite systems, both for the nucleon in terms of constituent quarks and the ³H/³He isodoublet in terms of nucleons. The results of the low energy theorems are recovered only if both size effects and internal excitation of the composite system are properly taken into account. In particular, the ground state has to be non-degenerate and the excitation spectrum has to have the proper symmetries of the many-body system.     

Off-diagonal quark distributions in pions in the effective single-instanton approximation

Nonperturbative functions that parametrize off-diagonal hadronic matrix elements of the light-cone leading-twist quark operators are considered. These functions are calculated within the proposed relativistic quark model allowing for the nontrivial structure of the QCD vacuum, special attention being given to gauge invariance. Hadrons are treated as bound states of quarks; strong-interaction quark-pion vertices are described by effective interaction Lagrangians generated by instantons. The parameters of the instanton vacuum, such as the effective radius of the instanton and the quark mass, are related to the vacuum expectation values of the quark-gluon operators of the lowest dimension and to low-energy pion observables.     

The number of neutral pions and fireballs with quark quantum numbers

The prediction of fireball models that for fixed total pion number the neutral versus charged pion distribution is binomial is confronted with experiment. The agreement is very good with available data. This restricts coupling ratios in the statistical model for fireball decay to coincide with those of a simple quark model.     

Tau decays into four pions

We compute branching ratios and invariant mass distributions of the tau decays into four pions. The hadronic matrix elements are obtained by starting from the structure of the hadronic current in the chiral limit and then implementing low-lying resonances in the different channels. Reasonable agreement with experiment is obtained both for the τ → v_τ+(4π) decay rates and the e⁺ e^? → (4π) cross sections. Furthermore we supply an interface to use our matrix elements within the Tauola Monte-Carlo program.     

Light quark masses from scalar sum rules

In this work, the mass of the strange quark is calculated from QCD sum rules for the divergence of the strangeness-changing vector current. The phenomenological scalar spectral function which enters the sum rule is determined from our previous work on strangeness-changing scalar form factors [1]. For the running strange mass in the scheme, we find . Making use of this result and the light quark mass ratios obtained from chiral perturbation theory, we are also able to extract the masses of the lighter quarks and . We then obtain and . In addition, we present an updated value for the light quark condensate. Received: 18 October 2001 / Revised version: 22 January 2002 / Published online: 12 April 2002     

Inclusive π+ to π− ratios and quark distributions of pions and nucleons

The ratio R of inclusive $\text{π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}$ production at low transverse momenta is measured in π⁺ n and pn interactions at 195 GeV/c.$\text{R (}\text{1}\text{R}\text{)}$ in the proton (neutron) fragmentation region increases with Feynman-x and is smaller than R from π⁺ fragmentation. The results agree with a quark-quark scattering model originally proposed for “hard” processes and disagree with a modified version of the model which includes gluon scattering.     

Light cone algebra of a selfinteracting quark field

In a gauge invariant model for a selfinteracting constituent quark field of subcanonical dimension $\text{1}\text{2}$ the current operators and the energy-momentum operator are explicitly constructed. The current-current commutator near the light cone is found to be distinctly different from the free quark case with corresponding implications on deep inelastic lepton-hadron scattering.     

Decay rate of τ into odd number of pions

Decay rate of τ into 3π and 5π is calculated using current algebra and PCAC. Assuming universality for ordinary leptons and τ, satisfactory agreement with experimental data is obtained with $\text{Г}{}_{\mathrm{<mtext>tot</mtext>}}\text{(}\text{A}{}_1\text{) ≈ 300?400}\text{MeV}$. The branching ratio for π → 5π + ν is found to be 1%.     

Light front approach to correlations in hot quark matter

We investigate two-quark correlations in hot and dense quark matter. To this end we use the light front field theory extended to finite temperature T and chemical potential μ. Therefore it is necessary to develop quantum statistics formulated on the light front plane. As a test case for light front quantization at finite T and μ we consider the NJL model. The solution of the in-medium gap equation leads to a constituent quark mass which depends on T and μ. Two-quark systems are considered in the pionic and diquark channel. We compute the masses of the two-body system using a T-matrix approach.     

N\bar N annihilation at rest into five pions

annihilation at rest into five pions was studied in the ASTERIX spectrometer by stopping antiprotrons from LEAR in a H₂ or D₂ gas target. In annihilations in H₂ leading to the ^+–+–0 final state, the invariant mass spectra of two, three, and four pions show no evidence for any new narrow states. In the difference of the four pion mass spectra recoiling against a ^– and those recoiling against a ⁺ shows a resonance-like structure ( (1480)), confirming a previous analysis with a smaller event sample. The (1480) mass is shown to depend strongly on the momentum of the spectator proton. In H₂, we searched for the (1480) in the reaction (1480), (1480) ^+–+– to clarify whether the (1480) is identical to the 4-mode of thef ₂ (1520). No evidence for this decay mode was found. A prominent resonance in innihilation into five pions is the meson. We find two quasi-two-body intermediate states: and (1235). The mass and width of theb ₁ (1235) are determined to be: