Pion Final-state Interactions Research Articles

ϕ(2170) Decaying to ϕππ and ϕKK¯

Within the framework of dispersion theory, we study the the processes e+e−→ϕ(2170)→ϕππ(KK¯). The strong pion–pion final-state interactions, especially the KK¯ coupled channel in the S wave, are taken into account in a model-independent way using the Omnès function solution. Through fitting the experimental data of the ππ and ϕπ invariant mass distributions of the e+e−→ϕ(2170)→ϕπ+π− process, the low-energy constants in the chiral Lagrangian are determined. The theoretical prediction for the cross sections’ ratio σ(e+e−→ϕ(2170)→ϕK+K−)/σ(e+e−→ϕ(2170)→ϕπ+π−) is given, which could be useful for selecting the physical solution, when the fit to the e+e−→ϕK+K− cross-section distribution is available in the future. Our results suggest that above the kinematical threshold of ϕKK¯, the mechanism e+e−→ϕK+K−, with the kaons rescattering to a pion pair, plays an important role in the e+e−→ϕπ+π− transition.

Towards resolving the [formula omitted]H lifetime puzzle

Recent Λ3H lifetime measurements in relativistic heavy ion collision experiments have yielded values shorter by (30±8)% than the free Λ lifetime τΛ, thereby questioning the naive expectation τ(HΛ3)≈τΛ for a weakly bound Λ hyperon. Here we apply the closure approximation introduced by Dalitz and coworkers to evaluate the Λ3H lifetime, using Λ3H wavefunctions generated by solving three-body Faddeev equations. Our result, disregarding pion final-state interaction (FSI), is τ(HΛ3)=(0.90±0.01)τΛ. In contrast to previous works, pion FSI is found attractive, reducing further τ(HΛ3) to τ(HΛ3)=(0.81±0.02)τΛ. We also evaluate for the first time τ(nΛ3), finding it considerably longer than τΛ, contrary to the shorter lifetime values suggested by the GSI HypHI experiment for this controversial hypernucleus.

Cross sections forνμandν¯μinduced pion production on hydrocarbon in the few-GeV region using MINERvA

Separate samples of charged-current pion production events representing two semi-inclusive channels $\nu_\mu$-CC($\pi^{+}$) and $\bar{\nu}_{\mu}$-CC($\pi^{0}$) have been obtained using neutrino and antineutrino exposures of the MINERvA detector. Distributions in kinematic variables based upon $\mu^{\pm}$-track reconstructions are analyzed and compared for the two samples. The differential cross sections for muon production angle, muon momentum, and four-momentum transfer $Q^2$, are reported, and cross sections versus neutrino energy are obtained. Comparisons with predictions of current neutrino event generators are used to clarify the role of the $\Delta(1232)$ and higher-mass baryon resonances in CC pion production and to show the importance of pion final-state interactions. For the $\nu_\mu$-CC($\pi^{+}$) ($\bar{\nu}_{\mu}$-CC($\pi^{0}$)) sample, the absolute data rate is observed to lie below (above) the predictions of some of the event generators by amounts that are typically 1-to-2 $\sigma$. However the generators are able to reproduce the shapes of the differential cross sections for all kinematic variables of either data set.

Singleπproduction in neutrino-nucleus scattering

We study one pion production in both charged and neutral current neutrino nucleus scattering for neutrino energies below 2 GeV. We use a theoretical model for one pion production at the nucleon level that we correct for medium effects. The results are incorporated into a cascade program that apart from production also includes the pion final state interaction inside the nucleus. Besides, in some specific channels coherent pion production is also possible and we evaluate its contribution as well. Our results for total and differential cross sections are compared with recent data from the MiniBooNE Collaboration. The model provides an overall acceptable description of data, better for NC than for CC channels, although theory is systematically below data. Differential cross sections, folded with the full neutrino flux, show that most of the missing pions lie on the forward direction and at high energies.

Double pion photoproduction off nuclei – Are there effects beyond final-state interaction?

Photoproduction of π0π0 and π0π± pairs from nuclei has been measured over a wide mass range (2H, 7Li, 12C, 40Ca, and natPb) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4π electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion–pion invariant-mass spectra for heavy nuclei to small invariant masses has been observed for π0 pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the σ-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for σ-in-medium modification.

Measurement of the [formula omitted] Dalitz plot distribution with the WASA detector at COSY

In the first production run of the WASA experiment at COSY, the eta decay into three neutral pions was measured in proton–proton interactions at a proton beam kinetic energy of 1.4 GeV. The Dalitz plot of the three pions was studied using 1.2×105 fully reconstructed events, and the quadratic slope parameter α was determined to be −0.027±0.008(stat)±0.005(syst). The result is consistent with previous measurements and further corroborates the importance of pion–pion final state interactions.

Scalar isoscalar pion pairs in nuclei and theA(π,ππ)Xreaction

The reaction A(pi,pi pi)X has been studied at low energies, paying particular attention to the interaction of the two final pions in the scalar isoscalar (I=J=0) channel. We have developed a microscopic model for the pion production, and then implemented the two pion final state interaction by using the results of a non-perturbative unitary coupled-channels method based in the standard chiral Lagrangians. The resulting model, describes well the reaction on the nucleon for all different isospin channels. Finally, we have considered the reaction in nuclei. Our calculation takes into account Fermi motion, Pauli blocking, pion absorption, and also the strong modification of the pi-pi interaction in the nuclear medium.

Pion reabsorption in heavy-ion collisions interpreted in terms of the Δ capture process

We have measured energy-differential cross sections for π 0 production in 36Ar+ 197Au collisions at 95 MeV/u. From an analysis of spectral features due to pion final-state interactions we have estimated the cross section of the capture process Δ + N → N + N in the center-of-mass energy range s ≃2.05−2.25 GeV . Within the frame of BUU calculations, our results support the extension of the detailed-balance principle to broad-width resonances.

On the surprisingly large Bose-Einstein enhancement observed in e +e -annihilation

The observed Bose-Einstein enhancement e +e − annihilation is well accounted for by a string model, ignoring resonance production and final state interactions. The data are scarcely compatible with the model when resonance production and like pion final state interactions are taken into account, for both these effects suppress the predicted enhancement. I show that multiparticle final state interactions should not be neglected and that isospin zero final state interactions enhance substantially the correlation, compensating for the repulsive isospin 2 effects.

Generalized Multiperipheral Models, Pion Exchange, and the Van der Waals Gas

It is shown that the ABFST (Amati, Bertocchi, Fubini, Stanghellini, and Tonin) multiperipheral model with a resonance kernel possesses an effective cutoff in ${\ensuremath{\sigma}}_{n}(s)$ at a maximum value of $n$ related to lns by ${n}_{max}\ensuremath{-}1=(\frac{1}{b})\mathrm{ln}(\frac{s}{{{m}_{0}}^{2}})$, where $\mathrm{ln}2<b<\mathrm{ln}4$, and $n$ is the number of pion pairs (resonances). It is shown that this is analogous to a Van der Walls-like (VdW) gas (in the Feynman gas analogy) with the parameter $b$ occurring in the equation of state $p(v\ensuremath{-}b)=1$, the pressure $p$ being taken equal to the Pomeron intercept $\ensuremath{\alpha}$, and ${v}^{\ensuremath{-}1}=\frac{〈n〉}{\mathrm{ln}s}$. The choice of $p=\ensuremath{\alpha}$ is shown to correspond to massless-pion exchange. The results are strong-coupling results and cannot be obtained in a weak-coupling (Poisson) limit in which $pv=1$. If the VdW parameter $a$ is introduced via the prescription $(p+\frac{4a}{{v}^{2}}) (v\ensuremath{-}b)=1$, we can establish a relationship with a generalized multiperipheral ABFST model, in which pairwise pion final-state interactions are taken into account in an average, though nontrival, way. We also examine an iterated resonance-plus-diffraction multiperipheral model (the "schizophrenic Pomeron" model), and find similar results for cutoffs, though an explicit analogy with the $a=0$ VdW model is not possible. The introduction of $a\ensuremath{\ne}0$ is shown to mix the resonance and diffractive couplings in this model. In all cases, leading-Regge-pole behavior is obtained if it is assumed that the critical point is not reached.