Pion Bremsstrahlung Research Articles

Dark photons from charged pion bremsstrahlung at proton beam experiments

Dark photons from charged pion bremsstrahlung at proton beam experiments

The ρ → γπ and ω → γπ decays in quark-model approach and estimation of coupling for pion emission by quark

In the framework of the relativistic and gauge invariant spectral integral technique, we calculate radiative decays rho(770)-> gamma pi(140) and omega(780)-> gamma pi(140) supposing all mesons (pi, rho and omega) to be quark-antiquark states. The q anti-q wave functions found for mesons and photon lead to a reasonably good description of data ($\Gamma^{(exp)}_{\rho^{\pm} \to\gamma\pi^{\pm}}=68\pm 30$ keV, $\Gamma^{(exp)}_{\rho^{0}\to\gamma\pi^0}=77\pm 28$ keV, $\Gamma^{(exp)}_{\omega\to\gamma\pi^0}=776\pm 45$ keV) that makes it possible to estimate the coupling for the bremsstrahlung emission of pion by quarks $g_\pi\equiv g_\pi (u\to d \pi)$. We have found two values for the pion bremsstrahlung coupling: $|g_\pi|=16.7 \pm 0.3 ^{+0.1}_{-2.3}$ (Solution I) and $|g_\pi|=3.0 \pm 0.3 ^{+0.1}_{-2.1}$ (Solution II). Within SU(6)-symmetry for nucleons, Solution I gives us for pi NN coupling the value $16.4 \le g_{\pi NN}^2/(4\pi) \le 23.2$ that is in qualitative agreement with the pi N scattering data, $g_{\pi NN}^2/(4\pi)\simeq 14$. For excited states, we have estimated the partial widths in Solution I as follows: $\Gamma (\rho_{2S}^\pm\to \gamma\pi)\simeq 10 - 130$ keV, $\Gamma (\rho_{2S}^0\to \gamma\pi)\simeq 10 -130$ keV, $\Gamma (\omega_{2S}\to \gamma\pi)\simeq 60 - 1080$ keV. The large uncertainties emphasise the necessity to carry out measurements of the meson radiative processes in the region of large masses.

The decays ω → γπ, ρ → γπ, ππ and confinement singularity

We consider the decays of ω and ρ mesons, the states of constituent quark model: ρ → γπ, ω → γπ and ρ → ππ. The wavefunctions used in calculations were found previously in the spectral integral equation approach for meson masses. The performed calculations for the radiative decays lead to a reasonably good description of data keV, keV, keV) that makes it possible to estimate the coupling for the bremsstrahlung emission of pion by quarks q → q + π. We have found two values for the pion bremsstrahlung coupling: |gπ| = 16.7 ± 0.3+0.1 − 2.3 (solution I) and |gπ| = 3.0 ± 0.3+0.1 − 2.1 (solution II). Within the SU(6) symmetry for nucleons, solution I gives us for πNN coupling the value 16.4 ⩽ g2πNN/(4π) ⩽ 23.2 that is in qualitative agreement with the πN scattering data, g2πNN/(4π) ≃ 14. The decay ρ → ππ, being determined by the transition , gives us information on the confinement interactions. One can specify in this decay two types of transitions: (i) the bremsstrahlung radiation of a pion q → q + π with a subsequent fusion , and (ii) the direct transition . With our knowledge of the bremsstrahlung radiation of the pion, gπ, we estimate the amplitude and its role in the confinement mechanism.

Role of resonances inρ0→π+π−γ

We study the effect of the $\ensuremath{\sigma}(600)$ and ${a}_{1}(1260)$ resonances in the ${\ensuremath{\rho}}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\gamma}$ decay within the meson dominance model. Major effects are driven by the mass and width parameters of the $\ensuremath{\sigma}(600)$, and the usually neglected contribution of the ${a}_{1}(1260)$, although small by itself, may become sizable through its interference with pion bremsstrahlung, and the proper relative sign can favor the central value of the experimental branching ratio. We present a procedure, using the gauge invariant structure of the resonant amplitudes, to kinematically enhance the resonant effects in the angular and energy distribution of the photon. We also elaborate on the coupling constants involved.

Charge distribution in soft chiral pion bremsstrahlung

Bremsstrahlung of soft chiral pions in high-energy processes is considered. The distribution over number of neutral and charged pions is shown to obey the inverse square root law. This law is argued to be generic for multiple coherent production of soft chiral pions.

Dilepton production in heavy-ion collisions

The dynamical evolution of nucleus-nucleus collisions is described by a transport equation of the Uehling-Uhlenbeck type. Our model evolves phase-space distribution functions for nucleons, Δ's, N ∗(1440)-resonances and pions with their isospin degrees of freedom. The equations are solved by the test-particle simulation method. We apply this model to proton-nucleus and nucleus-nucleus collisions from 400 MeV/ A to 2.1 GeV/ A. Our approach is found to correctly describe the spectra of baryons and pions. Furthermore, we calculate the production of dileptons considering proton-neutron and pion bremsstrahlung, Δ and N ∗ decay and π +π − annihilation. The calculations reproduce the data for p+ 9Be and for 40Ca + 40Ca at the bombarding energies of 1 GeV/ A and 2 GeV/ A except for small invariant masses ( M⩽ 200 MeV) where the situation is not fully understood. Above 300 MeV dilepton invariant mass the relevant contributions arise from proton-neutron bremsstrahlung, the Δ Dalitz-decay and π + π − annihilation. At 2 GeV/ A for 40Ca + 40Ca, however, we find the bremsstrahlung contribution to be higher than that from pion annihilation.

Theoretical angular distributions from coherent subthreshold pion production.

We have calculated theoretical angular distributions for exclusive ${\ensuremath{\pi}}^{0}$ production in C+C collisions at 84 MeV/nucleon using a quantum-coherent, microscopic, many-body formalism. We use a model spin, transition-spin, \ensuremath{\Delta}-isobar-formation interaction that is expanded in angular momentum multipoles and we now have the results from a complete code that includes these multipoles to all orders which couple to shell-model information. Also, we calculate pion contributions coming from both the target and projectile and find that these contributions are important in determining the shape of the theoretical angular distributions. We compare the theoretical calculations to ${\ensuremath{\pi}}^{0}$ data and find that our results are similar to the shape of the data at higher pion energies and bear some similarity in trend to the pion bremsstrahlung model. We also find that the shape of the theoretical angular distribution is sensitive to shell-model information.

Spin-isospin density fluctuations in nuclear collisions

The possible build-up of the spin-isospin fluctuation strength in nuclear collisions is investigated. In lack of quantum mechanical methods we rely on assumptions usually applied for heavy ion collisions. Based on a fluid dynamical picture we present our ‘isospin coupling in a nucleon cascade’. Within this framework it is possible to describe the mean isospin value as a function of the number of participants in a heavy ion collision and it is possible to study the effect of neutron excess. Since the pion field couples to the spin-isospin density a substantial size of this quantity gives rise to pion bremsstrahlung, the dominant subthreshold pion production mechanism.

Pion bremsstrahlung and critical phenomena in relativistic nuclear collisions

We investigate the dependence of pion bremsstrahlung on the deceleration of the nuclei in relativistic heavy ion collisions. The vicinity of an abnormal phase in nuclear matter can lead - owing to critical N-N scattering - to faster deceleration or decreasing transparency of the nuclei. This would result in a threshold enhancement of the pion bremsstrahlung cross section, which in turn can be used to search for abnormal nuclear states experimentally.