Nuclear Transparency Ratio Research Articles

Color transparency in $${{\bar{p}}} d \rightarrow \pi ^- \pi ^0 p$$ reaction

We consider exclusive two-pion production in antiproton-deuteron interactions at the beam momenta around 10 GeV/c in the kinematics with large momentum transfer in the underlying hard process $\bar p n \to \pi^- \pi^0$. The calculations are performed taking into account the antiproton and pion soft rescattering on the spectator proton in the framework of the generalized eikonal approximation. We focus on the color transparency effect that is modeled by introducing the dependence of rescattering amplitudes on the relative position of the struck and spectator nucleons along the momentum of a fast particle. As a consequence of the interplay between the impulse approximation and rescattering amplitudes the nuclear transparency ratio reveals a pretty complicated behaviour as a function of the transverse momentum of the spectator proton and the relative azimuthal angle between the $\pi^-$-meson and the proton. Color transparency significantly suppresses rescattering amplitudes which leads to substantial modifications of the nuclear transparency ratio moving it closer to the value obtained in the impulse approximation. By performing the Monte-Carlo analysis we determine that this effect can be studied at PANDA with a reasonable statistics.

Sub-threshold strangeness and charm production in UrQMD

We present recent results on the sub-threshold production of strange and charmed hadrons in nuclear collisions. In particular we highlight how the excitation and decay of heavy baryonic resonances can be used to describe the production yield of ϕ mesons at the SIS18 accelerator and show how this production mechanism also consistently describes the ϕ nuclear transparency ratio. Including even more massive baryonic resonances in the model we are able to extend our approach and make, for the first time, realistic predictions on the sub-threshold production of J/Ψ and Λc in nuclear collisions at the SIS100 accelerator at FAIR. We find that even at a beam energy of Elab = 6 A GeV, charm physics is feasible at the CBM experiment, which opens up new opportunities to study charmed hadron properties at large baryon densities.

Modification of the strange meson properties in nuclear matter

To make progress in understanding the properties of the strange mesons with open ( K − ) and hidden ( ϕ ) strangeness in the nuclear medium, the measurements of the production of K + K − pairs with invariant masses corresponding to both the ϕ and non- ϕ regions in proton collisions with C, Cu, Ag, and Au targets at an incident beam energy of 2.83 GeV were recently performed by the ANKE Collaboration at COSY. The data from the ϕ region of invariant masses provide information on the momentum dependence of the ϕ nuclear transparency ratio and the in-medium ϕ meson width over the momentum range of 0.6-1.6 GeV/c. Analysis of the data from the non- ϕ region of K + K − invariant masses, where differential cross sections for pair production on the four targets were obtained as functions of the laboratory K − momentum, allows to draw the conclusion of the strength of the K − -nuclear potential.

Sub-threshold ϕ and production by high mass resonances with UrQMD

We present a possible explanation for the deep sub-threshold, ϕ and production yields measured with the HADES experiment in Ar+KCl reactions at A GeV and present predictions for Au+Au reactions at A GeV. To explain the surprisingly high yields of ϕ and hadrons we propose new decay channels for high mass baryon resonances. These new decay channels are constrained by elementary cross sections, and production in p+Nb. Based on the fits to the elementary reactions one obtains a satisfactorily description of ϕ and production in deep sub-threshold Ar+KCl reactions as well as the observed nuclear transparency ratio in proton-induced ϕ production in cold nuclear matter. The results implicate that no new medium effects are required to describe the rare strange particle production data in low energy nuclear collisions.

Momentum dependence of theφ-meson nuclear transparency

The production of phi mesons in proton collisions with C, Cu, Ag, and Au targets has been studied via the phi -> K+K- decay at an incident beam energy of 2.83 GeV using the ANKE detector system at COSY. For the first time, the momentum dependence of the nuclear transparency ratio, the in-medium phi width, and the differential cross section for phi meson production at forward angles have been determined for these targets over the momentum range of 0.6 - 1.6 GeV/c. There are indications of a significant momentum dependence in the value of the extracted phi width, which corresponds to an effective phi-N absorption cross section in the range of 14 - 21 mb.

Recent results onpp→ppϕ/K+K−production and the momentum dependence ofϕ-meson nuclear transparency

Kaon pair production by 2.83 GeV protons has been investigated in two experiments using the ANKE magnetic spectrometer at COSY-Julich: (1) Differential and total cross sections for the pp → ppK + K − reaction are sep- arated into kaon pairs arising from the decay of the φ-meson and the remainder. The measurements show that higher partial waves represent the majority of the pp → ppφ cross section at an excess energy of 76 MeV. Taken together with existing low energy ANKE data, the energy dependence of the results seem to suggest some S-wave φp enhancement near threshold. The non- φK + K − data can be described in terms of the combined effects of two-body final state inter- actions, using the same effective scattering parameters already determined from lower energy ANKE data. (2) The inclusive production of φ mesons on C, Cu, Ag, and Au targets has also been studied using the K + K − decay to detect the φ. The momentum de- pendence of the nuclear transparency ratio, the in-medium φ width, and the differential cross section for φ meson production at forward angles have been determined for these nuclei in the momentum range of 0.6 - 1.6 GeV/c .T here are indications of a significant momentum dependence in the values extracted for the φ width, which on average exceed the free value by about an order of mag- nitude. The results are compared with data from other experiments at slightly higher momenta.

In-mediumϕmeson width extracted from proton-nucleus collisions

The inclusive production of ϕ mesons at small angles in proton collisions with C, Cu, Ag, and Au targets has been measured at an incident energy of 2.83 GeV at the ANKE-COSY facility. The ϕ mesons were registered via the ϕ → K + K − decay. The momentum dependence of the nuclear transparency ratio, the in-medium ϕ width, and the differential cross section for ϕ production have been determined in the momentum region p ϕ = 0.6 − 1.6 GeV/c . Comparison with different model calculations suggests a significant broadening of the in-medium ϕ width for normal nuclear density with evidence for a momentum dependence.

Medium modifications of theρmeson in nuclear photoproduction

We extend our recent study of dilepton invariant-mass spectra from the decays of $\ensuremath{\rho}$ mesons produced by photon reactions off nuclei. We specifically focus on experimental spectra as recently measured by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility using carbon and iron nuclei. Building on our earlier work, we broaden our description to a larger set of observables to identify sensitivities to the medium effects predicted by microscopic calculations of the $\ensuremath{\rho}$ spectral function. We compute mass spectra for several target nuclei and study the spectral shape as a function of the three-momentum of the outgoing lepton pair. We also compute the so-called nuclear transparency ratio, which provides an alternative means (and thus consistency check) of estimating the $\ensuremath{\rho}$ width in the cold nuclear medium.

Measurement of the incoherent [formula omitted] photoproduction near threshold

We report measurements of differential cross sections and decay asymmetries of incoherent ϕ-meson photoproduction from the deuteron at forward angles using linearly polarized photons at E γ = 1.5 – 2.4 GeV . The nuclear transparency ratio for the deuteron shows a large suppression, and is consistent with the A-dependence of the ratio observed in a previous measurement with nuclear targets. The reduction for the deuteron cannot be adequately explained in term of isospin asymmetry. The present results suggest the need of refining our understanding of the ϕ–N interaction within a nucleus.

ω attenuation in nuclei

We study inclusive ω photoproduction in nuclei and propose a measurement of the nuclear transparency ratio as a means to learn about the in-medium properties of the ω meson. To this end we are using the semi-classical coupled-channel BUU transport approach that allows for a consistent treatment of all nuclear effects. The conditions of our calculations are chosen such as to match the setup of existing experimental facilities at ELSA or MAMI C. We show that the observables (total production cross section and transparency) indeed are sensitive to the ωN interaction in the nuclear medium.

Diffractive exclusive electroproduction of ϱ0 mesons at HERMES

Recent HERMES measurements are presented on the diffractive slopes of the coherent and incoherent part of the cross section for exclusive diffractive production of ϱ 0 mesons. Based on data taken with a variety of targets, 1 H, 3 He, 14 N, 20 Ne and Krypton, the method to extract the coherent-to-incoherent cross section ratios in dependence of the coherence length l c and momentum transfer Q 2 is presented. These measurements provide important information about the interaction of quark-antiquark pairs with the nuclear medium. Using 2 H and 14 N targets, the nuclear transparency and coherent cross section ratio of ϱ 0 production has been measured versus coherence length and Q 2 . The data have been checked for a possible onset of color transparency and are compared with recent theoretical predictions.