arXiv subjects

P. Muehlich

Publications and source records attributed to P. Muehlich.

12 recordsLinked to original sources

arXiv2008

Photoproduction of eta-mesons off nuclei for Eg < 2.2 GeV

Photoproduction of $η$ mesons off $^{12}$C, $^{40}$Ca, $^{93}$Nb, and $^{nat}$Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2 GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S$_{11}$(1535) nucleon resonance and the study of the absorption properties of nuclear matter for $η$ mesons. Careful consideration was given to contributions from $ηπ$ final states and secondary production mechanisms of $η$-mesons e.g. from inelastic $πN$ reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross section $σ_{Nη}$ for $η$ mesons is constant over a wide range of the $η$ momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU-model show no unexplained in-medium modifications of the S$_{11}$(1535).

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arXiv2008

Light Vector Mesons in the Nuclear Medium

The light vector mesons ($ρ$, $ω$, and $ϕ$) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the $ρ$ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to $e^{+}e^{-}$. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The $ρ$ meson mass spectrum was extracted after the $ω$ and $ϕ$ signals were removed in a nearly model-independent way. Comparisons were made between the $ρ$ mass spectra from the heavy targets ($A > 2$) with the mass spectrum extracted from the deuterium target. With respect to the $ρ$-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.

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arXiv2007

Search for medium modification of the $ρ$ meson

The photoproduction of vector mesons on various nuclei has been studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory. The vector mesons, $ρ$, $ω$, and $ϕ$, are observed via their decay to $e^+e^-$, in order to reduce the effects of final state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the $ρ$ meson. The $ρ$ spectral function is extracted from the data on various nuclei, carbon, iron, and titanium, and compared to the spectrum from liquid deuterium, which is relatively free of nuclear effects. We observe no significant mass shift for the $ρ$ meson; however, there is some widening of the resonance in titanium and iron, which is consistent with expected collisional broadening.

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arXiv2006

The spectral function of the omega meson in nuclear matter from a coupled-channel resonance model

We calculate the spectral function of the omega meson in nuclear matter at zero temperature by means of the low-density theorem. The omega N forward scattering amplitude is calculated within a unitary coupled-channel effective Lagrangian model that has been applied successfully to the combined analysis of pion- and photon-induced reactions. While the peak of the omega spectral distribution is shifted only slightly, we find a considerable broadening of the omega meson due to resonance-hole excitations. For omega mesons at rest with respect to the surrounding nuclear medium, we find an additional width of about 60 MeV at saturation density.

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arXiv2006

Omega attenuation in nuclei

We study inclusive omega photoproduction in nuclei and propose a measurement of the nuclear transparency ratio as a means to learn about the in-medium properties of the omega 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 omega N interaction in the nuclear medium.

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arXiv2005

Attenuation of phi mesons in gamma A reactions

We present a theoretical analysis of inclusive photoproduction of phi mesons in nuclei. In particular the dependence of the total phi meson yield on the target mass number is investigated. The calculations are done using the semi-classical BUU transport approach that combines the initial state interaction of the incoming photon with the coupled-channel dynamics of the final state particles. The conditions of the calculations are chosen such as to match the set up of a recent experiment performed at SPring8/Osaka. Whereas the observables prove to be rather sensitive to the phi self energy in the medium, the attribution of deviations from the standard scenario to a particular in-medium effect seems to be impossible.

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arXiv2005

In-Medium properties of the omega meson in photon induced nuclear reactions

We discuss the possibility to study the in-medium changes of the properties of the omega meson in reactions on ordinary nuclei with elementary electromagnetic probes. We present a tree-level calculation of the elementary gamma p -> omega p process which is extended to describe also the photoproduction of medium-modified omega mesons in nuclear matter. Using a semi-classical transport approach we obtain results for e+e- and pi0 gamma photoproduction off heavy nuclei in the invariant mass range of the rho and omega mesons. Both reactions are also studied experimentally and are presently being analyzed at accelerator facilities in Bonn and at Jefferson Lab. We show that the in-medium signals expected can be as large as those obtained in heavy-ion reactions.

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arXiv2005

In-Medium Properties of Hadrons - Observables II

In this review we discuss the observable consequences of in-medium changes of hadronic properties in reactions with elementary probes, and in particular photons, on nuclei. After an outline of the theoretical method used we focus on a discussion of actual observables in photonuclear reactions; we discuss in detail $2π$- and vector-meson production. We show that the $2π^0$ photoproduction data can be well described by final state interactions of the pions produced whereas the semi-charged $π^0π^\pm$ channel exhibits a major discrepancy with theory. For $ω$ production on nuclei in the TAPS/CB@ELSA experiment we analyse the $π^0γ$ decay channel, and illustrate the strength of the method by simulating experimental acceptance problems. Completely free of final state interactions is dilepton production in the few GeV range. We show that the sensitivity of this decay channel to changes of hadronic properties in medium in photonuclear reactions on nuclei is as large as in ultrarelativistic heavy ion collisions and make predictions for the on-going G7 experiment at JLAB. Finally we discuss that hadron production in nuclei at 10 -- 20 GeV photon energies can give important information on the hadronization process, and in particular on the time-scales involved. We show here detailed calculations for the low-energy (12 GeV) run at HERMES and predictions for planned experiments at JLAB.

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arXiv2004

In-medium properties of the omega meson through omega photoproduction in nuclei

The feasibility to study the in-medium properties of the omega meson in photon induced reactions on nuclei is investigated by means of a semiclassical transport model. The calculations are dedicated to an experiment presently being analyzed by the TAPS/Crystal Barrel collaborations. A full coupled-channel treatment of final state interactions allows a quantitative analysis of the influence of absorption and elastic scattering of the produced hadrons and likewise the application of kinematic cuts on the observables. Within the scope of our calculations, the preliminary data by the TAPS/Crystal Barrel collaborations indeed seem to indicate a modification of the isoscalar spectral density in the strongly interacting environment.

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arXiv2003

In-medium Properties of Hadrons -- Observables

We first briefly review the theoretical basis for calculations of changes of hadronic properties in dense nuclear matter. These changes have usually been investigated by means of relativistic heavy-ion reactions. Here we discuss that observable consequences of such changes can also be seen in more elementary reactions on nuclei. Particular emphasis is put on a discussion of actual observables in photonuclear reactions; we discuss in detail $η$- and vector-meson production. We show that photoproduction of $η$'s can yield essential information on in-medium properties of the $S_{11}(1535)$ resonance while the $ϕ$ meson properties will probably not be accessible through the $K^+K^-$ decay channel. However, for $ω$ mesons the $π^0γ$ decay channel, due to its reduced final state interaction, looks more promising in this respect. Completely free of final state interactions is dilepton production in the few GeV range. We show that the sensitivity of this decay channel to changes of hadronic properties in medium in photonuclear reactions on nuclei is as large as in ultrarelativistic heavy ion collisions. Finally we discuss that hadron production in nuclei at 10 -- 20 GeV photon energies can give important information on the hadronization process.

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arXiv2003

Inclusive omega photoproduction off nuclei

We investigate inclusive omega photoproduction off complex nuclei, concentrating on the feasibility to examine a possible in-medium change of the omega meson properties by observing the pi^0 gamma invariant mass spectrum. The simulations are performed by means of a BUU transport model including a full coupled-channel treatment of the final state interactions. In-medium changes of the omega spectral density are found to yield a moderate modification of the observables as compared to the situation in free space. Also the effects of a momentum dependence of the strong omega potential are discussed.

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arXiv2002

Photoproduction of phi mesons from nuclei

We investigate the consequences of possible medium modifications of the phi meson at finite nuclear matter density on the K+K- mass distribution in photonuclear reactions. The inclusive cross sections for K+K- pair production are calculated within a semi--classical BUU transport model, which combines the initial state interaction of the incoming photon with the final state interactions of the produced particles. The effects of final state interactions on the invariant mass distribution of the observed K+K- pairs are discussed in detail. In addition we calculate the Coulomb correction and possible effects of hadronic kaon potentials on the K+K- mass spectrum. Due to the large cross sections for reactions of the final state particles with the surrounding nuclear medium and the influence of the Coulomb potential we find no measurable sensitivity of the observables to the phi properties at finite baryon density.

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