Fermi Motion Research Articles

Medium effects in λK+ pair production by 2.83 GeV protons on nuclei**Supported by the Ministry of Education and Science of the Russian Federation

We study ΛK+ pair production in the interaction of protons of 2.83 GeV kinetic energy with C, Cu, Ag, and Au target nuclei in the framework of the nuclear spectral function approach for incoherent primary proton–nucleon and secondary pion–nucleon production processes, and processes associated with the creation of intermediate Σ0K+ pairs. The approach accounts for the initial proton and final Λ hyperon absorption, final K+ meson distortion in nuclei, target nucleon binding, and Fermi motion, as well as nuclear mean-field potential effects on these processes. We calculate the Λ momentum dependence of the absolute ΛK+ yield from the target nuclei considered, in the kinematical conditions of the ANKE experiment, performed at COSY, within the different scenarios for the Λ-nucleus effective scalar potential. We show that the above observable is appreciably sensitive to this potential in the low-momentum region. Therefore, direct comparison of the results of our calculations with the data from the ANKE-at-COSY experiment can help to determine the above potential at finite momenta. We also demonstrate that the two-step pion–nucleon production channels dominate in the low-momentum ΛK+ production in the chosen kinematics and, therefore, they have to be taken into account in the analysis of these data.

Neutrino-nucleus cross sections in 12C and 40Ar with KDAR neutrinos

High intensity monoenergetic muon neutrinos of energy 236 MeV from the kaon decay at rest at the medium energy proton accelerator facilities like J-PARC and Fermilab are proposed to be used for making precision measurements of neutrino-nucleus cross sections in and and perform neutrino oscillation experiments in and modes. In view of these developments, we study the theoretical uncertainties arising due to the nuclear medium effects in the neutrino-nucleus cross sections as well as in the angular and energy distributions of the charged leptons produced in the charged current induced reactions by and in and in the energy region of MeV. The calculations have been done in a microscopic model using the local density approximation which takes into account the nuclear effects due to the Fermi motion, binding energy and long range correlations. The results are compared with the other calculations available in the literature.

Neutrino-nucleus cross-sections at supernova neutrino energies

The inclusive neutrino/antineutrino-induced charged and neutral current reaction cross-sections in [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] in the energy region of supernova neutrinos/antineutrinos are studied. The calculations are performed in the local density approximation (LDA) taking into account the effects due to Pauli blocking, Fermi motion and the renormalization of weak transition strengths in the nuclear medium. The effect of Coulomb distortion of the lepton produced in the charged current reactions has also been included. The numerical results for the energy dependence of the cross-section [Formula: see text] as well as the flux averaged cross-section and event rates for the charged lepton production in the case of some supernova neutrino/antineutrino fluxes recently discussed in the literature have been presented. We have also given the flux-averaged angular and energy distributions of the charged leptons corresponding to these fluxes.

First measurement of the polarization observable E and helicity-dependent cross sections in single π0 photoproduction from quasi-free nucleons

The double-polarization observable E and the helicity-dependent cross sections σ1/2 and σ3/2 have been measured for the first time for single π0 photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the π0 meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the π0N final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry E since the σ1/2 and σ3/2 components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.

Global study of nuclear modifications on parton distribution functions

A global analysis of nuclear medium modifications of parton distributions is presented using deeply inelastic scattering data of various nuclear targets. Two obtained data sets are provided for quark and gluon nuclear modification factors, referred as nIMParton16. One is from the global fit only to the experimental data of isospin-scalar nuclei (Set A), and the other is from the fit to all the measured nuclear data (Set B). The scale-dependence is described by DGLAP equations with nonlinear corrections in this work. The Fermi motion and off-shell effect, nucleon swelling, and parton–parton recombination are taken into account together for modeling the complicated x-dependence of nuclear modification. The nuclear gluon shadowing in this paper is dynamically generated by the QCD evolution of parton splitting and recombination processes with zero gluon density at the input scale. Sophisticated nuclear dependence of nuclear medium effects is studied with only two free parameters. With the obtained free parameters from the global analysis, the nuclear modifications of parton distribution functions of unmeasured nuclei can be predicted in our model. Nuclear modification of deuteron is also predicted and shown with recent measurement at JLab.

Nuclear medium effects in Drell–Yan process

We study the nuclear medium effects in Drell–Yan process using quark parton distribution functions calculated in a microscopic nuclear model which takes into account the effects of Fermi motion, nuclear binding and nucleon correlations through a relativistic nucleon spectral function. The contributions of π and ρ mesons as well as shadowing effects are also included. The beam energy loss is calculated using a phenomenological approach. The present theoretical results are compared with the experimental results of the E772 and E866 experiments. These results are applicable to the forthcoming experimental analysis of E906 Sea Quest experiment at the Fermi Lab.

Space-time development of in-medium hadronization : scenario for leading hadrons

Lepton deep inelastic scattering off nuclei at medium energies gives the opportunity to study the space-time development of hadronization. Indeed, for these kinematics, the production length is comparable to the nuclear size. Based on the Berger model [1] and dipole phenomenology, we built a model for vacuum and in-medium hadronization. The model, which includes vacuum energy loss, induced energy loss and nuclear absorption in a parameter-free way successfully describes Hermes data [2, 3]. In a future publication, Fermi motion will be taken into account and the model will be applied to CLAS Eg2 data, at Jefferson laboratory.

Nuclear parton distributions

We review a microscopic model of the nuclear parton distribution functions, which accounts for a number of nuclear effects including Fermi motion and nuclear binding, nuclear meson-exchange currents, off-shell corrections to bound nucleon distributions and nuclear shadowing. We also discuss applications of this model to a number of processes including lepton-nucleus deep inelastic scattering, proton-nucleus Drell-Yan lepton pair production at Fermilab, as well as W+- and Z0 boson production in proton-lead collisions at the LHC.

Probing the low and high density nuclear matter by hadron scattering

The subject of this report is connected with the widely discussed manifestation of in-medium effect in nuclear collisions. At present, the medium modification of hadron properties in the normal and in a dense nuclear matter is one of the fundamental questions of strong interaction physics. A new Glauber Monte Carlo model for hadron–nuclei interaction at intermediate energy is proposed. We utilized the principal assumptions as in the approaches of other authors describing nuclear collisions at high energy in the framework of the models without QGP. Yet, a number of new ingredients (noneikonal corrections, correlations of nucleons, in the nuclei, the nuclear Fermi motion, etc.) are introduced.

Near-threshold J/ψ-meson photoproduction on nuclei

On the basis of the first-collision model that relies on the nuclear spectral function and which includes incoherent processes involving charmonium production in proton–nucleon collisions, the photoproduction of J/ψ mesons on nuclei is considered at energies close to the threshold for their production on a nucleon. The absorption of final J/ψ mesons, their formation length, and the binding and Fermi motion of target nucleons are taken into account in this model along with the effect of the nuclear potential on these processes. The A dependences of the absolute and relative charmonium yields are calculated together with absolute and relative excitation functions under various assumptions on the magnitude of the cross section for J/ψN absorption, the J/ψ-meson formation length, and their inmedium modification. It is shown that, at energies above the threshold, these features are virtually independent of the formation length and the change in the J/ψ-meson mass in nuclear matter but are rather highly sensitive to the cross section for J/ψN interaction. The calculations performed in the present study can be used to determine the unknown cross section for J/ψ-meson absorption in nuclei from a comparison of their results with data expected from experiments in the Hall C of the CEBAF (USA) facility upgraded to the energy of 12 GeV. It is also shown that the absolute and relative excitation functions for J/ψ mesons in photon–nucleus reactions at subthreshold energies are sensitive to the change in the meson mass and, hence, carry information about the properties of charmonium in nuclear matter.

High-energy proton emission and Fermi motion in intermediate-energy heavy-ion collisions

An antisymmetrized molecular dynamics model (AMD-FM), modified to take into account the Fermi motion explicitly in its nucleon-nucleon collision process, is presented. Calculated high-energy proton spectra are compared with those of $^{40}\mathrm{Ar}+^{51}\mathrm{V}$ at 44 MeV/nucleon from Coniglione et al. [Phys. Lett. B 471, 339 (2000)] and those of $^{36}\mathrm{Ar}+^{181}\mathrm{Ta}$ at 94 MeV/nucleon from Germain et al. [Nucl. Phys. A 620, 81 (1997)]. Both of the experimental data are reasonably well reproduced by the newly added Fermi boost in the nucleon-nucleon collision process without additional processes, such as a three-body collision or a short-range correlation. The production mechanism of high-energy protons in intermediate-energy heavy-ion collisions is discussed.

Modelling the nuclear parton distributions

We review a semi-microscopic model of nuclear parton distributions, which takes into account a number of nuclear effects including Fermi motion and nuclear binding, nuclear meson-exchange currents and off-shell corrections to bound nucleon distributions as well as nuclear shadowing effect. We also discuss applications of the model to the lepton-nuclear deep- inelastic scattering, Drell-Yan process and neutrino total cross sections.

Near-threshold incoherentϕphotoproduction on the deuteron: Searching for traces of a resonance

We study the near-threshold incoherent $\phi$ photoproduction on the deuteron based on a model of $\gamma N \to \phi N$, consisting of Pomeron, $(\pi, \eta)$ exchanges, and a $J^P = 3/2^-$ resonance, which describes the low energy $\gamma p \to \phi p$ LEPS data well, including the peak in the forward differential cross section. The calculation is done up to double rescatterings, with the spin dependence of the elementary $\gamma N \to \phi N$ amplitude retained throughout the calculation. The Fermi motion and final-state interactions (FSI) are all properly treated as prescribed by realistic nucleon-nucleon interaction. The couplings of the resonance to $\gamma n$ and $\phi n$ channels are estimated with the help of a constituent quark model. The main features of the LEPS and CLAS data are described reasonably well except for some quantitative discrepancies at very low energies and low momentum transfers regions. It is found that contributions of Fermi motion, $pn$ FSI, and resonance are all indispensable in bridging the differences between the single-scattering results and the data. The off-shell rescattering is found to be important as it cancels out a large portion of the on-shell contribution. The discrepancies at low momentum transfer region might be related to the binning size of the data. No peak is found to be associated with the weak resonance as it gets smeared out by the Fermi motion and FSI with deuterium target. The problem at very low energy region hints at the possible contributions from other mechanisms and should be investigated in depth with the use of recent high statistics $\gamma p \to \phi p$ data from CLAS.

Nuclear medium effects in [formula omitted] and [formula omitted] structure functions

Recent phenomenological analysis of experimental data on DIS processes induced by charged leptons and neutrinos/antineutrinos beams on nuclear targets by CTEQ collaboration has confirmed the observation of CCFR and NuTeV collaborations, that weak structure function F2AWeak(x,Q2) is different from electromagnetic structure function F2AEM(x,Q2) in a nucleus like iron, specially in the region of low x and Q2. In view of this observation we have made a study of nuclear medium effects on F2AWeak(x,Q2) and F2AEM(x,Q2) for a wide range of x and Q2 using a microscopic nuclear model. We have considered Fermi motion, binding energy, nucleon correlations, mesonic contributions from pion and rho mesons and shadowing effects to incorporate nuclear medium effects. The calculations are performed in a local density approximation using a relativistic nucleon spectral function which includes nucleon correlations. The numerical results in the case of iron nucleus are compared with the experimental data.

Near threshold production of η-mesons in proton neutron collisions at ANKE

The interaction between hadrons and η-mesons is an intensively studied topic. Due to its stength it might lead to the formation of η-mesic nuclei. In order to study the characteristics of this interaction a measurement of the reaction p + d → d + η + p sp has been performed at the ANKE spectrometer at the COSY accelerator of the Forschungszentrum Julich. The two different beam momenta ( p 1 = 2.09 GeV/c and p 2 = 2.25 GeV/c) in combination with the Fermi motion inside the target deuteron grant access to the determination of total and differential cross sections in an excess energy range from threshold up to Q = 90 MeV. While the course of the total cross section, especially near threshold, will allow to extract the scattering length a dη via an s-wave final state interaction ansatz, the differential cross sections permit to verify the legitimacy of the s-wave assumption.

Nuclear medium effects in structure functions of nucleon at moderate [formula omitted

Recent experiments performed on inclusive electron scattering from nuclear targets have measured the nucleon electromagnetic structure functions F1(x,Q2), F2(x,Q2) and FL(x,Q2) in 12C, 27Al, 56Fe and 64Cu nuclei. The measurements have been done in the energy region of 1 GeV2<W2<4 GeV2 and Q2 region of 0.5 GeV2<Q2<4.5 GeV2. We have calculated nuclear medium effects in these structure functions arising due to the Fermi motion, binding energy, nucleon correlations, mesonic contributions from pion and rho mesons and shadowing effects. The calculations are performed in a local density approximation using a relativistic nucleon spectral function which includes nucleon correlations. The numerical results are compared with the recent experimental data from JLab and also with some earlier experiments.

Antihyperon potentials in nuclei via exclusive antiproton–nucleus reactions

The exclusive production of hyperon–antihyperon pairs close to their production threshold in p‾-nucleus collisions offers a unique and hitherto unexplored opportunity to elucidate the behavior of antihyperons in nuclei. For the first time we analyze these reactions in a microscopic transport model using the Gießen Boltzmann–Uehling–Uhlenbeck transport model. The calculation takes the delicate interplay between the strong absorption of antihyperons, their rescattering and mean field deflection as well as the Fermi motion of the struck nucleon into account. We find a substantial sensitivity of transverse momentum correlations of coincident Λ‾Λ-pairs to the assumed depth of the Λ‾-potential. Because of the high cross section for this process and the simplicity of the experimental method our results are highly relevant for future activities at the international Facility for Antiproton and Ion Research (FAIR).

Non-resonant kaon pair production and medium effects in proton–nucleus collisions

We study the non-resonant (non-ϕ) production of pairs by protons of 2.83 GeV kinetic energy on C, Cu, Ag and Au targets within the collision model, based on the nuclear spectral function, for incoherent primary proton–nucleon and secondary pion–nucleon creation processes. The model takes into account the initial proton and final kaon absorption, target nucleon binding and Fermi motion as well as nuclear mean-field potential effects on these processes. We calculate the antikaon momentum dependences of the exclusive absolute and relative pair yields in the acceptance window of the ANKE magnetic spectrometer, used in a recent experiment performed at COoler SYnchrotron (COSY), within the different scenarios for the antikaon–nucleus optical potential. We demonstrate that the above observables are strongly sensitive to this potential. Therefore, they can be useful to help determine the optical potential from the direct comparison of the results of our calculations with the data from the respective ANKE-at-COSY experiment. We also show that the pion–nucleon production channels dominate in the low-momentum , production in the considered kinematics and, hence, they have to be accounted for in the analysis of these data.

Parity violating asymmetry with nuclear medium effects in deep inelastic [formula omitted] scattering

Recently parity violating asymmetry (APV) in the Deep Inelastic Scattering (DIS) of polarized electrons (e→) from deuterons has been measured at JLab and there exist future plans to measure this asymmetry from nuclear targets. In view of this we study nuclear medium effects in APV in the DIS of e→ from some nuclear targets like 12C, 56Fe and 208Pb. The effects of Fermi motion, binding energy and nucleon correlations are taken into account through the nucleon spectral function calculated in a local density approximation for nuclei. The pion and rho cloud contributions have also been taken into account. This model has been earlier used to study nuclear medium effects in the electromagnetic and weak interaction induced processes. The results are presented and discussed in view of the future JLab experiments.

Investigating of the exclusive reaction of π+π− pair electroproduction on a proton bound in a deuteron

Preliminary results from analyzing π+π− pair electroproduction on a proton bound in a deuteron are presented. Procedures for considering the Fermi motion of the initial proton in the deuteron and assessing the effects of interaction in the final states are developed. The yield of the reaction ep(n) → e′p′(n′)π+π− is obtained for the first time.