Quasi-free Processes Research Articles

Measurement of polarization transfer in the quasi-elastic [formula omitted] process

Polarization transfer to a bound proton in polarized electron knock-out reactions, A(e→,e′p→), is a powerful tool to look for an in-medium modification of the bound proton. It requires comparison to calculations that consider the many-body effects accompanying the quasi-free process. We report here measured components Px′, Pz′, and their ratio Px′/Pz′, of polarization transfer to protons bound in Ca40, which is described well by the shell model and for which reliable calculations are available. While the calculations capture the essence of the data, our statistical precision allows us to observe deviations that cannot be explained by simple scaling, including by varying the proton electromagnetic form factor ratio GE/GM. We further explore the deviations of the ratio of the polarization transfer components from that of a free proton, (Px′/Pz′)A/(Px′/Pz′)H, and its dependence on the bound-proton virtuality.

Differential cross sections for neutron-proton scattering in the region of the d*(2380) dibaryon resonance

Differential cross sections have been extracted from exclusive and kinematically complete high-statistics measurements of quasifree polarized n⃗p scattering performed in the energy region of the d*(2380) dibaryon resonance covering the range of beam energies Tn=0.98−1.29 GeV (s=2.32−2.44 GeV). The experiment was carried out with the WASA-at-COSY setup having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process dp→np+pspectator. In this way the np differential cross section σ(Θ) was measured over a large angular range. The obtained angular distributions complement the corresponding analyzing power Ay(Θ) measurements published previously. A SAID partial-wave analysis incorporating the new data strengthens the finding of a resonance pole in the coupled D33−G33 waves.Received 3 April 2020Accepted 9 June 2020DOI:https://doi.org/10.1103/PhysRevC.102.015204Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNucleon induced nuclear reactionsNucleon-nucleon interactionsParticle interactionsPhysical SystemsBaryonsHadronsParticles & FieldsNuclear Physics

“K−pp”, a [formula omitted]-meson nuclear bound state, observed in 3He(K−,Λp)n reactions

We observed a distinct peak in the Λp invariant mass spectrum of He3(K−,Λp)n, well below mK+2mp, i.e., the mass threshold of the K− to be bound to two protons. By selecting a relatively large momentum-transfer region q=350∼650 MeV/c, one can kinematically separate the peak from the quasi-free process, K‾N→K‾N followed by the non-resonant absorption by the two spectator-nucleons K‾NN→ΛN. We found that the simplest fit to the observed peak gives us a Breit–Wigner pole position at BKpp=47±3(stat.)−6+3(sys.) MeV having a width ΓKpp=115±7(stat.)−20+10(sys.) MeV, and the S-wave Gaussian reaction form-factor parameter QKpp=381±14(stat.)−0+57(sys.) MeV/c, as a new form of the nuclear bound system with strangeness – “K−pp”.

Exclusive $$\varvec{\pi ^{-}}$$ Electroproduction Off the Neutron in Deuterium in the Resonance Region

The goal of our research is to provide the exclusive \({\gamma }^{*}n(p) \rightarrow p{\pi }^{-} (p)\) reaction cross section from deuterium data. The existing \({\gamma }^{*}n \rightarrow p{\pi }^{-}\) event generator based on MAID (Drechsel et al. in Nucl Phys A 645:145, 1999) is modified to include the spectator (proton) information based via the CD-Bonn potential (Entem and Machleidt in Phys Rev C 68:041001, 2003) to simulate the quasi-free production process. With this method, the exclusive quasi-free process has been isolated successfully as demonstrated by the comparison of the spectator momentum distribution of the simulation with the missing momentum distribution of the data, and a kinematical final-state-interaction contribution factor \(R_{FSI}\) is extracted directly from the data according to the ratio between the exclusive quasi-free and full cross sections. Furthermore, the corresponding structure functions are extracted from those cross sections as well. The experiment was done in Hall B at the Thomas Jefferson National Laboratory (JLab) by using the CEBAF Large Acceptance Spectrometer (CLAS) detector, the “e1e” run data off a liquid deuterium target will provide final results with a kinematic coverage for the hadronic invariant mass (W) up to \(1.825\,\text {GeV}\) and in the momentum transfer (\(Q^{2}\)) range of 0.4–\(1.0\,\text {GeV}/c^{2}\).

Proton distributions in dp dielectron production within Regge theory

The processes of dielectron production in deuteron–proton collisions at intermediate incident deuteron beam energies are analyzed in the spectator model within the one-pion exchange reggeized approach. We focus mainly on the momentum and angle distributions of the proton-spectator and the proton emitted in quasi-free NN processes at small angles in the laboratory frame. It is shown that the inclusion of many channels in quasi-free NN interaction allows us to describe the HADES data quite satisfactorily at incident deuteron kinetic energies of about 2.5 GeV.

Beam-energy dependence and updated test of the Trojan-horse nucleus invariance via a measurement of the H2(d,p)H3 reaction at low energies

The $\mathrm{^2H}(d,p)\mathrm{^3H}$ bare nucleus astrophysical S(E) factor has been measured indirectly at energies from about 500 keV down to several keV by means of the Trojan-horse method applied to the quasi-free process $\mathrm{^2H({}^6Li},pt)\mathrm{^4He}$ induced at the lithium beam energy of 11 and 9.5 MeV, which makes the virtual binary process incident energy $\mathrm {E}_{dd}^{qf}$ go much closer to the zero-quasi-free-energy point than that in the previous similar experiment. The obtained results are compared with direct data as well as with previous indirect investigation of the same binary reactions. It shows that the precision of S(E) data in low energy range extracted via the same Trojan horse nucleus ($\mathrm{^6Li}=(d \oplus \alpha)$ ) becomes better when the incident energy decreases from high value down to the zero-quasi-free-energy point. The very good agreement between data extracted from different Trojan horse nucleus ($\mathrm{^6Li}=(d \oplus \alpha)$ vs. $\mathrm{^3He}=(d \oplus p)$) gives a strong updated test for the independence of the binary indirect cross section on the chosen Trojan horse nucleus at low energies.

Measurement of theH2(d,p)H3reaction at astrophysical energies via the Trojan-horse method

The study of d(d,p)t reaction is very important for the nucleosynthesis in both standard Big Bang and stellar evolution, as well as for the future fusion reactors planning of energy production. The d(d,p)t bare nucleus astrophysical S(E) factor has been measured indirectly at energies from about 400 keV down to several keV by means of the Trojan horse method applied to the quasi-free process $\rm {}^2H({}^6Li,pt){}^4He$ induced at a lithium beam energy of 9.5 MeV, which is closer to the zero quasi-free energy point. An accurate analysis leads to the determination of the $\rm S_{bare}(0)=56.7 \pm 2.0 keV \cdot b$ and of the corresponding electron screening potential $\rm U_e = 13.2 \pm 4.3 eV$. In addition, this work gives an updated test for the Trojan horse nucleus invariance comparing with previous indirect investigations using $\rm {}^3He=(d+p)$ breakup.

Quasifree mechanism in theLi6+Li6→3αreaction at low energy

The $^{6}\mathrm{Li}+^{6}\mathrm{Li}$ $\ensuremath{\rightarrow}3\ensuremath{\alpha}$ reaction was measured in a kinematically complete experiment at 3.1 and 2.5 MeV to investigate the presence of quasifree processes at sub-Coulomb energies. Results from the present experiment show a very clear evidence of quasifree mechanisms with an $\ensuremath{\alpha}$-particle spectator either in the target or in the projectile. To check the occurrence of quasifree processes, ``energy sharing'' and ``angular correlation'' analyses have been performed in the framework of the plane wave impulse approximation, both leading to consistent results. The huge peak in the angular correlation spectra, previously ascribed to the ``anomalous quasifree'' process, corresponds instead to the population of the 22.2-MeV $({2}^{+},\ensuremath{\Gamma}=800$ keV) state of $^{8}\mathrm{Be}$.

Evidence for a New Resonance from Polarized Neutron-Proton Scattering

Exclusive and kinematically complete high-statistics measurements of quasifree polarized $\vec{n}p$ scattering have been performed in the energy region of the narrow resonance structure $d^*$ with $I(J^P) = 0(3^+)$, $M \approx$ 2380 MeV/$c^2$ and $\Gamma \approx$ 70 MeV observed recently in the double-pionic fusion channels $pn \to d\pi^0\pi^0$ and $pn \to d\pi^+\pi^-$. The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process $\vec{d}p \to np + p_{spectator}$. That way the $np$ analyzing power $A_y$ was measured over a large angular range. The obtained $A_y$ angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new $A_y$ data into the SAID analysis produces a pole in the $^3D_3 - ^3G_3$ waves as expected from the $d^*$ resonance hypothesis.

Investigation of thedd→3Henπ0reaction with the FZ Jülich WASA-at-COSY facility

An exclusive measurement of the dd -> 3He n \pi 0 reaction was carried at a beam momentum of p = 1.2 GeV/c using the WASA-at-COSY facility. For the first time data on the total cross section as well as differential distributions were obtained. The data are described with a phenomenological approach based on a combination of a quasi-free model and a partial wave expansion for three-body reaction. The total cross section is found to be \sigma(tot) = (2.89 +- 0.01(stat) +- 0.06(sys) +- 0.29(norm)) \mu b. The contribution of the quasi-free processes (with the neutron being target or beam spectator) accounts for 38% of the total cross section and dominates the differential distributions in specific regions of the phase space. The remaining part of the cross section can be described within a partial wave decomposition indicating the significance of p-wave contributions in the final state.

Breakup of deuterons on tritons

The breakup reaction T (d, p)nt at the projectile deuteron energy of 36.9 MeV was studied theoretically and experimentally. The measured inclusive spectra and angular distributions of product protons are presented. A theoretical description of the reaction in question is based on the microscopic nuclear-diffractionmodel and on the theory of resonance processes. It is shown that, in T (d, p) processes, a significant role is played not only by quasifree processes involving final-state interaction that are considered in the diffraction approximation but also by the production of 4H* and 4He* resonance states. The experimental results obtained in the present study for the angular and energy distributions of final-state particles can be satisfactorily interpreted with allowance for these resonances. It is shown that only upon taking into account two 4H* resonances do the calculated spectra of protons from T (d, p) reactions appear to be in good numerical agreement with experimental data.

Quasi-free inclusive processes in the two-particle splitting of tritons by deuterons with an energy of 37 MeV

The d + t → d + d + n reaction was experimentally and theoretically investigated at an incident deuteron energy of 36.9 MeV. The experiment was performed on the U-240 isochronous cyclotron at the Institute for Nuclear Research, National Academy of Sciences of Ukraine. Inclusive spectra of deuterons and their angular distributions in the range of 15° ≤ θd ≤ 52° were measured. Experimental spectra and angular distributions of the final deuterons from the d + t → d + d + n reaction are satisfactorily described in a diffraction approximation that considers interaction in the final state and the correct asymptotic of the wave function of colliding nuclei. The scattered deuterons are the main contributors to the cross section (up to 70–90%), while the deuterons from tritons can be seen well only at low and high energies, at the edges of the spectra.

Study of charged pion photoproduction on deuteron

Photoproduction of charged pion on deuteron, emphasis on channels γ d → π − p p and γ d → π + π − n p , were measured with the second generation of Neutral Kaon Spectrometer. The photon beam was provided from the tagged photon facility at the Laboratory of Nuclear Science, Tohoku University. The energy range of photon is 0.8-1.1 GeV. The aim is to investigate the pion photoproduction process on the nucleus in the second and third resonance regions. The quasi-free process inside deuteron and also non-quasi-free contributions were derived individually.

Study of double pion photoproduction on the deuteron

The π+π− photoproductions on the proton and deuteron have been studied in a photon energy range of 0.8–1.1 GeV at the Laboratory of Nuclear Science, Tohoku University. Charged pions and protons were detected using Neutral Kaon Spectrometer. We obtained the cross sections for the p(γ,pπ+π−) and d(γ,pπ+π−)n. The quasi-free process with a neutron spectator was extracted by the neutron momentum cut of pn>0.3 GeV/c. The cross section for the Δ++Δ− production was deduced in the non-quasi-free process of the γd→pnπ+π−. It was 13.4±0.4 μb at Eγ=0.82 GeV.

Polarization characteristics of nuclear quasi-free (p, dπ+) reaction

Nuclear reaction 16O(p, dπ+)15N is considered theoretically as a prototype of a specific class of nuclear quasi-free processes where deuteron, α-particle, 3H- or 3He-cluster are produced together with pion or η-meson in proton-nucleus interactions in the intermediate energy region.

The weak production of hypernuclei

We present a formalism for the description of hypernuclei production via the weak interaction. It is shown that the cross-section can be written as the contraction of a leptonic and hadronic tensor if we model the interactions as quasifree processes. The hadronic tensor is written in a model-independent way in terms of thirteen nuclear structure functions. A Born term model is used to describe the corresponding elementary hyperon production process. The bound state wave functions of the hyperon and nucleon are calculated within a relativistic mean-field approximation. Together with the relativistic kinematics a fully relativistic framework is constructed and we present cross-section predictions and comparisons for 2.0 GeV, 3.0 GeV and 4.0 GeV neutrino for the charged current reaction v + 12C → μ − + K + + Λ 12 C.

MESON PHOTOPRODUCTION ON THE NEUTRON AT GRAAL

Using a Deuterium target, the GRAAL experiment has collected meson photoproduction data. The data have been analysed in the quasi free regime. The results of the quasi free process on the proton reproduce quite well the results previously obtained with a Hydrogen target. The preliminary results on the quasi free neutron for π0 and η photoproduction are presented and discussed.

Extracting the spectral function of4Hefrom a relativistic plane-wave treatment

The spectral function of He-4 is extracted from a plane-wave approximation to the (e,e'p) reaction using a fully relativistic formalism. We take advantage of both an algebraic ``trick'' and a general relativistic formalism for quasifree processes developed earlier to arrive at transparent, analytical expressions for all quasifree (e,e'p) observables. An observable is identified for the clean and model-independent extraction of the spectral function. Our simple relativistic plane-wave calculations provide baseline predictions for the recently measured, but not yet fully analyzed, momentum distribution of He-4 by the A1-collaboration from Mainz. Yet in spite of its simplicity, our approach predicts momentum distributions for He-4 that rival some of the best nonrelativistic calculations to date. Finally, we highlight some of the challenges and opportunities that remain, both theoretically and experimentally, in the extraction of quasifree observables.

Coherent and incoherent π0 photoproduction from the deuteron

Differential cross-sections for the reactions d(gamma,pi (o))d and d(gamma,pi (o))pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV > E- > 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The (o) missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d(gamma, pi (o))X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d(gamma, pi (o))X cross-sections, on, the other hand, are significantly smaller than predicted by the quasi-free process for E-gamma greater than or similar to 250 MeV. Associating this failure with the pi (o) photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value.

Excitation of the isovector giant monopole resonances via thenatPb(3He,tp)reaction

The ${}^{\mathrm{nat}}\mathrm{Pb}{(}^{3}\mathrm{He},t)$ reaction and the subsequent decay of the excited nucleus by proton emission at backward angles were studied at ${E(}^{3}\mathrm{He})=177 \mathrm{MeV}$ with the aim to separate the $2\ensuremath{\Elzxh}\ensuremath{\omega}$ isovector monopole strength in Bi isotopes from the continuum background. Since the continuum is largely due to quasifree processes associated with forward-peaked high-energy protons, the observed resonance-to-continuum ratio is drastically improved when coincidences between ejectile tritons and protons emitted at backward angles are required. Furthermore, the cross section for the monopole resonances excited via ${(}^{3}\mathrm{He},t)$ is, unlike the cross sections for higher multipolarity excitations, strongly forward peaked. By comparing coincidence spectra at ${\ensuremath{\theta}}_{t}\ensuremath{\approx}0\ifmmode^\circ\else\textdegree\fi{}$ and at finite angles, monopole strength was found in the region $\ensuremath{-}45<Q<\ensuremath{-}30 \mathrm{MeV}.$ The measured monopole cross section is compared with distorted-wave Born approximation calculations for spin-flip and non-spin-flip isovector monopole excitations. It is found that 20% of the respective non-energy-weighted sum rules are exhausted in the proton-decay channel.