Two-nucleon Mechanism Research Articles

The effect of the in-medium [formula omitted] pentaquark on the kaon optical potential

The kaon nuclear optical potential is studied including the effect of the Θ+ pentaquark. The one-nucleon contribution is obtained using an extension of the Jülich meson-exchange potential as bare kaon–nucleon interaction. Significant differences between a fully self-consistent calculation and the usually employed low-density Tρ approach are observed. The influence of the one-nucleon absorption process, KN→Θ+, on the kaon optical potential is negligible due to the small width of the pentaquark. In contrast, the two-nucleon mechanism, KNN→Θ+N, estimated from the coupling of the pentaquark to a two-meson cloud, provides the required amount of additional kaon absorption to reconcile with data the systematically low K+-nucleus reaction cross sections found by the theoretical models.

Study of the mechanisms of cumulative-proton production in neutrino-nucleus interactions

The mechanisms of cumulative proton production are studied on the basis of data from the SCAT propane-freon bubble chamber exposed to a wideband neutrino beam at the Serpukhov accelerator. Kinematic correlations between the cumulative proton and muon, as well as pair correlations of the final protons, are analyzed. The former correlations are predicted by the two-nucleon correlation mechanism (TCM), whereas the latter are caused by the mechanism of secondary pion absorption (SPA) in the nucleus. It is shown that the TCM contribution to the cumulative proton production is most significant in the peripheral interactions (about 50%). The SPA contribution accounts for about a quarter of the inclusive cross section of cumulative proton production.

Double-beta decay in gauge theories

Neutrinoless double-beta decay is a very important process both from the particle and nuclear physics point of view. From the elementary particle point of view, it pops up in almost every model, giving rise among others to the following mechanisms: (a) the traditional contributions like the light neutrino mass mechanism as well as the j L –j R leptonic interference (λ and η terms), (b) the exotic R-parity-violating supersymmetric (SUSY) contributions. Thus, its observation will severely constrain the existing models and will signal that the neutrinos are massive Majorana particles. From the nuclear physics point of view, it is challenging, because (1) the nuclei, which can undergo double-beta decay, have complicated nuclear structure; (2) the energetically allowed transitions are suppressed (exhaust a small part of all the strength); (3) since in some mechanisms the intermediate particles are very heavy one must cope with the short distance behavior of the transition operators (thus novel effects, like the double-beta decay of pions in flight between nucleons, have to be considered; in SUSY models, this mechanism is more important than the standard two-nucleon mechanism; and (4) the intermediate momenta involved are quite high (about 100 MeV/c). Thus one has to take into account possible momentum-dependent terms of the nucleon current, like modification of the axial current due to PCAC, weak magnetism terms, etc. We find that, for the mass mechanism, such modifications of the nucleon current for light neutrinos reduce the nuclear matrix elements by about 25%, almost regardless of the nuclear model. In the case of heavy neutrino, the effect is much larger and model-dependent. Taking the above effects into account, the needed nuclear matrix elements have been obtained for all the experimentally interesting nuclei A=76, 82, 96, 100, 116, 128, 130, 136, and 150. Then, using the best presently available experimental limits on the half-life of the 0νββ decay, we have extracted new limits on the various lepton-violating parameters. In particular, we find 〈m ν〉 < 0.3 eV/c 2, and, for reasonable choices of the parameters of SUSY models in the allowed SUSY parameter space, we get a stringent limit on the R-parity-violating parameter λ′111<4.0×10−4.

The40Ca(t,p)42Careaction at triton energies near 10 MeV per nucleon

Two nucleon transfer reactions have been used to probe for correlations in nuclei but there have been doubts about the assumptions made and hence about the success of the analyses. An experimental study of the ${}^{40}\mathrm{Ca}{(t,p)}^{42}\mathrm{Ca}$ reaction at incident triton energies of 10\char21{}15 MeV/nucleon was made to determine if the one-step, two-nucleon transfer mechanism was dominant at these energies. Measurements were made with 28, 33, and 37.3 MeV incident energy and particular care was taken to measure absolute differential cross sections for transitions to several residual states in ${}^{42}\mathrm{Ca}.$ The analysis of all the data used the results of full finite-range calculations and care was taken to calculate all the one-step and two-step reaction channels in a consistent manner starting from a shell-model description of the relevant Ca isotopes with a single, large basis set of single-particle wave functions. Although a complete prediction of the data was not achieved, it is concluded that incident triton energies about 10 MeV/nucleon are still not enough for one-step, two-nucleon transfer to dominate the $(t,p)$ reaction mechanism. Even with a full analysis, it was not possible to reliably extract the reaction amplitudes by fitting the data because of the effects of the discrete potential ambiguity when describing elastic scattering. Hence it is not possible to extract accurate data about two-nucleon correlations in nuclei from $(t,p)$ reactions at these energies. Additionally, some measurements on the ${}^{8}\mathrm{Ca}{(t,p)}^{50}\mathrm{Ca}$ reaction with 37.3 MeV incident energy are presented. Using a similar analysis, calculations predicted the magnitude of the differential cross-section data and the structure of the angular distributions. The ${J}^{\ensuremath{\pi}}$ of the second excited state of ${}^{49}\mathrm{Ca}$ was shown to be ${7/2}^{\ensuremath{-}}.$

Pd↔π+treaction around theΔresonance

The $\mathrm{pd}\ensuremath{\leftrightarrow}{\ensuremath{\pi}}^{+}t$ process has been calculated in the energy region around the $\ensuremath{\Delta}$ resonance with elementary production/absorption mechanisms involving one and two nucleons. The isobar degrees of freedom have been explicitly included in the two-nucleon mechanism via $\ensuremath{\pi}$- and $\ensuremath{\rho}$-exchange diagrams. No free parameters have been employed in the analysis since all the parameters have been fixed in previous studies on the simpler $\mathrm{pp}\ensuremath{\leftrightarrow}{\ensuremath{\pi}}^{+}d$ process. The treatment of the few-nucleon dynamics entailed a Faddeev-based calculation of the reaction, with continuum calculations for the initial $p$-$d$ state and accurate solutions of the three-nucleon bound-state equation. The integral cross section was found to be quite sensitive to the $\mathrm{NN}$ interaction employed while the angular dependence showed less sensitivity. Approximately a 4% effect was found for the one-body mechanism, for the three-nucleon dynamics in the $p$-$d$ channel, and for the inclusion of a large, possibly converged, number of three-body partial states, indicating that these different aspects are of comparable importance in the calculation of the spin-averaged observables.

Negative pion absorption at rest in 3He.

The pnn and dn final states after the absorption of stopped pions in $^{3}\mathrm{He}$ have been measured in a kinematically complete experiment covering the whole phase space. Proton-neutron, deuteron-neutron, and neutron-neutron coincidences have been detected with a charged-particle hodoscope and large-area time-of-flight counters. For the first time, triple coincidences between the pionic K x rays and two particles have been measured thus selecting absorption from the atomic 1s state. It has been found that the absorption is dominated by the two-nucleon mechanism (2NA) on isoscalar nucleon pairs as indicted by the measured ratio of 6.3\ifmmode\pm\else\textpm\fi{}1.1 for back-to-back correlated nn and pn pairs. A substantial amount of the three-body final state pnn is affected by the final-state interaction (FSI) or leads to the two-particle final state dn. Ratios of 3.1\ifmmode\pm\else\textpm\fi{}0.4 for 2NA to FSI and of 4.2\ifmmode\pm\else\textpm\fi{}0.6 for the pnn to the dn final state are measured. No evidence has been found for three-nucleon absorption with an upper limit of %/${\mathrm{\ensuremath{\pi}}}_{\mathrm{stop}}^{\mathrm{\ensuremath{-}}}$.

The Two-Nucleon Mechanism for the Neutrinoless Double Beta Decay and the BCS Nuclear Wave Function11The project supported by the NSF of China and the Fujian Provincial Science Foundation of China

A method of using the Bardeen–Cooper–Schrieffer (BCS) nuclear wave function to treat the two-nucleon mechanism for neutrinoless double beta decay process is proposed. The neutrinoless decay mode and the neutrinoless decay accompanied by a Majoron emission mode of are studied. Our calculated results show that to reproduce the experimental value of for neutrinoless double beta decai of , the Majorana neutrino mass and the mixing parameter of right-handed current In the emission with a Majoron mode the effective Majoron coupling to neutrino is deduced from the experimental value of for with the result .

Two-nucleon production of hyperons and the search for strange dibaryons

The double charge exchange reaction3He(K−,π+)Xn was studied at 870 MeV/c. In the X missing mass range below the sigma-nucleon production threshold (2130 MeV/c2), events were detected which can be attributed to the two-nucleon process pp(K−,π+)λn. This reaction and mass range also offers good prospects for finding theI=1/2,l=1 (1 P1) spin-singlet dibaryon Ds suggested as the lowest massS=−1 dibaryon in the MIT Bag Model. Although the existence of the Ds is not ruled out by the present data, there is no need to invoke such an object to account for the observed events below σ production threshold. We show that the cross section level for these events is compatible with a dominant two-nucleon mechanism K−p→π0λ,π0p →π+n. We also offer an interpretation of the recent (K−,K+) data on nuclear targets from Iijimaet al., which display a broad peak centered around a K+ momentum of 600 MeV/c. We find that the two-nucleon mechanism K−N →πY,πN→K+Y produces cross sections which are at least an order of magnitude smaller than those observed, and we suggest that the one nucleon process K−p →Φλ, followed by the decayΦ → K+K−, accounts for the data.

Mass of Light Neutrino and Right-Handed Current in 76Ge and 82Se 0νββ Decays11The project supported by National Natural Science Foundation of China under Grant No. 18975038B.

We discuss the 0νββ decay mediated by a light neutrino. Two-nucleon mechanism and shell-model wave function are used in this paper. Also we take into account the influences of both nuclear short-range correlation term and finite nucleon size on the calculations of transition matrix elements. The relation between the mixing parameter η of right-handed current and the Majorana neutrino mass mν is obtained by fitting the experimental data of 0νββ decay lifetimes of 76Ge and 82Se.

Role of the Δ-isobar in the A(p, π) A+1 reaction

We have applied a microscopic model of the A(p, π) A+1 reaction that includes both the one-nucleon mechanism and the resonance p-wave rescattering part of the two-nucleon mechanism to the 16 O( p, π +) 17 O and 12,13,14 C( p, π −) 13,14,15 O reactions leading to two-particle one-hole final states to test separately the two-nucleon mechanism part of the model. The calculations reproduce fairly well the differential cross section and analyzing power data for 16 O( p ̄ , π +) transitions leading to high-spin, two-particle one-hole states in 17O at 7.76 MeV( 11 2 −) and 15.8 MeV( 13 2 −) at bombarding energies of 200, 250 and 354 MeV, but underestimate appreciably the differential cross sections for 12,13,14 C( p ̄ , π −) transitions leading to the ground states 13,14,15O and the 7.28 MeV( 7 2 +) excited state of 15O and fail to reproduce the analyzing-power angular distributions for these (p, π −) transitions. These results verify that NN → NN Δ → NN π is the underlying two-nucleon process in the 16O(p, π +) reactions, but that non-Δ channels dominate the 12,13,14C(p, π −) reactions. The latter conclusions is consistent with results of recent studies of the pn ↔ π −pp reaction.

Dominance of the two-nucleon mechanism in 16O( pi +,pp) at 115 MeV.

Singles $^{16}\mathrm{O}$(${\mathrm{\ensuremath{\pi}}}^{+}$,p) and coincidence $^{16}\mathrm{O}$(${\mathrm{\ensuremath{\pi}}}^{+}$,2p${)}^{14}$N measurements have been made at ${\mathit{T}}_{\mathrm{\ensuremath{\pi}}}$=115 MeV over an extended range of the phase space of the two final-state protons. The coincidence measurements have a missing-mass resolution of 4 MeV. The direct two-nucleon absorption cross section extracted from the data is 58\ifmmode\pm\else\textpm\fi{}8 mb. After corrections for final-state interactions, it was found that the two-nucleon ${\mathrm{\ensuremath{\pi}}}^{+}$+np\ensuremath{\rightarrow}pp process accounts for about 76% of the total absorption cross section, and thus is the dominant absorption mechanism. Evidence for absorption on s-p pairs is seen as a broad bump near 32 MeV of excitation. At backward angles the inclusive $^{16}\mathrm{O}$(${\mathrm{\ensuremath{\pi}}}^{+}$,p) spectra show a distinct absorption peak, an important feature not previously observed. Comparisons of distorted wave impulse approximation calculations to both the coincidence data and inclusive data are presented.

A Note on Two-Nucleon Mechanism in the Neutrinoless Double Beta Decay11The project supported by National Natural Science Foundation of China

One-pion exchange correction to the conventional two-nucleon mechanism in the nuclear neutrinoless double beta decay is discussed. A numerical calculation is carried out for decay. The results indicate that this correction is absolutely negligible, compared with the conventional two-nucleon mechanism.

Reaction 58Ni( pi +,pp) at T pi +=160 MeV.

The reaction {sup 58}Ni({pi}{sup +},{ital pp}) was studied with good energy resolution for an incident pion energy of 160 MeV. The angular correlation of the outgoing protons was measured for {theta}{sub lab}=30{degree}, 75{degree}, and 130{degree}. From these angular correlations the contribution from direct two-nucleon absorption was extracted. A Monte Carlo calculation modeling the absorption process in terms of initial-state scattering of the pion before absorption, two-nucleon absorption, and final-state scattering of the outgoing nucleons is compared to the data. We conclude that within this framework the two-nucleon mechanism can account for less than half of the total absorption cross section in {sup 58}Ni. We report results for the low-energy coincidence proton spectra (down to 5 MeV) which suggest that most of these protons come from evaporation. Angular correlation data for the reaction {sup 58}Ni({pi}{sup {minus}},{ital pp}) are also presented.

12,13C(p

The results of a comparative study of the $^{13}(\mathit{p}\ensuremath{\rightarrow}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) and $^{12}(\mathit{p}\ensuremath{\rightarrow}$,${\ensuremath{\pi}}^{+}$) reactions at ${T}_{p}$=200 MeV and 30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\ensuremath{\theta}}_{\mathrm{lab}{}^{\ensuremath{\pi}}\mathrm{\ensuremath{\le}}150\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$ are reported. The isospin-dependence of the NN\ensuremath{\rightarrow}NN\ensuremath{\pi} processes inside nuclei is investigated by comparing cross sections and analyzing powers for the $^{13}(\mathit{p}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) continuum production to those for corresponding NN\ensuremath{\rightarrow}NN\ensuremath{\pi} processes in the free case. The comparison favors the dominance of a quasifree two-nucleon mechanism in nuclear pion production. Relative populations of discrete final states in the $^{13}(\mathit{p}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) spectra are compared, confirming the dominance of the two-nucleon mechanism and suggesting the identification of some high-spin states in the mirror nuclei $^{14}\mathrm{C}$ and $^{14}\mathrm{O}$. Cross-section and analyzing power angular distributions are presented for discrete final states populated strongly in $^{13}(\mathit{p}$,${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$) and $^{12}(\mathit{p}$,${\ensuremath{\pi}}^{+}$) up to 24 MeV excitation energy. Cross-section data for a number of $^{12}$,13C(p,${\ensuremath{\pi}}^{+}$) transitions are compared to (p,${\ensuremath{\pi}}^{+}$) shell-model calculations employing the elementary pp\ensuremath{\rightarrow}d${\ensuremath{\pi}}^{+}$ amplitude along with realistic nuclear wave functions. Isolated anomalous features exhibited by strong $^{12}$,13C(p,${\ensuremath{\pi}}^{+}$) transitions to two highly excited discrete states in $^{13}\mathrm{C}$ and $^{14}\mathrm{C}$ are discussed and alternative possible interpretations for these states are presented.

The 3He( p, π+) 4He reaction near threshold

We apply a recently developed microscopic model of nuclear pion production to the 3 He( p, π +) 4He reaction at T p = 200 MeV and show the approximate equivalence for this reaction of a one-nucleon mechanism (ONM) calculation with full proton and pion distortions to a more microscopic calculation involving the coherent sum of the ONM amplitude with proton and nonresonant pion distorted waves and the pion-exchange projectile-emission part of the two-nucleon mechanism amplitude with proton and full pion distortions. A full microscopic calculation including target-emission and ρ-meson exchange contributions describes qualitatively the shapes of the differential cross section and analyzing power angular distributions but underestimates the magnitude of the measured cross sections. The sensitivity of the calculations to D-state admixtures in the bound-state wave functions is shown to be small.

Weak transitions in the quasi-elastic reaction 12C(e, e′p) 11B

In a high-resolution quasi-elastic 12C(e, e'p) 11B experiment several weak transitions have been observed to excited final states with spin and parity characteristic of direct knockout from orbitals above the 1p shell. The momentum distributions, which have been measured in parallel kinematics at an outgoing-proton energy of 70 MeV in the range of missing momentum −170 ⩽ p m ⩽ 210 MeV c, show the shape expected for a single-step knockout process. It is demonstrated that the interference between a direct-knockout process and a two-step process leading to the same final state in the (e, e'p) reaction may cause important modifications of the deduced spectroscopic factors. Explicit coupled-channels (CC) calculations show that the spectroscopic factor for the transition to the 7 2 − state at 6.743 MeV is reduced by a factor of 6, whereas the spectroscopic factors of the other weak transitions observed in the present experiment are uncertain by a factor of 2 due to CC-effects. Since the strength of these transitions is larger than can be explained by a pure two-step process, we interpret the observation of these transitions as direct evidence for the existence of ground-state correlations in 12C. The total spectroscopic strength in the E x region between 6 and 12 MeV amounts to 0.1, or 4.1 % of the observed strength for 1p knockout in the low E x region. Two peaks have been identified in the missing-energy spectrum that hitherto have not been reported: a narrow peak at E x = 9.82 (3) MeV with an l = 0 character and a broad structure centered at about 11.5 MeV with an l = 1 character. The missing-energy spectrum between E x = 12 and 24 MeV corresponding to 1 s 1 2 knockout has also been analyzed. The deduced momentum distribution shows evidence for the onset of a two-nucleon mechanism beyond the two-particle emission threshold.

0 + → 2 + neutrinoless ββ decay of 76Ge

The neutrinoless ββ decay rate of the process 76Ge(0 + 1) → 76Se(2 + 1) has been calculated taking into account both the two-nucleon and Δ-isobar mechanisms. Since there occurs a cancellation in the contribution of the two-nucleon mechanism, the Δ mechanism becomes important. Using the calculated nuclear matrix elements, the upper limits on the effective coupling constants of right-handed leptonic currents |〈λ〉| < 2 × 10 −3 and |〈η〉| < 2 × 10 −4 are deduced from recent experimental data.

Nuclear pion absorption in the Skyrme model

The description of pion absorption in nuclei in the Skyrme model is similar to that used in phenomenological meson theory. The main absorption mechanism can be described as a two-nucleon mechanism in which the initial pion is rescattered once prior to final absorption. The long range parts of the rescattering amplitudes for theS- andP-wave rescattering mechanisms have the conventional pion exchange form. The model predicts a strong increase of the isospin evenS-wave rescattering contribution to the absorption rate above threshold, which in the case of the deuteron would cause a rapid decrease of the netS-wave absorption rate with increasing pion energy.

Microscopic description of the A(p, π)A + 1 reaction

We have developed a flexible and thoroughly tested computer program for microscopic calculations of proton-induced pion production in nuclei at bombarding energies up to 500 MeV. The model used includes explicitly both the one-nucleon (pionic stripping) mechanism and the resonant p-wave rescattering part of the two-nucleon mechanism. Initial and final state interactions are included through proton-nucleus and pion-nucleus optical model distortions, with careful consideration of the multiple scattering series. The intermediate delta in the two-nucleon mechanism is treated as static with possible inclusion of a local density approximation for the delta-nucleus interaction. We explain the philosophy of the model, detail the parametrization of the microscopic dynamics, and give the formalism for the calculation of experimental observables. We also describe the testing of the computer program and present preliminary results for the 3He(p, π+)4He reaction.

Observation of a quasifree three-nucleon-absorption mode of pions in 4He.

A kinematically complete experiment on $^{4}\mathrm{He}$(\ensuremath{\pi},dN)N absorption in flight reveals an absorption mechanism in $^{4}\mathrm{He}$ with only one spectator nucleon. This adds to the evidence that modes beyond the two-nucleon mechanism contribute to the pion absorption process.