arXiv subjects

J. A. Niskanen

Publications and source records attributed to J. A. Niskanen.

At least 19 recordsLinked to original sources

arXiv2024

NN scattering with NDelta coupling: Dibaryon resonances without "dibaryons"?

It is known that at their threshold intermediate two-baryon NDelta states can produce resonance-like structures in some isospin one states, often interpreted as more exotic manifest six-quark states. This paper applies the coupled-channel method to study details of the NDelta effect in isospin one NN scattering with interactions constrained to exclude the influence of such extraneous hypothetical particles in two respects. Firstly, the calculated phase parameters are fitted below pion inelasticity, i.e. far below the region of "dibaryons". Secondly, the strength of the NN --> NDelta transition is constrained to agree with the pion production reaction NN --> dpi, fairly independent of the details of the pure "diagonal" NN potential and its effect practically as strong as the NN itself. The results may be considered as a necessary background for searches of additional dibaryon effects. Optimal conditions for resonant NDelta appearances involve a decrease of the orbital angular momentum in the transition NN --> NDelta, as is the case in the 1D2 and 3F3 waves. Also the effective NDelta threshold as well as its width are state dependent. Detailed complex phase shift results are presented for isospin one partial waves up to J=6, where NDelta excitation is still found to be comparable to or even larger than one pion exchange.

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arXiv2020

Centrifugal effects in N-Delta states

Recently it has been pointed out that in the two-baryon $NΔ$ or $ΔΔ$ system the width of the state is greatly diminished due to the relative kinetic energy of the two baryons, since the internal energy of the particles, available for pionic decay, is smaller. For nonzero orbital angular momenta this effect becomes state dependent. Also the real part of the $NΔ$ energy can get contribution from the centrifugal barrier leading to rotational series of diffuse states. Obviously, these states may have an interpretation as isospin one dibaryons. Direct and explicit calculations for this are presented with some details of the coupled state wave functions displayed. With finite expectation values of these repulsive effects it may be possible to define state dependent effective thresholds for $NΔ$ states and these, in turn, can show resonant like behavior.

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arXiv2016

On the width of N-Delta and Delta-Delta states

It is seen by a coupled-channel calculation that in the two-baryon N-Delta or Delta-Delta system the width of the state is greatly diminished due to the relative kinetic energy of the two baryons, since the internal energy of the particles, available for pionic decay, is smaller. A similar state dependent effect arises from the centrifugal barrier in N-Delta or Delta-Delta systems with non-zero orbital angular momentum. The double-Delta width can become even smaller than the free width of a single Delta. This has some bearing to the interpretation of the d'(2380) resonance recently discovered at COSY.

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arXiv2015

Eta-nuclear interaction: optical vs. coupled channels

The existence of etamesic nuclei has been speculated for a long time without firm experimental evidence. Much of the effort has taken place in final state interactions on production. One crucial factor in seeing a quasibound state is its width, which should be related to the imaginary part of the scattering length. Comparing two models for eta-N scattering giving the same elementary scattering length, a simple optical potential and an explicit coupling to the pionic channel, it is seen that the latter yields a much smaller imaginary part for eta-nucleus scattering. This decrease of absorption may also mean a possibility for narrow eta-nuclear states.

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arXiv2013

Low-energy eta-nuclear scattering parametres vs. binding properties in light nuclei

The connection of the binding energy and width of possible bound eta-mesic states is given to the complex scattering length for s-states in the hope that, with knowledge of the final state interaction this could be useful in searches of these states. In spite of the strong direct potential dependence of both observables this connection is seen to be very model independent even for various nuclei once the influence of also the effective range is considered. The importance of this term is pointed also for data analyses. Although the nucleus considered here is 12C, extension to other nuclei is implied in the background work.

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arXiv2013

Study of the $Λp$ Interaction Close to the $Σ^+n$ and $Σ^0p$ Thresholds

The $Λp$ interaction close to the $ΣN$ threshold is considered. Specifically, the pronounced structure seen in production reactions like $K^-d \to π^- Λp$ and $pp\to K^+ Λp$ around the $ΣN$ threshold is analyzed. Modern interaction models of the coupled $ΛN - ΣN$ systems generate such a structure either due to the presence of a (deuteron-like) unstable bound state or of an inelastic virtual state. % A determination of the position of the prominent peak as observed in various experiments for the two aforementioned reactions leads to values that agree quite well with each other. Furthermore, the deduced mean value of $2128.7\pm 0.3$ MeV for the peak position coincides practically with the threshold energy of the $Σ^+ n$ channel. This supports the interpretation of the structure as a genuine cusp, signaling an inelastic virtual state in the $^3S_1-^3D_1$ partial wave of the $ΣN$ isospin 1/2 channel. % There is also evidence for a second peak (or shoulder) in the data sets considered which appears at roughly 10-15 MeV above the $ΣN$ threshold. However, its concrete position varies significantly from data set to data set and, thus, a theoretical interpretation is difficult.

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arXiv2012

Parity nonconserving proton-proton elastic scattering

The parity nonconserving longitudinal analyzing power A_L is calculated in elastic pp scattering at the energies below the approximate inelastic region T_lab = 350 MeV. The short-ranged heavy meson rho and omega exchanges as well as the longer-ranged two pion exchanges are considered as the mediators of the parity nonconserving interactions. The DDH "best" coupling values are used as the parity nonconserving meson-NN couplings. Also three different parity nonconserving two-pion exchange potentials by various authors are compared.

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arXiv2010

Parity nonconservation in deuteron photoreactions

We calculate the asymmetries in parity nonconserving deuteron photodisintegration due to circularly polarized photons gamma+d to n+p with the photon laboratory energy ranging from the threshold up to 10 MeV and the radiative capture of thermal polarized neutrons by protons n+p to gamma+d. We use the leading order electromagnetic Hamiltonian neglecting the smaller nuclear exchange currents. Comparative calculations are done by using the Reid93 and Argonne v18 potentials for the strong interaction and the DDH and FCDH "best" values for the weak couplings in a weak one-meson exchange potential. A weak NDelta transition potential is used to incorporate also the Delta(1232)-isobar excitation in the coupled-channels formalism.

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arXiv2009

Precision measurements of the pp\to π^+pn and pp\to π^+d reactions: importance of long-range and tensor force effects

Inclusive measurements of pion production in proton--proton collisions in the forward direction were undertaken at 400 and 600 MeV at COSY using the Big Karl spectrograph. The high resolution in the $π^+$ momentum ensured that there was an unambiguous separation of the $pp\to π^+d/π^+pn$ channels. Using these and earlier data, the ratio of the production cross sections could be followed through the $Δ$ region and compared with the predictions of final state interaction theory. Deviations are strongly influenced by long-range terms in the production operator and the tensor force in the final $pn$ system. These have been investigated in a realistic $pp\toπ^+d/π^+pn$ calculation that includes $S \rightleftharpoons D$ channel coupling between the final nucleons. A semi-quantitative understanding of the observed effects is achieved.

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arXiv2007

Parity nonconserving two-pion exchange in elastic proton-proton scattering

Parity nonconserving two-pion exchange in elastic pp scattering is investigated in the presence of phenomenological strong distortions in various models. Parity violation is included in the nucleon-pion vertex considering NN and N Delta(1232) intermediate states in box and crossed box diagrams. Using the derived parity nonconserving two-pion exchange potential we calculate the longitudinal analyzing power A_L in elastic $pp$ scattering. The predicted effect is of the same order as vector meson exchanges.

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arXiv2006

Predictions of striking energy and angular dependence in pp --> (pp)_S-wave pi^0 production

A phenomenological calculation from threshold to 800 MeV of the initial proton beam energy is presented to describe recent data on the reaction pp --> (pp)_S-wave pi^0 with a low energy cut on the final state diproton excitation energy. A strong forward dip is obtained in the differential cross section as in the data from COSY at 800 MeV, although the absolute value of the forward cross section is too low. Earlier low energy data from CELSIUS are reasonably well reproduced. In the unexplored energy interval between these two experiments the model predicts a spectacular energy dependence both in the forward direction and in the angle-integrated cross section.

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arXiv2006

Realistic Few-Body Physics in the $dd\to απ^0$ Reaction

We use realistic two- and three-nucleon interactions in a hybrid chiral-perturbation-theory calculation of the charge-symmetry-breaking reaction $dd\toαπ^0$ to show that a cross section of the experimentally measured size can be obtained using LO and NNLO pion-production operators. This result supports the validity of our power counting scheme and demonstrates the necessity of using an accurate treatment of ISI and FSI.

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arXiv2005

Smallness of the imaginary part of the eta-nucleus scattering length

In two recent analyses of eta meson production a very small imaginary part was obtained for the eta-He3 scattering length, which is in contradiction with most theoretical predictions. In this report a plausible explanation is given to this unexpectedly weak absorption by suppression of the two main inelasticity channels. It is stressed that the real and imaginary parts of the elementary scattering length do not necessarily always have the same isospin algebraic and spatial properties in a nuclear environment and caution should be exercised in their use in multiple scattering calculations and even constructing simple optical potentials.

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arXiv2004

Bounds on the bound η^3He system

We investigate the relation between the η^3He binding energy and width and the (complex) η^3He scattering length. Following our systematic analysis of the eta 3He scattering length we set limits on the η^3He binding. If bound states exist the binding energy (width) should not exceed 5 MeV (10 MeV). In addition, we comment on a recently claimed observation of an η-mesic ^3He quasi-bound state by the TAPS collaboration based on ηphotoproduction data. Although our limits are in reasonable agreement with the values reported by this collaboration, our analysis of these data does not lead to a solid conclusion concerning the existence of an η-mesic bound state. More dedicated experiments are necessary for further clarification.

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arXiv2004

The $η{-}^3$He scattering length revisited

The possible existence of $η$-mesic nuclei poses an interesting and still open issue of research. Since the occurence of such $η$-nucleus bound states is reflected in the corresponding $η$-nucleus scattering length, we critically review the present knowledge for the $η$$^3$He system. Specifically, we scrutinize the available experimental information for the reaction $p{+}d \to η{+} ^3{\rm He}$ which is commonly used to extract the $η$$^3$He scattering length. We point out several striking discrepancies between the various measurements. Subject to those inconsistencies we deduce a value of $a{=}|4.3{\pm}0.3|{+}i(0.5{\pm}0.5)$ fm.

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arXiv2004

Survey of charge symmetry breaking operators for dd -> alpha pi0

The charge-symmetry-breaking amplitudes for the recently observed d d -> alpha pi0 reaction are investigated. Chiral perturbation theory is used to classify and identify the leading-order terms. Specific forms of the related one- and two-body tree level diagrams are derived. As a first step toward a full calculation, a few tree-level two-body diagrams are evaluated at each considered order, using a simplified set of d and alpha wave functions and a plane-wave approximation for the initial dd state. The leading-order pion-exchange term is shown to be suppressed in this model because of poor overlap of the initial and final states. The higher-order one-body and short-range (heavy-meson-exchange) amplitudes provide better matching between the initial and final states and therefore contribute significantly and coherently to the cross section. The consequences this might have for a full calculation, with realistic wave functions and a more complete set of amplitudes, are discussed.

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arXiv2003

Charge symmetry breaking in n p --> d pi^0

The forward--backward asymmetry in n p --> d pi^0, which must be zero in the center-of-mass system if charge symmetry is respected, has been measured to be [17.2 +/- 8 (stat) +/- 5.5 (sys)] * 10^{-4}, at an incident neutron energy of 279.5 MeV. This charge symmetry breaking observable was extracted by fitting the data with GEANT-based simulations and is compared to recent chiral effective field theory calculations, with implications regarding the value of the u d quark mass difference.

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arXiv2003

Charge symmetry breaking as a probe for the real part of eta--nucleus scattering lengths

We demonstrate that one can use the occurrence of charge symmetry breaking as a tool to explore the eta--nucleus interaction near the eta threshold. Based on indications that the cross section ratio of pi+ and pi0 production on nuclei deviates from the isotopic value in the vicinity of the eta production threshold, due to, e.g., pi0-eta mixing, we argue that a systematic study of this ratio as a function of the energy would allow to pin down the sign of the real part of the eta-nucleus scattering length. This sign plays an important role in the context of the possible existence of eta-nucleus bound states.

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