Double-pionic Fusion Research Articles

On sequential single-pion production in double-pionic fusion

Recently a two-step process has been proposed for the double-pionic fusion to deuterium pn(I=0)→dπ+π−. Its calculation is solely based on total cross section data for the two sequential single-pion production steps pn(I=0)→ppπ− followed by pp→dπ+. Though this sequential process was aimed to explain the dibaryon resonance d⁎(2380) peak in double-pionic fusion, we demonstrate that this is not the case. It rather fits to a possible broad bump at 2.31 GeV in the energy dependence of the pn→dπ0π0 reaction, which was recently interpreted as a consequence of dibaryonic excitations in isoscalar single-pion production.

Shedding Light on Hexaquarks

Several new findings in the four, five and six quark systems reheat the interest in the field of multiquark states (beyond the trivial [Formula: see text] and [Formula: see text]). A lot of progress has recently been made in the 6q sector, on both the theoretical and experimental side. A resonance like structure observed in double-pionic fusion to the deuteron, at M = 2.38 GeV with [Formula: see text] = 70 MeV and [Formula: see text] has been consistently observed in a wealth of reaction channels, supporting the existence of a resonant dibaryon state - the [Formula: see text]. These studies include measurement of all the principle strong decay channels in pn collisions in the quasifree mode by the WASA-at-COSY and HADES collaborations. The internal structure of the [Formula: see text] is largely unknown. It can contain various ”hidden color” 6q configurations, [Formula: see text] molecular states with angular momentum L = 0,2,4,6 as well as meson-assisted dressed dibaryon structures. The large set of experimental data obtained to date gives some constraints on the internal structure of the [Formula: see text] dibaryon, but does not settle the issue. The [Formula: see text] is the only multiquark state which can be produced copiously at current facilities, offering unique access to information beyond its basic quantum numbers, particularly its physical size and internal structure.

ABC effect and resonance d*(2380)

A new state in the two-baryon system with mass 2380 MeV and width 80 MeV has been detected in the experiments at the Juelich Cooler Synchrotron (COSY). The new particle denoted now d*(2380) has quantum numbers I(J p ) = 0(3+). The total cross sections for the d and 4He fusion reactions show similar to each other resonance-like energy dependence. The resonance-like structure is sensed in the double-pionic fusion channels and polarized np scattering.

Examination of the nature of the ABC effect

Recently it has been shown by exclusive and kinematically complete experiments that the appearance of a narrow resonance structure in double-pionic fusion reactions is strictly correlated with the appearance of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the ππ-invariant mass spectrum. Whereas the resonance structure got its explanation by the d⁎(2380) dibaryonic resonance, a satisfactory explanation for the ABC effect is still pending. In this paper we discuss possible explanations of the ABC effect and their consequences for the internal structure of the d⁎ dibaryon. To this end we examine and review a variety of proposed explanations for the ABC effect, add a new hypothesis and confront all of them with the experimental results for the np→dπ0π0 and np→npπ0π0 reactions, which are the most challenging ones for this topic.

Measurement of the $\overrightarrow{n} p\rightarrow d\pi^{0}\pi^{0}$ reaction with polarized beam in the region of the d*(2380) resonance

We report on a high-statistics measurement of the most basic double pionic fusion reaction $\vec{n}p \to d\pi^0\pi^0$ over the energy region of the $d^*(2380)$ resonance by use of a polarized deuteron beam and observing the double fusion reaction in the quasifree scattering mode. The measurements were performed with the WASA detector setup at COSY. The data reveal substantial analyzing powers and confirm conclusions about the $d^*$ resonance obtained from unpolarized measurements. We also confirm the previous unpolarized data obtained under complementary kinematic conditions.

From CELSIUS to COSY: on the observation of a dibaryon resonance

Using a high-quality beam of storage rings in combination with a pellet target and a hermetic WASA detector covering practically the full solid angle, two-pion production in nucleon–nucleon collisions has been systematically studied by exclusive and kinematically complete measurements—first at CELSIUS and subsequently at COSY. These measurements resulted in a detailed understanding of the two-pion production mechanism by t-channel meson exchange. The investigation of the ABC effect, which denotes an unusual low-mass enhancement in the ππ-invariant mass spectrum, in double-pionic fusion reactions led the trace to the observation of a narrow dibaryon resonance with about 80 MeV below the nominal mass of the conventional system. New neutron–proton scattering data, taken with a polarized beam at COSY, produced a pole in the coupled partial waves at () MeV, establishing thus the first observation of a genuine s-channel dibaryon resonance.

Is d* a candidate for a hexaquark-dominated exotic state?**Supported by National Natural Science Foundation of China (11475181, 11105158, 11035006, 11165005), Key Project of Chinese Academy of Sciences (KJCX2-EW-N01), IHEP Innovation Fund (Y4545190Y2), One Hundred Person Project of the University of Chinese Academy of Sciences

We confirm our previous prediction of a d* state with I(JP) = 0(3+) [Phys. Rev. C 60, 045203 (1999)] and report for the first time based on a microscopic calculation that d* has about 2/3 hidden color (CC) configurations and thus is a hexaquark-dominated exotic state. By performing a more elaborate dynamical coupled-channels investigation of the ΔΔ-CC system within the framework of the resonating group method (RGM) in a chiral quark model, we find that the d* state has a mass of about 2.38–2.42 GeV, a root-mean-square radius (RMS) of 0.76–0.88 fm, and a CC fraction of 66%–68%. The last may cause a rather narrow width for the d* which, together with the quantum numbers and our calculated mass, is consistent with the newly observed resonance-like structure (M≈2380 MeV, Γ≈70 MeV) in double-pionic fusion reactions reported by the WASA-at-COSY Collaboration.

ABC effect and resonance structure in the double-pionic fusion toHe3

Exclusive and kinematically complete measurements of the double pionic fusion to $^3$He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the $\pi\pi$-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observations in the basic $pn \to d \pi^0\pi^0$ reaction and in the $dd \to ^4$He$\pi^0\pi^0$ reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of $d^*$ resonance formation during the reaction process in addition to the conventional $t$-channel $\Delta\Delta$ mechanism. The deduced $d^*$ resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.

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.

ΔΔ ABC partner in theND̅system

We perform a study of the N ¯ D interaction within a chiral constituent quark model. This model has been completely tight in the description of the NN interaction and the baryon and meson spectra, therefore provides a parameter-free framework from where to address the problem of the charm −1 two hadron systems. We predict the existence of a unique bound state, ¯ D ∗ with (T, S) = (1,5/2), that would appear in the scattering of ¯ D mesons on nucleons as a D wave state. This resonance resembles our findings in the system, that offered a plausible explanation to the cross section of double-pionic fusion reactions through the so-called ABC effect.

Dibaryons – a new state of matter?

New results in the field of dibaryons are discussed concentrating on the d ∗ (2380) resonance with I(J P ) = 0(3 + ), which has been observed first in the double- pionic fusion to deuterium and meanwhile also in neutron-proton scattering — establish- ing it thus as a genuine s-channel resonance. Double-pionic fusion reactions to the He isotopes demonstrate that this resonance survives also in a nuclear surrounding. It also has been shown to contribute to dilepton production helping thus to resolve the so-called DLS puzzle. Moreover, the existence of dibaryons has also an impact on the equation of state for nuclear matter.

Isospin decomposition of the basic double-pionic fusion in the region of the ABC effect

Exclusive and kinematically complete high-statistics measurements of the basic double-pionic fusion reactions pn→dπ0π0, pn→dπ+π− and pp→dπ+π0 have been carried out simultaneously over the energy region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the dπ0π0 channel exhibits the ABC effect, i.e. a low-mass enhancement in the ππ-invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the dπ+π0 channel shows a smooth behavior consistent with the conventional t-channel ΔΔ process. The dπ+π− data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account.

Investigations of the Basic Double-pionic Fusion in the Region of the ABC Effect

Investigations of the Basic Double-pionic Fusion in the Region of the ABC Effect

Abashian-Booth-Crowe resonance structure in the double pionic fusion to4He

Exclusive and kinematically complete high-statistics measurements of the double pionic fusion reaction $dd \to ^4$He$\pi^0\pi^0$ have been performed in the energy range 0.8 - 1.4 GeV covering thus the region of the ABC effect, which denotes a pronounced low-mass enhancement in the $\pi\pi$-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observation in the basic $pn \to d \pi^0\pi^0$ reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. The maximum occurs at m=2.37 GeV + 2$m_N$, i.e. at the same position as in the basic reaction. The observed resonance width $\Gamma \approx$ 160 MeV can be understood from broadening due to Fermi motion of the nucleons in initial and final nuclei together with collision damping. Differential cross sections are described equally well by the hypothesis of a $pn$ resonance formation during the reaction process.

ND¯system: A challenge forP¯ANDA

We study the $N \bar D$ system by means of a chiral constituent quark model. This model, tuned in the description of the baryon and meson spectra as well as the $NN$ interaction, provides parameter-free predictions for the charm -1 two-hadron systems. The presence of a heavy antiquark makes the interaction rather simple. We have found sharp quark-Pauli effects in some particular channels, generating repulsion, due to lacking degrees of freedom to accommodate the light quarks. Our results point to the existence of an attractive state, the $\Delta \bar D^*$ with $(T,S)=(1,5/2)$, presenting a resonance close to threshold. It would show up in the scattering of $\bar D$ mesons on nucleons as a D wave state with quantum numbers $(T)J^P=(1)5/2^-$. This resonance resembles our findings in the $\Delta\Delta$ system, that offered a plausible explanation to the cross section of double-pionic fusion reactions through the so-called {\it ABC} effect. The study of the interaction of $D$ mesons with nucleons is a goal of the $\bar {\rm P}$ANDA Collaboration at the European facility FAIR and, thus, the present theoretical study is a challenge to be tested at the future experiments.

Two-pion production in nucleon–nucleon collisions—Back to dibaryons?

The two-pion production in nucleon–nucleon collisions has been investigated systematically by exclusive and kinematically complete measurements at CELSIUS/WASA and more recently at COSY-TOF and particularly WASA@COSY. The data for all pp-induced isovector production channels in the energy range from threshold up to Tp = 1.4 GeV can be well understood by t-channel Roper, ΔΔ and possibly Δ(1600) excitation. In contrast to that the purely isoscalar reaction channel pn→dπ0π0 exhibits a narrow resonance-like structure with m=2.37GeV and Γ=70MeV in the total cross section, which is correlated with an intriguing low-mass enhancement in the ππ invariant mass spectrum (ABC effect) as well as with the formation of a ΔΔ system in the intermediate state. From the angular distributions the spin and parity of this structure have been derived resulting in I(JP)=0(3+). The consequences of the resonance hypothesis for other reaction channels are discussed. The fact that also the double-pionic fusion reactions to 3He and 4He exhibit a corresponding resonance-like structure in the total cross section correlated with the ABC effect suggests that the underlying conjectured pn resonance is obviously robust enough to survive in nuclei.

ABC Effect in Double-Pionic Fusion – a New Resonance?

ABC effect, an intriguing low-mass enhancement in the π π invariant mass spectrum — known since more than 50 years from inclusive measurements of double-pionic fusion reactions — is reexamined. To this end exclusive and kinematically complete high-statistics experiments of the fusion reactions to d, 3 He and 4 He have been carried out with WASA at COSY. These measurements cover the full energy region, where the ABC effect has been observed previously. They also complement the systematic measurements of nucleon-nucleon induced two-pion production. An isospin decomposition of all three basic double-pionic fusion reactions leading to the deuteron uniquely shows that solely the isoscalar reaction part exhibits the ABC effect tightly correlated with a narrow resonance structure in the total cross section. The peak energy of the resonance structure is about 90 MeV below the nominal ΔΔ threshold of 2 m Δ and its width of only 70 MeV is much less than the 2 Г Δ expected from the conventional t -channel ΔΔ process. Based on angular distributions the quantum numbers I (J P ) = 0(3+ ) have been assigned. In the double-pionic fusion reaction dd →4 Heπ 0 π 0 again the ABC effect is observed to be correlated with the appearance of a resonance-like structure in the total cross section at the same excess energy. From this we conclude that this resonance structure obviously is strong enough to survive even in nuclei.

ABC Effect and Resonance Structure in the Double-Pionic Fusion to3He

Historically the double-pionic fusion to 3 He is the reaction, where the ABC effect (low-mass enhancement of the ππ -invariant mass spectrum) was observed for the first time more than 50 years ago. Meanwhile exclusive and kinematically complete high-statistics measurements with WASA at COSY revealed a strict correlation between the ABC effect and a narrow resonance structure with I (J P ) = 0(3+ ) in the total cross section of the most basic double-pionic fusion, the pn → dπ 0 π 0 reaction. In order to investigate the situation in the double-pionic fusion to 3 He the energy dependence of the ABC effect in the fusion to 3 He has been measured by fixed energy measurements of the type pd →3 Heππ as well as by quasifree and coherent measurements of the type dd →3 He n ππ .

Isospin Decomposition of the Basic Double-Pionic Fusion in the Region of the ABC Effect

With a proton beam of Tp = 1.2 GeV incident on the deuterium pellet target of the WASA detector setup all three basic double-pionic fusion reactions have been measured simultaneously. By use of quasifree kinematics the energy range 2.3 GeV ≤ √s < 2.5 GeV could be covered, which just coincides with the energy region, where the ABC effect and its associated resonance structure has been observed. From the isospin decomposition we see that the resonance effect is solely in the isoscalar part of the reaction process, whereas the isovector part exhibits a monotonic smoothly rising energy dependence and no ABC effect.

Abashian-Booth-Crowe Effect in Basic Double-Pionic Fusion: A New Resonance?

We report on an exclusive and kinematically complete high-statistics measurement of the basic double-pionic fusion reaction pn→dπ(0)π(0) over the full energy region of the ABC effect, a pronounced low-mass enhancement in the ππ-invariant mass spectrum. The measurements, which cover also the transition region to the conventional t-channel ΔΔ process, were performed with the upgraded WASA detector setup at COSY. The data reveal the Abashian-Booth-Crowe effect to be uniquely correlated with a Lorentzian energy dependence in the integral cross section. The observables are consistent with a narrow resonance with m=2.37 GeV, Γ≈70 MeV and I(J(P))=0(3(+)) in both pn and ΔΔ systems. Necessary further tests of the resonance interpretation are discussed.