Is d* a candidate for a hexaquark-dominated exotic state?

Abstract

We confirm our previous prediction of a d * state with I(J P ) = 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 ∆∆-CC system within the framework of resonating group method (RGM) in a chiral quark model, we found 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 to 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 WASA-at-COSY Collaboration.PACS numbers: 14.20. Pt, 13.75.Cs, 12.39.Jh, 24.10.Eq The ABC effect has drawn physicists' great attention since its observation in 1961 in the pd reaction [1]. In recent years, much experimental progress in exploring the nature of the ABC effect has been made. In 2009, the CELSIUS/WASA Collaboration measured the most basic double-pionic fusion reaction pn → dπ 0 π 0 with an incident proton energy of 1.03 GeV and 1.35 GeV [2], and found significant enhancements in the ππ invariant mass spectrum at ππ invariant mass below 0.32 GeV 2 and also in the dπ invariant mass spectrum at ∆ resonance region. To accommodate these data as well as the energy dependence of the total cross section at √ s < 2.5 GeV, the conventional t-channel ∆∆ intermediate state is found to be not sufficient, and a new structure, namely an s-channel resonance with mass of about 2.36 GeV and width of about 80 MeV, is expected. In 2011, the WASA-at-COSY Collaboration further measured the pn → dπ 0 π 0 reaction with the beam energies of 1.0−1.4 GeV which cover the transition region of the conventional t-channel ∆∆ process [3]. They found that neither the t-channel ∆∆ process nor the Roper resonance process can explain the data, and an s-channel resonance with quantum numbers of I(J P ) = 0(3 + ), mass of about 2.37 GeV and width of about 70 MeV is indeed needed to describe the data. Recently, the WASA-at-COSY Collaboration measured the polarized np scattering through the quasi-free process dp → p spectator np [4,5]. By incorporating the newly measured A y data into the SAID analysis, they obtained a pole in the 3 D 3 -3 G 3 waves at (2380 ± 10)+ i(40 ±5) MeV, which again supports the existence of a resonance, called d * , as mentioned in Ref. [3]. Further evidence of this resonance has also been reported in the quasi-free np → npπ 0 π 0 reaction [6]. Since its mass is above the threshold of ∆N π channel, while its width is much smaller than the decay width of ∆, this resonance must be a very interesting state involving new physical mechanisms and it is obviously worthwhile investigating.Theoretically, the possibility of the existence of dibaryon states was first proposed in 1964 by Dyson a...

Examination of the nature of the ABC effect

“…This is in accordance with meanwhile numerous theoretical work about this resonance [29][30][31][32][33][34][35][36][37][38][39]. It also is in agreement with the measured branchings of the d * decay into the diverse N N ππ channels [40].…”

“…A ∆∆ D-wave contribution has actually been predicted in quark model calculations recently [36,37]. The predicted d-wave percentage, however, is only 1.5%.…”

“…A D-wave admixture in the intermediate ∆∆ system has been predicted from quark-model calculations [36,37] -though a factor of three less than needed for an exclusive explanation of the ABC effect without the use of the vertex function. However, if also in this scenario a vertex function is incorporated -as needed for a consistent description of baryonic excitations, then a 1.5% d-wave admixture in agreement with model predictions is sufficient for a quantitative description of both key reactions.…”

Form factors of the d*(2380) resonance

“…In our previous studies [8,[10][11][12][13][14][15], we enlarged the Fock space to involve the next lowest QCD allowed configuration, proposed the possible structure of d à as a hexaquark dominated state and dynamically calculated it in the framework of the resonating group method (RGM) with a trial wave function of the ΔΔ þ C 8 C 8 structure in the chiral SU(3) constituent quark model. It should be stressed that the approach can explain most of the data available on the masses of the ground states of baryons, the scattering and reaction data of the baryon-baryon interactions, the binding energy of the deuteron and even H particle, etc.…”

Dibaryons at COSY

“…However it seems likely that the main part of the d * wave functions is the hidden color six-quark state, barely coupled to ordinary matter, which could provide the d * (2380) its narrow width. According to recent evaluations the d * (2380) spends 2/3 of its time as a hexaquark and the rest as a ∆ − ∆ molecule [12,13] (note that a perfect I(J P ) = 0(3 + ) hexaquark would have 4/5 of the six-quark component and 20 percent of ∆ − ∆ [14]). …”