Isoscalar Single-Pion Production in the Region of Roper and $d^*(2380)$ Resonances

Abstract

Exclusive measurements of the quasi-free pn → ppπ − and pp → ppπ 0 reactions have been performed by means of pd collisions at T p = 1.2 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region T p = 0.95 − 1.3 GeV ( √ s = 2.3 -2.46 GeV), which includes the regions of ∆(1232), N * (1440) and d * (2380) resonance excitations. From these measurements the isoscalar single-pion production has been extracted, for which data existed so far only below T p = 1 GeV. We observe a substantial increase of this cross section around 1 GeV, which can be related to the Roper resonance N * (1440), the strength of which shows up isolated from the ∆ resonance in the isoscalar (Nπ) I=0 invariant-mass spectrum. No evidence for a decay of the dibaryon resonance d * (2380) into the isoscalar (NNπ) I=0 channel is found. An upper limit of 180 µb (90 % C.L.) corresponding to a branching ratio of 9 % has been deduced.

The d ⁎ (2380) dibaryon resonance width and decay branching ratios

“…A new element in the present derivation is the ability, through the πD 12 channel, to evaluate the d * → NNπ decay width and BR. Our prediction is for a BR of order 8%, considerably higher than that obtained for a quark-based purely ∆∆ configuration [24], but consistently with an upper limit of 9% determined recently by the WASA-at-COSY collaboration [25]. A precise measurement of this decay width and BR will provide a valuable constraint on the πD 12 -∆∆ mixing parameter.…”

“…Next we allow α to vary by replacing the value of Γ < = 44 MeV that served as input through Eq. ( 4) to derive the value of α = 5 7 in use in 2, we have also provided here a cross-section estimate for σ(pn → d * → NNπ), with estimated uncertainties, to compare directly with the experimental upper limit of 180 µb [25]. Given these uncertainties, the NNπ production cross section could be as low as ∼ 120 µb, comfortably below the reported upper limit.…”

“…The NNπ partial decay width and BR listed for the purely πD 12 model (α = 0) were normalized to a total d * pionic width of 75−11=64 MeV. The relatively high value of ≈15% for the obtained BR is excluded by a recent determination of a 9% upper limit [25]. In contrast, a value of the NNπ BR smaller by almost a factor of two was obtained, by applying the πD 12 decay fraction (1 − α) to the NN partial decay width Γ D 12 NN = 0.18 × 120 MeV, in the specific mixing model version listing in Table 2.…”

Charge distribution ofd*(2380)

“…Even more, in terms of the third model (hexaquark dominant model), the predicted decay width of the single pion decay mode of d * → N N π is about "1 MeV" which is much smaller than the double-pion decay widths [18]. Here, we would particularly mention that this value is much smaller than that with the second scenario of D 12 π, but agrees with the experimental observation very recently [19]. All the outcomes from the third scenario support the idea that d * is probably a compact six-quark dominated exotic state due to its large CC component by Bashkanov, Brodsky and Clement [20].…”