Exclusive measurements of pd→He3ππ: The ABC effect revisited

Abstract

Exclusive measurements of the reactions pd → 3 He π + π − and pd → 3 He π 0 π 0 have been carried out at Tp = 0.893 GeV at the CELSIUS storage ring using the WASA detector. The π + π − channel evidences a pronounced enhancement at low invariant ππ masses -as anticipated from previous inclusive measurements of the ABC effect. This enhancement is seen to be even much larger in the isoscalar π 0 π 0 channel. The differential distributions prove this enhancement to be of scalar-isoscalar nature. ∆∆ calculations give a good description of the data, if a boundstate condition is imposed for the intermediate ∆∆ system. About 40 years ago first measurements onto the double-pionic fusion of protons and deuterons to 3 He particles led to a big surprise. In the momentum spectra of the 3 He particles detected by a magnetic spectrometer Abashian, Booth and Crowe [1] found an intriguing excess of strength close to the ππ threshold. Follow-up measurements of this group suggested this enhancement to be of isoscalar I ππ = 0 nature, since corresponding measurements on the isovector π + π 0 channel in pd → 3 HX yielded a much smaller cross section. Hence it has been speculated, whether some unknown isoscalar resonance (like, e.g., the σ meson) could be the origin of the observed enhancement. Later on the effect, meanwhile referred to as ABC effect after the initials of the original authors and interpreted as ∆∆ excitation [2], was confirmed in much more detailed studies 1

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

“…In both cases we observe a pronounced ABC effect, i.e. low-mass enhancement -similar to the situation in the pd → 3 Heπ 0 π 0 reaction [28]. This experimental finding strongly contravenes conventional calculations based on the mutual excitation of the two colliding nucleons to their ∆(1232) state by t-channel meson exchange, which rather predict a gentle two-hump structure relative to phase space (dotted lines in Fig.…”

Examination of the nature of the ABC effect

“…Another, alternative attempt to explain the ABCeffect by a final-state interaction (FSI) between the two ∆s in the intermediate state dates back to the first WASA measurements on this issue [13,56]. We note that this ansatz dates back to the time, when it was not yet known that the ABC effect is correlated with the presence of the s-channel resonance d * (2380).…”

“…[13,56] the ∆∆-FSI was parameterized in the Migdal-Watson ansatz [49][50][51] providing the factoranalogous to eq. (17) -…”

“…In addition, WASA measurements revealed that d * (2380) is also present in the double-pionic fusion reactions to the helium isotopes pd → 3 Heπ 0 π 0 , * corresponding author email address: mikhail.bashkanov@ed.ac.uk pd → 3 Heπ + π − , dd → 4 Heπ 0 π 0 and dd → 4 Heπ + π − [12][13][14][15]. This means that obviously d * (2380) is stable enough to survive also in a nuclear surrounding.…”

Recent Results from WASA-at-COSY

“…Since the ABC effect had been observed earlier also in the double-pionic fusions to 3 He and 4 He, the corresponding reactions pd → 3 Heππ [42,43] and dd → 4 Heπ 0 π 0 [44] have been measured as well by WASA in the region of the ABC effect. In fact, also here these reactions exhibit the d * (2380) resonance effect, however, being broadened by Fermi motion and collision damping.…”