The ABC effect, an intriguing low-mass enhancement in the $\pi\pi$ invariant mass spectrum, is known from inclusive measurements of two-pion production in nuclear fusion reactions to the few-body systems d, $^{3}$He and $^{4}$He. It was first observed 1960 by Abashian, Booth and Crowe in the inclusive pd $\to$ $^{3}$He X reaction. Its explanation has been a puzzle since then. In an effort to solve this long-standing problem by exclusive and kinematically complete high-statistics experiments, we have measured the fusion reactions to d, $^{3}$He and $^{4}$He with WASA-at-COSY. These measurements cover the full energy region, where the ABC effect has been observed previously in inclusive reactions. In a recent kinematically complete measurement of the $pn \to d\pi ^{0}\pi ^{0}$ reaction we have shown that the ABC effect in this basic double-pionic fusion reaction is correlated with a narrow structure in the total cross section with quantum numbers I(J$^{P}$) = 0(3$^{+}$), a mass of 2.37 GeV and a width of about 70 MeV. The mass is about 90 MeV below 2 times the mass of $\Delta$,  the mass of a $\Delta\Delta$ system, and the width is three times narrower than expected from a conventional t-channel $\Delta\Delta$ process. In the double-pionic fusion reaction to the helium isotope $dd \to ^{4}$He$\pi ^{0}\pi ^{0}$ 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 a previous exclusive experiment at CELSIUS-WASA it is known that the double-pionic fusion to $^{3}$He  also exhibits a pronunced ABC effect. New data from COSY on the $pd \to ^{3}$He$\pi ^{0}\pi ^{0}$ reaction scanning the full ABC region are presented as well as the status of measurements in other reaction channels, where the new resonance might contribute

Bashkanov, M.

Clement, H.

Doroshkevich, E.

Perez del Rio, Elena

Pricking, A.

Skorodko, T.

Wagner, G. J.

English

Jagiellonian Univeristy Repository

PoS(Bormio 2013)013Encounters with Di-Baryons - from the ABC Effect toa New Resonance?Elena Perez del Rio∗, M. Bashkanov, H. Clement, E. Doroshkevich, A. Pricking, T.Skorodko, G. J. Wagner, for the WASA-at-COSY collaborationPhysikalisches Institut der Universität Tübingen, GermanyE-mail: perez@pit.physik.uni-tuebingen.deThe ABC effect, an intriguing low-mass enhancement in the ππ invariant mass spectrum, isknown from inclusive measurements of two-pion production in nuclear fusion reactions to thefew-body systems d, 3He and 4He. It was first observed 1960 by Abashian, Booth and Crowe inthe inclusive pd →3He X reaction. Its explanation has been a puzzle since then. In an effort tosolve this long-standing problem by exclusive and kinematically complete high-statistics exper-iments, we have measured the fusion reactions to d, 3He and 4He with WASA-at-COSY. Thesemeasurements cover the full energy region, where the ABC effect has been observed previously ininclusive reactions. In a recent kinematically complete measurement of the pn→ dπ0π0 reactionwe have shown that the ABC effect in this basic double-pionic fusion reaction is correlated witha narrow structure in the total cross section with quantum numbers I(JP) = 0(3+), a mass of 2.37GeV and a width of about 70 MeV. The mass is about 90 MeV below 2 times the mass of ∆, themass of a ∆∆ system, and the width is three times narrower than expected from a conventionalt-channel ∆∆ process. In the double-pionic fusion reaction to the helium isotope dd→4Heπ0π0again the ABC effect is observed to be correlated with the appearance of a resonance-like struc-ture in the total cross section at the same excess energy. From a previous exclusive experimentat CELSIUS-WASA it is known that the double-pionic fusion to 3He also exhibits a pronuncedABC effect. New data from COSY on the pd→3Heπ0π0 reaction scanning the full ABC regionare presented as well as the status of measurements in other reaction channels, where the newresonance might contribute.51st International Winter Meeting on Nuclear Physics21-25 January 2013Bormio (Italy)∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(Bormio 2013)013Encounters with Di-Baryons - from the ABC Effect to a New Resonance? Elena Perez del Rio1. IntroductionThe ABC effect, an intriguing low-mass enhancement in the ππ invariant mass spectrum, hasfirst been observed in inclusive measurements of the pd →3He X reaction by Abashian, Boothand Crowe [1]. Subsequent bubble-chamber [2, 3] and single-arm magnetic spectrometer measure-ments [4, 5, 6, 7, 8, 9, 10, 11] suggested this enhancement to be correlated with the production ofan isoscalar pion pair in fusion reactions to light nuclei. Its explanation has been a puzzle sincemore than 50 years. Therefore it has been named just after the initials of those authors, who firstobserved this effect.In recent exclusive and kinematically complete measurements of the pn→ dπ0π0 reaction ithas been demonstrated [12, 13, 22] that the ABC effect in this basic double-pionic fusion reaction iscorrelated with a narrow structure in the total cross section with quantum numbers I(JP) = 0(3+),a mass of 2.37 GeV and a width of about 70 MeV. The mass is about 90 MeV below 2 m∆, the massof a ∆∆ system and the width is three times narrower than expected from a conventional t-channel∆∆ process.2. ExperimentsIn an effort to solve this long-standing problem by exclusive and kinematically complete high-statistics experiments, we have measured the fusion reactions to d, 3He and 4He with WASA atCOSY. These measurements cover the full energy region, where the ABC effect has been observedpreviously in inclusive reactions. They complement the systematic two-pion production measure-ments carried out at PROMICE/WASA [15, 16, 17], CELSIUS/WASA [13, 18, 19, 20, 21, 27, 28],COSY-TOF [23, 24], COSY-MOMO [29], WASA-at-COSY [25, 30, 31] and COSY-ANKE [26].The bulk of these experiments has been performed with the WASA detector featuring a solidangle coverage of nearly 4π , windowless hydrogen and deuterium pellet target systems [32, 33] aswell as a reliable particle identification for γ,π, p,n,d,3He and 4He ejectiles.Measurements at WASA have been performed by detection of all ejectiles of an event (withthe exception of spectator nucleons) allowing thus kinematic fits with several overconstraints inthe data analysis. The pn initiated reactions have been studied via the quasifree pd process witha spectator proton resulting from the target deuteron. By use of the Fermi motion of the activenucleon in the deuteron target the energy dependence of pn and pp initiated reactions could bemeasured over a range of more than 100 MeV with just a single beam energy. In case of the fusionreaction to 3He pd initiated reactions have been measured, either by hitting a deuteron target witha proton or a deuteron beam. The latter gives rise to quasifree pd collisions and through the Fermimotion gives acess to a broad region of energy, where the ABC effect is expected to take place.3. Discussion of ResultsFrom these measurements total as well as differential cross sections have been obtained overthe full energy region, where the ABC effect has been found previously.As an example we show in Figs. 1 and 2 the Dalitz plots for two different energies as well asits projections for the double pionic fusions to d and 4He, respectively [12, 30, 31].2PoS(Bormio 2013)013Encounters with Di-Baryons - from the ABC Effect to a New Resonance? Elena Perez del RioFigure 1: Top: Dalitz plots of M2dπ0 versus M2π0π0 at√s = 2.38 GeV (peak cross section) (left) and at√s = 2.5 GeV (right). Bottom: Dalitz plot projections M24Heπ0 (left) and M2π0π0 (right) axes at√s = 2.38GeV. The curves denote calculations for a s-channel resonance decaying into ∆∆ with JP = 3+ with (solid)and without (dash-dotted) form factor as well as for JP = 1+ (dashed). Hatched and shaded areas representsystematic uncertainties and phase-space distributions, respectively. From Ref. [12],Fig. 4.Figure 2: Top: Dalitz plot of M24Heπ0 versus M2π0π0 for data (left) at the energy of the peak cross section(√s = 4.24 GeV) and Monte Carlo calculation for an s-channel pn resonance with m = 2.37 and Γ = 124MeV including Fermi morion of the nucleons in the initial and final nuclei. Bottom: Distributions of M4Heπ0(left) and Mπ0π0 (right) at√s = 2.27 GeV. The shaded areas represent the phase-space distributions. Dottedlines give t-channel ∆∆ calculations, whereas the solid line shows a calculation of a pn s-channel resonancefor illustration. From Ref. [30, 31], Figs. 2 and 3.3PoS(Bormio 2013)013Encounters with Di-Baryons - from the ABC Effect to a New Resonance? Elena Perez del RioIn both cases we observe a pronounced ABC effect, i.e. low-mass enhancement — similar tothe situation in the pd →3Heπ0π0 reaction [27]. This experimental finding strongly contravenesconventional 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 structurerelative to phase space [12, 20, 30].The basic double-pionic fusion to deuterium features three different reaction channels:• pn→ dπ0π0, which is purely isoscalar• pp→ dπ+π0, which is purely isovector, and• pn→ dπ+π−, which contains both isoscalar and isovector contributions.They are connected by the following isospin relation [34] for their total cross sections:σ(pn→ dπ+π−) = 2σ(pn→ dπ0π0)+ 12σ(pp→ dπ+π0), (3.1)which allows for a cross check on the internal consistency of measurements.Based on our measurements of all three reactions we have carried out an isospin decompositionof the isospin-mixed pn→ dπ+π− reaction by use of both the isovector pp→ dπ+π0 and theisoscalar pn→ dπ0π0 channel. We find internal consistency of the measurements within theiruncertainties. The result of the isospin decomposition is shown in Fig. 3 [22, 35].The basic isovector fusion process pp→ dπ+π0 exhibits neither an ABC effect nor a narrowresonance structure [22, 14] in this particular energy region (black filled circles in Fig. 3) — inagreement with the observations in all other two-pion channels of pp initiated reactions [15, 16,17, 18, 19, 20, 21, 23, 24, 25, 26] and properly reproduced by conventional calculations of thet-channel ∆∆ process [22, 14].On the contrary the purely isoscalar fusion reaction pn→ dπ0π0 does not behave as expectedfrom conventional reaction dynamics. It rather exhibits a narrow resonance structure in the totalcross section (blue filled circles in Fig. 3), which is correlated with the appearance of the ABCeffect in the π0π0 invariant mass spectrum (Fig. 1) [12, 13, 22]. Its peak energy is about 90 MeVbelow the nominal ∆∆ threshold of 2 m∆ and its width of only 70 MeV is much less than the widthof 2 Γ∆ expected from the conventional t-channel ∆∆ process. From the angular distributions thequantum numbers I(JP) = 0(3+) have been assigned to this structure [12]. From Dalitz plots wemay deduce that this state decays first into a ∆∆ system before it reaches the dπ0π0 final system.Such kind of dibaryonic state has actually been predicted in some theoretical models, see e.g.[36, 37]. At present no conventional process is known, which could at least qualitatively explainthis phenomenon.In the double-pionic fusion reaction dd →4Heπ0π0 again the ABC effect, which exibits anarrower and more pronounced enhancement than in the basic case – see Fig. 1 and 2, is observedto be correlated with the appearance of a resonance-like structure in the total cross section at thesame excess energy, however, with an increased width due to Fermi motion in initial and finalnuclei as well as due to collision damping, see Fig. 4 [30, 31]. From this we conclude that thisresonance structure obviously is strong enough to survive even in light nuclei.4PoS(Bormio 2013)013Encounters with Di-Baryons - from the ABC Effect to a New Resonance? Elena Perez del Rio [GeV]s2.2 2.4 2.6 2.8 3 [mb]σ00.20.40.6Figure 3: Total cross sections of the basic double-pionic fusion reactions pN → dππ of different isospinsystems in dependence of the center-of-mass energy√s from threshold (√s = 2.15 GeV) up to 2.6 GeV.The data for the isospin mixed reaction pn→ dπ+π− are shown by the red full circles. The data for itsisovector part given by half the cross section of the pp→ dπ+π0 reaction are plotted by the black filledcircles, whereas the data for its isoscalar part given by twice the cross section of the pn→ dπ0π0 reactionare shown by the blue filled circles. From Ref. [35].The energy dependence of the reaction pd→3Heππ is under investigation. Preliminary results[38, 39] indicate a similar trend as observed for the 4He case. A pronunced ABC effect is observedat energies close to those, where the resonance is expected to reach its maximum and then it fadesaway at higher energies, where the t-channel ∆∆ process becomes more relevant – see Fig. 5.4. Conclusions and OutlookBased on the large amount of meanwhile available data we observe a scenario, which is surpris-ingly different, whether the two-pion production is initiated by isovector or isoscalar NN collisions.Whereas all pp, i.e. isovector initiated reaction channels may be well understood by conventionalt-channel processes, the two-pion production initiated by isoscalar pn collisions is not understoodby such processes alone. The outstanding finding here is that the ABC effect known for more thanfifty years is strictly correlated with a narrow resonance-like structure in the total cross section. Forthe purely isoscalar pn→ dπ0π0 reaction this phenomenon – interpreted as s-channel resonancewith I(JP) = 0(3+) – comprises most of the measured cross section. The same situation is foundfor the double-pionic fusion to the nuclei 3He and 4He, thus this isoscalar pn resonance structure isobviously strong enough to survive even in nuclei. In a more recent experiment the reaction chan-nels pn→ pnπ0π0 [40] and especially in pn elastic scattering are being investigated in search ofsuch a resonance. Also single-pion production channels could in principle contain effects from thisresonance. However, the large conventional cross sections in these channels make it very difficultto search for the resonance there.5PoS(Bormio 2013)013Encounters with Di-Baryons - from the ABC Effect to a New Resonance? Elena Perez del Rioexcess energy [GeV]0 0.2 0.4 0.6 [a.u.]σ0123this workCELSIUSCELSIUS-WASABirmimghamSaclay   WASA-at-COSY0π0π dFigure 4: Total cross sections of the pn→ dπ0π0 (black symbols) and dd →4Heπ0π0 reactions (coloredsymbols) in dependence of the excess energy above threshold. Both cross sections are given in relativeunits and scaled such as to have comparable heights in the peak cross section. The WASA-at-COSY results[30, 31] (solid circles) are compared to results from a previous exclusive measurement at CELSIUS/WASA[28] (square) as well as inclusive measurements at CELSIUS [41] (star), Birmingham [42] (inverted triangle)and Saclay [7] (triangle). The dotted line shows the energy dependence of the conventional t-channel ∆∆process, arbitrarily scaled in height. From Ref. [30, 31]Figure 5: Distributions of the π0π0 invariant mass Mπ0π0 observed in the reactions dd →3 Heπ0π0 (left)and pd→3Heπ0π0 (right) at incident beam energies of 1.7 GeV and 1.0 GeV respectively, corresponding to3.3 GeV ≤√s≤ 3.41 GeV.. The shaded areas represent the phase-space distributions.5. AcknowledgmentsWe acknowledge valuable discussions with C. Hanhart, E. Oset, A. Sibirtsev, F. Wang and C.Wilkin on this issue. This work has been supported by BMBF (06TU9193) and ForschungszentrumJülich (COSY-FFE).6PoS(Bormio 2013)013Encounters with Di-Baryons - from the ABC Effect to a New Resonance? Elena Perez del RioReferences[1] N. E. Booth, A. Abashian, K. M. Crowe, Phys. Rev. Lett. 7 (1961) 35; 5 (1960) 258; Phys. Rev. 132(1963) 2296ff[2] I. Bar-Nir et al., Nucl. Phys. B 54 (1973) 17[3] A. Abdivaliev et al., Sov. J. Nucl. Phys. 29 (1979) 796[4] R. J. Homer et al., Phys. Rev. Lett. 9 (1964) 72[5] J. H.Hall et al., Nucl. Phys. B 12 (1969) 573[6] J. Banaigs et al., Nucl. Phys. 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Encounters with di-baryons - from the ABC effect to a new resonance?

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Encounters with di-baryons - from the ABC effect to a new resonance?

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