Double-Pionic Fusion of Nuclear Systems and the ABC Effect: Approaching a Puzzle by Exclusive and Kinematically Complete Measurements

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1 Double-Pionic Fusion of Nuclear Systems an the ABC Effect: Approaching a Puzzle by Exclusive an Kinematically Complete Measurements M. Bashkanov, C. Bargholtz, M. Berłowski, 6 D. Bogoslawsky, H. Calén, H. Clement, L. Demiroers, E. Doroshkevich, D. Duniec, C. Ekström, K. Fransson, L. Geren, L. Gustafsson, B. Höista, G. Ivanov, M. Jacewicz, E. Jiganov, T. Johansson, O. Khakimova, S. Keleta, I. Koch, F. Kren, S. Kullaner, A. Kupść, K. Linberg, P. Marciniewski, R. Meier, B. Morosov, C. Pauly, 7 H. Pettersson, Y. Petukhov, A. Povtorejko, A. Pricking, R. J. M. Y. Ruber, K. Schönning, W. Scobel, B. Shwartz, 8 T. Skoroko, V. Sopov, J. Stepaniak, 6 P.-E. Tegner, P. Thörngren-Engblom, V. Tikhomirov, A. Turowiecki, 9 G. J. Wagner, M. Wolke, 7 J. Zabierowski, 7 I. Zartova, an J. Złomanczuk (CELSIUS/WASA Collaboration) Physikalisches Institut er Universität Tübingen, D-776 Tübingen, Germany Joint Institute for Nuclear Research, Dubna, Russia The Sveberg Laboratory, Uppsala, Sween Uppsala University, Uppsala, Sween Hamburg University, Hamburg, Germany 6 Soltan Institute of Nuclear Stuies, Warsaw an Loz, Polan 7 Forschungszentrum Jülich, Germany 8 Buker Institute of Nuclear Physics, Novosibirsk, Russia 9 Institute of Experimental Physics, Warsaw, Polan Institute of Theoretical an Experimental Physics, Moscow, Russia Department of Physics, Stockholm University, Stockholm, Sween (Receive July 8; revise manuscript receive October 8; publishe February 9) The ABC effect a puzzling low-mass enhancement in the invariant mass spectrum, first observe by Abashian, Booth, an Crowe is well known from inclusive measurements of two-pion prouction in nuclear fusion reactions. Here we report on the first exclusive an kinematically complete measurements of the most basic ouble-pionic fusion reaction pn! at beam energies of. an. GeV. The measurements, which have been carrie out at CELSIUS-WASA, reveal the ABC effect to be a ðþ I¼L¼ channel phenomenon associate with both a resonancelike energy epenence in the integral cross section an the formation of a system in the intermeiate state. A corresponing simple s-channel resonance ansatz provies a surprisingly goo escription of the ata. DOI:./PhysRevLett.. PACS numbers:.7.cs,..gk,..pt,..aq The ABC effect first observe by Abashian, Booth, an Crowe [ in the ouble-pionic fusion of euterons an protons to He stans for an unexpecte enhancement at low masses in the invariant mass spectrum M. Follow-up experiments [ reveale this effect to be of an isoscalar nature an to show up in cases when the two-pion prouction process leas to a boun nuclear system. With the exception of low-statistics bubblechamber measurements [,8, all experiments conucte on this issue have been inclusive measurements carrie out preferentially with single-arm magnetic spectrographs for the etection of the fuse nuclei. Initially, the low-mass enhancement ha been interprete as an unusually large scattering length an evience for the meson, respectively [. Since the effect showe up particularly clear at beam energies corresponing to the excitation of two s in the nuclear system, the ABC effect was interprete later on by a t-channel excitation in the course of the reaction process leaing to both a low-mass an a high-mass enhancement in isoscalar M spectra [ 7. In fact, the missing momentum spectra from inclusive measurements have been in support of such preictions. In orer to she more light on this issue an complementing our previous result on the ouble-pionic fusion to He [8,9, exclusive an kinematically overconstraine measurements have been carrie out on the most basic system for ouble-pionic fusion, the pn! reaction. To this en, we have measure this reaction in the quasifree moe p! p spectator at beam energies T p ¼ : an. GeV at CELSIUS using the WASA etector setup incluing the pellet target system [. The latter provies frozen euterium pellets of size m, which cross the beam perpenicularly with a frequency of 7 khz. The beam energies have been chosen to be in the region of the ABC effect as known -97=9=()=() - Ó 9 The American Physical Society

2 from inclusive measurements. The experimental results on the p! He an p! He þ reactions an first results from the measurements of the! He reaction are given in Refs. [8,9,. The euterons emerging from the reaction of interest here have been etecte in the forwar etector an ientifie by the E-E technique using corresponing information from a quirl an range hooscope, respectively (see, e.g., Fig. of Ref. [8). rays from the ecay have been etecte in the central etector. This way, the full four-momenta have been measure for all particles of an event except for the very low-energetic spectator proton, which i not reach any active etector element. Thus kinematic fits with overconstraints coul be performe for each event. Because of Fermi motion of the nucleons in the target euteron, the quasifree reaction procees via a range of effective collision energies. Base on the measure energies in the exit channel an the thus event-by-event reconstructe total energies in the pn system, the ata have been binne into small ranges of effective beam energy, in orer to reuce the kinematical smearing an also to allow the extraction of the energy epenence in the total cross section. The absolute normalization of these ata has been obtaine by normalization relative to the quasifree singlepion prouction pn! [ measure simultaneously with the same particle trigger. For the higher beam energy, we use in aition the pn! prouction process for an alternative calibration. Both calibration methos agree within %. However, since at the higher energy the phase-space coverage is less complete than at the lower energy, the etermination of the absolute cross section epens somewhat on the moel use in the MC simulation for acceptance an efficiency correction. Hence the cross sections erive for the higher beam energy are plotte in Fig. with an increase uncertainty. Also, in orer to avoi contaminations ue to the increase prouction an other backgroun, we introuce in the analysis of the high energy ata kinematic constraints on the spectator proton before kinematic fits. Results of our measurements are shown in Figs.. Since the two particles emerging from the reaction of interest are ientical particles, observables epening only on a single like M are calculate by averaging over both possible combinations. Figure isplays the Dalitz plots of the invariant mass squares M versus M an of the invariant mass square M of the euteron with one of the pions versus the same quantity with the other pion for both ata an Monte Carlo (MC) simulations of a moel ansatz iscusse below. The Dalitz plots are far from being flat, i.e., far from being phase-space-like. They rather exhibit very pronounce enhancements in the region of the resonance an at the low-mass kinematic limit of M. This unusual phenomenon is known as the ABC effect. In this reaction the channel is free of both isovector (I ¼ ) an isotensor (I ¼ ) contributions ue to vanishing isospin coupling coefficients. Hence the observation of the ABC effect here means that it must be of isoscalar (I ¼ ) character. Figure epicts the Dalitz-plot projections, the spectra of invariant mass squares M an M for the two bins of effective collision energies T p ¼ : : GeV an T p ¼ : :7 GeV, respectively. At both energies, low-mass M enhancement an excitation, respectively, are the preominating structures. Angular istributions are shown in Fig.. The istribution of the opening angle between the two pions in the (or, equivalently, pn) center-of-mass (c.m.) system peaks at small angles, in particular, if we select events with M : GeV=c. This means that the low-mass enhancement is associate with pions leaving the interaction vertex in parallel. The istributions of the euteron polar angles in the c.m. system an of the pion polar angles an in the an subsystems, respectively, are anisotropic an essentially symmetric about 9. The anisotropy observe for the latter correspons just to the one expecte from ecay. The anisotropy in the angular istribution in the subsystem signals some -wave amixture. It vanishes, if we consier only ata with M : GeV=c, i.e., in M π π Mπ M π oπ o Mπ M π o Mπ 8 6 M π o Mπ FIG. (color online). Dalitz plot of the invariant mass istributions for M versus M (first an secon on the left: ata an MC simulation, respectively, for collision energy.. GeV) an M versus M (first an secon on the right: ata an MC simulation, respectively, for collision energy.7.7 GeV) for the quasifree reaction process pn!. The color-coe scale (z axis) is in arbitrary units. -

3 Mπ π.. Mπ M π π Mπ FIG. (color online). Distributions of the invariant masses M an M from the exclusive measurements (full ots) of the pn! reaction at effective collision energies of.. (left) an..7 GeV (right). The shae areas show the pure phase-space istributions. Soli an otte curves give calculations with an without the assumption of an s-channel resonance, respectively. All curves are normalize to the experimental integral cross section. the region of the low-mass enhancement. From this we euce that the enhancement is of a scalar nature, i.e., in total of a scalar-isoscalar nature. The observe low-mass enhancement is consistent with the finings in previous inclusive single-arm measurements, where only the momentum of outgoing fuse nuclei was measure. Note that the momentum of the fuse ejectile is irectly relate to the invariant mass in the restricte kinematical range of the measure scattering angle (see Figs. in Ref. [8 an Refs. [ 7). The momentum spectra of inclusive single-arm measurements [7,9 exhibit also a strong central maximum (corresponing to large M values), if the euteron scattering angle lab is close to. A possible albeit controversial explanation has been given in Ref. [9 by associating this central maximum with an prouction. In our M ata, which cover all angles lab, we observe no apparent high-mass enhancement. The low-mass enhancements observe in the exclusive ata for the channels are much larger than preicte in previous calculations [ 6. As an example, we show in Figs. (ashe lines) calculations in the moel ansatz of Ref. [, where we aitionally inclue the pion angular istribution in ecay an the Fermi smearing of the nucleons boun in the final nucleus. We have chosen this moel ansatz, since in aition to its simplicity it gives the smallest high-mass enhancement, i.e., is closest to our observation in this respect. Since, on the one han, the available calculations obviously fail, but, on the other han, the ata clearly show the excitation in their M N spectra, a profoun physics piece appears to be missing in the interpretation. As we emonstrate in the following, such a missing piece is foun by inspection of the energy epenence of the total cross section isplaye in Fig.. Shown are the results of this work (soli circles) for pn! multiplie by the isospin factor of in comparison with those of Refs. [,8 for the pn! þ reaction (open symbols). The latter reaction is compose of I ¼ an contributions. The isovector part can be irectly erive from the known pp! þ cross sections by use of isospin relations [. Since the pp! þ reaction is very well escribe [ by conventional t-channel calculations for the formation of a system in the intermeiate state, also the isoscalar part of this conventional process can be erive by applying isospin relations to the intermeiate system. The thus obtaine I ¼ part is shown in Fig. by the otte line, whereas the noninterfering sum of π π δ π π [µb σos π π [µb σos π π [µb σos π π π FIG. (color online). Angular istributions in the reaction pn! at T p ¼ : GeV for the opening angle between the two pions, the angle of the euteron all in the c.m. system as well as the pion angles an in the an subsystems (Jackson frame), respectively. The meaning of symbols an curves is as in Fig.. In aition, ata (open squares) an calculations are plotte also with the constraint M < : GeV=c. -

4 σ [mb pn π π pn π + π s FIG. (color online). Energy epenence of the total cross section for the pn! þ pffiffi pffiffi reaction from threshol ( s ¼ : GeV) upto s ¼ : GeV. Experimental ata are from Refs. [8 (open squares) an [ (open triangles). The results of this work for the channel scale by the isospin factor of are given by the full circles. Dashe an otte lines represent the cross sections for þ an channels, respectively, as expecte from the isovector þ ata by isospin relations (see text). The soli curve inclues an s-channel resonance in the system ajuste to escribe the ABC effect in the channel. isoscalar an isovector parts is given by the ashe line. Inee, the p þ ffiffi ata are in reasonable agreement with this sum for s : GeV. At lower energies, however, the measure values are much larger than expecte from the conventional process. p ffiffi The so far unexplaine structure observe for s < : GeV in our ata is much larger an narrower than expecte from the conventional excitation by t-channel meson exchange [,,6. The cross section maximum is shifte substantially below the nominal threshol being, however, still MeV above the threshol. Since isospin is very unlikely to be broken on such a large scale, a mechanism ifferent from the t-channel excitation must be the reason for this resonancelike structure in the isoscalar sector. These consierations le us to the concept of an s-channel resonance, which couples to isoscalar pn an configurations. We note that such a kin of ibaryon resonance has been preicte by a number of theoretical investigations [ 9. For the escription of the observables by such a resonance, we use the following Breit-Wigner ansatz for the resonance amplitue: A R Fðq ÞD R D D ; () with the mass an with of the s-channel resonance being m R :6 GeV=c an R 8 MeV, respectively, an where D R an D stan for s-channel resonance an propagators, respectively. The form factor Fðq Þ of the vertex, which is chosen to be of monopole type, epens on the relative momentum q between the two s. Since q ¼ q, when neglecting the Fermi motion of the nucleons, this form factor is reflecte irectly in the M spectra an causes there the ABC effect by suppression of the high-mass region. Fitting the cutoff parameter of this monopole form factor to the ata in the M spectra results in : GeV=c, which correspons to the size of a euteronlike object. While other theoretical explanations cannot be rule out at this time, with such a simple ansatz we obtain a surprisingly goo escription of the ata (MC Dalitz plot in Fig. an soli lines in Figs. ) both in their energy epenence an in their ifferential behavior. In previous attempts [8, of ata interpretations, we ha to impose a strong attraction or even a boun state conition between the two s, in orer to obtain a satisfactory escription of ata on the ABC effect. Having learne now from the behavior of the total cross section, that possibly an s-channel resonance coul be the cause, these impositions woul fin a natural explanation. We finally note that, after the move of the WASA etector to COSY, new ata have been taken there very recently on this issue with orers of magnitue higher statistics. It is expecte that these will be able to finally solve the long-staning ABC puzzle. We acknowlege valuable iscussions with L. Alvarez- Ruso, V. Anisovich, D. Bugg, L. Dakhno, C. Hanhart, M. Kaskulov, V. Kukulin, E. Oset, I. Strakovsky, F. Wang, W. Weise, an C. Wilkin on this issue. This work has been supporte by BMBF (No. 6TU an No. 6TU6), COSY-FFE, DFG (European Grauiertenkolleg 68), an the Sweish Research Council. [ N. E. Booth, A. Abashian, an K. M. Crowe, Phys. Rev. Lett. 7, (96);, 8 (96); Phys. Rev., 96 (96). [ R. J. Homer et al., Phys. Lett. 9, 7 (96). [ J. H. Hall et al., Nucl. Phys. B, 7 (969). [ I. Bar-Nir et al., Nucl. Phys. B, 7 (97). [ J. Banaigs et al., Nucl. Phys. B67, (97). [6 J. Banaigs et al., Nucl. Phys. B, (976). [7 F. Plouin et al., Nucl. Phys. A, (978). [8 A. Abivaliev et al., Sov. J. Nucl. Phys. 9, 796 (979); Nucl. Phys. B68, 8 (98). [9 F. Plouin, P. Fleury, an C. Wilkin, Phys. Rev. Lett. 6, 69 (99). [ R. Wurzinger et al., Phys. Lett. B, (999). [ For a review, see A. Coino an F. Plouin, Report No. LNS/Ph/9-6. [ T. Risser an M. D. Shuster, Phys. Lett. B, 68 (97). [ I. Bar-Nir, T. Risser, an M. D. Shuster, Nucl. Phys. B87, 9 (97). [ J. C. Anjos, D. Levy, an A. Santoro, Nucl. Phys. B67, 7 (97). [ See, e.g., A. Garestig, G. Fält, an C. Wilkin, Phys. Rev. C 9, 68 (999); Phys. Lett. B, (998). -

5 [6 C. A. Mosbacher an F. Osterfel, nucl-th/996. [7 L. Alvarez-Ruso, Phys. Lett. B, 7 (999); Ph.D. thesis, Universitat Valencia, 999. [8 M. Bashkanov et al., Phys. Lett. B 67, (6). [9 M. Bashkanov, Ph.D. thesis, Universität Tübingen, 6. [ Chr. Bargholtz et al., Nucl. Instrum. Methos Phys. Res., Sect. A 9, 9 (8). [ S. Keleta, Ph.D. thesis, Uppsala Universitet, 8. [ J. Bystricky et al., J. Phys. (Paris) 8, 9 (987). [ F. Kren et al., in Proceeings of the th International Workshop on Meson Prouction, Properties an Interaction (MESON8) (to be publishe). [ J. Ping et al., Phys. Rev. C 6, (), an references therein; arxiv:86.8. [ T. Barnes et al., Phys. Rev. C 8, 9 (99). [6 T. Kamae an T. Fujita, Phys. Rev. Lett. 8, 7 (977). [7 M. Oka an K. Yazaki, Prog. Theor. Phys. 66, 7 (98). [8 V. I. Kukulin et al., Nucl. Phys. A689, 7 (). [9 R. D. Mota et al., Phys. Rev. C 6, 6 (). [ M. Bashkanov et al., in Proceeings of the th International Conference on Meson-Nucleon Physics an the Structure of the Nucleon (MENU 7), econf C79, 9 (7). -