This paper presents a large solid angle measurement of the positive pion absorption cross section on ^4He and its decomposition into partial channels. A large fraction of the absorption cross section at incident pion kinetic energies of T_π+ =70, 118, 162, 239, and 330 MeV is due to multinucleon channels

Androić, Darko

Backenstoss, Gerhard

Bosnar, Damir

Dooling, Tom

Furić, Miroslav

Gram, Peter

Gregory, Nick

Hoffart, Arnim

Ingram, Quentin

Klein, Andew

Koch, Klaus

Koehler, Joerg

Kotlinski, Boghdan

Kroedel, Mathias

Kyle, Gary

Lehmann, Albert

Mateos, Arthu

Michaelian, Karo

Petković, Tomislav

Planinić, Mirko

Redwine, Robert

Rowntree, David

Trezeciak, Ralf

Ullrich, Horst

Weyer, Heinz

Wildi, Markus

Wilson, Kewin

Šimičević, Neven

University of Zagreb Repository

ISSN1330–0016CODEN FIZBE7LETTER TO THE EDITORPION ABSORPTION IN 4HeM. PLANINIC´h, D. ANDROIC´h, G. BACKENSTOSSa, D. BOSNARh, T. DOOLINGf ,M. FURIC´h, P. A. M. GRAMc, N. K. GREGORYd, A. HOFFARTb,g,C. H. Q. INGRAMg, A. KLEINf , K. KOCHg, J. KO¨HLERa, B. KOTLIN´SKIg,M. KROEDELa, G. KYLEe, A. LEHMANNa,g, A. O. MATEOSd, K. MICHAELIANg,T. PETKOVIC´h, R. P. REDWINEd, D. ROWNTREEd, N. SˇIMICˇEVIC´d,R. TREZECIAKb, H. ULLRICHb, H. J. WEYERa,g, M. WILDIa and K. E. WILSONd(LADS collaboration)aUniversity of Basel, CH-4056 Basel, SwitzerlandbUniversity of Karlsruhe, D-7500 Karlsruhe, GermanycLAMPF, Los Alamos NM 87545, U. S. A.dMassachusetts Institute of Technology, Cambridge, MA 02139, U. S. A.eNew Mexico State University, Las Cruces, NM 88003, U. S. A.fOld Dominion University, Norfolk, VA 23529, U. S. A.gPaul Scherrer Institute, CH-5232 Villigen PSI, Switzerlandh University of Zagreb, HR-10000 Zagreb, CroatiaReceived 3 February 1999; Accepted 7 June 1999This paper presents a large solid angle measurement of the positive pion absorptioncross-section on 4He and its decomposition into partial channels. A large fractionof the absorption cross-section at incident pion kinetic energies of Tpi+=70, 118,162, 239, and 330 MeV is due to multinucleon channels.PACS numbers: 25.80.Hp, 25.10.+s UDC 539.126Keywords: positive pion absorption in 4He, cross-sections of partial channels, multinu-cleon channels: (pp)d/pn, (pn)pp, (pd)p, (ppp)n, (ppn)p, (ppd), (pppn)Pion absorption in nuclei is a very important reaction around the ∆ resonanceregion which has an explicit connection to certain CEBAF physics: it forms typ-ically about half of the pi-nucleus reaction cross-section; thus it is also importantin the interaction of other probes such as electrons in the GeV region. We knowthat an important mode in the absorption process involves an isoscalar nucleon pairFIZIKA B 8 (1999) 1, 113–116 113planinic´ et al.: pion absorption in4Hein the absorption process (2NA), but a significant fraction of the total absorptioncross-section leads to at least three energetic, unbound nucleons in the final state -multi-nucleon absorption (MNA). The existence of a three-nucleon pion absorption(3NA) mode was first established on the nucleus 3He [1]. To clarify the question,whether this could be due to a new coherent mechanism, it is also important toknow if and how the mode appears in heavier nuclei. In this context the nucleus4He is unique. Its nuclear density is already comparable to that of heavy nuclei andall important final state channels are open including the ones with fast deuterons.The LADS 4pi solid angle detector [2] was built at PSI to clarify the nature ofMNA. It is a 4pi detector, sensitive to protons, neutrons and deuterons, with thethreshold for nucleon detection about 20 MeV. It also has good kinematic definition(ca 10 MeV FWHM) and neutral pion/neutron separation. The lack of at least oneof these properties in all earlier experiments contributed significantly to ambiguityin the interpretation of their results.The main purpose of this experiment was to determine the composition of theabsorption cross-section. For this, but also to correct the raw data for detectorinefficiencies and acceptance (mainly threshold) limitations, simulations of variousreaction dynamics were used to generate raw distributions of protons, neutrons anddeuterons. These were input into a simulation of the detector and passed throughthe same analysis chain as the real data. The resulting distributions, over variouskinematic variables such as proton angle and energy, were then simultaneouslyfitted to the data with only their normalisations as free parameters. The integral(s)of the raw distributions thus normalised then provided total and partial absorptioncross-sections. It should be noted that the models used here were simple semi-classical ones, but they reproduced the overall distributions well, and since thefull correction for the detector inefficiency and acceptance limitations was typicallyonly a factor of 2 - 3, the resulting cross-sections should be reliable.The LADS results on 4He [3,4], show substantial 3NA strength that becomesstronger with increasing incident pion energy (from about 33 % at 70 MeV toroughly 44 % at 239 MeV) . An example of the composition of absorption on 4Heis given in Table 1 [4]. Here it can be seen that in the 3NA channel the 2p1n finalTABLE 1. Composition of the pi+-4He absorption cross-section at 162 MeV [4,5].Final state % of σabs Class % of σabs(pp)d/pn 47%(pn)pp 3% 2NA 54 ± 7%(pd)p 4%(ppp)n 12%(ppn)p 21% 3NA 43 ± 7%(ppd) 10%(pppn) 3% 4NA 3 ± 1%114 FIZIKA B 8 (1999) 1, 113–116planinic´ et al.: pion absorption in4Hestate is almost twice as important as the 3p one; this “preference” for neutronsover protons is a general feature observed also in the heavier nuclei and at highermultiplicities. This isospin selectivity through the preference for the ppn over theppp final state, is seen at all energies (Fig. 1) . It can also be seen that, while 3NAis strong, 4NA is weak.0 100 200 300 400Pion kinetic energy (MeV)024681012Cross section σ  (mb) (a) 4He(pi+, ppn)p4He(pi+, ppp)n4He(pi+, pppn)0 100 200 300 400Pion kinetic energy (MeV)0123456  (b) 4He(pi+, ppd)p4He(pi+, pd)pFig. 1. (a) Decomposition of the 4He pppn yield according to the energetic finalstate particles.(b) Cross-sections of the deuteron final states.The ppd final state consists primarily of contributions from the (pd)p and (ppd)channels ((pd)p is the channel where one proton was only a spectator).The distributions of the (pd)p channel could be described in two ways. In thefirst, they are consistent with the distributions of the model where pion is absorbedon a quasitriton [3H(pi+,pd)] with one proton acting as a spectator [5]. In the second,they are consistent with being due to pick-up by an outgoing nucleon. Comparisonof the differential distributions of the (pd)p channel with the model in which pionabsorption takes place on a quasideuteron pair, followed by the pickup of a neutronby one of the outgoing protons shows that the overall agreement is good [5]. Insuch a picture the proton and deuteron are emitted roughly back-to-back with thedeuteron preferentially at backward angles. The preference for the emission of thedeuteron at backward angles can be qualitatively explained as being due to thesmaller momentum transfer necessary for the pick-up reaction at these angles. Thecross-section attributable to the (pd)p process falls steadily with increasing incidentpion energy (Fig. 1) while the energy dependence of the (ppd) cross-section showsa broad resonancelike behaviour peaking somewhere around 160 MeV.From an investigation of specific differential cross-sections we could concludethat there is a significant signal of the ISI (pion quasi-free scattering before absorp-FIZIKA B 8 (1999) 1, 113–116 115planinic´ et al.: pion absorption in4Hetion) mechanisms in the 3NA(ppn,ppp,ppd) channels. However, our simple modelssuggest that this process (ISI) amounts to only some 30% of the full 3NA yield (or10% of σabs). The remaining 70% of the 3NA yield is rather featureless, resemblingthree-nucleon phase space, and with no strong indication of another process suchas proton-proton rescattering (FSI), and its origin remains unexplained. One maynote that if the relative strengths of ISI and FSI reflect the elementary pi- and N–Ncross-sections, one would expect the latter to be weaker; a recent calculation byKamada et al. [6] also suggests this.As a summary, LADS has produced accurate total absorption cross-sections, andthe first reliable partial cross-sections as a function of final state multiplicity forpion absorption on 4He, from 70 to 330 MeV. In order to understand the substantialMNA yield, it remains imperative to have a realistic calculation of 3NA.References1) G. Backenstoss et al., Phys. Rev. Lett. 55 (1985) 2782;2) T. Alteholz et al., Nucl. Instrum. Methods A 373 (1996) 374;3) A. Lehmann et al., Phys. Rev. C 56 (1997) 1872;4) A. O. Mateos et al, Phys. Rev. C 58 (1998) 942;5) M. Planinic´, Ph.D. thesis, University of Zagreb, in preparation;6) H. Kamada et al., Phys. Rev. C 55 (1997) 2563.PIONSKA APSORPCIJA U 4HeDaju se ishodi mjerenja pod velikim prostornim kutom udarnog presjeka za apsor-pciju pozitivnih piona u He4 i razdjela na pojedine kanale. Velik dio apsorpcijskogudarnog presjeka za energije upadnih piona od Tpi+=70, 118, 162, 239 i 330 MeVje posljedica viˇsenukleonskih kanala.116 FIZIKA B 8 (1999) 1, 113–116

Pion absorption in ^4He

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Pion absorption in ^4He

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