Positive pion absorption was studied in an almost 4p geometry allowing simultaneous measurements of various charge and neutral multiplicities. Total absorption cross sections and its decomposition into the most important channels is determined. The results are presented for N, Ar and Xe nuclei at incident pion energies of 118,162 and 239 MeV. The role of multinucleon emission in the absorption process is emphasized

Androić, Darko

Backenstoss, Gerhard

Bosnar, Damir

Dooling, Tom

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{0016CODENFIZBE7MULTINUCLEON EMISSION FOLLOWING THE PION ABSORBTIONIN N, Ar AND XeDARKO ANDROI¶Ch, G. BACKENSTOSSa, DAMIR BOSNARh, T. DOOLINGf,P. A. M. GRAMc, N. K. GREGORYd, A. HOFFARTb;g, C. H. Q. INGRAMg, A. KLEINf,K. KOCHg, J. KOEHLERa, B. KOTLI¶NSKIg, M. KRÄODELa, G. KYLEe,A. LEHMANN a;g, A. O. MATEOSd, K. MICHAELIANg, TOMISLAV PETKOVI¶Ch,MIRKO PLANINI¶Ch, R. P. REDWINEd, D. ROWNTREEd, NEVEN ·SIMI·CEVI¶Cd,R. TREZECIAKb, H. ULLRICHb, H. J. WEYERa;g, M. WILDIa and K. E. WILSONdaUniversity of Basel, CH-4056 Basel, SwitzerlandbUniversity of Karlsruhe, D-7500 Karlsruhe, GermanycLAMPF, Los Alamos, NM 87545, USAdMassachusetts Institute of Technology, Cambridge, MA 02139, USAeNew Mexico State University, Las Cruces, NM 88003, USAfOld Dominion University, Norfolk, VA 23529, USAgPaul Scherrer Institute, CH-5232 Villigen PSI, SwitzerlandhUniversity of Zagreb, HR-10000 Zagreb, Croatia(LADS Collaboration)Dedicated to Professor Kseno Ilakovac on the occasion of his 70th birthdayReceived 12 February 2002; Accepted 18 March 2002Online 6 April 2002Positive pion absorption was studied in an almost 4¼ geometry allowing simulta-neous measurements of various charge and neutral multiplicities. Total absorptioncross sections and its decomposition into the most important channels is deter-mined. The results are presented for N, Ar and Xe nuclei at incident pion energiesof 118,162 and 239 MeV. The role of multinucleon emission in the absorption pro-cess is emphasized.PACS numbers: 25.80.Hp, 13.75.-n UDC 539.126Keywords: positive pion absorption, 118,162 and 239 MeV, N, Ar and Xe targets, 4pi ge-ometry, total absorption cross sections, decomposition into the most important channels,multinucleon emission1. IntroductionFor quite some time [1, 2], studies of mechanisms for pion absorption in nucleihave been concentrated around the well-known discrepancy between the total pionFIZIKA B (Zagreb) 10 (2001) 4, 279{284 279androi¶c et al.: multinucleon emission following the pion absorption in . . .absorption cross section and the directly measured absorption cross section forthe absorption on a nucleon pair [3, 4, 5]. Namely, for a long time, absorptionon a nucleon pair was expected to dominate the absorption process. The simplestabsorption mechanism on a single nucleon is suppressed as the energy/impulse to bereleased on the account of the absorbed pion mass does not match the nucleon Fermimomentum within the nucleus. The smallness of the absorption cross section on asingle nucleon was also experimentally veri¯ed. Therefore, there was a prevailingexpectation of the dominance of the pionic absorption by the absorption mode ontwo nucleons. However, the mentioned discrepancy between the total absorptioncross section and its 2N-component, stimulated experimental developments aimingto carefully monitor the events with higher than two multiplicities. Instrumentssuch as LADS at PSI and BGO ball at LAMPF have been developed for thatpurpose.We report here the results on a large number of absorption channels accumulatedat three incident pion energies (118, 162 and 239 MeV) with N, Ar and Xe targets.The comparison of contributions of various multiplicities explains the compositionof the total absorption cross section. It also demonstrates the important role of themultinucleon emission that increases with the incident pion energy. This providesa crucial step in the solution of the long-standing puzzle.2. ExperimentThe data were accumulated at the Paul Scherrer Institute (PSI) using the LargeAcceptance Detector System (LADS) [6] built to investigate the multinucleon pionabsorption (see Fig. 1). The detector consisted of several laminated scintillatorcylinders with coaxial wire chambers. The system possessed ¯ne position resolution(e.g. 1◦ FWHM in angle). Gas targets were used to minimize the particle energylosses within the target ensuring clear vertex reconstruction. Applied gas pressurewere 39, 29 and 19 bars for N, Ar, Xe targets respectively.The details of our data treatment can be found elsewhere [7]; here we give onlythe basic procedures. Events with well de¯ned vertices within the target regionwere analyzed. The timing and gain parameters were obtained from the 2H and Heruns. Events with a charged or neutral pion in ¯nal state were cut out. Absorptioncandidates were tagged according to the number of charged particles and neutronsin the ¯nal state. For charged particles, only those with energy higher than 30 MeV(for protons) could be detected.Beside that, cut on energy for each neutron detected in ¯nal state is applied.So in ¯nal results 3N and 4N channels are populated with particles with energythreshold. This procedure ensures that low energy neutrons, especially present inxenon data, possibly comming from the highly excited nucleus, are cut out fromhigh multiplicity channels and added to lower ones. This conservative procedureensures that percentage of 2N contribution is certainly upper limit for two nucleonabsorption.280 FIZIKA B (Zagreb) 10 (2001) 4, 279{284androi¶c et al.: multinucleon emission following the pion absorption in . . .1.60 m4.50 m2.20 mEND CAPE-BLOCKSMWPCsCYLINDERE-BLOCKSBEAM MAGNETICSHIELDE-SCINTILLATORSPHOTOMULTIPLIERSLIGHT GUIDESTARGETMWPCsdE-SCINTILLATORSdE-SCINTILLATORFig. 1. The LADS detector.3. ResultsCommon feature for all targets and energies analyzed is the signi¯cant di®erencebetween the detected particles energy and the available reaction energy. This canbe explained either with undetected neutron(s) in ¯nal state or high excitationof recoiled nucleus. These two processes have no obvious signature in data forstraightforward distinguishing. Only nitrogen data have direct 2NA signature. Thatmeans that only in nitrogen data portion of events comes with small excitationenergy of remaining nucleus.Successive subtraction is performed on histograms ¯lled with detected eventsand ¯nal Monte Carlo correction is made for uncovered phase space. This procedureis main source of errors and depends on quality and quantity of available data.First, we present the data on the raw experimental multiplicities for the pionabsorption channels in the form of percentages with which a particular ¯nal absorp-tion channel contributes to the total absorption cross section. In the next step, acorrection is made for the migration of the events among these classes which occursbecause of the ine±ciency of the neutron detector. Finally, we lump together somechannels in order to emphasize the contributions of the two nucleon (2N), threenucleon (3N) and four nucleon (4N) ¯nal states in the total cross section.In Table 1, the starting percentages for various combinations of ¯nal particles(protons-p, deuterons-d and neutrons-n) are given. One can see that the three-nucleon emission provides a signi¯cant part of the absorption cross section.FIZIKA B (Zagreb) 10 (2001) 4, 279{284 281androi¶c et al.: multinucleon emission following the pion absorption in . . .TABLE 1. Experimental percentages of absorption channels. All protons have ki-netic energy greater than 30 MeV and detection polar angle 15◦ · µ · 165◦.Channel contributions smaller than 0.1% are not given.Channel Nitrogen Argon XenonTpi(MeV) 118 162 239 118 162 239 118 162 239pp 50.2 40.1 29.0 44.2 39.6 21.1 62.1 55.3 47.2ppn 10.3 16.5 17.4 18.1 16.8 20.4 7.1 10.1 10.9ppnn 1.1 2.8 2.9 3.4 6.1 10.1 1.2 0.8 0.7ppnnn 0.2 0.2 0.5 1.2 2.9 0.1 0.1pd 9.4 5.3 3.2 4.1 4.2 2.2 16.1 11.9 12.1pdn 1.4 2.0 3.0 2.8 3.0 4.6 1.2 3.1 3.2pdnn 0.1 0.2 0.4 0.9 1.9 0.1ppp 12.9 14.6 12.4 14.1 11.1 8.7 4.4 6.2 7.7pppn 1.9 4.5 8.3 2.8 3.6 7.9 0.2 3.3pppnn 0.2 2.1 0.3 1.1 2.3 1.3ppd 5.3 4.7 5.5 4.8 4.5 3.1 1.7 4.5 6.8ppdn 1.8 1.2 2.7 0.6 1.2 3.0 0.6 0.9ppdnn 0.6 0.3 0.9pppp 0.2 0.7 1.7 0.1 0.4 1.7 0.2 0.4 0.4ppppn 0.1 0.3 1.1 0.3 1.9pppd 0.1 0.9 2.0 0.4 1.7pppdn 0.2 1.2 0.1 0.6pn 4.9 5.2 6.1 3.5 5.0 4.9 5.1 4.8 5.9dn 0.4 0.5 0.4 0.3 0.2 0.1 0.7 0.8 0.8In order to count properly events with, e.g., two protons and a neutron inthe ¯nal state, with respect to those in which the detector registered two protonsonly, one must carefully treat events with one more neutron detected. Namely,their number should be compensated for the fact that in the simple counting ofevents with only two protons tagged, events with one emitted neutron are presentbut misidenti¯ed due to the ine±ciency of the neutron detector. We know theangular and energy dependence of the neutron detection from our 4He(¼+,pppn)measurements and we compensate for the ine±ciency e®ects.In Table 2, the total absorption cross sections are presented. In Fig. 2 is shownthe decomposition of the total cross section according to the number of emittednucleons. One observes that the 2N emission after pion absorption is never theonly contribution to the total cross section. Also, as the energy increases, multipleemission of nucleons becomes more probable.282 FIZIKA B (Zagreb) 10 (2001) 4, 279{284androi¶c et al.: multinucleon emission following the pion absorption in . . .TABLE 2. Total pion absorption cross sections for the full solid angle. The valuesfor argon and nitrogen are already published results [8]. Error di®erences are cor-related with quality and quantity of relevant experimental data and procedures ofextrapolation.Nitrogen Argon Xenon118MeV 182§ 10 mb 393§ 21 mb 623§ 140 mb162MeV 163§ 11 mb 366§ 22 mb 736§ 160 mb239MeV 107§ 10 mb 282§ 21 mb 652§ 100 mbNitrogen target shows common features of light nuclei. Pion absorption onthem predominantly leads to the low excitation modes of the remaining nucleus.For heavy nuclei, in contrast, much higher excitations of the remaining nucleus isachieved. For this reason, some similarity between the nitrogen and xenon resultsshould not be seriously pursued.Fig. 2. Graphical presentation of multinucleon emission after absorption of a posi-tive pion for di®erent channels.FIZIKA B (Zagreb) 10 (2001) 4, 279{284 283androi¶c et al.: multinucleon emission following the pion absorption in . . .4. ConclusionsWhile in the early experiments the total absorption cross section was measuredindirectly through the pion disappearance, we see through direct measurement thata large portion of the absorption events goes into the multiplicities higher than two.This component was previously unaccounted for in measurements with small solidangle detectors. While the detailed mechanisms of the multinucleon emission duringthe process of pion absorption still remain to be investigated, we now understandthe origin of controversy plagueing the earlier measurements.References[1] H. J. Weyer, Phys. Rep. 195 (1990) 295.[2] C. H. Q. Ingram, Nucl. Phys. A 553 (1993) 537.[3] A. Altman et al., Phys. Rev. C 34 (1986) 1757.[4] N. J. Burger et al., Phys. Rev. C 41 (1990) 2215.[5] D. J. Mack et al., Phys. Rev. C 45 (1992) 1767.[6] T. Alteholz et al., Nucl. Instr. Methods A 373 (1996) 374.[7] D. Kotlinski et al., Eur. Phys. J. A 1 (1998) 435.[8] D. Rowntree et al., Phys. Rev. C 60(1999)054610VI·SENUKLEONSKA EMISIJA NAKON PIONSKE APSORPCIJE U N, ArI XeProu·cava se pionska apsorpcija s blizu 4¼ detekcijom koja dozvoljava istovremenomjerenje raznih nabojskih i neutralnih vi·sestrukosti. Odreduju se ukupni udarnipresjeci i njihovo razlaganje u najva·znije kanale. Predstavljaju se rezultati za jezgreN, Ar i Xe na energijama 118,162 i 239 MeV. Isti·ce se uloga vi·senukleonske emisijeu procesu apsorpcije.284 FIZIKA B (Zagreb) 10 (2001) 4, 279{284

Multinucleon emission following the pion absorption in N, Ar and Xe

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Multinucleon emission following the pion absorption in N, Ar and Xe

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