Recent results from LEAR experiment PS187 are presented. Preliminary data for the inclusive production of ..pi../sup +/, K/sup +/, and p from the annihilation of 180 MeV antiprotpns in /sup 28/Si and /sup 238/U are compared with predictions of intranuclear cascade calculations. Proton and pion production data are well reproduced by the calculations, but kaon yields at low momenta appear to be strongly suppressed in the experimental data. 8 refs., 5 figs

Bol, K. D.

Clover, M. R.

DeVries, R. M.

DiGiacomo, N. J.

Kapustinsky, J. S.

McGaughey, P. L.

Smith, G. R.

Sondheim, W. E.

Sunier, J. W.

Yariv, Y.

UNT Digital Library

r.UR -85-1481. .j]’LA-uR--85-1481DE85 010729TITLE: ANTIPROTON ANNIHILATIONS IN NUCLEIAUTHOR(S): P. “ . McGaughey, K. D. 1301, M. R. Clover, R. M. DeVries,N. J. DiGiacomo, J. S. Kapustinsky, G. R. Smith,* J. W. Sunier,W. E. Sondheim, and Y. Yarivt ** Los Alamos Nation&l Laboratory,Los Aiamos, NM 87545M. Euenerd, J. Chauvin, D. Lebrun, and P. ?-fc~tin, Institut desSciences Nucl&air~s, 38C2.6 Grenoble, Cedex, FranceJ. C. Dousse, Universit6 de Fribour8, 1700 Fribourg, SwitzerlandSUBMITTED TO; AIP Conference Proceedings, Meeting at Tempe, A%, March 10-14, 1985,,? , ~r~l!!J&&Permanent addresses: *TRIUMF, Vancouver V6T 2A3, British Columbia.**Soreq Nuclear Research Center, Yavne 70600, Israel.L(XU4lalmoslllSTf!lllffllOflF NISIIOUF4EN1IS llN.lM1[0 , ~~Y\Los Alamos National LaboratoryLos Alamos,Nwv Mexico 875454ANTIPROTON ANNIHILATIONS IN NUCLEIP. L. McCaughey, K. D. Bolt ?4. R. Clover, R.*M. DeVries,N. J. DiGiacomo, J. S. Kapuatinaky, G. R. Sml~~, J. W. Sunier,W. E. Sondheim, and Y. YarivPhysics .Divfsion, Los Alamos National Laboratory,Los Alamos, NM 87545M. Buenerd, J. Chauvin, D. Lebrun, and P. MartinInstitut des Sciences Nucl~alres. 38026 Grenoble, Cedex, FranceJ. C. DousseUnlversit6 de Fribourg, 1700 Fribourg, SwitzerlandABSTRACTRecent results from LEAR experiment PS187 areliminary data for the inclusive production of r+~$resented pre-.annihilation of 180 MeV antiprotons in 2%1 and $38U’a;;dc&;;;;dthewith predictions of Intranuclear cascade calculations. Proton andpion production clata are well reproduced by the calculations, butkaon yields at low momenta appear to be strongly suppr~ssed in theexperimental data.INTRODUCTIONOne of the primary goals of LEAR exp$ri~nt PS187 is to measurethe features of light charged particle (W , K , p, d and t) produc-tion following the annihilation? of antiprotons on a variety ofnuclei. This study of the inclusive spectra is aimed at determiningwhether or not anything unusual (above and beyond the known 5-N, n-N,A-N and N-N interactions) takes place in ~-nucleus annihilations. Inparticular, the study of the K+ spectra is of int~’rest, since thesekaons are produced only in the annihilation process and should traveirelatively undisturbed through the nuclear medium, due to the smallK+-N interact.’.on cross sections. Thus, th? K+ may carry cleaner in-formation on the annihl aticm process Inside nuclei than is the casewith pions. These data are to be compared to the pr d ctions ofMonte Carlo Intra Nuclear Cascade (INC) calculations 7,$ which providea quantitative simulation of the conventional physics expected fromlow energy antiproton annihilations in nuclei.EXPERIMENTAL ARRANGEMENTA broad range spectrometer, depicted in Fig. 13,Zss developedfor the experiment, and te~ted at LAMPF and TRIUMF. It consieteof a circular dipole magnet, 81 cm in diameter, with a pole gap ofPermanent add~ses: *:TRI’UMF, Vancouver V61’ 2A3, British Columbiu.Soreq Nuclear Research Center, Yavm 70600,Israel.1-Fig. 1. Schemotic diagram of CALLIOPE spectrometer.12.7 cm and a ❑agnetic field of 1.2 Tesla. Targets were placed in avacuum chamber in the pole gap at the center of the magnet. sixdetector modules consisting of a pair of x,y position sensitive gascounters, 10 cm apart from one another, together with a ~last!c acin-tillator (AE) and a plastic threshold Cherenkov detector surroundthe pole gap. These detector modules provide position, time offlight, energy loss and Cherenkov information. Together with the mo-mentum and scattering angle obtained from the reconstruction of thepafticle trajectories, they aliow for the identification of n, K, p,d and t. The b~am 10 count~d by a thin 8CtntL~l&tor (S1), and eventtriggers are defined by the combination s ●620AE*63.iThe electronicsand data acquisition system are capable o acquiring data at asustained rate of 1000 ●vents per second, with minimal dead time.Thin spectrometer o ntem providenoangular accept~nce of ‘1/2 sr, an-%gular coverage of O < Olab < 180 and momentum acceptance of001 <p~a~< 1.5 “ck?v/c.INTRA NUCLEAR CASqADE MODELsnarch for any “exotic” effecte in the experimentaleclculation of the known physicq LO required. WeIn order todata, a rellablei)ave chosen to use the INC code of Clover et ●l.t’z which wae devel-oped to treat low enerRy ●ntiproton-ucleus interaction, This com-puter code ie ● mod f ed veroion of the INC code ISABEL written byi+Yariv and Fraenkel. ‘ Annihilations ware included by taking the ex-perimental characteriatlca of nucleon-antinucleon ●nnihilation andacatt~ring anu allowing the res~lting particles to cascade through, .the nucleus in the conventional way. All of the known pion ●nd kaonbranches are included in the annihilation channele.ACTIVE S1 TARCET/DETECTORThe use of an active target, like a Si detector, beetde offeringa clean way of scaling the beam on target and eff~ciently rejectingmost of the background events, promises to allow selection of centralannihilations. Using the results of the INC calculations, the totalamount of charged oarticle energy deposited in the Si detector s~lbse-quent to an annihilation can be determined. This is performed bycalculating the energy loss of each charged particle emitted from thenucleus as it propagates through the S1 wafer. The computation in-cludes $he $E/dx from: the incident ~ up to the point of annihila-tion: v-, K- and p emiasfon from the INC calculation; p, d, t, 3He,and a production from n~lclear evaporation; and the recoil energy ofthe residual nucleus.The result of this calculation indicates that in the cases Inwhich a large S1 pulse is obtained, most of the S1 signal is due tocascade protons and evaporation particles, thereby reflecting theWco.“d864200123456 78Annihilation Radius (fro)Fig. 2, Annihilation probabilityas a function of annihilation ra-ditiu calculated using the INC modelfor the interaction of 180 MeV ~ +2%1. The solid histogram io forall events , while the cl~shed his-togram is for ●vents which depou-ited greater than 80 MeV in the Sidetector.amount of excitation depositedin the nucleus by the annihi-lation process. This quantityIn turn is quite sensitive tothe depth at which the annf.hi-lation occurred inside thenucleus, since annihilationsnear the surface deposit theleast amaunt of excitationene~gy in the nucleus. Thecorrelation between the S1 de-tector signal and the annihila-tion radtus is shown in Fig. 2.It ia apparent that by choosingevents with greater than 80 MeVdepcsited in the St detector(the dashed histogram), theannihilation which take placeon the nuclear surface can berejected and the resulting dataset will contnin events forwhich the mean annihilationradius tI inmide the half den-sity nuclear surface.EXPERIMENTAL RESULTSIn FiK.+3 the angle integrated momentum distributions are pre-oented for w ●nd p produced in the interaction of 180 MeV ~ with28S1 and 238U. The solid histograms are the experimental data, whilethe dashed lines reprasant the INC calculations. A striking featureio observed in the pion momentum distribution; the spectra showId10-’\, ,.,,,,..10” Le. ~—. -10 mp4m(MeV/c)m mId~,....,; %(~,ff+)id “ :.,:..,;.;.;10-’ ,., ,.,,,,-, ~-ilo 402 aooaoolmp (MeV/c)Fig. 3. Angle integrated momentum distributions for n+ and pfollowing the interaction of 1S0 MeV ~ with 28S1 and 23%. Thesolid histograms are the experimental data while the dashedhistograms are the result of INC calculations.two bumps, one at high momentum which corresponds to primordial pionsproduced in the annihilation and another at low momentum which ispresumably due to plons that formed deltas and reappeared at lowerenergies. This structure is well reproduced by the INC calculationfor both target masses, with the exception of the fit at the highestpion momenta. The distributions from uranium and silicon show littledifference other than the slopes a~ high momenta being steeper foruranium, which reflect the surface nature of the annihilation and thrIwrger interaction probability for fast pions in uranium relative tosilicon, respectively. Angular distributions of the pionu (not shownhere) #how that the low energy piona are emitted Isotropically fromthe tar~ete, while the high energy ones appear to be coming from therest frame of the nucleon-antinucleon system. These distributionsare also well described by the INC results.The proton momentum distribution are relatively featurelesswith a broad maximum at low momenta. The ehape and magnitude of thediatributione from S1 and U ●re again well fit by the INC calcula-tion which confirms the belief that the protone receive their kinet-ic energy primarily via the path of delta formation and absorption:w+N+A (1)A+N+N+N (2)Little dependence of the shapes of theee distributions upcm target#mass is obse~ed other than the slope at high momenta again beingsteeper for uranium. Angular distributions of the protons ●regenerally fsotroplc, which is consistent with the mechanism ●hewnabove and is well reproduced by the INC calculations.I1%I(iip)E?mo\Fig. 6. Angle integrated momentum dis-tributions for m+ and p following theinteraction of 180 FksV ~ with 28S1,gated by an enerfzy depo8itiGn ofgreater than 8!) FleV in the S1 detector.The solid histograms are the selectedexperimental data and the dashed histo-grams are the selected results of INCcalculationt3.In Fig. 4 the momen-tum distribution for *+and p from S3 gated by alarge signal (E ~ 80 Hev)Sn t~e S1 detector arecompared with the gatedresults from the INCmodel. The shapes of bothexperimmtal distributionsare similar to the ungateddata discussed previouslybut are depleted at highmomenta and enhanced atlow momenta. Thus the neteffect of qating the sili-con 6pectra ro selectsmaller average annihila-tion radii is somewhatsimilar to choosing aheavier mass target, ashas been observed foruranium. The INC modelresults fit the selectedS1 data to the same degreeof accuracy cbtainedearlier for the unselecteddata. Nothing unexpectedis obgerved in the spectraas a result of selectingannihilations that occurprimarily inside the halfdensity nuclear radiusversus outside this radiusfor the ungated events.Thus by conaiderinqonly the experimentalmeasuremellta for protonsand pions, one would beforced to conclude thatnothing unexpected isoccurring in c-nucleus annihilations. Most of the annihilations aretaking pl~ce on the nuclear aurfacc and once the annihilation occurs,the plons formed appear to ir.teractprimarily via the delta resonancepathway. However, the preliminary ex perimenta l kaon momentum diatri-butione, mhown in FIR. 5, are not consi~tent with the predictions ofthe INC model; the experimental data are ‘L times lower than themodel calculations at %200 MeV/c momentum. At much larger momentathe agreement la ●xcellent. The shape of the experimental, apectrumla even inconslatenc with che 8hape of the known primordial kaon!.;lO-““”A.,,=”(’’K+)> .,fg ,.. ;,..:.;:1.! J10-’~.... , Jo eoo m 1000p4w(MeV/c)Fig. S. Angle integrated momentum dis-tributions for K+ production from theinteraction of 180 MeV ~ with 28Si and238U. The solid histograms are the ex-perimental data and the dashed histo-grams are the result of lNC calcula-tions.diatrlbutlon direct fromthe F-N annihilation;there #imply are too fewlow momentum kaontiin the●xperimental data. Thismay be a signal that thekaon branchee of theannihilation chanwl arestrongly affected by thenuclear medium, perhapsdue to a change in co~-finement scale as as l~eenrecently suggested k as thecauae of the EMC effect.It was not possible toselect the kaon eventsaccording to annihilationradius, using the Si de-tector pulse height, dueto inadequate statistics.CONCLUSIONSF’ion and prc,ton pro-duction data from the in-teraction of 180 MeV ~with 28S1 and 238U are nowavailable. In general,these experimental meas-urements are well repro-duced by conventionalIntra Nuclear Cascade cal-culations, even whenannihilations believed tooccur inside the half den-sity nuclear radius areeelected. Preliminarydata for K+ productio~ are inconsis~ent with INC calculations andwith free E-N annihilation data. K“ product~.on appears to be sup-pressed for low momentum kaons produced in ~-nucleus annihilations.REFERENCES1. M. R. Clover, R. M. l)eVries, N. J. DiGiacomo, and Y. Yariv,Phys. Rev. C26, 2138 (1982).2. N. J. DiGia=m and H. R. Clover, J. Phys. G1O, L119 (1984).3. N. J. DiGlacomo, M. R. Clover, R. M. D@Vrie~J. C. Dousse,J. !3. Kapuetinsky, P. L. McGaughey, W. E. Sondheim,J. W. Sunler, M. Buenerd, and D. Lebrun, Phys. Rev. C31, 292( 1985).4. J. W. Sunler, et al., aubmlttsd to Nucl. Inetr. Meth. (1985).5. W. E. Sondheim, R. H. DaVries, M. R. Clover, N. J. DiGiacomo,J. S. Kapustinsky, ●nd J. W. Sunier, Nucl. Instr. Ileth.219, 97(1984).6. Y. Yariv ●nd Z. Fraenkel, Phys. Rev. C24, 488 (1981).7. Y. Yariv and Z. Fraenkel, phys. Rev. ~, 2227 (1979).8. F. E. Close, R. L. Jaffe, R. G. Rober~and G. Gross,Phys. Rev. I&, 1004 (1985).. ..DISCLAIMERThis repwt was prepared as an account of work sponsored by an agency of the(J nitcd States Government. Neither the United States Government nor any agencythereof, nor’ arty of their employees, makes any Warranty, express or implied, or;issumes any legal lial~ility or responsibility for the accuracy, completeness, or use-fulness of any information, apparatus, product, or process disclosed. or representstha[ its use would not infringe privately owned rights. Reference herein to anv spe-cific comrrrercial product, process, or service by trade name, trademark, manufac-[urer, or 01hmwise dees not necessarily constitute or imply its endorsement, recom-mcrrd:it ion, or favoring by the United States Government or any agency thereof.‘The VICWS~nd opinions of :iuthors expressed herein do not necessarily state orreflect thmc of the Uni(ed Stutes Government or any agency thereof.

Antiproton annihilations in nuclei

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Antiproton annihilations in nuclei

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