И Ф В Э ПЭФ OBSERVATION OF A SPIN 4 NEUTRAL MESON WITH 2 GeV MASS DECAYING IN П П

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1 I N S T I T U T E F O R H I G H E N E R G Y P H Y S I C S И Ф В Э ПЭФ W.D. Apel, K. Augenstein, E. Bertolucci, S.V. Donskov, A.V. Inyakin, V.A. Kachanov, W. Kittenberger, R.N. Krasnokutsky, M. Kruger, G. Leder, A. A. Lednev, I. Mannelli, yu.v. Mikhailov, H. lluller, G.M. Pierazzini, Yu.D. Prokoshkin, И. Quaglia, H. Schneider, A. Scribano, F. Sergiampietri, R.S. Shuvalov, G. Slgurdsson, И. Steuer, H.L. Vlncelli OBSERVATION OF A SPIN 4 NEUTRAL MESON WITH 2 GeV MASS DECAYING IN П П Serpukhov 197S

2 W.D. Apel*, K. Augensteln*, E. Bertolucci +, S.V. Donskov**, A.V. Inyakin**, V.A. Kachanov**, W. Kittenberger ++, R.N. Krasnokutsky**, M. Kruger*, G. Leder ++,IA.A. ~Lednev * * I. Mannelli +, Yu.v. Mlkhailov**, H. Muller*T~G.M. Pierazzini +, Yu.D. Prokoshkin**, M. Quaglia*, H. Schneider*, A. Scribano +, P. Sergiampietrl +, R.S. Shuvalov**, G. Sigurdsson**, M. Steuer ++, M.L.Vincelli + OBSERVATION OF A SPIN 4 NEUTRAL MESON ТГГГН 2 GeV MASS DECAYING IN» п (Joint Experiment of the Institute for High Energy Physics, Serpukhov, USSR and the European Organization for Nuclear Research, Geneva, Switzerland) Submitted to Phys. Lett. + ФФ + + Institut fur Experimentalle Kernphysik, Universit'a't und Kernforschungszentrum, Karlsruhe, BED. Institute di Pisica dell'dniversita, Pisa and INEN, Sezione di Pisa, Italy. IREP, Serpukhov, USSR. Institut fur Hochenergiephysik, Wien, OAdW, Austria. CERN, Geneva, Switzerland ife

3 M-24 Аннотация Аполь ВЛ., А)геиштайн, К., Бертолуччи Е Винчеллн М.Л., Поисков СВ., Инякин А.В., Качанов В.А., Кяалья М., Кнттонборгор В., Краснокутошй Р.Н., Крюгер М., Леаер Г., Леанев А.А., Машшли И., Михайлов Ю.В., Мюллер X., Пьерацшши Г.М., Прокошкин ЮМ.., Сержиампьетри Ф Свгурдсон Г., Скрвбано А., Шнайдер X., Штоер М Шувалов Р.С. Наблюдение нейтрального h-мезона со сшшом J - 4 и массой 2 ГэВ. (Объединенный эксперимент Института фнаиш высоких энергий, Серпухов, СССР и Европейской организации ядерных исследований, Женева, Швейцария). Серпухов, стр. с рис. (ИФВЭ ПЭФ 75-84). Библвогр. 5. Сообщается о результатах исследования нейтральных меэонлых состояний, о"5раэс==чных в ггрсоудареннях при импульсе 40 ГэВ/с. Эксперименты выполнены на 70-Гэв ускорителе ИФВЭ. Для ре - гистрацни высокоэнергетичных у-квантов испот>зоваи годоскопический спектрометр в линию с ЭВМ. В области 2 ГэВ обнаружена четкая структура в спектре масс онпионов, образующихся в реакции г~р - ir o!r o n. Угловые распределения распада показывают в этой области изменения с ростом массы, характерные для резонанса со спином J 4, Масса найденного в работе мезона - М = ( ) МэВ, оценка полной ширины распада - (180 ± 50) МэВ. Abstract Apel W.D., Augeneteln К., Bartoluccl E., Donakov S.V., Inyctkin A.V., Kachanov V.A., Klttenberger W., Kraanokutsky B.H., Kruger II., Leder G., Lednev A. A., Mannelll I., Hlkhailov Yu.V., lluller Я., Plernzzlnl G.H., Prokoahkin Vu.D., Quaglla Ы., Schneider H., Scribano A., SerglMpletri P., Sbuvalov R.S., Sohnelder H., Steuer M., Vincelll H.L. Observation of a Spin 4 Neutral lleson with 2 GeV Haas Decaying In " ". (Joint Experiment of the Institute for High Energy Physics, Serpukhov, USSR and the European Organization for Nuclear Research, Geneva, Switzerland). Serpukhov, P. 14. (ШКР 75-84). Ref. 3. The Invariant шва apactrim of neutral meson states from ir~p lnteractlona at 40 Gev/c Incident momentum baa bom investigated In a high atatlatica experiment performed at the 70 GeV IHRP accelerator. To detect the high energy photons coming from the produced neutral states, a hodoacope spectrometer with a computer on-line waa used. A clear structure on the mass spectrum of dlplons produced In the reaction»~p - ir <r a la observed at 2 GeV. The decay angular distributions show In this mass region the variation with швов typical of a state with r apln J = 4. The maaa of the observed aeaon la found to be H = (2020 ±25) HeV and the estimate of the full width la (180 ± 50) HeV.

4 We have performed an experiment which studies the mass spectrum of neutral meson states from n~ p interactions at 40GeV/c incident momentum. The reactions investigated are of the type n~ + p - M + n (1) with the M meson states decaying in Y'S, ""' s and other neutral particles whose final decay products are y's. The invariant mass region above 2 GeV has been investigated in a previous work. In the following we report the results of a further search for heavy neutral mesons in the mass region up to 3 GeV. The experiment has been carried out with the NICE set-up described elsewhere' 2 '. The lay-out of the apparatus, which uses a negative secondary beam at the 70-GeV IHEP accelerator is shown in fig. 1. The H_ liquid target was surrounded by a set of veto counters selecting neutral final states of the type (1), with y-rays going in a forward cone. These high energy y-rays from the decay M -» my (m = 2, 3,...) were detected in a 648-channel

5 Fig. 1. Experimental lay-out. Horizontal and vertical tending magnets M-_, k^ and quadrupoles LJJ, Lgg are final part of the beam transport system. Sj - S 7 and % are scintillation counters defining the beam. C^ - C. are gas threshold Carenfcov countera for the Identification of n~, K" and p in the beam. PC. and PC 2 are three-coordinate proportional wire chambers which, together with the 1 em bin hodoscope H, define the Incoming particles. The liquid hydrogen target H 2 la surrounded by lead-sdntillator sandwich counters B], Bg, Aj - A 24, F :J - F 3. Counters Sg, Fj 2 and Sj, are in anticoincidence. BS is the 648-channel hodoscope spectrometer to measure К-rays. '

6 /о/ hodoscope spectrometer ', on-line with a computer. The distance between the spectrometer and the liquid hydrogen target was L=3 m which corresponds to a kinematical mass acceptance for my-systems in reaction (1) up to about 5 GeV. 7 During all the running time about 10 events were collected. Here we report results based on 1/3 of the total statistics corresponding to a sensitivity of 10~ 35 cm /event for reaction (1). The analysis of the my-systems has been done with the help of an automatic shower recognition program which scans the pulse height distributions in the 3 projections of the spectrometer looking for peaks that could be associated as belonging to the /2 4/ same shower '. The events thus found with 2, 3 or 4 showers have been kinematically reconstructed and invariant mass spectra have been produced. Before discussing these spectra some comments are necessary on the procedure to determine the multiplicity m of showers. Due to the small distance between the target and the spectrometer,gammas coming from fast Jf*s are not sufficiently separated ( 3 cm) to give two distinct showers. Thus an energetic shower found by program could mean а у detected as well as a ". Therefore a significant fraction of the decays IT + n n, namely those decaying asymmetrically, so having a forward n with momentum near the maximum, is detected as 3 shower events, the 3 showers being one

7 unresolved " plus 2 resolved y's from the other». We will indicate these decays as M * U» ) where the subscript "as" stands for asymmetrical. Fig. 2 shows the mass spectrum for 3 shower events (m = 3). For the reason previously mentioned this spectrum is a mixture of M - rr y and M * (n n i events. This explains the eimulta - US neous presence of the < > and f peaks in the spectrum with u> * п у and f -> (п п ) a a # Besides these well established states, a new peak is clearly visible at higher mass, around M = 2 GeV, which can be either a resonant state or a strong dynamical effect (cups etc.). The mass spectrum for events with shower multiplicity m=4, for those identified by program as i7~p -» n n n (2) is shown in fig. 3. Here too a shoulder is visible In the mass region around 2 GeV. This indicates that the events in the peak at 2 GeV in fig. 2 do not decay mostly into " y (such events should disappear in the spectrum in fig. 3, as is clearly illustrated by the U) case), but rather into (n n ) B B, so they belong; to reaction (2). Some indications about a possible structure in - + _ /5/ this mass region, have been reported for KK and я ir states It should be remarked that the structure at 2 GeV in the mass spectrum with и = 3 (fig. 2) is more pronounced than tho ov

8 дн/дм M.G«V Invariant maaa apectrua for 3-ahower event» a - 3, I.e. w y or (»*»").,dacaya. The paaka corraapond to th» decays of «a (783), f(oto) and Ь(ЭОаО). The solid carve la the remit of a l.a. fit to the data daaorlbad in the text. The dashed line describes the oontlnuoua part of the apectnm by polynomial. The dotted curve ahowa the b-peak aa it eoaea out from the fit. An atteapt to fit the apectrua above t веаоп with nonreaonant curve elve» the probability value Р(к а )«1(Г: fig- 3. Invariant aaaa apectrua for 4-«bower eventa, m - 4, I.e. ir c > s Btatea produced in reaction (2). The peak correaponda to the decay t -» "'.The olid curve la the reault of a l.a. fit.

9 visible in the spectrum with m = 4 (fig. 3); the number of events in the h-reglon being 10% of the f events in the former spectrum and 3% in the latter one.this Indicates that the decays vf 3 ~n''n o in the region of the observed structure, are concentrated at small cm. angles as it should be expected for the decay of a high-spin particle. In order to understand the structure observed in the mass spectra better, an angular analysis for 3 and 4 shower events added together has been made. For the events used in this analysis a cut in the four-momentum transfer -t < 0.16 (GeV/c) Z has been applied in order to favour events produced by OPE mechanism. In fig. 4 we present the experimental " " angular distributions evaluated in the Gottfried-Jackson frame (not corrected for efficiency) for masses from 0.9 to 2.5 GeV. The general behaviour of the low mass distributions in fig.4 is as expected. The isotropic angular distribution below 1 GeV gradually gets distorted by the growing amount of D-wave scattering. In the f region S- and D-wave produce the familiar pattern with a dip at cos0 = 0.57 typical tor spin J = 2. At masses higher GJ than f mass the rise of the angular distributions above this dip gradually disappears. The striking feature of the distributions at higher masses is the reappearance of a second maximum at cosd = 0.65 but 8

10 и 04 ot at ш С«Яво Ot ai as ю Qt OB COS&. Fig. 4. Bxperlaents.1 " " angular distributions, evaluated In the Gottfried- Jackson fraae (not corrected for efficiency) for virloup Bass Intervale. The dotted curves show the Ideal behaviour for decaying dlpions with spin J = 2 and J = 4, fully polarized and detected with 100% efficiency.

11 now the dip is shifted to lower cos в around 0.35, for the GJ h-peak mase region GeV. This behaviour cannot be explained in terms of S- and D-wave nn scattering alone. A strong contribution of G-wave spin 4irir scattering must be assumed. Above 2.2 GeV mass of the rr n -system the structure of the angular distributions again changes substantially; the dip is filled up, the second maximum flattens out. The spin 4 contribution to "" scattering is decreasing again. In conclusion the peak we observe in the invariant mass spectrum combined with the dip at cos " GJ = 0.35 that we see in the angular distributions between 1.9 and 2.2 GeV gives strung evidence for the observation of the expected spin 4 neutral h meson, produced in our experiment in the reaction and decaying in two n n~ + p -» h + n (3) h -» n ir. (1) The quantum numbers which have to be assigned to the h meson are P 4* spin-parity J = 4, positive C- and G-parity and isospin 1=0 or 2. In order to evaluate the mass and the width of the observed h meson a l.s. fit to the 3 shower mass spectrum has been using a third degree polynomial to describe the background done and gaussian curves for the peaks ( the widths of the peaks are determined essentially by the mass resolution of the apparatus). 10

12 The results of the fit are shown in the first column of the table. The mass of the ш is found to be ( ) MeV which is in agreement with the table value MeV within 1,3%. The difference does not exceed the systematical error which is due mainly to the uncertainty in the incident beam momentum. The table value of the ы mass is used to normalize the mass scale in our spectrum. The linearity of the scale is proved by the very good agreement of the ratios Mf/M for measured and table mass values: (M r /M bj ) meas /(M f /M CJ ) table = The fit gives as a mass value for the h meson И = ( ) MeV. The quoted error includes the scale uncertainty. The shape of the h peak is well described by a guassiar. curve. The intrinsic width calculated taking into account the mass resolution of the apparatus (ДМ/М = + 5%) Is Г= ( ) MeV. A fit similar to the one described for the m = 3 mass spectrum has been done also for the и = 4 mpss spectrum (fig. 3). The results are shown in the second column of the table and it can be seen that the mass value for the h peak is equal to the one obtained for m = 3. A preliminary evaluation of the angular «oments illustrates the presence of such a spin 4 state. Using simply the geometrical 11

13 TABLE Parameters of the fits for mass spectra with shower multiplicities m = 3 and 4 spectrum m = 3 (fig. 2) m = 4 (fig. 3 ) M* *** ( ) - Д* g f M л *** ( ) g M h Д g g h / g f o.xi + o.oi *> Masses M, and widths Д(FWHM), are given in MeV y g is a normalization factor. **) Table mass value, used for scale normalization. The measured mass value (not corrected) is given in brackets. 12

14 M t GeV Fig. S. Preliminary evaluation of the 8th unnormallzed spherical harmonic moment as a function or <" mass. The arrow shove the position or the peak maxlnun In fig

15 acceptance of our apparatus, but not taking into account tho shower recognition efficiencies, we fitted the contribution of the 8th unnormalized spherical harmonic moment N Yg to our data (fig. 5). W<i find it consistent with zero for masses of the ff ""system below 1.7 GeV, rising above this value and reschinf? its maximum at 2 GeV, then decreasing again to low values around 2.5 GeV. Reaction (3), associated with decay ( I), is a very low rate process. Our estimated production cross section, the unknown branching ratio for decay (4) included, is only some tens of nanobarns. The analysib of the data is continuing in order to study further characteristics of this new particle. R E F E R E N C E S 1. W.D.Apel et al. IHEP Preprint 72-28, Serpukhov (1975); Phys. Lett., 56B (1975) W.D.Apel et al. IHEP Preprint , Serpukhov (1974); XVII Intern. Conf. on High Energy Physics, paper 536; Proceedings, 1-35, London, Yu.B.Bushnin et al. IHEP preprints and Serpukhov (1974);Nucl.Instr.Meth.,106,497 (1973) and 120 (1974) w.d.apel et al. IHEP Preprint , Serpukhov (1974); XVII Intern. Conf. on High Energy Physics, paper 537; Proceedings, 1-35, London, 1974, 5. R.Wagner. XVII Intern. Conf. on High Energy Physics; Proceedings, 11-35, London, 1974; Dowell et al., Experimental Meson Spectroscopy Conference, Boston, 1974; Maener et al., ibid. Received 13 June

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