IL NUOVO CIMENTO VOL. 107A, N. 11 Novembre 1994

Study of the 0ZI-Rule Violation in Antiproton Annihi lat ion in Deuterium at Rest(*).

THE OBELIX COLLABORATION

V. G. ABLEEV(1), A. ADAMO(2), M. AGNELLO(3), F. BALESTRA(4), G. BELLI(9) G. BENDISCIOLI(5), i . BERTIN(6), P. BOCCACCIO(7), G. C. BONAZZOLA(4), E. BO2"rA(4) T. BRESSANI(4), M. BRUSCHI(6), M. P. BUSSA(4), L. BUSSO(4), D. CALVO(4) U. CAPPONI(6), B. CEREDA(6), P. CERELLO(4), C. CICALO(2), U. CORRADINI(9) S. COSTA(4), S. DE CASTRO(6), C. DE LEO(S), D. YU. DENISOV(1), F. D'ISEP(a) A. DONZELLA(9), L. FAVA(4), A. FELICIELLO(4), L. FERRERO(4), A. FILIPPI(4) V. FILIPPINI(5), A. FONTANA(5), D. GALLI(6), R. GARFAGNINI(4), V. GASTALDI(7) B. GIACOBBE(6), P. GIANOTTI(S), O. E. GORCHAKOV(1), A. GRASSO(4), C. GUARALDO(s) F. IAZZI(3), A. LANARO(S), E. LODI RIZZINI(9), M. LOMBARDI(7), V. LUCHERINI(s) A. MAGGIORA(4), S. MARCELLO(4), V. MARCONI(6), G. V. MARGAGLIOTTI(10) G. MARON(7), A. MASONI (2), I. MASSA(6), B. MINETTI (3), p. MONTAGNA(5) M. MORANDO(11), F. NICHITIU (1, 8), D. PANZIERI(4), G. PAULI(I°), M. PICCININI(6) G. PIRAGIN0(4), M. POLI(12), S. N. PRAKHOV(1), G. PUDDU(2), R. A. RICCI(7'11) E. ROSSETT0(4), i . ROTONDI(5), i . i . ROZHDESTVENSKY(1), A. SAINO(5) P. SALVINI(5), L. SANTI(13), i . G. SAPOZHNIKOV(1), i . SEMPRINI-CESARI(6) S. SERCI(2), R. SPIGHI(6), S. TESSARO(10), F. TOSELLO(4), V. T. TRETYAK (1' 5) G. USAI (2), L. VANNUCCI(7), S. VECCHI(6), G. VEDOVATO(7), L. VENTURELLI(9) M. VILLA(6), A. VITALE(6), A. ZnioiI(14), A. ZOCCOLI(6) and G. ZosI(a)

presented by M. G. SAPOZHNIKOV (1) Joint Institute for Nuclear Research - Dubna, SU-101000 Moscow, Russia (2) Dipartimento di Scienze Fisiche dell'Universit5 Cagliari

INFN, Sezione di Cagliari - 1-09100 Cagliari (3) Politecnico di Torino and INFN, Sezione di Torino - Torino (4) Dipartimento di Fisica dell'Universitd - Torino

INFN, Sezione di Torino - 1-10125 Torino (5) Dipartimento di Fisica Nucleate e Teorica dell'Universit5 - Pavia

INFN, Sezione di Pavia - 1-27100 Pavia (6) Dipartimento di Fisica deU'Universit~t - Bologna

INFN, Sezione di Bologna - 1-40100 Bologna (7) Laboratori Nazionali di Legnaro deU'INFN - 1-35020 Legnaro (Padova) (s) Laboratori Nazionali di Frascati deU'INFN - 1-00044 Frascati (Roma) (o) Dipartimento di Chimica e Fisica per i Materiali dell'Univerist5 - 1-25060 Brescia

INFN, Sezione di Torino - 1-10125 Torino (lo) Istituto di Fisica deU'Universit~t - Trieste

INFN, Sezione di Trieste - 1-34100 Trieste (11) Dipartimento di Fisica dell'Universit5 - Padova

INFN, Sezione di Padova - 1-35100 Padova

(*) Paper presented at the conference HADRON '93, Como, June 21-25, 1993.

2315

2316 M. G. SAPOZHNIKOV

(12) Dipartimento di Energetica dell'Universitd - 1-50100 Firenze INFN, Sezione di Bologna - 1-40100 Bologna

(13) Istituto di Fisica deU'Universit& - 1-33100 Udine INFN, Sezione di Trieste - 1-34127 Trieste

(14) Dipartimento di Chimica e Fisica per i Materiali dell'Universit& - 1-25100 Brescia INFN, Sezione di Pavia - 1-27100 Pavia

(ricevuto il 14 Aprile 1994)

Summary. - - The branching ratios of On and ~o~ final states were measured for the antiproton annihilation in gaseous deuterium at rest. A significant violation of the OZI rule was found.

PACS 14.40 - Mesons and meson resonances. PACS 13.75 - Hadron-induced low- and intermediate-energy reactions and scattering (energy ~< 10 GeV). PACS 01.30.Cc - Conference proceedings.

The main interest in O-meson production in NN annihilation stems from the problem of the OZI-rule violation and a possible contribution from ~s pairs in the nucleon wave function[i-3]. Such interest has been stimulated by the experimental data [4-6] demonstrating that the yield of • in ~p annihilation is remarkably high and the ratio between O-meson and (o-meson production is larger than what predicted by the theory. Indeed, knowing that the vector mesons are nearly ideally mixed and assuming that the N and N have no strange quark components, one may expect [3] that R=OX/o)X=tg 2 8=(1-3)-10 -8 (8 is the deviation from the ideal mixing angle).

For ~p annihilation at rest there are data from the ASTERIX Collaboration [5] on the production of 0X and o~X final states with X = ~, ~, ~o, p, ~ from S- and P-wave states of the protonium. It occurs that the experimental values of R for different annihilation channels are slightly higher (by a factor 2-3) than the naive OZI-rule prediction. The only exception is for the channel with X = ~ for annihilation from S-wave where the ratio R is approximately 20 times higher than what follows from the OZI rule. Recently, the Crystal Barrel Collaboration has reported [6] the measurements of R for X = ~o, ~ channels in antiproton annihilation in liquid hydrogen. They confirmed the unusually high ratio for 0~ /o~ found by the ASTERIX group. Moreover, there are some preliminary data from the Crystal Barrel detector on the 0~,/o)~, ratio which turns out to be as large as for 0~/o~.

Production of 0-mesons in ~n-annihilation was studied in the old bubble chamber experiments [7, 8]. A substantial yield of the O::- channel was found. If one uses the branching ratio for the ~o7:- channel from the measurements of [9,10], then the ratio 0~- /o)~- becomes even larger than for ~p annihilation. But the statistics in these experiments was rather scarce. For instance, in [7] only 54 events of the O~- channel were found. We intend to verify the bubble chamber results [7,8] on substantially larger statistics studying the antiproton annihilation in deuterium at rest using the OBELIX spectrometer at LEAR (CERN)[11].

STUDY OF THE OZI-RULE VIOLATION IN ANTIPROTON ANNIHILATION ETC. 2317

We have measured the branching ratios of Ou- and o)~- channels

(1) ~ + d o ~ - + ¢ + p ,

(2) ~ + d - * ~ - +o~ + p ,

for two regions of proton momenta: p < 200 MeV/c and p > 400MeV/c. A description of the apparatus can be found in[ll]. Here we give only a short

description of the detectors relevant to the present experiment. OBELIX is a magnetic spectrometer for the detection of charged and neutral

particles. It consists of four subdetectors arranged inside and around the Open-Axial Field Magnet (OAFM), providing a field of 0.5 T in an unobstructed volume of ~ 3m 3 . These detectors are:

1) The Spiral Projection Chamber (SPC): an imaging vertex detector with three-dimensional readout for charged tracks and X-rays detection.

2) The Time-Of-Flight (TOF) system: two coaxial barrels of plastic scintillators for charged-particle identification at the trigger level. The first barrel consists of 30 slabs located at 18 cm from the beam axis, the second barrel consists of 90 slabs at 136 cm from the beam axis. The intrinsic time resolution of the system is 500 ps FWHM.

3) The Jet Drift Chamber (from CERN AFS Spectrometer) for tracking and particle identification by dE/dx measurement. The detector consists of two half-cylinders (160 cm in diameter, 140 cm long). The 3280 sense wires are organized into 82 azimuthal sectors of 4 ° and are equipped with a 100 MHz 8 bit FADC readout system on both sides. The intrinsic spatial resolution is zz = 12 mm, z~¢ = 200 ~m. The momentum resolution, which was measured at 928 MeV/c for monoenergetic pions from the reaction ~p ---~:÷ ~-, was found to be 3.5%.

4) The High-Angular-Resolution Gamma-Detector (HARGD): an electromagnetic calorimeter consisting of four modules made by layers of (3 × 4)m 2 Pb converter foils by planes of limited streamer tubes.

The HARGD modules were under test during the data acquisition and their data are not included in this analysis.

The antiprotons of 105MeV/c supplied by LEAR were stopped in a cylindrical gaseous target filled with deuterium at NTP. The thickness of the target mylar walls was 12 ~m. The length of the target was 62 cm and the antiprotons stopped in a rather limited central part of the target. The spread of the annihilation vertex distribution along the beam axis is Zz = 5 cm.

The data used in this analysis were collected with a special trigger dedicated to the selection of two kaons from O decays. The trigger conditions were as follows:

1) Three or four hits in inner and outer layers of the TOF system.

2) Angular correlations.

The topology of the trigger was chosen after Monte Carlo simulation of the reaction

~ + d---~@ + 7:- + P s ,

O---) K + + K - .

2318 M.G. SAPOZHNIKOV

I t turns out that in the lab system the open angle between two kaons is r a the r small 25 +_ 13 °, whereas the angle between the pion and a kaon is r a the r large ~ 179 +-

--- 19 °. So the typical event looks like a pion recoiling back-to-back with respect to a pair of strongly correlated kaons.

So, to select events with such angular correlations the following coincidence between the signals from counters of inner TOF barrel was requested: there should be two hits in any three adjacent slabs correlated with one hit in an opposite slab.

3) Request of a slow particle. To increase the percentage of kaons among the t r iggered events it was requested that the time difference between any hit in inner TOF counters and a hit with maximum time in outer TOF barrel should be g rea te r than 7 ns. This condition should reject pions which have an average t ime difference of the order of 3 ns.

The overall statistics consists of 1.1 • 10 ~ events with trigger. We also have a sample of 142 000 events of the minimum bias trigger, requiring only the disappearance of an antiproton in the ta rge t and a sample of 18700 events with t r igger on multiplicity (from 3 to 8 hits in TOF barrels).

~ + d ~ - + ~ + p ~ .

We search for O-mesons in their decays to K ÷ K - . To select this final s tate the events with 3 tracks in JDC and negative total charge were analysed. I t was required that, the kinematical fit probability for this reaction should be at least 5%. To

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Fig. 1. - Effective-mass distribution of K ÷ K- in the reaction ~ + d ~ =- + K ÷ + K + p for spectator proton region (p < 200MeV/c) (a)) and for proton momenta p > 400MeV/c (b)). Effective-mass distribution of ~+ ~- ~o for the reaction ~ + d --* 2~ + ~+ + ~0 + p for spectator proton region (c)) and for high proton momenta (d)).

STUDY OF THE OZI-RULE VIOLATION IN ANTIPROTON ANNIHILATION ETC. 2319

separate kaons a cut on time of flight was applied. To select events with proton- spectators the cut on total momentum of tracks in JDC Ptot < 200 MeV/c was introduced.

In fig. la) the effective mass of the K ÷ K- system for events passing through all selection criteria is shown. A clear narrow peak in the (I) region is seen superimposed on some background. We fit this distribution with the Breit-Wigner function smeared by a Gaussian reflecting the experimental resolution. The background was approximated by the expression BG(m) = (m - too) al exp [a2 + a3m + a4m2], where m0 is the threshold for the K ÷ K- spectrum, and ai are free parameters. There are 831 ± 50 events under the ~ peak.

The momentum distribution of negative pions from these events has a maximum at 650 MeV/c as expected from kinematics of the ~n--~ ~ - two-body reaction. The Fermi motion of the neutron in a deuteron smeared the momentum of pions and the final distribution may be approximated by a Gaussian with ~ = (27.5 _+ 1.3)MeV/c. The distribution of the total momentum of particles in JDC, which should be equal to the momentum distribution of the proton-spectator, indeed demonstrates a typical Hulthen-like behaviour. It is remarkable that the angular distribution between a kaon and the direction of ¢ is proportional to sin 2 0 as expected for annihilation from sS 1 initial state.

~ + d ---~- + (I) + p.

To select this reaction in the high-momentum proton region (p > 400 MeV/c) we used the sample of events with 4 tracks in JDC, having the kinematical fit probability of at least 3%, with all particles obeying time-of-flight cuts for protons, kaons and pions.

The effective mass distribution of K ÷ K- for events passing through the applied cuts is shown in fig. lb). There are 17 ± 5 events under the ~ peak.

It is interesting that the branching ratio of this reaction in the high-momentum region of the proton spectrum is rather high, at least two times larger than the predictions based on the calculations of Zou and Locher where different meson rescattering diagrams were considered.

~ + d---~ ~- + oJ + ps.

To test the OZI rule it is desirable to compare the measurements of the ~ and ~o mesons yields in the same experimental conditions. However, for our trigger, dedieated to the selection of events with at least one slow partiele in the final state, this channel was not suitable. The proton spectators could not pass through the inner barrel of the TOF system and the pions in this reaction are mainly fast. On the other hand, there were no measurements of this reaction for annihilation in gas. So we have to use the minimum-bias sample to determine the branching ratio of this reaction.

The effective-mass distribution of the ~+ ~-n0 system is shown in fig. lc). To calculate the neutral-pion momentum it was assumed that p~o = -Ptot. There are 126 _+ 34 events under the oJ peak.

~ + d--) 7:- + ¢o + p.

For the selection of this reaction in the high-momentum proton region (p > 400 MeV/e) we used the data from our trigger sample, with 4 traeks in JDC, having the 1 C fit probability at least 3%, with all particles obeying time-of-flight cuts for protons and pions.

2320 M. G. SAPOZHNIKOV

The effective-mass distribution of the =÷ ~- n ° system is shown in fig. ld). There are 319 _+ 40 events under the ~o peak. Contrary to the situation with the ~ channel, the branching ratio for ~o~ at high proton momenta is in agreement with the predictions of the meson rescattering calculations of Lev and Buzatu.

Finally, it was obtained that the ratio R = ~ / ( o ~ is

R(¢~-/¢o~ ) = 0.133 + 0.033 p < 200 MeV/c,

R ( O ~ - / o ~ - ) = 0.113 +_ 0.040 p > 400 MeV/c.

These values are considerably higher than the OZI rule prediction R = ( 1 - 3)" 10 -~ . In fact, for annihilation on a quasi-free neutron, when the momenta of the spectator proton are less than 200 MeV/c, the ratio R should be multiplied by the phase space factor f = k~/k¢ = 1.18 which worsens the situation with the OZI rule for ~n annihilation. The similar value for R was obtained in antineutron-proton annihilation into O~ + and co~ + channels[12].

For the explanation of the apparent violation of the OZI rule in the antiproton annihilation several suggestions were done:

- Admixture of strange sea quarks in the proton [1, 2].

- Final-state interaction of the kaons [13].

- Resonances in the q)u system, like C(1480)[14].

To disentangle among these possibilities a systematic study of annihilation into kaons is needed.

We are very grateful to D. Buzatu, F. M. Lev, M. P. Locher and B. S. Zou for providing us with the results of their calculations. The J I N R group acknowledges the support from the Russian Fund of Fundamental Research under grant No. 93-02-3997. One of us (MGS) would like to thank the organizers of the Conference for the financial support at Como.

R E F E R E N C E S

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Phys. A, 558, 269c (1993). [7] R. BIZZARRI et al.: Nuovo Cimento A, 20, 393 (1974). [8] A. BETTINI et al.: Nuovo Cimento A, 63, 1199 (1969). [9] R. BIZZARRI et al.: Phys. Rev. Lett., 25, 1385 (1970).

[10] A. BETTINI et al.: Nuovo Cimento A, 47, 642 (1967). [11] A. ADAMO et al.: Proceedings of the NAN-91 Conference, Moscow, Sov. J. Nucl. Phys., 55,

1732 (1992). [12] V. LUCHERINI et al.: these proceedings, p. 2271. [13] Y. Lu, B. S. Zou and M. P. LOCHER: Z. Phys. A, 345, 207 (1993). [14] S. I. BITYUKOV et al.: Phys. Lett. B, 188, 383 (1987).