Z. Phys. A 359, 435–444 (1997) ZEITSCHRIFTFUR PHYSIK Ac© Springer-Verlag 1997

Study of the OZI selection rule in hadronic processes

S.V. Golovkin1, A.P. Kozhevnikov1, V.P. Kubarovsky1, A.I. Kulyavtsev1, V.F. Kurshetsov1, L.G. Landsberg1, V.V.Molchanov1, V.A. Mukhin 1, I.N. Nikitin 1, V.I. Solyanik1, D.V. Vavilov1, V.A. Victorov 1, M.Ya. Balatz2, G.V. Dzubenko2,G.S. Lomkatsi2, V.T. Smolyankin2

1 Institute for High Energy Physics, Protvino2 Institute of Theoretical and Experimental Physics, Moscow

Received: 5 June 1997Communicated by V.V. Anisovich

Abstract. In the experiments with LEPTON-F and SPHINXspectrometers the pion-induced charged exchange reactionsπ− + p → φ + n andπ− + p → ω + n atPπ−=32.5 GeV, aswell as proton-induced diffractive reactionsp +N → [pφ] +N and p + N → [pω] + N at Ep=70 GeV were studied.The comparison of the cross-sections forφ andω-productionin these reactions is used for testing the OZI selection rulein hadronic processes. It has been demonstrated that in pionreactions the ratio of the yields ofφ andω-mesonsR(φ/ω) =(3± 1) · 10−3 is in a good agreement with naive quark modelprediction based on the mixing in vector meson nonet and onthe OZI rule (R(φ/ω)OZI = tg2∆θV ' 4·10−3). At the sametime in proton reactions the effective ratio ofφ andω yieldsis ∼ (4÷ 7) · 10−2, i.e. a strong violation of the OZI rule isobserved in proton-nucleon interactions. This violation can bein favor of possible existence of some exoticss component inthe quark structure of protons.

PACS: 13.85.Fb; 14.40.Cs; 14.40.Ev

1 The OZI rule

The selection rule for connected and disconnected quark dia-grams that is referred to as the OZI rule has been known fora long time [1-3]. According to this rule, processes involvingthe annihilation or creation of a quark-antiquark pair enter-ing into the composition of the same hadron are forbidden or,strictly speaking, strongly suppressed. The OZI rule may beillustrated by the diagrams for the production and decay of thevector mesonsω andφ (see Fig. 1).

Let us consider exclusive reaction ofqq pair productionA + B → C1 + C2 + ... + Cn + (qq) (q = u; d; s) with parti-clesA, B, Ci without strange quarks (see [4]). The ratio ofamplitudes of the corresponding reactions

Z =

√2M [(A + B → C1 + C2 + ... + Cn + (ss)]

M [A + B → C1 + C2 + ... + Cn + (uu)] + M [A + B → C1 + C2 + ... + Cn + (dd)](1)

determines the violation of the OZI rule in these processes (theOZI rule demandsZ = 0).

It is well known that neutral mesons from meson nonetsare the mixture of SU(3) octet and singlet states

|ω8 > = | 1√6(uu + dd− 2ss) >

|ω1 > = | 1√3(uu + dd + ss) >

}.

For vector meson nonet

|φ > = cosθV · |ω8 > −sinθV · |ω1 >= −cosα · |ss > −sinα · | 1√

2(uu + dd) >

|ω > = sinθV · |ω8 > +cosθV · |ω1 >= −sinα · |ss > +cosα · | 1√

2(uu + dd) >

.

Here θV is a mixing angle for vector nonet;α (or αV ) =∆θV = θV − θoV ; θoV = arc tg 1√

2= 35.3o is an ideal mixing

angle (for this ideal mixing|φ >= −|ss >; |ω >= 1√2(uu +

dd) >), as on diagrams in Fig. 1).For the ratio of amplitudes ofφ and ω production the

following expression was derived [4]:

β =M (A +B → C1 +C2 + ... +Cn + φ)M (A +B → C1 +C2 + ... +Cn + ω)

= − Z + tgα1− Z · tgα (2)

or

Z = (β + tgα)/(β · tgα− 1). (3)

Thus the ratio of corresponding cross sections is

Fig. 1. Diagrams for:a,b OZI-allowed andc,d OZI-suppressed productionand decay processes:a reactionπ− + p → ω + n, b decayω → π+π−π0

(connected quark diagrams);c reactionπ− + p → φ + n, and d decayφ→ π+π−π0 (disconnected quark diagrams)

436

R(φ/ω) =σ(A +B → C1 +C2 + ... +Cn + φ)σ(A +B → C1 +C2 + ... +Cn + ω)

= |β|2 · f

=( Z + tgα

1− Z · tgα)2· f, (4)

where f is a kinematical phase space correction factor.Because the phase ofβ is unkown it is only possible to

estimate the limits for the OZI violation factor|Z|:

| −|β| + tgα1 + |β| · tgα | < |Z| <

|β| + tgα1− |β| · tgα . (5)

Many measurements of theR ratio and testing of the OZI rulein different reactions were made in the last thirty years. It is thegeneralization of the OZI rule for the processes with charmedquarks which explaines the anomalously small decay widthof theJ/ψ particle, representing a boundcc state (in analogywith φ meson).

Recent years have seen a revived interest in the OZI rule.First, intensive searches for exotic hadrons (see the surveysin [5-10] and references therein) are greatly facilitated bychoosing production reactions in which the formation of con-ventional particles is suppressed by the OZI rule [11-14]. Sincethis rule may be significantly violated for exotic hadrons be-cause of their complex color structure, signals from exotic andcryptoexotic particles are expected to benefit from more fa-vorable background conditions in OZI-suppressed productionprocesses. While theφπ andφρ decays of isovector mesonswith conventional quark structure|(uu− dd)/

√2> are sup-

pressed by the OZI rule (the corresponding probabilities arereduced more than by two orders of magnitude), the same de-cay channels may prove much more probable for exotic mul-tiquark mesons with hidden strangeness|(uu− dd)ss/

√2>

and hybrids states like|(uu − dd)g/√

2 >. Farthermore, theunexpected results obtained in deep-inelastic lepton scatter-ing on polarized nucleons led to the well-known nucleon-spincrisis [15, 16]. To explain this phenomenon, it has been hy-pothesized that nucleons involve an enchanced quark com-ponent with hidden strangeness, which may induce substan-tial violations of the OZI rule in nucleon processes [17-20].Strong violations of the OZI rule were indeed observed in therelativeφ andω yields from some channels of ¯pp annihila-tion (the reactions ¯pp → φπo, pp → ωπo, pp → φγ andpp→ ωγ [19-21]).

The above arguments suggest that the OZI rule must befurther tested in various production and decay processes in-volving conventional particles. Toward this, we analyzed anumber of processes studied in experiments with LEPTON-Fand SPHINX detectors. In particular, in earlier measurementson the LEPTON-F setup the investigation of this selectionrule was performed in the OZI-suppressed charge-exchangereaction

π− + p→ φ + n (6)

at a relatively high primary momentum ofPπ−=32.5 GeV [22],as well as in the decay of the isovector mesonB/b1(1235) withconventional quark structure1√

2(uu− dd) on the channel:

B/b1(1235)→ φπo (7)

(see [23]). Preliminary results of these studies were reportedin [24]. In the experiments with the SPHINX spectrometer

(which evolved from the LEPTON apparatus) a comparativeanalysis of [pφ] and [pω] systems produced inpN diffractive-like reactions was carried out and the OZI rule in nucleonprocesses was tested by these data [25].

2 Charge exchange reactionπ− + p→ φ + n atPπ=32.5 GeV [22]

Reaction (6) was studied in several experiments at differentenergies in the region between 2.1 and 16 GeV (see [26], [27]and the summary of other data in [26]). There is a strong en-ergy dependence of the cross sections for (6) andR(φ/ω) ratioin these data and some inconsistency for theR(φ/ω) values indifferent experiments. This makes it quite desirable to measurethe cross section of (6) andR(φ/ω) at higher energy. This mea-surement was made in our experiment atPπ=32.5 GeV [22]. Inthis experiment reaction (6) was studied with the LEPTON-Fspectrometer simultaneously with the main research programfor this setup aimed at the investigation ofφπo system in me-son reaction [12, 24, 28], as well as at the search for rare mesonradiative decays onφγ andπ+π−γ channels [24, 28-31].

The LEPTON-F setup included a magnetic spectrometerwith proportional chambers and scintillator hodoscopes andmultichannelγ-spectrometer with total absorption lead glassdetectors. Three gas Cherenkov countersC1 − C3 were usedto identify beam particles, while a wide-aperture thresholdgas counterC4 placed downstream the target in front of mag-netic spectrometer identified charged secondaries. A detaileddescription of the LEPTON-F setup was published in [28, 32].

The material and geometry of the target of the LEPTON-F setup were chosen so as to maximize the overall yields ofφπo, φγ andπ+π−γ production in the corresponding chargeexchange reactions (π−+p→M+n; M → φπo orM → φγ;π+π−γ). A 40 cm thick LiH target (28 g/cm2) was found to beoptimal for these purposes. The target was enclosed by specialguard veto system (scintillator counters, lead-scintillator sand-wiches) aimed at selecting exclusive processes under study.In these measurements 4× 1011 π− and 8× 109 K−-mesonspassed through the target [24, 28-31]1.

In the analysis of two-track events detected in the LEPTON-F magnetic spectrometer and identified by Cherenkov detec-tors the reactions

π− + p→ [K+K−] + (n; ∆, N∗) (8)

and

K− + p→ [K+K−] + Y (9)

were selected after the “elastic” restriction 29< EK+ +EK− < 34 GeV was imposed. In the invariant mass spec-tra on reactions (8) and (9) clear peaks corresponding to theφ-meson production in processes

π− + p→ φ + (n, ∆, N∗) (10)

and

K− + p→ φ + Y (11)

were observed (Figs. 2 and 3). In selection of the OZI forbiddenprocesses (10) a special attention was paid to suppress the

1 The fraction ofK−-mesons in the negative beam withPπ = 32.5 GeVwas' 2%

437

Fig. 2.Effective mass spectrum ofK+K− system in OZI-suppressed reaction(8): π− + p→ K+K− + (n;∆;N∗)

background from the OZI allowed reaction (11) which hastwo orders of magnitude higher cross section as comparedwith (10). Special choice of the working regime forC1 −C3 counters allowed one to suppress this background to anegligibly small level.

In order to determine the number ofφ mesons, producedin “elastic” processes (10) (i.e. the number of events in theφ-peak in Fig.2), the corresponding distribution was fitted bythe sum of polynomial background and the convolution ofa P -wave Breit-Wigner resonance with a Gaussian functionof the spectrometer resolution. Reaction (11), in which thebackground under theφ-peak is very small (see Fig. 3), wasused to determine the shape of the peak. Fixing the standardvalue of theφmeson width (Γ = 4.22 MeV), the spectrometerresolution was found to beσφ = (3.0± 0.3) MeV. Finally, thetotal number of events of reactions (10) was determined to be:

Nn;∆;N∗ (φ) = 2890± 160. (12)

At the next step of data processing it was necessary tosingle out reaction (6) with neutron in the final state. In orderto separate contributions from the reactionsπ−+p→ φ+nandπ− +p→ φ+ (∆;N∗) the information from the veto countersarranged around the target was used. The special procedure forsuch analysis and for absolute normalization of cross section(6) with the help of the known value of cross section forρo

Fig. 3. Effective mass spectrum ofK+K− system in OZI-allowed reaction(9):K− + p→ K+K− + Y

Fig. 4.Total cross sections for the reactionsπ− + p→ φ +n andπ− + p→ω + n as functions of the beam momentumPπ− . Results for the reactionπ− + p → ω + n correspond to the left-hand scale: (o) low-energy data[35],(•) data from [33],∇ data from [34].Straight linescorrespond to power-law parametrizationsσ ∼ P−2.3

π−andσ ∼ P−1.4

π−. Results for the reaction

π− + p→ φ +n correspond to the right-hand scale:×— data from [35] forrelatively low energies;¥— results of this investigation with the LEPTON-Fdetector

meson production in the charge exchange reactionπ− + p→ρo+n are detailed in [22]. As a result, the number of the eventsfor π− + p→ φ + n

Nn(φ) = 1670± 410 (13)

and the value of the total cross section for OZI suppressedreaction (6) atPπ−=32.5 GeV

σ(π−+p→ φ+n)|Pπ=32.5 GeV = (11.5±3.3)·10−33 cm2(14)

were obtained. The quoted error is largely due to systematicuncertainties associated with the normalization of the crosssections and with the separation of the contributions from dif-ferent reaction channels of (10).

The cross-sections for the reactionπ− + p→ ω + n weremost accurately measured in two experiments [33, 34]. Takingthe average value from these measurements, we can determinethe cross-section ratio

R(φ/ω) =σ(π− + p→ φ + n)σ(π− + p→ ω + n)

=(11.5± 3.3) · 10−33

(4.0± 0.5) · 10−30= (0.29± 0.09) · 10−2. (15)

This ratio characterizes the OZI suppression of theφ mesonproduction in the pion charge exchange reaction (6) — seediagrams on Fig. 1. The available data on the cross-sections ofthe reactionsπ− +p→ φ+n andπ− +p→ ω +n at differentenergies [33-35] are presented in Fig. 4.

Ratio (15) can be used in the naive quark model by theOZI rule to determine the mixing angleθV for the vector me-son nonet from the relationtg2αV = R(φ/ω), which can beobtained from (5) in the assumption of the OZI rule accom-plishing, i.e. forZ = 0 (see also [36]). HereαV = θV − θoVandθoV = 35.3o is the angle of ideal mixing. It follows from

438

(15) that|αV | = (3.1± 0.5)o, in excellent agreement with thedata on the radiative widthsΓ (φ → πoγ) andΓ (ω → πoγ)(|αV | = (3.0± 0.2)o [37]) and on the quadratic mass formulafor the vector nonet (αV = (3.7± 0.4)o [38]). In another ap-proach we can fix the value of the angleαV = (3.7± 0.4)o

from quadratic mass formula and estimate from (5) and (15)the upper limit for the OZI rule violation parameterZ:

| − (0.054± 0.008) + (0.065± 0.007)| 6 |Z|6 (0.054± 0.008) + (0.065± 0.007) (16)

or

|Z| < 0.13 (95% C.L.). (17)

Let us compare the data forR(φ/ω) obtained in our exper-iment with the data on lower energies. From the ZGS exper-iment [26, 27] it is possible to obtainR(φ/ω)|Pπ−=6 GeV =

(3.2 ± 0.4) · 10−3 in good agreement with the LEPTON-Fdata. But this ratio increases by about an order of magnitudeasPπ decreases from 6 GeV to 2.1 GeV. Such strong violationof the OZI rule at lower energies was explained in [39] withintroduction of s-channel box diagrams with the threshold andduality cancellation in the intermediate states of the box. Theagreement between 6 GeV data [27] and 32.5 GeV data (ourexperiment) and the upper limit of Z (17) are in favor of agood accomplishing of the OZI rule in pion charge exchangereactionsπ− + p → φ + n andπ− + p → ω + n at relativelyhigh beam momenta.

3 Search for the OZI suppressed decayB/b1(1235)→ φπo [23]

For isovector neutral mesons with quark structure|MI=1 >=|(uu−dd)

√2> the decay toφπo is OZI-forbidden and should

be strongly suppressed with respect toωπo decay channel.Here are presented the LEPTON-F data on the search for thedecayB/b1(1235)→ φπo (7). It is known that the main de-cay channel of theB/b1(1235) meson (M = 1231± 3 MeV,Γ = 142± 8 MeV, JPC = 1+−, IG = 1+) is B/b1(1235)→ωπo [38]. The search for decay (7) was performed in the studyof the charge exchange reaction

π− + p→ (φπo) + n. (18)b→ K+K−

In these measurements (see [12, 24, 28] for more details) thenew vector meson C(1480) was observed in the reaction

π− + p→ C(1480) +n, (19)b→ φπo

which made a dominant contribution to (18) and which was amajor source of background for the decay signal (7). Reaction(19) is largely due to one-pion exchange mechanism (OPE)and is therefore characterized by a narrow|t′| distribution. Atthe same time OPE is forbidden for the reaction

π−p→ [B/b1(1235)]o + n, (20)

because of theB/b1(1235)-meson quantum numbers (oppo-site parity and charge parity). Reaction (20) is expected to

Fig. 5. Effective mass spectrum of theφπ0 system in the reactionπ− +p→ (φπ0) + n (the results are weighted with detector efficiency). An “anti-OPE” selection|t′| > 0.1 (GeV)2 is applied, which affects only slightly theefficiency for the reactionπ− + p → B/b1(1235) +n (not proceeding viaOPE exchange), but which reduces the background from the OPE-mediatedreactionπ− + p→ C(1480)0 + n by a factor close to 5

be dominated by theA2 exchange leading to a broader|t′|distribution.

Therefore, the “anti OPE-cut”|t′| >0.1 GeV2 decreasesthe efficiency ofB/b1(1235) detection in reaction (20) by nomore than 25%, whileC(1480) production and other OPEbackground processes are suppressed by a factor of about5 [12, 28].

The effective mass spectrum of theφπo-system in reac-tion (18) at|t′| >0.1 GeV2 weighted with the efficiency ofthe setup is shown on Fig. 5. For the evaluation of the up-per limit for the cross sectionσ[π− + p → B/b1(1235)o +n] · BR[B/b1(1235)o → φπo] the number of events in theinterval 1150< M (φπ) < 1330 MeV was determined to be78± 167. Therefore the upper limit for the cross section of(20) with decay mode (7) is

σ[π− + p→ B/b1(1235)o + n]|pπ−=32.5 GeV·BR[B/b1(1235)→ φπo] < 5 nb (95% C.L.). (21)

In the experiment with the GAMS-2000 facility [40] the crosssection ofB/b1(1235) production was determined to be

σ[π− + p→ B/b1(1235)o + n]|pπ−=38 GeV·BR[b1(1235)→ ωπo] = 0.8± 0.2 µb. (22)

Taking into consideration the energy dependence of thecross section for this processσ ∼ P−1.5±0.2

π [40], the upperlimit for the ratio of decay branchings forB/b1(1235)-mesoncan be obtained from (21) and (22) to be

RB = BR[B/b1(1235)→ φπo]/BR[B/b1(1235)→ ωπo]

< 4 · 10−3 (95%C.L.). (23)

Since the decayB/b1(1235)o → ωπo proceeds mostly withzero orbital angular momentum [38], we can represent (23) inthe form

RB = [p(B)φ /p(B)

ω ]|g2Bφπ/g

2Bωπ| < 4 · 10−3 (24)

or

|g2Bφπ/g

2Bωπ| 6 1 · 10−2 (25)

439

Fig. 6. Selection of the reactionsp + N → [pφ] + N (26) andp + N → [pω] + N (27) and il-lustration of sideband method forbackground subtraction to separatethese processes:a effective massspectrumM (K+K−) in p +N →[pK+K−] + N (29); b effec-tive mass spectrumM (π+π−πo) inp + N → [pπ+π−πo] + N (30).The regions of theφandω peaks andsideband regions for backgroundsubtraction to obtain all distribu-tions in (26) and (27) are shown onthese spectra

(after some small correction for possible contributions fromD-wave decays ofB/b1(1235) meson). Here,p(B)

φ andp(B)ω

are the decay momenta ofφ andω mesons in the rest frameof theB/b1(1235) meson, andgBφπ andgBωπ are the corre-sponding coupling constants. Thus, in a framework of naivequark model it is possible to obtain from (25) the limitation|gBφπ/gBωπ|2 ' tgα2

V < 1 · 10−2 for the mixing angleαV = ϑV − ϑoV and to conclude that the decays of the or-dinary isovector mesonB/b1(1235) do not contradict the OZIselection rule for connected and disconnected quark diagrams.

4 Study the OZI rule in the nucleon diffractive-likereactionsp +N → [pφ] + N and p +N → [pω] + N

The test of the OZI selection rule forφandωmeson productionin proton-nucleon processes was performed in the experimentswith the SPHINX spectrometer in the study of diffractive re-actions

p +N → [pφ] + N (26)

and

p +N → [pω] + N. (27)

Process (26) should by suppressed by the OZI rule becauseof disconnected character of quark diagram forφ production.In the naive quark model (assuming the strict OZI rule) theratio of cross sections forφ andω production is related toφ− ω mixing and should beR(φ/ω)|OZI = tg2αV ' 0.4 · 10−2. (28)

HereαV = θV − θoV andθoV = 35.3o is the angle of idealmixing in vector nonet (see Sect. 1).

Toward separating transitions (26) and (27), we investi-gated the reactionsp +N → pK+K− +N, (29)

p +N → pπ+π−πo +N. (30)

The SPHINX detector used in these measurements hadalready been described in detail [41]. This detector is a newmodification of the LEPTON-F setup. It comprises a wide-aperture magnetic spectrometer instrumented with scintil-lation hodoscopes and proportional and drift chambers, a

beam-determinating system, a veto system, and a multichan-nel γ spectrometer consisting of lead-glass full-absorptionCherenkov counters. Charged secondaries were identified byusing a differential RICH device detecting several rings ofCherenkov light in a matrix of small-size photomultipliersFEU-60 and two multichannel gas threshold Cherenkov coun-ters employed mostly for triggering.

A target (12-cm-long polyethylene of effective thickness0.48×1024 CH2/cm2) was irradiated by 0.9×1011 protons of70 GeV energy. The trigger for reaction (29) selected eventswith three charged secondaries, some of which were requiredto be slow (γ < 43, see [41]). The trigger for (30) did not re-quire any slow secondaries and was implemented with a sup-pression factor of 4 because of technical limitations imposedby the data-acquisition system.

Toward isolating reaction (29) geometrically reconstructedevents were required to satisfy the following selection criteria:

(1) the presence of three charged secondaries that areidentified asp, K+ andK− in the RICH and in thresholdCherenkov counters and which originate from a common ver-tex within the target;

(2) the absence of photons with energy above the threshold(Eγ > 0.65 GeV) detected in theγ spectrometer;

(3) fulfillment of the “elasticity” condition 65 GeV< Ep+EK+ +EK− < 75 GeV.

TheK+K− effective-mass spectrum for reaction (29) se-lected in this way is illustrated in Fig. 6a. This spectrum ex-hibits a distinct peak associated withφ production and whichpermits isolation of reaction (26). The observedφmass agreeswell with the tabular value [38], while the shape of theφ peakis consistent with that predicted by a Monte Carlo simulationtaking detector resolution into account. The simulation wasbased on the GEANT package [42]. The detector responsesto generated events with required kinematics were recordedon a tape in the real-event format. These simulated eventswere then subjected to the same selections as real events. Inplotting distributions for reaction (26), we used the standardintegral technique of background subtraction. According tothis technique, the half-sum of the distributions for theφ-masssidebands (0.995-1.005 and 1.035-1.045 GeV) was subtractedfrom that for the centralφ region 1.010-1.030 GeV. The abovemass intervals are indicated in Fig. 6a.

440

Fig. 7.Distribution of thepφ system in the square of the transverse momentumP 2T for reaction (26)

Reaction (23) was isolated throught the following selectioncriteria:

(1) the presence of three charged secondaries that are iden-tified as p, π+ and π− in the RICH and in the thresholdCherenkov counters and which originate from a common ver-tex within the target;

(2) the presence of two photons above the energy threshold(Eγ > 0.65 GeV) in theγ detector with an effective mass com-patible with aπo mass (0.11 GeV< M (γγ) < 0.16 GeV);

(3) fulfillment of the “elasticity” condition 65 GeV< Ep+Eπ+ +Eπ− +Eπo < 75 GeV.

Theπ+π−πo effective mass in reaction (30) is plotted inFig. 6b. The spectrum shows anω peak permitting the extrac-tion of reaction (27). The observed position and shape of thepeak are in good agreement with the tabularω mass value andwith mass resolution simulated by the Monte Carlo method.As before, the integral technique was used for backrgroundsubtraction involving the mass intervals 0.737-0.825, 0.671-0.715, and 0.847-0.891 GeV, as illustrated in Fig. 6b.

The distributions in the squared transverse momentumP 2T

for reactions (26) and (27) are shown in Figs. 7 and 8, re-spectively. Each of them is adequately described by the sumof two exponentials with nearly equal weights. The slope

Fig. 8.Distribution of thepω system in the square of the transverse momentumP 2T for reaction (27)

Fig. 9. Weighted distributions of events in the massM (pV ) (here and in thenext figures of this subsection all distributions are weighted with efficiencyof the setup).Closedandopen circlesrepresent, respectively, the distributiononpω and the scale distribution ofpφ (scale factor is 100)

of the first exponential cannot be determined precisely be-cause of a finite experimental resolution, and its visible value[b > 50 (GeV)−2] suggests a coherent transition on a carbonnucleus. Since the character of these distributions is identicalfor reactions (26) and (27), we usually present the results forthe entireP 2

T region (except the data on Fig. 13, see below).The distributions of events from reactions (26) and (27)

in M (pφ) andM (pω) were corrected for detection efficien-cies and for theφ → K+K− andω → π+π−πo branchingratios. The results are illustrated in Fig. 9. For convenience,the distribution inM (pφ) is scaled up by a factor of 100.The quoted errors are purely statistical. Detection efficienciesfor transitions (26) and (27) were estimated by using sam-ples of simulated events with uniform distributions inM (pφ)andM (pω) andP 2

T distributions similar to those observedexperimentally. The angular distributions for the systemspφandpω and for the decayφ → K+K− were assumed to beisotropic. The normal vector to theω → π+π−πo decay planewas also assumed to be isotropically distributed. On the whole,the above assumptions lead to an adequate description of ex-perimental data, except for some limited inconsistencies (inparticular, the observed distribution in the cosine of the polar-angle in the Gottfried-Jackson frame depends on the mass ofthe pω system and is not uniform). Many systematic errorsare expected to be canceled out in the ratio of cross sectionsR(φ/ω), since events from reactions (29) and (30) were de-tected simultaneously under similar experimental conditions.The above assumptions lead to an estimated systematic uncer-tainty inR(φ/ω) smaller than 20% (due to the different finalstates in these reactions).

Derivation of the ratioR(φ/ω) for pN collisions and theOZI rule testing in these processes are not as straightforwardas in charge exchange pion reactions (Sect. 2), meson decays(Sect. 3) or in nucleon-antinucleon annihilation [19-21]. Theproblem is that a significant contribution to the cross sectionfor reaction (27) comes from the kinematical region below thethreshold ofpφ production. Thus, the simple ratio of the crosssections for reactions (26) and (27)

R(φ/ω) = σ(26)/σ(27) = (1.55± 0.05± 0.31) · 10−2 (31)

441

Fig. 10. The weighted distributionsdN/dPv forreactions (26) and (27) as functions of momentumPv in the rest frame ofpV system. The distributionfor pφ is scale by a factor of 100

is not very meaningful for the OZI rule testing.Bearing in mind this difficulty we used two approaches to

the OZI rule test in proton reactions.A. On Fig. 10 we present the data for the distributions

dN/dPV for reactions (26) and (27) as a function of vectormeson momentumPV in the rest frame forpV system withthe invariant massM (pV )

PV =

{[M (pV )2− (mV +mp)2][M (pV )2− (mV −mp)2]}1/2

2M (pV )(32)

(here and everywhereV denotes vectorφ or ω mesons).For testing the OZI rule we obtain the ratio of the cross

sections for reactions (26) and (27) as a function of someminimal value of momentumPVmin = Po (see Fig. 11)

R1(φ/ω)|PV >Po =

∫ PmaxPo

(dσ/dP )φ · dPφ∫ Pmax

Po(dσ/dP )ω · dPω

'∑Pφ>Po

Nφ(Pϕ)∑Pω>Po

Nω(Pω). (33)

To avoid the influence of possible resonances or resonance-like threshold structure effects which can be different for reac-tions (26) and (27) we use the valuePo >1 GeV (“asymptoticregion” forR1(φ/ω), see Fig. 11). For this value ofPo

R1(φ/ω)|Pv>1 GeV = (4.0± 0.04± 0.08) · 10−2. (34)

This approach is justified if the cross sections of (26) and (27)are functions ofPV only.

B. Another approach can be used if these cross sectionsare functions of invariant massM (pV ) and phase space. For

this case we can obtain the values ofρ2(φ/ω) as a functionof massM (pV ) atM > Mp +Mφ with correction for phasespace

ρ2(φ/ω) =dσ/dM )φdσ/dM )ω

· PωPφ

= ρ2(φ/ω;M ). (35)

The data onρ2(φ/ω) are presented on Fig. 12.2

For a more precise testing of the OZI rule in proton re-actions (26) abd (27) in this approach we use weighted aver-age values of the ratioρ2(φ/ω) in different regions of massesM (pV ) > Mo. The mass thresholdMo is introduced to avoidthe influence of some near threshold peculiarities or resonanceeffects in the invariant mass spectraM (pφ) andM (pω) (seeFig. 9). Thus we define

< ρ2(φ/ω) > |M (pV )>Mo=

1ξ

Mmax∑Mi>Mo

ρ2(φ/ω;Mi)ξi. (36)

Hereξi = 1/σ2i are statistical weights;σ2

i — variance ofρ2(φ/ω;Mi); ξ =

∑ξi. Variance of< ρ2(φ/ω > is 1/ξ.

Weighted average values< ρ2(φ/ω) > |M (pV )>Mowere ob-

tained from the data of Fig. 12 for different values ofMo. Wegive here several numbers for this ratio (for differentMo):

< ρ2(φ/ω) > |M (pV )>Mo

=

(7.3± 0.5± 1.5) · 10−2 (Mo = 2.5 GeV);(6.5± 0.3± 1.3) · 10−2 (Mo = 2.3 GeV);(5.8± 0.2± 1.2) · 10−2 (Mo = Mp +Mφ = 1.96 GeV).

(37)

2 We use designationρ(φ/ω) for the ratio of differential cross sections andR(φ/ω) – for the ratio of integrated cross sections

442

Fig. 11. Integrated ratioR1(φ/ω) (see (33)) as a function ofminimal momentumPvmin = P0

Fig. 12. Differential ratioρ2(φ/ω;M ) (see (35)) as a function of effectivemassM (pV )

The valuesR1(φ/ω) (34) and< ρ2(φ/ω) > (37) mustbe compared with the OZI rule predictionROZI = 0.4 ·10−2 (28). Thus, we see that in proton-nucleon reactions theOZI rule is significantly violated — by more than an or-

Fig. 13.Weighted ratio< ρ2(φω) > |M>Mp+Mφ as a function of squared

transverse momentumP 2T of thepV system

der of magnitude for< ρ2(φ/ω) >, in a strong contradic-tion with data on pion reactions. This violation is practicallyindependent of transverse momenta, as is demonstrated for< ρ2(φ/ω) > |

M >mp+mφ

in Fig. 13. From (5) and (37) it is

443

possible to estimate the value of the parameterZ for the OZIrule violation in proton reaction (26)

0.2. |Z| . 0.3. (38)

Theφ/ω ratio had also been measured inpp collisions at lowenergies of 10 and 11.75 GeV. The reported values exceed theOZI prediction by a factor varying between 2 and 5 [43, 44].

5 Aparent violation of the OZI rule and hidden intrinsicstrangeness in nucleons

Summing up the main results of studying theφ andω mesonproduction processes we must stress that there is a significantviolation of the OZI rule in proton-induced reactions and inparticular, in high energy diffractive production processes. Atthe same time the data on pion charge exchange reactions arein a good agreement with this selection rule and with the valueof mixing angle in the vector meson nonet. True enough, from(17) we must bear in mind that even in pion reactions it isimpossible to exclude a perceptible OZI rule violation with anaccidental compensation between this amplitude and mixingeffects. But such interpretation seems to be unlikely becauseof a good agreement between the value of|αV | obtained from(15) (in the assumption ofZ = 0 in expression (4) for procesesunder study) and other independent data on this mixing angle.Thus, it seems that pion-induced charge-exchange reactionsare in a good agreement with the OZI rule, but in proton-induced data this rule is strongly violated. Very large OZI-violation effects (with|Z| ∼ 0.2÷0.4) were also observed ina number of antiproton annihilation reactions [17-21].

A large apparent violation of the OZI selection rule inproton and antiproton reactions can significantly change ourconcept of quark structure of these particles. According to thenaive quark model the valence quark constituents (uud forproton) give a good general description of hadron structureat large distances (small momenta transfer region) and hadronquantum numbers. Hadronic probes of shorter distances (largemomentum transfers) reveal more consistuents which formthe “sea” of quark-antiquark pairs and gluons. This “sea” andits evolution with momentum transfer is in qualitative agree-ment with QCD. However, the data on strong OZI-violationin nucleons may be considered as possible manifestation ofadditionalss pairs in nucleon wave function even at large dis-tances. Another experimental indications of possible hiddenstrangeness component in nucleons come from the data onπ−N sigma term and from “spin crisis” effects in polarizedµp scattering. The interpretation of all these data in terms ofdirect intrinsicss component in the nucleon wave function(higher Fock-space component) was discussed in [17-20].

In the intrinsic hidden strangeness model the wave func-tion of proton can be presented as follows [20]:

|p >= α∑X

|uudX > +β∑X

|uudssX >, (39)

whereX stands for any numbers ofqq pairs and gluons (nu-cleon “sea”) and|α|2 + |β|2 = 1. The possible way for unsup-pressedφmeson production by this small exotic component ofthe proton|uudssX > is presented on Fig. 14. This connectedquark diagram (not suppressed by the OZI rule) can explain anapparent OZI-violation effect in the reactionp+N → [pφ]+N ,

Fig. 14. φ production in proton-nucleon or proton-nucleus diffractive re-actions:a disconnected diagram for ordinary proton wave function (OZI-suppressed process);b connected quark diagram for the model in which pro-ton wave function has small exotic component with hidden strangeness (see(39)). This process can explain apparent violation of the OZI rule in protonreactions

which was observed in the SPHINX experiment, if|β/α| isof an order of 0.1–0.2. This value of|β/α| is compatible withannihilation data.

Possible existence of small exotic component with hiddenstrangeness in parton wave function is very exciting and itmust be thoroughly investigated in other processes, some ofwhich were considered in [20].

We are indebted to L.Montanet for attracting our attention to the possibleviolation of the OZI rule in proton-induced reactions. It is a pleasure to expressalso our gratitude to N.Isgur, A.Kaidalov and M.Sapozhnikov for helpfuldiscussions.

This work was supported in part by the ISF (grant JA 2100) and by theRFBR (grant 96-02-16759a).

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