Search for exotic baryons with hidden strangeness in diffractive production processes

$Page 1: Search for exotic baryons with hidden strangeness in diffractive production processes$
Physics Reports 320 (1999) 223}248

Search for exotic baryons with hidden strangeness indi!ractive production processes

L.G. Landsberg

State Research Center Institute for High Energy Physics, Protvino, Moscow region 142284, Russia

Abstract

Experimental results of the SPHINX Collaboration on studying proton di!ractive production processesare presented. Evidences for new baryon states with masses Z2 GeV were obtained in hyperon-kaon e!ectivemass spectra in several reactions. Unusual properties of these massive states (small enough decay widths,large branching ratios for their decays with strange particles in "nal states) make them serious candidates forcryptoexotic pentaquark baryons with hidden strangeness Dqqqss6 T. New results for these states are presentedhere, as well as recent data on large violation of the OZI rule in proton di!ractive dissociation. ( 1999Elsevier Science B.V. All rights reserved.

PACS: 14.40.Gk; 13.85.Fb

Keywords: Exotic baryon; Di!ractive production; OZI rule

1. Introduction

The last two decades have been marked by a signi"cant increase in studies in the "eld ofspectroscopy of hadrons, i.e., the particles participating in strong interactions. It has been "rmlyestablished that hadrons are not truly elementary, but composite particles. Similar to atomicnuclei, which consist of nucleons, hadrons are bound systems composed of fundamental particlesknown as quarks. Quarks are those structural elements that de"ne the variety of the hadronicmatter. In addition to a fractional electric charge and baryon number, di!erent #avors (isospin,strangeness, charm, etc.) quarks have a `strong-interaction chargesa called colors.

The interactions between quarks go through the exchange of color virtual particles, gluons, ina similar way as the interactions between electric charges are e!ected by exchange of virtualphotons. However, in contrast to neutral photons, which bear no charge and do not interactdirectly with each other, gluons are characterized by color `chargesa and are capable of interactingnot only with color quarks but between themselves, thus forming even bound gluon states

0370-1573/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 0 - 1 5 7 3 ( 9 9 ) 0 0 0 6 0 - 5

} glueballs. The interaction between color quarks and gluons is described by quantum chromo-dynamics.

Apparently, color quarks and gluons cannot exist in a free state. Such a hypothesis is experi-mentally based on the long-standing unsuccessful e!orts to "nd free quarks. Therefore, the conceptof con"nment (`the quark imprisonmenta) arose, according to which only the particles withoutcolor charge can freely exist, the so-called `colorlessa or `whitea hadrons. All these hadrons aredivided by their quantum numbers and their quark content into two large groups: baryons andmesons. Baryons are characterized by their baryon number B (B"1 for baryons and B"!1 forantibaryons) and are produced in pairs to conserve the baryon number of the whole system.Mesons have B"0. Rapid development of hadron spectroscopy has led to a signi"cant advance inthe systematics of ordinary hadrons with simplest valence quark structure: qq6 for mesons andqqq for baryons.

Here, we consider only the so-called valence quarks, which determine the hadron nature and itsmain characteristics (quantum numbers). According to the current theoretical perceptions, wellcon"rmed by numerous experiments, the valence quarks in a hadron are surrounded by a `cloudaof virtual quark}antiquark pairs and gluons, which are constantly emitted and absorbed by thevalence quarks. This `clouda or, as one says now, the `seaa of quark}antiquark pairs and gluons, isphysical reality determining many properties of hadron (for example, space distribution of itselectric charge and magnetic moment, intrinsic distribution of quark and gluon constituents overmomenta, etc.).

A very important question is whether there exist `colourlessa hadrons with a more complexinner valence structure, such as multiquark mesons (M"qqq6 q6 ) and baryons (B"qqqq6 q),dibaryons (D"qqqqqq), hybrid states (M"qq6 g, B"qqqg), and glueballs, which are mesonscomposed only of gluons (M"gg, ggg). Of course, in such new forms of hadronic matter, known asexotic hadrons, e!ects of the quark}gluon `seaa must also appear in addition to the valence quarkand gluon structure.

The discovery of the exotic hadrons would have a far-reaching consequences for quantumchromodynamics, for the concept of con"nement and for speci"c models of hadron structure(lattice, string, and bag models). Detailed discussions of exotic hadron physics can be found inrecent reviews [1}7].

Exotic hadrons can have anomalous quantum numbers not accessible to three-quark baryonsor quark}antiquark mesons (open exotic states), or even usual quantum numbers (cryptoexoticstates). Cryptoexotic hadrons can be identi"ed only by their unexpected dynamical properties(anomalously narrow decay widths, anomalous decay branching ratios and so on).

As is clear from review papers [1}7], in the last decade searches for exotic mesons have led toa considerable advance in this "eld. Several new states have been observed whose properties cannotbe explained in the framework of naive quark model of ordinary mesons with qq6 valence quarkstructure. These states are serious candidates for exotic mesons (most of them are of cryptoexotictype).

At the same time the situation for exotic baryons is far from being clear. There are also someexamples of possible unusual baryon resonances [8}11], but these data are not su$cently preciseand are not supported by some new experimental results [2,12}16].

Extensive studies of the di!ractive baryon production and search for cryptoexotic pentaquarkbaryons with hidden strangeness (B

("Dqqqss6 T, here q"u, d quarks) are being carried out by the

224 L.G. Landsberg / Physics Reports 320 (1999) 223}248

SPHINX Collaboration at the IHEP accelerator with 70GeV proton beam. This program wasdescribed in detail in reviews [2].

The recent data of the SPHINX experiment [16}25] gave new important evidence of possibleexistence of cryptoexotic baryons with hidden strangeness X(2050)`PR(1385)0K` andX(2000)`PR0K`. We shall summarize these data in Section 3, after a general description of thenature and expected properties of cryptoexotic baryons, as well as some promising ways for theirproduction and observation. We also present here the SPHINX results in favour of strongviolation of the OZI rule in proton di!ractive dissociation reactions [26}28] which may beconnected with direct strangeness in the nucleon quark structure.

2. Exotic baryons and possible mechanisms of their production

There arise three main questions tightly connected with the exotic searches in the SPHINXexperiments:

1. How to identify cryptoexotic B("Dqqqss6 T baryons without open exotic values of their

quantum numbers and how to distinguish them from several dozens of well-known NH andD isobars?

2. How to produce the exotic baryons in the most e!ective way?3. How to reduce background processes and to make easier the exotic baryon observation?We will try to "nd some qualitative answers to these questions because of the lack of theoretical

models for the description of exotic hadrons.

2.1. Properties of exotic baryons with hidden strangeness

As has been stated before, cryptoexotic baryons do not have external exotic quantum numbers,and their complicated internal valence structure can be established only indirectly by examiningtheir unusual dynamical properties that are quite di!erent from those of ordinary baryons DqqqT. Inthis connection, we consider the properties of multiquark baryons with hidden strangeness Dqqqss6 T.

If such cryptoexotic baryon structure consists of two color parts spatially separated by a centri-fugal barrier, its decays into the color-singlet "nal states may be suppressed because of a complic-ated quark rearrangement in decay processes. The properties of multiquark exotic baryons with theinternal color structure

Dqqqqq6 T1#"D(qqq)

8#](qq6 )

8#T (1)

(color octet bonds) or

Dqqqqq6 T1#"D(qqq6 )

6#](qq)

6#T (2)

(color sextet}antisextet bonds) are discussed in [29}32]. Here, subscripts 1c, 8c, and so on specifyrepresentations of the color S;(3)

#group. If the mass of a nonstrange baryon with hidden

strangeness is above the threshold for decay modes involving strange particles in "nal states, themain decay channels must be of the type

B("Dqqqss6 TP>K#kn (3)

L.G. Landsberg / Physics Reports 320 (1999) 223}248 225

(k"0, 1,2). Another possibility is associated with the decays

B(PDqqqss6 TP/N(gN; g@N)#kn (4)

which involve the emission of particles with a signi"cant ss6 component in their valence-quarkstructure. It should also be emphasized that g and, particularly, g@ mesons are strongly coupled togluon "elds and, hence, to the states with an enriched gluon component. Therefore, baryon decaysof the type BPNg,Ng@ may be speci"c decay modes for hybrid baryons (see, for example, [2]).

The nonstrange decays of baryons with hidden strangeness, B(PN#kn, must be suppressed by

the OZI rule. Thus, the e!ective phase-space factors for the decays of the massive baryons B(would

be signi"cantly reduced because of this OZI suppression (owing to a high mass threshold for theallowed decays B

(P>K with respect to the suppressed decay B

(PNn,Dn). The mechanism of

quark rearrangement of color clusters in the decays of particles with complicated inner structure oftype (1) or (2) can further reduce the decay width of cryptoexotic baryons and make themanomalously narrow (their widths may become as small as several tens of MeV). Here, theoreticalpredictions are rather uncertain. For this reason, only experiments can answer conclusively thequestion of whether such narrow baryon resonances with hidden strangeness really exist.

Thus, it is desirable to perform systematic searches for the cryptoexotic baryons B(

withanomalous dynamical features listed below.

(i) The dominant OZI-allowed decay modes of the baryons B(are those with strange particles in

the "nal states:

R(Dqqqss6 T"BR[Dqqqss6 TP>K]/BR[Dqqqss6 TPpnn;Dn]Z1. (5)

For ordinary DqqqT isobars, R(D;NH)&(several %) [33].(ii) The cryptoexotic baryons B

(can simultaneously possess a large mass (M'1.8}2.0GeV) and

a narrow decay width (C450}100MeV). This is due to a complicated internal color structure ofthese baryons, which leads to a signi"cant quark rearrangement of color clusters in decayprocesses, and due to a limited phase space for the OZI-allowed decays B

(P>K. At the same time,

typical decay widths of the well-established DqqqT isobars with similar masses are not less than300MeV.

2.2. Diwractive production mechanism and search for exotics

It was emphasized in a number of studies [1,2,8,10,30,32,34] that di!ractive production pro-cesses featuring Pomeron exchange o!er new possibilities in searches for exotic hadrons. Origin-ally, interest was focused on the model of Pomeron with a small cryptoexotic (qqq6 q6 ) component[30,32]. According to modern concepts, the Pomeron is a multigluon system owing to which exotichadrons can be produced in gluon-rich di!ractive processes (see Fig. 1).

It is apparent that only the states with the same charge and #avor as those of the primaryhadrons can be produced in di!ractive processes. Moreover, there are some additional restrictionson the spin and parity of the formed hadrons which are stipulated by the Gribov}Morrisonselection rule. According to this rule, the change in parity *P occurring as a result of the transitionfrom the primary hadron to the di!ractively produced hadronic system, is connected with thecorresponding change in the spin *J through the relation *P"(!1)*J. For example, because ofthis rule, in the proton di!ractive dissociation (for proton JP"1/2`), only baryonic states with


Fig. 1. Diagrams for exotic baryon production in the di!raction processes with the Pomeron exchange. The PomeronP is a multigluon system.

natural sets of quantum numbers 1/2`, 3/2~, 5/2`, 7/2~, etc., can be excited. The Gribov}Morrisonselection rule is not a rigorous law and has an approximate character.

The Pomeron exchange mechanism in di!ractive production reactions can induce the coherentprocesses on the target nucleus. In such processes the nucleus acts as descreet unit. Coherentprocesses can be easily identi"ed by the transverse momentum spectrum of the "nal-state particlesystem. They manifest themselves as di!ractive peaks with large values of the slope parametersdetermined by the size of the nucleus: dN/dP2

TKexp(!bP2

T), where bK10A2@3 GeV ~2. Owing to

the di!erence in the absorption of single-particle and multiparticle objects in nuclei, coherentprocesses could serve as an e!ective tool for separation of resonance production against non-resonant multiparticle background (see e.g. [35]).

The conditions for coherent reactions are destroyed by absorption processes in nuclei. Thus, thecoherent suppression of nonresonant background takes place:

p#0)

(res)p#0)

(nonres. BG)'

p/0/#0)

(res)p/0/#0)

(nonres. BG). (6)

The separation of coherent reactions can be achieved by studying dN/dP2T

distributions forprocesses under investigation.

As is seen from dN/dP2T

spectra in the SPHINX experiments, there are strong narrow forwardcones in these distributions with the slope bZ50GeV~2, which correspond to a coherent di!ractiveproduction on carbon nuclei (see, for example, Fig. 2). For identi"cation of the coherently producedevents we used `softa transverse momentum cut P2

T(0.075}0.1 GeV2. With this cut noncoherent

background in the event sample can be as large as 30}40%. It is possible to reduce this backgroundwith more stringent P2

Tcut at the cost of some decrease of the signal statistics.


Fig. 2. dN/dP2T

distribution for typical di!ractive production reaction (p#NP[R0K`]#N). The P2T-regions for

coherent reaction on carbon nuclei (P2TS0.075}0.1 GeV2) and for nonperipheral processes (P2

T'0.3 GeV2) are shown.

2.3. Processes with large transverse momenta

As was discussed above, coherent di!ractive production reactions with small transverse momen-ta seem quite promising for the search for exotic hadrons, but, of course, they do not exhaust theexisting opportunities. Certainly, these searches can be also performed for all di!ractive-typeprocesses (e.g. without coherent cuts for small P2

T). And of special interest is the study of

nonperipheral production reactions which can be the most e!ective way to seek for certain exoticstates, especially those that are formed at short ranges and are characterized by broad enoughtransverse momentum distributions. In this case, the most favorable conditions for identifyingexotic hadrons are achieved at higher transverse momenta (P2

T'0.3}0.5GeV2), where the back-

ground from peripheral processes is strongly suppressed. For instance, the unusually narrowmeson states X(1740)Pgg [36] and X(1910)Pgg@ [37] were observed in studying the charge-exchange reactions n~#pP[gg]#D0 and n~#pP[gg@]#n after the selection of events with


Fig. 3. Diagram for di!ractive-type reaction with multiple Pomeron rescattering (these processes might be signi"cant atP2TZ0.3 GeV2).

P2T50.3 GeV. These anomalous states are good candidates for cryptoexotic mesons. The rescat-

tering mechanism involving multipomeron exchange in the "nal state (a gluon-rich process) mayexplain X(1740) and X(1910) nonperipheral production [38].

In the very high primary energy region which, for example, corresponds to the search for exoticstates with heavy quarks, di!ractive production reactions with rescattering can be used, instead ofthe charge-exchange processes with rescattering (see the diagram in Fig. 3). The cross sectionsof these di!ractive processes also do not die out with energy rise. The nonperipheral P2

Tregion

for these processes is shown in Fig. 2.

2.4. Electromagnetic mechanism

The search for exotic hadrons with hidden strangeness can be also carried out in another type ofhadron production processes caused by electromagnetic interactions. The example of such processis the formation reaction with s-channel resonance photoproduction of strange particles

c#NPDqqqss6 TP>K (7)

(see diagram in Fig. 4a). It is possible in principle to study the s-channel resonance production bydetailed energetic scanning of the cross sections and angular distributions for Eq. (7) and byperforming the subsequent partial-wave analysis. As is seen from Fig. 4a and from VDM (with itssigni"cant coupling of photon with ss6 pair through /-meson), reaction (7) can provide a naturalway to embed the ss6 quark pair into intermediate resonance state and to produce the exotic baryonwith hidden strangeness.

The existing data on reactions (7) are rather poor and insu$cient for such systematical studies.But one can hope that good data would be produced in the near future in the experiments onstrong current electron accelerators CEBAF and ELSA (see, for example, [39]).

Electromagnetic production of exotic hadrons can be searched for not only in the resonancephotoproduction reactions but in the collisions of the primary hadrons with virtual photons of theCoulomb "eld of the target nuclei [40}44], e.g. in the Primako! production reactions

h#ZPa#Z (8)


Fig. 4. Electromagnetic mechanisms for production of exotic baryons with hidden strangeness: (a) formation reactionwith s-channel resonance photoproduction; (b) Coulomb production reaction h#ZPa#Z.

(see diagram in Fig. 4b). The Coulomb production mechanism plays the leading role in the regionof very small transfer momenta, where it dominates over the strong interaction process [40}42].The total cross section of the Coulomb reaction (8) is

p[h#ZPa#Z]DC06-0."

Kp0

2Ja#1

2Jh#1

C (aPh#c) . (9)

The value p0

is obtained in the QED calculations. In the narrow width approximation for theresonance as in Eq. (9) p

0has the form

p0"8naZ2C

Ma

M2a!m2

hD

3

Pq2

q2.*/

[q2!q2.*/

]q4

DFz(q2)D2dq2 . (10)

Here Z is the charge of nucleus; a"1/137 is the narrow structure constant; C(aPh#c) is theradiative width of a;J

a, J

hand M

a, m

hare the spins and masses of a and h particles; F

z(q2) is

the electromagnetic formfactor of nucleus; q2.*/

"[[M2a!m2

h]/2E

h]2 is the minimum square

momentum transfer q2; Eh

is the primary hadron energy. In the high-energy region, q2.*/

is verysmall and q2"P2

T#q2

.*/KP2

T.

It must be borne in mind that in the Coulomb production reactions with primary protonsone studies the same processes as in ordinary photoproduction reactions. But theCoulomb production in the experiments with unstable primary particles (pions, kaons,hyperons) opens the unique possibility to study the photoproduction reactions with these unstable`targetsa.


3. The experiments with the SPHINX setup

The search for exotic baryons with hidden strangeness was performed in the experiments on theproton beam of the 70 GeV IHEP accelerator with the SPHINX spectrometer. The SPHINX setupincludes the following basic components:

1. A wide aperture magnetic spectrometer with proportional wire chambers, drift chambers,drift tubes, scintillator hodoscopes.

2. Multichannel c-spectrometer with lead}glass Cherenkov total absorption counters.3. A system of gas Cherenkov detectors for identi"cation of secondary charged particles

(including a RICH detector with photomatrix equipped with 736 small phototubes; this is the "rstcounter of this type used in the experiments).

4. Trigger electronics, data acquisition system and on-line control system.The SPHINX spectrometer works in the proton beam with energy E

p"70 GeV and intensity

I+(2}4)]106p/cycle. The measurements were performed with a polyethylene target to optimizethe acceptance, sensitivity and secondary photon losses.

The "rst version of the SPHINX setup was described in [12]. The next version of this setup afterpartial modi"cation (with a new c-spectrometer and with better conditions for K and R0 identi"ca-tion) was discussed in [21].

To separate di!erent exclusive reactions, a complete kinematical reconstruction of events wasperformed by taking into account information from the tracking detectors, from the magneticspectrometer, from the c-spectrometer, and from all Cherenkov counters of the SPHINX setup. Atthe "nal stage of this reconstruction procedure, the reactions under study were identi"ed byexamining the e!ective-mass spectra for subsystems of secondary particles.

Several photon-induced di!ractive production processes were studied in the experiments of theSPHINX Collaboration [12}25,27,28]:

p#NP[pK`K~]#N , (11)

P[p/]#N

vK`K~(12)

P[K(1520)K`]#N ,

vK~p(13)

P[RH(1385)0K`]#N

vKn0(14)

P[RH(1385)0K`]#N#(neutrals) ,

vKn0(15)

P[R0K`]#N ,

vKc(16)

P[R` K0]#N

vpn0vn`n~(17)


P[pg]#N,

vn`n~n0(18)

P[pg@]#N,

vn`n~gPn`n~2c(19)

P[pu]#N,

vn`n~n0(20)

P[pn`n~]#N, (21)

P[D``p~]#N ,

vpp`(22)

and several other processes. Here N is nucleon or C nucleus for the coherent processes. Theseparation of coherent di!ractive processes is obtained by studying their dN/dP2

Tdistributions, as

is shown in Fig. 2.

4. Previous data on the coherent di4ractive reactions p1CP[R0K1]1Cand p1CP[R*(1385)0K1]1C

One of the major results obtained with the SPHINX setup was the study of R0K` systemproduced in di!ractive process (16).

These data were obtained in two di!erent runs on the SPHINX facility:(a) the "rst run with the old version of this setup (òld runa, [12,17}20]);(b) the second run with partially upgraded SPHINX apparatus (`new runa [21}23]).As a result of this upgrading the detection e$ciency and purity of K and R0 events were

signi"cantly increased.The main results of these measurements can be summarized as follows:(1) Old [16}20] and new [21}23] data from coherent di!ractive reaction (16) were obtained

under di!erent experimental conditions, with a signi"cantly modi"ed apparatus, with di!erentbackground and systematics. Nevertheless, the R0K` invariant mass spectra from both runs are ina good agreement which makes them more reliable.

(2) The combined mass spectrum M(R0K`) for coherent reaction (16) from the old and new data(with P2

T(0.1GeV2) is presented in Fig. 5. This spectrum is dominated by the X(2000) peak with

parameters in Table 1.(3) There is also some near threshold structure in this M(R0K`) spectrum in the region of

&1800MeV (see Fig. 5 and Table 1). Such a shape of the R0K` mass spectrum (with an additionalstructure near the threshold) proves that the X(2000) peak cannot be explained by a non-resonantDeck-type di!ractive singularity. Therefore, most likely this peak has a resonant nature.

A strong in#uence of P2T

cut for the production of this X(1810) state was established: thisstructure is produced only at very small P2

T([0.01}0.02GeV2) } see below.


Fig. 5. Combined mass spectrum M(R0K`) for coherent di!ractive reaction (16) in old and new runs on the SPHINXsetup (P2

T(0.1GeV2). The parameters of X(2000) peak in this spectrum are: M"1997$7MeV; C"91$17 MeV.

We have also some data in studying the RH(1385)0K` system in reaction (14), which wereobtained only in the old run (data on this reaction from the new run are now in process of analysis).Coherent events of (14) were singled out in the analysis of dN/dP2

Tdistribution as a strong forward

peak with the slope bZ50 GeV~2. In order to reduce the noncoherent background and to obtainthe R(1385)0K` mass spectrum for the `purea coherent production reaction on carbon nucleia `tighta requirement P2

T(0.02 GeV2 was imposed and the mass spectra of R(1385)0K` for the

coherent events of (14) were obtained (see, for example, Fig. 6). In these spectra some very narrowstructure X(2050) was observed. The "ts of the spectra with Breit}Wigner peaks and polynomialsmooth background were carried out, and the average values for the main parameters of X(2050)structure are presented in Table 1. Certainly, one needs further con"rmation of the existence ofX(2050) in the new data with increased statistics. Up to now it is impossible also to excludecompletely the feasibility for X(2000) and X(2050) to be in fact two di!erent decay modes of thesame state.

In studying coherent reactions (21) p#CP[pp`p~]#C and (22) p#CP[D(1232)``p~]#C under the same kinematical conditions as of processes (14) and (16) a search for other decayschannels of the X(2000) and X(2050) baryons was performed [18,19]. No peaks in 2 GeV massrange were observed in the mass spectra of pp`p~ and D(1232)``p~ systems produced in reactions(21) and (22), respectively. Lower limits on the corresponding decay branching ratios R (see (5))


Table 1The main results of the previous SPHINX data for coherent di!ractive reactions p#CP[R0K`]#C andp#CP[RH(1385)0K`]#C

1. Coherent di!ractive production reaction p#CP[R0K`]#C with coherent cut P2T(0.1GeV2 was studied in the

old and new runs. The combined mass spectrum M(R0K`) is presented in Fig. 5. This spectrum is dominated byX(2000) state with parameters

M"1997$7MeV

C"91$17MeV

statistical significance of the peak is 7 SD

2. There are also some near threshold structure X(1810), which is produced only in the region of very smallP2T([0.01}0.02GeV2). The parameters of this peak are

M"1812$7MeV

C"56$16MeV

3. Coherent di!ractive production reaction p#CP[RH(1385)K`]#C with tight coherent cut P2T(0.02 GeV2 was

studied in the old run ([12,17}20]). The mass spectra of M[RH(1385)0K`] are in Fig. 6. The peak was observed in thesespectra with average value of parameters

M"2052$6MeV

C"35`22~35

MeV

(with the account of the apparatus mass resolution); statistical CL of the peak55 SD

4. The data of (14) and (16) were analyzed together with the data from (21) and (22) to obtain the branching ratios ofdi!erent decay channels (with strange particles and without strange particle in "nal state). The lower limits of theratios were obtained from this comparative analysis (with 95% CL):

R1"

BRMX(2050)`P[RH(1385)K]`NBRMX(2050)`P[D(1232)n]`N

'1.7

R2"

BRMX(2050)`P[RH(1385)K]`NBRMX(2050)`Ppn`n~N

'2.6

R@2"

BRMX(2050)`PRH(1385)0K`NBRMX(2050)`Ppn`n~N

'0.86

R3"

BRMX(2000)`P[RK]`NBRMX(2000)`P[D(1232)n]`N

'0.83

R4"

BRMX(2000)`P[RK]`NBRMX(2000)`Ppn`n~N

'7.8

R@4"

BRMX(2000)`PR0K`NBRMX(2000)`Ppn`n~N

'2.6.


Fig. 6. Invariant mass spectra M[RH(1385)0K`] in the coherent reaction (14) at tight transverse-momentum cutP2T(0.02GeV2 for various bin widths: (a) *M"10MeV; (b) *M"30 MeV. The spectra are "tted to the sum of

a smooth polinomial background and X(2050) Breit}Wigner peak. The parameters of X(2050) peak are: (a)M"2053$4MeV, C"40$15MeV; (b) M"2053$5MeV, C"35$16MeV.

were obtained from this comparative analysis:

R[X(2000);X(2050)]Z1}10 (95% CL) (23)

(more details are in Table 1).The isotopic relations between the decay amplitudes of I"1

2particles were assumed in these

calculations (the X(2000) and X(2050) states belong to isodoublets since they are produced ina di!ractive dissociation of proton). In accordance with these relations

BR[XÌ/1@2

PR0K`]"13BR[X`

I/1@2P(RK)`] , (24)

BR[XÌ/1@2

PD``p~]"12BR[X`

I/1@2P(*p)`] . (25)

The ratios R1}R

4of the widths of the X(2000) and X(2050) decays into strange and nonstrange

particles are much larger than those for ordinary (qqq)-isobars [18,33].A narrow width of the X(2000) and X(2050) baryon states as well as anomalously large

branching ratios for their decay channels with strange particle emission (large values of R) arethe reasons to consider these states as serious candidates for cryptoexotic baryons with a hiddenstrangeness Duudss6 T.


5. New analysis of the data for reaction p1NP[R0K1]1N

In what follows we present the results of a new analysis of the data obtained in the run withpartially upgraded SPHINX spectrometer where conditions for K and R0 separation were greatlyimproved as compared with an old version of this setup. The key element of the new analysis lays ina detailed study of the R0PK#c decay separation, which makes it possible to reach the reliableidenti"cation of this decay and reaction (16) with the increased e$ciency in comparison with theprevious analysis of Bezzubor et al. [21].

In this new analysis the data for (16) were studied with di!erent criteria for R0PK#cseparation (with larger e$ciency and larger background or with the reduced background at theprice of lower e$ciency). We will designate these di!erent criteria for photon separation as soft,intermediate and strong photon cuts (the details will be presented in [45]). Reaction (16) wasstudied in di!erent kinematical regions. It was found that improved background conditions wereimportant for the investigation of the region of small mass M(R0K`) and very small transversemomenta.

The e!ective mass spectra M(R0K`) in p#NP[R0K`]#N for all P2T

are presented in Fig. 7.The peak of X(2000) baryon state with M"1986MeV and C"98$20 MeV is seen very clearlyin these spectra with a very good statistical signi"cance. Thus, the reaction

p#NP X(2000)#N ,

vR0K`(26)

is well separated in the SPHINX data. The cross section for X(2000) production in (26) is

p[p#NPX(2000)#N] )BR[X(20000)PR0K`]"95$20 nb/nucleon (27)

(with respect to one nucleon under the assumption of pJA2@3, e.g. for the e!ective number ofnucleons in carbon nucleus equal to 5.24). The parameters of X(2000) are not sensitive to thedi!erent photon cuts, as is seen from Table 2. The dN/dP2

Tdistribution for reaction (26) is shown in

Fig. 8. From this distribution the coherent di!ractive production reaction on carbon nuclei isidenti"ed as a di!raction peak with the slope bK63$10 GeV~2. The cross section for coherentreaction is determined as

p[p#CPX(2000)`#C]C0)%3%/5

) BR[X(2000)`PR0K`]

"260$60 nb/C nucleus . (28)

We must bear in mind that it is more convenient to use other relations for cross sections:

p[p#NPX(2000)`#N]BR[X(2000)`P(RK)`]"285$60 nb/nucleon , (29)

p[p#CPX(2000)`#C]BR[X(2000)`P(RK)`]"780$180 nb/nucleus , (30)

which were obtained from Eqs. (27) and (28) using branching ratio (24).The errors in the cross sections of Eqs. (27)}(30) are statistical only. Additional systematic errors

are K$20% due to uncertainties in the cuts, in Monte Carlo e$ciency calculations and in theabsolute normalization.


Fig. 7. Invariant mass spectra M(R0K`) in the di!ractive reaction p#NP[R0K`]#N for all P2T

(with soft photoncut): (a) measured mass spectrum; (b) the same mass spectrum weighted with the e$ciency of the setup. Parameters ofX(2000) peak are in Table 2.

In the mass spectra M(R0K`) in Fig. 7 there is only a slight indication for X(1810) structure whichwas observed earlier in the study of coherent reaction (16) } see Fig. 5 and [21]. This di!erence iscaused by a large background in this region for the events in Fig. 7 (all P2

T, soft photon cut). To clarify

the situation in this new analysis we investigated also the M(R0K`) mass spectra for coherentreaction (16), e.g. for P2

T(0.1GeV2. In these mass spectra not only the X(2000) peak is observed, but

the X(1810) structure as well. These spectra (see [45]) are compatible with the data in Fig. 5.The yield of the X(1810) as function of P2

Tis shown in Fig. 9. From this "gure it is clear that

X(1810) is produced only in a very small P2T

region (P2T(0.01}0.02GeV2). For P2

T(0.01 GeV2 the

M(R0K`) mass spectrum demonstrates a very sharp X(1810) signal (see Fig. 10) with the para-meters of the peak

X(1810)PR0K`GM"1807$7 MeV ,

C"62$19MeV(31)

which is in a good agreement with the previous data of Table 1. The cross section for coherentX(1810) production is

p[p#CPX(1810)`#C]DP

2T:0.01 G%V2 BR[X(1810)`PR0K`]

"215$45 nb/C nucleus . (32)


Table 2Data on M(R0K`) in reaction p#NP[R0K`]#N, R0PKc with di!erent photon cuts (for all P2

T)

Photon cut Soft Intermediate Strong

N events in X(2000) peak 430$89 301$71 190$47Correction factor for photon 1.0 1.4 2.25e$ciencyParameters of X(2000)

M (MeV) Weighted spectrum 1986$6 1991$8 1988$6Measured spectrum 1988$5 1994$7 1990$6

C (MeV) Weighted spectrum 98$20 96$26 68$21Measured spectrum 84$20 94$21 68$20

p[p#NPX(2000)#N] 100$19 93$25 91$21BR[X(2000)PR0K`](nb/nucleon)

SMT MeV 1989$6SCT MeV 91$20

Average values Sp[p#NPX(2000)#N]T 95$20 (statist.) $20 (system)BR[X(2000)PR0K`]nb/nucleon

Sp[p#CPX(2000)#C]T 285$60 (statist.) $60 (system)BR[X(2000)PR0K`]nb/C nucleus

The additional systematic error for this value is $30%. It increased as compared with the sameerrors in Eqs. (27)}(30) due to the uncertainty in the evaluation of P2

Tsmearing in the region of

P2T(0.01 GeV2, which is more sensitive to P2

Tresolution.

It is possible also to demonstrate the coherent di!ractive X(2000) production in the clearest wayby using the `restricted coherent regiona 0.02(P2

T(0.1GeV2 (see Fig. 11) where there is no

in#uence of X(1810) structure.To explain the unusual properties of X(1810) state in a very small P2

Tregion the hypothesis of the

electromagnetic production of this state in the Coulomb "eld of carbon nucleus was proposedearlier [46]. It is possible to estimate the cross section for the Coulomb X(1810) production from(9) and (10):

p[p#CPX(1810)`#C]DP

2T:0.01 G%V2_ C06-0."

"(2Jx#1)MC[X(1810)`Pp#c][MeV]N2.8]10~30cm2/C nucleus

55.6]10~30 cm2MC[X(1810)`Pp#c][MeV]N (33)

(Jx51

2is the spin of X(1810)).


Fig. 8. dN/dP2T

distribution for the di!ractive production reaction p#NPX(2000)#N. The distribution is "ttedin the form dN/dP2

T"a

1exp(!b

1P2T)#a

2exp(!b

2P2T) with the slope parameters b

1"63$10GeV~2;

b2"5.8$0.6GeV~2.

Fig. 9. The P2T

dependence for the X(1810) structure production in the coherent reaction p#CPX(1810)#C.


Fig. 10. Invariant mass spectra M(R0K`) in the coherent di!ractive production reaction p#CP[R0K`]#C in theregion of very small P2

T(0.01 GeV2 (with strong photon cut) weighted with the e$ciency of the setup.

Let us compare this Coulomb hypothesis prediction with the experimental value

p[p#CPX(1810)`#C]DP

2T:0.01G%V2Z645nb/C nucleus . (34)

To obtain (34) we assumed in (16) that X(1810) is isodoublet, and then we use from (24) thebranching BR(X`PR0K`)[1

3(here K means that BR[X`P(RK)`]K1, i.e. this decay is

dominating).If the value of radiative width C[X(1810)Pp#c] is around 0.1}0.3MeV and the branching

BR[X(1810)`P(RK)`] is signi"cant, then the experimental data for cross section of the coherentX(1810) production (34) can be in agreement with the Coulomb mechanism prediction (33). Itseems that such value of radiative width is quite reasonable. For example, the radiative width forD(1232) isobar is C[D(1232)`Pp#c]K0.7MeV. The value of radiative width depends on theamplitude of this process A and on kinematical factor: C"DAD2 ) (Pc)2l`1 (Pc is the momentum ofphoton in the rest frame of the decay baryon and l is orbital momentum). For X(1810)Pp#cdecay the kinematical factor may be by an order of magnitude larger than for D(1232)`Pp#cbecause of the large mass of X(1810) baryon. Certainly, the predictions for amplitude A arequite speculative. But if, for example, (X1810) is the state with hidden strangeness Dqqqss6 T,then the amplitude A might be not very small due to a possible VDM decay mechanism


Fig. 11. Weighted invariant mass spectrum M(R0K`) for the reaction p#CP[R0K`]#C in the `restricted coherentregiona 0.02(P2

T(0.1 GeV2 (with intermediate photon cut).

(qqqss6 )P(qqq)#/7*35

P(qqq)#c. Thus, it seems that the experimental data for the coherentproduction of X(1810) (34) is not in contradiction with the Coulomb production hypothesis.

It is possible that the candidate state X(2050)PRH(1385)0K` which was observed in coherentreaction (14) in the region of very small transverse momenta (P2

T(0.02 GeV2) is also produced not

by di!ractive, but by the electromagnetic Coulomb production mechanism [46].The feasibility to separate the Coulomb production processes in the coherent proton reactions at

Ep"70 GeV on the carbon target in the measurements with the SPHINX setup was demonstrated

recently by observation of the Coulomb production of D(1232)` isobar with I"32

in the reaction

p#CPD(1232)`#C (35)

(see [46]).

6. Reliability of X(2000) baryon state

The data on X(2000) baryon state with unusual dynamical properties (large decay branchingwith strange particle emission, limited decay width) were obtained with a good statistical signi"-cance in di!erent SPHINX runs with widely di!erent experimental conditions and for several


Fig. 12. The e!ective mass spectrum M(R`K0) in reaction (17) for P2T(0.1GeV2.

kinematical regions of reaction (16). The average values of the mass and width of X(2000) state are

X(2000)PR0K`GM"1989$6 MeV ,

C"91$20MeV .(36)

Due to its anomalous properties X(2000) state can be considered as a serious candidate forpentaquark exotic baryon with hidden strangeness: DX(2000)T"Duudss6 T. Recently some newadditional data have been obtained which are in favor of the reality of X(2000) state.

(a) In the experiment of the SPHINX Collaboration reaction (17) was studied. The data for thee!ective mass spectrum M(R`K0) in this reaction are presented in Fig. 12. In spite of limitedstatistics the X(2000) peak and the indication for X(1810) structure are seen in this mass spectrumand are quite compatible with the data for reaction (16).

(b) In the experiment on the SELEX(E781) spectrometer with the R~ hyperon beam of theFermilab Tevatron the di!ractive production reaction

R~#NP[R~K`K~]#N (37)


Fig. 13. Study of M(R~K`) in the reaction R~#NP[R~K`K~]#N in the SELEX experiment. Here the spectrumM(R~K~) with open exotic quantum number used for subtraction of nonresonance background in M(R~K`) after somenormalization. One presents in this "gure M(R~K`)!0.95M(R~K~) (here 0.95 } normalization factor): (a) all theevents; (b) after subtraction of the events in / band to suppress the in#uence of the reaction R~#NP[R~/]#N.

was studied at the beam momentum PR~K600 GeV. In the invariant mass spectrum M(R~K`)for this reaction a peak with parameters M"1962$12MeV and C"96$32 MeV wasobserved (Fig. 13). The parameters of this structure are very near to the parameters ofX(2000)PR0K` state which was observed in the experiments on the SPHINX spectrometer. Itseems that the real existence of X(2000) baryon is supported by the data of another experiment andin another process.

Preliminary results of studying reactions (17) and (37) were discussed in the talks at the lastconferences [24,25,47] and are now under detailed study.

7. Nonperipheral processes

As was discussed above (Section 2.3) the search for new baryons in proton-induced di!ractive-like reactions in the nonperipheral domain, with P2

T'0.3}0.5GeV2, seems to be quite promising.

Here we present the very "rst results of these searches in the invariant mass spectra of the R0K`

and pg systems produced in the reactions p#NP[R0K`]#N (16) and p#NP[pg]#N (18)for P2

T'0.3GeV2 (see [20,21]). Combined data on reaction (16) from the old and new runs are

shown in Fig. 14a. The data from the old run for reaction (18) are shown in Fig. 14b. Despite limitedstatistics, a structure with mass M+2350 MeV and width C&60 MeV can be clearly seen in thesetwo mass spectra. They require a further study in future experiments with larger statistics. Thesame statement seems true for the intriguing data on the invariant mass spectrum M(pg@) forreaction (19) in the region P2

T'0.3 GeV2 (see Fig. 14c). It must be stressed that reaction (19) is the

only one (among more than a dozen of other di!ractive-like reactions studied in the SPHINXexperiments) in which a strong coherent production on carbon nuclei was not observed (theabsence of the forward peak in dN/dP2

Tdistribution with the slope value bZ50 GeV~2; the slope

for forward cone in Eq. (19) is b&6.5 GeV~2 } see [20,22]).


Fig. 14. (a) Invariant mass spectrum of the R0K` system produced in the reaction p#NP[R0K`]#N (16) forP2T'0.3GeV2 (combined data from the old and new runs). (b) Invariant mass spectrum of the pg system produced in the

reaction p#NP[pg]#N (18) for P2T'0.3 GeV2 (old data, see [20]). A narrow structure with M&2350MeV and

C&60 MeV in the nonperipheral region of reactions (16) and (18) is seen in these mass spectra. (c) Invariant massspectrum M(pg@) for the reaction p#NP[pg@]#N (19) in the region P2

T'0.3GeV2 (old run). This spectrum is

dominated by a threshold structure with M&2000MeV and C&100MeV.

8. OZI rule and di4ractive proton reactions

The selection rule for connected and disconnected quark diagrams, which is referred to as theOZI rule, has been known for a long time [48}50]. According to this rule, processes involving theannihilation or creation of a quark}antiquark pair entering into the composition of the samehadron are forbidden or, strictly speaking, strongly suppressed. The OZI rule can be illustrated byresults of studying the charge exchange reactions

p~#pP/#n (38)

(OZI forbidden process) and

p~#pPu#n (39)

(OZI allowed process).As is well known, the quark structure of vector / and u mesons has the form

D/T"!cos a ) Dss6 T!sin a ) D1

J2(uu6 #ddM ) ,

DuT"!sin a ) Dss6 T#cos a ) D1

J2(uu6 #ddM ) .

(40)


Here a"*0V"0

V!00

V; 0

Vis the mixing angle in the vector meson nonet and 00

V"arctg (1/J2)"

35.33 is ideal mixing angle. For ideal mixing a"0 and D/T"ss6 ; DuT"D(1/J2)(uu6 #ddM ). Fromseveral data the value of DaD for vector nonet is DaDK(3}43). Thus, in accordance with the OZI rule,the ratio of the cross sections for reactions (38) and (39) is

R(//u)OZI

"p(/)/p(u)"tg2 a&(3}5)]10~3 (41)

in a very good agreement with several experimental data. For example, the recent result ofLEPTON-F Collaboration for charge exchange reactions (38), (39) at P

p~"32.5GeV [26] is:

R(//u)%91._p

"(2.9$0.9)]10~3 . (42)

Recent years have seen revived interest in the OZI rule. First, intensive searches for exotichadrons (see the surveys in [1}7] and references therein) are greatly facilitated by choosingproduction processes in which the formation of conventional particles is suppressed by the OZIrule [51}53,34]. Since this rule may be signi"cantly violated for exotic hadrons because of theircomplex color structure, signals from exotic and cryptoexotic particles are expected to bene"t fromthe more favorable background conditions in OZI-suppressed production processes. While the /pand /o decays of isovector mesons with conventional quark structure (uu6 !ddM )/J2 are suppressedby the OZI rule (the corresponding probabilities are reduced by more than two orders ofmagnitude), the same decay channels may prove much more probable for exotic multiquarkmesons with hidden strangeness [(uu6 !ddM )ss6 /J2] and hybrid states like (uu6 !ddM )g/J2. Further-more, unexpected results obtained in polarization measurements for deep-inelastic lepton scatter-ing on nucleons give rise to the well-known problem of nucleon-spin crisis [54,55]. To explain thisphenomenon, it was hypothesized that nucleons involved an enhanced quark component withhidden strangeness (direct strangeness in nucleons). This may induce signi"cant violations of theOZI rule in nucleon processes [56,57]. Strong violations of the OZI rule were indeed observed inthe relative / and u yields from some channels of p6 annihilation (reactions p6 pP/p0, p6 pPup0,p6 pP/c, and p6 pPuc [57,58]).

The above arguments suggest that the OZI rule should be further tested in various productionand decay processes and "rst of all in the nucleon reactions. In the experiments of the SPHINXcollaboration this test was performed in studying di!ractive production reactions with / andu mesons (12) and (20) [27,28]. It was observed that the average value of the e!ective ratio of yieldsof / and u mesons in these reactions is

SR(//u)T%91._p

"(4}7)]10~2 . (43)

Thus, the strong violation of the OZI rule (by more than an order of magnitude) is observed inproton di!ractive production reactions.

The intriguing large violation of the OZI rule in di!erent nucleon reactions and, particularly, inproton di!ractive production processes may suggest an enhanced component with hidden strange-ness in a quark structure of nucleons (the model of direct strangeness in nucleon). The OZI rulein proton reactions and the possible existence of direct strangeness are illustrated by diagrams inFig. 15.


Fig. 15. / production in proton}nucleon or proton}nucleus di!ractive reactions. (a) Disconnected quark diagram(suppressed by OZI rule). (b) Connected diagram for / production in the model with some small direct component of ss6 inproton wave function. In this model one observes the apparent OZI rule violation due to a small exotic ss6 component(model with direct strangeness in nucleon).

9. Conclusion

The extensive research program of studying di!ractive production in Ep"70GeV proton

reactions is being carried out in experiments with the SPHINX setup [2,12}28,45}47]. Thisprogram is aimed primarily at searches for cryptoexotic baryons with hidden strangeness (Duudss6 T).Only a part of these experiments is discussed here.

The most important results of these searches were obtained in studies of the hyperon-kaonsystems produced in proton di!ractive dissociation processes and "rst of all in reactionp#NPR0K`#N (16).

New data for this di!ractive production reaction were obtained with the partially upgradedSPHINX detector (with new c-spectrometer and with better possibilities to detect KPpn~ andR0PKc decays). New data are in a good agreement with previous SPHINX results on theinvariant mass spectrum M(R0K`) in this reaction.

A strong X (2000) peak with M"1989$6GeV and C"91$20MeV together with a narrowthreshold structure (with M&1810MeV and C&60MeV) are clearly seen in the (R0K`) invariantmass spectra. The latter structure is produced at very small transverse momenta, P2

TS0.01!0.02

GeV2. The unusual properties of the X (2000) baryon state (narrow decay width, anomalouslylarge branching ratio for the decays with strange particle emission) make this state a seriouscandidate for a cryptoexotic pentaquark baryon with hidden strangeness Dqqqss6 T. Preliminary datafor DRKT states in other reactions (17) and (37) con"rm the real existence of X (2000) baryon.

The OZI selection rule was investigated by comparing the cross sections for pion-inducedcharge-exchange reactions n~#pP/#n and n~#pPu#n at Pn~"32.5 GeV, as wellas the cross sections for the proton-induced di!ractive reactions p#NP[p/]#N andp#NP[pu]#N at E

p"70 GeV, in experiments with the LEPTON-F and SPHINX spectrom-

eters. It was shown that in pion reactions the ratio R(//u)K(3$1)]10~3 is in good agreementwith the naive-quark model and with the OZI rule prediction (R(//u)

OZI"tg2Dh

VK4]10~3).


At the same time, this ratio in proton reactions is SR(//u)TK(4}7)]10~2. That is, a strongviolation of the OZI rule is observed in proton}nucleon interactions. The large violation of OZIrule in proton di!ractive reactions may be in favor of some enhanced hidden strange component innucleons (the model with direct strangeness).

Several other interested phenomena were observed in the nonperipheral domain (withP2TZ0.3GeV2), in the study of di!erent reactions (for example, reactions (14), (18) and (19)). But

they need experimental veri"cation with much better statistics.Now the "rst stage of the experimental program on the SPHINX setup has been completed. In

the last years the SPHINX spectrometer was totally upgrated. The luminosity and the data takingrate were greatly increased. In the recent runs with this upgrated setup we obtained large statisticswhich is now under data analysis. In the near future, we expect to increase statistics by an order ofmagnitude for the processes discussed above and for some other proton reactions.

Acknowledgements

It is a great pleasure and a great honor for me to participate in this volume dedicated to the 70thanniversary of the birth of the outstanding scientist Lev Okun. I am very lucky to share withhim many years of close contacts and a real friendship. I am deeply obliged to Lev for manyilluminating discussions which helped me tremendously in many ways and, in particular, in thestudies which are covered in this paper. Even my original interest in exotic hadrons was initiatedmany years ago by his brilliant lectures at physics schools in IHEP, ITEP and MEPI (see, forexample, Ref. [59]). I wish Lev many further productive and happy years in his life and work.

This work is partially supported by Russian Foundation for Basic Research (grant 99-02-18251).

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Search for exotic baryons with hidden strangeness in diffractive production processes