UCLEAR PHYSIC~

P R O C E E D I N G S SUPPLEMENTS

ELSEVIER Nuclear Physics B (Proc. Suppl.) 56A (1997) 299-302

R e c e n t Resu l t s on O Z I Ru le Vio la t ions

Ulrich Wiedner CERN, PPE-Division, CH-1211 Geneva 23, Switzerland

Recent data from PP annihilation at LEAR indicate large violations of the OZI rule for certain final states. The final state dependance together with results on polarized lepton-nucleon scattering led to a model that assumes that there is a polarized ~s component in the nucleon wave function. The data and this model will be presented and in the end a little phenomenological model is used to explain part of the strangeness enhancement that is seen in ultra-relativistic heavy ion reactions.

If the current picture of hadronic interactions expressed in the theory of Quantum Chromodynamics (QCD) is valid, we should observe the effect of chiral symmetry breaking. The concept of chiral SU(3) symmetry constrains possible effective theories. The pattern of chiral symmetry breaking, however, does not seem to fit well with the picture that we have in the constituent quark model, which predicts that the proton is composed solely of up and down quarks. To solve the problems connected with chiral SU(3) symmetry breaking, a radical alternative has been suggested in the last years: that the proton contains a sizeable amount of ss quarks even at hadronic scales. There are several experimental indications for this hypothesis. For a recent review of the experimental and theoretical situation see Ref. [1].

The annihilation of antiprotons with protons could be one testing ground for the strangeness content of the nucleon. In the constituent quark model, pp-annihilation at rest is a reaction containing only up and down quarks in the initial state. The vector mesons ~ and to produced in such an annihilation process mix nearly ideally. The

m E

production ratio pp--~ OX over pp ~ toX can be calculated, and with the quadratic Gell-Mann-Okubo mass formula it is expected to be 1/240. Studies of this ratio done mainly at the LEAR storage ring at CERN by the ASTERIX, Crystal Barrel and OBELIX collaborations in different final states show deviations from this result [2-16], violating the so called OZI rule [17].

A possible strange quark content of the proton may explain the observed deviations from the expected value of the naive OZI rule [ 18]. A model

RaUo ~

04

[] [] [] pp (S-wave) pn l~p 15p (P-wave)

Fig. 1: Production Ratio #X/toX

has been put forward that interprets the observed violations of the OZI rule in terms of a 'shake-out' and 'rearrangement' of an intrinsic ~s component of the nucleon wave function [ 19-20]. The proton wave function would then look like:

IP) = v ~ luudX) + z ~ luudgsX) +. . . x = 0 X = 0

X stands for any number of gluons and light non- strange qq pairs and the dots denote components with more than one ~s pair.

Furthermore this model predicts that the nucleon's ~s component is polarized opposite to the

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300 lA Wiedner/Nuclear Physics B (Proc. Suppl.) 56A (1997) 299-302

nucleon spin. Such a polarized sea of is quarks is indeed indicated by deep inelastic lepton-nucleon scattering experiments [21-28] with polarized gluons being an alternative explanation [29-31]. The simplest possibility for the wave function of the component containing is which preservs the quantum number of the vacuum is a spin-triplet S- wave is pair. This is pair is polarized with Sz = -1 and couples with an angular momentum Lz = +1 to the usual Sz = 1/2 luud> valence state.

The consequences of this model for ~p annihilations are the following: a) if the annihilation takes place in a ~S0 state the production of a spin triplet state like the ~ meson is possible by a 'shake-out process'. b) if the annihilation takes place in a 3Sl state both 'shake-out' and 'rearrangement' contribute to the production of t~ mesons. Therefore one expects a maximum enhancement for

production in the 3S~ channel and a weaker enhancement in the tSo channel. Table 1 shows the contributing partial waves for certain final states in a

ISo 3S l ip~ 3Po 3P2

0~' x X X

~ o X X

¢ w x x

~p x X X

0~0 x X X

t~'q x x

Table 1: Initial and final states

pp annihilation process at rest. The polarized intrinsic strangeness model can

explain why there is so much tim ° production and relative little ~to and d~p production (mainly 1S0). The model cannot make any prediction for the ~rl final state, because it is unclear how additional diagrams caused by the strangeness content of the rl would influence the production ratio. It clearly fails to explain the large OF/ production ratio. Alternative explanations for the violation of the OZI rule assume rescattering processes or other final state interactions [32-36].

The intrinsic strangeness model predicts that the enhancement of the Crdorrc production ratio should decrease if the fraction of S-wave annihilation decreases. The OBELIX collaboration [I0] indeed sees indications of this effect by varying the target density in order to achieve more P-wave annihilation. Crystal Barrel observes a similar effect by studying annihilations in flight at moderate p momenta of 600 MeV/c. At this momentum the S-wave fraction of the annihilation is reduced to -15-25% and we have increased P-wave annihilation. This makes it interesting to investigate another channel. The possibility of rearrangement diagrams from P-wave annihilation states in the intrinsic strangeness model would lead to an increased f2'(1525)~f2(1270)~ production ratio at 600 MeV/c compared to annihilation at rest. A preliminary analysis of the Crystal Barrel data [37] gives a value for this ratio between 1/10 and 1/20. The expectations from mixing angles vary between 1/38 (linear mass formula) [38], 1/62 (quadratic mass formula) [38] and 1/1000 (2 photon physics) [39]. Crystal Barrel measurements at rest [40] on the other hand agree with the value from the linear mass formula and again we see a dependance of the production of a s~ meson on the quantum numbers of the initial state.

There are further tests for the intrinsic strangeness model. The PS 185 collaboration at LEAR is going to measure the polarization of A's in the pp annihilation process. In the naive constituent quark model the strange quarks are believed to carry the spin of the A and 7[. By using a polarized proton target the p ---) A polarization transfer (or the depolarization D) can be measured. The intrinsic strangeness model predicts the polarization of the strange quark to be opposite to the polarization of the proton and therefore negative depolarization D<0 [41]. In case of polarized gluons one expects the spin of the gluons to be parallel to the proton spin and the quark-antiquark pair arising from these gluons hence to have their spins parallel to the proton spin. The depolarization in this case would be positive. Previous measurements of the PS 185 collaboration indicate that the A~ pairs are produced mainly in a spin-triplet state, which would be in accordance with the intrinsic strangeness model [32-44].

Similar measurements of the depolarization parameter D in deep-inelastic charged lepton-proton scattering in the target fragmentation region are

u Wiedner/Nuclear Physics B (Proc. Suppl.) 56A (1997) 299-302 301

planned to be done with the new COMPASS experiment at CERN [45] at the beginning of next century. In the proposal made in ref. [46] a polarized muon beam couples to a proton target preferentially through a polarized virtual boson (W or "/) which will strike one particular quark (antiquark) polarization state inside the target nucleon. The fragment left behind will contain some memory of the spin that was removed. This may be transferred to the final state in the target fragmentation region. For example, would a positively-polarized u quark - in the scope of the intrinsic strangeness model - tend to leave behind a negatively polarized is pair, which then leads to a negative polarization of A's produced in the target fragmentation region. Such measurements of the A polarization were in fact already done in neutrino scattering experiments on an unpolarized target [47-50]. The results of the WA59 collaboration [47] show a large negative (with respect to the direction of the momentum transfer) polarization of the A in the order of 0.7, a number which is well reproduced by the polarized intrinsic strangeness model.

The enhancement of strangeness production in ultra-relativistic heavy-ion collisions is of considerable physics interest. In the following an attempt is made to understand how a s~ component in the nucleon's wave function could influence hte results. Let us assume that the proton consists of three valence quarks and a string junction made of gluons as proposed in ref. [51]. Taking this picture, a recent paper [52] has pointed out that the effect of string junction interaction should be visible at ultra- relativistic energies. The idea behind that is that the valence quark distributions will be Lorentz contracted to thin pancakes in the center-of-mass system at sufficient high enough energies, because the valence quarks carry most of the nucleon's momentum. There is a certain time needed for the valence quarks to interact with valence quarks from another nucleon in a collision. At sufficient high enough energies it may happen that the collision time of the valence quark distributions from two nucleons is much smaller than the time needed for an interaction because of the Lorentz contraction. In such a case no valence quark interaction is possible.

The same argument might not hold for an interaction of the gluons of the string junction. Such a string junction contains an infinite number of

gluons which, by virtue of momentum conservation, can carry only a very small fraction of the nucleon's momentum. Therefore a string junction may not be Lorentz contracted and the interaction of two high energy nucleons could be the interaction of string junctions which have in contrast to the valence quarks enough time to interact. The string junctions may even be stopped and the valence quarks be stripped off, causing jets in the fragmentation regions. This effect is partly counteracted by the fact that pp collisions at high energies become more and more peripheral and the string junctions, which are confined inside the valence quarks pass away from each other in the impact parameter plane and do not interact. Nevertheless Ref. [52] claims that a substantial amount of string junction stopping is observed event at highest available energies.

What w o u ld now be the consequences of a strange quark contribution to the proton wave function? In case of a string junction stopping and a 'stripping off' of the valence quarks the strings break and the end of the string will get immediately dressed up by sea quarks. If this sea contains a sizeable amount of s~ pairs, we will observe the productions of strange hadrons, especially visible in the central rapidity region. From this model we expect to see an increase of strangeness production in proton-proton collisions with increasing energies. This effect has been well established in experiments at the ISR [53,54] but of course other mechanism to produce strangeness have to be taken into account as well and string junction stopping would count for only part of the observed increase.

What would happen in case of heavy ion collisions? First of all there would be an increase of strangeness production with energy similar to the one observed in pp collsions. Secondly the strangeness production per nucleon would be higher compared to pp collisions. This is due to the dense packing of string junctions in heavy ions where one has less peripheral interactions Indeed, this is what has been observed [52,53]. Of course other physics processes may explain the results as well, but it might be worth to look at these and forthcoming data e.g. from RHIC under the aspect that a strangeness enhancement in heavy ion collisions may reflect partly the structure of the nucleon.

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