Strangeness production with hadrons

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A623 (1997) 294c-303c

Strangeness Production with Hadrons R. Bertini LNS CEN Saclay and Torino University

1. I N T R O D U C T I O N The discovery of the first hypernucleus [1], showed that stable systems could be obtained adding to the nuclear matter A hyperons. Since then the study of strangeness became an important clue to understand hadronic processes. The addition of strangeness increases the stability of systems, not only built up with nucleons and hyperons, like in the case of hypernuclei, but also when more elementary constituents are involved, like in the case of quark systems. Examples of such a stability for quark aggregates are the tt dibaryon and the fact that the first hyperon resonances have widths much narrower as compared to that of the first nucleon resonances, both I=1/2 and I=3/2. In the case of nuclear systems, in addition to the A- hypernuclei, for which a large amount of experimental data exist, we have other possible aggregates like the I3-hypernuclei, the AA-hypernuclei and the E-hypernuclei, that need experimental confirmation. In all these systems the strange object (hyperon or quark) can play the role of a tagged probe to study the hadronic interaction. Examples of the results obtained, taking advantage of this property, are the clear evidence for color provided by the discovery of the f~ and the measurement [2] of the A-nucleus spin-orbit force for the description of the short range nuclear forces. When the spin dependence of the interaction is studied an other property of the hyperons plays an important experimental role. ttyperons decay mainly via parity non conserving weak decay. Then the hyperon polarisation can be measured directly through the angular distribution of the decay products. This measurement can usually be performed with much higher efficiency than in the case of protons and neutrons, where polarimeters based on secondary scattering provide the measurement of the polarisation at the price of a very low efficiency. An additional question of large interest is to know if strangeness is present and at what level inside the nucleon, before the interaction. The study of the production of the ¢ meson, also detecting spirt observables, can be a powerfull tool to clarify this point. In spite of the potentialities of this field, all the questions have not been, so far, fully investigated, mainly because of the experimental difficulties to provide beams of high quality to collect data. Object of this talk is to review some of the open questions, that look more 0375-9474/97/$17.00© 1997-Elsevier ScienceB.V. All rights reserved. PII: S0375-9474(97)00448-X

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promising, and to provide some suggestions on possible experimental approaches to increase the number of data and to improve their quality. These open questions concern: the baryon-baryon interaction, the hyperon resonances, the polarisation of hyperons, the propagation of the hyperon resonances in nuclear matter. Some of these topics are in the program of the DISTO collaboration. We will discuss also this program and present some preliminary data. 2. B A R Y O N - B A R Y O N

INTERACTION

A theory of Baryon-Baryon (BB) interaction has to be intended as an extension of the description of the nucleon-nucleon interaction, that includes also, both in the short range and long range regime, the forces leading the nucleon-hyperon and hyperon-hyperon interaction. As in the nucleon-nucleon case two regions, the high energy and the low energy regions, represent the two extreme cases of the quark models and of the meson exchange representation. In the high energy domain powerfull tests of quark models can be provided ( [3], [4]) by the comparisons of total cross-sections , in different reactions. For example, in the case of the additive quark model, as first noted by Lipkin and Scheck [5], one can obtain relations among meson-baryon and baryon-baryon total cross-sections. Examples of these relations are: cr(Ap) - ~'(pp) = o r ( K - n ) - q ( r + p )

(1)

and

~ ( ~ - p ) + ~(r,-,~) = 2~(hp)

(2)

that should be tested at energies above those at which prominent meson-baryon resonances occur. The missing data are mainly on the hyperon-nucleon total cross-sections. A possible range of momenta for hyperon beams, where to perform measurements, is 6 GeV/c < pHY < 21 GeV/c. Dedicated experiments with high flux machines, like the JHP [6] now in project, will, hopefully, in the future, produce the lacking cross-sections. In this momentum range, scattering experiments should be easier because hyperons travel a longer distance before decaying and sizable polarisations can be obtained. One can then envision scattering between polarised hyperons and polarised target, learning then about the spin dependent hyperon-nucleon cross-sections. On the low energy side, at momenta comparable with those of nucleons in nuclei ( Fermi momenta ), the lack of data is impressive. A summary of the existing data is shown in Fig. 1. The motivation to provide more and better data is given not only by the need to check more general BB interaction theories but also to confirm the SU(3) symmetry breaking suggested [7] by the existing BB scattering data. This symmetry breaking and the possible hindering of the Z ~ A conversion in nuclear matter is related to the question of the existence of narrow ~-hypernuclei states [8]. Although the experimental situation on ~-hypernuclei states remains unclear, as

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Figure 1. Total cross-sections of nucleon-nucleon and hyperon-nucleon scattering at momenta corresponding to those allowed in Fermi motion in nuclei [7]. The solid lines are fits with the Nijmegen model D [9].

shown by the previous speaker, improvement in quality of the data is essential and requires a dedicated set-up that can be used also for the excitation of the first hyperon resonances and therefore discussed in that context.

3. H Y P E R O N

RESONANCES

As already mentioned in the introduction, an important characteristics, from the experimental point of view, of the first hyperon resonances is their width. This is about 10 times smaller then the width of the corresponding first excited states of the nucleon. That feature makes feasible the measurement of the properties (branching ratios, angular distributions etc.) of the electromagnetic transitions (e.m.) between the neighboring hyperon states. The approach is similar to that followed for the first excited states of ordinary nuclei, where even small contributions to the wave functions of the individual states could be probed that way. Take as an example the A*(1520). The width of this resonance is 15.6 MeV. It can be excited in the K - p reaction at 395 MeV/c. The e.m. transitions A(1520) --* A(1116) (70) and A(1520) ---+~](1192) (Tz) have energies larger than 300 MeV, far above the background. Predictions for the partial widths rr0 and F~l have been given in two different

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approaches. In one [10], the description of the wave functions is given in the framework of the MIT bag model and the radiative decay widths are calculated assuming a single quark transition. The predicted widths are r~0 = 46 KeV and P,1 = 17 KeV, taking the complete A*(1520) wave function, and Fro = 27 KeV and r.~l = 102 KeV, taking only the singlet component of the A*(1520) wave function. The non relativistic quark model of Isgur and Karl [11] has been very usefull for the understanding of a large amount of data in hadron physics. In this model the relevant configurations for the initial and final states for 3'0 and 71 transitions have been determined. With these configurations, the predictions for the partial widths are [12] r~0 = 96 KeV and 1~1 = 74 KeV, respectivelly. The relativeUy large differences between the numerical values of these partial widths show how important is, in order to make possible the choice between different theoretical approaches, to provide experimental values, with good statistical significance, for Fro and r~l. Intense and clean low energy kaon beams can be available in a near future with high flux machine. High resolution 7 ray spectrometers are now available for the energies to be detected here and therefore these e.m. transitions and those connecting other hyperon states should definiteUy be measured. The nature of the A*(1405), F = 50 MeV, is a puzzle since more than 35 years. This is mainly due to the fact that, lying about 30 MeV below the N K threshold, the A*(1405) can be observed directly only as a resonance bump in the (E~r)° subsystem in final states of production experiments. A new approach, made possible by the new high intensity machines, would be to construct an intersection ring where protons and antiprotons would interact at the center of mass energies convenient to produce hyperon and antihyperons or their first excited states. The production reaction here would be p~ ~ Y Y or p~ ~ Y'Y*. The particle (antiparticle) decay would be used to tag the partner particle produced in the p~ reaction. This technique as been successfully used in the fixed target experiment PS185 [13] at LEAR. The tagged Y (Y) can then be used to study the YN or YN scattering to provide the missing cross-sections in the low energy domain discussed in the previous section. The Y* or Y* can be studied through their decay. For example the e.m transitions from the A*(1405) to the A(1116) and to the ~(1192) could be studied that way. Such an experiment will provide the needed informations on the nature of the A*(1405) resonance. 4. T H E D I S T O P R O G R A M Some of the topics, discussed in the introduction, are in the program of the DISTO experiment [14], now taking data with the polarised proton beam of the Saturne accelerator in Saclay. The main goals of this program [15,16] are the following: 1) To measure the differential cross-sections &r/d.w and the polarization P of the A and ~0 hyperons (Y) produced by the reaction ~p ~ pK+Y.

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To study the dependence on the beam polarization of these observables getting the analyzing power A u and the depolarisation parameter D ~ . To obtain the relationship between these observables and the Y transverse momentum PT. To have a complete kinematical reconstruction of the reaction products in order to distinguish the Y's directly produced from those coming from the decay of Y* or N* resonances. In this sense this will be the first attempt to have a complete study (including spin) of the reaction mechanism through an exclusive experiment. 2) To measure the differential cross-section and the analyzing power Ay of the reaction gp --* pp¢ near the production threshold. 3) To determine the central part of the E°-nucleus potential via the measurement of the momentum distribution spectra of the ~0 hyperons produced in quasi-free scattering on various nuclei. 4) To study the behavior of the Y* resonances in nuclear matter comparing branching ratios for the decay of Y* in free space and inside the nuclear matter. The primary aim of the DISTO program remains that of probing the reaction mechanism for producing polarized hyperons in proton-proton collisions at intermediate energies. Do the sizable polarization effects observed in high-energy inclusive experiments [17] at momentum transfers PT ~>1 GeV/c persist closer to threshold? Are any of the simple models that have been suggested to account for these sizable spin effects viable here? In order to separate such possible effects as final-state interactions, formation of intermediate N* or Y* resonances, or transfer of polarization from the strange quark sea in the colliding protons from, for example, effects of ~r- or K-exchange, it is important to perform a kinematically complete experiment, spanning essentially all of phase space, with a polarized incident beam. Furthermore, the different spin coupling of strange quarks to other constituents in A vs. 2C hyperons makes it highly desirable to resolve A and E0 production [18]. Such resolution is also critical to distinguish directly produced A's from E ° production and decay (E0 __. AT). Predictions of the polarization observables for ~p --* p K + A and gp --* pK+'~ ° within the DISTO kinematic range have been made within a meson-exchange model [19]. The first DISTO measurements are being made at the maximum energy at which Saturne can deliver reliable polarized beam, but subsequent measurements at one or two lower energies are foreseen to provide a more stringent test of model calculations. The DISTO apparatus [20], designed to identify and track four charged products (p, K +, and p, ~r- from A decay) from A or E0 production through a strong magnetic field, can also be used for a number of other important investigations related to strangeness. The production of hidden strangeness in the pp --+ pp¢ reaction presumably involves quite a different mechanism than hyperon production, because the s and

R. Bertini /Nuclear Physics A623 (1997) 294c-303c quark lines leading to the ¢ are, to zeroth order, :'disconnected" from those in the interacting protons. Thus, ¢ production ought to be strongly suppressed by the OZI rule [21]. However, the OZI rule can be evaded if there is a substantial s~ content of the proton sea [22], and it can be violated by multi-step mechanisms [23] proceeding through OZI-aUowed intermediate states, such as pp --o p p K + K -. One interpretation that has been suggested [24] for unexpectedly large and channel-dependent OZI violations observed at LEAR in low-energy ffp --~ e X reactions [25] attributes the observations to the polarized-gs sea in a polarized proton. This suggestion can eventually be checked via polarization measurements (either spin correlation or spin transfer) in pp --+ pp¢ reactions. The DISTO experiment will provide a feasibility test for such experiments, revealing whether the cross section for pp ~ pp¢ near threshold is sufficient to yield a clear signal above background in the K+K - channel. There is also considerable interest in the quasifree production of hyperons from nuclei. If we are able to separate A from ~0 production on the moving nucleons inside nuclei, then we can extract information about the controversial ~-nucleus mean field [8] from final-state interactions in the pA -* pKr,°(A-1) reaction. There is currently insufficient knowledge of the strength of this interaction even to judge whether quasi-bound ~-hypernuclear states should exist. Furthermore, measurements for this case might elucidate the puzzle concerning the A-nucleus mean field, whose value is well determined from hypernuclear structure investigations, but not easily understood on the basis of YN potential models. Saturne has sufficient energy also to produce the narrow A'(1520) resonance in both free pp and quasifree pA collisions. If the production is sufficiently strong at 2.9 GeV to cleanly tag the A*(1520) in the pK + missing mass spectrum for both free and quasifree production, then a comparison of the decay properties in the two cases could provide the very first information on the propagation of a narrow resonance through nuclear matter. The experimental set-up, shown in Fig. 2, includes the magnet $170 from C E R N which provides a maximal magnetic field of 14.7 kGauss, an angular acceptance A~ = +120 ° in the horizontal plane and A¢ = +20 ° in the vertical plane. The liquid hydrogen target (LH2), placed in the center of the set-up has a cylindrical shape 20 mm long and with a diameter of 20 mm. The detection system consists of 16 detectors in two arms, symmetrically placed at the two sides of the beam direction. They cover a scattering angle of 45 degrees and a dip angle of 4-15.5 degrees. The inner detectors, placed at distances of 20 and 40 cm from the target center, respectively, are plastic scintillating fiber chambers made of one horizontal layer and two stereo layers at +45 degrees. These detectors have a cylindrical shape. The 12 detectors distributed in four sets of three detectors each, placed at 90 cm and 120 cm from the target center and on each side of the beam direction, are planar MWPC.

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The next set of detectors, also in two arms, with cylindrical shape and placed at 143 cm from the target center, consists of two planes of scintillation counter hodoscopes. Finally, behind the hodoscope (at 160 cm from the target), are set two arms of water Cerenkov hodoscopes. Each arm, of planar shape, consists of 12 vertical slabs. Only the fast readouts from the scintillating fiber detectors and from the scintillator hodoscope are used in the hardware of the first level trigger. The informations from the other detectors are needed in the retracking.

3

Figure 2. Layout of the DISTO experimental set-up

In the Fig. 2 are also shown simulted tracks of one event of the reaction pp -~ pK+Y.

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Data taking is now in progress at the Saturne accelerator on the items 1) and 2) of the DISTO program. Preliminary results, that represent a small part of the data so far analysed, are shown in Fig. 3. On the upper side of the figure,the invariant mass distribution, calculated from the momenta at the vertex between the negatively (assuming a 7r-) and positively (assuming a proton) charged tracks, has a peak corresponding to the A mass. Candidate A events must satisfy the criterion that the distance between the previous vertex and the primary reaction vertex into the target is > 2cm. The primary reaction vertex is obtained from the tracks of the proton and of the K +, partners of the A in the event. The kaon identification is obtained making use of the time of flight and pulse height (from the sdntillator and cerenkov hodoscopes) correlation with the particle momenta. The missing mass distribution, calculated from the momenta at the vertex of these tracks and with a cut on A mass, is given in the lower side of the Fig. 3 . The two peaks correspond to the A mass and to the G° mass respectiveUy. A cut on these two peaks will allow to select directly produced A's from those coming from the decay of the ~0. Even from the limited statistics of this bunch of data one can see that one of the objectives of the DISTO experiment, that to provide exclusive events, can be fulfilhd. New data are also available on $ production.

5. C O N C L U S I O N Mainly for lack of space, two important topics, also related to the increased stability of the systems, due to the addition of strangeness, and quoted in the introduction, have not been discussed here. These are the H dibaryon,for quark systems, and the related question of the AA-hypernuclei, for baryon systems. The study of these hypernuclei toghether with the study of the X-hypernuclei will provide information on the A-A and A- A-N interaction. The first exclusive experiment, with large acceptance, the DISTO program, is now producing data. The correlation between the A polarisation and its production channels is now being investigated for the first time. To understand why the A, and other hyperons, are polarised at high transfer momenta this kind of eperiments should be pursued at higher energies. Some new experimental approaches have been suggested to improve the quality of the data in hyperon-nucleon scattering and on the hyperon resonances. They require high quality tagged beams. A first example of dean low energy tagged kaon beams is provided here at DA~NE by the FINUDA experiment, that will start data taking soon. The new facilities, now in project, like the JHF accelerator, will make possible, with the high beam intensities available, the design of experiments on the guidelines proposed here.

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