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The Physics of Hypernuclei

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The Physics of Hypernuclei FEATURES nuclei (with new quantum numbers), in direction ofother exotic nuclear systems (charmed nuclei and so on). The physics of A hypernucleus is generally indicated with the symbol ofthe parent nucleus with the suffix A, indicating that a A particle has replaced a neutron. ~ C means a nuclear system composed of6 hypernuclei protons, 5 neutrons and one A particle. Three events ofdouble hypernucleiwere observed, inwhich 2 nucleons are substitutedby Aldo Zenoni, Universitii di Brescia and INFN Sezione di Pavia, 2 A particles. One example is ,J. He, a system composed of2 pro­ Brescia, Italy tons, 2 neutrons and 2 A particles. Paola Gianotti, INFN Laboratori Nazionali di Frascati, Frascati, The concept ofhypernuclei can be extended to nuclei where a Italy nucleon is replaced by hyperons other than the A one. Experi­ mental evidence was claimed oftheexistence ofE-hypernuclei,in which a nucleon is replaced by a L hyperon. This is rather sur­ ypernuclear physics was born 50 years ago, in 1953, when prising since, in nuclear matter, the L can decay through a strong H the Polish physicists M.Danysz and J. Pniewski [1] observed, interaction process (L + N ~ A + N) givingverylargewidths for in a stack ofphotographic emulsions exposed to cosmic rays at the hypernuclear states, unless a strongsuppression mechanism is around 26 km above the ground, the event shown in Figure 1. A at work. However, the experimental evidence ofthe existence of high energy proton, colliding with a nucleus ofthe emulsion, narrow L-hypernuclei is rather controversial and only the case of breaks it in several fragments forming a star. All the nuclear frag­ tHe is considered unambiguous. ments stop in the emulsion after a short path, but one After the very first evidence ofhyperfragments in cosmic ray disintegrates, revealing the presence inside the fragment, stuck interactions with nuclei, starting from the end ofthe Sixties, the among the nucleons, ofan unstable particle decaying weakly, the installation ofbeams ofK- mesons atparticle accelerators made it A hyperon. For this reason, this particOular nuclear fragment, and possible to study the formation ofhypernuclei in the laboratory. the others obtained afterwards in similar conditions, were called Hypernuclei were produced copiouslywithlittle background, the hyperfragments or hypernuclei. required negative strangeness for A hyperon production being The A hyperon is a baryon, like the nucleons (proton and neu­ present in the beam. The typical reaction utilized was the 2 tron), with mass 1115.684 ± 0.006 MeV/c , 20% greater than the "strangeness exchange reaction", where a neutron hit by a K- is mass ofthe nucleon, zero charge and isospin 1=0. It carries a new changed into a A hyperon emitting a 1t- (K- + n ~ A + rr-). The quantum number, not contained normally inside the nuclei, the experimental techniques used in these experiments were photo­ strangeness S =-1. The A hyperon is unstable and decays with graphic emulsions and bubble chambers (filled with He or heavy lifetime 263 ± 2 ps , typical ofthe weak interaction that doesn't liquids) exposed to K- beams. These experiments mainly mea­ conserve strangeness and makes a free A mainly disintegrate in a sured the hyperon binding energies, bymeans ofthekinematical nucleon~pion system. analysis ofthe disintegration star, and observed the principal However, since strangeness is conserved in the strong interac­ decaymodes ofthe hypernuclei. tion and the A particle is the lighter particle in the family of Starting from the Seventies, the study ofhypernuclei was con­ hyperons (baryons with tinued with K- beams by means of counter techniques with ,. strangeness), it can stay magnetic spectrometers and the introduction ofnew particle in contact with nucleons detectors (multiwire proportional chambers and drift chambers). inside nuclei and form The identification ofwell defined excited hypernuclear levels, by hypernuclei. The mere means ofthe kinematical analysis ofthe production reaction, existence ofhypernuclei was one ofthe most important results of a first series ofexperi­ is ofgreat scientific inter­ ments started at CERN and continued at Brookhaven (USA). est; it gives indeed a new The physical interpretation ofthese spectra was possible in terms dimension to the tradi­ ofmicroscopic descriptions of the A-nucleus and A-nucleon tional world ofnuclei by potentials. '. revealing the existence of In the Eighties, a new technique for hypernucleus production a new type of nuclear was introduced atthe Brookhaven laboratory,bymeans ofintense matter and generating beams ofhigh energy 1t+. With this technique the A hyperon is new symmetries, new produced inside the nucleus by an"associated production reac­ selection rules, etc. tion" (1t+ + n ~ A + K+). This reaction has a reduced cross - Hypernuclei represent section, compared to the"strangeness exchange" reaction, howev­ ~". .. .. ; . the first kind offlavoured : .- er this drawback is over compensated bythe greater intensities of j • .. the 1t+ beams. The technique was fully exploited at the KEK labo­ ". .EJg,j,; The first ratory in Japan where, for more than ten years, a great wealth of ,;~ excellent hypernuclear data were produced, concerning both to •••• ; .r hypernuclear event •I ,... observed in a nuclear spectroscopyand decay ofhypernuclei. .,,,( a ._-----... ----._o£-~ .::;••O'<:..'__ __1 emulsion [1]. (A) Theinterestinthese new data has triggeredthe attention ofthe I indicates the star of the primary interaction of the high energy nuclear physics community and, in the last few years, different I' cosmic ray (p) coHiding with a nucleus ofthe emulsion; (f) is the laboratories aroundthe worldhave started a hypernuclear physics track of the produced hyperfragment; (B) is the vertex of the program: COSY at Jiilich in Germany, TJNAF at NewportNews in I, I decay of the hyperfragment.The other non-strange nuclear I USA, Nuclotron in Dubna (Russia). Inparticular,itisworthmen­ I fragments stop in the matter, tioningthe Italian hypernudearproject,the FINUDA experiment ~ ~ _.~ L_... ~_ .. .•_._"'"....._.. _ .. __..•.._._... ..... _... -1 (acronim ofFlsica NUcleare a DA<I>ne) at the Laboratori Nazion- europhysics news SEPTEMBER/OCTOBER 2002 157 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn:2002501 FEATURES ali di Frascati of INFN, which is going to start collecting data next year. The experiment, which has an ambitious physics pro­ gram, has some special features compared to traditional CHART OF A·HYPERNUQIDES .' ~ffiJ hypernuclearphysics experiments. Infact, interestingly enough,it ,~/~~Y will operate at a collider, rather than on an extracted beam. / e+e- ~. What follows will describe it in some detail. z lI Structure of A-hypernuclei r //~AI AA-hypernucleus t Z is a boundstate ofZ protons, (A-Z-l) neu­ trons and a A hyperon. The ground state of such a system is r.:-~r:~~ '~N 1~. made bythe (A-l) nucleons accommodated inthe groundstate of l~C l~C '~CI l~B the nucleus (A-l)Z and the A hyperon in its lower energy state. ~B '~B I~B The A hyperon, carrying the strangeness quantum number, is a • ~8e ~Be ~r.s. 3 distinguishable baryon and is not subject to the limitations ~ll ~ll ~ll :ll 2 imposed bythe Pauli principle, therefore it can occupy all quan­ ;,H. ;'H. ~H. ~ ~. 1 tum states alreadyfilled up with nucleons. This feature makes the ~H ~" A hyperon, embedded in a hypernucleus, a unique means to explore nuclear structure. ... Fig. 2: "5egre tab/e" ofthe 35 known A-hypernuclei. The : The binding energy BA ofa A particle in the hypernucleus t Z 1 hypernucleus is formed by Zprotons, Nneutrons and a A hyperon. in its ground state is defined as: ! quality, due to thelimited beam intensities available for hypernu­ clear studies from the existing facilities and the modest energy 2 where M core is the mass (in MeV/c ) ofthe nucleus (A-l)Z ,MA is resolution ofthe presentexperiments.Nevertheless, the gross fea­ the mass ofthe A particle and M Iryp the mass ofthe hypernucleus tures ofthe data are reasonably well reproduced by effective t Z ,experimentally measured. BA varies linearly with A with a one-bodyhyperon-nucleus interactions constructed on poten­ slope ofabout 1 MeV/(unit ofA) and saturates at about 23 MeV tial models ofthe hyperon-nucleon YNinteraction. for the heavy hypernuclei. This behavior suggests a simple model The starting point ofthese models is a good NNinteraction inwhichthe A particleis confinedin a potentialwell witha radius generated by the exchange ofnonets ofmesons, withSU(3)f con­ equalto the nuclear radius anda depth of28 MeV; to be compared strainst for the coupling constants. Their predictions are fitted to the 55 MeV typical value ofthe nucleon potential well. simultaneouslyto the abundant NNdataand the veryscarce YN This is consistentwith a A-nucleon interaction weaker than the free scattering data. However, untilnow, ithas notbeenpossible to nucleon-nucleon one. Indeed, in a meson exchange model ofthe obtain a reliable and unambiguous YN interaction model. interaction, the zero isospin ofthe A prevents the exchange of The improvement ofthe YNinteraction models would need isovector mesons like the 1t or the p with a nucleon and deter­ precise data on the free YN interaction,which are verydifficult to mines the lack ofstrong tensor components in the interaction. Therelative weakness ofthe A-nucleon interaction entails thatthe shell structure is not disrupted by the insertion ofthe A in the nucleus and the lack ofPauli effects allows all the nuclear single 1.6 H'" .... \ ••.• ..1 particle states to be populated bythe A. In Figure 2, the so called 1.4 "Segre table" ofthe hypernuclei shows the 35 hypernuclei known ·······1·· at present. ~ .. i Experiments ofhypernucleus production by"strangeness "'" 1.2 ·········l .. ······ exchange" and"associated production" processes can produce ~ ci ··········!·············:·.. ··········1···········4~········1 hypernuclei in which the A populates different single particle ~ states. Tht; latter technique is particularly suitable for populating ~ 0.8 .. ·······"i·· ., ..•....: ~ . low lying A states, thanks to the high recoil momentum trans­ ...... L _.. L t _.. ferred to the A particle in the reaction.
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