Today ’ s View on Strangeness

There are several different experimental indications, such as the pion-nucleon Σ term and polarized deep-inelastic scattering, which suggest that the nucleon wave function contains a hidden ss̄ component. This is expected in chiral soliton models, which also predicted the existence of new exotic baryons that may recently have been observed. Another hint of hidden strangeness in the nucleon is provided by copious φ production in various NN̄ annihilation channels, which may be due to evasions of the OkuboZweig-Iizuka rule. One way to probe the possible polarization of hidden ss̄ pairs in the nucleon may be via Λ polarization in deep-inelastic scattering. PACS. 12.39.Dc Skyrmions – 13.75.Cs Nucleon-nucleon interactions – 14.20.-c Baryons CERN-PH-TH/2004-231 hep-ph/0411369 1 How Strange is the Nucleon? Some people might argue that this is a “strange” question: why should the nucleon be strange at all after all, is it not just made out of three up and down quarks? We should not jump to such a näive conclusion. For a start, even the vacuum is strange: chiral symmetry for π,K mesons tells us that [1] < 0|s̄s|0 >= (0.8± 0.1) < 0|q̄q|0 > . This hidden strangeness cannot be expected to disappear when one inserts a set of three quark ‘coloured test charges’ into the vacuum. Moreover, hidden strangeness will be generated in perturbative QCD: quark → gluon → s̄s pair. There are also non-perturbative mechanisms for generating s̄s pairs in the nucleon, such as instanton effects [2]. Another objection to this ‘strange’ question is the fact that (at least some) experiments do not see very much strangeness in the nucleon. For example, CCFR measures a strange momentum fraction: Ps = 4% atQ 2 = 20GeV [3], the HAPPEX measurement of a combination of strange electric and magnetic form factors gives a small value: GE+0.39GM = 0.025±0.020±0.014 atQ 2 = 0.48GeV [4], SAMPLE finds a small strange contribution to the nucleon magnetic moment: −0.1 ± 5.1% [5], and the A4 Collaboration finds small strange contribution to another combination of form factors: GE +0.225GM = 0.039±0.034 [6]. Send offprint requests to: On the other hand, a few experiments indicate quite large matrix elements for some hidden-strangeness operators. One prominent example is the π-nucleon Σ term, whose value is related to the strange scalar density: