2A search for neutron-antineutron (n −n) oscillation was undertaken in Super-Kamiokande using the 1,489 day livetime or 2.45 × 10 34 neutron-year exposure data. This process violates both baryon and (baryon−lepton) numbers by two units and is predicted by a large class of hypothetical models where the seesaw mechanism is incorporated to explain the observed tiny neutrino masses and the matter-antimatter asymmetry in the universe. No evidence for n −n oscillation was found, the lower limit of the lifetime for neutrons bound in 16 O, in an analysis that included all of the significant sources of experimental uncertainties, was determined to be 1.9 × 10 32 years at the 90% confidence level. The corresponding lower limit for the oscillation time of free neutrons was calculated to be 2.7 × 10 8 s using a theoretical value of the nuclear suppression factor of 0.517 × 10 23 s −1 and its uncertainty.
The DEAR (DANE exotic atom research) experiment measured the energy of x rays emitted in the transitions to the ground state of kaonic hydrogen. The measured values for the shift and the width ÿ of the 1s state due to the K ÿ p strong interaction are 1s ÿ193 37 (stat) 6 (syst) eV and ÿ 1s 249 111 (stat) 30 (syst) eV, the most precise values yet obtained. The pattern of the kaonic hydrogen K-series lines, K , K , and K , was disentangled for the first time. DOI: 10.1103/PhysRevLett.94.212302 PACS numbers: 13.75.Jz, 25.80.Nv, 36.10.Gv Over 40 years, chiral symmetry breaking has been recognized as the essential aspect of nuclear low-energy phenomena. The outline of how the breaking plays a vital role is well known, yet its detailed dynamics is uncertain. The existence of the eight pseudoscalar mesons (; K; ) is believed to arise from spontaneous symmetry breaking of the flavor global symmetry represented by the group SU3 L SU3 R , which generates the mesons as Nambu-Goldstone bosons, leaving the vacuum only SU(3) symmetric [1]. Furthermore, the mass spectrum of these mesons reflects the explicit breaking of this symmetry [2]. In the quark model, the squares of the meson masses are proportional to the small current quark masses with the multiplicative factors of the chiral quark condensate in vacuum. The large mass difference between the mesons and the current quarks then suggests that the condensate is playing a significant role in the structure of the mesons [3].A similar situation is expected to occur in the structure of baryons and to be manifested in the baryon-pseudoscalar meson interaction [4]. In this case, the corresponding relation is that the baryon sigma terms are proportional to the current quark masses with the factors of the chiral quark condensate for the baryons [3]. The sigma terms thus serve as the measure of the significance of the condensate in the structure of the baryons. Especially of interest here is how the SU(3) flavor symmetry is realized in this aspect of the nucleon structure, but more specifically, how high is the strangeness content of the nucleon. The resolution of these issues depends quite sensitively on the value of the kaonnucleon (KN) sigma terms [5]. As the basic symmetry of QCD is SU(3), the KN sigma terms play the central role in various nuclear phenomena, such as strangeness production in heavy-ion collision and chiral restoration in nucleon matter, a topic of astrophysical interest [6].The KN sigma terms are closely related to the lowenergy KN and antikaon-nucleon (KN) scattering amplitudes [7], but the value of the KN sigma terms continues to remain with a large uncertainty [6,7] in spite of the recent efforts in lattice [8] and chiral perturbation [9] calculations, where the information on the KN and KN scattering lengths is vital. In this work, we report an accurate measurement of the ground-state x-ray transitions in kaonic hydrogen atoms. The shift and width of the atomic ground state is known to provide the most accurate information of the K ÿ -proton scattering l...
The results of the second phase of the Super-Kamiokande solar neutrino measurement are presented and compared to the first phase. −0.15 (sys.)) × 10 6 cm −2 sec −1 and the day-night difference is found to be (−6.3 ± 4.2(stat.) ± 3.7(sys.))%. There is no evidence of systematic tendencies between the first and second phases.
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