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Register a new userOn the nature of new baryon state X(2000) observed in the experiments with the SPHINX spectrometer
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L. G. Landsberg
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In the experiment with the SPHINX spectrometer in 1995-99 a new baryon state X(2000) -> Sigma^0K^+ was observed in the proton diffractive reactions p + N(C) -> Sigma^0K^+ + N(C). The main parameters of X(2000)^+ baryon are M = 1986 +- 6 MeV, Gamma = 98 +- 21 MeV. Unusual features of massive X(2000) state (narrow decay width Gamma/M simeq 0.05, anomalously large branching ratio with strange particle emission) make it serious candidate for cryptoexotic pentaquark baryon with hidden strangeness. New data on the SPHINX with by an order of magnitude enlarge statistics are in agreement with previous data and are supported our conclusions. We propose further studies X(2000) baryon production in the meson reactions with baryon exchange: pi^{+-} + p -> X(2000)^+ + pi^{+-} OZI - forbidden suppressed reaction for the state |qqqsbar{s}>) and K^{+-} + p -> X(2000)^+ + K^{+-} (OZI - allowed reaction). If the value of R=BR[pi^{+-} + p -> X(2000)^+ + pi^{+-}]/BR[K^{+-}+p -> X(2000)^+ + K^{+-}]<<1 it will be the crucial argument in favor of X(2000)= |qqqsbar{s}>) structure. These experiments may be done on IHEP separated kaon beam with OKA spectrometer.
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The paper describes the SPHINX facility which includes a wide-aperture magnetic spectrometer with scintillation counters and hodoscopes, proportional chambers and drift tubes, multichannel electromagnetic and hadron calorimeters, a guard system, a RICH velocity spectrometer and a hodoscopical threshold Cherenkov detector for the identification of charged secondary particles. The SPHINX spectrometer, in its last modification, had the possibility to record 3000-4000 trigger events per an accelerator burst. The spectrometer was used during the last decade in experiments with the 70GeV proton beam of the IHEP accelerator U-70.
The radiative decay Lambda(1520) --> Lambda gamma was measured directly in the study of exclusive diffractive-like reaction p + N --> Lambda(1520) K^+ + N, Lambda(1520) --> Lambda gamma with the SPHINX spectrometer. The values of the branching and partial width of this radiative decay were obtained: BR[Lambda(1520) --> Lambda gamma] = (1.02+/-0.21(stat)+/-0.15(syst))x10^{-2} and Gamma[Lambda(1520) --> Lambda gamma] = 159+/-33(stat)+/-26(syst) keV.
Experimental results of the SPHINX Collaboration on studying proton diffractive production processes are presented. Evidences for new baryon states with masses >1.8 GeV were obtained in hyperon-kaon effective mass spectra in several reactions. New data for the diffractive reaction p+N->[Sigma0 K+]+N at Ep=70 GeV were obtained with partially upgraded SPHINX setup. The data are in a good agreement with the results of our previous study of this reaction. In the mass spectrum M(Sigma0 K+) a structure at the threshold region with a mass about 1800 MeV and a distinct X(2000) peak with M=1989+/-6 MeV and Gamma=91+/-20 MeV are observed. Unusual features of the massive X(2000) state (narrow decay width, anomalously large branching ratio for the decay channel with strange particle emission) make it a serious candidate for a cryptoexotic pentaquark baryon with hidden strangeness |qqqs sbar>. We also present new results on the narrow threshold structure X(1810) with M=1807+/-7 MeV and Gamma=62+/-19 MeV which is produced in the region of very small pt**2<0.01 GeV**2. The possibility of the Coulomb production mechanism for X(1810) is discussed.
A new baryon state is observed in the $Lambda_b^0pi^+pi^-$ mass spectrum with high significance using a data sample of $pp$ collisions, collected with the LHCb detector at centre-of-mass energies $sqrt{s}=7, 8$ and 13TeV, corresponding to an integrated luminosity of 9fb$^{-1}$. The mass and natural width of the new state are measured to be begin{eqnarray*} m & = & 6072.3 pm 2.9 pm 0.6 pm 0.2 MeV,, Gamma & = & 72 pm 11 pm 2 MeV,, end{eqnarray*} where the first uncertainty is statistical and the second systematic. The third uncertainty for the mass is due to imprecise knowledge of the $Lambda_b^0$ baryon mass. The new state is consistent with the first radial excitation of the $Lambda_b^0$ baryon, the $Lambda_b(2S)^0$ resonance. Updated measurements of the masses and the upper limits on the natural widths of the previously observed $Lambda_b(5912)^0$ and $Lambda_b(5920)^0$ states are also reported.
An experiment designed to investigate the strangeness photoproduction process using a tagged photon beam in the energy range of 0.90 -1.08 GeV incident on a liquid deuterium target was successfully performed. The purpose of the experiment was to measure the production of neutral kaons and the lambda particles on a deuteron. The generation of photo produced particles was verified by the measurement of their decayed charged particles in the Neutral Kaon Spectrometer 2. The reconstructed invariant mass distributions were achieved by selecting events where two or more particles tracks were identified. Preliminary results are presented here.
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