We search for a charged partner of the X(3872) in the decay B --> X- K, X- --> J/psi pi- pi0, using 234 million BBbar events collected at the Y(4S) resonance with the BaBar detector at the PEP-II e+e- asymmetric-energy storage ring. The resulting product branching fraction upper limits are BR(B0 --> X- K+, X- --> J/psi pi- pi0) < 5.4 x 10(-6) and BR(B- --> X- K0bar,X- --> J/psi pi- pi0) < 22 x 10(-6) at the 90% confidence level.
We find no evidence for high-energy photoproduction of pentaquarks at 1540 MeV/$c^2$, 1862 MeV/$c^2$, or 3099 MeV/$c^2$ using decay modes $pK_S^0$, $\Xi^-\pi^-$, and $D^{(*)-}p$, respectively.
Recent results from H1, ZEUS, HERMES and HERA-B on searches for exotic baryons in ep collisions, eD scattering and pA scattering at HERA are reviewed. Evidence for the production of the strange pentaquark Theta^+ and of a narrow anti-charmed baryon decaying to D^{*-}p together with negative results of pentaquark searches at HERA are presented.
Evidence for the production of exotic hadron states composed of five quarks (pentaquarks) has been searched for in data collected by the ALEPH, DELPHI, and L3 experiments at LEP. No significant signal is observed. Several 95% C.L. upper limits are set on the production rates of such particles.
Belle data for the pi-pi mass spectrum in X(3872) -> pi\pi + JPsi are well fitted by rho + J/\Psi with JPC = 1++, but are poorly fitted by the pi-pi S-wave. Formulae for partial wave amplitudes are given for all likely JPC and decay modes of the 3941 MeV peak in omega + J/Psi. Angular correlations involving up to five angles may allow a spin-parity determination, even with quite low statistics. Special attention is given to the case where X(3872) is a cusp or quasi-bound state with the same quantum numbers as the 3941 MeV peak.
The three pentaquark candidates Theta^+(1530), Xi(1862) and Theta^0_c(3100) have been studied in ep collisions at a centre-of-mass energy sqrt{s}=300-318 GeV using the full luminosity of the HERA-I data. Searches for narrow baryonic states in the decay channels K^0_s p, K^+ p, Xi^- pi^{+-}, anti-XI^+ pi^{+-} and D^{*+-}p^{-+} are reported. The results support the existence of a narrow resonance decaying into K^0_s p and K^0_s anti-p, consistent with the Theta(1530) state. No signals are seen in the K^+ p, Xi^- pi^{+-}, anti-Xi^+ pi^{+-} and D^{*+-}p{-+} channels.
We have searched for Theta+(1.54) -> K0,p decays using data from the 1999 run of the HyperCP experiment at Fermilab. We see no evidence for a narrow peak in the K0,p mass distribution near 1.54 GeV/c among 106,000 K0,p candidates, and obtain an upper limit for the fraction of Theta+(1.54) to K0,p candidates of <0.25% at 90% confidence.
Recently there has been revival of interest in exotic baryon spectroscopy triggered by experimental evidence for pentaquarks containing u,d,s and c-quarks. We report results of the searches for pentaquark states in decays to p K0S, Xi- pi+,- and D*- p performed at CDF detector using 220 pb-1 sample of pp= interactions at sqrt(s) of 1.96 TeV. No evidence for narrow resonances were found in either mode.
We study the $K^+p\to \pi^+KN$ reaction with kinematical condition suited to the production of the $\Theta^+$ resonance. It is shown that in this reaction with the polarization experiment, a combined consideration of the strength at the peak and the angular dependence of cross section can help determine the $\Theta^+$ quantum numbers.
The existence of an anti-decuplet of pentaquark particles has been predicted some year ago within the chiral soliton model. In the last year, several experimental groups have reported evidence for a S=+1 baryon resonance, with mass ranging from 1.52 and 1.55 GeV and width less than 25 MeV, by looking at the invariant mass of the $K N$ system. This resonance, has been identified with the lowest mass of the anti-decuplet, the $\Theta^+$. At the same time, there are a number of experiment, mostly at high energies, that report null results. An overview of the experimental results so far obtained will be given here together with a review of the second generation experiments currently ongoing and planned at Jefferson Lab Hall B.
Last year's X(3872) discovery was confirmed with the CDF II detector in pbarp collisions. We measure its mass to be $3871.3 \pm 0.7 \pm 0.4 MeV/c2$. The source of X-mesons in the large CDF sample is resolved by studying their vertex displacement. We find $16.1 \pm 4.9 \pm 2.0$% of our X-sample comes from decays of b-hadrons, and the remainder from prompt sources: either direct production or by decay of (unknown) short-lived particles. The mix of production sources is similar to that observed for the psi(2S) charmonium state.
We study the reactions \gamma d --> \pi\gamma NN with the aim to search for six-quark states, the decay of which into two nucleons is forbidden by the Pauli exclusion principle. Such states predicted by a variety of QCD inspired models and recent evidence from Proton Linear Accelerator of INR (Moscow) strongly suggests the existence of such states with masses 1904, 1926, and 1942 MeV. We propose an experiment at MAMI-B which will provide a unique opportunity to observe such dibaryon states in mass region up to 2000 MeV and determine their masses and quantum numbers.
We report recent results on the properties of the X(3872) produced via the B+ --> K+ X(3872) decay process. We observe X -->pi+pi-pi0 J/psi decays where the 3pi invariant masses cluster near the upper kinematic boundary suggesting that they originate from sub-threshold decays to virtual omega(782) mesons. This is consistent with expectations for a DD*-bar bound state interpretation for the X(3872). In addition, we constrain the possible charmonium-state assignments for this particles. Results are obtained from a 253/fb data sample that contains 274 million BB-barr pairs that was collected near the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric energy e+e- collider.
We have searched for Theta^+(1540) and Xi^{--}(1862) pentaquark candidates in proton-induced reactions on C, Ti and W targets at mid-rapidity and \sqrt{s} = 41.6 GeV. In 2x10^8 inelastic events we find no evidence for narrow (sigma~5 MeV) signals in the Theta^+ -> pK_s and Xi^{--} -> Xi^-pi^- channels; our 95% CL upper limits (UL) for the inclusive production cross section times branching fraction Bx(dsigma/dy)|_{y~0} are 3.7 and 2.5 microb/N. The UL of the yield ratio of [Theta^+ / Lambda(1520)] < 2.7% is significantly lower than model predictions. Our UL of [BxXi^{--} / Xi(1530)^0] < 4% is at variance with the results that have provided first evidence for the Xi^{--} signal.
We have performed a search for the $D^*_{sJ}(2632)^+$ state recently reported by the SELEX Collaboration at FNAL. This preliminary analysis makes use of an integrated luminosity of 125 ${\rm fb}^{-1}$ collected by the BaBar detector at the PEP-II asymmetric-energy $e^+e^-$ collider. The resulting $D_s \eta$ and $D^0 K^+$ mass spectra show no evidence for the $D^*_{sJ}(2632)^+$ state. In addition, no signal is observed in the $D^{*+} K_S$ mass spectrum.
We present an inclusive search for strange pentaquark production in e+e- interactions at a center-of-mass energy of 10.58 GeV using 123 fb-1 of data collected with the BaBar detector. We look for the states that have been reported previously: the Theta+(1540), interpreted as a ududsbar state; and the Xi--(1860) and Xi0(1860), candidate dsdsubar and uss(uubar+ddbar) states, respectively. In addition we search for other members of the antidecuplet and corresponding octet to which these states are thought to belong. We find no evidence for the production of such states and set preliminary limits on their production cross sections as functions of c.m. momentum. The corresponding limits on the Theta+(1540) and Xi--(1860) rates per e+e- to qqbar event are well below the rates measured for ordinary baryons of similar mass.
We report the results of a search for the Theta*++ pentaquark in the decay B+ -> Theta*++ pbar where Theta*++ -> pK+ using 81fb^-1 of data collected on the Upsilon(4S) resonance with the BaBar detector at PEP-II. We find an upper limit on the branching fraction of B+ -> Theta*++ pbar where Theta*++ -> pK+ to be 1.5x10^-7 for 1.43
Experimental results of a search for the Xi_(3/2)(1860) cascade pentaquark state in data collected with the CDF 2 Detector in Run II at the Tevatron are presented. No evidence for these states in the neutral Xi-pi+ and doubly charged Xi-pi- modes has been found. Preliminary upper limits on yields at 1862 MeV/c**2 relative to the well established resonance Xi*0(1530) are presented.
We report recent results on the properties of the X(3872) produced via the B-->KX(3872) decay process in the Belle detector. We compare these properties with expectations for possible charmonium-state assignments.
A search for narrow Theta(1540)^+, a candidate for pentaquark baryon with positive strangeness, has been performed in an exclusive proton-induced reaction p+C(N) \to Theta^+ \bar{K}^0 + C(N) on carbon nuclei or quasifree nucleons at E_{beam}=70 GeV (sqrt{s} = 11.5 GeV) studying nK^+, pK_S and pK_L decay channels of Theta(1540)^+ in four different final states of the Theta^+ \bar{K}^0 system. In order to assess the quality of the identification of the final states with neutron or K_L we reconstructed Lambda(1520)\to nK_S and phi\to K_LK_S decays in the calibration reactions p+C(N)\to Lambda(1520)K^+ + C(N) and p+C(N)\to p\phi + C(N). We found no evidence for narrow pentaquark peak in any of the studied final states and decay channels. Assuming that the production characteristics of the Theta^+ \bar{K^0} system are not drastically different from those of the Lambda(1520)K^+ and p\phi systems, we established upper limits on the cross section ratios sigma(Theta^+\bar{K}^0)/sigma(Lambda(1520)K^+) < 0.02 and sigma(Theta^+\bar{K}^0)/sigma(p\phi) < 0.15 at 90% CL and a preliminary upper limit for the forward hemisphere cross section sigma(Theta^+\bar{K}^0) < 30 nb/nucleon.
We present preliminary results of the STAR experiment at RHIC on exotic particle searches in minimum bias Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We observe a narrow peak at 1734 $\pm$ 0.5 $\pm$ 5 MeV in the $\Lambda K^0_s$ invariant mass with width consistent with the experimental resolution of about 6 MeV within the errors. The statistical significance can be quantified between 3 and 6 $\sigma$ depending on cuts and methods. If this peak corresponds to a real particle state it would be a candidate for the $N^0$ or the $\Xi^0$ I=1/2 pentaquark states.
We study the decay B- --> J/psi K- pi+ pi- using 117 million BBbar events collected at the Y(4S) resonance with the BaBar detector at the PEP-II e+ e- asymmetric-energy storage ring. We measure the branching fractions Br(B- --> J/psi K- pi+ pi-) = (116 +- 7 (stat.) +- 9(syst.))* 10-5 and Br(B- --> X(3872)K-)* Br (X(3872) --> J/psi pi+ pi-) =(1.28+- 0.41)* 10-5 and find the mass of the X(3872) to be 3873.4 +- 1.4 MeV/c2. We search for the h_c narrow state in the decay B- --> h_c K-, h_c --> J/psi pi+ pi- and for the decay B- --> J/psi D0 pi-, with D0 --> K- pi+. We set the 90% C.L. limits Br(B- --> h_c K-)* Br (h_c --> J/psi pi+ pi-) <3.4 * 10-6 and Br(B- --> J/psi D0 pi-) <5.2 * 10-5.
We report on a high statistics search for the $\Xi^{--}(1860)$ resonance in $\Sigma^-$-nucleus collisions at 340GeV/c. No evidence for this resonance is found in our data sample which contains 676000 $\Xi^-$ candidates above background. For the decay channel $\Xi^{--}(1860) \to \Xi^-\pi^-$ and the kinematic range 0.15$
Searches for narrow baryonic states in the K0s-p, K+p, Xi-pi and bar{X}+pi decay channels are reported. The data were collected with the ZEUS detector at HERA using an integrated luminosity of 121 pb-1. The searches were performed in the central rapidity region of inclusive deep inelastic scattering at an $ep$ centre-of-mass energy of 300--318 GeV for exchanged photon virtuality, Q2, above 1 GeV2. The results support the existence of a narrow baryonic state with strangeness in K0s-p and K0s-\bar{p} decay channels, consistent with the pentaquark prediction. No pentaquark signals were found in the K+p, Xi-pi and bar{X}+pi channels.
An analysis of $K^+$-d total cross section data is undertaken to explore possible effects of the recently observed resonance in the S=+1 hadronic system with mass around 1.55 GeV. It is found that a structure corresponding to the resonance is visible in the data. The width consistent with the observed deviation from background is found to be $0.9\pm 0.2$ MeV and the mass is $1.559\pm 0.003$ GeV for spin-parity $\h^+$ and $1.547\pm0.002$ GeV for $\h^-$.
Motivated by the recent experimental evidence of the exotic B = S = +1 baryonic state Theta(1540), we examine the older existing data on K+ N elastic scattering through the time delay method. We find positive peaks in time delay around 1.545 and 1.6 GeV in the D03 and P01 partial waves of K+ N scattering respectively, in agreement with experiments. We also find an indication of the J=3/2 Theta* spin-orbit partner to the Theta, in the P03 partial wave at 1.6 GeV. We discuss the pros and contras of these findings in support of the interpretation of these peaks as possible exotics.
We report the observation of the X(3872) in the J/psi pi+pi- channel, with J/psi decaying to mu+mu- in p-p(bar) collisions at sqrt(s) = 1.96 TeV. Using approximately 230 pb^-1 of data collected with the Run II D0 detector, we observe 522 +/- 100 X(3872) candidates. The mass difference between the X(3872) state and the J/psi is measured to be 774.9 +/- 3.1 (stat.) +/- 3.0 (syst.) MeV/c^2. We have investigated the production and decay characteristics of the X(3872), and find them to be similar to those of the psi(2S) state.
We report a search for low-lying exotic baryons via double radiative capture on pionic hydrogen. The data were collected at the TRIUMF cyclotron using the RMC spectrometer by detecting gamma-ray pairs from pion stops in liquid hydrogen. No evidence was found to support an earlier claim for exotic baryons of masses 1004 and 1044 MeV/$c^2$. We obtain upper limits on the branching ratios for double radiative capture via these exotic states of $< 3 \times 10^{-6}$ and $< 4 \times 10^{-6}$ respectively.
A narrow resonance in D*- p and D*+ pbar invariant mass combinations is observed in inelastic electron-proton collisions at centre-of-mass energies of 300 GeV and 320 GeV at HERA. The resonance has a mass of 3099 \pm 3 (stat.) \pm 5 (syst.) MeV and a measured Gaussian width of 12 \pm 3 (stat.) MeV, compatible with the experimental resolution. The resonance is interpreted as an anti-charmed baryon with a minimal constituent quark composition of uuddcbar, together with the charge conjugate.
The hadronic reaction pp --> Sigma+ K0 p was measured exclusively at a beam momentum of 2.95 GeV/c using the TOF detector at the COSY storage ring. A narrow peak was observed in the invariant mass spectrum of the K0 p subsystem at 1530 +- 5 MeV/c2 with a significance of 4 - 6 standard deviations, depending on background assumptions. The upper limit of 18 +- 4 MeV/c2 (FWHM) for its width is given by the experimental resolution. The corresponding total cross section is estimated to be about 0.4 +- 0.1(stat) +- 0.1(syst) microbarn. Since a resonance in this subsystem must have strangeness S = + 1 we claim it to be the Theta+ state for which very recently evidence was found in various experiments.
The gamma + D -> Lambda + Theta+ and gamma + D -> Sigma + Theta+ reactions can be used to determine the width of Theta+ almost model-independently. We calculate the differential cross sections of the gamma +D -> Lambda + Theta+, gamma + D -> Sigma + Theta+ and relevant background reactions in the photon energy range 1.2 < E_{\gamma} < 2.6 GeV. We determine the most favorable kinematic conditions and observables for the experimental studies of Theta+ in the considered processes. We argue that a comparison of the gamma + D -> Lambda + Theta+ and gamma + D -> Sigma + Theta+ cross sections should unambiguously determine isospin of Theta+.
Results are presented on $\psi(2S)$ and $J/\psi$ hadronic decays to $K^0_SpK^-\bar n$ and $K^0_S\bar p K^+n$ final states from data samples of 14 million $\psi(2S)$ and 58 million $J/\psi$ events accumulated at the BES II detector. No $\Theta(1540)$ signal, the pentaquark candidate, is observed, and upper limits for ${\cal B}(\psi(2S)\to\Theta\bar\Theta\to K^0_S p K^-\bar n + K^0_S \bar p K^+ n) < 0.84\times 10^{-5}$ and ${\cal B}(J/\psi\to\Theta\bar\Theta\to K^0_S p K^-\bar n + K^0_S \bar p K^+ n) < 1.1\times 10^{-5}$ at the 90% confidence level are set. For single $\Theta(1540)$ production, the upper limits determined by our analysis are also on the order of $10^{-5}$ in both $\psi(2S)$ and $J/\psi$ decays.
In this note I recall some features of two-body decay kinematics which can be effectively applied, in particular, in experimental searches for pentaquark states.
SVD-2 experiment data have been analyzed to search for an exotic baryon state, the $\Theta^+$-baryon, in a $pK^0_s$ decay mode at $70 GeV/c$ on IHEP accelerator. The reaction $pA \to pK^0_s+X$ with a limited multiplicity was used in the analysis. The $pK^0_s$ invariant mass spectrum shows a resonant structure with $M=1526\pm3(stat.)\pm 3(syst.) MeV/c^2$ and $\Gamma < 24 MeV/c^2$. The statistical significance of this peak was estimated to be of $5.6 \sigma$. The mass and width of the resonance is compatible with the recently reported $\Theta^+$- baryon with positive strangeness which was predicted as an exotic pentaquark ($uudd\bar{s}$) baryon state. The total cross section for $\Theta^+$ production in pN-interactions for $X_F\ge 0$ was estimated to be $(30\div120) \mu b$ and no essential deviation from A-dependence for inelastic events $(\sim A^{0.7})$ was found.
Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1 (syst) MeV. Depending on the background model,the naive statistical significance of the peak is 4--6 standard deviations and its width may be somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s)) pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed in the pK^+ invariant mass distribution. The absence of such a signal indicates that an isotensor Theta is excluded and an isovector Theta is unlikely.
The reaction $\gamma p \to \pi^+K^-K^+n$ was studied at Jefferson Lab using a tagged photon beam with an energy range of 3-5.47 GeV. A narrow baryon state with strangeness S=+1 and mass $M=1555\pm 10$ MeV/c$^2$ was observed in the $nK^+$ invariant mass spectrum. The peak's width is consistent with the CLAS resolution (FWHM=26 MeV/c$^2$), and its statistical significance is 7.8 $\pm$ 1.0 ~$\sigma$. A baryon with positive strangeness has exotic structure and cannot be described in the framework of the naive constituent quark model. The state is consistent with the mass predicted by a chiral soliton model \cite{Diakonov} for 5-quark baryon states. In addition, the $pK^+$ invariant mass distribution was analyzed in the reaction $\gamma p\to K^-K^+p$ with high statistics in search of doubly-charged exotic baryon states. No resonance structures were found in this spectrum.
Several recent experiments have reported evidence for a narrow baryon resonance with positive strangeness ($\Theta^+$) at a mass of 1.54 GeV/$c^2$. Baryons with $S=+1$ cannot be conventional $qqq$ states and the reports have thus generated much theoretical speculation about the nature of possible $S=+1$ baryons, including a 5-quark, or pentaquark, interpretation. We show that narrow enhancements in the $K^+n$ effective mass spectrum can be generated as kinematic reflections resulting from the decay of mesons, such as the $f_2(1275)$, the $a_2(1320)$ and the $\rho_3(1690)$.
We made an analysis of the pentaquark $\Theta^+$ photoproduction with polarized photon beams. Kinematical and dynamical aspects were examined for the purpose of determining $\Theta^+$'s spin and parity. It shows that the polarized photon beam asymmetry in association with certain dynamical properties of the production mechanism would provide further information on its quantum numbers. Facilities at SPring-8, JLab and ELSA will have access to them.
Given the growing evidence for an exotic S = +1 resonance, seen in kaon, photon and neutrino induced reactions, we reexamine the existing K+p and K+d database in order to understand how such a state could have been missed in previous studies. The lack of structure in this database implies a width of an MeV or less, assuming a state exists near 1540 MeV.
Results of resonance searches in the Xi- pi-, Xi- pi+, antiXi+ pi- and antiXi+ pi+ invariant mass spectra in proton-proton collisions at sqrt{s}=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi- pi- baryon resonance with mass of 1.862+/-0.002 GeV/c^2 and width below the detector resolution of about 0.018 GeV/c^2. The significance is estimated to be 4.0 sigma. This state is a candidate for the hypothetical exotic Xi_(3/2)^-- baryon with S = -2, I = 3/2 and a quark content of (d s d s ubar). At the same mass a peak is observed in the Xi- pi+ spectrum which is a candidate for the Xi_(3/2)^0 member of this isospin quartet with a quark content of (d s u s dbar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.
The K+ nucleon elastic scattering process has been reexamined in light of recent measurements which have found a narrow exotic S=+1 resonance in their KN invariant mass distributions. We have analyzed the existing database in order to consider the effect of a narrow state on fits to K+ nucleon observables.
CLAS photoproduction data on deuterium and hydrogen targets have been analyzed in a search for an exotic baryon state with strangeness S=+1, the \Theta^+ (originally named the Z^+). This resonance was predicted recently in theoretical work based on the chiral soliton model as a lowest mass member of an anti-decuplet of 5-quark states. The reaction gamma d --> pK^-K^+n, which requires a final state interaction inside the deuteron, was used in the analysis of deuteron data. In the analysis of proton data, the reaction gamma p --> pi^+K^-K^+n was studied. Evidence for the \Theta^+ state is found in both analyses in the invariant mass distribution of the nK^+. Our results are consistent with previously reported results by LEPS/Spring-8 collaboration (Japan), and by the ITEP (Moscow) group.
The pentaquark state of $\Theta^{+}(uudd\bar{s})$ has been observed to decay with two decay modes: $\Theta^+\to n K^+$ and $\Theta^+ \to p K^0$. The decay probability ratio of the two decay modes is studied with general symmetry consideration of isospin, spin, and parity. We arrive at a result of the ratio $\frac{\Gamma(\Theta^+\to nK^+)}{\Gamma(\Theta^+\to pK^0)} =\frac{(\alpha-\beta)^2}{(\alpha+\beta)^2}(\frac{k_1}{k_2})^{2L+1}$, which is valid for the $\Theta^+$ state to be a pure isoscalar or isovector state, or an isotensor state with mixture of isoscalar and isovector components with coefficients $\alpha$ and $\beta$. The dependence on spin and parity of the pentaquark $\Theta^+$ state is found to be small due to small difference between the center of mass decay momenta $k_1$ and $k_2$ of the two decay modes. Future experimental results about the decay probability ratio may provide information about the isospin configuration of the pentaquark $\Theta^+$ state.
An exclusive measurement of the reaction $\gamma d \to K^+ K^- p n$ is reported from the CLAS collaboration at Jefferson Lab. A narrow peak, which can be attributed to an exotic baryon with strangeness $S=+1$, is seen in the $K^+n$ invariant mass at $1542\pm 5$ MeV/c$^2$ with a measured width of 21 MeV FWHM that is consistent with the instrumental resolution of the CLAS detector. The statistical significance of the peak is $5.3 \pm 0.5 \sigma$, for a Gaussian peak shape on top of a smooth background. This result is consistent with inclusive measurements of a narrow $S=+1$ baryon reported by other experimental groups.
In the music of the subatomic sphere, elementary particles bond together like barbershop quartets in precisely determined patterns - and the "pentaquark" simply hasn't been one of them.
The gamma n -> K+ K- n reaction on 12C has been studied by measuring both K+ and K- at forward angles. A sharp baryon resonance peak was observed at 1.54 +- 0.01 GeV with a width smaller than 25 MeV and a Gaussian significance of 4.6 sigma. The strangeness quantum number (S) of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon resonance or alternatively as an exotic 5-quark state (uudd{s_bar}) that decays into a K+ and a neutron. The resonance is consistent with the lowest member of an anti-decuplet of baryons predicted by the chiral soliton model.
We give reasons for the existence of the NN decoupled dibaryon resonance $d^*_1$(1956). Strong evidence for its presence has first been found in the energy spectrum of coincident photons emitted at $\pm 90^0$ from the $pp \to \gamma\gamma X$ process at 216 MeV measured by the DIB2$\gamma$ collaboration at JINR. As further experimental indications of the $d^*_1$(1956) existence we present those found in the available photon energy spectra of $np\gamma$, $pd\gamma$, and $pA\gamma$ reactions below the pion threshold. It is noted that serious discrepancies between the $np\gamma$ and $pd\gamma$ experimental cross sections and theoretical calculations can reasonably be explained by the fact that latter did not take into account the $d^*_1$ effect.
A searche for supernarrow dibaryons (SND) and exotic baryons with small masses is reviewed. As a result of the study of the reaction pd->p+pX_1, three narrow peaks at M_{pX_1}=1904, 1926, and 1942 MeV have been observed. The analysis of the angular distributions of the protons from the decay of the pX_1 states showed that the peaks found can be explained as a manifestation of the isovector SNDs, the decay of which into two nucleons is forbidden by the Pauli exclusion principle. The observation of the peaks in the missing mass M_{X_1} spectra at 966, 986, and 1003 MeV is an additional indication that the dibaryons found are the SNDs. The possible interpretation of these peaks in the M_{X_1} mass spectra and peaks observed in the reaction pp->pi^+pX at M_X=1004, 1044, and 1094 MeV as new exotic baryon states with small masses is discussed. The mass equation has been constructed which was used to calculate the masses and determine parities of the exotic baryons. The obtained values of the masses are in a good agreement with the experimental data. Two new exotic baryon states bellow the pi production threshold have been predicted.
The yields of dibaryon (Omega-Omega) in relativistic heavy ion collisions, especially at RHIC energies, are estimated by statistical model. The yields of hyperon Omega- and the ratio of dibaryon to Omega are also given.
Based on the measured transverse mass spectra of $\pi^-$, $K^-$ and $\bar p$ at the RHIC energy $\sqrt {s_{NN}}$ =130GeV, di-omega productions from baryon-baryon reactions in hadronic matter are studied. Results about the $(\Omega \Omega)_{0^+}$ number show that the deeply bound state $(\Omega \Omega)_{0^+}$ can be observed at RHIC energies.
The energy spectrum for high energy $\gamma$-rays ($E_\gamma \geq 10$ MeV) from the process $pp \to \gamma \gamma X$ emitted at $90^0$ in the laboratory frame has been measured at an energy below the pion production threshold, namely, at 216 MeV. The resulting photon energy spectrum extracted from $\gamma-\gamma$ coincidence events consists of a narrow peak at a photon energy of about 24 MeV and a relatively broad peak in the energy range of (50 - 70) MeV. The statistical significances for the narrow and broad peaks are 5.3$\sigma$ and 3.5$\sigma$, respectively. This behavior of the photon energy spectrum is interpreted as a signature of the exotic dibaryon resonance $d^\star_1$ with a mass of about 1956 MeV which is assumed to be formed in the radiative process $pp \to \gamma d^\star_1$ followed by its electromagnetic decay via the $d^\star_1 \to pp \gamma$ mode. The experimental spectrum is compared with those obtained by means of Monte Carlo simulations.
An analysis of experimental data, obtained at the Proton Linear Accelerator of INR, is carried out with the aim of searching for supernarrow dibaryons in the pd->p+pX_1 and pd->p+dX_2 reactions. Dibaryons with masses 1904\pm 2, 1926\pm 2, and 1942\pm 2 MeV have been observed in invariant mass M_{pX_1} spectra. In missing mass M_{X_1} spectra, the peaks at M_{X_1}=966\pm 2, 986\pm 2, and 1003\pm 2 MeV have been found. The analysis of the data obtained leads to the conclusion that the observed dibaryons are supernarrow dibaryons, the decay of which into two nucleons is forbidden by the Pauli exclusion principle. A possible interpretation of exited nucleon states with small masses is suggested.
An analysis of new experimental data, obtained at Linear Accelerator of INR, is carried out with the aim of searching for supernarrow dibaryons in the reactions pd->p+X and pd->p+pX_1. Dibaryons with masses 1904\pm 2, 1926\pm 2, and 1942\pm 2 MeV have been observed in missing mass M_X spectra. In missing mass M_{X_1} spectra, the resonancelike states X_1=\gamma+n at M_{X_1}=966\pm 2, 986\pm 2, and 1003\pm 2 MeV have been found. The analysis of the data obtained leads to the conclusion that the observed dibaryons are supernarrow dibaryons, the decay of which into two nucleons is forbidden by the Pauli exclusion principle. A possible interpretation of exited nucleon states with small masses is suggested.
Talk presented at the Conference of RAS Nuclear Physics Department, Moscow, ITEP, November 16-20, 1998. Minor corrections Journal-ref: Phys.Atom.Nucl. 62 (1999) 2021-2023; Yad.Fiz. 62 (1999) 2191-2023
The reaction pd-->pX at 305 MeV is studied with aim to search for supernarrow dibaryons a decay of which into two nucleons suppressed by the Pauli principle. The experiment was carried out at Moscow Meson Factory using the spectrometer TAMS which detected the proton spectator in coincidence with another charged particle (either p or d) from a decay of X. A detection of the second charged particle at angles closed to an emission angle of the particle X with the mass M allows to suppress essentially a contribution of background processes and to increase a relative contribution from a possible production of the dibaryon under consideration. Narrow structures in missing mass spectra at 1905 and 1924 MeV with the statistical significance of ~4.5 S.D. have been observed. Widths of both peaks are smaller than the experimental resolution (3 MeV). A comparison of the obtained data with the theoretical predictions leads to a conclusion that most likely the found structure at 1905 MeV corresponds to the supernarrow dibaryon with the isotopic spin equal to 1. The possible origin of the structure at 1924 MeV is also discussed.
The reactions p+d -->p+pX and p+d -->p+dX at 305 MeV are studied with aim to search for supernarrow dibaryons, decay of which into to nucleons is suppressed by the Pauli principle. The experiment was carried out at Moscow Meson Factory using a spectrometer TAMS which detected the proton spectator in coincidence with the second charged particle (the proton or deuteron) at various angles. Narrow structures in missing mass spectra at 1905 and 1924 MeV with the widths smaller than the experimental resolution (3 MeV) have been observed. An analysis of the angular dependence of the experimental data shows that the resonance at M=1905 MeV corresponds, with the most probability, to the production of the isovector supernarrow dibaryon.
The reactions p+d -->p+pX and p+d -->p+dX at 305 MeV are studied with aim to search for supernarrow dibaryons, decay of which into to nucleons is suppressed by the Pauli principle. The experiment was carried out at Moscow Meson Factory using a spectrometer TAMS which detected the proton spectator in coincidence with the second charged particle (the proton or deuteron) at various angles. Narrow structures in missing mass spectra at 1905 and 1924 MeV with the widths smaller than the experimental resolution (3 MeV) have been observed. An analysis of the angular dependence of the experimental data shows that the resonance at M=1905 MeV corresponds, with the most probability, to the production of the isovector supernarrow dibaryon.
The reaction $pp \to pp2\gamma$, proposed earlier to probe for the NN--decoupled dibaryon resonances, has been studied by the DIB$2\gamma$ Collaboration (JINR) and their preliminary data seem to give evidence for the resonance effect at about 1920 MeV. We discuss some other two-photon processes: the double radiative capture reaction in pionic deuterium, photoabsorption sum rules and the nuclear Compton scattering, which we consider to be especially feasible for further test and invesigation of possible low-lying, exotic six-quark states.
The possibility of observation of supernarrow dibaryons, decay of which into two nucleons forbidden by the Pauli principle, produced by photons in processes of charged pion photoproduction by polarized photons on the deuteron is analyzed. It is shown that the expectable dibaryon yield may exceed the background by a factor of 10--100.