Formation of a narrow baryon resonance with positive strangeness in K + collisions with Xe nuclei

The data on the charge-exchange reaction K ⁺Xe → K ⁰ pXe′, obtained with the bubble chamber DIANA, are reanalyzed using increased statistics and updated selections. Our previous evidence for formation of a narrow pK ⁰ resonance with mass near 1538 MeV is confirmed. The statistical significance of the signal reaches some 8 (6) standard deviations when estimated as $ {S \mathord{\left/ {\vphantom {S {\sqrt B \left( {{S \mathord{\left/ {\vphantom {S {\sqrt {B + S} }}} \right. \kern-0em} {\sqrt {B + S} }}} \right)}}} \right. \kern-0em} {\sqrt B \left( {{S \mathord{\left/ {\vphantom {S {\sqrt {B + S} }}} \right. \kern-0em} {\sqrt {B + S} }}} \right)}} $ . The mass and intrinsic width of the Θ⁺ baryon are measured as m = 1538 ± 2 MeV and Γ = 0.39 ± 0.10 MeV.     

Further evidence for formation of a narrow baryon resonance with positive strangeness in K + collisions with Xe nuclei

We have continued our investigation of the charge-exchange reaction K ⁺Xe → K ⁰ pXe’ in the bubble chamber DIANA. In agreement with our previous results based on part of the present statistics, formation of a narrow pK ⁰ resonance with mass of 1537 ± 2 MeV/c ² is observed in the elementary transition K ⁺ n → K ⁰ p on a neutron bound in the xenon nucleus. The visible width of the peak is consistent with being entirely due to instrumental resolution and allows one to place an upper limit on its intrinsic width: Γ < 9 MeV/c ². A more precise estimate of the resonance intrinsic width, Γ = 0.36 ± 0.11 MeV/c ², is obtained from the ratio between the numbers of resonant and nonresonant charge-exchange events. The signal is observed in a restricted interval of incident K ⁺ momentum that is consistent with smearing of a narrow pK ⁰ resonance by Fermi motion of the target neutron. The statistical significance of the signal is some 7.3, 5.3, and 4.3 standard deviations for the estimators and , respectively. This observation confirms and reinforces our earlier results, and offers strong evidence for formation of a pentaquark baryon with positive strangeness in the charge-exchange reaction K ⁺ n → K ⁰ p on a bound neutron. (The DIANA Collaboration) The text was submitted by the authors in English.     

An evidence for a possible stable dibaryon

A search for stableH-dibaryon ofI=0,Y=0,J ^P =0⁺ was undertaken in negative pion-, neutron-,¹²C-nucleus- and proton-propane interactions at 4.0, 7.0, 50.4 and 10 GeV/c, respectively, using 55 cm and 2 m propane bubble chambers. This technique permite one to examine the whole range of possibleH-mass value. In the range ofM _H >2M _Λ=2231.2 MeV/c² an enhancement ofM _ΛΛ=(2365.3±9.6) MeV/c², Γ _ΛΛ =(47.2±15.1) MeV/c² with production cross section σ=(24.2±7.0) μb per carbon nucleus was found. The mass rangeM _Σ?p<M _H <2M _Λ was explored via the search for a weak decay modeH→Σ^?+p. AV ⁰-particle was found pointing to a two-prong interaction and satisfying the kinematics of the above two-body decay mode with a high confidence level. The possibility of imitation of the observed event by background processes was investigated for a series of two-step strong reactions and weak decays. We succeeded to show the significance of theH-production hypothesis on a deuteron-mass nuclear fluctuon via thepd→HpK ⁺ K ⁰ reaction. The mass of the event treated as aH→Σ^?+p weak decay appeared to be (2173.94±1.32) MeV/c², its life-time being 0.668·10^?10 s. Both are in agreement with predictions of the MIT Bag Model for the so-calledH-dihyperon. The formally determined production cross section of theH in proton-propane interactions at 10 GeV/c is estimated to be 40 nb. Perhaps, this event can be considered as a first evidence for a six-quark bound state-H-dihyperon. An independent way for the search for theH-particle, based on a specific conversion processHN→YYN, peculiar to its intranuclear strong interactions was proposed and tried. For this purpose all events with two Λ-hyperons and identified protons were tested for the hypothesesH _p →ΛΛp at three fixed masses:M _H =1880.00, 2173.94 MeV/c² and the ΛΛ invariant mass for the event in question. The kinematics of this reaction fits successfully only one event and only atM _H =2173.94 MeV/c². It seems worthy of note that the final state ΛΛ invariant massM _ΛΛ=2344 MeV/c² is rather close to the observed peak at 2365 MeV/c². It is shown that the intranuclear conversion processes Ξ^? p→ΛΛ and Ξ⁰ n→ΛΛ fail to imitate the observed event. The ΛΛp invariant mass of this event equal to 3363 MeV/c² is out of a 3568 MeV/c² peak region of the ΛΛp invariant mass spectrum observed earlier. TheH conversion cross section exceeds the production one. Thus, we consider this event as providing a new evidence for the stableH-dibaryon ofM _H =(2173.94±1.32) MeV/c².     

A study of the Regge recurrence of the Δ(1236) produced in 10 GeV/c K+p interactions

Data are presented on a high mass $\text{I}\text{3}\text{2}\text{Δ}$ resonance produced in 10 GeV/c K⁺p interactions. The mass and width of the resonance, determined by a maximum likelihood fit to the pπ⁺ spectrum in the final state K⁺pπ⁺π^?, are 1895±15 MeV/c²230±50 MeV/c² respectively. The differential cross sections, dσ/dt, for the quasi two-body final states K¹(890)Δ(1236) and K¹(890)Δ(1900) are well described by the one pion exchange model. In addition the moments of the Gottfried-Jackson angular distribution of the pπ⁺ system agree with those computed from on-shell π⁺p elastic scattering data. We conclude that the two final states are produced mainly by π exchange and associate the structure observed in the moments up to NH(6,0) in the 1900 MeV region with the $\text{7}\text{2}{}^{\mathrm{+}}$ Regge recurrence of the Δ(1236). The dominant decay mode of the resonance is Δ⁺⁺(1900)→pπ⁺ but decays to pπ⁺π⁰ and nπ⁺π⁺ are also observed. Cross sections are determined for the production of the quasi two-body final states K¹(890)Δ(1236), K¹(890) Δ(1900), K¹(1420)Δ(1236) and K¹(1420)Δ(1900).     

Properties of the φ and f′ produced in the channel Kp → KpKK̄ at 10 GeV/c

Data are presented and cross sections given for ф and f′ production in the channel K⁺p → K⁺pKK? at 10 Gev/c. The resonance parameters obtained from a fit to the KK? effective mass distribution are M = (1020.4 ± 0.5) MeV/c² and Γ = (5.0 ± 1.3) MeV/c² for the ф meson and M = (1514 ± 4) MeV/c² and Γ = (28 ± 15) MeV/c² for the f′. The resonance widths are corrected for experimental resolution. The branching fraction for the rate (ф → K⁰₁K⁰₂)/(ф → K⁺K⁻) is determined to be 1.15 ± 0.15. The angular distributions for the decay of the f′ have unnormalized moments _H(2,0) and H(4,0) different from the values for the nearby background, indicating spin 2 or greater for the f′.     

Backward production of a spin parity 1+ϱπ enhancement at 1.04 GeV

A spin-parity 1⁺?π enhancement is observed for the 3π mass spectrum in the reaction K^?p → Σ^?π⁺π⁺π^? where events with a small (K^? → Σ^?) momentum transfer are selected. The mass (1040 MeV) and width (230 MeV) of this enhancement are reminiscent of the A₁ → ?π bump which has mainly been observed in the diffractive-like processes. The cross section for this enhancement at 4.15 GeV/c incident K^? momentum is (3.6 ± 0.5) μb.     

Evidence for two strangeonium resonances with JPC=1++ and 1+− in K−p interactions at 11 GeV/c

The results of a partial wave analysis of the low mass KK̄π system produced in the reactions K⁻p→K⁰_sK^±π^±Λ at 11 GeV/c using the LASS spectrometer are presented. Evidence is given for two ss̄ axial-vector meson states: one with quantum numbers J^PC = 1⁺⁺, mass ∼ 1530 MeV/c², and width ∼ 100 MeV/c², and the other with quantum numbers J^PC=1⁺⁻, mass ∼ 1380 MeV/c², and width ∼ 80 MeV/c². These states are good candidates to be the mostly strangeonium members of the ground state 1⁺⁺ and 1⁺⁻ nonets predicted by the quark model.     

Measurement of K production in the reaction Kp → Kp

The K^∗− spectrum in the reaction K⁻+p → K^∗−+p has been measured at beam momenta 10.9, 13.4 and 15.9 GeV/c using the missing mass technique. Production of the L(1770), and a Q-K^∗(1420) enhancement are observed. Differential cross sections in the range of momentum transfer 0.12 < | t_pp | < 0.40 (GeV/c)² are given. The L meson is observed with a width Γ = 100 ± 26 MeV. The mass spectrum between the L and 2.5 GeV does not show significant structure.     

Observation of a KK enhancement at 1.85 GeV in the reaction K−p → KKΛ at 8.25 GeV/c

In a high statistics $\text{(}\text{～150 eV}\text{μb}\text{)}$ bubble chamber experiment on K^?p interactions at 8.25 $\text{GeV}\text{c}$, the study of the reaction K^?p → KKΛ provides evidence for an enhancement in the KK system with a mass of (1850 ± 10) MeV and a width of (80_?30⁺⁴⁰) MeV. Its possible identification with the missing isosinglet of the 3^? nonet is discussed. A K¹$\text{K}$ decay mode is also observed.     

A new study of Kπ scattering between 0.7 and 2 GeV/c2

We present new data on the reaction K⁺d → K⁺π^?p(p), at 5.44 GeV/c. This reaction is dominated by absorbed pion exchange and we have used the data to study K⁺π^? scattering between 0.7 and 2 GeV/c². We find the $\text{I=}\text{1}\text{2}$ s-wave Kπ phase shift increases approximately linearly with mass up to 1.45 GeV/c², and have evidence for a more rapid variation of the s-wave amplitude in the region of 1.5 GeV/c². The so-called “up” branches in the region of the K¹(890) and K¹(1400) are excluded, save for the possibility of narrow s-wave resonances of width <10 MeV/c². We have explained quantitatively every aspect of the data up to 2 GeV/c² with s-, p-, d- and f-waves, finding an f-wave resonance at a mass of ≈1.76 GeV/c² and with a width ≈300 MeV/c². We have obtained new resonance parameters for the K¹(1400) which differ substantially from the currently accepted values.     

Diffractive production of the charmed baryon Λ+c at the CERN ISR

In a sample of diffractive events of high multiplicity a sharp five standard deviation signal is observed at M = 2255 MeV/c² in the K^?pπ⁺ mass distribution and, although with less statistical strength, at the same mass in the Λ⁰π⁺π⁺π^? channel. These signals are identified as being due to the decay of the charmed baryon Λ⁺_c which is produced with a cross section times branching ratio σ_cB in the range 0.7?1.8 μb for the K^?π⁺p decay and 0.3?0.7 μb for the Λ^-π⁺π⁺ π^? system.     

Investigation of the diffractive reaction p + N → [∑+ K 0] + N at the proton energy of E p =70 GeV and observation of the decay process X(2000) → ∑+ K 0

The diffractive reaction p + N → [∑⁺ K ⁰] + N induced by 70-GeV protons is investigated at the SPHINX facility. The measured mass spectrum of the ∑⁺ K ⁰ system originating from this reaction shows a resonance structure with a mass of M=1995±18 MeV and a width of Γ=90±32 MeV. The cross section for the formation of this structure is found to be σ=182±38 nb per target nucleon. These data comply well with the results that we obtained previously in studying the reaction p + N → [∑⁰ K ⁺] + N, where we observed the formation of the X(2000) → ∑⁰ K ⁺ state, a candidate for an exotic baryon with hidden strangeness. The measured ratio R of the two branching fractions, R = Br[X(2000) → ∑⁺ K ⁰]/Br[X(2000) → ∑⁰ K ⁺] = 1.91 ± 0.38, is consistent with the value of R=2 expected for an isospin-1/2 baryon.     

Observation of a narrowK s 0 K s 0 π+π− state at 1.97 GeV produced inK + p interactions at 32 GeV/c

Evidence is provided for a new resonance in theK _s ⁰ K _s ⁰ π⁺π^? system with a mass of 1.97±0.01 GeV and width 40±20 MeV. It is produced with an inclusive cross section of ～6μb in the beam fragmentation process of 4-prongK ⁺ p interactions with at least 2 associatedV ⁰'s at 32 GeV/c and decays dominantly into theK ^{* ±} (892)K _s ⁰ π^? mode.     

An amplitude analysis of the K+K− system produced in the reaction π−p→K+K−n at 10 GeV/c

We present results on an amplitude analysis of the K⁺K^? system produced in the reaction π^?p→K⁺K^?n from threshold up to 2.2 GeV. The branching ratios of f⁰ and f' to K$\text{K}$ have been determined. In the low mass region of the K$\text{K}$ system the observation of an S-wave enhancement at 1.3 GeV and the interference of the f⁰-A₂-f' mesons are studied. We observe a 3^? structure in the mass region of 1.7 GeV which is consistent with g⁰ production. With this interpretation the branching ratio $\text{g→}\text{K}\text{K}$ has been determined. Evidence for a new structure in the J^p = 2⁺ wave around 1.8 GeV with a width of ～200 MeV is presented.     

First observation of the production and decay of the Σc+

An event with the decay chain Σ_c⁺ → Λ_c⁺ + π⁰, Λ_c⁺ → K^? + p + π⁺, has been observed in an exposure of BEBC, equipped with a track sensitive target, to the wide band neutrino beam from the SPS at CERN. The event has a unique three constraint kinematic fit to the ΔS = ?ΔQ reaction v + p → μ^? + p + K^? + π⁺ + π⁺ + π⁰ with both gammas from the π⁰ decay detected. The proton and other final state particles are identified. The masses are M(Λ_c⁺) = 2290 ± 3 MeV/c², M(Σ_c⁺) = 2457 ± 4 MeV/c² and M(Σ_c⁺) ? M(Λ_c⁺) = 168 ± 3 MeV/c². Including other data one obtains M (Σ_c⁺⁺) ? M(Σ_c⁺) = 0 ± 4 MeV/c².     

Forward K1+ (892) production in K+p → π+Ks0p at 12.7 GeV/c

We have measured 618 K⁺p → π⁺K_S⁰p events at 12.7 GeV/c incident lab momentum, mass range 790 ? m_π⁺Ks⁰ ? 990 MeV and t range 0.01 ? ?t ? 0.60 (GeV/c)². The π⁺K_S⁰ mass spectrum is dominated by the K¹⁺ (892) resonance and a Breit-Wigner fit yields a mass m = 893.5 ± 1.1 MeV and a width Γ = 33.2 ± 4.1 MeV which is much narrower than measured hitherto. The t distribution of K¹⁺ (892) events shows a dip in the forward direction and an exponential fall off thereafter, consistent with dominance of helicity flip amplitudes. The spin density matrix is almost saturated by ρ₁₁ and ρ_1?1 which are very close to their maximum allowed value of $\text{1}\text{2}$ throughout the measured t range except in the very forward direction where ρ₀₀ and Re ρ₁₀ deviate from zero. Natural parity exchanges, therefore, dominate with unnatural parity exchanges being restricted to a small region in the forward direction. A Regge pole analysis of the differential cross sections of the present measurement in conjunction with previously measured total cross sections supports the f-coupled-pomeron hypothesis.     

Generalization of multiperipheral model with long range correlations; Van Der Waals approximation

Three enhancements are observed in the final state ω^°π⁺π⁺π⁺⁺π^?π^? selected from the channel $\text{p}\text{p → 3π}{}^{\mathrm{+}}\text{3π}{}^{\mathrm{?}}\text{π}{}^{\mathrm{°}}$ at 715 MeV/c: one in ω^°π^± at 1040 MeV (～ ≈ 55MeV) ω^°π⁺π^?, respectively near 1315 MeV (～ ≈ 100 MeV) and 1405 MeV (～ ≈ 40 MeV). The first two effects are strongly correlated and are interpreted in terms of a sequential decay A^°₂ through a new object, the B₁. The second (ωππ) enhancement seems to be an ?^°?^° effect below threshold and is attributed to a pionic decay of the $\text{K}\text{K}\text{)}{}_{\mathrm{I}}\text{=1}$ effect seen around the same mass in other reactions.     

Search for the P11Σ(1660) resonance in the reactions K− + p → Σ± + π∓ in the momentum range 597 to 888 MeVc

The reaction cross sections, differential cross sections and Σ ⁺ polarization for the reactions K^? + p → Σ^± + π^? at 8 momenta in the range 688 to 833 MeV/c were obtained. A partial-wave analysis was performed over the momentum range 597 to 888 MeV/c using the present data together with data in the neighboring momentum regions. A satisfactory solution was obtained with the well-established S₀₁ Λ(1670), D₀₃ Λ(1690) and D₁₃ Σ(1670) resonances. The addition of a P₁₁ resonance with a mass, width, and branching ratio of 1665 MeV/c², 82 MeV/c² and ?0.13 improved the χ² per degree of freedom by an insignificant amount, indicating that the coupling of the p¹¹Σ(1660) to the Σ^±ν^? channels is at best marginal.     

Q 1 production by hypercharge exchange in π- p interactions at 3.95 GeV/c

Q-meson production is studied in the hypercharge exchange reaction π^- p → (Kππ)Λ at 3.95 GeV/c by selecting events witht(π^- →Kππ)>1.2GeV². An enhancement with a mass of 1294±10 MeV and a width of 66±15 MeV is observed in the (Kππ) mass distribution. A spin-parity analysis of the (Kππ) decay Dalitz plot shows the enhancement to be in theJ ^P=1⁺ S(K?) wave and is therefore attributed toQ ₁-meson production. No evidence is found for the decayQ ₁→K ⁰ω but limited statistics allow only placing an upper limit of 30% for the decay ratioKω/K?⁰. TheQ ₁ production cross section fort(π^- →Kππ)>1.2GeV² is 8±1.3 μb. No evidence is found for the process π^- p→Q ₂Λ withQ ₂→K ^*π for which the partial wave analysis gives an upper cross section limit of 2.5 μb at the 95% confidence level.     

Measurement of pp annihilation cross sections into charged particles in the momentum range 374–680 MeV/c

$\text{p}\text{p}$ annihilation cross sections into 2-, 4- and 6-prong topologies, and the exclusive annihilation cross sections for the π⁺π^?, K⁺K^?, π⁺π^?π⁰, π⁺π⁺π^?π^?, π⁺π⁺π^?π^?π⁰, π⁺π⁺π⁺π^?π^?π^?, and π⁺π⁺π⁺π^?π^?π^?π⁰ channels in the momentum range 374–680 MeV/c were measured. No prominent structure was observed in the momentum dependence of any of the above cross sections, but a small enhancement in the cross section was observed at 490 MeV/c corresponding to the S meson mass in the topological, and π⁺π^?π⁰ and π⁺π⁺π^?π^?π⁰ cross sections. If the extensive of the S meson with a mass and width of 1935.5 and 2.8 MeV/c² is assumed, our measurement gives a total resonant annihilation cross section into charged particles of 10.0±3.0 mb.