Charmed baryon production at the CERN intersecting storage rings

The production of a Λ⁺_c charmed baryon has been observed in pp collisions at the CERN ISR. A sharp peak at 2.26 GeV/c² was found in the decay mode Λ⁺_c → $\text{K}{}^{10}$p in events triggered with a forward K^? meson. Also the decay mode Λ⁺_c → K^? Δ⁺⁺ seems to be present. The estimation of the total cross section for Λ⁺_c production is given. However, the resulting value depends on the assumed form of the differential cross section. In addition, Λ⁺_c production in events triggered by the presence of a direct electron is discussed.     

An analysis of the Λ (1520) observed in a K− neutron production experiment

The mass, width and branching fractions of the Λ(1520) into $\text{K}\text{N, σπ, Λππ}\text{and}\text{σππ}$ have been determined using a Λ(1520) sample produced in the reaction K^?d→Λ(1520)π^?p_s at K^? -neutron centre-of-mass energies between 1.9 and 2.2 GeV. A Dalitz plot analysis of the Λ⁰π⁺π^? decay mode shows that there is a substantial Σ(1385)π component. The SU(3) octet-singlet mixing angle for the $\text{3}\text{2}{}^{\mathrm{?}}$ baryons calculated from the partial widths for the s-wave σ(1385)π decay of the Λ(1520) and Λ(1690) is found to be consistent with that obtained from the d-wave decays of the singlet and octet members, within experimental limits.     

Study of proton dissociation into ΛK+ in 3-body final states at 10 and 16 GeV/c

Results are presented for p → (ΛK⁺) dissociation in the reactions K^?p → ΛK⁺K^? and and π^±p → ΛK⁺π^±at 10 and 16 GeV/c. The cross sections for the low-mass ΛK⁺ enhancement are compatible with the energy dependence σ ∝ p_lab^?0.3. The t′ spectra or the (ΛK⁺) threshold enhancement are exponential in shape. Its decay angular distribution reveals neither s-channel nor t-channel helicity conservation. The relative probabilities of the processes $\text{p}\text{→}\text{p, p}\text{→ (}\text{N}\text{π)}{}_{\mathrm{<mtext>I=</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{and p}\text{→ (Λ}\text{K}{}^{\mathrm{+}}\text{)}$ dissociation are in the ratios 100 : 10 : 0.2, independent of the nature of the incident particle.     

Diffraction dissociation of the proton into ΛK+ and (ΣK)+in π+ p interactions at 10.3 GeV/c

We observe a threshold ΛK⁺ enhancement in the reaction π⁺p → ΛK⁺π⁺ at 10.3 GeV/c. The production characteristics are very similar to those of the diffractive ΛK⁺ enhancement observed at the CERN ISR. The ΣKπ channels show no similar (ΣK)⁺ enhancement, as predicted by SU(6).     

Study of the reactions K−p → K+K−Λ and K−p → ppΛ, at 10 and 16 GeV/c

The reactions (1) K^?p → K⁺K^?Λ and (2) $\text{K}{}^{\mathrm{?}}\text{p}\text{→}\text{p}\text{p}\text{Λ}$ have been studied on samples of 109 and 64 events, respectively, at 10 GeV/c and 125 and 69 events at 16 GeV/c, reasonably free from contaminations. The investigation of the first reaction uses also 84 events of the $\text{K}{}^0\text{K}{}^0\text{Λ}$ final state at 10 GeV/c. Analysis of the Van Hove plots indicates that the K⁺K^?Λ and $\text{p}\text{p}\text{Λ}$ final states are produced by two main mechanisms: (i) a ΔQ = 0 process, with a strong diffractive component near threshold, involving the dissociations p → K⁺Λ in reaction (1) and $\text{K}{}^{\mathrm{?}}\text{→}\text{p}\text{Λ}$ in reaction (2) and (ii) a ΔQ = 1 process involving hypercharge exchange, and producing K⁺K^? and $\text{p}\text{p}$ systems in reactions (1) and (2), respectively, recoiling off the Λ. With increasing energy, this hypercharge exchange process decreases slowly when K⁺K^? is produced, but fast where the production of $\text{p}\text{p}$, violating the Zweig rule, occurs.     

Λ production in K−p interactions at 32 GeV/c

Lambda production is studied in K^?p interactions at 32 GeV/c. The total Λ cross section is 2.31±0.03 mb. Using the measured Λγ combinations we find that (31±4)% of all Λ's are produced via the Σ⁰→Λγ decay. About 60% of the Λ's are associated with either a $\text{N}\text{N}\text{or K}\text{K}$ pair; about 40% of the Λ's are produced through the hypercharge annihiltion reaction K^?p→Λ+π'a. The two-peak structure of the invariant x distribution can be related to fragmentation processes. The Λ is found to be unpolarized in the target fragmentation region, whereas a transverse polarization is observed for forward produced Λ's. As a function of p⊥, a polarization effect is measured at medium p⊥.     

Study of quasi two-body final states in K−n reactions at 8.25 GeV/c

Cross sections for various channels in 3 prong + V₀ final states of K^?n interactions are obtained at 8.25 GeV/c. An energy dependence study of the quasi two-body reactions Σ^?(1385) + vector meson and ΛB^? seem to imply the presence of cuts in the Regge exchange formalism, whereas the reaction Δ(1236) $\text{K}$¹ (890) does not require such cuts. Upper limits of 1 and 2 μb are found for the reactions K^?n → ΛA₁ and K^?n → ΛA₂.     

Λp interactions below 24 GeV/c

Λp interactions have been studied at momenta up to 24 GeV/c using a new bubble chamber technique. We find that the Λp elastic cross section has fallen to approximately 6 mb at the highest momentum. Results are presented on Λp→Λpπ⁺π^? for the first time above 5 $\text{GeV}\text{c}$.     

Phase-shift analysis of π N → K Σ

We have performed an energy-independent phase-shift analysis of πN → KΣ by analyzing differential cross-section and polarization data for the reactions π⁺p → K⁺∑⁺ and

at fifteen energies from threshold to 2137 MeV invariant mass. The single-energy solutions were linked by shortest-path methods with and without constraints implied by the Odorico zeros for the reaction π^?p → K⁺Σ^?. Similar to π^?p → KΛ it has been found that both isostates are dominated below 1900 MeV by resonances in the spin $\text{1}\text{2}$ waves. The F₃₇ resonance couples to KΣ with √x_πNx_KΣ= 0.04.     

Strange particle production in three- and four-body final states of 16 GeV/cπ±p reactions

Three- and four-body final states with strange particles are studied in π⁺p and π^?p interactions at 16 GeV/c. We present cross sections and investigate their energy dependence. Production mechanism, resonance production and quantum number transfer are discussed. Strong Y¹(1385) production is found in the reaction π⁺p → ΛK⁺π⁺, while the corresponding π^?p reaction is dominated by production of K¹(890). In the $\text{NK}\text{K}\text{π}$ channels, the K and $\text{K}$ are produced mainly at the same vertex, i.e. non-strangeness exchange ΔS = 0 is dominant (about 75% of the cases), whereas in the ΛKππ channels, the Λ and K are more frequently produced at opposite vertices, i.e. |ΔS| = 1 exchange is important (about 60% of the cases). Results on the polarization of the lambdas produced in the π⁺p reactions are given.     

Inclusive strange particle production in π+p interactions at 16 GeV/c

The production of $\text{K}{}_{\mathrm{S}}{}^0\text{, Λ}\text{and}\text{Λ}$ is measured in π⁺p reactions at 16 GeV/c. The total strange particle cross section is found to be 4.0 ± 0.3 mb, about 20% of the inelastic cross section. Cross sections for single strange particles and for strange particle pairs are determined, both inclusively and as functions of the charged multiplicity. Relative production rates for different strange particle combinations are compared with the prediction of the isospin statistical model. Inclusive spectra for single particles are studied and it is found the K_S⁰ are produced mostly in the forward hemisphere, most probably by fragmentation of the incident pion into $\text{K}\text{K}\text{π}$. The Λ are mostly backwards, probably deriving from fragmentation of the proton into ΛK pairs. The $\text{Λ}$ tend to be produced forwards, but evidence is found for central $\text{Λ}\text{Λ}$ production. Distributions in rapidity of the Λ particles from π⁺p interactions are compared in terms of the factorisation hypothesis. Results are given on the Λ transversal polarisation.     

Charmed meson decays and right-handed currents

In the presence of right-handed currents including $\text{(}\text{c}\text{s}\text{)}{}_{\mathrm{<mtext>R</mtext>}}$, it is emphasized that the dominant parity-violating (-conserving) Hamiltonian for nonleptonic decays of charmed hadrons transforms like $\text{45}\text{+}\text{45}{}^1\text{(}\text{20″ +}\text{84}\text{)}$ of SU(4), and leads to distinctive results especially in D⁺ → K_Sπ⁺, F⁺ → μ⁺ν_μ and D⁰ → K^?e⁺ν_e, etc.     

Backward K10 production in the reaction π−p→Λ0K+π− at 9 and 12 GeV/c

We have analysed about 85 000 fast Λ⁰ events, obtained in a fast proton triggered experiment performed at the CERN-Ω spectrometer at 9 and 12 GeV/c incident π^? beam. Nearly 2500 Λ⁰K⁺π^? events have been isolated. We find strong production of quasi-two-body processes $\text{Λ}{}^0\text{K}{}^{10}$ and $\text{∑}{}^{\mathrm{1?}}\text{K}{}^{\mathrm{+}}$ consistent with u-channel hyperon exchange. Results on Λ⁰ polarization, K¹⁰ decay parameters and differential cross sections are given for $\text{Λ}{}^0\text{K}{}^{10}\text{(892)}$ and $\text{Λ}{}^0\text{K}{}^{10}\text{(1430)}$ final states. A comparison is made with the associated backward $\text{Λ}{}^0\text{(1520)}\text{K}{}^{10}$ production seen in the four-prong reaction π^?p→pK^?K⁺π^? obtained in the same experiment.     

Λ and Λ production in K+p interactions at 32 GeV/c

A study of the inclusive and semi-inclusive Λ and $\text{Λ}$ production in K⁺p interactions at 32 GeV/c is presented. The inclusive cross sections for Λ and $\text{Λ}$ amount to 0.78 ± 0.05 and 0.42 ± 0.04 mb thus showing a remarkable growth between 16 and 32 GeV/c with a factor of 1.7 for Λ and 2.8 for $\text{Λ}$. Target and beam fragmentation processes are found to be dominant for Λ and $\text{Λ}$ production respectively with the following lower limits for the corresponding cross sections: $\text{σ(p}\text{→}\text{K}{}^{\mathrm{+}}\text{?) > 0.5}\text{mb}$ and $\text{σ(K}{}^{\mathrm{+}}\text{→}\text{p}\text{?}\text{) > 0.3}\text{mb}$. Although the early scaling conditions are fulfilled for the Λ production in the target fragmentation region, and $\text{Λ}$ production in the beam fragmentation region, scaling is not observed between 16 and 32 GeV/c in the x and p_T² Feynman variables. The Λ production is found to be very similar in the K⁺p inclusive reaction at 32 GeV/c and in the semi-inclusive reaction $\text{K}{}^{\mathrm{?}}\text{p}\text{→ Λ}\text{K}\text{K}\text{X}$ at the same energy. The $\text{Λ}\text{Λ}$ pair production cross section increases significantly in K⁺p interactions from 16 to 32 GeV/c where it reaches the value $\text{σ}{}_{\mathrm{<mtext>\Lambda </mtext><mtext>\Lambda </mtext>}}\text{= 47 ± 11 μ}\text{b}$. The cross sections for Λ or $\text{Λ}$ produced in association with an identified proton are also given and discussed.     

Differential cross-sections for pp → π−π+, K−K+ between 0.8 and 2.4 GeV/c

We present measurements from a spark chamber experiment of the differential cross-sections for $\text{p}$p → π^?π⁺, K^?K⁺ at 20 momenta in the range 0.8–2.4 GeV/c (c.m. energy 2.02 to 2.57 GeV). The c.m. angular range was $\text{?0.95 <}\text{cos}\text{θ}{}^1\text{< 0.95}$. There are about 2000π^?π⁺ events and about 300 K^?K⁺ events at each momentum.     

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 partial-wave analysis of the 3π system in the reaction K−p → π+π−π0Λ at 4.2 GeV/c

A partial-wave analysis of the (3π)⁰ system produced peripherally in the reaction K^?p → π⁺π^?π⁰Λ at 4.2 GeV/c is presented. The observation of the weak Λ decay allows a determination of all the transversity production amplitudes except for two phases. The production of known resonances having decay modes other than 3π is used to test the isobar model ansatz. Significant ω(783), φ(1020) and A₂(1310) production is observed. The spin parity of the $\text{ω}{}^1\text{(1675)}$ is established as 3^?. No evidence for production of other resonances, such as axial vector-mesons, is found.     

Diffraction dissociation in the reaction p+p→Λ°+K++p at the CERN ISR

The exclusive reaction pp→Λ°K⁺p has been studied at 0.1<?t_pp<0.6 GeV² and energies √s=45 GeV and 53 GeV at the CERN ISR. Diffractive excitation p→Λ°K⁺ occurs predominantly in the mass range M(Λ°K⁺)≈2.1 GeV and peaks at 1.7 GeV. The cross section for pp→Λ°K⁺p is 10±3μb for M(Λ°K⁺)<2.5 GeV, and the t_pp dependence is exp (bt) with b=7.0±0.5 GeV^?2.     

Study of the production of a strange (S = −1) dibaryon in the interactions of K− and π+ on deuterium below 1.5GeV/c

The production of a strange dibaryonic system called H⁺₁ (M = 2.13 GeV/c², S = ?1), has been studied with a missing mass spectrometer, at the CERN Proton Synchrotron, in the reaction K^?d → π^?H⁺₁ and in the line-reversed reaction π⁺d → K⁺H⁺₁ between 0.9 and 1.4 GeV/c.The reactions

$\text{K}{}^{\mathrm{?}}\text{d}\text{→ π}{}^{\mathrm{?}}\text{X}{}^{\mathrm{+}}$

,

$\text{π}{}^{\mathrm{<mtext>d</mtext>}}\text{→}\text{K}{}^{\mathrm{+}}\text{X}{}^{\mathrm{+}}$

,have been studied in a missing mass spectrometer at CERN. The experiment (PS159) is well adapted to search for a signal in the missing mass X⁺ (B = 2, S = ?1) produced in the backward c.m.s. direction, between 2.0 and 2.3 GeV/c². The two reactions have been analysed at three different beam settings: 1.4, 1.06 and 0.92 GeV/c for reaction (1) and 1.4, 1.2 and 1.06 GeV/c for reaction (2).     

Charged particle multiplicity distributions for 32 GeV/c K±p and pp interactions and for 50 GeV/c π±p interactions

Topological cross sections are presented for K⁺p, K^?p and $\text{p}\text{p}$ interations at 32 GeV/c and for π⁺p and π^?p interactions at 50 GeV/c observed in the 4.5 m Mirabelle hydrogen bubble chamber at Serpukhov. Parameters characterizing the behaviour of the multiplicity distributions, namely 〈n_c〉, D and f₂ and the normalized moments are given. The dependence of these parameters on the energy and the nature of the beam particle is discussed. The multiplicity distributions obtained cannot be described by a single function in the KNO variables.