Search

The ratios of the production rates K*(892)/K, phi/K, rho^0/pi, omega/pi, Delta^{++}/p, Sigma+(1385)/Lambda, Xi^-/Lambda and their x_p dependences obtained from results of the LEP and SLD experiments in Z hadronic decays are analysed. The corresponding ratios for promptly produced mesons are estimated at x_p -> 1. A comparison of the LEP results with those from the Mirabelle and BEBC K+p experiments at 32 and 70 GeV/c shows striking similarity in fragmentation of the strange valence quark of the incident K+ and strange quarks produced in Z decays. The JETSET model describes the LEP, Mirabelle and BEBC results. The model of Pei is consistent with the data for mesons, but presumably underestimates the fractions of primary octet baryons. The quark combinatorics model of Anisovich et al.is incompatible with the data.

The production rates of light-flavour hadrons at LEP 1 are set by their masses, spins, strangeness suppression, and strongly influenced by the spin-spin interactions of their quarks. Similarities in production of pseudoscalar mesons and octet baryons, vector mesons and decuplet baryons are observed and explained by the relative spin configurations of their quarks. Similarity in production of tensor mesons and Λ (1520) is also seen. The vector-to-pseudoscalar meson and decuplet-to-octet baryon suppressions are the same and explained by the hyperfine mass splitting. The strangeness suppression factor, λ =0.295±0.006, is the same for mesons and baryons. It is related to the difference in the constituent quark masses, λ= e −(m s − m )/T , where m =m u =m d , at the temperature, T =142.4±1.8 MeV/ c 2 . No evidence for additional suppression of strange baryons is observed. The LEP 1 data are compared with hadronic reactions and heavy nuclear collisions.

DELPHI results on the spin density matrix analysis of the ϱ0, K∗0 (892) and o produced in Z0 hadronic decays at LEP are discussed. There is no spin alignment of the K∗0 (892) and o in the region xp ≤ 0.3 (xp = ppbeam). In the fragmentation region, the evidence is presented for noticeable spin alignment of the ϱ0, K∗0 (892) and o, with the diagonal matrix element ϱ00 increasing with xp. The off-diagonal matrix element ϱ1−1 is consistent with zero in all cases. These results are compared with those obtained by OPAL for the K∗0 (892) and o.

With the production rates, 〈 n part 〉, of the K 0 , K + , p and Λ and average charged particle multiplicities, 〈 n ch 〉, recently measured at LEP1.5–LEP2, scaling of the relative particle production rates 〈 n part 〉/〈 n ch 〉 is seen in a broad center of mass energy range from s = 29–35 to 183 GeV. The quantity ξ 0 /〈 n ch 〉 1/2 , where ξ 0 is the position of the maximum of the ξ=− ln (2p/ s ) distribution and p is the particle momentum, also scales from 14 to 183 GeV. It is shown that the gaussian shape of the ξ distribution is a simple consequence of longitudinal phase space. The slope parameter in the logarithmic increase of ξ 0 with the center of mass energy must be smaller than 1, as observed experimentally, but this does not necessary imply a manifestation of coherence.

Arguments are presented that available experimental data on the inclusive momentum distributions of particles produced in multihadronic events in e+e− annihilation and on evolution of the spectra with center of mass energy provide no evidence for the sensitivity of the hadronic spectra to soft gluon interference. The QCD shower Monte Carlo programs are used to study the dependence of the inclusive momentum distributions of different particles on flavours of initial quarks in the process e + e − →q q and to investigate the influence of coherence on the momentum spectra of gluons. The significance of the agreement of the experimental data with the analytical calculations based on the Modified Leading Logarithmic Approximation is discussed.

3-jet events of reaction e + e − → Z 0 →hadrons generated by the Monte Carlo program HERWIG 5.4 are used to compare the properties of the true jets initiated by primary partons with the properties of the jets reconstructed from the same events by the JADE and k T jet-finding algorithms. It is shown that the jet-finding algorithms attribute an important fraction of particles emitted in the backward direction and of particles with large p t with respect to the jet direction to other jets and thus distort the multiplicity distribution of particles in the jets. The influence of this systematics on the interpretation of some experimental results obtained with the use of these algorithms is discussed.

Multiplicity distributions for negatively charged particles in e + e − → hadrons measured by the ALEPH, DELPHI and OPAL Collaborations at √ s =91.2 GeV, with the combined statistics of 200 000 events, and those obtained at lower energies are perfectly described by the modified negative binomial distribution with the generating function M(x)={ [1+Δ(1−x)] [1+r(1−x)]} 7 with two parameters Δ and r . Possible models of branching processes leading to this distribution are discussed.

Factorial correlators are studied in 250 GeV/c π + p and K + p collisions as a function of the distance in rapidity. The correlators are found to increase with decreasing correlation distance, independently of the rapidity resolution. The increase approximately follows a power law, but the power is considerably larger than expected from a log-normal approximation in the simple α model of intermittency. Also the FRITIOF results are independent of the resolution, but slopes and p T behavior cannot be reproduced by this model.

By redefining, for p ± p collisions, the energy available for particle production, we show that the energy dependence of the average charged particle multiplicity in e + e − and p ± p collisions in the presently available energy range is strikingly similar.

A simple modification of the negative binomial distribution is obtained under the assumption that multiparticle production at high energies can be described as a stationary branching process with immigration. The charged particle multiplicity distributions for e + e − annihilation into hadrons and for inelastic ¯pp collisions at S¯ppS energies are shown to be in excellent agreement with the proposed distribution.

We present a search for the third generation up type quark t and a possible fourth down type quark b' in hadronic Z0 decays observed in DELPHI at the LEP collider. For any scenario with a decay through the charged current or into a charged Higgs with a mass at least 6 GeV/c2 below the t and 3 GeVc2 below the b' mass, we set a lower limit for the t quark mass at 44.0 GeV/c2 and for the b' mass at 44.5 GeV/c2. For specific scenarios the mass limits are slightly higher, e.g. for charged current decays the limits are 44.5 and 45.0 GeV/c2 respectively, where all limits are given at a 95% confidence level. © 1990., 0, SCOPUS: ar.j, info:eu-repo/semantics/published

It is shown that asymptotic KNO scaling is a property inherent to a broad class of stationary branching processes. An approach to asymptotics of reduced factorial and normalized moments is discussed.

It is shown that relative production rates, 〈n (part) 〉 〈n ch 〉 , of the pseudoscalar and vector mesons, and octet and decuplet baryons in e + e − annihilations are determined only by particleisospins, spins and their masses and approximately energy independent in the PETRA/PEP-LEP 1 energy range. From this and extrapolating the energy dependence of the average charged particle multiplicity, 〈 n ch 〉, the empirical estimates for the meson and baryon production rates in e + e − annihilations at LEP 2 energies are obtained.

The P/V ratio of directly produced pseudoscalar (P) to vector (V) mesons is analysed using the data on the K 0 S and K * (892) total inclusive cross sections in pp, π + p and K ± p reactions. The indication for a change of P/V from a value of about 1 at low energies, where the fragmentation processes dominate, to a value of 1/3, suggested by spin-statistics, at high energies is discussed.

It is shown that e + e − data on the production rates of all particles (apart from pions) belonging to the SU (3) nonets of the pseudoscalar and vector mesons and to the octet and decuplet of the baryons are determined only by particle spins, isospins and their masses. Contrary to the general belief and popular models, such as the Jetset model, the 2 J + 1 spin counting factor works perfectly. The meson and baryon production rates at the Z 0 pole at LEP as a function of particle mass squared lie on the one universal curve described by a simple formula 〈N( particle )〉 = 2J+1 2I m +1 a exp (−bM 2 ) with parameters a = 11.28 ± 0.31 and b = 3.872 ± 0.027 ( GeV c 2 ) −2 and X 2 NDF = 15 10 , where I m is the isospin for baryons or the modified isospin I m for mesons introduced in this paper. A similar universal dependence with the same value of the slope b is observed at PETRA and PEP energies.

The multiplicity distributions of charged particles produced in hadronic collisions are considered from the point of view of stationary branching processes. It is shown that the linear dependence of the dispersion on the average multiplicity D=A+B - the empirical Wroblewski relation - is an intrinsic property of a wide variety of branching processes. The partially coherent laser distribution is also deduced from one particular case of the branching process.

The average charged particle multiplicity, 〈 n ch ( M X 2 )〉, in the reaction K + p→K o X ++ is studied as a function of the mass squared, M X 2 , of the recoil system X and also as a function of the K o transverse momentum, p T , at incident momenta of 5.0, 8.2 and 16.0 GeV/ c . The complete data samples yield distributions which are not independent of c.m. energy squared, s , They exhibit a linear dependence on log ( M X 2 X / M o 2 )[ M o 2 =1 GeV 2 ] with a change in slope occurring for M X 2 ≈ s /2, and do not agree with the corresponding distributions of 〈 n ch 〉 as a function of s for K + p inelastic scattering. Sub-samples of the data for which K o production via beam fragmentation, central production and target fragmentation are expected to be the dominant mechanisms show that, within error, the distribution of 〈 n ch ( M X 2 )〉 versus M X 2 is independent of incident momentum for each sub-sample separately. In particular in the beam fragmentation region the 〈 n ch ( M X 2 )〉 versus M X 2 distribution agrees rather well with that of 〈 n ch 〉 versus s for inelastic K + p interactions. The latter result agrees with recent results on the reactions pp → pX and π − p → pX in the NAL energy range. Evidence is presented for the presence of different production mechanisms in these separate regions.

Inclusive production of Λ and Λ in K+ p interactions is studied at incident momenta of 8.2 and 16.0 GeV/c. Cross sections and single-particle distributions are presented, the correlation between longitudinal and transverse momentum is investigated, and the dependence of average charge multiplicity on missing mass measured. For Λ production, early scaling is observed in the target fragmentation region when the data are presented in terms of (M2-Mth2)/s and t, where Mth is the threshold value of the missing mass M. Furthermore, a triple-Regge analysis in these variable yields an effective exchange trajectory which passes through the K, Q and L mesons. There is evidence for beam fragmentation in Λ and Λ production, but the contributions seem not to be dominant in the fragmentation region. Nevertheless, the parameter values in a triple-Regge description are estimated, and together with those for target fragmentation in Λ production, provide a complete description of the fragmentation contributions to the two reactions. Integration of the resultant distribution functions over the complete Chew-Low plot yields fragmentation cross sections increasing approximately as log s; in addition the observed features of the x, pL and pT2 projections and of the pL-pT correlation are well-described in the fragmentation regions. Central production contributions are isolated by subtracting the calculated fragmentation distributions

The reaction K + p→Δ ++ (1236) + anything is studied at a beam momentum of 16.0 GeV/ c . The total and topological cross sections for K + π − interactions are estimated by a Chew-Low extrapolation from threshold to the c.m. energy of 3.4 GeV by the maximum likelihood method.

We present results on the inclusive polarization of Λ hyperons produced in K + p interactions at 32 and 70 GeV/ c . A large positive Λ polarization is observed in the kaon fragmentation region. The polarization is energy independent, increases strongly with increasing x , but shows essentially no p T -dependence.

The reaction K + p→ Δ ++ (1236)+ anything is studied at beam momenta 8.2 and 16 GeV/ c and compared with other Δ ++ producing reactions. We find that the low-mass Δ ++ π − enhancement affects the shapes of Δ ++ inclusive distributions. The triple-Regge formula is found to be consistent with the data. Dual properties of this formula are successfully tested.

The inclusive reaction K + p → K 0 + X is studied at 5, 8.2 and 16 GeV/ c . The energy dependence and the shapes of inclusive spectra in the central region are found to be consistent with double-Regge expansion. With the values obtained for the parameters of the Regge expansion, prediction are made for the behaviour of the cross section at higher energies.

Results are presented on the production of K0, K∗+(892) and π± in the interaction K+p → K0 + π± + anything at 8.2 and 16 GeV/c. The π± forward and backward d σ d x distributions are of the form exp (−Ax). It is shown that the forward and backward values of A in a quark-quark rest frame change after subtraction of the contributions due to the resonances produced. A comparison is made of d σ d x inclusive cross sections for K0's and K∗+(892) with the predictions of a quark model in the fragmentation region. The experimental cross section matches the theoretical predictions for K0's but disagree with those for K∗+(892).

The reaction K + p → Δ ++ (1236) + anything is studied at beam momentum 8.2 GeV/ c . The spin density matrix of the Δ ++ shows than one-pion exchange is dominant at low-momentum transfer. The total and topological K + π − cross sections have been determined by a Chew-Low extrapolation in the cm energy range from threshold to 1.8 GeV, using three different methods of extrapolation.

The valence-quark distributions in strange and charmed particles are determined by introducing different Regge trajectories for different quark flavours in order to take into account the strong SU(3) and SU(4) symmetry breaking. The valence-quark distributions in π, K and D mesons are shown to be quite different. The K-meson valence-quark distributions and the quark fusion mechanism of particle production have been used to calculate the inclusive spectra of vector mesons in K+p interactions at 32 GeV/c which are found to be in a good agreement with experimental data.

The inclusive reactions K + p → K 0 + X ++ at 5.0 and 8.2 GeV/ c and K + p → K 0 + π + + X + at 8.2 GeV/ c are analysed in terms of angular variables θ and o. The data are compared and found consistent with various theoretical bounds on the s -dependence of the inclusive reaction cross sections. Experimental evidence is presented that d σ /dcos θ = a exp (− b sin θ ) with b ∼ √ s and a increasing with energy. Correlations between the angular variables and between the variable o and the K 0 and π + momenta in the c.m.s. are observed and discussed.

The assumption that multiparticle production can be considered as a stationary branching process leads, through the use of the Kolmogorov reverse differential equation, to the relation 1 k =a+b ln m between the parameters 1 k and m= n k of the negative binomial describing the multiplicity distribution of charged particles. The evidence for such a relation follows also from the analysis of the experimental data.

We present an analysis of the reaction K + p → K 0 + anything at incident momenta of 5, 8.2 and 16 GeV/ c in terms of the Bjorken variables gw and x . Assuming dominance of ϱ exchange, we derive the structure function F 2 ( x , y ) which is found similar to the one obtained in deep inelastic electroproduction.