Leading Particle Effect Research Articles

A hydrodynamic model for hadron production ine +?e ? annihilation

We consider hadron production ine+-e− annihilation within the framework of a hydrodynamic model. The two key assumptions in the moddel are (i) the initial shape of a Lorentz-contracted disc for the hadronic fireball, with the jet axis providing the necessary longitudinal direction and (ii) the criterion of a critical size of hadronic dimension for the expanding fireball to break up into finalstate particles. Experimental data on charged multiplicity, inclusive distributions, dispersions and particle spectra upto the highest centre-of-mass energy, presently accessible in colliding-beam machines, are explained satisfactorily with only three energy-independent parameters. The present analysis brings out the similarity in the mechanism for hadron production inp-p collision ande+-e− annihilation and attributes the apparent differences to the presence of leading-particle effect in the former process.

Can QCD be tested in high-energy low-p⊥ hadronic physics?

Summary. - It is suggested that the broadening of the pdistribution which is predicted by QCD and which has been seen in e+e - at highest PETRA energies should be reached in low-pproton-proton collisions (using the technique of removal of the leading-particle effect) at c.m. energies ~/s~400 GeV. The preliminary data from the pp Cern collider support this estimate which agrees with cosmic-ray analysis.

Leading particle effect on antiproton annihilation in neon

Semiclassical transport theory is applied to the pion emission from antiproton annihilation in Ne. In the annihilation of a moving antiproton in nuclei, we find that the leading particle effect is apparent.

On the role of the QCD evolution of parton distributions in explaining diffractive production of charm

We calculate charmed quark spectra resulting from the QCD evolution of parton distributions which enter into the contributions of peripheral parton subprocesses to pp → Λ c + + X. It turns out that at the low Q 2 ≈ M c 2 of interest the spectrum of spectator charmed quarks is much harder than the charm structure function which remains instead essentially soft as at large Q 2. This opens the possibility that perturbative QCD alone may explain the leading particle effect observed in pp → Λ c + + X.

Hadron-nucleus interactions and the leading particle effect in a dual-parton model

We describe hadron-nucleus interactions at smallPT in the framework of a dual-parton model which is a generalization of the two-chain dual-partonmodel of hadron-hadron interactions introduced recently in the literature. The model (of a multiple scattering type) is formulated in the framework of the 1/N expansion in dual theories. Regge singularities are used to determine the (joint) momentum distribution functions of the various sets of constituents (valence and sea quarks) which yield hadron jets in successive orders of the 1/N expansion. These jets result from a standard color separation mechanism, inspired by QCD. We study the inclusive spectra of various types of secondaries for various projectiles and targets. Particular emphasis is laid on theA-dependence in the projectile fragmentation region. A comparison with available experimental data is presented.

On the origin of the leading-particle effect in high-energy hadronic processes

Within a very crude approach, we show that the multiparticle unitarity constraint can be invoked to account for the origin of the leading-particle effect in high-energy hadronic processes. The quasielastic peak of inclusivep⊥ distributions and the diffractive behaviour around |x| ≃ 1 (x being the Feynman normalized momentum) emerge naturally. The limitations of the approach are briefly discussed. Nell’ambito di un modello molto rozzo, si mogtra che la condizione di unitarieta a molte particelle puo essore considerata responsabile dell’origine dell’efletto di particella guida in processi adroniei di alta energia. II picco quasi elastico inp⊥ delle distribuzioni inclusive e il comportamento diffrattivo vicino a |x| ≃1 (x essendo la variabile di Feynman) emergono in maniera naturale. I limiti del modello sono brevemente discussi.

A cluster algorithm for the study of jets in high energy physics

A cluster algorithm using angular correlations and leading particle effects is presented which is applicable to the study of jets produced in high energy storage ring collisions. The algorithm uses the concept of a minimal spanning tree and is computationally very efficient. Events are classified by their cluster number and the cluster number frequency distribution can be used for comparison with particle production models. Individual particles are assigned to the clusters and the vector sum of their momenta generate a cluster axis. These cluster properties permit the study of the dynamics of the jet production and fragmentation processes. The example of two and three jet production at PEP and PETRA energies is used to illustrate this technique.

The unitarity constraint in multiple production as a possible origin of the leading-particle effect

The unitarity constraint in multiple production as a possible origin of the leading-particle effect

A new parton model description of soft hadron-nucleus collisions

We present a parton model description of small p T hadron nucleus collisions based on a two-chains parton model for hadron-hadron collisions. The model provides a quantitative description of single particle spectra — including the leading particle effect — in terms of standard quark fragmentation and momentum distribution functions.

Two-pion correlations in K+p interactions at 32 GeV/c

We present results obtained from a study of two-particle correlations in K+p interactions at 32 GeV/c. The correlations are shown as a function of rapidity and mass for different charge multiplicities. We make a particular effort to compare our results with a simple Monte Carlo model which embodies the fundamental aspects of cylindrical phase space, the leading-particle effect and the experimental topological distribution and, in the light of this, we try to estimate the qualitative significance of our results.

A coherent state model for electron-positron annihilation

A phenomenological model is discussed for electron-positron annihilation in which the centrally produced particles are in a coherent state. This is analogous to the two-dimensional Schwinger model for e +e − annihilation. The leading particle effects associated with the initial quark and antiquark are described by including wave functions for the leading mesons. By using the asymptotic methods of De Groot for calculating phase-space integrals, all the properties of the model are investigated analytically. It is shown by an appropriate choice of leading meson wave function that all the results of the Schwinger model and of the iterative-type cascade models are obtained. Interference effects between leading and non-leading mesons are calculated and are shown to be significant. The general properties expected of heavy quark fragmentation are also investigated. The results obtained in this paper are also applicable to other processes with leading particle effects.

Neutron fragmentation in the reaction pn→pX at 19 GeV/c

Data on the reaction pn→pX are extracted from a pd experiment in the CERN 2m DBC at 19 GeV/c. The cross-section for neutron fragmentation events with |t|<1 (GeV/c)2 is determined and compared with data at high energies at fixedMx2/s andt. We find that the cross-section can be described with the triple-Regge formula taking into account only the pion exchange contributions,i.e. the ππP and ππR terms. A leading-particle effect consistent with the pion exchange model is observed in the longitudinal-momentum distribution of the negative pions in the final state of the reaction pn→pX, when transformed to the rest frame of the recoiling system X.

Factorization in the central region of inclusive reactions

Detailed factorization predictions for inclusiveπ± production in the central region are compared with the available data. If account is taken of possible leading-particle effects extending into the central region at low energies, there is substantial agreement with factorization for both the energy-independent and the energy-dependent parts of the inclusive cross-sections.

Study of proton-nucleus collisions at high energies based on the hydrodynamical model

We study proton-nucleus collisions at high energies using the one-dimensional hydrodynamical model of Landau with special emphasis on the effect of the size of the target nucleus and of the magnitude of velocity of sound of excited hadronic matter. We convert a collision problem of a proton and a nucleus with a spherical shape into that of a proton and a one-dimensional nuclear tunnel whose length is determined from the average impact parameter. By extending the methods developed by Milekhin and Emelyanov, we obtain the solutions of the hydrodynamical equations of proton-nucleus collisions for arbitrary target tunnel length and arbitrary velocity of sound. The connection between these solutions and observable physical quantities is established as in the work of Cooper, Frye, and Schonberg. Extensive numerical analyses are made at ${E}_{\mathrm{lab}}=200$ GeV and for the velocity of sound $u=\frac{1}{\sqrt{3}}$ of a relativistic ideal Bose gas and $u=\frac{1}{{(7.5)}^{\frac{1}{2}}}$ of an interacting Bose gas. In order to compare proton-nucleus collisions with proton-proton collisions, all the analyses are made in the equal-velocity frame. We find the following results. (1) In comparing the number of secondary particles produced in $p$-$A$ collisions ${N}_{\mathrm{pA}}$ with those in $p$-$p$ collisions ${N}_{\mathrm{pp}}$ while most of the excess of ${N}_{\mathrm{pA}}$ over ${N}_{\mathrm{pp}}$ is concentrated in the backward rapidity region, there exists also an increase of ${N}_{\mathrm{pA}}$ with $A$ in the forward rapidity region. This result is at variance with the predictions of the energy-flux-cascade model and of the coherent-production model. (2) The excess energies are contained exclusively in the backward region. We also find evidence for the following new phenomena in proton-nucleus collisions. (3) The existence of an asymmetry of average energies of secondary particles between forward and backward regions, in particular, $〈{E}_{\ensuremath{\pi}}^{b}〉\ensuremath{\gg}〈{E}_{\ensuremath{\pi}}^{f}〉$ for larger nuclear targets. Thus, energetic particles are predominantly produced in the backward region. (4) The existence of an asymmetry in the transverse masses between forward and backward regions, ${m}_{T}^{b}g{m}_{T}^{f}$ for $u=\frac{1}{{(7.5)}^{\frac{1}{2}}}$ and ${m}_{T}^{b}l{m}_{T}^{f}$ for $u=\frac{1}{\sqrt{3}}$. This asymmetry implies a similar asymmetry in the transverse momenta ${p}_{T}$. (5) The disappearance of the leading-particle effect in the forward region for $u=\frac{1}{{(7.5)}^{\frac{1}{2}}}$. Some other new findings are also discussed. The energy dependence of the above phenomena is studied in the range ${E}_{\mathrm{lab}}=200\ensuremath{-}1500$ GeV.

Evidence for leading particle effects in charm fragmentation from neutrino data

Recent data of the CERN-Dortmund-Heidelberg-Saclay collaboration on opposite sign dimuon events produced in neutrino interactions are shown to require a fragmentation function D( z) for charmed quarks non-negligible near z = 1, in contrast with the general current belief. Although the published data already suggest that the gross features of D( z) are to be described by a constant or slowly rising (near z = 1) behaviour, we find that a much more decisive test would be provided by the E + (E H + E +) distribution, where E + and E H are the energies of positive muon and of the hadron shower, respectively.

University of Mean Charged Hadron Multiplicities in Particle and Nuclear Collisions

We propose that mean charged hadron multiplicities in various particle collision processes including hadron-nucleus collisions are described in terms of a universal function F( ), where is the mean square invariant mass of the final state subsystem X which consists of all the produced hadrons other than the leading particles. Leading particle effects in general and nuclear mass effects in hadron-nucleus collisions are most important factors which deter· mine at a given incident energy. It is found that in proton-emulsion interaction the leading particle effects depend strongly on the number of heavily ionizing prongs. Within the framework of our model, the observed values of mean shower particle multiplicities in proton-emulsion interaction indicate that F( ) obeys a power-law.

Model for high-energy baryon-antibaryon annihilation

A model for annihilation is presented, based on a simple multiperipheral interpretation of the corresponding dual unitarity diagrams. The model exactly conserves internal quantum numbers and leads to characteristic correlations and leading particle effects associated with the topology of the dominant diagrams. Uncertainties about non-leading terms prevent a direct comparison with annihilation data at low energies, but a fit to high-energy data on p p- pp cross-section differences, motivated by the similarities between the differences studied and genuine annihilation data where these can be compared, shows excellent agreement when the mesonic objects produced in the model are taken to be clusters of the same type as those produced in non-annihilation processes.

A study of events with a secondary of high transverse momentum in 16 GeV/ cπ+-p and GeV/ cπ−p collisions

Events with a charged secondary having high transverse momentum (>1 GeV/ c) have been investigated in π +p reactions at 16 GeV/ c and π −p reactions at 16 and 100 GeV/ c. The charge asymmetry (the ratio π + π − ) increaes strongly as p t increases in π +p interactions, while in π −p interactions it is the ratiok π + π − that increases strongly with p t in the forward hemisphere. This dominance of primary charge is related to leading-particle effects. We find evidence for long-range correlations when pions produced opposite to a high p t proton are selected.

What semi-inclusive data say about clusters

Our global analysis of inclusive high-energy multi-particle production data in the cluster model framework is extended to semi-inclusive data. Our cluster model embodies leading particle effects and kinematical constraints, which are shown to be of great importance. It appears that models with light clusters decaying on average into 〈n c 〉 ⋍ 2 charged particles, with a rapidity width δ ⋍ 0.6–0.7 and a distribution much narrower than a Poisson-type, allow one to fit in a nice way both inclusive and semi-inclusive data. It is pointed out that the most constraining semi-inclusive data are those regarding longitudinal correlations, which definitely exclude heavy cluster models, whereas the data on zone characteristics only bear out that a non-negligible percentage of clusters have to carry an electric charge.

Hydrodynamical Model of Multiparticle Production at Large Transverse Momentum

The large transverse momentum phenomena in the central region can be understood by considering the hydrodynamical expansion of a fireball which is formed in the collision of hadrons. The hydrodynamical equation of the hadronic flmd with large transverse velocity is solved analytically in the central region under the appropriate boundary condition. For the case of the relativistic three-dimensional ideal fluid, the resultant particle spectrum EdN/d'p at large transverse momentum has the form s'1'p'i' which is consistent with the ISR data. § l. Introduction Recent experiments of high energy hadron collisions accumulate a large amount of data and reveal many interesting features of multiple production phenomena. One of them is the phenomenon at large transverse momentum. It has been found in the measurements at CERN ISR and NAL that the differential cross section of produced pi-meson at large transverse momentum (Pr>2 Ge V /c) falls less steeply (say, fJr-) than what is expected from an extrapolation of the exponential behavior at low transverse momentum, and the cross section at large fixed tranverse momen­ tum increases strongly with the incident energy, which is also a contrast to the low transverse mom en tun1 results. D The charged particle nmltiplicity as a function of the transverse momentum of photons (rr 0) detected at a center-of-mass angle of 90° is measured. 2> It is reported that the total associated multiplicity increases with the transverse momen­ tum of detected 7r0 and this increase is found almost at a wide polar angle with respect to the beam direction (17c.m.~90') and is observed even in the direction of the detected 7Z0 as well as its opposite side. This fact that a particle with large transverse momentum is accompanied by many particles, is considered to be con­ sistent with the following picture. The particles with large transverse momentum are produced from a fireball which is formed by the collision of particles and is at rest in the center-of-mass frame. In what follovvs, this picture is employed only in the central region, since the largest transverse momentum particles are found m this region and the leading particle effect must be taken into account particularly in the non-central region. It is the purpose of this paper to understand the large transverse momentum phenomena by considering the hydrodynamical expansion