Multiparticle azimuthal correlations of negative pions in nucleus-nucleus collisions

Multiparticle azimuthal correlations of ?? ^? mesons have been studied in dC, HeC, CC, CNe, MgMg, (d, He)Ta, CCu, CTa, and OPb collisions at momentum of 4.2, 4.5 GeV/c per nucleon within the standard transverse momentum analysis method of P. Danielewicz and G. Odyniec. The data were obtained by SKM-200-GIBS and Propane Bubble Chamber Collaborations of JINR. The axis has been selected in the phase space and with respect to this axis ?? ^? meson correlations were observed. The values of the coefficient of the correlations linearly depend on the mass numbers of projectile (A _P ) and target (A _T ) nuclei. The Quark-Gluon String Model satisfactorily describes the experimental results.     

Small-angle correlations of identical particles in carbon-carbon interactions atP=4.2 GeV/c per nucleon

Two-particle small-angle correlations between negative pions and between protons in carbon-carbon collisions atP=4.2 GeV/c per nucleon have been studied, both for an unbiased sample and for “central” events. A comparison of experimental π^? π^? andpp correlation functions with theoretical predictions has been made. A possible evidence for the existence of two fireballs in C+C interactions atP=4.2 GeV/c per nucleon is presented.     

Formation of dense groups of particles in intermediate-energy nucleus-nucleus collisions

The formation of dense groups (fluctuations) of particles produced in the central CCu and MgMg collisions at the projectile momenta of, respectively, 4.5 and 4.3 GeV/c per nucleon is analyzed. The distributions of the maximum densities and of the centers of charged-particle fluctuations in pseudorapidity space are studied in searches for dynamical multiparticle correlations. The distributions of the centers show two peaks above the statistical background with a structure similar to that which is expected in the model of coherent gluon emission and which was observed in hadronic interactions. The charge independence of the distributions in question and an azimuthal isotropy of events involving pseudorapidity fluctuations are observed. The distributions of events with respect to the maximum density of fluctuations are governed primarily by the statistical contribution, although the behavior of the distributions in CCu collisions is in qualitative agreement with the prediction of the one-dimensional intermittency model. It is found that the resulting distributions are of a non-Poisson character both in CCu and in MgMg collisions. The results of this study indicate that, in describing local dynamical fluctuations in multiparticle production processes, the coherent and the stochastic approach supplement each other. The procedure employed in the analysis described here makes it possible to draw a direct comparison of the present results with the results of similar investigations of different reactions.     

Production of pseudoscalar mesons in 300 GeV central π+ N collisions

The exculsive production of π⁰, η, and η′ mesons in π⁺ N central collisions has been studied with 300 GeV incident pions. The experiment has been performed at the CERN SPS. The gammas from decaying mesons were detected in the hodoscope multiphoton spectrometer GAMS-4000. The measured differential cross sections show a plateau in the longitudinal momentum interval 0.1F<0.3.     

Study of inclusive production of π± and protons in\bar p p interactions at 32 GeV/c

Results are presented on the inclusive production of π^± mesons and protons in \(\bar p\) p interactions at an incident antiproton momentum of 32 GeV/c in the MIRABELLE bubble chamber, based on a sample of 55,000 inelastic events. The spectra of π⁺(π^?) andp( \(\bar p\) ) are separated in the whole kinematically admitted region of phase space by a statistical method. The semi-inclusive and inclusive cross sections and the main average characteristics of π andp production are calculated. The invariant differential cross sections are studied as functions of the Fevnman scaling variablex, the rapidity and the transverse momentum. Thex distribution of π^± mesons in the proton fragmentation region does not show any significant contribution from quark exchange or annihilation processes. A possible contribution of hard quark scattering on mesons or baryons is observed for large transverse momenta.     

Inclusive particle production in π− n interactions at high energies

Results on inclusive particle production in π^? n interactions at 21, 205 and 360 GeV/c are presented. The invariant cross sections in both neutron fragmentation and central regions are found to be equal within errors to the corresponding cross sections for π^? p collisions and exhibit the same energy dependence. A strong energy dependence of the invariant cross section ratios of negative and positive pions is observed. There is also an indication of transverse momentum dependence of these ratios. The η^? correlations in the neutron fragmentation region show little variation with energy and are in agreement with the predictions of the naive quark recombination scheme.     

Multiplicities in highp t hadron-nucleus interactions

We present results obtained in π^? A (A=C, Cu, Pb) — collisions at 38 GeV/c. A single particle trigger selects events with one charged particle in the central region and large transverse momentum. The effect of this trigger on the multiplicities of all charged particles and of protons is shown.     

Features of carbon-carbon interactions at a momentum of 4.2 GeV/c per nucleon that are accompanied by the production of Λ hyperons and K S 0 mesons versus the degree of nuclei collision centrality

Experiemntal data obtained by using the 2-m propane bubble chamber of the Joint Institute for Nuclear Research (JINR, Dubna) are used to study the effect collision centrality on the spectra of Λ hyperons and K _S ⁰ mesons produced in carbon-carbon interactions at 4.2 A GeV/c. The multiplicity of participant protons having momenta in excess of 300 MeV/c is taken to be a measure of collision centrality. The features of pions and protons accompanying strange-particle production are also presented. The experimental data in question are compared with the prediction of a modified version of the FRITIOF model. It is shown that strange particles are predominantly produced in central and semicentral collisions. The average kinematical features of K _S ⁰ mesons are found to be indepedent of collision centrality. At the same time, the average transverse momentum of Λ hyperons and the average value of their emission angle increase slowly with increasing degree of collision centrality. The anisotropy of the angular distributions of both Λ hypersons and K _S/0 mesons in the c.m. frame of nucleon-nucleon collisions decreases with increasing collision centrality. The average transverse momentum of K _S ⁰ mesons is approximately 1.6 times higher than the average transverse momentum of π ^? mesons.     

A study of high transverse momentum η and π0 mesons at the CERN ISR

The production of high transverse momentum η mesons from proton-proton collisions has been studied at the CERN ISR. The decay η→γγ was identified in an array of lead-glass ?erenkov counters. The preliminary results, obtained at a total centre-of-mass energy √s=52.7 GeV, show an η signal, which amounts to approximately one half of the π⁰ cross-section for centre-of-mass transverse momenta between 3 and 5 GeV/c. The production of high transverse momentum π⁰'s was also measured and compared with previous results.     

Fine structure in theα decay of192Po

The production of energetic π^? at 0° has been measured in Ne+NaF and Ni+Ni collisions with incident energies between 1.3 and 2AGeV. In Ne+NaF collisions the investigation was extended to extreme subthreshold processes with lab momenta up to 4.5 GeV/c. In both systems at all incident energies the π^? production cross sections deviate in a systematic way from thermal distributions.     

Analyzing the features of π− mesons and protons from AC interactions at a momentum of p=4.2 GeV/c per projectile nucleon on the basis of the FRITIOF model

The mean multiplicities of π^? mesons and protons originating from pC, dC, αC, and CC interactions at a momentum of p=4.2 GeV/c per projectile nucleon and the distributions of these particles in kinematical variables are presented. These experimental distributions are compared with the corresponding predictions obtained on the basis of the FRITIOF model. It is shown that the FRITIOF version used in the present analysis describes satisfactorily our experimental data.     

The π N → e +e−N reaction close to the vector-meson production threshold

The π^? p→e ⁺ e ^? n and π⁺ n→e ⁺ e ^? p reaction cross sections are calculated below and in the vicinity of the vector-meson (?⁰,ω) production threshold. These processes are largely responsible for the emission of e ⁺e^? pairs in pion-nucleus reactions and contribute to the dilepton spectra observed in relativistic heavy ion collisions. They are dominated by the decay of low-lying baryon resonances into vector-meson-nucleon channels. The vector mesons materialize subsequently into e ⁺ e^? pairs. Using πN→?⁰ N and πN→ωN, amplitudes calculated in the center of mass energy interval 1.4 < √s<1.8 GeV, we compute the π^? p→e ⁺ e ^? n and π⁺ n→e ⁺ e ^? p reaction cross sections in these kinematics. Below the vector-meson production threshold, the π⁰?ω interference in the e ⁺ e^? channel appears largely destructive for the π^? p→e ⁺ e ^? n cross section and constructive for the π⁺ n→e ⁺ e ^? p cross section. The pion beam and the HADES detector at GSI offer a unique possibility to measure these effects. Such data would provide strong constraints on the coupling of vector-meson-nucleon channels to low-lying baryon resonances.     

Factorial moments and correlations in meson-nucleus interactions at 250 GeV/c

A sample of 8000 interactions of 250 GeV/c π⁺ andK ⁺ mesons on Al and Au nuclei, is used to search for intermittency effects by calculating the normalized factorial moments of order two to four. No significant effect is observed in the higher moments. Two-body rapidity correlations and Bose-Einstein correlations in these interactions are presented.     

Production of meson pairs,involving tensor and pseudotensor mesons,in photon-photon collisions

Starting from a bound-state model of weakly bound quarks for (qq¯) mesons, we derive a formalism for computing the production or decay of such mesons, whatever the value of their internal orbital angular momentum L. That approach appears as a natural generalization of the Brodsky-Lepage formalism (valid only for L = 0) that has been widely used in recent years for the computation of exclusive processes in perturbative QCD. We here apply it to the production, in photon-photon collisions, of: i ) tensormeson pairs; ii ) pseudotensor-meson pairs; iii ) hybrid pairs made of a pion and a pseudotensor meson. The numerical results we obtain allow for some hope of experimentally identifying such pairs, in the charged channels, at high-energy e ⁺ e ^?. colliders of the next generation, provided integrated luminosities as high as ~10⁴⁰ cm^?2 can be reached.     

Structure of final states with a high transverse momentum π0 in proton-proton collisions

We present a study of the final state structure in proton-proton collisions (√s = 53 GeV) where a large transverse momentum π⁰ (p_t > 2 GeV/c) is produced at an angle of 90°. Charged secondaries have been detected and momentum analysed in the split field magnet detector at the CERN Intersecting Storage Rings. The large angular coverage of this detector extends over ±2.5 units of rapidity and ±30° of azimuth with respect to the trigger π⁰, both towards and away from it. In each of these directions, where we observe similar strong correlations, we present charged particle distributions, in rapidity and momentum. In the hemisphere containing the trigger π⁰ we have measured the cross section for inclusive production of large transverse momentum ?^± mesons. In the opposite hemisphere the data exhibit several features predicted by hard scattering quark-parton models: coplanarity and short-range rapidity correlation for the large transverse momentum secondaries as well as a transverse momentum sharing distribution similar to that observed in deep inelastic electro-production and in e⁺e^? collisions.     

On spectral temperatures of negative pions produced in d 12C, 4He12C, and 12C12C collisions at 4.2 A GeV/c

The experimental transverse momentum distributions of negative pions produced in d ¹²C, ⁴He¹²C, and ¹²C¹²C collisions at 4.2 A GeV/c were analyzed in the framework of Hagedorn Thermodynamic Model. The spectral temperatures of π ^? mesons as well as their relative contributions to the total multiplicity of π ^? mesons were extracted from fitting the p _t spectra by two-temperature Hagedorn function. The results were compared systematically with the earlier results obtained from analysis of non-invariant center-of-mass energy spectra of negative pions produced in the analyzed collisions.     

Photoproduction of π+ mesons on polarized protons at photon energies between 0.5 and 2.2 GeV

The target asymmetry T = (σ↑ ? σ↓)/(σ↑ + σ↓) for the reaction γp → π⁺n has been measured at the Bonn 2.5 GeV electron synchrotron for a pion c.m. angle of 40° and γ energies between 0.5 and 2.2 GeV. Butanol was used as the target material. About 35% of the protons could be polarized using the dynamic-polarization method in a continuous-flow cryostat operating at 1°K and 25 kG. The π⁺ mesons were detected in a magnetic-spectrometer system. Considerable structure in the asymmetry was observed.     

Correlations of secondaries in events with a forward negative hadron at the ISR

Data obtained from p-p collisions at centre-of-mass energies between 31 and 63 GeV are presented on correlations between momentum analysed forward π^?, K^? and $\text{p}$ and charged particles observed in an omnidirectional hodoscope. The data show that significant correlations are present over the whole rapidity range for all three types of negative particles. The dependence on various kinematic variables suggests a cluster mechanism for the production of particles. In this picture, pions would be produced in clusters emitted in the fragmentation region while K^? and $\text{p}$ emanate from non-leading clusters.     

Investigation of the narrow structure K(1630) decaying into the K S 0 π+π− system

The narrow structure K(1630) is singled out in the effective-mass spectrum of the K _S ⁰ π⁺π^? system originating from π^? p collisions at 16 GeV/c. This structure is predominantly formed in inelastic collisions accompanied by high momentum transfers. For events from the K(1630) region, kinematical distributions show special features that may be associated with spin manifestations and with correlations between the products arising from the decays of the hypothesized exotic resonance state. Experimental observations of narrow hadronic structures formed at high momentum transfers are reviewed.