The forward-backward (FB) charged particle multiplicity correlations between windows separated in rapidity and azimuth are analyzed using a model that treats strings as independent identical emitters. Both the short-range (SR) contribution, originati
ng from the correlation between multiplicities produced from a single source, and the long-range (LR) contribution, originating from the fluctuation in the number of sources, are taken into account. The dependencies of the FB correlation coefficient, $b$, on the windows rapidity and azimuthal acceptance and the gaps between these windows are studied and compared with the preliminary data of ALICE. The analysis of these dependencies effectively separates the contributions of two above mechanisms. It is also demonstrated that traditional definitions of FB correlation coefficient $b$ have a strong nonlinear dependence on the acceptance of windows. Suitable alternative observables for the future FB correlation studies are proposed. The connection between $b$ and the two-particle correlation function, $C_2$, is traced, as well as its connection to the untriggered di-hadron correlation analysis. Using a model independent analysis, it is shown that measurement of the FB multiplicity correlations between two small windows separated in rapidity and azimuth fully determine the two-particle correlation function $C_2$, even if the particle distribution in rapidity is not uniform.
In the framework of the classical Glauber approach, the analytical expressions for the variance of the number of wounded nucleons and binary collisions in AA interactions at a given centrality are presented. Along with the optical approximation term,
they contain additional contact terms arising only in the case of nucleus-nucleus collisions. The magnitude of the additional contributions, e.g., for PbPb collisions at SPS energies, is larger than the contribution of the optical approximation at some values of the impact parameter. The sum of the additional contributions is in good agreement with the results of independent Monte Carlo simulations of this process. Due to these additional terms, the variance of the total number of participants for peripheral PbPb collisions and the variance of the number of collisions at all values of the impact parameter exceed several multiples of the Poisson variances. The correlator between the numbers of participants in colliding nuclei at fixed centrality is also analytically calculated.
The correlation between multiplicities in two separated rapidity windows, the so-called long-range correlation (LRC), is studied in the framework of the model with independent identical emitters. Its shown that the LRC coefficient, defined for the sc
aled (relative) variables, nevertheless depends on the absolute width of the forward rapidity window and does not depend on the width of the backward one. The dependence of the LRC coefficient on the forward rapidity acceptance is explicitly found with only one theoretical parameter. The preliminary comparison with ALICE 7TeV pp collisions data shows that the multiplicity LRC in the data can be described in the framework of the suggested approach.
It is shown that naive two stage scenario of the soft multiparticle production in hadronic and nuclear collisions at high energy, when at first stage the colour strings are formed and at the second stage these strings, or some other (higher colour) s
trings formed due to fusion of primary strings, are decaying, emitting observed particles, encounters some difficulties at the attempt to analyse the space-time picture of the process. Simple analysis shows the dominant is the process when the formation and the decay of a string occur in parallel - a string breaks into two parts already at rather small length (about 1-2 fm in its c.m. system), then the process repeats in the pieces and so on. Nevertheless it is proved to be possible to agree the string fusion idea with the space-time picture of a string decay. In the framework of the Artru-Mennessier model of a string fragmentation the simple interpretation of the homogeneity of the rapidity distribution for hadrons produced from the decay of a single string at high energy is presented and the analytical estimate for the density of this rapidity distribution is obtained.
