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long - range correlation studies between observables in two separated rapidity windows are considered @xcite as a tool for investigation of the initial stages of the hadronic and nuclear collisions , preceding the creation of a hot and dense medium .
because of the non - perturbative nature of multiparticle production in a soft region , one has to apply the various semiphenomenological approaches , such as the model of quark - gluon string formation . at high energies , due to multiparton interactions
, the formation of several pairs of strings becomes possible .
the interaction between the strings could be observed as a collective phenomena in pp collisions .
experimentally , the multiplicity correlation coefficient , defined as @xmath0 , has been measured in a wide energy range @xcite as a function of pseudorapidity windows width , their position and transverse momentum region . in the present paper ,
we study @xmath1 using the monte carlo model , that incorporates string collectivity effects in the form of string fusion @xcite , and compare the results with the data at @xmath2 from 200 to 7000 gev .
the monte carlo model @xcite is based on the partonic picture of nucleon interaction .
it preserves the energy and angular momentum conservation in the nucleon initial state and uses the dipole approach @xcite for description of elementary partonic collisions .
multiplicity and transverse momentum are obtained in the approach of colour strings , stretched between projectile and target partons .
the interaction of strings is realized in accordance with the string fusion model prescriptions @xcite .
namely the mean multiplicity @xmath3 and the mean transverse momentum @xmath4 of the particles produced from a cluster of @xmath5 overlapping strings are related to those ( @xmath6 ) from a single string : @xmath7{k } { p_t}_{1}.$ ] for realization of the string fusion prescription , we have used the discrete approach , in which a lattice with the cell area equal to the string transverse area @xmath8 is introduced .
the strings are thought to be fused if their transverse position centres belong to the same cell . for the multiplicity from one string ( or a cluster of fused strings ) we used poisson distribution , with gaussian transverse momentum spectra of produced particles . however , in order to provide to provide the possibility of a direct comparison with experimental data , the correct description of the transverse momentum spectra is required . for this purpose
the mc model @xcite has been extended by taking into account the hardness of elementary collision . for this
the mechanism similar to the one in dipsy event generator @xcite , has been incorporated in our model with the string fusion .
it was assumed that the hardness an elementary collision is inversely proportional to the transverse size of the interacting dipoles : @xmath9 the mean transverse momentum of particles produced by a single string has the contributions from both edges of the string plus the additional constant term @xmath10 , corresponding to the intrinsic string transverse momentum : @xmath11 .
accordingly , in the version with string fusion , the transverse momentum of a cluster of strings : @xmath12 where @xmath13 .
parameters of the model are constrained from the data on total inelastic cross - section and multiplicity@xcite . in the present study , for the case with string fusion
we have used @xmath14 ( in the case with string fusion ) . for the intrinsic string transverse momentum
we have used @xmath15 , which provides a reasonable description of the transverse momentum distribution in pp collisions at the lhc energies .
) at midrapidities ( @xmath16 ) .
lines  the results of calculation in the model with and without string fusion , points  the experimental data @xcite.,scaledwidth=90.0% ]
figure [ fig : mv ] shows the dependence of the correlation coefficient on the width of the pseudorapidity windows at three energies . the cuts on the transverse momentum ( @xmath17 ) applied in mc model calculations ,
enable direct comparison with the alice experimental data @xcite .
it was found that the general trends , like the growth of @xmath18 with collision energy and width of pseudorapidity windows ) are well described by the model .
the role of string fusion raises with @xmath2 , but using only midrapidity experimental data on multiplicity correlation coefficient at present energies it is hard to distinguish between cases with and without string fusion . also , it should be noted that at the small gap between rapidity windows there is a contribution of short - range correlation effects in the data ( such as the decays of resonances ) , which are currently not accounted by the model .        in fig .
2 the dependence of @xmath18 on the transverse momentum region of accounted particles is shown .
the correlation coefficient is studied as the function of the lower bound of the @xmath19 interval , and compared to the atlas experimental data @xcite .
the qualitative agreement of @xmath18 with experimental data is found .
the increase of @xmath20 is accompanied by the decrease of the multiplicity in the given transverse momentum region , which restricts the phase space for particle production and the number of `` active '' strings , which leads to the decrease of correlation coefficient .
numerically , the model calculations overestimate the value of the correlation coefficient in the hard transverse momentum area .
it could be an indication that the direct approach with the soft strings is applicable at the @xmath19 region below @xmath21 , and different processes ( such as jet fragmentation ) begin playing a role at higher @xmath19 .
on the other hand , this dependence is very sensitive to the shape of the transverse momentum spectra . whereas , the description of the @xmath19 spectra in our approach has an effective character and does not account the jet production and other hard phenomena accurately .
figure  [ fig:3 ] shows the dependence of @xmath18 on the @xmath22 at four energies , calculated in the monte carlo model , with a comparison to the experimental data @xcite .
the model calculations do not discriminate pp and @xmath23 scattering .
the model reproduces the growth of the correlation coefficient with collision energy and qualitatively describes the decrease of the correlation coefficient with with increase of the gap between pseudorapidity windows .
note that the short - range effects , such as the resonances decays and jets , which could contribute to the correlation coefficient at small @xmath22 are not accounted by the model .
the results indicate that taking into account of the string fusion effects improves the agreement with the data .
the forward - backward multiplicity correlation strength in pp collisions at high energy is studied in the monte carlo model with string formation and fusion .
the monte carlo model reasonably describes the main features of the behaviour of the correlation coefficient in a wide energy range , such as general growth of the correlation coefficient with collision energy and with increase of pseudorapidity window size .
the decrease of @xmath18 with the increase of the gap between windows and with increase of the lower @xmath24 bound is also qualitatively described .
it is found that the version of the model with inclusion of string fusion effects is better supported by the data compared to the case without string fusion .
the authors acknowledge saint - petersburg state university for the research grant 11.38.197.2014 . v. kovalenko acknowledges saint - petersburg state university for the special rector s scholarship .
he is also grateful to the dynasty foundation .
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the correlations between multiplicities in two separated rapidity windows , is studied in the framework of the monte carlo model based on the picture of string formation in elementary collisions of colour dipoles . 
 the hardness of the elementary collisions is defined by a transverse size of the interacting dipoles . 
 the dependencies of the forward - backward correlation strength on the width and position of the pseudorapidity windows , as well as on transverse momentum range of observed particles were studied . 
 it is demonstrated that taking into account of the string fusion effects improves the agreement with the available experimental data .