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we present a new monte carlo generator of events for neutrino interactions . the original motivation for our work was to improve nux+fluka scheme where no separate resonance contribution is present @xcite .
the aim of nux+fluka was to describe interactions of neutrinos of higher energies and from that point of view the resonance part was of minor importance .
usually mc generators contain a resonance contribution described by means of rein - sehgal model covering the kinematical region of hadronic invariant mass @xmath1 . if for neutrino reactions the quark - hadron duality holds true one can assume that contributions from higher resonances are averaged by deep inelastic scattering ( dis ) structure functions and that only the dominant @xmath0 resonance has to be treated separately .
the current version of the generator includes various dynamical models : quasi - elastic @xcite , @xmath0 excitation @xcite , and dis for which we use grv94 parton distribution functions ( pdf ) @xcite with modifications proposed by bodek and yang @xcite .
the total cross section for the neutrino scattering is assumed to be the incoherent sum : @xmath2 where @xmath3 is the sum of cross sections for single pion production ( spp ) and cc and nc denote charge and neutral current reactions respectively .    the mc generator is organized around the event structure which contains three vectors of particles : incoming , temporary and outgoing .
it also contains a structure with all the parameters used and a set of boolean flags tagging the event as qel , dis , cc , nc etc .
the input parameters are read at startup from a text file and the events are stored in the root tree file to simplify further analysis .
different interactions are implemented as functions acting on the event structure reading the incoming particles and producing the temporary ones .
the type of the interaction is chosen according to the ratio of the total cross sections .    in our presentation
we focus on single pion channels .
we present many comparisons with the existing experimental data . in the near future
the generator will be supplemented with a module with nuclear effects .
[ pn ] . data points taken from @xcite are connected by lines .
the results of our simulations are shown as separated points .
, title="fig : " ]    [ fig : spp_vs_el ]   and @xmath0 resonances . for neutron
the sum of 1-pion functions for two exclusive channels is shown.,title="fig : " ]    [ nu_n_mu_x ]   interaction .
total cross section is split into contributions from quasi - elastic , spp ( @xmath4 gev ) and more inelastic processes .
data points are taken from @xcite , @xcite , @xcite , @xcite , @xcite , title="fig : " ]    [ nubar_n_mu_x ]   interaction .
total cross section is split into contributions from quasi - elastic , spp ( @xmath4 gev ) and more inelastic processes .
data points are taken from @xcite , @xcite , @xcite , @xcite , @xcite , @xcite , @xcite , title="fig : " ]    in our mc we use the dis formalism to generate events in the whole kinematical region where inelastic reactions are possible . in order to get the event record for the final state
we assume that interaction occurs always on a particular parton and then the fragmentation of interacting quark and spectator is performed by means of pythia6 @xcite routines .
the inclusive cross section for the scattering off nucleon is given by @xmath5 @xmath6.\ ] ]    structure functions are assumed to be those defined in the parton model i.e. the combinations of a pdfs @xmath7 } \nonumber \\
f_3 \left ( { x , q^2 } \right ) & = & 2\sum\limits_j { \left [ { q_j \left ( { x , q^2 } \right ) - \bar q_j \left ( { x , q^2 } \right ) } \right ] }   \\
( { x , q^2 } \right ) & = & 2xf_1 \left ( { x , q^2 } \right ) \nonumber\end{aligned}\ ] ] using structure functions ( [ fs ] ) the cross section ( [ dis ] ) is rewritten in terms of contributions from separate partons @xmath8
@xmath9    where @xmath10 is a kinematic factor for parton @xmath8 . +
a probability of reaction on a given quark is : @xmath11    pythia fragmentation routines require a system of quark and diquark and perform fragmentation and hadronization using the lund algorithm . depending on the interacting parton ,
we distinguish several cases @xcite :    * in the case of the scattering off the valence quark , a string is formed from the created quark and the remaining diquark . * in the case of scattering off a sea quark u or d , the remaining anti - quark annihilates with appropriate valence quark , and the created quark forms a string with the the remaining diquark , exactly as in the previous case . *
if scattering off an anti - quark u gives an anti - quark d or scattering off an anti - quark d gives an anti - quark u , the created parton annihilates with a valence quark .
* if scattering off an anti - quark u gives a strange anti - quark s or scattering off an anti - quark d gives an anti - quark c , it creates with one of valence quarks a strange or a charm meson and the remaining quarks form a string for the fragmentation .
* in the cases of scattering off a strange quark or anti - quark , the remaining strange constituent creates a strange meson with one of valence quarks and the remaining quarks form a string for the fragmentation .
we fine tuned the pythia6 generator parameters of the fragmentation . in fig .
( 1 ) the comparison of the charged particles multiplicities    @xmath12    as obtained from our simulation with the data from the fermilab bubble chamber @xcite is shown .
[ fig : total_cc_nu_proton_pionplus ] . for data points and for simulations
only events with hadronic mass @xmath4 gev were included .
data points are taken from @xcite , @xcite , @xcite , @xcite , @xcite , @xcite , title="fig : " ]    [ fig : total_cc_nu_neutron_pionplus ] .
for data points and for simulations only events with hadronic mass @xmath4 gev were included .
data points are taken from @xcite , @xcite , @xcite , @xcite , title="fig : " ]    [ fig : total_cc_nu_neutron_pionzero ] . for data points and for simulations
only events with hadronic mass @xmath4 gev were included .
data points are taken from @xcite , @xcite , @xcite , @xcite , title="fig : " ]
the only resonance we consider is the @xmath0 and we have to estimate the single pion production cross section as a fraction of the inclusive dis cross section extrapolated into the resonance region .
this is done separately for each spp channel and the obtained fractions are called 1-pion functions .
they are the probabilities that in a given point in the kinematically allowed region the final state is that of spp .
@xmath13    in our generator @xmath14 are reconstructed using the lund fragmentation algorithm .
they turn out to be functions of @xmath15 only and are shown in fig .
we see that up to the threshold for two pion production , 1-pion function for proton and the sum of functions for neutron are equal 1 . in more common langauge 1-pion functions
can be recognized as average elasticities of resonances @xmath16 @xcite . in fig .
( 2 ) we see that in fact the values of 1-pion functions are close to resonance elasticities in a wide range of hadronic invariant mass .
[ fig : total_nc_nu_neutron_pionplus ]   gev were included .
data points are taken from @xcite , title="fig : " ]    [ fig : diff_proton_pionplus ]
our model of spp combines in a smooth way the @xmath0 excitation model with the spp part of the dis cross section .
we choose a linear transition with respect to hadronic invariant mass @xmath17 gev . as a bonus
we obtain an artificial resonance - like behavior of the cross section at @xmath18 gev which closely resembles the contribution from the @xmath19 resonances @xcite .
we describe the non - resonant background as a small admixture of the dis spp contributions at low values of @xmath15 .
our mc reproduces the following analytical expression for the cross section : @xmath20    [ fig : diff_neutron_pionplus ]    where @xmath21 @xmath22 , depending on the channel .
we note that a similar value for the division line in the hadronic invariant mass between resonance and dis contributions ( @xmath23 ) was found by naumov et al.@xcite by fitting procedure to the existing set of experimental data .
the performance of our generator is presented on a series of plots .
first we show contributions to the inclusive cross section for neutrino and anti - neutrino interaction on isoscalar target ( figs .
3 - 4 ) from three theoretically separated dynamical mechanism .
the spp contribution is restricted by a cut @xmath24 gev .
[ fig : diff_neutron_pionzero ]    the cross sections for cc spp channels are shown in figs .
we conclude that the agreement with the data is satisfactory . in fig .
8 we show the plots for nc spp channels .
we also compared the distribution of events in hadronic mass for spp channels with the data from the bnl experiment .
it is an important test because our procedure of modelling spp channels is different from what is done in other mc codes .
we used the bnl neutrino beam and generated the same number of events as reported in @xcite .
the results are shown in figs .
( 8 - 10 ) . in the case of neutrino - proton reaction
the agreement is excellent . in the case of @xmath25 reaction
the agreement is very good but our simulations give too high @xmath0 peak . in the case of @xmath26
there is an experimentally measured access of events with small invariant mass ( smaller then @xmath27 mev ) which is not reproduced by our simulations .
we find our results for spp encouraging .
an improvement in the @xmath28 channel can probably be achieved by a more accurate treatment of the non - resonant background .
the collaboration of krzysztof graczyk at early stages of the mc generator and many useful discussions are acknowledged with pleasure .
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we construct a new monte carlo generator of events for neutrino interactions . 
 the dynamical models for quasi - elastic reactions , @xmath0 excitation and more inelastic events described by the dis formalism with the pdfs modified according to recent jlab data are used . 
 we describe in detail single pion production channels , which combine the @xmath0 excitation and dis contribution . 
 many comparisons of the outcome of simulations with experimental data are presented .